Concepts in Computer Assisted Total Knee Replacement Surgery

Total knee arthroplasty (TKA) is commonly considered to be a reliable procedure, with implant survival rates higher than 90% at 10 to 15 years of follow-up. The goal of total knee replacement surgery is to relieve pain and obtain better knee function, those achieved by correct patient selection, pre-operative deformity, implant design, correct surgical technique and patient participation in the rehabilitation protocol (Nizard et al, 2002). Several technical requirements during TKA are important to obtain good results:  correction of deformities;  achievement of functional joint motion and stability;  optimal balancing of soft tissues;  satisfactory alignment in the frontal, sagittal and horizontal planes. From literature data alignment in frontal plane must be into 2o or 3o range around a neutral alignment; this thought is demonstrated by Ritter at al who observed that prostheses implanted in varus position had a lower survival rate than prostheses implanted in a neutral or valgus position (Ritter et al, 1994); moreover Jeffery at al observed that when mechanical axis was in 3o valgus-varus range, the loosening rate was 3%, whereas it’s 24% when the alignment was out of this range (Jeffery et al, 1991). The alignment in the horizontal plane is of particular importance for extensor mechanism stability, patellar wear, tilted patella, prostheses dislocation or loosening. In a study of Berger et al it was observed that patients with extensor mechanism problems have internal rotation of the femoral and tibial components (Berger et al, 1998). Technically, there is a definite relationship between the accuracy of implant positioning and long-term durability (Jeffery et al, 1991; Stulberg et al, 2002): the position of prosthetic components and, consequently the alignment of mechanical axis, could be the cause of polyethylene wear due to overload stresses, ending finally by prosthetic loosening. The postoperative mechanical axis of the lower limb should be a straight line passing through the center of the hip, the center of the knee, and the center of the ankle; so that satisfactory