Total knee replacement (TKR) has become the standard procedure in management of degenerative joint disease with its success depending mainly on two factors: three dimensional alignment and soft tissue balancing. The aim of this work was to develop and validate an algorithm to indicate appropriate medial soft tissue release during TKR for varus knees using initial kinematics quantified via navigation techniques.

Kinematic data was collected intra-operatively for 46 patients with primary end-stage osteoarthritis undergoing TKR surgery using a CT-free navigation system. All patients had preoperative varus knees and medial release was made using the surgeon’s experience. From this data an algorithm was developed to define the medial release based on the pre-operative mechanical femoral-tibial angle with valgus stress;

No release (tibial cut only) when valgus stress > −2/3°. Moderate release (medial aspect of tibia +/− semimembranosous tendon) when valgus stress > −5° and < −2°. Extensive release (proximal) when valgus stress < −5°. If there was a fixed flexion deformity > 5° then a posterior release was performed.

This algorithm was validated on a further set of 35 patients where it was used to determine the medial release based only on the kinematic data. The post-operative varus and valgus stress angles for the two groups were compared and showed good outcomes in terms of distribution and outliers.

The results showed that the algorithm was a suitable tool to indicate the type of release required based on intra-operatively measured pre-implant valgus stress and extension deficit angles. It reduced the percentage of releases made and the results were more appropriate than the decisions made by an experienced surgeon.