Patellofemoral positioning CT protocol has diagnostic ability to differentiate patellar maltracking phenotype

Traditional radiographs often fail to capture the dynamic nature of patellar maltracking in patellofemoral pain syndrome (PFPS) and patellar instability, necessitating improved diagnostic protocols. This study aimed to: (1) introduce a CT protocol with scans at three knee positions (45° flexion, extension, and extension with quadriceps contraction), (2) assess how positioning influences patellofemoral indices measured from radiographs and CT, and (3) to evaluate the protocol's ability to classify maltracking phenotypes: dislocator, subluxator, or symptomatic without dislocation/subluxation (Neither).

Patients who underwent surgery for PFPS from April to December 2022 were retrospectively reviewed. Patellofemoral indices from the three scans within the CT protocol were compared among themselves and with standard radiographs. Patients were grouped by maltracking phenotype, and their patellofemoral indices on radiographs and CT were compared to determine which imaging modality best distinguished the phenotypes. Statistical analyses included bivariate and multivariate logistic regression.

The study included 65 patients (51 females, 14 males) with mean age of 27. Patellofemoral indices measured on CT-45° versus CT-Extended differed significantly (p < 0.05), indicated the influence of knee position. Quadriceps contraction further worsened most indices, highlighting the importance of load-bearing conditions. Radiographs and CT-45° had limited capability to differentiate Dislocator, Subluxator, and Neither, but CT-Extended and CT-Quad showed significant differences among these groups. Multivariate analysis identified four independent predictors of patellar maltracking severity (p < 0.05): (1) Lateral Offset and (2) Insall-Salvati Ratio measured on CT-Extended, as changes in (3) Lateral Offset and (4) Lateral patellofemoral angle (LPFA) between extension and quadriceps contraction.

Radiographs alone cannot reliably distinguish Dislocator, Subluxator, and Neither. A dedicated CT protocol featuring scans in neutral extension and with quadriceps contraction better delineates patellofemoral maltracking phenotypes and offers improved diagnostic accuracy in PFPS. This approach may guide tailored interventions to address distinct underlying mechanics of each phenotype.

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