Risk of Redislocation After Primary Patellar Dislocation: A Clinical Prediction Model Based on Magnetic Resonance Imaging Variables

Arendt EA, Askenberger M, Agel J, & Tompkins MA. Am J Sport Med. 2018; 46(14):3385-3390. DOI:10.11177/0363546518803936


Take Home Message: A patient with a primary patellar dislocation who is skeletally immature and has a sulcus angle > 154 and patella alta is more likely to redislocate over the next 2 years.

People who suffer patellar dislocations often opt for conservative management of the injury. However, many people suffer subsequent redislocations, which increases the risk for more damage and surgery. It would be helpful if clinicians could predict who is at risk for redislocation to guide more effective management decisions. Therefore, the authors used 2 cohorts to determine if there are factors that differentiate people who redislocate their patella after an initial dislocation versus those who do not redislocate. The authors also tested if this information could help predict the risk of redislocation. The authors followed 145 patients with a lateral patellar dislocation for the 2 years after an initial patellar dislocation. All participants had no concomitant repairable chondral lesions, magnetic resonance (MR) images within 6 weeks of an injury, and managed their injury without surgery. The first prospective cohort included 37 patients with open growth plates and followed for 2 years. The second cohort of 108 patients were contacted by phone and email at least 2 years after injury. The authors analyzed MR images from within 6 weeks of injury for ligamentous, cartilage, and bone injury locations/severity along with other anatomical measurements to determine characteristics that were more prominent in those who suffered a redislocation. Over 2 years, 61 (42%) participants suffered a redislocation. A participant was more likely to redislocate if they were skeletally immature, had increased sulcus angles (>154 degrees), or had patellar alta (Insall-Salvati ratio >1.3). The presence of all three of these risk factors led to ~80% predicted chance of redislocation while a having no risk factors led to <10% predicted chance of redislocation.

These researchers found 3 risk factors that may help determine the risks of subsequent early redislocation. This may help clinicians better understand ideal candidates for conservative versus surgical interventions. All of these patients were from the same institution, so it may be interesting to see if their model can be applied to other healthcare systems. Furthermore, the 37 patients who were prospectively followed were all skeletally immature (with open growth plates). It would be interesting to see if their results were consistent and the model remained as strong within each of the 2 cohorts independently. These researchers also indicated that this study failed to consider physical examination measurements and that it would have been helpful to understand some of these measures in patients. For example, any strength or mobility measures may help determine who is at risk for redislocation The authors did not address rehabilitation protocols, and it would be interesting to combine some of the physical examination measures to determine those who are not being compliant with rehabilitation protocols and what impact that may have on their subsequent redislocation risk. In the meantime, clinicians should be aware that patients who are skeletally immature and have an increased sulcus angle and patella alta may be more likely to fail conservative care after an initial patellar dislocation.

Questions for Discussion: Are there any clinical risk factors for subsequent redislocation that you have seen with your patients? What are your thoughts on using imaging characteristics to help guide your clinical decisions?

Written by: Nicole Cattano

Reviewed by: Jeffrey Driban

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