The Recurrent Instability of the Patella Score: A Statistically Based Model for Prediction of Long-Term Recurrence Risk After First-Time Dislocation.

PURPOSE: To describe the clinical history of a series of primary, lateral patellar dislocations and determine long-term predictors of recurrent instability while accounting for patients undergoing early operative management.

METHODS

A large geographic database of more than 500,000 patients was used to identify patients who sustained a first-time lateral patellar dislocation between 1990 and 2010. Charts were individually reviewed to document demographics, radiographic measures including tibial tubercle to trochlear groove distance (TT-TG) and patellar length (PL), recurrent episodes of instability, and patellar stabilization surgery. A risk score that accounted for early surgical management was calculated using Fine and Gray competing risk regression, and its ability to stratify patients was examined using cumulative incidence curves.

RESULTS

Eighty-one patients (mean age 19.9 ± 9.4 years, 38 male, 43 female) were identified and followed for a mean of 10.1 years (range 4.1-20.2). Thirty-eight patients (46.9%) experienced an episode of recurrent instability and 30 (37.0%) underwent patellar stabilization surgery, including 7 who did so before recurrent dislocation. A multivariate, statistically derived scoring system, the Recurrent Instability of the Patella Score (RIP Score), that employed age, skeletal maturity, trochlear dysplasia, and TT-TG/PL ratio to predict recurrent instability while accounting for patients managed surgically, was generated. The resulting RIP score stratified patients into low-, intermediate-, and high-risk categories, with 0.0%, 30.6%, and 79.2% 10-year recurrent instability rates, respectively (P = .000004), and an area under the curve of 0.875 (P = .00002).

CONCLUSIONS

Patients who sustain a first-time, lateral patellar dislocation can be readily classified into low-, intermediate-, and high-risk categories employing the RIP score based on age, skeletal maturity, trochlear dysplasia, and TT-TG/PL ratio. This long-term risk stratification holds significant potential clinical utility for determination of patients who are at high risk for recurrent instability after primary patellar dislocation. LEVEL OF EVIDENCE: Level III, retrospective comparative study.