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Objectives and questions: Abnormal patellar position, particularly patella alta, has been associated with anterior knee pain, patellofemoral instability and altered cartilage loading. Several indices have been established to quantify patellar height, including the Insall–Salvati Index (ISI), Modified Insall–Salvati Index (mISI), Caton–Deschamps Index (CDI), Blackburne–Peel Index (BPI), and the Patellotrochlear Index (PTI). Although these parameters are widely used, most reference values are derived from conventional radiographs. MRI, however, is routinely applied in the evaluation of the patellofemoral joint and can provide different measurements due to variations in imaging landmarks. Furthermore, currently available norm values are often derived from relatively small sample sizes or pathological samples. The primary objective of this study was to assess patellar height parameters on knee MRI in a large, healthy population cohort and to define norm reference values. Furthermore, the influence of anthropometric parameters on patellar height was examined.

Material and methods: The ISI, mISI, CDI, BPI, and PTI were measured on bilateral knee MRI scans of 928 participants (462 females) from the Study of Health in Pomerania (SHIP), aged 56 ±13 (28–89) years. SHIP drew a representative sample of the adult general population of Northeastern Germany. Participants were on average 171 ± 9 (145–195) cm tall and 79±14 (44–124) kg in weight. All measurements were performed on standardized sagittal MRI sequences using predefined anatomical landmarks. Reference values for each patellar height parameter were assessed using regression models, using the 2.5 and 97.5 percentile. Associations between patellar height indices and sex, age, body height and body weight were analyzed using linear regression models.

Results: Patellar height indices differed between males and females significantly (p = < 0.001–0.002) with a higher patella position for females. Mean values were for female and male as the following: ISI 1.05 ± 0.14 (0.65–1.47); 0.98 ± 0.15 (0.58–1.47), mISI 1.57±0.15 (1.22–2.07); 1.52±0.15 (1.19–2.08), CDI 1.08±0.12 (0.77–1.46); 1.05±0.13 (0.71–1.60) BPI 0.87±0.13 (0.54–1.26); 0.84±0.13 (0.49–1.28) and PTI 0.28±0.09 (0.01–0.65); 0.31±0.09 (0.07–0.70). In males and females, increasing body height was positively associated with higher patella position measured with mISI, BPI, CDI, PTI (p<0.001–0.0049). Age and body weight was significantly associated with a decrease of patellar height in males for all indices (p<0.001–0.032) except ISI. However, no homogenous association between age/body weight and patellar height were found in females.

Discussion and conclusions: Adjusted reference values for patellar height indices measured on MRI scans were provided to enable patient-specific diagnostics. Based on these patient-individualized and gender specificreference values, patellofemoral pathologies could be evaluated and cut-off values re-considered.