Abstract
Goals of the study
(1) to investigate the relationships between the bony contours of the knee and the Popliteus Tendon (PT) in the healthy knee and after implantation of a TKA and (2) to analyze the influence of implant sizing.
Hypothesis
With an apparently well-sized TKA, the position of the PT during knee flexion is modified compared with the preoperative situation.
Method
In 4 fresh frozen cadavers we injected the PT with Barium-Sulfate and a CT-scan was performed from 0° to 140°. We implanted copies of TKAs’ obtained from the manufacturer, made with a non radio-opaque polymer (Acrylonitrile butadiene styrene) with additive manufacturing technology. Each cadaver received either a normosized (cortical fit), oversized (3mm overhang), undersized (3mm under-coverage) or mobile bearing (normosized) prosthesis. The limb was CT-scanned again. 3D-reconstructions were created using Mimics software (Fig 1). The pre-post operative position of the PT was analyzed with Matlab software. We quantified the postoperative posterior deviation of the tendon (PDT).
Results
In the normal knee the PT overlaps the posterolateral corner of the tibial plateau, between 0° and 100° of flexion with a maximum overlapping distance of 5.5mm (Fig2).
After implantation of a normosized TKA, the PT was displaced posteriorly from full extension to 100° of flexion (Fig 3). Mean PDT was 6.2mm (range 0 to 13; SD=1.2) in extension and 4.8mm (range −1 to 9.8; SD=1.1) at 20° of knee flexion.
After implantation of an oversized TKA, PDT was significantly greater than with a normosized TKA, at each angle of flexion: mean PDT was 16.7mm (range 4.4 to 23; SD=0.6) knee in extension (p<0.0001) and 10mm (range 4.4 to 15.7; SD=1.1) at 20° (p<0.0001). The deviation of the PT decreased during knee flexion but remained significant up to full flexion.
When an undersized plateau was implanted, the PDT was significantly decreased compared with a normosized implant and the deviation was non significant compared with the preoperative knee. Mean DPT in extension was −0.8mm (range; −3.1 to 1.8; SD=0.3) (p<0.001). This absence of deviation of the PT with an undersized implant was confirmed during the full range of flexion.
With a mobile-bearing implant, the deviation of the PT was also decreased compared with the normosized TKA. The mean PDT in extension was −0.6mm (range; −4.6 to 5.9; SD=2.6) (p<0.001).
Conclusions
This work demonstrates that the optimal sizing in TKA is very challenging due to the non-anatomic design of current implants. The main finding is that surgeons must analyze sizing in term of volume rather than in term of surface. In other words, most apparently «normosized» TKA, in term of surface coverage are in fact oversized in term of prosthetic volume.
It may be advantageous to aim atundersizing tibial implants and to preserve an uncovered area in the posterolateral corner of the resected tibia.
