Abstract
Aim: To introduce a new concept of Envelope of Laxity (EoL) in knee arthroplasty surgery for balancing a total knee replacement (TKR).
Methods: Twenty consecutive patients with varus knees undergoing TKR were included in the study. All operations were performed by the senior author using the Stryker Navigation system and the Scorpio cruciate retaining (CR) TKR. After registration with the navigation system initial dynamic varus/valgus curves were recorded from 0–120° flexion to give an EoL of the native knee. Repeated measurements were taken after trial components were initially inserted, then after any soft tissue releases and finally after insertion of actual tibial and femoral components. All measurements were taken with the patellar in situ.
Results: The average deformity in the varus group initially was 6.9° varus at 0°, 8.9° varus at 30°, 6.9° varus at 60° and 5° varus at 90° of knee flexion. Postoperatively values were found to be 0.1°, 0°,0.3°and 0.7°respectively. The initial EoL curves showed a mean increase in laxity of 4° between 30° and 60°compared to 0°–30° and 60°–90° through the range of knee flexion. This was seen less in the outcome curves which tended to show more uniform laxity with only an average of 2° difference throughout flexion.
Conclusions: Traditional balancing devices used in TKR surgery balance knees at 0° and 90°, often with the patellar everted which produces errors. The use of EoL curves allows knees to be balanced throughout the arc of movement from 0–150° with the patellar in situ. This study demonstrates the successful use of the EoL concept and that even when knees are balanced at 0° and 90° they may not be balanced at the mid flexion position where clinical problems often arise. This problem becomes worse with the use of poly radii TKR designs.
Correspondence should be addressed to Mr T Wilton, c/o BOA, BASK at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE, England.
