Measurement of soft tissue imbalance in total knee arthroplasty using electronic instrumentation

The existence of soft tissue contractures in arthritis and the presence of soft tissue imbalance at the time of a total knee arthroplasty causing deformity in the coronal plane has been debated extensively. This discussion was based on the use of instrumentation which tensed the medial and lateral soft tissues in an uncontrolled manner during the operation. Previous work by this research team has developed a surgical instrument to quantify soft tissue imbalance independently of the compressive passive loads through the knee. In order to validate this assumption, an electronic measuring system was developed to record the soft tissue imbalance at 0.25 mm distraction intervals of the knee. This soft-tissue measuring system consists of a surgical instrument containing electronic transducers, an analogue conditioning unit and a portable computer. The surgical instrument introduces a pivot to the centre of the knee in the coronal plane so that the clockwise and counterclockwise moments produced by the collateral soft tissues produce an angular deviation at the equilibrium position. Measurements of angular deviation and separation gap are recorded by the electronic transducers. Eight patients were measured whilst undergoing total knee replacement at Bretby Hall Orthopaedic Hospital. The mean change in angular deviation over an average distraction of the knee of 7.15 mm was 0.4 degrees with a standard deviation of 0.4. It is concluded that this is an acceptable error band for surgical measurement, and soft tissue imbalance can be defined as angular deviation independently of the passive compressive loads through the knee.