Mobile-bearing unicompartmental knee arthroplasty (UKA) was designed so that flexion and extension gap adjustments could achieve isometric function of the ligaments throughout ROM to prevent complications. However, achieving accurate knee balancing using a spacer block technique remains difficult since determination of the thickness of the spacer block is determined according to the feeling of the individual surgeon's hand. The objective of the study was to investigate flexion and extension medial unicompartmental knee gap kinematics in mobile-bearing UKA and to reveal the accuracy of spacer block measurement technique using a gap tensor device.
Mobile-bearing UKA was performed in 40 knees of 31 subjects using generally accepted spacer block technique so that the extension gap was made equal to the flexion gap. The extension and flexion gaps of the medial knee compartment were measured using the tensor device with 25, 50, 75, 100, 125, and 150 N of joint distraction force. The interplay gap was calculated by subtracting the thickness of the tibial prosthesis and the thickness of the selected size of bearing from the measured extension and flexion gaps. Medial compartmental joint interplay gap differences were compared among flexion and extension gaps.
The mean flexion interplay gap was 25 N: 0.5 mm, 50 N: 1.5 mm, 75 N: 2.4 mm, 100 N: 3.1 mm, 125 N: 3.6 mm, 150 N: 4.0 mm. The mean extension interplay gap was 25 N: -0.2 mm, 50 N: 0.81 mm, 75 N: 1.7 mm, 100 N: 2.5 mm, 125 N: 3.1 mm, 150 N: 3.5 mm. The measured extension gap was shown to be significantly smaller compared with the flexion gap at every joint distraction force (P < 0.01).
These results suggest that gap measurement using a spacer block in UKA has the potential risk that the resulting extension gap may be smaller than the flexion gap. Surgeons should adjust the flexion and extension gaps with caution to achieve good ligament function when performing mobile-bearing UKA.