Summary

Gap planning at 10 degrees of flexion produced equal and symmetric gaps from 10-90 degrees that were similar in patterns to those reported in the native knee while planning at 0 degrees resulted in larger gaps and lower joint forces at full extension, but increased knee laxity in mid-flexion which may contribute to mid-flexion instability.

Abstract

Introduction

The aim of gap-balancing in total knee arthroplasty (TKA) is to produce equal and symmetric gaps throughout the range of motion (ROM). The technique references the native flexion and extension gaps to plan the femoral bone resections to achieve a balanced knee. However, the native tibiofemoral gaps are significantly different between 0o and 10o of flexion [1,2]. Therefore, planning for equal and symmetric gaps at 0o and 90o of flexion could result in a different femoral plan, and thus a different knee laxity profile throughout flexion, including in mid-flexion, than when planning at 10o and 90o of flexion. This study therefore aims to quantify the change in the post-operative tibiofemoral gap throughout the ROM when varying the planning extension angle between 0 and 10 degrees flexion.

Methods

40 patients (mean age: 71±10, BMI: 28.6±7.7) undergoing robotic-assisted TKA were included. After resecting the tibia, the knee joint was tensioned using a computer-controlled ligament tensioning tool (Fig. 1). The system applied a load ranging between 80-100N of tension equally to the medial and lateral compartments as the limb was manually taken through a ROM. The femoral implant position and size was then planned to have equal and symmetric knee gaps in extension and flexion. Patients were divided into two sequential groups: Group-A, the knee was planned to have equal and symmetric gaps at 0o and 90o (18 knees), Group-B the knee was planned to have equal and symmetric gaps at 10o and 90o (14 knees). The femur was resected, and a femoral trial was inserted and the postoperative gaps were measured throughout the ROM while the tensioning tool applied equal tension to the ligaments. Mean and standard deviation of the post-operative gaps were calculated for each group. T-tests were used to identify significant differences between the two groups.

Results

In both group-A and group-B, the post-operative extension and flexion gaps were balanced to within 1mm of each other on average (Fig. 2). Significantly larger gaps were seen in mid-flexion for o group-A than for group-B however, with a maximum laxity increase of 3-4mm occurring around 25-30 in group A. The gap profiles between 20-60o were significantly different from the gaps at the extension and flexion planning angles in group-A, but not in group-B.

Discussion

Gap planning at 10o of flexion produced equal and symmetric gaps from 10-90o that were similar in patterns to those reported in the native knee [1]. Gap planning at 10o resulted in smaller gaps and increased tension at full extension, however, which may result in a flexion contracture requiring a posterior capsule release or distal femoral recut to achieve full extension. Planning at 0o resulted in larger gaps and lower joint forces at full extension, but increased knee laxity in mid-flexion which may contribute to mid-flexion instability. Depending on the clinical circumstances of the case, the implications of planning at both 0o and 10o in gap balancing TKA should be taken into consideration.