Radiological risk factors in patellar instability: a comparative analysis of single-episode, recurrent, and habitual patella dislocation.

Abstract

Background: Patellar instability is a multifactorial condition with varying severity, categorized into single-episode, recurrent, and habitual dislocations. This study aims to assess the association and the strength of association between clinical subtypes of patellofemoral instability (PFI) (single-episode, recurrent, and habitual patellar dislocation) and the frequency, severity, and cumulative presence of three key radiological risk factors: trochlear dysplasia, tibial tubercle-trochlear groove (TT-TG) distance, and the Caton-Deschamps (CD) ratio.

Methods: This was a retrospective observational study conducted from January 2018 to December 2024 on 106 patients reported in the outpatient department (OPD) with various type of patellar instability (124 knees; 39 knee SPD, 73 knee RPD, and 12 knee HPD). Three radiological parameters (trochlear dysplasia, tibial tuberosity-trochlear groove (TT-TG) distance with > 15 mm defined as high), and Caton-Deschamps (CD) ratio (> 1.2 indicating patella alta) were evaluated using magnetic resonance imaging (MRI). Chi-squared test and Cramér's V statistical methods were applied for analyzing the strength of association.

Results: The study identified prevalence of trochlear dysplasia in 63.7%, high TT-TG distance in 32.2%, and patella alta in 45.2% of knees with varying types of patellar instability. Trochlear dysplasia showed the strongest association, particularly with habitual dislocations (100%). Prevalence of high TT-TG progressively increased from single-episode (17.9%) to recurrent (32.9%) to habitual patella dislocation (75%). In contrast to trochlear dysplasia and high TT-TG distance, patella alta was found to have a weaker association in characterizing the patellar instability. Notable, correlation was observed with severity of patellar instability and the number of risk factors, with cases with habitual patella dislocation most frequently showing multiple contributing factors.

Conclusions: Our study found that patients with HPD, the most severe clinical form of patellar instability, had pronounced trochlear dysplasia, higher TT-TG distance, and a greater number of radiological risk factors, followed by RPD and finally SPD. Among the parameters, trochlear dysplasia and TT-TG distance showed stronger association with clinical types, while patella alta had a weaker correlation.