Mechanical properties of orthopedic plaster bandages

Abstract

This paper is a continuation of the investigations started by Schenck. Somerset and Porter (1968) to evaluate the stresses in orthopedic plaster bandage casts. Experiments have been conducted to determine and verify the mechanical properties of the molded plaster bandage material as well as its individual components, the plaster and the gauze. The modulus of elasticity, the yield stress, and the ultimate strength as functions of moisture content were measured for the plaster bandage material and the plaster without the gauze. Modulus of elasticity and ultimate strength tests were also conducted for various brands of bandage gauze. Using information obtained in the previous study regarding the location of maximum stresses in lower leg casts, volunteer experiments were conducted on completely dried casts to measure the stresses developed in lower leg casts subjected to normal and abnormal use. Material properties are explained by the plaster-gauze interaction and conclusions are drawn regarding the suitability of plaster bandage material. It is found that plaster bandage achieves its optimal mechanical characteristics at its natural dry weight of 21 per cent water content, a state which is usually not achieved in the prescribed 24 hr drying period. Various “normal” activities subject lower leg walking casts to stresses which may be expected to cause tensile failure of the plaster composite material, expecially before the cast is completely dry. The failure of the casts in tension is due primarily because the effective elastic modulus of the gauze is so much lower than that of the plaster.