Measurement of soft tissue imbalance in total knee replacement: system design and development

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 has been based on instrumentation which tensed the medial and lateral soft tissues maximally during the operation. however viscoelastic structures with Soft tissues are non-linear load elongation curves, be used as a force been developed without any distinct reference point to datum point. Surgical instrumentation has to estimate soft tissue imbalance independently of the compressive passive loads through the knee joint. A homeostatic datum point is assumed when constant force is applied to both the medial and lateral soft tissues structures. In order to validate this assumption and redefine the datum point for measurement of soft tissue imbalance, an electronic measuring system was developed to record the soft tissue imbalance at O.25mm distraction intervals of the knee. This soft tissue measuring system consists of a instrument containing electronic transducers, an condi tioning unit and a portable computer. The surgical analogue 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. Ten 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 7.15mm was 0.68' with a standard deviation of 0.8. It is concluded that this is an acceptable error band compared to current methods of measurement and soft tissue imbalance can be measured independently of the passive compressive loads through the knee. The development of the instrumentation within this thesis has resulted in patent applications in both the U.K and the U.S.A.