Design and evaluation of a 3D printed mechanical balancer for soft tissue balancing in total knee replacement

Soft tissue balancing is an important step in a total knee procedure, carried out manually, or using an indicator. The purpose of this study was to evaluate our design of 3D printed Balancer, and demonstrate how it could be used at surgery.

When inserted between the femur and tibia, the Balancer displayed the forces acting across the lateral and medial compartments, indicated by pointers at the end of the handle. A loading rig was used to measure the pointer deflections for different forces applied at different locations on the condyle surfaces. Repeatability and reproducibilty were evaluated. The Balancer was tested in six fresh knee specimens using a surgical simulation rig.

Pointer deflections of up to 12 millimeters occurred for less than 1 mm displacements at the condyle surfaces. Reproducibility tests showed a standard deviation of 14% at lower loads, reducing to only 4% at higher loads. Mean pointer deflections were within 8% for forces applied at ±10 mm AP, and +5/-3 mm in an ML direction, relative to the neutral contact point. In specimens, most lateral to medial force differences could be corrected by a 2° change in frontal plane angle of the tibial resection. Effects of ligament releases were also demonstrated.

The 3D printed Balancer was easy to use, and provided the surgeon with lateral and medial force data over a full range of flexion, enabling possible corrective procedures to be specified.