Summary

Anatomical risk factors associated with patellar instability were assessed to create a predictive model that could identify patients that will have a persistent (postoperative) J-Sign after a distalizing TTO + MPFL-R for recurrent lateral patellar dislocation and patella alta

Abstract

Background

The J-sign is typically regarded as a marker of abnormal patellar tracking and is associated with bony abnormalities. When patella alta is present, tibial tubercle distalization (d-TTO) can position the patella more distally and deeper within the groove, often eliminating the J-sign. Evaluating the anatomic factors present in patients with a persistent J-sign after d-TTO and medial patellofemoral ligament reconstruction (MPFL-R) may offer insights into a more satisfactory surgical solution.
Objectives:
To identify which anatomic findings are associated with a persistent J-sign after MPFL-R with d-TTO in patients with recurrent lateral patellar dislocation (RLPD).

Methods

A retrospective chart review was conducted on 137 skeletally mature patients (168 knees) with patella alta who underwent surgical patellar stabilization for R-LPD using MPFL-R and d-TTO, between August 2009 and December 2021. Patients with a preoperative J-sign were eligible for the study. Measurements included Caton-Deschamps Index (CDI), patello-trochlear index (PTI), TT-TG, Patellar Tendon-Lateral Trochlear Ridge (PT-LTR) distance, lateral patellar tilt (LPT), tibiofemoral joint rotation (TFJR), lateral trochlear inclination angle (LTIA), trochlear depth (TD), sulcus angle (SA), and sagittal bump height. Postoperative J-sign presence was assessed during follow-up, and patients were categorized into persistent J-sign (PJS) or resolved J-sign (RJS) groups. Independent samples t-tests were used to compare means between groups. ROC curve analysis was conducted, and optimal cut-off values to identify groups were determined using Youden’s index. Continuous variables were categorized based on the identified threshold values, and a regression analysis of categorical variables was performed to create a predictive model for identifying a persistent J-sign using preoperative measurements.

Results

Ninety-three knees in 77 patients were included. The mean age was 20.1 years (±6.8); 75 (80.6%) were female. The J-sign was not observed postoperatively and was reported as resolved in 56 cases (60.2%). Significant differences between groups were noted for PT-LTR, TT-TG, LPT, TFJR, LTIA, TD, SA, and bump height. Multivariate analysis found that PT-LTR ≥13mm (OR: 8.4, 95% CI: 2.5–29.9), TFJR ≥6º (OR: 8.0, 95% CI: 2.4–27.1), and LTIA ≤10º (OR: 4.4, 95% CI: 1.3–15.7) were significant predictors and independent risk factors for a persistent J-sign. The frequency of persistent J-sign was only 3.8% (95% CI: 0.1%–19.6%) in cases where none of these three risk factors were present. This increased to 10.5% (95% CI: 1.3%–33.1%) with one risk factor, 63% (95% CI: 42.4%–80.6%) with two risk factors, and 87.5% (95% CI: 61.7%–98.4%) if all three risk factors were present.

Conclusions

In patients with recurrent patellar dislocation and patella alta, MPFL-R combined with d-TTO can treat a preoperative J-sign. However, postoperative J-sign may persist if excessive extensor mechanism lateralization (PT-LTR >13mm), external tibiofemoral joint rotation (TFJR >6º), or trochlear dysplasia (LTIA >10º) are present.