Evaluation of patella height in native knees and arthroplasty: an instructional review

Total knee arthroplasty (TKA) is the gold standard for treating advanced knee osteoarthritis. Among the postoperative complications of TKA are true patella infera (TPI) and pseudo patella infera (PPI), which should be differentiated since TPI exhibits significantly worse clinical outcomes. Multiple radiological patella height indices (PHI) exist; some were modified or originally designed for knees with implanted endoprostheses. However, there is no consensus on measuring and comparing patella height. Due to the lack of established, simple, reliable, and reproducible concepts for assessing patella height for arthroplasty, measuring patella height and the change of patella height by or after TKA have been challenging tasks for clinicians and researchers. This is a review of the current literature on methods for measuring patella height, with special attention to the ability to differentiate between the TPI and PPI after TKA. All literature on the topic was retrieved, and references from relevant articles were investigated until the end of April 2022.