Quadriceps moment arm is one of the factors determining quadriceps force. Total knee arthroplasty designs with larger quadriceps moment arms should generate less quadriceps and patellofemoral forces. A study was conducted to measure knee kinematics, quadriceps and patellofemoral forces in two knee designs with differing centers of rotation. In addition, the effect of a central dome-shaped versus a medialized patella component was determined. Six human cadaver knees were tested before implantation and after sequential implantation with two posterior cruciate retaining designs: Scorpio and Control. The quadriceps moment arm of the Scorpio design was 1 cm longer than that of the Control design. Knee kinematics was measured with an eletromagnetic tracking device while the knee was put through dynamic simulated stair climbing under peak flexion moments of 40 N-m. Quadriceps tension and patellofemoral compressive and shear forces were measured for both conditions and for the central and medialized patella components. The normal unimplanted condition showed increasing rollback with flexion while both implanted conditions displayed relatively less rollback. Overall, quadriceps tension was highest in the unimplanted condition and lowest in the Scorpio condition. The Scorpio design showed a 10-20% reduction in quadriceps tension at angles greater than 40° when compared to the Control design. Patellofemoral forces were also significantly reduced in the Scorpio design when compared to Control. There were no differences noted between the central and medialized patella component. The Scorpio design, with its more posterior center of rotation, reduced quadriceps tension and patellofemoral forces. Reduced quadriceps forces may facilitate postoperative rehabilitation and activities such as stair climbing. Reduction in patellofemoral forces could reduce patellar complications such as anterior knee pain, component wear and loosening. These results are currently undergoing validation with a prospective clinical study.

Polyethylene (PE) wear affects survivorship in the long term while dislocation remains a significant factor in the short term. Increasing head size can reduce impingement and dislocation. However, this increases wear rates and reduces the net thickness of the liner. Several reports have demonstrated significant reduction in wear in cross-linked PE. This study reports wear rates in crosslinked PE liners with increased head size. Four groups of PE liners were tested against cobalt-chrome heads in a hip wear simulator: highly crosslinked liners with head size 28mm (28XPE) and 32mm (32XPE), and minimally crosslinked liners with head size 28mm (28PE) and 32mm (32PE). Additional liners were used as load-soak controls to monitor weight gain due to fluid absorption. Gravimetric analysis was performed every 500,000 cycles for a total of 5,000,000 cycles. 28PE and 32PE liners had mean wear rates of 12.5(±1.0) and 17.45 (±2.6) mg/million cycles. Both highly crosslinked PE liners (28XPE and 32XPE) had significant less wear rates that regular polyethylene 1.49 (±0.72) and 2.55 (±0.19) mg/million cycles respectively. Increasing head size resulted in increased wear, which is consistent with previous reports. Highly crosslinked PE significantly reduced wear rates in both head sizes. Although there was a small increase in wear in the 32XPE group compared to the 28XPE group, wear was significantly less than both 32PE and 28PE groups. These encouraging results suggest that a dual benefit (reduced wear and reduced dislocation rate) might be achieved using 32XPE liners. Further studies that evaluate fatigue damage, crack propagation and impingement are necessary.