Consequences of an interference fit on the fixation of porous-coated tibial components in total knee replacement.

Abstract

A linearly elastic, axisymmetrical finite-element model was developed in an attempt to explain observed long-term patterns of growth of bone into tibial components. This model, which represents a portion of the tibial tray, one peg, and the surrounding cancellous bone, was used to examine two conditions of fixation in the immediate postoperative period. The first was characterized by the use of an interference fit for initial fixation of the component and the second, by the use of an interference fit with the hole in the bone deeper than the length of the peg. Two conditions of long-term fixation were also examined. In one, the bone was assumed to have grown into all of the porous coating. In the other, the bone was assumed to have grown only into the peg, and a layer of fibrous tissue was assumed to have developed between the tray and the bone. An interference fit between the peg and the cancellous bone produced considerable residual radial stresses in the bone. These stresses provide conditions that are favorable for ingrowth of bone into the pegs because the bone at the interface is stressed, and these stresses inhibit relative motion at the bone-peg interface. However, the interference fit of the peg relieved the stresses in the cancellous bone under the tray of the implant. Lack of stress at this interface is consistent with relative motion and subsequent formation of a layer of fibrous tissue. Deepening of the hole for the peg in the cancellous bone did not diminish the effects of the interference fit. Stresses in the bone under the metal tray were relieved when a layer of fibrous tissue under the tibial tray was represented in the model.