Silicone Elastomer Cushioning For The Painful Heel Syndrome: A Preliminary Report
Ralph Lusskin, M.D
The painful heel syndrome is a somewhat obscure affliction, which often persists despite local treatment. This pain syndrome can be relieved by heel elevation, redistribution of body weight and support, and pressure relief under the heel. Use of a new type of heel pad may offer a simpler and more effective method of treatment than has been available in the past and may obviate the need for local injections to relieve the pain patterns typical of the condition.
The painful heel syndrome should be thoroughly evaluated prior to treatment. Local skeletal disease in the calcaneus and talus is occasionally present; and tumor, sepsis, and arthritis, if present, will be revealed by radiographic examination. The ossified insertion of the plantar fascia into the calcaneus must not be considered abnormal or the cause of pain, for its presence has been demonstrated in normal distribution with and without heel pain. Systemic rheumatoid arthritis and lupus erythematosus may be heralded by heel pain. The calcaneal branches of the posterior tibial nerve also can be a source of pain. Any trigger points on the nerve should be studied by local nerve blockade.
In recent years implantation of rubber in humans has expanded rapidly due to the substance's inertness and physical simulation of living mesenchymal structures, Its mechanical properties combine impact resistance with gradual dissipation of applied forces. It is able to diffuse impact loads both along the lines of force and transverse to that line, thus distributing large kinetic loads over a broad area with minimal thickness of material.
Resistance Properties of Heel Pads
The load-resisting properties of the human heel pad depend on columns of fat cells within tubes of heavy fibrous connective tissue. Compression force is cushioned by vertical and lateral deformation of these columns. Pressure is balanced by tension on the vertical fibers of adjacent areas, and impact force is distributed over a wide area. Pain seems to develop in these connective tissue tubes as a result of overloading. Fat hypertrophy in obesity, and connective tissue atrophy from disuse, may play a part in the failure of the heel pad structure which leads to local pain.
Silicone as Cushioning Material
In an effort to find a synthetic replacement for the heel pad and provide relief for a load-bearing structure which has failed, silicone rubber has been investigated. Internal placement of the device is not essential for cushioning, and the silicone can be placed in the shoe with an excellent mechanical result if no rigid structure intervenes between the foot and the cushion.
The pad that we have used for our patients is constructed from a 1/4-inch-thick block of medical-grade silicone rubber* of 30 durometer hardness (Fig. 1 ). Purity is not a requirement for a device used externally, and a lower grade of rubber would doubtless suffice. However, if cost considerations are not too great, it would probably be better not to introduce a lower grade of silicone block into the therapeutic environment, as possibilities of error do exist. If this low-grade silicone rubber were inadvertently used in surgical implantation, It would be most unfortunate .
The pad is cut to conform to the heel seat and is skived at its leading edge (Fig. 2 and Fig. 3 ). Its length is 3 1/2 to 4 inches. No contouring of the pad has been performed, since a full thickness of silicone beneath the heel is indicated for maximum control of shock at heel strike and diffusion of weight during midstance. The block silicone should be of maximum softness, but structural rigidity of the shoe is important to prevent instability and heel shift during the gait cycle. Thus, liquid-filled devices would not be satisfactory for use as a heel cushion. Lateral deformation would permit the heel to impact on the shoe unless excessive thickness was present. Pistoning and horizontal shift would take place even with a semisolid filling to the heel pad.
At this center the silicone pad has been used in four cases. All experienced immediate relief or improvement of pain and have continued to wear the pad. A felt elevation in the opposite shoe has been used to balance the patient. Other solutions to the problem of leg-length discrepancy, such as raising the contralateral heel or lowering the ipsilateral heel, could be equally effective.
Further studies to determine the duration of need for the device and to determine the limits of application are under way. Certainly its use following calcaneal fracture and traumatic disruption of the fatty and fibrous structure of the heel pad would seem appropriate. Various modifications of the silicone heel pad and the leather heel seat to eliminate forward shift of the pad are under study, but the basic structure is effective and comfortable with the simple fitting to the shoe as described.
*Silastic 370 medical-grade elastomer, supplied by Dow Corning.
Ralph Lusskin, M.D is the Associate Professor of Clinical Orthopaedic Surgery School of Medicine and Post-Graduate Medical School New York University, New York, New York