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The Effect of Malleolar Prominence on Polypropylene AFO Rigidity and Buckling

Wesley Golay, C.O.
Thomas Lunsford, M.S.E., C.O.
Brenda Rae Lunsford, M.S.
Jack Greenfield, C.O.

Introduction

A number of catastrophic clinical conditions (e.g., lower extremity fracture, brain injury, stroke, spinal injury) result in severe impairment of the foot and ankle complex during walking. Manifestations of these conditions often include paresis, spasticity, and disturbance of normal phasicmuscle control. Many foot and ankle disabilities can be improved with the aid of an appropriate orthosis.^1-4

The primary goal of a properly fitting orthosis is to realign the foot-ankle complex and maintain that alignment during all phases of gait. Management of genu recurvatum or excessive knee flexion is often accomplished by stabilizing the ankle.^5-7 Excessive knee flexion during stance is the most critical to control, since the patient might fall if the knee buckles.

Polypropylene ankle-foot-orthoses (AFOs) are a popular choice among physicians and orthotists for treating patients with impaired gait, but there are disadvantages with their use. Polypropylene AFOs lack adjustability in plantar and dorsiflexion, and often the AFOs tend to buckle at the ankle during stance phase where control is needed the most. The ability of the orthosis to restrain dorsiflexion under the dynamic loading of full body weight is a measure of the appropriateness of the design and choice of materials.

Generally, it is desired to fit a patient with a rigid polypropylene AFO which will restrain excessive dorsiflexion during stance phase. Patients whose malleoli are markedly prominent require polypropylene AFOs with adequate relief about the ankle to avoid excessive pressure. This relief may have a negative effect on the rigidity of the polypropylene AFO. It has been observed that plastic AFOs buckle more easily in the region near the relief, at the ankle. When a plaster cast is prepared for fabrication of a polypropylene AFO the orthotist builds-up the plaster where there are bony prominences. The apices of the medial and lateral malleoli on the cast require the largest build-ups. Unfortunately, this is also the area where the maximum compressive stress (buckling) occurs in the orthosis when the patient walks.⁹ The prominence or sharpness of the contours around the ankle seems to affect the ability of the orthosis to restrain dorsiflexion in terminal stance.

The purpose of this study, therefore, is to investigate the degree to which plastic AFOs buckle and lose rigidity as the malleolar prominence is increased. Rigidity is best characterized by the angle of collapse between the foot and the tibial sections versus applied torque or force. The maximum rigidity challenge placed on a plastic AFO occurs in late stance where dorsiflexion restraint is required by a patient with a weak calf. Therefore, rigidity shall refer to AFO stiffness in the sagittal plane against biomechanical forces tending to collapse the AFO into dorsiflexion.

Buckling, conversely, is the tendency of a plastic AFO to expand along an medio-lateral line through the contours of the malleoli. A convenient characterization of this phenomenon is the relationship between angle of dorsiflexion collapse versus the percentage increase in malleolar diameter. The percentage increase in malleolar diameter is called "diametrical strain." If this bulging at the malleoli disappears after the applied force is removed, then the diametrical strain is elastic. However, if the malleolar diameter is slightly larger after the load is removed, then the diametrical strain is inelastic. In this latter case, internal and irreversible material changes occur.

Specific Aims

1. Fabricate four uniform groups of test AFOs with increasing amounts of malleolar prominence (no build-up, 1/4" build up, 1/2" build-up, and 3/4" build-up).

2. Measure the change in the diameter at the malleoli and the applied force as the AFOs are forced into 16° of dorsiflexion in 2° increments.

3. Compare the characteristics of rigidity and buckling among the four groups of varying malleolar build-up.

Method

Procedure

Four plaster casts were made from an impression of the below-knee prosthesis portion of the AFO testing apparatus. ¹⁰ The first of the four casts had no malleolar buildups. The other three casts had 1/4", 1/2", and 3/4" total medial and lateral malleolar buildups.

Four groups of AFOs were fabricated (Figure 1) , all from the same sheet stock of 3/16" polypropylene material and all trimmed identically, the only variable being the diameters between the apices of the malleolar build-ups. Three AFOs were fabricated over each of the four casts, which were created to allow determination of reliability of fabrication. To ensure what each of the three AFOs in each of the four build-up groups were similarly drape vacuum formed, the medial and lateral sagittal wall thicknesses in the area of the malleoli were measured. These thickness measurements were taken at seven points (Figure 2) on the medial and lateral sides near the ankle area using a thickness gauge (Figure 3) .*

The diameter at the apex of the malleoli was measured on all AFOs before any force was applied. Malleolar diameter measurements were recorded at eight other dorsiflexion angles in 2° increments up to 16°. Reference marks were placed on the anterior edges of the AFOs to ensure that the diameter measurements were taken at the same place. For every two degrees of additional dorsiflexion the corresponding applied force value was recorded. Each AFO was tested three times to obtain average values for force and buckling diameter. One hour elapsed between tests of the AFOs for a given buildup configuration. This allowed the AFO to return to its original shape before retesting.

Instrumentation

The testing apparatus ¹⁰(Figure 4) was developed from a below knee prosthesis using a solid foot articulated with the shank of the prosthesis via a free moving hinge (Figure 5) . A 2" belt of cotton webbing served as the calf strap to which the tensiometer^** was attached. The tensiometer (Figure 6a) measured the applied dorsiflexion force on the system needed to achieve the eight specified dorsiflexion angles. The applied force was supplied by turning a small winch (Figure 4, bottom right) mounted to the base of the apparatus. A cable and pulley system changed the line of pull so that it was perpendicular to the tibial axis of the below-knee prosthesis and 10" proximal to the mechanical ankle joint axis. Toe-in/toe-out was referenced on the baseplate with black tape to maintain a consistent line of pull. The change in the malleolar diameter was measured with an outside vernier caliper (Figure 7) .^***

The angle of dorsiflexion collapse was indicated by a 1 mm diameter pin attached to the proximal-lateral edge of the AFO (Figure 8) . This indicator could be seen through the stationary transparent angular scale (Figure 8) . Each mark on the scale represented a single degree increment.

Data Analysis

The statistical programming package, CRUNCH,^**** was used to perform all data analysis. The data were screened by using univariate summaries. A paired T-TEST was used to compare medial and lateral sagittal wall thickness. Analysis of variance was used to compare differences between the malleolar build-up groups for both rigidity and buckling. Plots of a third order regression were used to demonstrate the relationships of applied force and diametrical strain to dorsiflexion collapse angle. All testing was done at a .05 significance level.

Results

The results consist of three parts: sagittal wall thickness, rigidity, and buckling.

Sagittal Wall Thickness

The mean (+/- standard deviation) of the medial and lateral sagittal wall thicknesses are shown in Table I for the four groups of AFOs. For Group I (no build-up) the mean (+/-s.d.) medial and lateral wall thickness was .150" (+/-.006) and .145" (+/-.009), respectively. For Group II(1/4" build-up) the mean (+/-s.d.) medial and lateral wall thickness was .153" (+/-.006) and .142" (+/-.009), respectively. Similarly the thicknesses were .153" (+/-.007) and .142" (+/-.011) for Group III(1/2" build-up), and .153" (+/-.009) and .148" (+/-.009) for Group IV (3/4" build-up). The medial wall thickness for all four groups were not significantly different. The lateral wall thicknesses for all four groups were also not significantly different. However, the difference between the medial and lateral wall thicknesses were significantly different for Groups I, II, and III.

Rigidity

As expected, the AFOs with greater malleolar build-up were less rigid (Table II) . For a dorsiflexion collapse angle of 6° the AFOs with no build-up, 1/4" build-up, 1/2" buildup, and 3/4" build-up required 39.2 (+/-4.44), 33.2 (+/-4.76), 26.4 (+/-3.54), and 25.0 (+/- 6.84) pounds, respectively.

An analysis of variance of the mean forces indicated that the AFOs with no build-up were significantly more rigid than any of the other three versions (Table II and Table III ). Further, the AFOs with 1/4" malleolar build-up were significantly more rigid than the AFOs with either 1/2" or 3/4" malleolar build-up. However, the rigidity of the AFOs with 1/2" and 3/4" build-up were not significantly different (Table III) . Four of the rigidity comparisons (Table II) were not significantly different at dorsiflexion collapse angles of 2° and 4°

The non-significance between the AFOs with 1/2" and 3/4" malleolar build-up is easily seen (Figure 9) .

Buckling

The measure for buckling is the percentage increase in the diameter of the AFO in the region of the malleolus. Technically this increase is called "diametrical strain."

The AFOs with greater malleolar build-up had less diametrical strain (Table IV) . For a dorsiflexion collapse angle of 6°, the AFOs with no build-up, 1/4" build-up, 1/2" buildup, and 3/4" build-up exhibited 12.6% (+/-.023), 10.7% (+/-.020), 11.8% (+/-.016), and 8.5% (+/-.024) diametrical strain, respectively. An analysis of variance of the mean diametrical strains indicated that all of the AFOs were significantly different except those with 1/4" and 1/2" malleolar build-ups (Table IV and Table V ). There were scattered cases at relatively small dorsiflexion angles (2-6°) where some of the AFO groups were not significantly different. The non-significance between the AFOs with 1/4" and 1/2" malleolar build-up is obvious (Table V and Figure 10 ).

Discussion

Sagittal Wall Thickness

Many subtle factors can change the thickness and resulting characteristics of a drape vacuum formed plastic AFO. Excessive heat tends to result in "thinned" areas such as the malleoli, while excessive vacuum and operator vigor can have the same result in areas such as the heel. This is a possible explanation for the significant difference between the overall medial and lateral wall thicknesses, .152" (+/-007) versus .144" (+/-.010). Although this difference is small (5.5%), it was consistent. One possible explanation is that the plastic tends to be thinner at the more posterior areas, such as the lateral malleolus. Using multiple thickness measurements on the medial and lateral sagittal walls of the plastic AFOs was a suitable means for screening out inconsistently fabricated orthoses.

Rigidity

Sagittal walls without bulging contours to stantially more rigid AFO. For example, the anteriorly directed force produced by the tibia of a patient with a weak calf is approximately one third body weight.¹³ Therefore, for a patient weighing 150 pounds, the AFO must restrain a force of 50 pounds. The AFO group with no build-up collapsed 7.5°; whereas the group with 1/4" and 1/2" buildups collapsed 8.7° and 11.5°, 16% and 52% additional rigidity loss, respectively. This is a relatively large loss in AFO rigidity, which diminished as the malleoli build-up increased beyond 1/2". When the build-up was increased from 1/2" to 3/4", the rigidity loss increased from 52% to 60%.

If the biomechanical objective of the prescription is to restrain motion of the ankle joint, then every effort should be made to avoid bulging contours of the sagittal walls in the region where the tibial portion of the AFO transitions to the foot section. The penalty of flat and parallel sagittal walls is excessive pressure on the malleoli during single limb stance. Therefore, the design challenge to the orthotist is to produce maximum rigidity without excessive skin pressure. The greater the AFO collapses, the greater the demand on the patient's quadriceps. Further, if the collapse is too great, then the patient will have a smaller window within which to walk.

Buckling

The results obtained for buckling (i.e., increase in malleolar diameter) were surprising and enlightening. For a dorsiflexion collapse angle of 10°, the group of AFOs with no build-up buckled 25% (i.e., diametrical strain .25), whereas the groups with 1/4" build-up and 1/2" build-up each buckled 19%. However, the 1/4" build-up group required 30% more force to collapse. Statistically, the 1/4" and 1/2" build-up groups were indistinguishable considering only buckling. The group with 3/4" build-up buckled only 13% for a 10° dorsiflexion angle (Table IV) . This result seemed contrary to what was expected.

Furthermore, there are two important considerations. First, the group with no build-up require 65 pounds to collapse into 10° of dorsiflexion, whereas the 3/4" group required only 40 pounds. Secondly, the group with 3/4" build-up behaved as though a "natural hinge" existed at the ankle region. Conversely, the group with no build-up and parallel sagittal walls were the most rigid and when forcefully collapsed into dorsiflexion, tended to demonstrate more buckling (diametrical strain).

By creating malleolar contours in the me- dial and lateral walls, the orthotist is "prebuckling" the AFO. A substantial amount of rigidity is lost and the AFO easily collapses into dorsiflexion without appreciably increasing the malleolar diameter. This contrasted to the AFOs with minimal malleolar build-up which are substantially more rigid and difficult to buckle. However, when buckling finally occurs, greater geometric distortion is observed.

Clinical Implications

If the treatment objective of the prescription is to protect a weak calf by limiting dorsiflexion in stance then a rigidly designed 3/16" polypropylene AFO is indicated. Furthermore, if the patient has prominent malleoli (more than 1/4"), then an attempt to stiffen the AFO should be made (e.g., with metal or carbon composite inserts). However, if the patient does not have prominent malleoli (1/4" or less), then judicious modification of the plaster model will suffice (i.e., build-up of the area around the malleoli until virtually straight sagittal walls exist). This will reduce the tendency for the AFO to buckle, but will add conspicuous bulk. Obviously, as either the weight and/or walking velocity of the patient increases, the easy solutions must be abandoned in favor of the use of thicker, more anteriorly trimmed plastic, the use of metal insert reinforcements, or the use of a heavy duty standard metal AFO.

The confusing point in these findings occurs when buckling is observed while evaluating a stressed orthosis. The buckling can not be used as an indicator of lack of dorsiflexion restraint; in fact, more buckling often occurs with greater dorsiflexion restraint. The true indicator of rigidity is the dorsiflexion collapse angle of the orthosis.

The important observation to be made, therefore, is the dorsiflexion angle throughout stance, especially at terminal stance. If excessive (greater than 10°) dorsiflexion is observed, then the orthosis is inadequately rigid. If less than 10° is observed, then the orthosis has adequate rigidity, no mattcr how much buckling (bulging) is observed.

Summary

Four groups of 3/16" thick polypropylene AFOs with increasing levels of mallcolar prominence were tcsted for rigidity and buckling. Those AFOs with greater mallcolar prominence were less rigid. However.the AFOs with less build-up required more force to buckle, but once buckling occurred there was a greater expansion of the sagittal walls.

Wesley L. Golay is a graduate of the Orthotics and Prosthetics Baccalaureate program at California State University Dominguez Hills and participated in this study as partial fulfillment of his graduation requirements.

Thomas R. Lunsford is Chief Orthotist at Rancho I-os Amigos Medical Center, Orthotics Department, 7450 Leeds Street, Downey, California 90242, and Clinical Director of the Baccalaureate program in Orthotics and Prosthetics at California State University Dominguez Hills.

Brenda Rae Lunsford is Statistician, Physical Therapy Department. Rancho Los Amigos Medical Center.

Jack Greenfield is Assistant Chief of the Orthotic Department at Rancho I-os Amigos Medical Center.

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