Abstract
Introduction
In order to achieve good clinical results in TKA, soft tissue balance is important. Soft tissue balance is closely related to knee kinematics which affects clinical results.
Modified gap balancing technique is one of the standard techniques for posterior stabilized (PS) TKA. On the other hand, appropriate load for the measurement of gap balance has not been established.
The purpose of the present study is to measure the mechanical properties of soft tissue structure of knee sleeve in flexion and extension during PS TKA using newly developed balancer. The understanding of the mechanical properties is crucial. In particular if these properties are used as input for surgical procedures, standard technique for many surgeons will be established.
Materials and Methods
Medial compartmental osteoarthrosis (OA) patients (13 female and 7 male) were evaluated. Average age, BMI, and Varus deformity were 72.1 years, 26.9, and 12 degrees, respectively.
The newly developed center paddle balancer consists of a built-in spring (Fig. 1). Figure 2 shows the sequence of surgery and measurements. In the surgery, we measured the balance (degrees in Figure 1, A) and distance (mm in Figure 1, B) in extension with a load (Figure 1,C) at transition zone of toe region to linear region. Then, applying the load until flexion gap was the same as that in extension with a patella reduction, we measured the femoral component rotation from the balancer (degrees in Figure 1, A). The anterior and posterior femoral cuts were performed according to measured femoral component rotation which angle is parallel to tibial cut surface.
Results
Load deformation curves of a knee sleeve structures showed toe and linear regions. The average stability range (transition zone of toe region to linear region) is 150 to 160N in extension and 130 to 140N in flexion. The distance of stability range between tibia and femur in extension is almost the same as the thickness of tibial component and femoral component (21mm). The distance of stability range between the tibia and femur in flexion is the same as the thickness of tibial component (10mm).
Discussion
In the present study, load deformation curves of knee sleeve structures showed bimodal patterns that is the same as ligaments and tendons. It has been reported that a load on ligament is below the transition zone during 80% of normal daily activity. The results indicated that the so called “palpable endpoint” is stability range. According to the present data, we propose a standard modified gap balance technique in PS TKA for medial compartmental OA. The ligament balance is confirmed in extension with 160N of distracting force after soft tissue release and distal femur and proximal tibial cut. The femoral component rotation is then decided with the load that will open the distance to the thickness of the tibial component in flexion.
