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http://fai.sagepub.com/ Foot & Ankle International http://fai.sagepub.com/content/9/3/111 The online version of this article can be found at: DOI: 10.1177/107110078800900303 1988 9: 111 Foot Ankle Int James W. Brodsky, Sohrab Kourosh, Mel Stills and Vert Mooney Objective Evaluation of Insert Material for Diabetic and Athletic Footwear Published by: http://www.sagepublications.com On behalf of: American Orthopaedic Foot & Ankle Society can be found at: Foot & Ankle International Additional services and information for http://fai.sagepub.com/cgi/alerts Email Alerts: http://fai.sagepub.com/subscriptions Subscriptions: http://www.sagepub.com/journalsReprints.nav Reprints: http://www.sagepub.com/journalsPermissions.nav Permissions: What is This? - Dec 1, 1988 Version of Record >> at UCSF LIBRARY & CKM on December 7, 2014 fai.sagepub.com Downloaded from at UCSF LIBRARY & CKM on December 7, 2014 fai.sagepub.com Downloaded from

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Page 1: Objective Evaluation of Insert Material for Diabetic and Athletic Footwear

http://fai.sagepub.com/Foot & Ankle International

http://fai.sagepub.com/content/9/3/111The online version of this article can be found at:

 DOI: 10.1177/107110078800900303

1988 9: 111Foot Ankle IntJames W. Brodsky, Sohrab Kourosh, Mel Stills and Vert Mooney

Objective Evaluation of Insert Material for Diabetic and Athletic Footwear  

Published by:

http://www.sagepublications.com

On behalf of: 

  American Orthopaedic Foot & Ankle Society

can be found at:Foot & Ankle InternationalAdditional services and information for    

  http://fai.sagepub.com/cgi/alertsEmail Alerts:

 

http://fai.sagepub.com/subscriptionsSubscriptions:  

http://www.sagepub.com/journalsReprints.navReprints:  

http://www.sagepub.com/journalsPermissions.navPermissions:  

What is This? 

- Dec 1, 1988Version of Record >>

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Page 2: Objective Evaluation of Insert Material for Diabetic and Athletic Footwear

0198-021 1/88/0903-111$02.00/0 FOOT & ANKLE Copyright 0 1988 by the American Orthopaedic Foot and Ankle Society, Inc

Objective Evaluation of Insert Material for Diabetic and Athletic Footwear

James W. Brodsky, M.D.,* Sohrab Kourosh, Ph.D.,t Me1 Stills, C.O.,$ and Vert Mooney, M.D.5 Dallas, Texas

ABSTRACT Five of the most commonly used materials for shoe inserts (soft Plastazote, medium Pelite, PPT, Spenco, and Sor- bothane) were objectively evaluated in the laboratory to characterize their behavior in the following three specific functions that correspond to clinical use: (1) the effect on the materials of repeated compression. (2) the effect of a combination of repetitive shear and compression. (3) the force-distribution (force-attenuation) properties of these materials, both when new and after repeated compres- sion. The last function represents a model for relief of pressure beneath plantar bony prominences, a topic of special concern for the insensitive foot.

All materials were effective in reducing transmitted force over the simulated bony prominence with a rank order of effectiveness. Other factors considered were: amount and rate of permanent deformation offset by con- siderations of enhanced moldability when comparing the neoprene and urethane materials with the polyethylene foams. The ideal insert represents a combination of ma- terial to achieve both durability and moldability.

Shoe inserts, or orthotics, represent a common method of conservative treatment for various foot dis- orders. They are prescribed to athletes to correct foot mechanics, to provide cushioning for impact loading, and to relieve pain.

Shoe inserts are widely recognized and recom- mended by authorities as valuable in the treatment and prevention of plantar ulcers in the insensitive foot. For the diabetic, for example, their purpose is to protect and cushion the sole, and they function by distribution

Assistant Clinical Professor, Orthopedic Surgery, University of Texas Southwestern Medical Center; Director Orthopedic and Dia- betic Foot Clinics, Dallas Veterans’ Administration Hospital. Address requests for reprints to 3600 Gaston Ave., Suite 303, Dallas, TX 75246.

t Assistant Professor, Orthopedic Surgery, University of Texas Southwestern Medical Center; Director of Bio-Engineering Labora- tory, Dallas Veteran’s Administration Hospital, Dallas. * Assistant Professor, Health Care Sciences, and Instructor, Or- thopedic Surgery, University of Texas Southwestern Medical Center, Dallas.

9 Professor and Chairman, Department of Orthopedic Surgery, University of Texas Southwestern Medical Center, Dallas.

1 1 1

of weightbearing force, thereby diminishing the pres- sure concentration on the plantar skin beneath bony prominences.

there is little in the literature to guide the orthopedic practitioner in the selection of materials for the fabrication of these inserts. The rational basis for shoe insert instruction has been largely superceded in favor of subjective and anecdotal formulas. Little attention has been given to those cases in which the inserts have failed to relieve pain in athletes or prevent ulceration in diabetics, and still less attention has been given in the orthopaedic literature to analysis of the materials themselves.

The purpose of this study was to take a “first step” in the rational formulation of shoe inserts by evaluation of the materials used in their fabrication. We wish to characterize the nature of these materials as they relate to three specific functions that correspond to their clinical use. These functions are as follows: (1) the effect of repeated compression, (2) the effect of a repeated shear and compression combined, and (3) the force distribution properties of these materials, both when new and after repeated compression.

In spite of a few preliminary

MATERIAL AND METHODS

Five commonly used, commercially available mate- rials were selected: Plastazote (soft grade), Pelite (me- dium grade), Spenco, PPT, and Sorbothane.

Plastazote (Apex Foot Products, South Hackensack, NJ) is an expanded closed cell polyethylene foam that is available in three color-coded grades of density. We chose to evaluate the softest density because it is the most commonly used. Pelite (Durr-Fillauer Medical Inc., Chattanooga, TN) is an expanded cross-linked, closed cell, thermoplastic polyethylene foam supplied in graded densities. We selected medium grade, because it is used most in our orthotic department. PPT (Langer Biomechanics Group, Inc., Deer Park, NY) is an open- cell urethane foam. Sorbothane (Sorbothane, Inc., Kent, OH) is a Visco-elastic polymer. Spenco (Spenco Medical Corp., 6301 Imperial Drive, Waco, TX) is a neoprene rubber foam with nylon covering.

Each material was tested in three ways: (1) compres-

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112 BRODSKY ET AL. Foot & Ankle/Vol. 9, No. StDecember 1988

sion, (2) combined shear-compression, and (3) force distribution. Forced distribution studies were done on materials both when new and at intervals after they had been subjected to cyclic compression.

Testing was done on the lnstron material testing machine in our laboratory at the Division of Orthope- dics, Dallas V.A. Hospital, University of Texas South- western Medical Center.

Two jigs (Figs. 1 and 2) were designed and fabricated by one of the authors (S.K.) specifically for the shear- compression and force transmission testing. The ma- terials to be tested were cut into pieces to fit the jigs. These were 43 x 56-mm rectanges for the shear- compression and pressure distribution tests and 50- mm diameter circles for the compression testing.

A force of 4 kg/cm was chosen for the force on the material during compression and shear-compression testing, based on the previous study by Campbell et aI.* and related to estimated normal peak plantar pres- sures. The repetitive motions were performed at a

Compression Testing

Each material was placed under the lnstron machine and subjected to cyclic loading. Before testing and after intervals of testing, the insert material was measured with a micrometer. The absolute and percentage change in insert thickness was recorded. Where appli- cable, the material was remeasured after periods of "rest" to determine "rebound" in thickness.

Shear-Compression Testing

The shear-compression jig consisted of a metal tray to hold the rectangle of insert material. The force of the

speed of 1 c/s.

I J Fig. 1. Schematic design of shearcompression jig.

Fig. 2. Force transmission jig with center piece removed.

lnstron is transmitted through a plunger that is set at an angle of 15' from the horizontal. It both slides and compresses to simulate the forces that occur with foot movement within the shoe while a person is walking on an insert.

Each material was placed in the sliding jig (Fig 2) and subjected to cyclic loading. Micrometer measurements were made before the test and then after the testing at various intervals. Because of the nature of the shear force, uneven reduction of thickness in the material in each sample occurred. Each sample was measured at its top, bottom, and midpoint. Reduction in thickness at the top was used for the purpose of calculations, because that represented the area of maximum change.

Force-Distribution Testing

The pressure distribution device is a metal box with a removable top cover (Fig. 2). A force transducer (Model SM-500, Interface Manufacturing Corporation, Scottsdale, AZ) was located inside the box. A cover on top of the box was designed to support the sample of insert material. The box has a 20 x 20-mm removable center piece for the force distribution testing. The load cell was 0.0625 cm (0.025 in) lower than the undersur- face of the top tray of the box. The removable center

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Foot & Ankle/Vol. 9, No. 3lDecernber 1988

cut-out piece on the cover was 0.075 cm (0.030 in) thicker than the cover itself. In this way, the center piece projected upward above the surface of the cover to transmit force to the load cell beneath, while resting on the load cell surface below (Fig. 3). This jig was constructed as a model of a plantar bony prominence.

The first test on the lnstron machine was against the load cell itself to verify that the applied pressure from the lnstron machine equaled the pressure measured by the transducer. Single blows of as great as nearly 6 kg/ cm were applied by the lnstron machine to the material in the force-distribution jig to measure the amount of transmitted force in the transducer below. Curves were recorded that demonstrated the behavior of the mate- rial. Each material was then retested for force-transmis- sion after intervals of cyclic compression to demon- strate the change in ability of the material to dissipate force after simulated use.

INSERT MATERIAL FOR FOOTWEAR 113

RESULTS

Compression

The results of cyclic compression testing for the five materials are displayed graphically in Fig. 4. With meas- urements of as great as 10,000 cycles, soft Plastazote compressed the most of all the materials. Moderate changes were seen in Pelite and Spenco but with slightly different curves. Minimal change was seen with Sorbothane and no change was seen in PPT.

A rebound phenomenon was exhibited by Plastazote and Pelite but not by any of the other three materials. For example, Plastazote with an original thickness of 6.6 mm was allowed to rest after 5,000 cycles of compression. Its thickness rebounded from 4.55 mm back to 6.0 mm, a 70% regain of lost thickness. Similar rebound was demonstrated by Pelite, which rebounded

Fig. 3. Schematic design of force transmission jig.

I 7 A

A-- - A-A- A-A-A-A- A-A

PELITE (Medium) A PPT

SORBOTHANE ' 31 A SPEN; , , -':

2O 1 2 3 4

Log Number of Cycles

Fig. 4. Results of cyclic compression as measured by change of thickness of the materials.

from 4.15 mm (original thickness was 5.00 mm) back to 4.90 mm after a 12-hour rest period. When these two materials were subjected to cyclic compression again, they experienced accelerated compression com- pared with the original trial.

Shear-Compression

The results of shear-compression are seen in Fig. 5. The greatest loss in thickness was again seen with Plastazote, whereas moderate change was seen with medium Pelite and Sorbothane. Minimal change was seen with Spenco and none with PPT. Changes in Pelite, Sorbothane, and Spenco were greatest early in the testing and then tapered.

The rebound phenomenon was demonstrated by Pelite and Plastazote under shear-compression in a manner comparable to compression. Sorbothane showed rebound under shear-compression which was not seen after compression testing alone. After 10,000 cycles, it rebounded from 5.70 mm nearly to its original thickness of 6.55 mm.

Compression and Shear-Compression Compared

In Table 1 and Fig. 6, the comparison between the two types of stress on the insert materials is seen. Maximum loss in insert thickness is shown as percent- age of the original material thickness. Most of the materials showed consistent response to the two types of stress. For example, Plastazote compressed the most and PPT did not change at all. Spenco and Sorbothane showed inconsistent response to the two types of stress: Spenco resisted shear better than compression, whereas the opposite was true of Sor- bothane.

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114 BRODSKY ET AL. Foot & Ankle/Vol. 9, No. 3lDecember 1988

PELITE (Medium) A P.PT A SPENCO

SORBOTHANE

1 2 3 4

Log Number of Cycles

Fig. 5. Change in thickness of the materials as a result of cyclic shear and compression combined.

TABLE 1 Maximum Loss in Thickness Expressed as Percentage of

Original Thickness After 10,000 Cycles

Material Compression (YO) Shear-compression (%)

Plastazote (soft) 55 Pelite (medium) 15 Spenco 15 Surbothane 3 PPT 0

45 16 3.6

10.2 0

70r

n 13 Compression

PLASTA- PEL‘ITE SPENCO SORBO- P ~ T 0-

ZOTE THANE

Mate r i a I s

Fig. 6. Comparison of the loss of thickness due to cyclic compres- sion versus cyclic shear-compression.

Force Distribution

A reference line was experimentally produced to verify that the applied pressure from the lnstron ma- chine did equal the pressure measured by the trans- ducer. Force distribution testing of the five virgin ma- terials is shown in Fig. 7. The abscissa represents applied force by the Instron, and the ordinate repre-

150 -

125 - - z g 100- e 3 0

+ - ; 75- 0 LL

0 e

z 50 .c-SORBOTHANE

25

0 25 50 75 100 125 150 Applied Force (Instron )/Lbs

Fig. 7. compression stress).

Force distribution of five materials when new (prior to

sents the force measured by the load cell. The reference line indicates the linearity of applied force from the lnstron and measured force on the load cell.

All five materials reduced the force transmitted from the lnstron to the load cell compared with the reference line. The overall shapes of their curves is comparable. At the midregion, they are all essentially the same except for Sorbothane, which transmits notably higher amounts of force. At the low end of the scale, Pelite transmits the highest amount of force, while transmit- ting the least force at high pressures. At peak pres- sures, the materials were successful in force distribu- tion, i.e., reduction of transmitted pressure, in a clear rank order, although these were rather close together. From most effective to least, the rank order was as follows: Pelite, Plastazote, Spenco, PPT, and Sorboth- ane. Each graph in Fig. 7 represents the hysteresis

In the second phase of force distribution studies, the tests were repeated on the material samples after intervals of cyclic compression. All materials showed some increase in transmitted force (i.e., loss of force distribution) after cyclic compression loading. This change was small (<1O0/o) for all materials except soft Plastazote, however.

The soft Plastazote showed the greatest changes in transmitted force after cyclical loading (Fig. 8). The Plastazote is shown to transmit more than double the amount of force of the virgin sample after 25,000 cycles of compression.

When new, however, the Plastazote distributes force better than all materials except Pelite, but this advan- tage is gradually lost after cyclic compression. Between 10,000 and 15,000 cycles, Plastazote becomes less effective than the virgin Sorbothane for force distribu- tion.

loop.

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Foot & Ankle/Vol. 9, No. 3/December 7988

Reference

150 / INSERT MATERIAL FOR FOOTWEAR 115

125 /

z- I ,O 75 100 125 150 175 200 225

Applied Force (Inslron l / L b r

Fig. 8. Diminished force distribution properties of soft Plastazote (increase in transmitted force) after cyclical compression.

DISCUSSION

Loss of insert thickness due to both compression and shear-compression stress was greatest in the soft- foamed polyethylene, soft-grade Plastazote. The me- dium-grade foamed polyethylene, medium Pelite, con- sistently ranked second, but with total change at 10,000 cycles of about one-third of the changes seen in the soft Plastazote. This appears to represent signif- icantly greater durability. Plastazote and Pelite are dif- ferent brands of closed cell polyethylene foams. The differences between them appear to be a function of the different grades of density of each that were se- lected rather than the actual brand, although this dis- tinction could be tested at a later date.

Spenco showed the same moderate level of loss of thickness with compression as seen in Pelite, whereas minimal change was experienced by Sorbothane. The minimal deformation produced in Sorbothane is much closer to the absence of deformation in PPT. The force distribution testing shows a more clinically relevant difference between these two materials, however. Sor- bothane is notably more rigid than the PPT and, there- fore, transmits more force at a given pressure than the PPT. This corresponds to the clinical observation that, although PPT springs back to its original shape readily, it also can be compressed more easily than Sorboth- ane.

Resistance to shear-compression combination stress for the two types of foamed polyethylene, Plastazote and Pelite, roughly paralleled their responses to pure compression. It is interesting to note that the Plastazote resisted the shear-compression somewhat better than pure compression, losing about 20% less thickness. This may be clinically relevant because some authors have postulated that most of the stress on the insert is the result of the shear-compression combination during

walking, whereas pure compression occurs only with standing.

Spenco was found to resist shear-compression bet- ter than all of the materials except PPT. It also resisted shear better than it resisted compression. One expla- nation for this, in addition to the nature of the foam itself, may be the function of the nylon cloth that is bonded to one side, providing a slick surface. It is manufactured in this way with the cloth intended to face the foot.

PPT is certainly the most durable of all five materials tested; it showed no permanent deformation in either of the two types of cyclic stress. One can postulate both advantages and disadvantages in its resistance to permanent deformation. The advantage is its long life. The disadvantage may lie in its lack of “moldability.” The fact that it does not conform and cannot be made to conform to the shape of the foot may in future clinical studies diminish its effectiveness in terms of force distribution once it has been made into the actual clinical insert.

Sorbothane and Spenco have the same disadvan- tage, namely, the inability to be molded and shaped in the way that the polyethylene foams can be. Sorboth- ane has the additional disadvantage, not examined in this study, of being a relatively heavy material.

Soft Plastazote fatigued the most of all materials and also fatigued most quickly, both in compression and in shear. This rapid and extreme loss of thickness corre- sponds to the clinically recognized phenomenon of “bottoming out.” Its rapid occurrence leads the authors to conclude that there are marked limitations to its usefulness in a clinical situation unless it is combined with other materials to “back it up.” However, this property is also an advantage for the insensitive foot, since the force of walking will dissipate through compression of the insert rather than through break- down of the plantar skin. This is a function of that same easy compression of the material.

When one looks at the change demonstrated in force distribution in Fig. 8, one can extrapolate and approxi- mate clinical lifetime of soft polyethylene foam were it to be used by itself. Assuming an average cadence of 90 steps per minute, there would be 45 steps per foot per minute on the Plastazote insert. After 555 minutes of continual walking, 25,000 cycles would be reached. For a diabetic patient who is relatively inactive and walks 15 minutes in total per day, 25,000 cycles would be reached in approximately 5 weeks, or, alternatively, after 9% hours of continuous walking. Because of the rebound phenomenon, this level of permanent defor- mation would probably be reached in a slightly longer period, but it is apparent that such a level of deformation would occur within approximately 2 months at most, even in a relatively inactive person.

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116 BRODSKY ET AL. Foot ti AnkleIVol. 9, No. 3lDecember 1988

cerations develop. In the midrange of pressure, all materials other than Sorbothane appear to be relatively equivalent.

Future laboratory and clinical studies should concen- trate on the ideal combinations of insert materials for pressure reduction (force distribution) as well as resist- ance to compression and shear. The relative impor- tance of durability compared with moldability has yet to be tested. The change in behavior of the materials clinically when combined with different shoes and when used with stockings, also remains to be determined in clinical trials.

The force distribution study was the most important part of this project. It represents a type of investigation that previously had not been done. Moreover, clinical relevance is based on the model of a plantar bony prominence. Although previous studies such as those done by Campbell et al.' have tested the effects on insert materials of cyclic compression, sustained compression, and heat, our study tested the shear properties as well as those of force distribution or force- attenuation.

All five materials were successful in force distribution, i.e., in reducing the transmitted force over the simulated bony prominence. Their rank order of effectiveness has been demonstrated. Most of the differences were small among the virgin materials. The Sorbothane was noted to be the most rigid and, therefore, the least effective. The differences among the four other materials is rela- tively small. The clinical significance of these differences remains to be shown in future clinical studies. We postulate that these differences should be significant, given the assumption that there are greater than normal pressures in the areas of bony prominence where ul-

REFERENCES

1. Beach, R.B., and Thompson, D.E.: Selected soft tissue re- search: an overview from Carville. Phys. Ther., 59:30-33, 1979.

2. Campbell, G.J., McLure, M., and McWell, E.N.: Compressive behavior after simulated service conditions of some foamed materials intended as orthotic shoe insoles. J. Rehabil. Res. Dev., 2157-65, 1984.

3. Pratt, D.J., Rees, P.H., and Rodgers, C.: Assessment of some absorbing insoles. Prosthet. Orthot. Int., 10:43-45, 1986.

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