Medial Joint Gap Kinematics in Mobile-Bearing Unicompartmental Knee Arthroplasty

Introduction Accurate soft tissue balancing in knee arthroplasty is essential in order to attain good postoperative clinical results. In mobile-bearing UKA (Oxford Partial Knee unicompartmental knee arthroplasty, Biomet), since determination of the thickness of the spacer block depends on the individual surgeon, it will vary and it will be difficult to attain appropriate knee balancing. The first objective of the present study was to investigate flexion and extension medial unicompartmental knee gap kinematics in conjunction with various joint distraction forces. The second objective of the study was to investigate the accuracy of gap measurement using a spacer block and a tensor device. Methods A total of 40 knees in 31 subjects (5 men and 26 women) with a mean age of 71.5 years underwent Oxford UKA for knee osteoarthritis and idiopathic osteonecrosis of the medial compartment. According to instructions of Phase 3 Oxford UKA, spacer block technique was used to make the extension gap equal to the flexion gap. Adequate thickness of the spacer block was determined so that the surgeon could easily insert and remove it with no stress. Following osteotomy, the tensor devise was used to measure the medial compartmental gap between the femoral trial prosthesis and the tibial osteotomy surface (joint component gap) (Fig. 1 and 2). The medial gap was measured at 20° of knee flexion (extension gap) and 90° of knee flexion (flexion gap) with 25N, 50N, 75N, 100N, 125N, 150N of joint distraction force. Corresponding size of bearing was determined for the prosthesis. 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. Results The selected bearing size was 3 mm: 3 knees, 4 mm: 20 knees, 5 mm: 15 knees and 6 mm: 2 knees. The mean flexion gap in the medial compartment was 25N: 8.4 ± 1.6 mm, 50N: 9.4 ± 1.6 mm, 75N: 10.4 ± 1.5 mm, 100N: 11.0 ± 1.4 mm, 125N: 11.6 ± 1.5 mm, 150N: 11.9 ± 1.4 mm. The mean extension gap was 25N: 7.8 ± 1.6 mm, 50N: 8.8 ± 1.6 mm, 75N: 9.7 ± 1.6 mm, 100N: 10.4 ± 1.5 mm, 125N: 11.1 ± 1.5 mm, 150N: 11.4 ± 1.5 mm. The mean flexion interplay gap was 25N: 0.5 ± 1.2 mm, 50N: 1.5 ± 1.2 mm, 75N: 2.4 ± 1.1 mm, 100N: 3.1 ± 1.0 mm, 125N: 3.6 ± 1.1 mm, 150N: 4.0 ± 1.1 mm. The mean extension interplay gap was 25N: −0.2 ± 1.2 mm, 50N: 0.8 ± 1.1 mm, 75N: 1.7 ± 1.2 mm, 100N: 2.5 ± 1.2 mm, 125N: 3.1 ± 1.2 mm, 150N: 3.5 ± 1.2 mm. When flexion and extension of the interplay gap were compared, the extension interplay gap was shown to be significantly smaller compared with the flexion interplay gap at every joint distraction force (p Conclusion The mean extension interplay gap was shown to be significantly smaller compared with the flexion interplay gap at every joint distraction force even though the extension gap was adjusted to the flexion gap using the spacer block. This suggests that in the actual UKA operative technique using a spacer block there is a potential that the extension gap will be smaller than the flexion gap. Surgeons should be aware of this fact and adjust the flexion and extension gaps with caution when performing mobile-bearing UKA.