Radiological risk factors in patellar instability: a comparative analysis of single-episode, recurrent, and habitual patella dislocation
Abstract
A retrospective analysis of 124 knees stratifying radiological risk factors across severity phenotypes. Data confirms that Habitual Patellar Dislocation (HPD) is a distinct, severe entity driven by ubiquitous trochlear dysplasia and excessive TT-TG distance.
Profiling the Habitual Dislocator
Key Finding
"100% of habitual dislocators exhibited high-grade trochlear dysplasia (p = 0.003) and 75% had a high TT-TG distance (> 15mm), compared to only 17.9% in single-episode cases (p = 0.001)."
Background: Patellar instability ranges from single traumatic events to habitual dislocation (occurring with every flexion). While risk factors like Trochlear Dysplasia and Patella Alta are known, their specific correlation with the clinical severity (Single vs. Recurrent vs. Habitual) remained undefined, leading to one-size-fits-all surgical planning.
Methods: A retrospective study of 106 patients (124 knees) categorized into Single-Episode (SPD, n = 39), Recurrent (RPD, n = 73), and Habitual (HPD, n = 12) dislocation. MRI analysis measured Trochlear Dysplasia (Dejour classification), TT-TG distance, and Caton-Deschamps (CD) ratio. Statistical strength was assessed using Cramer's V.
Results: Trochlear dysplasia was the strongest predictor, present in 100% of HPD cases (moderate association, V = 0.35). TT-TG distance progressively increased with severity: SPD (11.69 mm) < RPD (13.81 mm) < HPD (19.37 mm). Interestingly, Patella Alta showed a weak association (V = 0.24) and was actually less prevalent in the habitual group (33.3%) than the single-episode group (51.3%).
Conclusion: Habitual dislocation is not just more recurrent instability; it is a distinct patho-anatomical beast defined by severe trochlear dysplasia and lateralized tuberosities (high TT-TG). Patella Alta is a less reliable marker for this severe phenotype. Surgical reconstruction for HPD must address the bony geometry (trochleoplasty/tubercle transfer), not just the ligament.