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Thermoplastic Elastomer (TPE) -- The TPE Ankle-Foot Orthosis and The TPE Biomechanical-Foot Orthosis

Ron Sutton, B.S.E., C.O.,

A common experience of orthotists and prosthetists is to observe the abnormal gait patterns of patients. As clinicians, our goal is to restore gait patterns to or as close to normalcy as possible. From this perspective, a new plastic material was desired which would provide both improved comfort and gait for those candidates for the plastic ankle-foot orthosis (AFO) as well as meet the needs of those requiring a biomechanical foot orthosis.

At present, most plastic AFOs are made with either polypropylene or copolymer plastic. The selection of thermoplastic elastomer (TPE) as a preferred plastic material for orthotic application was based upon several of its unique performance characteristics. The performance characteristics of TPE - its semi-rigidity, flexibility and durability - were felt to be of biomechanical advantage when applied to the function of the AFO and the biomechanical foot orthosis. TPE belongs to the category of plastics referred to as ethylene propylene thermoplastic vulcanite's, and therefore, has performance characteristics which are similar to that of vulcanized rubber.

Criteria for Evaluation

In order to evaluate the TPE AFO, a set of criteria were established. The criteria were based upon a selection of biomechanical motions of both the ankle-foot and knee, and the goal was to improve the client's gait with an AFO with enhanced performance.¹ The criteria are as follows:

• To allow a loading response at heel strike whereby, plantar flexion of the ankle-foot followed by knee flexion will preserve the forward progression of the limb through midstance.
• To allow dorsiflexion of the ankle-foot at midstance through terminal stance whereby the tibia may advance beyond neutral over the supporting foot and preserve a forward progression of the limb. In early midstance knee flexion should be allowed to persist while at late midstance knee extension will occur.
• To control coronal plane ankle-foot instabilities and flexible varus or valgus tendencies of the ankle-foot. To hold the ankle-foot in a rigid neutral position during swing phase.

Available Orthotic Designs

Three existing polypropylene AFO designs and two TPE AFO designs were then compared and the data evaluated. Below is a brief description of each AFO used.

1. A semi-rigid polypropylene ankle-foot orthosis with supporting medial and lateral trim lines. (Figure 1)
2. A dorsiflexion assist polypropylene ankle-foot orthosis with supporting medial and lateral trimlines and plantar flexion resistant straps. (Figure 2) .
3. An articulating ankle joint polypropylene ankle-foot orthosis with supporting medial and lateral trimlines; free dorsiflexion and plantar flexion-stop design. (Figure 3) .
4. The TPL semi-rigid ankle-foot orthosis with supporting medial and lateral trimlines. (Figure 4).
5. The TPE ankle-foot orthosis with supporting medial and lateral trimlines and enhanced passive dorsiflexion. (Figure 5) .

Observation and Evaluation

The following are the observations and subsequent evaluations of the five ankle-foot orthoses designs. These observations were made during a video filming and review of the functional operation of each ankle-foot orthosis design. Performance was based upon the ability of an AFO to achieve the various criteria mentioned.

The Semi-Rigid Ankle-Foot Orthosis Design with Supporting Medial and Lateral Trimlines.

1. With this AFO design there is no ability of the ankle-foot to plantar flex from heel-strike through midstance. Instead there is the premature occurrence of knee flexion caused by the AFO at heel strike (Figure 6) . A SACH or beveled heel must be used to reduce the premature knee flexion.
2. There is no dorsiflexion of the anklefoot from midstance through terminal stance. Instead there is the occurrence of a knee extension moment caused by the AFO at midstance (Figure 7) . A rocker bottom sole must be used to reduce the knee extension moment caused by the AFO at midstance.
3. There is control of the ankle-foot in the coronal plane by the material proximal and distal to the ankle malleoli.
4. The ankle-foot is held in a rigid neutral position during swing phase.

1. With this AFO design there is no ability of the ankle-foot to plantar-flex from heel-strike through midstance. A SACH or beveled heel must be used to reduce the premature knee-flexion caused by the AFO at heel strike. The plantar flexion resistant straps prevent the natural plantar-loading of the ankle-foot from heel strike through midstance.
2. There is passive dorsiflexion of the ankle-foot from midstance through terminal stance. This is the result of the reduced trimlines posterior of the malleoli on this design.
3. There is control of the ankle in the coronal plane.
4. The plantar flexion resistant straps hold the foot in a rigid neutral position during swing phase.

Other disadvantages of this design are: the undesirable appearance of the plantarflexion resistant straps, the eventual deterioration of these straps and the fatigue-stress of the polypropylene or copolymer at its flexure points (Figure 8) .

The Articulating Ankle-Foot Orthosis with Supporting Medial and Lateral Trimlines, Free Dorsiflexion and Plantar Flexion Stop Design.

1. With this AFO design again the anklefoot has no ability to plantar flex after heel-strike through midstance. The plantar flexion stop design prevents natural plantar flexion of the ankle-foot from heel strike through midstance. A SACH or beveled heel must be used.
2. There is free dorsiflexion of the anklefoot from midstance through terminal stance. This is the result of the articulating ankle joint in this design.
3. The plastic proximal and distal to the malleoli does allow control of the ankle in the coronal plane. Particular care must be given to avoid malleoli contact with the articulating ankle joint and its hardware. This contact can occur with the presence of ankle-foot varus or valgus.
4. The plantar flexion stop holds the foot in a rigid neutral position during swing phase.

A disadvantage of this AFO design is its bulkiness around the malleoli due to the clearance space needed between the malleoli and the articulation, and the thickness of the bivalved plastic (Figure 9) .

The TPE Semi-Rigid Ankle-Foot Orthosis with Supporting Medial and Lateral Trimlines.

It should be noted that this TPE design may have plastic trimlines located about apices of the malleoli in one of three places: anterior to the apices, at the apices, or just posterior to the apices. Each successive plastic trimline increases passive dorsiflexion from midstance through terminal stance.

1. Plantar flexion of the foot at heel-strike is present due to the elongation of the plastic around the ankle malleoli and proximally along the medial and lateral trimlines. This is a result of the lever arm action created at the heel upon heel-strike. The heel acts as the fulcrum point of the lever arm between the plantar-flexing foot and the leg proximal to the ankle joint. The TPE elongates about this point because of its flexibility. As a result, there is less knee-flexion torque and no need for either SACH or beveled heel.
2. There is passive dorsiflexion of the ankle-foot from midstance through terminal stance. The TPE flexes about the ankle during this gait phase.
3. There is control of the ankle-foot in the coronal plane by the plastic about the malleoli. Additional coronal plane control of the ankle-foot is rendered by the ability of TPE to comfortably hug the ankle-foot region without creating pressure points.
4. The foot is held in a rigid neutral position during swing phase. Another advantage of this AFO design is that TPE does not demonstrate the same fatigue-stress as polypropylene and copolymer.

The TPE ankle-foot orthosis with supporting medial and lateral trimlines and enhanced passive dorsiflexion.

This design functions the same as the former TPE design except for allowing a greater degree of passive dorsiflexion from midstance through terminal stance. This TPE AFO has a posterior slot in the plastic. This slot is located at the level of the anatomical ankle joint and terminates in deflection holes posterior to the malleoli. When the posterior slot opens, increased passive dorsiflexion of the ankle-foot from midstance through terminal stance is permitted. When the posterior slot closes, a plantar flexion stop results during swing phase. The performance characteristics of TPE that permit these functions are its flexibility and durability. A possible disadvantage of this design is the increased flexure of the plantar flexion stop when increased stress from a knee hyperextension problem is present (Figure 10) .

Results

When the semi-rigid polypropylene AFO is used, the degree of dorsiflexion and plantar flexion of the foot is moderated by the placement of the plastic trimlines about the apices of the malleoli. As the plastic trimlines are placed more posterior to the apices of the malleoli, the plastic begins to flex and two changes occur: the ankle-foot is allowed more passive dorsiflexion from midstance through terminal stance and the foot is held in a less rigid neutral position during swing phase. Gradually a third change may occur: the polypropylene bends and causes the foot to remain in a plantar flexed position (Figure 11) . This change occurs because of the repeated lever force executed at heel-strike during loading response of the foot.

As mentioned earlier, with the TPE AFO design there are three possible placements of the trimlines around the apices of the malleoli. The trimlines may be placed anterior to the apices, at the apices. or just posterior to the apices. The TPE AFO with these three respective plastic trimlines will allow passive dorsiflexion of the ankle-foot from midstance through terminal stance with increasing ease, and yet maintain a rigid plantar flexion position during swing phase.

The results of evaluating these five AFO designs reveal that the TPE AFO provides a smoother and more normal gait cycle from heel-strike through terminal stance, increased capacity to control the ankle in the coronal plane, and a rigid plantar flexion position during swing phase. All of the above contribute to the enhancement of the three components of walking: progression, standing stability and energy conservation.

Approximately 38 patients are now using the TPE ankle-foot orthosis. Patients who formerly used the semi-rigid polypropylene or copolymer AFOs are pleased with both the comfort and performance of their new TPE ankle-foot orthosis. When interviewed they all preferred the TPE design over the standard polypropylene or copolymer plastic AFO These results are a direct consequence of the three performance characteristics of TPE: its semi-rigidity, flexibility and durability. In addition, there has been no evidence of fatigue-stress among the TPE AFOs in the 13 months used.

Two contraindications exist for the TPE AFO. The TPE AFO should not be used when a locked ankle and a resulting locked knee extension are desired. Secondly, the posterior slotted TPE AFO should not be used on patients with mild knee hyperextension problems because the severe leverage in this circumstance overcomes the plastic plantar-stop. However, a solid ankle TPE design is highly recommended for clients with mild knee hyperextension problem because of the advantages cited.

Fabrication

To prepare a positive mold of a leg for the TPE AFO with the posterior slot, apply the conventional bilateral malleoli buildups. In addition to this, the following steps are needed to prepare the positive leg mold for a TPE AFO with a posterior slot.

1. Locate the anatomical ankle joint height.
2. At this height, locate two points one centimeter posterior to the medial and lateral malleoli.
3. On the posterior of the leg draw a line connecting these two points.
4. Make a slot in the plaster along this line with an old hack saw blade (Figure 12).
5. Place a piece of cardboard in this slot and trim it so that it projects out from the positive mold one centimeter.
6. At the anterior ends of the cardboard piece a section of tubing, 1 cm x 5/16" O.D. x .035 wall, secured to the model with nails (Figure 13) .
7. Add plaster about the tubing and cardboard to secure them in place and then smooth the edges, flaring the plaster on both sides of the cardboard as little as possible (Figure 14) .
8. After vacuum-forming the TPE over the positive leg mold, drill a 1/4" hole where the tubing is located at the ends of the slot.
9. The slot is then slit with a sharp utility knife from one terminal deflection hole to the other (Figure 15) .

TPE having 3/16" or .187 thickness should be heated at a temperature of approximately 375° to 390° Fahrenheit for six to eight minutes before vacuum forming. Note that overheating will result in the plastic thinning out at the heel. The TPE will also thin out at the heel if it is stretched too much around the positive mold. It is recommended that two people slowly and deliberately drape the TPE around the ankle-foot region avoiding wrinkling. TPE is not as subject to spreading as polypropylene and copolymer and can be removed from the positive mold after it is cooled. TPE is easy to trim, rout and buff to a smooth finish and is cosmetically pleasing in appearance. TPE can be obtained in two colors (black and fleshtone) and can be obtained directly from a plastic extruder, Ligon Brothers Manufacturing, Almont, MI (313) 798-3921. TPE is an FDA approved plastic and should be used with the same professional discretion commensurate to that of other plastics used in orthotics and prosthetics.¹

The TPE Biomechanical-Foot Orthosis

When applied to the biomechanical foot orthosis, the semi-rigidity quality of TPE has filled in the gap between the available materials now used, which range from the rigid plastics to the high density foams. Unique to TPE is its ability to enable the foot to flexibly adapt to the ground at heel-strike through midstance, then to support the foot at midstance and allow it to propel as necessary from midstance through terminal stance. Although it is a semi-rigid material, TPE is resistant to cracking and breakage as it performs as a vulcanized thermoplastic rubber. TPE bonds well to posting material and is easy to adjust by heat gun. TPE biomechanical-foot orthoses have performed well for patients with corrective foot deformities, as well as for those clients desiring an excellent sports orthosis (Figure 16) . Over 20 patients have been fit with such orthoses.

Conclusion

The TPE ankle-foot orthosis is a worthy addition to the present armamentarium of plastic ankle-foot orthoses. As mentioned, the TPL AFO provides a smoother gait during stance phase. TPE provides greater control of the ankle in the coronal plane because of its capacity to hug and conform more comfortably to the anatomy of the leg. Patients experience greater ease when ascending and descending stairs as well as a greater ease when bending at the ankles. The use of the TPL AFO eliminates costly shoe alterations and patient expense for multiple shoe alterations such as SACH heels and rocker-bottom soles. The TPF AFO also eliminates the bulk size and timely two-part fabrication of an articulating AFO in order to achieve passive dorsiflexion and a plantar-flexion stop. The three components of walking - progression, standing stability and energy conservation - are improved by the use of the TPE AFO compared to the similar conventional AFO designs made of polypropylene and copolymer.

Note: Although the information and recommendations set forth herein are believed to be correct, both Ligon Brothers Manufacturing Company and myself make no representations or warranties expressed or implied regarding the use of this plastic. In no event will either of the above parties be responsible for damages of any nature resulting from the use of TPE or reliance upon information in reference here.

Ron Sutton, B.S.E.. CO.. Is a member of the Department of Physical Medicine Rehabilitation staff of The University of Michigan Medical Center. 1500 E. Medical Center Drive, University Hospital 1D203, Ann Arbor, Michigan 48109-0042.

References:

1. Jacquelyn Perry. "Normal and Pathologic Gait," Atlas of Orthotics, 2nd Edit.. 1985, pp. 76-86.