Does Kinematic Alignment Increase Polyethylene Wear Compared With Mechanically Aligned Components? A Wear Simulation Study

Abstract

Background: 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?

Methods: 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.

Results: 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).

Conclusion: In the current wear simulation study, the wear rates of mechanical alignment and kinematic alignment of 4° were in a comparable range.

Clinical relevance: 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.

Fig. 1.

This illustration shows an experimental flow chart of the three alignment groups, test conditions, and analyzed parameters.

Fig. 2.

A-C This illustration shows an implant and represents (A) mechanical alignment, (B) kinematic alignment of 4°, (C) and malalignment of 4°. The mechanical axes of the simulator correspond to the implant’s flexion-extension and internal-external rotation axes for mechanical and kinematic alignment. For the malalignment condition, the machine axes do not correspond to the implant’s flexion-extension and internal-external rotation axes; Flex-ex axis = flexion-extension axis; I-E rotation axis = internal-external rotation axis.

Fig. 3.

A-B These illustrations show (A) the typical femoral setup to simulate mechanical alignment and (B) adapted femoral setup to simulate kinematic alignment of 4°. For the 4° kinematic alignment condition, different bearing sockets with an angle of 4° were used; 1 = jointless connection; 2 = radial needle bearing of 0°; 3 = cardan joint; 4 = machine connection; 5 = swing arm; 6 = angular bearing bushing of 4° with a radial needle bearing; 7 = angular bearing bushing of 4° with a spherical roller bearing.

Fig. 4.

A-C These illustrations show (A) the typical tibial setup to simulate mechanical alignment and (B) adapted tibial setup to simulate kinematic alignment of 4° with (C) the corresponding cross-section; Fz = compression force; 1 = cardan joint; 2 = machine connection; 3 = implant plate; 4 = mold with tibial plateau; 5 = hollow cylinder with bevel of 4°; 6 = pin; 7 = pivoting bearing; 8 = angular contact ball bearings; 9 = linear bearing.

Fig. 5.

A-B These illustrations represent the (A) mechanical alignment condition and (B) malalignment condition using the test setup of the kinematic alignment condition, but with bearings of 0°.

Fig. 6.

This graph shows the wear rates of the mechanical alignment group, the kinematic alignment group with tilt of 4°, and the malalignment group with a tilt of 4°.

Fig. 7.

This illustration shows the worn areas of the three tested polyethylene inserts of the mechanical alignment, kinematic alignment, and malalignment groups.