A biomechanical evaluation of sorbothane.

Sorbothane (I.E.M. Orthopaedics, Aurora, Ohio), a viscoelastic polymer, has been developed for orthopedic application, but there has been little substantiation of the claim that it is capable of absorbing 95% of the impact energy in foot strike. Sorbothane has been used extensively in orthotic insoles in order to minimize the symptoms of joint degeneration, prosthesis loosening, and various soft tissue disorders resulting from the impact generated at foot strike during walking and running. The viscoelastic properties of Sorbothane were investigated by Durometer tests at various temperatures as well as compression tests of thin discs of material with different degrees of lateral constraints and varying rates of loading. Sorbothane has the characteristics of a viscoelastic solid with a relaxation time of two seconds. The effects of lateral constraint were significant, indicating that the properties of this material will change when bonded to other substances in the production of insoles. The final transmitted stress over the duration of foot strike would not be reduced by more than 10%, which may have some therapeutic implication, but not of the magnitude suggested by other authors.