Abstract
Introduction
Whether anterior referencing (AR) or posterior referencing (PR) are optimal to position and size the femoral component in Total Knee Arthroplasty (TKA) remains controversial. This controversy stems, in part, from a lack of understanding of whether one technique more consistently balances the medial/lateral collateral ligaments (MCL & LCL) in flexion and extension. Therefore, our goal was to compare AR and PR in terms of: (1) maximum MCL and LCL forces in passive flexion, and (2) medial and lateral gaps at full extension and 90‖ of flexion. In addition, we identified geometric landmarks that could help predict the ligament forces during flexion.
Methods
Computational models of six knees were virtually implanted with TKAs based on our previously-developed framework. AR and PR were simulated in each of the six models. A Posterior Stabilized implant was utilized. Standard AR and PR cuts and component positioning were simulated with the femoral component aligned parallel to the transepicondylar axis. In both AR and PR models, the distal femoral cut and the proximal tibial cut were perpendicular to the femoral and tibial mechanical axis, respectively. The amount of posterior bone resected with AR knees ranged from 4.2 to 10.8 mm, and with PR knees ranged from 4.2 to 8 mm. Ligament properties were standardized to reflect a balanced knee at full extension. Passive flexion under 500 N of compression was applied and the MCL and LCL forces were predicted. A new measure, the MCL ratio, that incorporated the femoral insertion of the anterior fiber of MCL relative to the posterior and distal femoral cuts was estimated (Fig. 1). A varus/valgus moment of 6 Nm was applied at full extension and 90‖ of flexion, and the corresponding lateral and medial gaps were measured.
Results
In passive flexion, the maximum MCL force ranged from 2 to 87 N in AR and from 17 to 127 N in PR (Fig. 2). The LCL forces decreased to zero before 25‖ of flexion in all knees. The MCL ratio corresponded to the MCL force; the larger the MCL ratio, the larger was the maximum MCL force (Fig. 2). At full extension, AR and PR knees were balanced with a maximum difference in medial-lateral gap < 1 mm. However, in flexion, only two out of the six AR and PR knees produced a difference in medial-lateral gap < 2 mm.
Conclusion
Neither AR nor PR consistently produced higher or lower maximum MCL force in flexion despite being well balanced in extension and aligned with the transepicondylar axis. Rather, the more the posterior bone resection, independent of AR or PR, the less was the maximum MCL force in flexion. Knees that produced symmetrical gaps at full extension and 90° of flexion were the ones with the lowest maximum MCL force. Therefore, our findings suggest that less MCL force in flexion promotes a more balanced knee. The MCL ratio corresponded to the variations in maximum MCL force in flexion; it may be used to help produce a more well-balanced knee joint.
For any figures or tables, please contact authors directly.
