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ViscoelasticProperties of Plastic Pediatric AFOs

Thomas R. Lunsford, MSE, CO
Thomas Ramm, RTO
Joseph A. Miller, CP

ABSTRACT

The viscoelastic behavior of pediatric-sized polypropylene ankle-foot orthoses (AFOs) has been investigated by measuring the stiffness and buckling of AFOs before, during and after cyclic loading. Cyclic loading simulates normal gait by forcing the AFOs into dorsiflexion (10 degrees) and plantarfiexion (15 degrees). Mechanical properties were measured after three 24-hour periods of 187,200 cycles. After cyclic loading, mechanical properties were again measured at six discrete time intervals to determine if the plastic recovered or remained permanently degraded. The cycling rate simulated a relatively rapid walking pace of 130 steps per minute.

The AFOs exhibited a significant reduction in mechanical properties - 30 percent reduction in stiffness and 4.4 percent increase in malleolar diameter - after just one 24-hour period of cycling. Within 15 minutes after cycling had ceased, however, the AFOs began to recover their mechanical integrity and within an hour had recovered fully.

Microfracturing (crazing) was observed in the sagittal walls of the AFO specimens. This was attributed to excessive stress as the plastic curved outward to accommodate the malleoli prominences.

Introduction

Stress-strain properties of plastics have been well documented (1). Creep, cold flow and elastic recovery all result from applying fixed loads on test specimens. When subjected to constant loads, plastic deforms quickly and eventually fails. This is known as creep.

Constant strain (elongation) occurs during this stress period and is referred to as cold flow. If the load is removed before creep rupture occurs, a slow period of recovery known as elastic recovery begins (1). When both elastic and viscous behaviors are observed, the plastic is said to exhibit viscoelasticity (2). A viscoelastic polypropylene AFO combines features of a perfect elastic solid and a perfect fluid. Under short duration loads the plastic AFO behaves elastically. However, with steady loads it will stretch or elongate, recovering when the load is removed.

Information is readily available on polypropylene's viscoelastic properties and its reaction to constant stresses (1,2). Information on the viscoelastic behavior of polypropylene AFOs, however, is lacking. For instance, does the stiffness of a plastic AFO degrade with typical loads and rates of loading? Do AFOs become softer? This research explores and documents the softening nature of polypropylene pediatric AFOs under the cyclic stresses of walking.

The solid ankle Rancho polypropylene AFO was investigated (3). This AFO is designed to translate ground reaction forces, thereby providing stability in the sagittal and transverse planes during stance phase (4). Degradation in AFO stiffness can undermine control of the impaired lower limb by allowing, for instance, excessive dorsiflexion in terminal stance or knee hyperextension in midstance.

In a prior study Miller showed that intermittent stresses degrade adult polypropylene AFOs (5). Measurements taken after 11,400 flexures (from 10 degrees of dorsiflexion to 15 degrees of plantarfiexion) show stiffness was reduced by 9 percent. After 22,800 flexures, stiffness was reduced by 38 percent. For purposes of this study, stiffness is defined as force required to collapse the AFO to 10 degrees of dorsiflexion.

From the resting state to 10 degrees of dorsiflexion, the ankle diameter increased by 28.6 percent at pretest, 30 percent at 11,400 flexures and 30.3 percent at 22,800 flexures (5).

Diametrical Strain

The measurement taken for "buckling" in the malleoli region is technically referred to as diametrical strain (6). Diametrical strain is expressed as:

In the equation, E represents diametrical strain in percent, D₁₀ represents malleolar diameter at 10 degrees dorsiflexion collapse and D₀ is the original unstressed M/L diameter. D₁₀ values were measured before, during and after cycling (recovery period).

Stiffness

An AFO's ability to resist sagittal plane collapse under load is a measure of its stiffness. Because plastic resists tension better than compression, the AFO's medial and lateral walls absorb plantarfiexion loads better than dorsiflexion loads (1,2). To evaluate stiffness the AFOs are forced into discrete dorsiflexion angles. The corresponding anteriorly directed forces at the calf cuff are then recorded, and the resulting graph of force versus angle becomes the stiffness signature for that AFO. It is hypothesized that simulating walking (cyclic loading) will degrade stiffness and soften the AFO. Moreover, it is hypothesized that removing the cyclic loads will permit the AFO to recover its original stiffness.

A pilot study was performed to assess appropriate time interva1s for measuring dorsiflexion collapse force and malleolar diameter strain(5). Extending cycling periods from several hours to intervals of 24 hours was found to be necessary. The study also found that similar measurements for the elastic recovery period occur logarithmically at time intervals of 15, 30, 45, 60, 240 and 540 minutes.

Perry demonstrated that the range of ankle motion for normal subjects during walking is 10 degrees of dorsiflexion to 15 degrees of plantarfiexion (7). Each AFO in this study was cyclically stressed throughout this range of motion at the rate of 130 cycles per minute, 7,800 cycles per hour and 187,200 cycles per 24-hour period.

Specific Objectives

The study had five objectives:

1. To document the pediatric surrogate leg and dynamic (cyclic) testing apparatus.
2. To fabricate and document three similar standard (Rancho) 1/8-inch thick polypropylene (pediatric-sized) AFOs.
3. To measure thickness, stiffness and diametrical strain of the three AFOs at the following stages:
4. pre-cycling sagittal wall thickness at predetermined points (see Figure 1 ).
5. pre-cycling stiffness (force to collapse AFO to 10 degrees of dorsiflexion)
6. pre-cycling diameter at apexes of malleoli, and
7. stiffness and diameter measurements for each AFO before cycling and at intervals of 24, 48 and 72 hours during cycling and 15, 30, 45, 60, 240 and 540 minutes after the end of cyclic loading (recovery period)
8. To calculate mean (+/- sd) stiffness and diametrical strain for the three AFO specimens before cycling, at 24-hour intervals during cycling and at 15-, 30-, 45-, 60-, 240- and 540-minute intervals during recovery.
9. To compare the stiffness and diametrical strain of the AFOs for differences before, during and after cycling and identify any material softening and subsequent recovery.

Methodology

Apparatus

The surrogate leg, modeled after that of a 10-year-old male, was made of polyester resin^a with a nylon stockinette^b outer lamination. It has a single-axis, free-motion mechanical talocrural joint located 6 cm proximal to the plantar surface and 5 cm anterior to the posterior coronal plane. The overall height of the surrogate leg is 30.5 cm (see Figure 2 ).

The dynamic AFO cycling apparatus consists of a steel frame, electric motor, and crank and push-rod (see Figure 3 ). The frame is 34.3 cm high, 91.5 cm long and 30.5 cm wide. It is made of 3.2-cm by 0.6-cm-thick angle iron and 0.6 cm steel plating that has been welded and bolted together. The electric motor^c will drive a 2.5-cm pulley to a 35-cm pulley (14:1 speed reduction), attached to the 3.2-cm diameter crank and 28-cm adjustable push-rod of the dynamic tester. This device is capable of flexing a surrogate leg with an AFO 187,200 times a day.

AFO Fabrication and Design

A plaster bandage impression was taken of the surrogate leg, and a model cast was produced following the guidelines described in the Rancho-type polypropylene AFO fabrication manual (8).

The three pediatric Rancho solid ankle AFO specimens were fabricated from 1/8-inch (3.2-mm)-thick polypropylene^d , Resinol Type 0 (stress relieved) with a mean (+/- sd) thickness of .1252 (+/- .001) inches. Fabrication adhered to the guidelines described in the Rancho-type polypropylene AFO fabrication manual (8). The polypropylene was heated at 350°F until clear, draped over the plaster model and vacuum applied (approximately 25 inches of hg). The plastic remained undisturbed on the model for at least 24 hours before trimming. Trimlines are depicted in Figure 2 .

To ensure similarity among the three specimens, identical parent materials and fabrication procedures, trimlines and sagittal wall thicknesses were used. Sagittal walls were measured with a thickness caliper^e at specific locations (see Table I and Figure 1 ). These data ensured minimal variations in the vacuum forming of the polypropylene.

Procedure

In this study, the amount of force required to attain 10 degrees of dorsiflexion is referred to as force. Each AFO was mounted to the dynamic testing apparatus, the push-rod was removed from the crank mechanism, and a force gauge^f with handle was applied (see Figure 4 ). The surrogate leg (including well-delineated midline) with AFO attached was collapsed from neutral to 10 degrees of dorsifiexion using an angular scale to ensure accurate measurement duplication (see Figure 5 ).

The mean force value from three trials with the force gauge was recorded. The push-rod was reattached and continuous cycling from 10 degrees of dorsiflexion to 15 degrees of plantarfiexion began. The procedure was repeated at intervals of 24, 48 and 72 hours, and force values were recorded. Cycling only stopped while measurements were taken and resumed immediately. Cycling ended after 72 hours. At 15 minutes of recovery (postcycling) the force to collapse the AFO to 10 degrees of dorsiflexion was measured and recorded in identical fashion. This value was also obtained at 30, 45, 60, 240 and 540 minutes following recovery.

AFO malleolar diameters were measured with a precision calipers at points corresponding to the malleoli apexes (see Figure 6 ). The dynamic tester was advanced by hand to its maximum dorsiflexion angle capability (10 degrees). The malleolar diameter was recorded and continuous cycling began. Diameter values were obtained at the same time intervals as the force measurements.

Means and standard deviations of the dorsiflexion collapse forces and diametrical strain data were calculated using the CRUNCH^h statistical programming package. ANOVA was used to compare the restraining forces (strain) of the AFOs, and the Scheffe' post hoc test was used to identify pairs of significant differences at the (p < 0.05) level.

Results

Before cyclic loading, the mean force required to collapse the three AFOs into 10 degrees of dorsiflexion was 366 Newtons (see Table II ). After three 24-hour periods of cyclic loading the mean force degraded to 254, 239 and 237N, respectively. AFO stiffness decreased significantly (over 30 percent) after the first 24 hours of continuous cycling. The two additional 24-hour periods did not significantly weaken the AFOs further.

After 72 hours of cyclic loading, AFOs remained motionless for 15 minutes (see Figure 7 ). In that period the stiffness improved 23 percent, from 237 N to 292 N. After three more 15minute periods the stiffness had recovered to 338 N. After 72 hours of cyclic loading stiffness at the most weakened point had recovered 43 percent. After 60 minutes the AFOs had completely recovered from the deleterious effects of 72 hours of cyclic loading. No further improvement was noted.

The diametrical strain (bulging) of the diameter across the malleoli was not as dramatic as the stiffness (see Table III and Figure 8 ). Before cyclic loading the mean increase in the malleolar diameter was 11.8 percent as the AFOs were forced into 10 degrees of dorsiflexion.

After 24 hours of cycling, the mean malleolar apex diameter at 10 degrees of dorsiflexion collapse increased 14.4 percent. Two additional 24-hour periods of cycling did not cause a significant worsening of the malleolar bulging.

After 72 hours of cyclic loading, the diametrical strain was measured every 15 minutes for an hour. A mean increase in malleolar diameter of 12.9 percent was noted. This latter value for diametric strain was not significantly (p < 0.05) different from the original value.

Conclusion

Stiffness

In the first 24 hours of continuous cycling, the ability to resist 10 degrees of dorsiflexion decreased significantly for each of the three AFOs tested. Minimal degradation occurred in the next 48 hours of cycling. Within the first 15 to 60 minutes of recovery this softening effect improved, and the force required to collapse the AFO into 10 degrees of dorsiflexion returned to pretest values (see Table II and Figure 7 ). In essence, cyclically stressing a inch-thick polypropylene pediatric AFO will cause it to soften. Allowing the AFO to rest will cause spontaneous and full recovery of its viscoelastic properties.

Clinically, if a patient is able to stress an AFO to extremes, the dorsiflexion resistance values have been exceeded, and the biomechanical effectiveness of the orthosis is questionable. During times of no activity (e.g., at night) the AFO will regain its original mechanical properties.

On the molecular level, polypropylene resembles long, semi-parallel strings of beads held together by two bonds, one between the individual beads in the string (called primary or covalent bonds) and one at string intersections (called secondary or Van der Waals bonds)(2). When plastics are heated or stressed the weaker secondary bones dissociate, and the thermoplastic becomes flexible (moldable). This phenomenon occurs routinely when thermoplastics are heated in preparation for vacuum forming.

Visual evidence of this dissociation is obvious with polypropylene-it becomes transparent when the glass transition temperature is reached at the core of the plastic (2). These secondary bonds between the strings reassociate during cooling, when the plastic returns to an opaque state. It is thought that this process of dissociating and reassociating secondary bonds, albeit on a smaller scale, is responsible for the softening and spontaneous recovery exhibited in the AFOs in this study.

Diametrical Strain

The malleolar diameter increased significantly after 24 hours of cycling and then underwent a measurable decrease 15 minutes following recovery (see Table III and Figure 8 ). After being subjected to cyclic stresses the three AFOs exhibited an increase in the diameters at the malleoli (diametric strain). Although there was a small decrease in this measurement during the first 15 minutes of recovery, permanent deformation occurred in all three test specimens. This deformation, however, did not effect the AFOs' primary objective-to offer resistance to dorsiflexion equal to that of original unstressed design after a recovery period was permitted.

Microfracturing or crazing occurred with each AFO specimen during cycling and appears as a whitish area near the proximal malleolus (see Figure 9 ). The location of this defect was the same for each AFO. This could be attributed to excessive stress in this area or the local curvature and buildup size of the positive model. Crazing, which indicates the dissociation of primary bonds, is considered to be a form of permanent plastic deformation.

Thomas R. Lunsford, MSE, CO, is director of the orthotic department at the Institute for Rehabilitation Research in Houston. He also is immediate past president of the Academy.

Thomas Ramm, RTO, and Joseph A. Miller, CP, were students at California State University - Dominguez Hills, Carson, Calif., when this research was conducted.

References:

1. Shah V. Handbook of Plastic Testing Technology. Philadelphia: John Wiley & Sons, 1984:14-31.
2. Mills K. Engineered Materials Handbook. Vol 2. Metals Park, Ohio: ASM International, 1988.
3. Stills M. Thermoformed ankle-foot orthoses. Orthotics and Prosthetics 1975; 29:4:41-51.
4. Lin R, Gage J. Neurological control system for normal gait. JPO 1989;2:1:1-13.
5. Miller J. Dynamic Testing of Plastic AFOs with Mechanical Ankle Joints. Carson, Calif. :Archives of California State University-Dominguez Hills.
6. Golay W, Lunsford T, Greenfield J. The effects of malleolar prominence on polypropylene AFO rigidity and buckling. JPO 1989;1:4:231-41.
7. Perry J. Normal and pathological gait. Atlas of Orthotics. 2nd ed. St. Louis, Mo.: C.V. Mosby Co., 1985:76-111.
8. Lunsford T. Rancho-Type Polypropylene AFO Fabrication Manual. HEA34OD3. Carson, Calif. :Orthotic and Prosthetic Baccalaureate Program, CSUDH, 1982.