Treatment of Proximal Femoral Focal Deficiency by Using a Hybrid Orthotic and Prosthetic Design

Mark F. Devens, CO

ABSTRACT

A hybrid device using a thermoplastic solid-ankle ankle-foot orthosis and an attached distal prosthetic-type pylon and foot plate has been designed to enhance ambulation and multiplane control of the extremity for the patient with proximal femoral focal deficiency, congenital short femur.

Key Words: hybrid device, proximal femoral focal deficiency, instability, orthosis

Introduction

For many years, treatment of the child with femoral focal deficiency (PFFD) prior to surgical intervention, such as a Van Ness rotationplasty or amputation and subsequent prosthetic fitting, has been to apply a lift to the shoe on the affected side. This is an effective and inexpensive solution for many patients, yet it becomes cumbersome and unstable when the lift exceeds approximately 7 cm. This instability is due to the effect of a long lever arm distal to the foot (the lift), which the young patient must control via an insecure attachment mechanism (the shoe). Control in both the sagittal and coronal planes is diminished, and the patient walks by elevating the extremity and setting it down with a foot-flat gait.

Along with a decrease in proximal hip and knee control, the foot tends to rotate in the shoe because of the long distal lever arm the lift provides, as well as the inflexibility the lift brings to the shoe. Thus, the shoe/lift combination, although leveling the patient's leg-length discrepancy, does not lend itself to an optimal gait pattern and muscle use and development.

To control these instabilities inherent in a long shoe lift, a solid ankle ankle-foot orthosis (AFO) is combined with a distal pylon and foot plate, thus enabling direct control of the ground forces through a rigid system, allowing also the ability for multiplane adjustment.

Methods

Technique

Over the past 8 years, a number of models have been fabricated for various patients. Because there are no prosthetic components specifically designed for this type of application, we used a combination of custom fabricated and commercially available modified componentry. Suffice it to say, the practitioner's ingenuity is as important a factor in fabrication as any other aspect. Currently, we are fabricating two types of hybrid designs, one that can use available prosthetic componentry, and one that uses fabricated components. The extent of the leg-length inequality will dictate which design can be used. One must have more than 10 cm to work with to use commercially available components.

Casting

The patient's foot and leg is casted in the usual fashion for the AFO section. In patients with knee-flexion contractures or significant proximal instability, a KAFO cast can be taken. The positive model is modified as for a solid AFO, keeping the foot plate completely distal to the toes and the foot/ankle complex at 90°.

Custom-Design Hybrid System

If prosthetic components are not used, a pylon made of 1" diameter DelrinR nylon (E.I. DuPont de Nemours & Co., Wilmington, DE) is used. The pylon is aligned on the positive model just proximal to the ankle joint and midline in the sagittal plane. A small hole is drilled in the center of the pylon and a nail is placed in the positive mold corresponding to the placement of the pylon. The plastic used is a copolymer, 3/16-inch thick. Two pieces are used for the vacuum forming. The first piece of plastic is applied to the positive mold covering the entire foot just proximal to the ankle. This is for the added stability and enclosure of the pylon. The pylon is then placed on the positive mold and the second full sheet of plastic is vacuum formed over the entire mold incorporating the pylon in the plastic.

The AFO is trimmed out and the pylon is drilled through the center to accept a 3/8-16 SACH foot bolt, (Kingsley Manufacturing Co., Costa Mesa, CA). A SACH foot adapter nut (Kingsley Manufacturing Co., Costa Mesa, CA) is then heated with a propane torch and set in the AFO and pylon from the plantar surface of the AFO with the teeth pointing down, flush with the AFO (Figure 1 ).

An aluminum foot plate is fabricated, 1/4-inch thick, approximately the length and width of the patient's foot. This is curved slightly to allow easier rollover (Figure 1 ). The foot plate is drilled to accept the foot bolt then attached to the AFO/pylon. The head of the SACH foot bolt on the underside of the plate is covered with crepe and the proximal portion is built up with Plastazote^TM (Kingsley Manufacturing Co., Costa Mesa, CA) so that it fits in the shoe snugly. A smaller hole is drilled through the foot plate and pylon, after final "dynamic alignment," for the insertion of a small screw to inhibit rotation of the foot plate (Figure 2 ).

The aluminum plate and 'pylon' can then be covered with foam, either prosthetic foam or Plastazote. Enough foam should be used to make the foot plate secure in the shoe. The shoe is then applied, laced tightly for optimal security and control and usually left on the device (Figure 3 ).

Design Using Available Componentry

When length allows, we utilize readily available prosthetic modular componentry from any manufacturer. The fabrication is similar yet in lieu of a nylon pylon, a standard aluminum pylon, pyramid attachments, and adapters are used (Figure 4 ). The pyramid is placed on the positive mold and vacuum formed into the AFO. This is the same technique as for the Delrin pylon (Figure 5 ). A pylon and prosthetic SACH foot can then be applied using components that best suit the individual patient and discrepancy. Leg lengths can be equaled, of course, by cutting the pylon to the appropriate length.

By using modular components, we can make angular changes in both the coronal and sagittal alignment of the foot and/or pylon, allowing the practitioner to dynamically adjust the orthosis as the patient walks, as they would a prosthesis (Figure 6 ). In this way, push off, varus/valgus moments at the knee and ground-reaction forces can be adjusted. The patient has enhanced control over the ground-reaction forces, increased proprioception due to the rigid system, and subsequent better regulation of the gait speed, stride length, and rotational control (Figure 7) .

Discussion and Conclusion

By applying the combined technology of orthotics and prosthetics and their biomechanical principles, we can more effectively treat a patient with significant leg-length discrepancy by using a hybrid "prosthosis" than by simply applying a shoe lift. By using a securely attached, rigid system, the patient more easily controls the ground-reaction forces, which are more optimally transferred to the extremity. This enhances not only stability, but also gait speed and agility. Adjustments can be made to extend the length of the device to maximize its use. The patient's enhanced proprioception and muscle use assists in development of a more normal gait.