The Use of Multiple Thermoplastics in the Design of Lower Extremity Orthoses and Their Application in Pediatric Case Studies

D. P. BLOCKA, C.O.(c), B.Sc. AND M. M. MILLER, C.O.(c), B.Sc.

Ankle Foot Orthoses (AFO) are commonly implemented to control and stabilize the biomechanical position of the ankle-foot complex, improve detrimental gait variables, and influence spasticity. A wide range of molded thermoplastic designs have been implemented clinically to deal with these issues. These range from the basic rigid and flexible designs to hinged or single-axis ones. The single-axis designs can incorporate a wide range of ankle joints that have various features. The most common types of thermoplastics used in the fabrication of these orthoses has been polypropylene or various copolymers.

Recently the use of low density polyethylene in a circumferential or tubular manner have been implemented to deal with extreme and unstable positional problems of the subtalar and midtarsal joints and are incorporated in orthoses relating to upper motor neuron disorders such as Cerebral Palsy. Benefits of using this material relate to its inherent flexibility allowing the characteristics of the orthosis to be softer and more dynamic. References in literature relate to its potential in tone reduction due to improved comfort in bony anatomical regions and its ability to allow controlled and normal incremental movements.

Many of the ankle joints available for single-axis AFOs reduce the amount of total contact available to control position. The orthoses lack the beneficial qualities of low density polyethylene and rigid thermoplastics such as polypropylene must be used. These rigid thermoplastics supply the single-axis AFO with the necessary structural strength required and allow joint motion controls to be used. Problems result when using this design to control extreme positional problems involving varying degrees of spasticity as they do not supply the volume or circumferential control and other qualities desired that low density polyethylene is able to.

Circumferential rigid AFOs fabricated from low density polyethylene are at times structurally ineffective when either the patients' body mass or the demands placed on the orthosis are too high. In many of these cases buckling or fracture of the plastic has occurred.

The incorporation of multiple thermoplastics in rigid and single-axis AFOs has rendered promising results. Hybrid designs utilizing low density polyethylene inner shells with outer polypropylene or colene frames have been implemented in a number of case studies. The designs have allowed benefits of both types of thermoplastics to be derived from the orthoses. The total contact and volume control of low density polyethylene and the structural strength of polypropylene or colene can be combined in many variations. This allows much latitude in the final design of orthoses applicable to cases indicating the use of the system.

A negative factor in implementing this design clinically is increased time and cost of fabrication. At the present time more orthoses of this nature are being fabricated to make further design refinements and develop substantial criteria indicative for its implementation.

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