Abstract
Introduction
Although gap balancing technique has been reported to be beneficial for the intra-operative soft tissue balancing in posterior-stabilized (PS)-TKA, excessive release of medial structures for achieving perfect ligament balance would be more likely to result in medial instability, which would deteriorate post-operative clinical results. We have modified conventional gap balancing technique and devised a new surgical concept; named as “medial gap technique” aiming at medial stability with permitting lateral looseness, as physiologically observed in normal knee.
Objective
We compared intra-operative soft tissue balance between medial gap technique (MGT) and measured resection technique (MRT) in PS-TKAs.
Materials and Methods
The subjects were 210 female patients with varus type osteoarthritic knees, underwent primary PS TKA. The surgical techniques were MGT in 96 patients and MRT in 114 patients. The extension gap was made in the same manners in both groups with medial releases limited until the spacer block could be easily inserted. The residual lateral laxity was permitted. In the MGT group, before posterior femoral osteotomies, varus angles (°) and center gaps (mm) at extension and flexion were measured using an offset type tensor with applying 40 lbs. (177.9N) of joint distraction force. The level and external rotation angle of posterior femoral osteotomies were determined based on the difference of center gaps and varus angles between extension and flexion respectively. Intra-operative joint gap kinematics was measured with femoral trial in place and patello-femoral joint reduced. We measured varus angle and component gap at 8 different knee flexion angles from 0° to 135°. From these component gaps and varus angles, we calculated a medial and lateral compartment gaps (MCG and LCG) by using a trigonometric function. Also we calculated the increase of both compartment gaps from those at full extension, named as joint gap loosening (mm). Both compartment gaps and joint gap loosening were compared between 2 groups using unpaired t-test, and the difference between MCG and LCG in each group were compared using paired t- test (p<0.05).
Results
The mean MCGs showed significantly smaller value than LCGs at all flexion angles in both groups (Fig.1). Both medial and lateral joint gap loosening were significantly smaller in MGT group than MRT group from mid-flexion to deep flexion (Fig. 2, 3).
Discussion
We have reported the joint distraction force affected varus imbalance due to the stiffness difference between medial and lateral structures. This might be a reason why gap technique was performed less quantitatively and with higher risk of medial instability. In MGT, we allowed persistent lateral looseness and applied the difference in varus angle between extension and flexion to the external rotation angle of femoral component. Results showed no medial looseness were observed in MGT like in MRT. The less joint gap loosening with knee flexion were achieved by MGT because the advantage of conventional gap balancing was also incorporated. We found “medial gap technique” was effective for quantitative soft tissue balancing with more stable joint gap kinematics and no medial looseness.
