Trajectory of bearing movement during Oxford mobile-bearing unicompartmental knee arthroplasty using a kinematic alignment technique.

To the Editor: Dislocation of the mobile bearing is the most common reason for revisions in Asian patients who undergo Oxford mobile-bearing unicompartmental knee arthroplasty (Oxford MB UKA), with an incidence rate as high as 7.9%.[1] Multiple studies have shown that the bearing dislocation rate is significantly higher in Asian than Western patients.[2] A poor bearing movement trajectory was considered one of the risk factors. Malpositioning of the component results in a wider space between the margin of the bearing and the lateral wall of the tibial component, leading to a poor bearing movement trajectory and even bearing rotation. The superior surface is designed with high surrounding rims (higher in the anterior and posterior parts, lower in the medial and lateral parts). When rotation occurs, the lower rim loses its restriction to mutual movement of the bearing and femoral component, which leads to dislocation.[3] A recent study showed that the widely used microplasty (MP) surgical instruments could reduce the separation between the bearing and the lateral wall. However, an ideal trajectory is still not achieved in half of the cases.[4] In 2020, we introduced a kinematic alignment technique and confirmed that the prosthesis position using this method was similar to that using traditional MP devices.[5] The present study was performed to determine whether this technique can improve the bearing movement trajectory. This study was performed in line with the principles of the Declaration of Helsinki. This study was approved by the Institutional Ethics Board of China-Japan Friendship Hospital (No. 2020-50-k28). Informed consent was obtained from all individual participants included in the study. We analyzed patients who underwent Oxford MB UKA from January to June 2019. In total, 23 patients with 30 knees treated by UKA using the kinematic alignment technique were enrolled in the study group according to the surgical records. Another 25 patients with 30 knees treated by UKA using MP instrumentation during the same period were matched as controls. The surgeries were performed using the kinematic alignment technique introduced by Zhang et al[5] in 2020. After precise resection of the tibial plateau, a feeler gauge was inserted into the flexion joint gap to confirm that the ligament tension was restored and that the relationship between the tibial plateau surface and lower limb anatomical axis was normal. Starting from the midpoint of the gauge, a vertical line was drawn on the surface of the femoral condyle with an electronic scalpel, which represented the midline of the femoral condyle. The knee joint was then fully extended, and an 8-mm gauge was inserted into the joint gap to restore the natural tension of the medial collateral ligament and correct the varus deformity. If the 8-mm gauge was still too loose, a thicker one was inserted. After the appropriate gauge was inserted, another reference line (line A) was drawn on the front surface of the femur perpendicular to the midpoint of the insertion. This line is an extremely important reference for alignment in knee extension; therefore, it was checked repeatedly, as shown in Figure 1A. The knee was flexed back to 90°, and the femoral drill guide was placed parallel to line A and located on the central line of the medial condyle. After the position of the guide was checked and fixed, 4- and 6-mm holes were drilled through the guide. A posterior condyle resection guide was inserted to remove the posterior condyle. The rest of the procedure was performed in accordance with the instructions for Oxford UKA.[6] After balancing the flexion and extension gap, we used the method introduced by Kawaguchi et al[4] to measure the movement trajectory of the mobile bearing. We created a set of tibial prosthesis trials with a mesh scale, which was printed vertically and horizontally at 2-mm intervals, and the first, third, fifth, seventh, and ninth lateral cells were marked in Arabic numbers to help identify the position of the bearing. With the knee in full extension and 90° of flexion, we observed the bearing position by marking the A point of the anterior midpoint and B point of the front lateral corner of the bearing trial. Therefore, the distance of the bearing movement during the knee joint flexion could be calculated, as shown in Figure 1D. The bearing moving patterns were similar to those in the report by Kawaguchi et al.[4] The most common bearing movement patterns were as Figure 1F. Anteroposterior lower limb radiographs were obtained both pre-operatively and post-operatively. We measured the contiguity of the prosthesis [Figure 1E] to analyze the distance between the mobile bearing and lateral wall of tibial prosthesis.Figure 1: (A and B) The positioning of the femoral drill guide depended on the central line of the medial condyle In flexion and line A in extension, both of which should be started from the midpoint of the feeler gauge and perpendicular to the tibial resection surface. (C) The figure included the tibial trial that we created to record the bearing movement. The grids were printed vertically and horizontally with an interval of 2 mm. The first, third, fifth, seventh, and ninth small grids on the anterolateral side were marked in Arabic numbers to help identify the position of the bearing. And the figure was taken at knee flexion of 90°. The position of the bearing was recorded according to the position of points A and B. (D) Contiguity between the lateral margin of the femoral component and lateral wall of the tibial component. (E) Bearing movement trajectory patterns. Type (a) The distance between the bearing and the lateral wall of the tibial trial was constant and within 2 mm during knee extension. Type (b) During extension, the bearing moved anteriorly and gradually away from the lateral wall, and the distance between the two was >2 mm when the knee was fully extended. Type (c) The bearing moved anteriorly, starting far away from the lateral wall and moving toward the wall as the knee extended, but only two such cases were observed in the control group. Type (d) The bearing moved anteriorly without contacting the lateral wall, and the distance remained >2 mm.The contiguity between the femoral and tibial prosthesis in the study group was 4.8 ± 1.6 mm, whereas that in the control group was 6.3 ± 1.8 mm; the difference was statistically significant (P < 0.05). The difference in the intraoperative bearing movement trajectory was significant, with an ideal trajectory in 23 of 30 cases in the study group and 14 of 30 cases in the control group (P < 0.05). The distance between point A and the lateral wall of the tibial prosthesis was 0.8 ± 0.6 mm in the study group and 3.7 ± 2.3 mm in the control group (P < 0.05). The bearing movement distance was 8.5 ± 3.0 mm in the study group and 8.1 ± 3.2 mm in the control group, with no significant difference (P > 0.05). The most important finding of this study is that a higher ratio of ideal moving trajectories was observed in the study group with less contiguity of the implants on the anteroposterior knee extension X-ray. Therefore, we considered that more optimal bearing movement can be achieved with this kinematic alignment technique. The recommended movement trajectory of the mobile bearing was 2 mm away from the lateral wall, without contact, and not substantially separated from the wall. However, multiple studies have shown that most movement patterns are characterized as follows: during the whole motion of knee extension, the bearing is parallel to the lateral wall from 90° to 60° but is more likely to become separated from the lateral wall from 60° to 0°.[4] Other studies have shown that the bearing rotation is easier when the contiguity between the margin of the bearing and the wall increases, which might be one of the causes of bearing dislocation.[7] Therefore, close attention has been given to methods of improving the bearing movement trajectory. Kawaguchi et al[4] reported that the ratio of the optimal bearing movement was 44.8%, which is similar to the finding in our control group. The kinematic alignment technique was modified from the conventional surgical technique using MP instrumentation.[6] Our past research showed that UKA performed using this technique can be as reliable and accurate as the conventional technique in terms of the prosthesis position but that it has a shorter operating time, less blood loss, and more rapid recovery without intramedullary interruption.[5] Moreover, the results of the present study showed that this method can improve the bearing movement trajectory, which is another advantage. Our radiographic assessment showed less contiguity of the femoral and tibial components in the study group than in the control group, which also confirmed that the reference line A could reduce the separation of the components when the knee was extended. Therefore, the potential rotation and dislocation of the bearing could be reduced. This technique is different from the calipered kinematic alignment technique described by Rivière et al,[8] in which the tibial cut plane is determined according to the flexion and extension axis of the medial femoral condyle. Our method is indeed a hybrid technique of both measurement resection of the tibia and kinematic alignment resection of the femur, and the goal of this technique is to increase the compliance of the position of the femoral prosthesis with the tibial resection surface. Therefore, this technique mainly depends on an accurate tibial cut, which is also very important to the bearing trajectory. This study demonstrated that the kinematic alignment technique in Oxford MB UKA provides more optimal bearing movement trajectories and smaller contiguities between the femoral and tibial prosthesis than that obtained with conventional MP instrumentation. Funding This work was supported by the grants from the Capital Health Research and Development of Special (Nos. 2020-24067), the National Natural Science Foundation of China (No. 81972130 and 82072494), and Elite Medical Professionals project of China-Japan Friendship Hospital (No. ZRJY2021-GG08). Conflicts of interest None.