Incidence of second-time lateral patellar dislocation is associated with anatomic factors, age and injury patterns of medial patellofemoral ligament in first-time lateral patellar dislocation: a prospective magnetic resonance imaging study with 5-year follow-up.

PURPOSE: To examine the predictors of the second-time lateral patellar dislocation (LPD) in patients after acute first-time LPD in a 5-year follow-up.

METHODS: Data were collected prospectively from patients after acute first-time LPD with conservative treatment. Factors included sex, age at the first-time LPD, anatomical variants [trochlear dysplasia, patellar height, tibial tuberosity-trochlear groove (TT-TG) distance], and injury patterns of medial patellofemoral ligament (MPFL) in acute first-time LPD. Logistic regression was carried out to identify the independent risk factors for the incidence of the second-time LPD.

RESULTS: The incidence rate of a second-time LPD was 35.5% (59 of 166) in the 5-year follow-up. Univariate analysis revealed significant differences in the incidence rate of the second-time LPD among age at the first-time LPD (P = 0.04), trochlear dysplasia (P = 0.003), patella height (P = 0.017) and the TT-TG distance (P = 0.027). Risk factors for the second-time LPD were age < 18 years at the first-time LPD [odds ratio (OR) 4.088], low-grade trochlear dysplasia (OR 7.214), high-grade trochlear dysplasia (OR 18.945), patella alta (OR 8.416), elevated TT-TG distance (OR 12.742), complete MPFL tear at its isolated femoral-side (OR 6.04) and complete combined MPFL tear (OR 5.851).

CONCLUSIONS: Trochlear dysplasia, elevated TT-TG distance, patella alta, age < 18 years at the first-time LPD, complete MPFL tear at its isolated femoral-side and complete combined MPFL tear in the first-time LPD are independently associated with a higher incidence rate of the second-time LPD.

LEVEL OF EVIDENCE: III.