Knee Society Radiographic Evaluation System Update: Standardizing Assessment of Patellar Translation in Resurfaced and Unresurfaced Patellae.

The Modern Knee Society Radiographic Evaluation System was published in 2015, when routine patella resurfacing in total knee arthroplasty (TKA) was the norm and little attention was given in the United States to either selective patella resurfacing or leaving the patella unresurfaced as a matter of course in all cases. The method advocated for assessing patella translation at that time referenced the midpoint of the native patella relative to the trochlear sulcus. Recognizing the growing preference for leaving the patella unresurfaced in TKA when there is no major lateral patellar facet wear and the tendency to medialize the patella prosthesis when resurfacing it, referencing from the midpoint of the native patella may erroneously suggest that the patella is translated laterally, even when it is not. We offer an update to the methodology for assessing the translational position of the patella, referencing either the median ridge of the native patella when unresurfaced or the midpoint of the patellar prosthesis when resurfaced, rather than the midpoint of the native patella.