The contact condition in the human knee joint must play important roles especially in dynamic loading situations where the loads transfer in the knee. In this study, the impact stress propagations through the inside of the knee joint were simulated using the three-dimensional finite element analysis (FEA). And the differences in the stress distribution were investigated between the intact knee and the total replacement condition.

The finite element (FE) models of an intact human knee joint and a total replaced knee joint were constructed with high shape fidelity. The intact model included the cortical bone, cancellous bone, articular cartilage, bone marrow, and meniscus. And the total replacement knee FE model, which is consisted of the artificial femoral and tibial components were also prepared to compare the impact propagations with the intact model (Figure 1). Impact load were applied to the proximal femur of the FE models under the same conditions as those of the weight-drop experiments with the knee joint specimens.

The FEA results showed that the impact stress propagated to the tibia through the knee joint for several milliseconds. The values and the time dependent change of the compressive strain on the cortical surface had good agreement with the experimental results. The compressive stress mainly propageted at the medial side, with 1.0 MPa at 1.2 milliseconds.

Especially, the impact stress propagated not only in the cortical surface area which has hard material property but also in the soft cancellous bone region inside the knee joint. The mass density of the cancellous bone has similar to that of the cortical bone, and thus the role of the load bearing in the cancellous area must be much increasing under the impact condition.

In the total replacement model, concentration of the impact compressive stress was observed with 2.8 MPa at the tibial region, while not under the normal intact conditions (Figure 2). Since the total replacement model is formed of different materials and the impact propagations were inhibited by the interfacial condition, such as sliding or debonding, it is considered that the contact condition between such materials have a great effect on the stress propagation.

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 PSTKA and CR.-TKA.

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

Newer prosthetic total knee arthroplasty (TKA) designs as well as unicondylar TKAs spare the anterior cruciate ligament (ACL). Although success of these procedures requires near normal ACL function, little has been written about the arthritic ACL.

This study was designed to evaluate the relationship between cross sections of the intercondylar notch and the macroscopic condition of ACL degeneration. Thirty osteoarthritic patients who underwent TKA as a result of severe osteoarthritis were randomly selected. Occupation rate of the osteophytes to the notch width were measured at the anterior 1/3, middle 1/3, and posterior 1/3 notche images obtained from preoperative tunnel view. Macroscopic conditions of the ACL and PCL were classified into four types of Normal, Frayed, Partial rupture, and Absent.

The macroscopic ACL conditions were Normal: 9 cases, Frayed: 9 cases, Partial rupture: 9 cases, and Absent: 3 cases. The macroscopic PCL conditions were Normal: 24 cases, Frayed: 3 cases, Partial rupture: 3 cases, and Absent: 0 case.

Occupation rate of the osteophytes to the notch correlated to the preoperative femorotibial angle (p< 0.05). In terms of ACL, the occupation rate of the osteophytes to the notch were 22.9%, 28.8%, 46.0%, and 81.8% in Normal, Frayed, partial ruptured, and Absent, respectively. The patients with more than 40% occupation rate showed either partial rupture or absent of the ACL during the surgery.

We conclude that occupation rate of the osteophytes to the notch is a good predictor of evaluating the ACL degeneration in osteoarthritic knee.

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.

Introduction: Soft tissue balancing remains the most subjective and most artistic of current techniques in total knee arthroplasty. The flexion gap is traditionally measured at approximately 45 degree of hip flexion and 90 degree of knee flexion on the operation table. Despite of aiming equal joint gaps or tensions in flexion and extension, influence of the thigh weight on the flexion gap has not been documented. Therefore, the purpose of this study was to examine the flexion gaps in the 90-90 degree flexed position and the traditional 45-90 degree flexed position of hip-knee joints.

Materials and methods: Thirty patients with osteoarthritic knee underwent total knee arthroplasty. After the PCL sacrifice, soft tissue releases, and bone cuts, the specially designed tenser which has two load cells was employed. 160N was applied to open the joint gaps in the traditional 45-90 degree flexed position and the 90-90 degree flexed position of hip-knee joints.

Results: The flexion gap in the 90-90 degree flexed position of hip-knee joints was 2.1±1.2mm wider than that in the traditional 45-90 degree flexed position of hip-knee joints. The flexion gap had significant difference between the two different hip flexion angles (p< 0.001).

Discussions: In the traditional 45-90 degree flexed position of hip-knee joints on the operation table, the flexion gap is approximately 45 degree to the gravitation and influenced by the thigh weight. To avoid the influence of the thigh weight and obtain equal joint gaps or tensions in flexion and extension, the flexion gap should be checked in the 90-90 degree flexed position of hip-knee joints.

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.