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NHS Executive West MidlandsTechnological Developments: Partnership Funding
A Partnership Project to produce an orthotic hip joint providing ambulatory function for a variety of pathological conditions.
ORLAU, OswestryConsort Engineering, Hyde
EXECUTIVE SUMMARY
This project was undertaken by two organisations who have a history of collaboration in the field of walking rehabilitation, were committed to achieving a successful outcome and have the necessary physical and staffing resources to enable a thorough approach to all aspects of its completion.
Project management was undertaken by ORLAU who convened an initial meeting at which the division of responsibilities and a timetable for the various activities were agreed. Work was scheduled to begin in January 1997 and to be completed by December of that year. Good progress was maintained for the first six months and an Interim Report was submitted on time; however delays were encountered during the second half of the project and, following a dispensation from the West Midlands NHS Executive, the Final Report was submitted at the end of February 1998.
It should be noted that the delays, suffered by both partners, were entirely due to outside influences and not to difficulties with any aspect of the project itself.
The project proposal specified that a technical audit be undertaken of the orthotic hip joints currently available. This was achieved by conducting a search of manufacturer's catalogues, by contacting leading distributors and by attending the Naidex Exhibition at the NEC in May, 1997. The audit revealed that no joint was available which met more than eight of the twelve design criteria laid down by the project. Detailed structural calculations on a selection of the strongest and most versatile joints found were conducted to serve as a benchmark for comparison against the new joint to be developed.
The audit also allowed the compilation of information concerning the range of caliper and truncal side members in widespread use. This data, coupled with an assessment of possible future requirements, was an important element in the design process since the new joint must be capable of interfacing with the full range of such products.
An analysis of the potential maximum loads, together with the pattern of such loading, was undertaken, again with the aim of informing the design process. Research papers were consulted for data though it was noted that no information concerning magnitude of the potential rotational forces at the hip was available.
Considerable attention was paid to the issue of bearing selection for the new joint. Almost all existing joints employ quite simple arrangements and it was recognised that a much more sophisticated engineering approach was essential to achieve the functions demanded in the project brief. Conventional bearing technology was found to be sufficient to address the needs only in part, the major problem being that of their bulk. A solution was proposed which employed conventional rolling element thrust races coupled with a 'new technology' polymer based composite journal bearing. Much advice was given by the importers of this German manufactured material who, in turn, have expressed interest in publishing an account of this application in a future edition of their technical newsletter. With all of the above information now available the detailed design work could begin. Computer aided drawing software was extensively used in translating the emerging concepts into working drawings. These were further refined to take account of manufacturing constraints following extensive consultation between the project partners. The design which emerged incorporated sufficient features to address all of the design objectives laid down in the project proposal. Eleven of these objectives were fully incorporated, the twelfth partially so. The brief calls for the facility to control the flexion/extension stops to permit walking, standing and sitting. The ability to switch easily between walking and sitting is provided for but a standing stop function can be achieved only by the use of workshop facilities to vary the position of the stop adjusting screws. The possibility remains of future design work to improve this feature.
The first batch of prototypes were produced with virtually no manufacturing difficulties, a testament to the merit of the use of CAD and the thorough prior debate of the technical aspects of the design. These joints were then subjected to a programme of testing to destruction at ORLAU's premises using the format of test described in the relevant British Standard.
The outcome of this test programme exceeded expectations. The strength of the joint in almost all planes tested and the nature of their failure, sometimes only under severe overload, gave ample grounds for confidence that the basic design is very sound. Comparison with the values of the likely maximum service
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loadings derived previously were made. Factors of safety ranging from 1.4 to 4.8 were found to exist and the joint failed in a ductile, i.e. 'fail-safe' manner in every test.
One feature of the joint is the provision of adjustment of the range of movement into extension. Whilst this constituted one of the design objectives it was recognised that the facility would offer clinical benefit in a somewhat narrow range of applications. It was thus decided to contemplate a second manifestation of the joint which dispensed with the feature. This was achieved with minimal alteration to the main components of the assembly but offering the advantage of a notable simplification and hence a potential reduction in cost of the finished article. The project partners anticipate a future in which both forms of the joint will be manufactured and marketed.
A revised set of drawings was thus produced and the second batch of prototypes manufactured without the adjustable extension stop. A number of minor refinements which had been omitted in the first batch were incorporated in the second set of six joints. On this occasion, however, a number of technical difficulties in manufacture and assembly emerged which must be resolved in the pre-production development work which will follow the conclusion of this funded project.
Testing of the second batch of joints served to confirm the findings of the initial programme in most aspects of the joint's performance. A modification to improve the strength during bending in flexion was proved to have been successful, though the removal of the adjustable extension stop led to a reduction in performance in this direction. Further work is needed to determine the optimum arrangement of this aspect of the design. Once again, generous factors of safety were encountered in all other phases of the test programme and the fail-safe ductile modes of failure were repeated throughout.
A simulated accelerated lifetime test was devised and conducted using a purpose built test rig in order to assess the durability of the joint, in particular of the composite bearing material. This was loaded to a degree exceeding the likely maximum service condition by a factor of more than 12 and the cycling rate gradually increased from near normal walking pace to a value some 5 times greater. Once again the outcome of the test gave good grounds for confidence in this aspect of performance.
The project partners are extremely pleased with the outcomes of this project. There are substantial indicators that a successful design has been generated and that a new orthotic hip joint will in future be commercially available which offers significant advantages over those currently marketed. Some early discussions have already taken place between the partners on ways in which it can be exploited for the benefit of users.
Task 1 - Project Management
Introduction
The proposal document for this project identified ORLAU as the partner who would be responsible for Project Management.
ORLAU and Consort Engineering are particularly well placed to collaborate on a venture of this nature, there being a well established background of working together over a period of many years. Much of this is based around the fact that Consort is a manufacturer and supplier of the Swivel Walker, an HKAFO orthosis which is assembled from a kit of parts to fit individual patients. Consort manufacture the kit of parts which, when assembled, provides the patterns of walker designed and developed by ORLAU over a period of years. Consort has also collaborated with Salford University to produce a similar device to their design, and has undertaken its own development work to produce other similar devices with its own novel features.
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The Managing Director of Consort Engineering, Mr. Brendan Lomas, has many year's experience of working with orthotists in the supply and fitting of these devices and has developed a good understanding of many of the clinical issues associated with this work.
An initial meeting was convened between ORLAU's Technical Director, John Stallard, Product Liaison Engineer, Peter Woollam and Brendan Lomas on 13th January, 1997. The purpose of this meeting was to agree the division of responsibility for the completion of the tasks identified in the proposal document and to establish a proposed timetable for the completion of these tasks. Several subsequent meetings took place at which technical matters and other aspects of progress were reviewed. The task of compiling the interim and final reports was allocated to ORLAU, with financial statements of Consort's project costs to be inserted as annexes.
Task 2 - Technical Audit of currently available orthotic hip joints
Introduction
The orthotics industry in this country is estimated to have an annual turnover of some £70 000 000, with a structure involving manufacture through to patient supply being achieved by some 150 companies.(1) Of these, however, only a small number are engaged in the manufacture of orthotic joints.
ORLAU has held a prominent position in the industry for some 20 years during which time it has gathered considerable detailed knowledge and understanding of its structure. At the same time ORLAU has become widely known by the industry and good contact exists between staff within the unit and senior staff in virtually all of the major British companies working in this field. The past years have seen many co-operative, collaborative and consultative activities between ORLAU and domestic manufacturers, experience which has also led to a knowledge of the availability in this country of products manufactured abroad.
In order to conduct this technical audit it was therefore possible to establish a list of those companies who are currently marketing orthotic hip joints in this country, a list which includes British, European and American manufacturers and their import agents. Copies of the catalogues produced by these companies were obtained in order to gather information about the joints currently available. Telephone conversations were conducted with leading distributors to ascertain if any new joints were available which did not appear in catalogues (one such was found). As a further measure, staff from ORLAU attended the NAIDEX Exhibition at the NEC on 13th May 1997 and visited the stands of appropriate exhibitors. It is therefore believed that the evaluation which follows is based on the latest information available.
(1) Managing Director, Remploy Healthcare Ltd.General
A study of the hip joints currently on the market quickly reveals a number of conclusions.
1. The majority of joints are of a simple type and fall into one of three categories:
a) Freely hinged: i.e. the joint is free to move with no provision for governing the range of such movement.
b) Single locking: i.e. the joint is either free to move or can be locked in the 1800 position for standing.
c) Double locking: i.e. the joint is either free to move, or can be locked at 1800 for standing and at 900 for sitting.
Joints of this type are available from all manufacturers, typical among them being the Masser joint, Part No. 32SD
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Fig.1 Masser hip joint 32SD
2. Becker Orthopaedic produce a joint which allows greater control over the range of joint movement. Their Model No. 1096CP incorporates a stop mechanism which permits complete freedom in
flexion but resists the degree of movement into extension. The limit of this movement can be adjusted in increments of 60, an adjustment which is achieved by stripping and re-assembling the joint. In addition, a locking ring action is incorporated which allows the joint to be fixed at any position between the limit of extension and the range of flexion, again at intervals of 60.
3. The OTS Corporation (USA) have recently introduced the new "Notch-Lock" joint into their range which incorporates very similar features to the Becker joint described above and having the additional feature that the locking mechanism is lever operated. This is an important benefit in that the locking rings fitted to all other joints are so small as to demand good hand function for their operation.
4. No other principal design features are found amongst the range of joints currently on the market.
Analysis
The purpose of this audit is to measure the performance of the joints currently available on the market against the criteria specified in the proposal document for this project. These criteria are:
1. An adjustable range of flexion/extension control
2. Override on stops to permit sitting
3. Stop settings which provide no flexion/extension to permit secure standing
4. Stop mechanisms which can be operated by patients with compromised control of the hands.
5. High lateral rigidity
6. No lateral bearing play
7. Very high rigidity in the fore and aft plane
8. Low friction bearings
9. High resistance to torque about the vertical axis
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10. A facility to take a variety of side member sections
11. A facility to accept a variety of sacral cross bracing arrangements which do not intrude into the
space between patient and orthosis
12 A size envelope which is cosmetically acceptable
On the basis of the information presented thus far, several of these can now be addressed.
Criteria 1The survey reveals that there is no joint currently available which satisfies this requirement. One joint, from Becker Orthopaedic, allows coarse adjustment of extension control but none in flexion.
Criteria 2All joints currently available meet this condition and many are available which can be locked in this position.
Criteria 3All of the joints found in the survey can be locked at 1800 to permit secure standing.
Criteria 4Only one joint, from OTS Corporation, offers a lock mechanism which can be operated by patients with compromised hand control.
Criteria 10It is a characteristic of every joint found that it is either:
a) fabricated with solid rectangular cross-section upper and lower members forming an integral part of the joint, or
b) machined with recesses to accept a specific size of rectangular cross-section upper and lower members.
No joint has been found which offers any flexibility in the choice of cross-section of upper and lower members to which it may be attached.
Criteria 12All of the joints found use hinge assemblies which present a low profile. Many patterns of joint are available in a range of sizes such that the purchaser can select the cross-section of upper and lower member to match the weight of the patient. In most cases the hinge assembly is only slightly larger, in both width and thickness, than these members.
A number of the above criteria refer to the strength of the joint. In criteria 5 and 8 the need for rigidity in the two vertical planes is highlighted and in criteria 9 the resistance to torque, or twisting, is mentioned.
The ability of a structural member of an orthotic (or any other kind of) device to withstand the loads placed upon it will depend on the dimensions of that member and the material of which it is made. In predicting the behaviour of components under load, engineers use a number of values calculated from their physical dimensions. The Second Moment of Area, I, is used when assessing the extent to which a member will bend under the action of a known force acting at a known point and thus producing a calculated Bending Moment, M. The Polar Second Moment of Area, J, is used to determine the twist which will be generated by a Torque (turning moment), T.
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The ability of a given material to resist distortion when stresses are induced under the action of applied loads is expressed by the Modulus of Elasticity, E, in bending and by the Shear Modulus, G, in torsion. (For a large number of the family of metals and alloys in widespread use in manufacturing the values of E and G are the same, or differ by only very small margins.)
In order to assess the combined effect of a chosen set of dimensions for a structural member and a chosen material, the values of I and E are multiplied to evaluate the effects in bending and the values of J and G are multiplied when assessing torsion.
Table 1 overleaf presents data calculated for a sample of the joints listed by manufacturers and the side member sections which accompany them.
Criteria 6 and 8These refer to the function of bearings within a joint assembly. In the design of a hip joint the manufacturer must consider how best to ensure the required freedom of movement in the flexion/extension plane whilst at the same time resisting any tendency for lateral play.
All of the joints found in this survey are of similar basic character in that the lower member is drilled near its upper end, inserted between the jaws of a yoke formed in the upper member with the assembly secured by a hinge pin which is screwed or riveted into place. In many cases these components are machined to fine tolerances and bear directly against one another. Such an arrangement is capable of providing a joint with the required ease of movement and lack of play when new, but the absence of any materials with bearing properties will leave the joint vulnerable to rapid wear of components when carrying heavy loads. The levels of stress encountered by, say, a knee joint are likely to be such that the joint will give a satisfactory life expectancy but the greater stresses placed on a hip joint by a severely handicapped patient demand a more sophisticated arrangement.
Becker Orthopaedic address this issue by incorporating rolling element thrust races into their assemblies, whilst UCMS provide thrust washers and a nylon bush fitted over the hinge pin. These joints are likely to prove far more durable in service, though it is unfortunate that their designs do not incorporate means to compensate for any wear which may occur over periods of time.
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Becker Masser OTS Corp Otto Bock UCMS ORLAU
Part Number 1096-A 32SD Notch Lock NL616A
17H19 058 Prototype
Dimensions
11/4”x1/4
”
11/16”x3/16
” 5/8”x1/4
”
20mmx5mm 1”x1/4” 11/4
”x3/8”
Material Aluminium Alloy
SteelStainless
Steel Stainless Steel
Duralumin Aluminium Alloy
Material Modulii (E)&(G)kN/mm2
70 205 200 200 70 70
Section Modulus (I)
Lateral Plane677 157 338 208 542 2290
EI for the Lateral Plane
47400 32000 67600 41600 37900 160000
Section Modulus (I)
Sagittal Plane16900 2110 2120 3330 8760 25400
EI for the Sagittal Plane
1186000 433000 423000 667000 607000 1780000
Torsional Modulus (J)
17600 2150 2460 3540 9210 277000
GJ for torsion about
Long Axis1230000 440000 491000 708000 1840000 1940000
For Rectangular Cross Sections I = BD3 J = BD(B 2 +D 2 ) 12 12
Table 1. Structural Values for a sample of Hip Joints(including data on the new ORLAU design for comparison)
Criteria 11An orthosis which is designed to give support in pathologies where motor function of the lower limbs is deficient is likely to be constructed with cross bracing in the lower thoracic region. The upper members of the hip joint must allow for the fixing of such bracing. The joints found in this survey use solid rectangular cross-section upper members and the flat surfaces presented allow for this ease of fixing. Some caution must be exercised here, however, as any drilling of these upper members will, by virtue of the removal of metal, weaken the member. Referring to Table 1, the section with the greatest lateral rigidity was that used in the OTS Notch Lock joint. The drilling of a hole through the centre of this section to allow passage of, say, a 6mm screw would reduce the value of I (and hence its structural performance) from 338 to 203 mm4 , a reduction of 40%.
Conclusion
The survey of hip joints currently available reveals a somewhat disappointing picture of a lack of variety of provision. All manufacturers produce the simplest type of joint and most offer this in a range of sizes. There is, however, quite a noticeable absence of innovation in design concept but some evidence that American manufacturers in particular are beginning to address this issue.
The experience of ORLAU and others working in the orthotics field is that joint manufacturers are competent engineers and that all of their products are well made and perform their function efficiently. The lack of design innovation, however, places limitations on the uses to which any of the joints can be put.
Some of the twelve design criteria specified in the proposal document for this project are met by every joint studied, but even the best joint, the OTS Notch Lock, offers a high performance rating in only eight of these.
The technical demand in designing a new joint which presents a substantial improvement on the above is not inconsiderable. Inevitably perhaps, as with most design exercises, some compromises will have to be struck in arriving at a final specification. ORLAU believes that a successful conclusion to this project will yield a product which has the potential to deliver improved orthoses to patient groups already receiving treatment, and to widen the range of pathologies for which treatment regimes can be devised.
Task 3 - Identify potential side member sections to be connected to the joint
Introduction
In compiling the proposal document for this project, ORLAU had in mind the needs of:
1. The patient groups for which innovative treatment regimes are being devised within ORLAU. These often use a combination of an orthotic device and appropriate physiotherapy. One of the most significant categories which is currently receiving our attention is that of the quadriplegic cerebral palsy sufferer. In order to permit ambulation by such patients it is believed that a reciprocal walking orthosis is required to give support to compensate for inefficient muscular activity and provide control of involuntary muscle spasms.
2. The potential for the wider application of a new hip joint in other clinical centres working in similar fields. Any such applications of the new joint will necessitate its use within an orthosis where it
interfaces with truncal and lower limb elements within the overall design.
Task 3 in the proposal document calls for the need to establish the likely range of side member sections to which the new hip joint could be expected to connect. This report deals with this question.
Lower limb membersIt is difficult to envisage a use of the new joint which will not involve the need to link its lower member with a lower limb caliper section. All of the manufacturers referred to in the Interim Report on Task 2 produce a range of knee joints which are designed to be incorporated in such calipers. These joints, without exception, use solid rectangular cross-sections in the construction of their structural members. A review of the range of knee joints available will therefore establish the largest and smallest cross-sections which might be connected to the hip joint.
Manufacturers catalogues were examined, with the result that the largest section width is found in the Otto Bock joint, Part No. 17K43, measuring 21mm. The greatest thickness is used by Becker Orthopedic, namely ¼" (6.35mm). Similarly the smallest width and thickness are both found in the Otto Bock joint, Part No. 17K32, which measures 12 x 3mm.
Provision will thus need to be made for connection to this range of sizes, with an allowance being made for overlap of the mating components to ensure security of fastening. ORLAU's experience highlights the merit of including sufficient such overlap to allow for adjustment in the fixing of these mating components. Large orthoses are, by nature, relatively expensive to produce and the interests of good economics are served when such an orthosis allows for adjustment to match patient growth.
Truncal membersIt is much more difficult to predict the nature of the upper side members to which the new joint may be connected. The report on Task 2 revealed that the entire range of hip joints currently available use solid rectangular cross-sectional members above the hinge assembly. The sizes employed range from a maximum 1¼" x ¼" (31.75 x 6.35mm) used by Becker Orthopedic to the minimum 12mm width (OTS Corp.) and 1/8" (3.175mm) thickness (Becker Orthopedic).
ORLAU has considerable experience in the design of large orthoses, and many of the successful features of, for example, its ParaWalker will inform the work to be undertaken on this project. Amongst these features is the nature of the body brace and the manner of its fixing to the hip joint. The solid rectangular cross-sections described above have the disadvantage that the drilling of holes to affix adjacent components can significantly weaken the structural capacity of a member. (This fact was noted and supported by calculations in the Interim Report on Task 2, p.8.) In the design of the ParaWalker this weakness is overcome by the use of upper members made from extruded aluminium alloy channel sections. Such sections provide a large 'projected' area having thin wall sections where the structural capacity is much less affected by the drilling of holes. The design of the new hip joint should therefore anticipate the choice of such sections for the upper side members of orthoses. The ParaWalker employs a channel section whose dimensions are 1" x ½" x 1/8", a size which is unlikely to be exceeded in any application of the new joint.
Task 4 - Establish the loads and loading patterns which an
ambulatory orthotic hip joint might be required to accomodate.
Introduction
It is a relatively simple matter to identify the nature of the potential loading patterns which the hip joint may be required to sustain but, by contrast, quite difficult to establish with confidence the magnitude of such loads.
The essential purpose of a hip guidance orthosis is to provide control of the movement of the anatomical hip joint. In those pathologies where the relevant musculature is inactive the orthosis will provide a rigid exoskeleton to maintain the appropriate relative position of body segments and these positions must be maintained against the effects of gravity. Where muscular activity is present but the control of such activity is compromised the orthosis permits movement of the anatomical joint within an externally determined limited range. This range of movement is permitted against the effects of gravity and against the influence of muscle activity directed in an inappropriate manner.
Any patient, regardless of pathology, may use their orthosis to provide partial body weight bearing during standing and ambulation. The joint will be subjected to bending loads in both coronal and sagittal planes during walking and it may form part of a larger structure which is intended to resist internal or external limb rotation.
Levels of loading
The initial target group for whom the joint is envisaged is the junior patient suffering cerebral palsy with total body involvement. In this respect the term junior will be taken to include those with a body weight up to 50Kg. The category will thus include virtually the entire population up to the age of 10 years, more than three-quarters of 12 year olds and a substantial minority of those up to the age of 16 years. (2)
In order to determine the potential maximum loadings the 'worst case scenarios' will be considered for each of the types of loading referred to above.
1. Support of body weight:
The worst case which could arise is that where the entire weight were brought to bear through a single joint. A body mass of 50Kg presents a vertical force due to gravity of approximately 500 Newtons.
.2. Bending Moments in Coronal and Sagittal Planes:
The greatest forces which will tend to produce bending in either of these planes can best be measured by evaluating the horizontal components of the ground reaction forces which occur during walking. Such forces could be transmitted through a long leg caliper structure to the hip joint. By considering forces in this manner, the condition in which the longest moment arm which can arise (i.e. the full length of the leg) is used.
Stallard and Major measured the crutch forces exerted in walking by patients using a hip guidance orthosis. These forces were measured at the point of contact between the crutch tip and the ground and in the direction of inclination of the crutch. They found the maximum such force to be approximately equivalent to one-third of body weight. (3) Assuming that during walking activity the
crutch is inclined, at its lowest position, at 600 to the horizontal, then the horizontal component of the ground reaction force will be half that of the force in the direction of inclination (Cos 600 = 0.5). It
follows that the maximum horizontal component of the ground reaction force generated by a 50Kg patient will be approximately 82N.
This horizontal component will itself be in the plane of the inclined crutch and will normally have further components in the directions of the coronal and sagittal planes. However, it is conceivable that the crutch(es) could be placed in a position directly ahead (or behind) the user or directly to the side, in which cases the whole of the horizontal component would be directed in these body planes. Any such component will be resisted by an equal and opposite component generated between the patient's foot and the ground. (the absence of such a reaction would constitute a lack of equilibrium and lead to 'skidding' of either the crutch or the foot).
(2) 'People Size' - Computerised Database of Human Anthropometric Data compiled by Loughborough University
(3) "The Dynamics of walking using the Hip Guidance Orthosis with Crutches.Prosthetics & Orthotics International, 1981 Vol.5It is these equal and opposite reactions at the foot which may be transmitted via the orthotic
structure to the hip joint. The greatest bending moment can occur will be in the case of the tallest patient. The source of anthropometric data referred to above reveals that 92% of British boys will achieve a weight of 50Kg by their sixteenth birthday. It further reveals that the greatest leg length of a 16 year old of 50Kg weight is approximately 750mm. When this figure is coupled with the horizontal force
derived above it yields a maximum bending moment of 61.5Nm.
3. Rotational loading:
No reliable data has been found which describes the forces generated in correcting rotational deformities of the lower limb. The proposal document for this project highlights the fact that there is at present no orthosis which is widely available which addresses the problem of uncontrolled hip rotation. At this stage of the project, therefore, the aim will be to produce a joint which presents at least as good a resistance to torque as any other joint identified in the technical audit described in
the response to Task 2 within this report.
The proposal document further suggests that a successful outcome to this project may enable the issue of the control of rotational deformities to be addressed with greater prospects of success than
might otherwise have been the case.
Task 5 - Identify appropriate bearing components
Introduction
The response to Task 2 in this interim report refers to design criteria 6 and 8 which are concerned with the need to incorporate appropriate bearing arrangements in the new joint. The comments made in that response imply some disappointment that manufacturers of joints currently on the market rely largely on somewhat simplistic bearing arrangements in their designs.
Task 5 calls for the examination of a more sophisticated approach to this issue in an attempt to ensure low friction and good wear properties for both journal and thrust loadings, these objectives to be achieved within a compact size envelope. In addition, it is considered essential that the whole assembly be designed in a manner which permits easy dismantling and re-assembly, interchangeability of components and hence simplicity of repair and replacement of worn or damaged parts. It is recognised that whilst the original manufacture of the joint will take place in controlled conditions where engineering expertise and excellence can be assured, the same cannot be guaranteed amongst orthotic suppliers and hospital departments where patients are fitted.
General
Bearings are so widely used in engineered devices that the technology which surrounds them is well understood, readily accessible and has spawned a wide range of products and materials to satisfy an enormous variety of applications. Nonetheless, the need to ensure that the new hip joint can be produced at reasonable cost demands that the bearings employed be of a type which is readily available "off the shelf" rather than being the subject of special manufacture. Care in making the selection will be essential in order to ensure that they can withstand the levels of loading discussed in the response to Task 4 whilst remaining within a compact size envelope.
Before such a selection process can begin it is necessary to identify the basic layout and form of the major components of the joint. The practice of using an upper 'limb' machined to present a two-pronged fork which will accept the lower limb is, perhaps, the most obvious, widely used and practical arrangement. Such a form is envisaged for the new joint with a layout which incorporates a journal bearing to ensure controlled freedom of joint flexion and extension, and thrust bearings to minimise lateral play as shown in Fig. 2.
Bearing Selection
The need to produce a joint which hinges with as little resistance to movement as possible, coupled with the levels of loading discussed earlier, would normally lead to the conclusion that rolling element bearings would be the preferred choice.
Fig. 2 reveals that the journal bearing must be large enough to accommodate a hinge pin of sufficient diameter to withstand the anticipated loading. The inner diameter of the thrust races must also be no less than the outside diameter of the selected journal bearing and it follows that increasing the inner diameter of the thrust race leads to a larger outside diameter, which impinges on the desire to produce a compact assembly. A search through catalogues of such bearings did not reveal a combination which could satisfy these constraints and hence the 'perceived wisdom' of the use of such types had to be re-examined.
Plain bearings in a variety of materials are widely available and the possibility of their use was thus considered for the new joint. In order to produce an assembled joint which has minimal 'play' it is necessary either to ensure manufacture to very close tolerances or to incorporate provision for adjustment during assembly. Manufacturing techniques allow the machining of the hole in the lower limb to a close tolerance to accept the journal bearing with relative ease. The same can be said of the outside diameter of the hinge pin where it fits inside the journal bearing. The machining of the faces of components to provide a close fit for the thrust races is considerably more difficult and potentially expensive. It was therefore decided to avoid close tolerancing in an attempt to achieve this, but rather to permit adjustment during assembly. Such provision carries with it the risk of over tightening, a risk which would become more critical were plain thrust washers used as such an arrangement would be much more inclined to 'lock up' if overtightened.
The solution thus arrived at was that of retaining a rolling element type of thrust race but using a plain journal bearing. The newest generation of materials employed for such journals are constructed of thermoplastic alloys which are reinforced with composite fibres and impregnated with solid lubricants. They offer good wear and pressure resistance and maintenance free operation at low cost. They are available in a wide range of sizes which have in common a small wall thickness, all of which makes them very suitable for this application.
Task 6 - Outline Design Proposals.
On the basis of the information collected and described in the foregoing chapters, the initial design proposal was formulated and the essential geometry of the major components determined using Computer Aided Drawing software. Once the major features had been identified in the initial drawings the details of individual components then followed.
These drawings contain all the information necessary for manufacture and copies were first distributed for perusal and comment by ORLAU engineering staff before being sent to Consort Engineering. The drawings reveal that the basic nature of the joint is that of a body machined to provide a 'fork' profile to accept the lower member. Control of the degree of movement of the lower member is provided by adjustable screws, the tips of which bear against a stop slide which is mounted at the root of the fork in the body. In walking mode these screws permit adjustment of the degree of hip extension between approximately 00 and 240, and the degree of hip flexion between approximately 120 and 300. The stop slide is mounted in such a manner that it can move between two conditions, namely a position in which the above controlled range is enabled and a second position where, due to the profiling of the underside of the slide, the limit on extension is removed and the lower member is free to rotate through some 1050 from neutral to allow sitting.
The stop slide is also mounted in such a way that spring loaded 'click stops' positively locate the slide in one of these two conditions. A spring bias is incorporated for the purposes of safety such that if the slide is left between either of these two positions it will automatically move to the walking mode setting.
In arriving at the detailed design proposal, account has been taken of the range of stock sizes of raw materials available in order to minimise the demand for machining of components. Where possible, use has been made of standard 'bought-in' components, again for the purpose of simplifying manufacture and assembly and hence reducing costs.
The following drawings have been compiled:
Dwg.No. JHJ1 General Assembly Issue 1 30.5.97Dwg.No. JHJ2 Body Issue 1 30.5.97Dwg.No. JHJ3 Lower Member Issue 1 30.5.97Dwg.No. JHJ4 Hinge Pin Issue 1 30.5.97Dwg.No. JHJ5 Stop Slide Issue 1 30.5.97Dwg.No. JHJ6 Extension Stop Block Issue 1 30.5.97Dwg.No. JHJ7 Extension/Flexion Adjusting Screws Issue 1 30.5.97Dwg.No. JHJ16 Bought-in Parts Issue 1 30.5.97
N.B. One complete set of these drawings was supplied with the interim report on the project.
Task 7 - Calculation of Load Distribution, Material Selection
Introduction
The report on task 2 in this interim submission described the nature of the calculations which must be performed in order to determine the resistance of a structural member to bending and twisting. The drawings of the body and lower member components presented under task 6 reveal the dimensions to be used in the manufacture of the initial prototypes.
From these drawings it will be seen that lower member is the component having the smaller dimensions, and is thus more vulnerable to failure. The following calculations are therefore based on the lower member only.
I = 20300 (Sagittal Plane)I = 1820 (Coronal Plane)J = 27713
Comparison of these figures with the data contained in Table 1, page 7 of the response to Task 2 reveals that the proposed new joint offers greater inherent load resistance than any other currently on the market.
Load distribution
Using the values above in conjunction with the data concerning the maximum loads which it is estimated must be sustained in service (see response to Task 4), it is possible to evaluate the stresses which will be generated using the 'Bending Equation':
M = sI y
M is the Bending Moment, y is half the thickness of the member (*) and s is the stress.
Hence, in bending in the Coronal Plane: smax = 161 N/mm2
in the Sagittal Plane: smax = 48 N/mm2
in compression: smax = Load = 500 = 1.7 N/mm2 Area 303
(*) For rectangular cross-sections
Material Selection
In contemplating the choice of material for the manufacture of the components of the joint, two conditions must be satisfied to ensure patient safety. The material must have the inherent capability to withstand the anticipated stresses which will be placed upon it and, in the event of unexpected circumstances, should provide a benign mode of failure should its maximum strength be exceeded.
Other factors, such as weight, cost, availability, ease of machining and finishing also need to be considered but with secondary status of importance.
Taking account of all of the factors encountered thus far, the first prototypes will be machined using aluminium alloy for the major components, namely the body and lower member. BS 1474 (1987) recommends the figure of 195 N/mm2 to be taken as the value of the Ultimate Tensile Strength for the standard extruded sections of this material. This value exceeds the maximum predicted in the stress calculations above and the material offers a ductile failure mode.
Task 8 - Modifications to design proposals.
The detail drawings of the proposed new hip joint compiled by ORLAU under Task 6 take account of the data previously gathered and discussed in earlier chapters of this interim report.
Once these drawings were compiled, two copies of each were supplied to Consort Engineering in order that they might consider any implications concerning the manufacturing processes to be employed. A meeting was then convened between the Technical Director and the Product Liaison Engineer of ORLAU, Messrs. John Stallard and Peter Woollam, and the Managing Director of Consort Engineering, Mr. Brendan Lomas.
The meeting took place at the premises of Consort Engineering and the following alterations to details contained within the drawings discussed and agreed:
1. Hip Joint Body, Drawing No. JHJ2
The drawing shows the outer face of the body machined with a curved profile, a feature which has been incorporated for cosmetic purposes. It was agreed that, since a special milling machine cutter would be required to produce this shape, it would not be required on the first batch of prototype joints. However, the removal of material associated with this shaping will have an influence on the strength of the component. It was therefore agreed that the first prototypes would be chamfered to
the same depth as the curved profile in order to enable realistic assessment of the load bearing capacity of the joint in tests.
It was also noted that the dimension of the circular 'spotface' which will be the location of the ORLAU decal in production joints was not stated on the drawing.
2. Lower Member, Drawing No. JHJ3
This component is to be produced in aluminium alloy. An anodised finish is called for in the drawing in order to increase the durability of the item. It was agreed that the first prototypes need not receive this treatment. It was further noted that good engineering practice determines that once the joint is in
production, the reaming of the 10mm diameter hole to receive the journal bearing bush should be undertaken after the anodising process.
Caution was expressed about the drilling of the holes for the M4 threads which will locate the flexion and extension adjusting screws. If these holes are too deep they may extend beyond the zone covered by the outline of the body once the joint is assembled. It is intended that the drilling be within this zone in order not to compromise the load bearing capacity of the lower member. The holes must therefore be drilled to a depth no greater than 24mm.
3. Hinge Pin, Drawing No. JHJ4
The domed shape of the head is a cosmetic feature and may be replaced with a flat head in prototype versions. There are no implications for strength with this change.
The hexagonal socket will offer a more robust and durable means of tightening the pin during assembly. Machining of such a recess calls for a dedicated broaching tool. First prototypes
may be machined with a 3mm wide screwdriver slot.
4. Stop Slide, Drawing No. JHJ5
The slot in which the bias spring and spring loaded plunger are located is dimensioned as 4.4mm wide. Standard milling cutters are available to machine a 4.5mm width. A permanent alteration to the drawing to this effect was agreed.
Each end of the stop slide is to be machined with a spherical depression, the purpose of which is to act as a guide for the fingers when the joint is being switched between walking and sitting modes. It was agreed that this feature need not be incorporated on the initial prototypes.
5. Extension Stop Block, Drawing No. JHJ6
The 2mm radius called for is not a critical dimension. A linishing process may be used to form this radius provided that, on assembly, there is sufficient clearance for this component to swivel freely.
The drawing specifies Steel, grade EN1A, but it was agreed that stainless steel would be a preferred choice once production commences. The drawing will be amended for production purposes.
6. Flexion and Extension Adjusting Screws, Drawing No. JHJ7
The dimension of radius 3 for the domed head is not critical, a linishing process may be used to produce this feature.
7. Grub Screw, Drawing No. JHJ16
The M3, 10 long grub screw shall be of the type with a cone point.
Task 9 - Production of 4 initial Prototypes
Consort Engineering undertook the manufacture of four prototype hip joints, i.e. two pairs of joints, on the basis of the design compiled and reported in Task 6 and the outcomes of the reviews described in Tasks 7 and 8.
No unexpected difficulties were encountered during the manufacture which was completed in accordance with the specifications defined and referred to above. Consort Engineering supplied the hip joints to ORLAU on 22nd July, 1997.
Task 10 - Four Point Bending Structural Tests.
The purpose of conducting these tests was two-fold:
a) To establish the behaviour and strength of the joint under varying conditions of load application with reference to the appropriate British Standard, BS 2574 : Part 3 : 1990
b) To determine the mode of failure of the joint when subjected to overload.
The figures below are reproduced from the British Standard and illustrate the manner in which bending loads should be applied and the need to conduct tests in four planes to verify joint behaviour in all circumstances.
ORLAU has the necessary facilities to conduct tests after the manner illustrated and the photograph below is extracted from a video recording made during one of the tests.
Fig 3. Four-Point Bending Test.
Test Procedure.
Figure 4 overleaf shows illustrations taken from BS 2574 which describes the correct procedure for tests of this nature. This procedure was employed with all four tests with the wider load span set to 350mm and the narrower span to 100mm. The load beams were mounted in ORLAU's Testometric 500 Test Machine fitted with a 10KN load cell. Prior to the conduct of the tests the machine was calibrated by an independent NAMAS accredited test house who verified its performance to BS EN 10002-2.1992. The data generated during each test were collected via an interface to a PC and processed using Microsoft Excel software.
Outcome of the structural tests.
All four joints were tested to failure in the manner described above. However, in order to gain the maximum information from the test programme the coronal plane, outward bending test was omitted in favour of conducting two tests in the sagittal plane, flexion bending orientation. It was believed that the results of outward bending in the coronal plane would not differ substantially from those in inward bending, though both tests will be conducted with the second set of prototypes to verify this.
This additional flexion test was necessary in order to assess the strength of the joint in a variety of service conditions. Given the wide range of adjustment of the flexion stop, it is likely that the nature of the failure of the joint will differ when this stop is adjusted towards the opposite ends of its range. The two tests in this orientation were therefore conducted to reflect this range of service use.
Fig 4. Extracts from BS 2574
Coronal plane - Inward Bending
The graph of the test results was as follows:
Orthotic Hip Joint 4-point bend test. Coronal plane, inward bending
0
500
1000
1500
2000
2500
3000
3500
0 5 10 15 20 25 30
Deflection (mm)
Loa
d (N
)
As the test progressed, the lower member of the joint was seen to suffer permanent bending without fracture. This observation, together with a study of the shape of the graph above, confirms that the failure follows closely the inherent ductile failure mode of the aluminium alloy of which the major components are made. This 'safe mode' of failure is regarded as a vital characteristic of the joint assembly. If a condition should arise in service where the joint is overstressed, the result will be a clearly visible distortion which, while representing a failure of the joint, will not compromise the safety of the user. (N.B. The vertical line at the end of the above graph shows the unloading of the joint at the culmination of the test.)After the completion of the test the joint was removed from the test machine and subjected to close visual examination. The first stage of this was undertaken without stripping the joint when it was observed that it retained freedom of movement, with only slight resistance, in flexion/extension over its full range. It was also noted that the sitting/standing stop function was fully maintained as normal. Stripping of the joint revealed no visible damage to the body, hinge pin or bearing assembly.
The point at which permanent deformation begins corresponds to that point on the graph above where the early (approximately) linear portion changes to a curved nature. The point is often referred to as the Limit of Proportionality and its exact position can be difficult to establish; in this particular case it is read as 2200N. The maximum load sustained is accurately established from the Excel data. These values may be used in conjunction with the figures for the geometry of the test rig given earlier to determine the bending moments at the onset of failure and at the maximum load sustained. Table 2 (page 20) summarises the results of such calculations for all four tests.
Sagittal Plane, extension bending
Orthotic Hip Joint 4-point bend test. Sagittal plane, Extension bending
0
500
1000
1500
2000
2500
0 10 20 30 40
Deflection (mm)
Load
(N)
Once again, the shape of the graph illustrates a progressive failure, the nature of which presents no threat to user safety. Observations made during the test, and confirmed by examination after the test, reveal that the failure occurred as a result of damage and distortion of the adjustable extension stop and the sliding stop block, components which are forced against one another in this loading condition. It was particularly encouraging to discover that the hinge pin remained intact throughout this test.
Sagittal plane, flexion bending
Orthotic Hip Joint 4-point bend test. Sagittal plane, flexion bending. Test No.1
-200
0
200
400
600
800
1000
1200
1400
1600
1800
0 5 10 15 20
Deflection (mm)
Load
(N)
As stated previously, two tests were conducted under this loading condition. In the first test the flexion adjusting screw was extended to the point where the joint range was limited to movement from extension into a neutral position. The adjusting screw was thus considerably extended and vulnerable to failure. It is likely, though, that this condition would only rarely be met in service and would not then be subject to high dynamic loadingThe purpose of the second test was to assess the strength and failure mode of the remaining elements of the joint with the flexion adjusting screw set to a position allowing a normal range of movement in the walking mode. For reasons of practicality in conducting the test, a steel wedge was fitted between the lower member and the sliding stop block and surmounting the flexion adjusting screw so that the joint could be mounted in the test machine in a neutral position.As expected, failure in the first of these tests occurred at a relatively low load. The adjusting screw was seen to bend and, within a short space of time, to fracture completely. This failure mode is not ideal and it is clear that there is merit in revisiting the joint design in an attempt to improve this performance. It is worth noting in Fig. 5 below, however, that with the value of load at the Limit of Proportionality taken from the graph as 1400N, the corresponding bending moment on the joint is 87.5Nm. This value exceeds the maximum likely service load found in Task 4 of 61.5Nm by a factor of over 1.4. Corresponding figures at failure are 108Nm and a factor of almost 1.8. It is also important to note that the loss of the adjusting screw has a limited impact on the function of the joint. Freedom of movement in flexion and extension was maintained and the only impact was the inability to limit the range of flexion movement. In many potential applications of the joint the loss of this element of control would have only a minor impact on the user. Examination of the joint after completion of the test revealed no damage to any other components.
Ortotic Hip Joint 4-point bend test. Sagittal plane, Flexion bending, Test No. 2.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 10 20 30 40
Deflection (mm)
Load
(N)
The graph above shows that the second test yielded the highest values of applied load and, not surprisingly, subsequent examination revealed the greatest damage caused. Once again a safe mode of failure was observed, there being an ample period of time where distortion due to the overload was evident. In this case, the vertical line at the end of the graph signals a more dramatic event than any previously observed.
Examination of the joint after the test revealed that movement could only be achieved against considerable resistance. Stripping the joint revealed that the hinge pin had broken, the fracture occurring at the junction of the bearing surface and the threaded tip. It was very pleasing to discover that, despite this fracture, the pin remained in place and the body and lower members did not separate, a valuable performance characteristic in serving to protect a user from danger. This important safeguard arises in large part from the effective use of Loctite 601 adhesive around the thread and the head of the pin. A data sheet with instructions for orthotic contractors using the joint will need to be produced to emphasise the importance of this feature of the assembly process.
Other damage to the assembly included considerable distortion of the plastic journal bush and a 'tearing out' of the flexion adjusting screw.
At Limit of Proportionality At Maximum Load Sustained
Load BendingMoment
Factor of Safety
Load Bending Moment
Factor of Safety
Coronal Plane,Inward bending.
2200 138 2.2 3069 192 3.1
Sagittal Plane,Extension bending.
1750 109 1.8 2340 146 2.4
Sagittal Plane,Flexion bending 1.
1400 87.5 1.4 1734 108 1.8
Sagittal Plane,Flexion bending 2.
3700 231 3.8 4730 296 4.8
N.B. The Factors of Safety are calculated by comparison with the maximum predicted service loading described in the response to Task 4.
Table 2. Summary of Results
Task 11 - Design Modifications,
As reported in response to Task 10, the performance of the joint exceeded expectations and yielded comfortable factors of safety at almost all stages of the initial test programme. Substantial modifications are therefore not called for.
The weakest performance was seen with the flexion stop extended to an excessive degree and, whilst this condition is unlikely to be met in normal service, it is appropriate to consider means of improving the outcome of this test. The 'second generation' of prototypes were therefore manufactured with a larger diameter adjusting screw, increased from M4 to M5. This change yielded a stronger screw but, with a reduced wall thickness remaining, weakened the lower member. Further testing revealed the optimum compromise.
Discussions amongst clinical staff within ORLAU confirmed the value of a joint offering the opportunity to adjust the permitted range of movement into extension. It was acknowledged, however, that this feature might only be employed in the treatment of a limited number of pathological conditions. It was agreed that, in many cases, this feature would therefore be redundant. The inclusion of this adjustment adds considerably to the complexity and hence the cost of the joint and may also yield a product which is less durable in service. Modifications were therefore proposed which dispensed with this facility and which were achieved by simplifying the design of the lower member and removing the need for the extension stop block.
It is envisaged that when the joint goes 'to market' it will be offered in two forms - with or without the adjustable extension range feature. The second batch of prototypes was manufactured without the facility and hence enable data on both manifestations of the joint to be collected within this project programme. It should be noted that most of the essential design features are common to both joints and the expectation was that they would yield very similar performances.The drawings call for a hexagonal slot to be machined in the head of the hinge pin to facilitate fitting and removal. Consultations between Consort Engineering and a specialist tool manufacturer have revealed a difficulty in producing this feature. The pilot hole required prior to the use of a broaching tool must be of sufficient depth to permit clearance for the build up of metal shavings. The depth required is such that the hole would approach the junction of pin head and shank and compromise the strength of the pin. The obvious solution might seem to be to use a screwdriver slot instead of a hexagonal recess. However, this may not provide sufficient 'purchase' to permit removal of a pin which has been effectively 'Loctited' into position.
The possibility of designing a dedicated extraction tool for use on a pin head with screwdriver slot has been discussed, but the issue has been identified as one to be dealt with in the production engineering phase prior to marketing the joint. As such, it is work which will be undertaken after completion of the contractual phase of this project.
Copies of the amendments to the drawings which detail all of the above are attached as an annex to this Final Report.
Task 12 - Production of 6 Pre-production Prototypes.
All of the above engineering and related issues were discussed between the project partners and, following the production of revised drawings by ORLAU, Consort Engineering manufactured the second batch of prototypes.
No unexpected difficulties were encountered during manufacture and the completed joints were delivered to ORLAU on 3rd February, 1998 where technicians assembled the components in readiness for the conduct of 4-point bend tests. Once assembled it became clear that certain features of the joints were not functioning in precisely the manner envisaged in the design. The following points were noted:
a) The sliding stop block which switches the joint from sitting to standing/walking mode is a somewhat ‘sloppy’ fit. The precise location of the stop block, the curved roof of the fork in the body in which the stop block ‘nests’ and the position of the spring plunger which locates the stop block need to be re-examined. Alterations to the dimensions and/or the tolerances governing the location
of these features may have to be considered to improve the action of the stop block. The design philosophy has been proven but this issue will need to be addressed upon completion of the funded project and during the pre-production phase of development.
b) The spring plunger and the compression spring fitted inside the stop block are intended to be ‘tuned’ during assembly. The correct condition is one in which the stop block is held at either of the selected positions, i.e. in sitting or standing/ walking mode, but if it should be moved to any intermediate position it will automatically slide to the standing/walking position. This is intended for reasons of safety for the user but the compression spring appears not to be strong enough for the purpose. Although the principle shows promise, further work on the design will be necessary to achieve reliable performance.
c) One of the joints was inadvertently assembled in an incorrect manner. Loctite 601 assembly adhesive had been used in the correct manner, i.e. applied to the head and the thread of the hinge pin and when an attempt was made to remove the pin to correct the error it was found impossible. Stripping of the joint was made possible only by drilling out the pin which had subsequently to be replaced. The ability to strip and re-assemble the joint is considered to be an important design philosophy and this experience demonstrates a further issue to be addressed in the refining of detail prior to achieving a marketable product.
Tasks 13 & 14 - Design and Build a Cyclic Loading Test Rig,
Introduction
This joint has been designed in a manner which makes use both of conventional engineering technology and of new materials. The engineering issues associated with its manufacture, together with an understanding of the behaviour of a majority of the materials used, are well tried, tested and understood. Against this background the 4-point bending test programme used with the prototypes is the most appropriate for the assessment of the strength of the joint assembly in its entirety.
An important feature of the assembly is the use of a polymer based composite material for the journal bearing. This material is new and largely unexplored by engineers. The manufacturer publishes data in the bearing catalogue which specifies the correct methods of fitting, the maximum loadings which can be sustained, and indications of the durability of the material. Some of the latter data uses laboratory test results, some uses anecdotal information gained in particular applications in which the bearings have been used; all support the view that the material is durable in nature. Whilst the information provided gave encouragement for its potential performance, the fact that the technology is new and that practical experience is extremely limited meant there was no directly relevant data on which to predict long term performance. This being so, it was considered appropriate to conduct a simulated lifetime cyclic test in order to explore the durability of the journal bearing arrangement.
(N.B. The bearing manufacturer, Igus UK Ltd, has been kind enough to supply the bearings for this project free of charge. They have been sufficiently interested in the nature of this application of their product to request that, on conclusion of the project, they be allowed publish a brief account of our work in their technical newsletter.)
A cyclic test may be conducted such that a component or an assembly is subjected to the expected maximum level of service load in a manner which closely replicates the actual mode of operation. Alternatively, it may be appropriate to exceed the service condition, both in terms of the stress applied and in terms of the speed at which the test is conducted, in order to compress the duration of the exercise. The project proposal anticipated that testing of two joints would be undertaken using each of these approaches. However, the publication of additional data by the bearing manufacturer, coupled with the need to obtain appropriate data within the timescale of this project, led to a decision to use the latter approach only. This was judged acceptable in view of the fact that the actual stresses which could be applied in the 'overload' test exceeded the service condition by a factor of 12.
The figures quoted in the response to Task 4 identify the maximum patient weight which the joint is intended to serve as 50kg. The bearing surface has a ‘presented area’ of 64mm2 (length 8mm x diameter 8mm) which yields a maximum potential pressure on the bearing of a little below 8N/mm2. However, since the joint will be used in ambulating orthoses a dynamic factor must be taken into account. ORLAU has considerable experience in the design and supply of ambulating orthoses and has studied dynamic effects extensively over a period of many years. The outcome of this experience is that the additional loads generated by a mobile patient will exceed those in a static situation by a factor of approximately 1.2. The maximum dynamic service load on the bearing should therefore be taken as some 9.2N/mm2.
The cyclic test was conducted with the gas spring used in the test rig adjusted to generate a stress of 120N/mm2 at the bearing surface, a value which was chosen to coincide with the manufacturer's stated maximum permissible level.
Design of a Cyclic Test Rig.
ORLAU does not possess a dedicated cyclic testing machine. Such equipment is widely available offering a range of sizes, maximum load capacities and power sources. Enquiries revealed that the lowest cost systems are available for sums of the order of £15 000. Test facilities are also offered by several commercial test ‘houses’, but experience advised that the cost implications of purchasing such a service exceeded the budget for this project.
It was therefore decided to harness the equipment in ORLAUs own mechanical workshop and to design a rig to exploit existing facilities. The rig must offer the following features:
a) ensure rigid location of the joint under test,
b) provide the facility to preload the bearing assembly to known levels and to maintain the load throughout
the cyclic movement of the joint,
c) cause the joint to oscillate repeatedly through a selected range of flexion/extension movement,
d) provide a means to monitor the number of completed cycles of oscillation,
e) incorporate appropriate health & safety safeguards for the operator.
Build a Cyclic Test Rig.
The test rig was constructed by ORLAU technicians within the unit's own mechanical workshop. An annotated drawing of the rig is presented in the annex to this report from which it can be seen that the structure was fabricated from standard rolled steel angle section. The rig was of a robust nature and designed in a manner which allowed it to be fitted to the bed of a large metalworking lathe, thus harnessing the rotation of the lathe chuck to generate the oscillating motion of the hip joint.
The body of the joint was mounted at one end of the rig and the gas spring (familiar to many as the device used to support the open boot lid of a car) at the other. The gas spring, nominally rated at 1200N, was of a type having a valve which allowed gas to be released, thus varying the applied force. This was adjusted to provide sufficient force to stress the bearing to the maximum level recommended by the manufacturer, a value in excess of 100N/mm2, and hence a factor of more than 10 times greater than the predicted maximum service load referred to above.
The crank and turnbuckle linkage were machined and adjusted to provide a range of movement from 6° extension to 9° flexion, values consistent with the expected service requirement of an ambulating orthosis. Note that it was reported earlier that two manifestations of the joint have been produced, with and without the adjustable extension stop feature. The cyclic test was conducted on an example of the joint which was fitted with this feature. The flexion and extension stops were adjusted such that, at the ends of the range of movement within the test rig, these stops came under pressure from the load exerted by the gas spring. In this way the test successfully replicated the most severe loading pattern which is likely to occur in service.
Counting of the number of oscillations was achieved by setting the lathe chuck in motion at selected spindle speeds and by timing the duration of the various phases of the test. Operator safety was secured by a combination of the standard machine guards fitted to the lathe and by the use of additional screens surrounding the installation.
A detailed description of the conduct of the test is contained in the final section of this report.
Task 15 - Results of Final Test Programme
4 Point Bend Tests
A series of 4 point bend tests were conducted on the second batch of prototypes on 17th February. Four joints were used to conduct a test in each of the four planes illustrated in Fig 4. This test programme included the coronal plane, outward bending, which was omitted in the earlier series and included a single test in the sagittal plane, flexion bending. The results were as follows:
Coronal Plane
Orthotic Hip Joint 4-Point Bend Test. Coronal Plane, outward bending
-500
0
500
1000
1500
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2500
3000
3500
0 10 20 30 40
Crosshead Movement (mm)
Load
(N)
Orthotic Hip Joint 4-Point Bend Test. Coronal Plane, inward bending
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1000
1500
2000
2500
3000
3500
0 5 10 15 20 25 30 35
Crosshead Movement (mm)
Load
(N)
The second graph above coincides with that from the first set of tests and, as anticipated in the account of those, the behaviour of the joint in outward bending proved to be virtually identical to that in inward bending. The two graphs above demonstrate a high degree of consistency in the three separate tests of the joint performance in this plane. All of the earlier comments concerning the safe mode of failure are emphasised by these results.
Sagittal Plane, extension bending
Orthotic Hip Joint 4-Point Bend Test. Sagittal Plane, extension bending.
0
200
400
600
800
1000
1200
1400
1600
0 5 10 15 20 25 30 35
Crosshead Movement (mm)
Load
(N)
In the response to Task 11 (Design Modifications) it was reported that the second batch of prototypes would not feature the adjustable extension stop. One outcome of this is that when the joint moves into full extension the lower member, rather than the extension stop, bears against the sliding stop block. The action of loading the joint forces these two components together and the lower strength and lower Young's Modulus aluminiumlower member is seen to distort at a lower loading in the graph above than was seen in the first set of test results. Though Table 3 reveals that the joint remains intact beyond the predicted maximum service loading, the factor of safety is reduced to a much lower level. Further discussion with clinical staff may clarify the extent to which users load an orthotic joint in extension and whether the performance recorded here may present a risk to user safety, bearing in mind the limited extension range of the anatomical hip joint. This in turn will determine whether additional design work is needed to improve this version of the joint.
Sagittal Plane, flexion bending.
The second batch of joints was modified to accommodate a larger flexion adjusting screw following the results in the first group of tests. The graphs below illustrates the success of this modification in improving the performance by a substantial margin, with further data given in Table 3.
Two tests were conducted in this plane, the first with the adjusting screw extended to limit flexion movement only to the neutral position. Though this condition is unlikely to be used in practice, the test result provides useful data concerning the strength of the enlarged screw. The second test used a joint adjusted to permit 180 flexion, the condition thought most likely to occur in service.
Orthotic Hip Joint 4-Point Bend Test. Sagittal Plane, flexion bending (from neutral).
0
500
1000
1500
2000
2500
0 2 4 6 8 10 12
Crosshead Movement (mm)
Load
(N)
Orthotic Hip Joint 4-Point Bend Test. Sagittal Plane, flexion bending (from 18 flexion)
-500
0
500
1000
1500
2000
2500
3000
0 10 20 30 40
Crosshead Movement (mm)
Load
(N)
It is worthy of note that all the results of the second phase of testing yielded ductile, and hence inherently safe, modes of failure.
At Limit of Proportionality At Maximum Load Sustained
Load(Newtons
)
BendingMoment
(Nm)
Factor of Safety
Load(Newtons)
Bending Moment
(Nm)
Factor of
Safety
Coronal Plane,Outward bending.
2700 169 2.7 3079 192 3.1
Coronal Plane,Inward bending.
2500
2200
156
138
2.5
2.2
3100
3069
194
192
3.2
3.1
Sagittal Plane,Extension bending.
1100
1750
69
109
1.1
1.8
1549
2340
97
146
1.6
2.4
Sagittal Plane,Flexion bending.(adjusted to neutral)
1700
1400
106
87.5
1.7
1.4
2080
1734
130
108
2.1
1.8
Sagittal Plane,Flexion bending.(adjusted to 180 flexion)
2300 144 2.3 2917 182 3.0
Sagittal Plane,Flexion bending.(joint wedged in neutral)
3700 231 3.8 4730 296 4.8
N.B. The figures in italics are those values obtained in tests on the first batch of prototypes.
Table 3. Summary of all 4-Point Bend Test Results
Cyclic Test Results.
The test rig was mounted as described and illustrated previously using the centre lathe installed in ORLAU's mechanical workshop to enable the following test sequence to be undertaken. The lathe spindle speed selected determined the rate at which the joint oscillated. (The initial speed of 38 cycles per minute corresponds approximately with normal walking pace.)
Day 1 Test commenced. Lathe spindle speed set at 38 r.p.m.
Day 2 Test maintained at above rate. Failure occurred in of one of the joints in the connecting rod of the rig. Test interrupted and a replacement joint fitted.
Day 3 25 000 cycles completed. Test interrupted, joint stripped and examined, no visiblesigns of wear detected.
Test re-commenced. Lathe spindle speed set at 59 r.p.m.
Day 4 50 000 cycles completed. Test interrupted, joint stripped and examined, no visiblesigns of wear detected.
Test re-commenced. Lathe spindle speed set at 112 r.p.m.
Day 5 100 000 cycles completed. Test interrupted, joint stripped and examined, no visiblesigns of wear detected.
Test re-commenced. Lathe spindle speed set at 190 r.p.m.
Day 6 200 000 cycles completed. Test ended, joint stripped and examined. Slight trace of plastic bearing material deposit found on the stem of the
hinge pin, otherwise no visible signs of wear detected.
ORLAUThe Orthotic Research and Locomotor Assessment Unit,The Robert Jones and Agnes Hunt Orthopaedic and District Hospital NHS Trust,Oswestry,Shropshire SY10 7AG
Tel: 01691 404531Fax: 01691 404058
Consort Engineering Ltd.Commercial Brow,Hyde,Cheshire SK14 2JR
Tel: 0161 366 6883Fax: 0161 366 9703
