Many recent knee prostheses are designed aiming to the physiological knee kinematics on tibiofemoral joint, which means the femoral rollback and medial pivot motion. However, there have been few studies how to design a patellar component. Since patella and tibia are connected by a patellar tendon, tibiofemoral and patellofemoral motion or contact forces might affect each other. In this study, we aimed to discuss the optimal design of patellar component and simulated the knee flexion using four types of patellar shape during deep knee flexion.

Our simulation model calculates the position/orientation, contact points and contact forces by inputting knee flexion angle, muscle forces and external forces. It can be separated into patellofemoral and tibiofemoral joints. On each joint, calculations are performed using the condition of point contact and force/moment equilibrium. First, patellofemoral was calculated and output patellar tendon force, and tibiofemoral was calculated with patellar tendon force as external force. Then patellofemoral was calculated again, and the calculation was repeated until the position/orientation of tibia converged.

We tried four types of patellar shape, circular dome, cylinder, plate and anatomical. Femoral and tibial surfaces are created from Scorpio NRG PS (Stryker Co.). Condition of knee flexion was passive, with constant muscle forces and varying external force acting on tibia. Knee flexion angle was from 80 to 150 degrees.

As a result, the internal rotation of tibia varied much by using anatomical or plate patella than dome or cylinder shape. Although patellar contact force did not change much, tibial contact balances were better on dome and cylinder patella and the medial contact forces were larger than lateral on anatomical and plate patella. Thus, the results could be divided into two types, dome/cylinder and plate/anatomical. It might be caused by the variations of patellar rotation angle were large on anatomical and plate patella, though patellar tilt angles were similar in all the cases. We have already reported that the anatomical shape of patella would contact in good medial-lateral balance when tibia moved physiologically, therefore we have predicted the anatomical patella might facilitate the physiological tibiofemoral motion. However, the results were not as we predicted. Actually our previous and this study are not in the same condition; we used a posterior-stabilized type of prosthesis, and the post and cam mechanism could not make the femur roll back during deep knee flexion.

It might be better to choose dome or cylinder patella to obtain the stability of tibiofemoral joint, and to choose anatomical or plate to the mobility.

Treatment of the femoral head necrosis with severe extensive collapse in young adults and adolescents are still challenging. We thought preserve the joint and bone stock were important factor for the treatment of femoral head necrosis in young patients. We reviewed the posterior rotational osteotomy for younger patients with severe osteonecrosis. The advantages of posterior rotational osteotomy are; the necrotic area is moved to non-weight bearing portion. The posterior column artery is shifted medially without vascular damage by rotation. Postoperative uncollapsed anterior viable areas are moved to the loaded portion below the acetabular roof in flexed positions. Eighty five hips of 66 young adults (less than 50 years old, mean age; 31 years) with extensive necrosis treated by posterior rotational osteotomy were reviewed with more than 5 year follow up with a mean of 9 years. Results of 13 hips of 12 adolescents (mean age; 14 years) with extensive collapsed necrosis treated by this procedure were also studied with a mean of 6.5 years. A mean degree of posterior rotation was 121. Recollapse was prevented in 77 hips (91%) of adults, and all 13 hips of adolescents on final AP radiographs. Collapsed lesion was remodeled well and resphericity of the postoperative transferred medial collapsed femoral head on final AP radiographs was observed. However, some of the cases were out of indication of the joint preserving procedure showing extensive lesion. In these cases, we performed the MAYO conservative stem for preserving bone stock. Radiological results of 26 hips with osteonecrosis treated by MAYO stem (mean age 42 years, minimum 5 year follow-up. mean; 6.7 years) showed that 2 mm subsidence in one, osteointegration of zone 2, 6 in 93%, no entire lucent line. No hips were revised for late loosening associated with osteolysis. CT imaging indicated that spot welds of zone 2, 6 were found in 100ï¼ï¿½, stress shielding of zone 1, 5 (23.5ï¼ï¿½). These operations were useful particularly for younger patients.