Prediction of Contralateral Patellar Instability After Ipsilateral Medial Patellofemoral Ligament Reconstruction

About 10% to 16% of patients with patellar instability have bilateral involvement. The risk factors for future contralateral patellar instability in patients who have ipsilateral patellar instability are not known. Knowledge related to the possibility of future contralateral patellar instability would be helpful, as previous studies have shown suboptimal outcomes and increased complication rates in patients with bilateral patellar instability.

The purpose of the current study was to identify the risk factors for future contralateral patellar instability and develop a prediction model for contralateral patellar instability.

Case series; Level of evidence, 4.

Using a hospital-based surgery database from 2012 to 2022, all patients who underwent medial patellofemoral ligament (MPFL) reconstruction were identified. Age, sex, skeletal maturity, body mass index, generalized joint hypermobility (defined as a Beighton score ≥5), and first-time versus recurrent ipsilateral patellar dislocation were noted. There were 4 anatomic risk factors-trochlear depth, Caton-Deschamps index, tibial tubercle-trochlear groove distance, and patellar tilt-assessed on magnetic resonance imaging of the ipsilateral knee. Multivariable backward conditional logistic regression analysis was performed to identify the risk factors for contralateral patellar instability. A simplified prediction model for contralateral patellar instability was developed based on the number of risk factors.

During the study period, 380 knees in 293 patients underwent MPFL reconstruction and formed the study cohort. The mean age at surgery was 14.7 ± 2.7 years. 243 ipsilateral knees (63.9%) were female, and 168 (44.2%) were skeletally immature. 83 knees (21.8%) had a first-time dislocation, while 297 (78.2%) had a recurrent dislocation. Of these 380 knees, 130 (34.2%) had future contralateral patellar instability. On multivariable regression analysis, skeletal immaturity (odds ratio [OR], 1.90 [95% CI, 1.20-3.00]), generalized joint hypermobility (OR, 2.80 [95% CI, 1.50-5.10]), recurrent patellar instability in the ipsilateral knee (OR, 2.00 [95% CI, 1.10-3.70]), trochlear dysplasia in the ipsilateral knee (OR, 1.90 [95% CI, 1.05-3.40]), and patella alta in the ipsilateral knee (OR, 1.80 [95% CI, 0.96-3.30]) comprised the final model to predict contralateral patellar instability. As per the prediction model, the risk of future contralateral patellar instability was 9.2% if there was no risk factor present and 77.8% if all 5 risk factors were present.

Skeletal immaturity, recurrent patellar instability in the involved knee, generalized joint hypermobility (Beighton score ≥5), trochlear dysplasia in the involved knee, and patella alta in the involved knee could help to predict contralateral patellar instability.