Abstract
Introduction:
Adequate coverage of the resected tibial plateau with the tibial tray is necessary to reduce the theoretical risk of tibial subsidence after primary total knee arthroplasty (TKA). Maximizing tibial coverage is balanced against avoiding excessive overhang of the tray causing soft tissue irritation, and establishing proper tray alignment improving implant longevity and patella function1. Implant design factors, including the number of tray sizes, tray shape, and tray asymmetry influence the ability to cover the tibial plateau2. Furthermore, rotating platform (RP) tray designs decouple restoring proper tibial rotation from maximizing tibial coverage, which may enhance the ability to maximize coverage. The purpose of the current study was to assess the ability of five modern tray designs (Fig. 1), including symmetric, asymmetric, fixed-bearing, and RP designs, to maximize coverage of the tibial plateau across a large patient population.
Methods:
Lower limb computed-tomography scans were collected from 14,791 TKA patients and the tibia was segmented. Virtual surgery was performed with an 8-mm tibial resection (referencing the high side) made perpendicular to the tibial mechanical axis in the frontal plane, with 3° posterior slope, and aligned transversely to the medial third of the tibial tubercle. An automated algorithm placed the largest possible tray on the plateau, optimizing the ML and AP placement (and I-E rotation for the RP tray), to minimize overhang. The largest sized tray that fit the plateau with less than 2-mm of tray overhang was identified for each of the five implant systems. The surface area of the tibial tray was divided by the area of the resected plateau and the percentage of patients with greater than 85% plateau coverage was calculated.
Results:
The percentage of patients with greater than 85% plateau coverage across the tray designs ranged from 17.0% to 61.4% (Fig. 1). The tray with the greatest number of size options (Tray 4, 10 sizes) had the best coverage among the fixed-bearing trays. The RP variant of the same tray had the best overall coverage. Tibial asymmetry did not significantly improve the overall tibial coverage across the patient distribution for both asymmetric designs. Incorporating a broader medial condyle improved fit along the posterior medial corner for Tray 2, but increased the average under-hang along the posterior lateral plateau offsetting any improvement in total coverage.
Discussion:
This analysis represents the most comprehensive assessment of tray coverage to date across a large TKA-patient population. Large variations exist in the size and shape of the proximal tibia among TKA patients3. Developing a tray design which provides robust coverage despite this variation remains challenging. This analysis suggests that tibial asymmetry may not robustly improve coverage. Conversely, incorporating an increased number of tray sizes and utilizing an RP implant to decouple coverage from alignment may provide the most reliable solution for maximizing coverage across the patient population.
