QUANTIFICATION OF SOFT-TISSUE IMBALANCE IN CONDYLAR KNEE ARTHROPLASTY

Soft-tissue balance has been debated in recent publications in connection with the long-term survival of the 'condylar-type' knee prostheses. Present methods of assessment have all assumed that the soft tissues around the knee are inelastic strings. The authors have developed two instruments to quantify soft-tissue imbalance, at the time of the operation, with the assumption that the soft tissues are viscoelastic structures. These two soft-tissue balancing devices were consequently used on 121 patients undergoing condylar knee arthroplasties at the Derbyshire Royal Infirmary and Bretby Hall Orthopaedic Hospital. The first instrument consisted of two flat plates separated by four standard compression springs and provided a qualitative measure of imbalance assuming that the soft tissues were viscoelastic. It was used on 55 patients before being replaced by the quantitative measure of the second instrument. The authors have redefined soft-tissue imbalance, to take into account the viscoelastic nature of the soft tissues, as the resultant trapezoidal geometry of the knee after the bony cuts have been made and when the knee is tensed by equal forces both medially and laterally. The second balancer eliminates the requirement to quantify the individual tensions in the medial and lateral structures by introducing to the system a low-friction, central pivot in the coronal plane. Once the pivot is situated at the centre of the knee, an equilibrium position is achieved where the clockwise and counter-clockwise moments are equal. The tensions exerted by soft tissues can be assumed to be equal and opposite as their moment arms are the same. Imbalance is quantified by the angular displacement of the top plate of the instrument in finding its equilibrium position. This instrument was subsequently used on 66 knees at the Bretby Hall Orthopaedic Hospital.