The Effect of Extra-Articular Deformity on the Femoral Component

Abstract

It is widely accepted that long-term satisfactory function and survival of both the prosthetic bone interface and the plastic surface of the tibial component requires accurate alignment in total knee arthroplasty. In fact, most patients present with some degree of preoperative malalignment and several chapters in this book deal with problems associated with severe preoperative varus and valgus deformity. However, the majority of these deformities are due to articular cartilage and bone loss of the distal femur, the proximal tibia, or both (Fig. 87.1). Occasionally, the deformity may be due to soft tissue instability with or without intra-articular bone loss. In both situations, the apex of the deformity is at the joint. With these kinds of deformities, ligamentous contractions or laxities may also be present. Regardless of the deformity, the distal femoral and proximal tibial bone cuts are made referencing the anatomical axis of the respective bones. Proper cuts will automatically restore proper alignment. Separate procedures may be necessary to reestablish soft tissue balance but the orientation of the bone cuts to the anatomic axis is not influenced by the direction or degree of the preoperative deformity. Moreover, in most instances when the proper bone cuts (level and orientation) have been made, ligamentous stability is reestablished without the need for any supplemental ligamentous balancing procedures (Fig. 87.2). When patients present with significant extra-articular deformities, the foregoing observations concerning alignment and ligament balance no longer apply. First, the anatomical references of the femoral shaft and the tibial shaft no longer apply and second, when the distal femoral and proximal tibial bone cuts result in alignment of the mechanical axis (center of the femoral head to the center of the ankle passing through the center of the knee) ligamentous instability has been created. It is the purpose of this and the following chapter to address the implications of extra-articular deformity on the femoral cuts and on the tibial cuts, and the consequences on reestablishing soft tissue stability.