A new cast immobilizer that is heat-shrunk to conform to an injured ex
tremity was examined. The purpose of these studies was to compare pres
sures beneath the thermoplastic cast with those beneath fiberglass cas
ts on a laboratory model and on the forearms of human volunteers. Pres
sures measured beneath fiberglass casts on metal cylinders averaged 36
mm Hg. Thermoplastic casts on the smaller cylinder that allowed 42% s
hrinkage produced a mean pressure of 25 mm Hg; those placed on the lar
ger cylinder that allowed 17% shrinkage produced a mean pressure of 39
mm Hg. Pressures measured on the forearms of healthy volunteers avera
ged 22 mm Hg beneath fiberglass casts and 31 mm Hg beneath the thermop
lastic casts. These pressures sere considerably less than pressures th
at 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 com
promise to the limb than previously used fiberglass materials.