The effect of calcar contact on femoral component micromovement. A mechanical study.

Extra-long-neck, large-stem Trapezoidal-28 femoral components with standard and modified large flanges were cemented into fresh cadaver femora and measurements were made of the movement of the prostheses relative to the bone as load was applied to the head of the prosthesis. In the presence of collar-calcar femoral contact, the prosthesis subsided very little under load; displacements for a load of 2000 newtons were less than 0.1 millimeter. Removal of bone and acrylic support beneath the undersurface of the flange by sawing a thin gap caused no increase in the displacement of the stem tip in most specimens, but the displacements of the collar relative to bone for the standard flange design increased by an average of 0.014 millimeter (8 to 28 per cent) under a 2000-newton force when contact was removed, while the displacements for the prostheses with a modified large flange that extended to the cortical rim of the resected neck increased 0.047 to 0.060 millimeter (56 to 432 per cent), indicating greater support from the collar due to the increased area of contact. An increase of the stem-flange angle of 22 degrees (from 30 to 52 degrees), making the flange more horizontal, had no significant effect on the results for large-flanged units. Prostheses with a standard flange and a one to two-millimeter Silastic liner between cement and bone subsided as much as twenty times more than rigidly cemented units. In the presence of the liner, removal of collar-calcar contact increased the collar displacement by 29 to 75 per cent, indicating a possibly important function of the flange for components cemented in soft bone or components that may become loose or be surrounded by a fibrous membrane. Repacking of the gap between the collar and bone with acrylic after removal of part of the calcar femoral reduced subsidence in one specimen, while the acrylic in a second specimen cracked out during reloading. Based on our laboratory experience with sixteen specimens, we believe that it would be extremely difficult to achieve an effective degree of uniform calcar femorale-collar contact at operation and that one or two localized contact areas would be the more common situation after total hip replacement.