Pressure Generation Beneath a New Thermoplastic Cast

A new cast immobilizer that is heat-shrunk to conform to an injured extremity was examined. The purpose of these studies was to compare pressures beneath the thermoplastic cast with those beneath fiberglass casts on a laboratory model and on the forearms of human volunteers. Pressures measured beneath fiberglass casts on metal cylinders averaged 36 mm Hg. Thermoplastic casts on the smaller cylinder that allowed 42% shrinkage produced a mean pressure of 25 mm Hg; those placed on the larger cylinder that allowed 17% shrinkage produced a mean pressure of 39 mm Hg. Pressures measured on the forearms of healthy volunteers averaged 22 mm Hg beneath fiberglass casts and 31 mm Hg beneath the thermoplastic casts. These pressures were considerably less than pressures that have been shown to occlude the microcirculation of the skin. Acute compartment syndromes result from swelling within a limited space and remain a serious concern clinically when swelling is anticipated under any type of constraining cast. The results of these studies indicate that the new cast should not produce a greater risk of circulatory compromise to the limb than previously used fiberglass materials.