Balanced Flexion and Extension Gaps Are Not Always of Equal Size

It has been widely accepted in total knee arthroplasty (TKA) that flexion and extension gaps in the disarticulated knee during surgery should be equalized. We hypothesized that tensioning during assessment of the flexion gap can induce temporary widening of the gap due to posterior tibial translation. We aimed to describe posterior tibial translation at flexion gap (90°) assessments and assess the correlation of tibial translation with laxity (flexion space increase) using constrained and non-constrained inserts.

Imageless navigation was used to measure flexion angle, tibial position relative to the femoral axis, and lateral/medial laxity in 30 patients undergoing primary TKA. Trialing was conducted using posteriorly stabilized and cruciate retaining trials of the same size to elucidate the association of posterior tibial translation with changes in joint capsule laxity at 90° knee flexion.

All patients demonstrated posterior tibial translation during flexion gap assessment relative to their subsequent final implantation [mean ± standard deviation (range), 11.3 ± 4.4 (4-21) mm]. Positive linear correlation [r = 0.69, 95% confidence interval (CI) 0.44-0.84, P ≤ .001] was demonstrated between translations [8.7 ± 2.4 (3-13) mm] and laxity changes [2.9° ± 2.0° (-0.7° to 7.4°)] at 90° of flexion.

Posterior tibial translation can cause artifactual widening of the flexion gap during gap balancing in posteriorly stabilized TKA, which can be of sufficient magnitude to alter femoral component size selection for some patients. Recognition and management of these intra-operative dynamics for optimal kinematics could be feasible with the advent of robotic applications.