Abstract
Introduction
Optimal knee joint function obviously requires a delicate balance between the osseous anatomy and the surrounding soft tissues, which is distorted in the case of joint line elevation (JLE). Although several studies have found no correlation between JLE and outcome, others have linked JLE to inferior results. The purpose of this in vitro investigation was to evaluate the effect of JLE on tibiofemoral kinematics and collateral ligament strains.
Materials and Methods
Six cadaver knees were equipped with reflective markers on femur and tibia and CT scans were made. A total knee arthroplasty (TKA) was performed preserving the native joint level. The knees were then tested in passive flexion-extension and squatting in a knee kinematics simulator while marker positions were recorded with an optical system. During squatting quadriceps forces were measured as well as tibio-femoral contact pressures. Finally, a revision TKA was performed with JLE by 4 mm. The femoral component was downsized and a thicker insert was used. The knees were again tested as before.
Based on the bony landmarks identified in the CT scans and the measured trajectories of the markers, relative tibiofemoral kinematics could be calculated as well as distance changes between insertions of the collateral ligaments.
Statistical tests were carried out to detect significant differences in kinematic patterns, ligaments elongation, tibiofemoral contact pressures and quadriceps forces between the primary TKA and after JLE.
Results
Tibiofemoral kinematics are shown in Figure 1. For both passive flexion and squatting, tibial external rotation and adduction were similar before and after JLE.
In passive flexion, JLE decreased the posterior translation of the femoral medial and lateral condyle centres, especially beyond 40 degrees of flexion.
A slight 5% anterior shift of both centres was noted after JLE during squatting, but this was not significant.
Strains in the collateral ligaments are shown in Figure 2. The collateral ligament lengths remained constant during passive flexion and were unaffected by elevation of the joint line. During squatting, the sMCL stretched with flexion after primary TKA and this behaviour stayed constant when the joint line was elevated. The LCL showed a similar loosening trend in both TKA configurations.
Also tibiofemoral joint kinetics were not affected by JLE: quadriceps force and contact pressures all remained essentially unchanged during squatting before and after JLE.
Discussion and conclusion
Although clinical observations have indicated that JLE is associated with inferior clinical results, the effects of JLE on knee biomechanics which might explain these outcomes remain relatively unknown. In this study, we specifically evaluated those effects on tibiofemoral kinematics and kinetics, as well as elongation of the collateral ligaments.
As our current study did not detect any effect of JLE in tibiofemoral kinematics, kinetics, and strains of collateral ligaments in revision TKA, it is possible that these effects may be limited to or triggered at the patello-femoral joint, and more significant with higher joint line elevations than the 4-mm level tested in the current study. This hypothesis needs to be further investigated in future in-vitro and in-vivo studies.
