Radiological assessment of patellar instability: comparative analysis of patients with and without instability and the impact of patellar stabilisation surgery

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Abstract
Background: Patellar instability (PI) is a clinical diagnosis often complicated by symptom overlap with other knee pathologies and spontaneous relocation of the patella. Anatomical variations are key risk factors, highlighting the role of radiological investigations in assessing underlying pathology. Inconsistent measurement parameters and limited comparative data reduce diagnostic reliability. This study aimed to compare radiological parameters between patients with and without PI, and to assess how these parameters change following patellar stabilisation surgery. Methods: This case-control study compared magnetic resonance imaging (MRI) and X-ray radiological parameters between patients with recurrent PI and controls. Pre- and postoperative imaging was analysed in the PI group. Eleven validated radiological measurements, including patellofemoral joint angles and patellar height indices, were evaluated. Results: Fifty-five knees in the PI group and 50 knees in the Control group were analysed. Preoperatively, significant differences were found for all patellar height indices ( p < 0.001) and patellofemoral measurements ( p < 0.05), except for the congruence angle. Postoperatively, all patellar height measurements improved in the PI group ( p < 0.05), with only the sulcus angle improving ( p = 0.005) for patellofemoral measurements. Significant differences were observed between MRI and X-ray measurements ( p < 0.05). Conclusion: Radiological parameters differ significantly between unstable and stable knees, with improvement following surgery. Patellar height was the most consistent marker of instability. Discrepancies between MRI and X-ray findings suggest a need for modality-specific normative values. These results reinforce the value of radiological assessment in PI and support standardisation to improve diagnostic accuracy.

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