Accuracy of the visual assessment of patellar tracking is poor and is influenced by trochlear dysplasia

TL;DR

The visual J-sign test for patellar tracking is not very accurate and becomes less reliable in patients with trochlear dysplasia.

Contribution

This study quantifies the poor accuracy of the J-sign and identifies trochlear dysplasia as a key influencing factor.

Findings

Binary J-sign assessments had 70.6% accuracy, while quantitative grading had only 36.8%.

Lateral trochlear inclination significantly improved diagnostic accuracy by 20.9% per degree.

TTTG and CDI did not significantly affect the accuracy of the J-sign assessments.

Abstract

The J‐sign is commonly used to assess patellar tracking in patellofemoral instability, though reliability is limited. This study analysed the diagnostic performance and reliability of digitally aided visual J‐sign assessments by patellofemoral specialists compared to dynamic kinematic computed tomography scans (DKCT).

A diagnostic study was conducted with 20 experts assessing maltracking (≥2 quadrants lateral translation) in 17 standardised single‐knee videos of patients with and without patellofemoral instability. A digital reference line was added to guide visualisation. Experts performed qualitative (binary) assessments and quantitative gradings (0–3) of patellar tracking. Inter‐ and intra‐rater reliability was evaluated using kappa statistics. Diagnostic performance was compared to DKCT‐based patellar lateralisation (bisect offset [BO]). Logistic regression analysed the influence of tibial tuberosity–trochlear groove (TTTG) distance, lateral trochlear inclination (LTI) and Caton–Deschamps Index (CDI) on accuracy.

Moderate inter‐rater (κ = 0.52) and substantial intra‐rater (κ = 0.71) reliability were found for binary assessments, while quantitative grading showed substantial inter‐rater (κ = 0.62) and intra‐rater (κ = 0.69) reliability. Mean accuracy for binary assessments was 70.6%, whereas grading accuracy was lower at 36.8%, with variability across grades. LTI significantly influenced accuracy (β = 0.19, p = 0.048), with a 20.9% increase in correct diagnoses per degree of LTI. TTTG and CDI had no significant effect.

Despite substantial intra‐rater and moderate inter‐rater reliability, accuracy in grading patellar tracking was low, particularly in quantitative assessments. Trochlear dysplasia affected diagnostic accuracy, suggesting the J‐sign is less reliable in dysplastic patients. More advanced imaging methods may be necessary to improve patellar tracking evaluations.

Level III, diagnostic study.

Figures