Radiographic Measures of Patellar Tracking After Total Knee Arthroplasty: Comparing the Native Patellar Midpoint versus Median Ridge as Reference Landmarks.

Abstract

Accurate radiographic assessment of patellar tracking in total knee arthroplasty (TKA) is important. A commonly used method references the midpoint of the native patella (MPNP), which assumes central patella implant positioning and overlooks asymmetric patellar morphology in TKAs in which the patella is not resurfaced. Alternative measurement techniques that reference the patellar median ridge (MR) or center of the patellar prosthesis (CPP) may more accurately reflect true patellar position. This study compared MPNP with MR/CPP methods to determine whether landmark selection meaningfully alters the measured magnitude or direction of patellar translation. A retrospective review of 413 primary TKAs, including 213 unresurfaced and 200 resurfaced patellae, was conducted. Using pre- and postoperative Merchant view radiographs, patellar translation relative to the trochlear sulcus was measured using (1) the MPNP method and (2) MR for unresurfaced patellae or CPP for resurfaced patellae. Magnitudes of shift and directional classifications were compared, with subgroup analyses performed by resurfacing status. Across the full cohort, MPNP consistently measured greater lateral translation compared to MR/CPP, both preoperatively (5.01 ± 4.72 versus -0.12 ± 3.01 mm, P < 0.001) and postoperatively (2.92 ± 3.47 versus 0.48 ± 0.50 mm, P < 0.001). Differences were most pronounced in unresurfaced patellae, with mean pre- and postoperative differences between methods of 4.74 and 4.41 mm. All groups saw high reclassification of shift direction from lateral by the MPNP method to neutral or medial by the MR/CPP methods. The choice of reference landmark significantly impacts radiographic assessment of patellar tracking. The MPNP method tends to overestimate lateral translation due to patellar morphological asymmetry and medialized prosthetic positioning. The MR method for unresurfaced patellae and CPP for resurfaced patellae provide more anatomically accurate and consistent assessments and should be considered for standardized reporting of patellar position after TKA.