Patella Alta on X-Ray and MRI: Diagnostic Agreement and Clinical Correlations

Background/Objectives: Patella alta contributes to patellofemoral (PF) maltracking and anterior knee pain, but the agreement of patellar-height indices across imaging modalities and their clinical relevance remain uncertain. We aimed to estimate the prevalence of patella alta when using each index, quantify inter-method agreement, and quantify patella alta’s associations with Hoffa’s fat-pad impingement, patellar chondromalacia, and tibial tubercle–trochlear groove (TT–TG) distance. Methods: This retrospective cross-sectional study included 250 patients aged 18–38 years who underwent both knee radiography and MRI. Patellar height was assessed using the Insall–Salvati (IS) and modified Insall–Salvati (mIS) ratios on radiographs and MRI, and the Patellotrochlear Index (PTI) on MRI. Associations with fat-pad impingement, chondromalacia grade, and TT–TG abnormality were analyzed using the chi-square or Fisher’s exact test and as odds ratios (ORs) adjusted for age, sex, and imaged side; agreement was assessed with Cohen’s kappa (κ). Results: Patella alta prevalence was 13.2% using IS on radiographs and 10.8% using IS on MRI, but markedly lower with mIS (≤1.6%). Patella alta was more frequent in patients with Hoffa’s fat-pad impingement (31.1% vs. 5.7% on IS radiography; adjusted OR 8.83, 95% CI 3.82–20.45) and advanced chondromalacia (p = 0.025). Agreement was moderate between IS radiography and IS MRI (κ = 0.47, 95% CI 0.33–0.61) and between IS MRI and the PTI (κ = 0.43). Conclusions: The prevalence and clinical relevance of patella alta depend strongly on the measurement method. The IS ratio was the most consistent index, but radiography–MRI agreement was moderate rather than high, so the two are not interchangeable. The PTI adds information on trochlear engagement from the same MRI examination. These cross-sectional associations do not establish causation.