Paper 217: Posterior Stabilized TKA's are Less Sensitive to Implantation Tightness than PCL-Retaining Designs: A Biomechanical Study

Abstract

Restricted range of motion or excessive laxity are potential complications of total knee arthroplasty (TKA). In contrast patients demand more function for daily activities. To achieve a good function after TKA is important but the aim for all patients should be a better flexion to kneel, work and live without restrictions. Hence the present study was to investigate the effects of implantation tightness on soft tissue strain during passive range of motion. Fourteen fresh frozen cadaver knee joints mounted in a passive flexion rig. Passive flexion was applied to the tibia in 15 deg increments from 0 to 150 deg of flexion. Tibiofemoral force and ligament strain of the medial and lateral collateral ligaments were measured using a custom-built force platform inserted beneath the polyethylene inlay. The standard implantation technique and two variations (2 mm tighter, 2 mm looser) were tested for four prosthesis types: fixed-bearing PCL-retaining (with and without PCL) (PFC), mobile-bearing PCL-sacrificing (LCS), posterior-stabilised (PS), posterior-stabilised with HiFlex femoral component (HF). As expected all prostheses showed a significant increase in tibiofemoral force at full extension when implantation tightness was increased. By comparison in the midrange between 15-90 deg the tibiofemoral force dropped down and was independent of variation in implantation tightness in all designs. Beyond 90 deg the fixed bearing design (PFC) and in the LCS knee were sensitive on implantation tightness. In contrast the two posterior stabilised prosthesis types (PS and HF) showed a different pattern of tibiofemoral force during flexion, with low force throughout the range of motion. As a result in full extension and deep flexion both the PFC prosthesis and the LCS knee represent sensitive designs when considering implantation tightness. Whereas posterior stabilised knee implants are associated with lower tibiofemoral force during the range of motion and more forgiving designs in relation to implantation tightness. The data also suggest that soft tissue structures other than the collaterals tighten as the knee flexes and cause an increase in tibiofemoral compressive force.