Laboratory of Biomechanics and Implant Research, Department of Orthopaedics, Heidelberg University Hospital, Heidelberg, Germany.
Clin Orthop Relat Res. 2022 Sep 1;480(9):1790-1800. doi: 10.1097/CORR.0000000000002245. Epub 2022 May 17.
Kinematic alignment is an alternative approach to mechanical alignment. Kinematic alignment can restore the joint line to its prearthritic condition, and its advocates have suggested it may be associated with other benefits. But this alignment approach often results in tibial components that are placed in varus and femoral components that are placed in valgus alignment, which may result in an increased risk of component loosening because of wear. Like malaligned implant components, kinematically aligned knee implants could increase wear in vivo, but we lack comparative data about wear behavior between these approaches.
QUESTIONS/PURPOSES: (1) Do the different alignment approaches (kinematic, mechanical, and purposefully malaligned components) result in different wear rates in a wear simulator? (2) Do the different alignment approaches lead to different worn areas on the polyethylene inserts in a wear simulator? (3) Do the different alignment approaches result in different joint kinematics in a wear simulator?
Mechanical alignment was simulated in a force-controlled manner with a virtual ligament structure according to the International Organization for Standardization (ISO 14243-1) using a knee wear simulator. To simulate kinematic alignment, flexion-extension motion, internal-external torque, and the joint line were tilted by 4°, using a novel mechanical setup, without changing the force axis. The setup includes bearings with inclinations of 4° so that the joint axis of 4° is determined. To verify the angle of 4°, a digital spirit level was used. To simulate malalignment, we tilted the implant and, therefore, the joint axis by 4° using a wedge with an angle of 4° without tilting the torque axes of the simulator. This leads to a purposefully malaligned tibial varus and femoral valgus of 4°. For each condition, three cruciate-retaining knee implants were tested for 3.0 x 10 6 cycles, and one additional implant was used as soak control. Gravimetric wear analyses were performed every 0.5 x 10 6 cycles to determine the linear wear rate of each group by linear regression. The wear area was measured after 3.0 x 10 6 cycles by outlining the worn areas on the polyethylene inserts, then photographing the inserts and determining the worn areas using imaging software. The joint kinematics (AP translation and internal-external rotation) were recorded by the knee simulator software and analyzed during each of the six simulation intervals.
Comparing the wear rates of the different groups, no difference could be found between the mechanical alignment and the kinematic alignment (3.8 ± 0.5 mg/million cycles versus 4.1 ± 0.2 mg/million cycles; p > 0.99). However, there was a lower wear rate in the malaligned group (2.7 ± 0.2 mg/million cycles) than in the other two groups (p < 0.01). When comparing the total wear areas of the polyethylene inserts among the three different alignment groups, the lowest worn area could be found for the malaligned group (716 ± 19 mm 2 ; p ≤ 0.003), but there was no difference between kinematic alignment and mechanical alignment (823 ± 19 mm 2 versus 825 ± 26 mm 2 ; p > 0.99). Comparing the AP translation, no difference was found between the mechanical alignment, the kinematic alignment, and the malalignment group (6.6 ± 0.1 mm versus 6.9 ± 0.2 mm versus 6.8 ± 0.3 mm; p = 0.06). In addition, the internal-external rotation between mechanical alignment, kinematic alignment, and malalignment also revealed no difference (9.9° ± 0.4° versus 10.2° ± 0.1° versus 10.1° ± 0.6°; p = 0.44).
In the current wear simulation study, the wear rates of mechanical alignment and kinematic alignment of 4° were in a comparable range.
The results suggest that kinematic alignment with up to 4° of component inclination may give the surgeon confidence that the reconstruction will have good wear-related performance when using a modern cruciate-retaining implant. The malaligned group had the lowest wear rate, which may be a function of the smaller worn area on the inserts compared with the other two alignment groups. This smaller articulation area between the femoral condyles and polyethylene insert could increase the risk of delamination of malaligned components over longer test durations and during high-load activities. For that reason, and because malalignment can cause nonwear-related revisions, malalignment should be avoided. Further in vitro and clinical studies must prove whether the wear simulation of different alignments can predict the wear behavior in vivo.
运动学对线是一种替代机械对线的方法。运动学对线可以将关节线恢复到关节炎前的状态，其支持者认为它可能与其他益处相关。但是，这种对线方法通常会导致胫骨组件放置在内翻位置，股骨组件放置在外翻位置，这可能会增加由于磨损导致的组件松动的风险。与对线不良的植入物组件一样，运动学对线的膝关节植入物可能会增加体内的磨损，但我们缺乏关于这些方法之间的磨损行为的比较数据。
问题/目的：（1）在磨损模拟器中，不同的对线方法（运动学、机械和故意对线不良的组件）是否会导致不同的磨损率？（2）在磨损模拟器中，不同的对线方法是否会导致聚乙烯插入物上不同的磨损区域？（3）在磨损模拟器中，不同的对线方法是否会导致不同的关节运动学？
根据国际标准化组织（ISO 14243-1）的规定，采用力控制方式模拟机械对线，使用膝关节磨损模拟器模拟虚拟韧带结构。为了模拟运动学对线，通过新颖的机械设置将屈伸运动、内外扭矩和关节线倾斜 4°，而不改变力轴。该设置包括倾斜 4°的轴承，以确定 4°的关节轴。为了验证 4°的角度，使用数字水准仪。为了模拟对线不良，我们使用楔形物将植入物和关节轴倾斜 4°，而不倾斜模拟器的扭矩轴，从而导致故意的胫骨内翻和股骨外翻 4°。对于每种情况，三个交叉韧带保留膝关节植入物进行了 3.0 x 10 6 次循环测试，另外一个植入物用作浸泡对照。每 0.5 x 10 6 次循环进行一次重量磨损分析，通过线性回归确定每组的线性磨损率。在 3.0 x 10 6 次循环后，通过勾勒聚乙烯插入物上的磨损区域，然后拍摄插入物并使用成像软件确定磨损区域，来测量磨损区域。关节运动学（AP 平移和内外旋转）由膝关节模拟器软件记录，并在每个六个模拟间隔期间进行分析。
比较不同组的磨损率，机械对线和运动学对线之间没有差异（3.8 ± 0.5 mg/million cycles 与 4.1 ± 0.2 mg/million cycles；p > 0.99）。然而，对线不良组的磨损率较低（2.7 ± 0.2 mg/million cycles），明显低于其他两组（p < 0.01）。当比较三个不同对线组的聚乙烯插入物的总磨损区域时，对线不良组的磨损区域最小（716 ± 19 mm 2 ；p ≤ 0.003），但机械对线和运动学对线之间没有差异（823 ± 19 mm 2 与 825 ± 26 mm 2 ；p > 0.99）。比较 AP 平移，机械对线、运动学对线和对线不良组之间没有差异（6.6 ± 0.1 mm 与 6.9 ± 0.2 mm 与 6.8 ± 0.3 mm；p = 0.06）。此外，机械对线、运动学对线和对线不良组之间的内外旋转也没有差异（9.9° ± 0.4° 与 10.2° ± 0.1° 与 10.1° ± 0.6°；p = 0.44）。
在当前的磨损模拟研究中，机械对线和 4°的运动学对线的磨损率处于可比范围内。
结果表明，在使用现代交叉韧带保留植入物时，对线倾斜 4°的运动学对线可能会让外科医生有信心，重建将具有良好的与磨损相关的性能。对线不良组的磨损率最低，这可能是由于与其他两个对线组相比，插入物上的磨损区域较小所致。这种股骨髁和聚乙烯插入物之间较小的关节面积可能会增加对线不良组件在更长测试时间和高负荷活动期间分层的风险。出于这个原因，并且因为对线不良会导致非磨损相关的修订，所以应该避免对线不良。还需要进行更多的体外和临床研究，以证明不同对线的磨损模拟是否可以预测体内的磨损行为。
