Femoral Trochlear Length Mismatch between Native and Current Implants: A Magnetic Resonance Imaging Study

Aim: Total knee arthroplasty (TKA) is a successful surgery, yet it is associated with a small but significant dissatisfaction rate.It has been shown that the native femoral trochlear length and its mismatch from the implant are related to anterior knee pain post-TKA.We aim to (a) measure the femoral trochlear length in normal human knees using magnetic resonance imaging (MRI) and (b) compare this with commonly used TKA designs.Materials and methods: We retrospectively reviewed 26 normal human knee 3 Tesla MRI scans obtained using a 3D SPACE sequence to better visualize cartilage and improve the accuracy of measurements.We measured the anterior trochlear length (AT), posterior lateral femoral condyle length (PL), and the posterior medial femoral condyle length (PM).We also calculated the average posterior condyle length (PC) by calculating the average of PL and PM for each knee.We calculated mean, median, standard deviation (SD), and 95% confidence intervals for each of the three measurements.Results: The mean AT was 36.2 mm (SD 3.5), PL was 35.1 mm (SD 3.3), PM was 36.5 mm (SD 3.0), and PC was 35.8 mm (SD 2.9).The mean ratio of AT to PL was 1.03 (SD 0.08), AT to PM was 0.99 (SD 0.07), and AT to PC was 1.01 (SD 0.07).When looking at some commonly used implant designs, the minimum AT was 44.6, and the maximum was 68.57, while the minimum AT:PC was 1.40 and the maximum was 1.89.Conclusion: Current knee implants have considerably longer femoral trochlear flange compared to the native trochlea.The ratio of the normal trochlear length to the posterior condyle is consistent, and adapting implant design to closely match this ratio can reduce anterior lengthwise overstuffing of the trochlea across all implant sizes and potentially reduce dissatisfaction post-TKA.However, further larger studies in a different population are required to confirm our findings.