MEASUREMENT OF SOFT-TISSUE IMBALANCE IN TOTAL KNEE ARTHROPLASTY USINGELECTRONIC INSTRUMENTATION

The existence of soft tissue contractures in arthritis and the presenc e of soft tissue imbalance at the time of a total knee arthroplasty ca using deformity in the coronal plane has been debated extensively. Thi s discussion was based on the use of instrumentation which tensed the medial and lateral soft tissues in an uncontrolled manner during the o peration. Previous work by this research team has developed a surgical instrument to quantify soft tissue imbalance independently of the com pressive passive loads through the knee. In order to validate this ass umption, an electronic measuring system was developed to record the so ft tissue imbalance at 0.25 mm distraction intervals of the knee. This soft-tissue measuring system consists of a surgical instrument contai ning electronic transducers, an analogue conditioning unit and a porta ble computer. The surgical instrument introduces a Pivot to the centre of the knee in the coronal plane so that the clockwise and counterclo ckwise moments produced by the collateral soft tissues produce an angu lar deviation at the equilibrium position. Measurements of angular dev iation 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 deviat ion 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 acc eptable error band for surgical measurement, and soft tissue imbalance can be defined as angular deviation independently of the passive comp ressive loads through the knee.