Design of load bearing joints

7/28/2019 Design of load bearing joints

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DESIGN OF LOAD BEARING JOINTS ANDCHALLENGES

BEL110-PROF.PRASHANT MISHRA

SUBMITED BY:

ROHIT GOTHWAL

(2010ME20796)


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DESIGINING OF LOAD BEARING JOINTS

INTRODUCTION:

The major load- bearing joints of the body are hip, knee and ankle. The human body may be subjectto loads externally applied in additional to gravitational and inertial force actions.at any section of limb or body part, the resultant force and momentum may be calculated from a consideration of relevant loading. These resultant loads will be transmitted by stress in anatomical structurestraversing the section. If junction between body sections is considered, the relevant body structuresare articulating surfaces of bone, the ligament in the region of joints and muscle and tendonstraversing the section.

In this term paper we will studying about ankle joint , its mechanical properties i.e. stress and forces,structure and stability. The ankle joint act as a link between leg and foot and play an important role intransferring load from leg to foot.

THE ANKLE JOINT

Bod y conguration

The ankle consists of three bones (tibia, bula and talus), collateral and syndesmotic ligaments and isa dynamic and highly congruent joint.

Ligamentous conguration

The deltoid ligament stabilizes the ankle medially [10]. The contribution of the deltoid ligament toankle joint contact has been reported that deltoid ligament sectioning produces the greatest changesin both contact area size (decreased up to 43%) and peak pressure values (increased up to 30%) andthis emphasises the fundamental role in ankle mobility played by the deltoid ligament .

The syndesmotic ligaments join the tibia to bula and consist of an anterior and posterior tibio bular ligament in addition to the transverse tibiobular ligament and the interosseous ligament .


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BIOMECHANICS:

Mechanical properties of bone and cartilage

The average thickness of the ankle cartilage is approximately1.6 mm whereas the thickness of the knee cartilage is 6 8 mmThe yield strain of human trabecular tibial bone is 0.73 -0.06% in compression and 0.65 -0.05% intension. The mean E (Youngs modulus) of the tibial cortex was found to be 34.11 GP a. Morgan andKeaveny showed that the Youngs modulus (E) of human trabecular tibial bone was 1091 -634 MPain compression and 1068-634 MPa in tension while Lowery found that the subchondral bone of thedistal tibia had an elastic modulus of 300 450 MPa. After removal of the subchondral plate,compressive resistance was lowered by 30 50%, and with sectioning of the subchondral bone 1 cm

proximal to the subchondral plate, by 70 90%. It is found talar bone to be 40% stronger than tibial bone, It is also found that removing part of the cortical shell of the talus placed abnormal increasedstress on the remaining talar cancellous bone. Therefore, ideally, the talar component of the

prosthesis should be anatomically sized, fully cover the talar body, and have a wide support on talar neck. Thus, as much superficial subchondral talar bone should be saved as possible, particularly theanterior part of the talar body and talar neck. It is that also found an eccentricity of the area of maximal bone strength of the distal tibia that is posteromedial and not central. This area of stiffer

bone could act as a pivot point, with the risk of overloading the surrounding weaker anterolateral bone. To avoid off centre forces on a prosthesis and possible collapse of the we ak lateral tibia, proper alignment of the prosthesis and adequate ligament balancing of the ankle must be achieved. In particular, valgus misalignment should be corrected.

Axis and range of motion

The normal range of motion in the ankle ranges from 238 to 568 of plantar flexion, and from 138 to338 of dorsiflexion. Three distinct axes of the ankle joint have been reported during various motions

based on the curvature of the talar trochlea with the axis inclined upwards medially duringPlantarflexion (PF) and upwards laterally during Dorsiflexion (DF). Although the ankle joint axis atthe neutral position is often regarded as a single axis, dorsiflexion plantarflexion hinge, the axisorientation may vary. The axis has been described as a changing axis or changing instant centres of rotation due to the shape of the talar trochlea and the action of soft tissues. In cadaveric and gait

studies, the rotation has been shown to range between 108 and 128. It is the varying centre of rotationthat allows the talus to glide and slide within the ankle mortise during PF and DF. Also, the curvature


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of the talus and the distal tibia show varying radii that allow horizontal rotations to occur in the footor leg with movements of the ankle and thus transverse plane motion is coupled with sagittal planemotion. The axial rotation of the tibia with respect to the talus is reported to be between 68 and128.Lundberg et al. Observed 8.98 of external rotation of the talus as the ankle moved from neutral

position to 308 of dorsiflexion, whereas a small amount of internal rotation occurred with plantarexion from neutral to 108, followed by external rotation at terminal plantarexion .

Michelson and Helgemo reported that dorsiexion resulted in an average of 7.28 3.88 of exte rnalrotation of the foot relative to the leg with ankle dorsiexion, and 1.98 4.128 of internal rotationwith plantarexion. Leardini et al. developed a \ mathematical model to explain the multiaxial motionof the ankle in the sagittal plane. A four-bar linkage model was described (Fig. 4) showing thetalus/calcaneus and tibia/bula rotating about one another on inextensible line segments thatrepresent the calcaneobular (line AB in Fig. 4) and tibiocalcaneal (deltoid) (line DC in Fig. 4)ligaments without resistance. Motion between the polycentric, polyradial trochlea consisted of acombination of rolling and sliding motions. In this model, rotation was dictated by the mostanterior bers of the anterior talobular and calcaneo - bular ligaments. Leardini [28] also showedthat these specic ber bundles were isometric through the range of sagittal motion of the ankle. Theinstant centre of rotation (shown by star in Fig. 4) translated from a posteroinferior to asuperoanterior position, which is consistent with several studies that suggest that the ankle iscongruent and rotates about a transient centre of rotation.


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Restraints of ankle motion

The tibiotalar articulating surface contributes to 70% of the antero-posterior stability, 50% of inversion/eversion stability and 30% of internal/external rotation stability. The rest of the Stability is

provided by the ligaments. Renstrom et al. found that during various motions of the ankle joint, theanterior talofibular and calcaneofibular ligaments were synergistic, that is when one ligament isstrained the other one is relaxed and vice versa and thus providing stability.

Forces and stresses in the ankle

The ankle has a load bearing surface area of 11 13 cm2 the tibiotalar area, however, accounts for only approximately 7 cm2 while Calhoun et al. found that during weight Bearing 77 90% of the load is transmitted through the tibial plafond to the talar dome. With aninterface area of 7 cm2, the average compressive load per unit area at the interface during gait would

be approximately 3.5 MPa in a patient of 700 N body weights. A vertical load on the ankle of 5.2times body weight was found during gait. The peak resultant force acting at the ankle joint during thestance phase during running was 9.0 13.3 BW, Landing from a jump generated 2 12 BW while heel-toe running at 4.5 m/s generated a force of 2.8 BW.In diseased ankles, the joint load decreased toapproximately three times body weight; the same values were noted in replaced ankles and

anteroposterior and lateral shear forces during gait were estimated to reach levels of two and threetimes body weight, respectively. The vertical load that is transmitted to the trabecular bone at the

prosthesis-bone interface may exceed the inherent trabecular bone strength in normal daily activities,having knowledge of the normal ankle joint, a discussion can now be carried about the various TotalAnkle Replacement designs.

KNEE JOINT


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The knee consists of three basic types of structures. Ligaments are passive elastic structures and can beloaded in tension only. Musculotendinous units are active elastic structures and can act only under tension.Bone is essentially non-elastic and serves to take the compressive loads in the joint.

The largest and most complex joint structure.

Some typical functions:

1. Transmit Loads. 2. Participate in motion. 3. Aids conservation of momentum. 4. Provides a force couple for body activities.

We will consider the knee as being composed of two joints, the patellofemoral joint and thefemorotibial joint. The relative position of the bones in a loaded joint is partially controlled by theshape of the joint surfaces in apposition with one another. Under weight bearing, the tibial spineinserts into the femoral intercondylar notch, creating an effective bony stabilizer. The shape of thisspine provides self-centring (laterally) during the transition from nonweight bearing to full weight

bearing. The shape of the femorotibial joint does not provide great stability in the anterior-posterior direction: the femur will slide off the tibia either anteriorly or posteriorly if there is no hindrancefrom any rigidly attached bony structure. Unfortunately, there is no bony surface to prevent thefemur from sliding posteriorly off the tibia. But the patella serves effectively as a bearing surface tokeep the femur from sliding forward off the tibia. Under these circumstances, the patella can beconsidered a part of the tibia, connected to the tibia by an elastic tendon. This combination of patellaand tibia cradles the femur and keeps it from sliding anteriorly off the tibial surface. The elastictendinous connection of the patella to the tibia, in harmony with the active quadriceps femorismechanism proximal to the tibia, serves as a shock absorber in buffering the patellofemoral jointfrom high deceleration forces. The rest of the joints in the body that would be subjected to theseshock loadings during rapid change in acceleration are all constrained by a rigid bone-to-bonecontact, such as the hip joint, with the ball-and-socket construction, or the spinal column, with thevertebral bodies aligned on top of one another. Such joints are all protected by the correct action of the knee joint in allowing these high-shock loads to be absorbed by the quadriceps femorismechanism and the patellar tendon.


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The ligaments of the knee also keep the relative positions of the femur and the tibia within bounds sothat contact between these surfaces from nonweight bearing to weight bearing occurs at theappropriate places. Figure 1 shows a schematic diagram of the cruciate ligaments in the knee. Whenthe tibia is suspended below the femur, the forces applied to the femur are resolved into vertical andhorizontal components. The horizontal components, being in direct opposition to the verticalcomponents, serve as a self centering mechanism to keep the femur and tibia in a good relative

position to one another. The ligaments also serve as passive load-carrying structures to back up theactive load-carrying elastic structures, the musculotendinous units.

The muscles of the thigh control rotation and deceleration as well as function as primary movers. Themusculotendinous units are generally separated into two synergistic muscle groups the quadricepsfemoris group and the hamstrings. The quadriceps femoris muscles are responsible for extension of the knee and deceleration of the forward motion of the femur on the tibia; the hamstring muscles'

primary functions are flexion of the knee and some basic rotational control of the femur on the tibia


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PATELLOFEMORAL JOINT FORCES

The patellofemoral joint is unique in that it protects the body's other joints by the way it distributesshock loadings in the knee. First, compressive forces from the femur are absorbed by the patella.Then, rather than being transferred directly as a compressive load, these forces are transformed into

tension forces in the quadriceps femoris and patellar tendons. This transformation allows the very powerful quadriceps femoris muscle to act as a retainer for the femur. The viscoelastic properties of the quadriceps femoris musculotendinous unit and the patellar tendon provide excellent shock absorption. During vigorous activity, very high deceleration forces are imposed on the body .

Figure 2 shows the patellofemoral forces relative to the quadriceps femoris tendon force as a functionof the knee joint angle.5 As shown, the patellofemoral force is zero with the knee in full extension(180) and is nearly 1.5 times the quadriceps femoris tendon force at 90 degrees of flexion. Thisincrease in patellofemoral force emphasizes the importance of controlling the compressive forcesdirected upon the patellofemoral joint as the tibia follows its helicoid path upon the femoralcondyles.


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Types of motion at knee joint

1. Rolling Motion - Initiates flexion.2. Gliding Motion- Occurs at end of flexion

ROLLING MOTION

GLIDING MOTION


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REFERENCES:

http://bonesmart.org/joint-replacement-surgery

http://www.engin.umich.edu/class/bme456/artjoint/artjoint.htm

http://www.ncbi.nlm.nih.gov/pubmed/7408313

http://www.pnas.org/content/83/9/2879.short


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Design of load bearing joints