Purpose: The effect of patellar position on soft tissue balancing in total knee arthroplasty (TKA) is under debate. We developed the digital tensor system to measure the load (N) and the distance (mm) of extension and flexion gaps in medial and lateral compartment separately with setting of femoral component trial. The gap load and distance in extension and flexion position of posterior stabilized (PS) and cruciate retaining (CR) TKA in both patella everted and reset position were measured.

Materials and Methods: Thirty-four patients who underwent primary TKA for medial type osteoarthritis using medial parapatellar approach were included. The load was measured at the gap distance, which is equal to the sum of implants including polyethylene insert.

Results: In extension, there was no significant difference between the load in patella everted and reset position in both PS-TKA and CR.-TKA. In flexion, there was a significant decrease of the load, which is comparable to the increase of gap distance of approximately 2mm, by resetting the patella from eversion in PS-TKA. There was, however, no significant difference in CR-TKA by resetting the patella. There was no significant difference in the ratio of medial/lateral load in both PS-TKA and CR.-TKA.

Conclusion & Significance: Soft tissue balancing of PS-TKA with medial parapatellar approach should be performed after resetting the patella.

The changes of stress distribution in the femoral head with Perthes disease were observed under several condition. Finite element models were constructed referring to X-ray images and magnetic resonance images of the intact hip joint. The model was divided into five parts: cancellous bone, articular cartilage, necrotic bone, cortical bone, physeal cartilage. Material properties were alloted to these components by the past literature. The body weight and abductor muscle force were applied as loading. The model was altered to study the effect of age, the extent of necrosis, and lateralization of the fomoral head. Analysis were performed on a digital computer PC-9821(NEC) using the finite element program. There was no significant difference in stress distribution patterns regardless of age or extent of necrosis. However, compressive stresses were concentrated on the lateral portion of the epiphysis by lateralization of femoral head. The femoral head deformity in Perthes disease was more affected by the lateralization than by the age and the extent of necrosis.