In vitro measurement of bone-acrylic interface pressure during femoral component insertion.

Bone-acrylic interface pressure measurements were recorded at the medial interior rasped surfaces of fresh cadaver femurs during digital packing of acrylic bone cement and during insertion and seating of a Trapezoidal-28 femoral total hip component. Plugging of the femoral canal below the tip of the prosthesis stem was an effective means for increasing pressure in the distal femoral canal when the stem was inserted in the early stages of acrylic polymerization (i.e. immediately after dough time). At surgery, this can be accomplished by inserting a small bolus of acrylic down the canal to a depth below the tip of the seated stem and allowing it to polymerize in place. This forms an effective seal which prevents distal extrusion of the acrylic when the cavity is then packed prior to prosthesis insertion. Elimination of cement in the distal canal also avoids any future difficulty of acrylic removal should revision become necessary due to loosening or infection. No significant pressure differences were observed between one and two millimeter thicknesses of acrylic between the metal and bone. Interface pressures developed during finger packing were of the same order of magnitude as those achieved during seating of the femoral component. Use of a rubber diaphragm stretched tightly over the margins of the rasped femoral cavity helped to contain the acrylic and prevent extrusion during finger packing but was ineffective in increasing interface pressure. When this method is used, the acrylic can be poured or injected into the canal and packed before dough stage and thus facilitate increased cancellous penetration when the acrylic is in a state of low viscosity.