Inter-rater Reliability of the Classification of the J-Sign Is Inadequate Among Experts

Anatomic Factors Influencing a Persistent J-Sign After Medial Patellofemoral Ligament Reconstruction and Distal Tibial Tubercle Osteotomy in Patients With Recurrent Patellar Dislocations and Patella Alta: A Retrospective Cohort Study

BACKGROUND

The J-sign is a marker of abnormal patellar tracking and is associated with bony abnormalities. When patella alta is present, distal tibial tubercle osteotomy (dTTO) can enable the patella to engage in a more distal/deeper groove, often eliminating the J-sign.

PURPOSE

To determine which anatomic findings are associated with a persistent J-sign after medial patellofemoral ligament reconstruction (MPFL-R) and dTTO in patients with recurrent lateral patellar dislocations and patella alta.

STUDY DESIGN

Cohort study; Level of evidence, 4.

METHODS

A retrospective cohort study of 93 knees (77 patients) with recurrent lateral patellar dislocations and the J-sign, treated by a single surgeon with MPFL-R and dTTO without trochleoplasty, was conducted. Demographic, imaging, and surgical data were obtained from medical records. The following measurements were obtained: Caton-Deschamps index (CDI), patellotrochlear index, tibial tubercle-trochlear groove (TT-TG) distance, patellar tendon-lateral trochlear ridge (PT-LTR) distance, lateral patellar tilt, tibiofemoral joint rotation (TFJR), lateral trochlear inclination (LTI), trochlear depth, sulcus angle, and sagittal bump height. The postoperative J-sign was assessed. Patients were categorized into the resolved J-sign group or persistent J-sign group. Binary logistic regression was performed to identify significant predictors of a postoperative J-sign. Cutoff values were determined by receiver operating characteristic curve analysis using the Youden index. The Fisher exact test was used to compare frequencies.

RESULTS

The J-sign was not observed postoperatively in 56 cases (60.2%) and was thus considered resolved. Preoperative characteristics revealed differences between the resolved J-sign and persistent J-sign groups for mean lateral patellar tilt, PT-LTR distance, TFJR, sulcus angle, trochlear depth, TT-TG distance, sagittal bump height, and LTI. The mean amount of distalization, patellotrochlear index, and preoperative and postoperative CDI were similar between the groups. Logistic regression identified TFJR, PT-LTR distance, and LTI as significant predictors of a persistent J-sign. An increased risk of a persistent J-sign was found for a TFJR ≥6° (odds ratio [OR], 14.9 [95% CI, 5.4-41.6]), PT-LTR distance ≥13 mm (OR, 12.3 [95% CI, 4.3-35.5]), and LTI ≤10° (OR, 4.1 [95% CI, 1.6-10.4]). The frequency of a persistent J-sign was 3.8% for cases with no risk factors above the threshold value, 10.5% with 1 risk factor, 63.0% with 2 risk factors, and 87.5% with all 3 risk factors present.

CONCLUSION

A persistent J-sign was associated with imaging measurements of a more lateralized extensor mechanism (greater PT-LTR distance), trochlear dysplasia (lower LTI), and increased external TFJR.

Anatomic Drivers of J-Sign Presence and Severity: If There Is a Jump, Look for a Bump

BACKGROUND

Medial patellofemoral ligament reconstruction is frequently indicated for recurrent lateral patellar instability. The preoperative presence and severity of a J-sign have been associated with poorer postoperative outcomes.

PURPOSE

To determine the underlying anatomic factors that contribute to the presence, severity, and jumping quality of the J-sign.

STUDY DESIGN

Cross-sectional study; Level of evidence, 3.

METHODS

All patients undergoing evaluation for patellar instability at a single institution between 2013 and 2023 and healthy controls without patellar instability were included. Patients with a history of knee osteotomies were excluded. The presence of a jumping J-sign and its relationship to patellofemoral measures including the Caton-Deschamps Index (CDI), trochlear dysplasia (Dejour grade), tibial tubercle-trochlear groove (TT-TG) distance, tibial tubercle lateralization, trochlear bump height, mechanical alignment, femoral anteversion, tibial torsion, trochlear medialization, patellar width, axial patellar/trochlear overlap, patellar height, trochlear height, and knee rotation angle (KRA) were measured using standardized 1.5-T magnetic resonance imaging (MRI). Univariate pairwise and multivariable analyses were performed to determine the factors associated with J-sign presence, severity, and quality.

RESULTS

Of the 130 knees with patellar instability, 89 (68.5%) demonstrated a J-sign on physical examination. In total, 44 (33.8%) patients demonstrated a 1-quadrant J-sign, 32 (24.6%) demonstrated a 2-quadrant smooth J-sign, and 13 (10.0%) demonstrated a jumping J-sign. A total of 22 control, noninstability cases were included. On multivariable analysis, increasing TT-TG distance (OR, 1.1 increase per millimeter; P = .04), external KRA (OR, 1.1 increase per degree; P = .02), and increasing CDI (OR, 1.3 increase per 0.1 increase in CDI; P = .02) were associated with J-sign presence. Increasing bump height (OR, 1.72 increase per millimeter; P = .007) and decreasing patellar width (OR, 0.89 decrease per millimeter; P = .076) were associated with a larger J-sign, when present. Increasing bump height (OR, 1.80 increase per millimeter; P = .018), increasing patellar width (OR, 1.33 increase per millimeter; P = .047), and decreasing CDI (OR, 0.009 decrease per 0.01 increase in ratio; P = .008) were associated with a jumping J-sign in comparison with a smooth 2-quadrant J-sign. A KRA of 10° (AUC, 0.70) and a cartilaginous bump height of 6.6 mm (AUC, 0.73) were thresholds associated with jumping J-sign presence.

CONCLUSION

The presence of a J-sign is associated with MRI findings of relatively greater external tibiofemoral rotation, increased TT-TG distance, and increased patellar height, while J-sign severity and jumping quality are associated with the presence of additional underlying trochlear factors such as increased bump height. The anatomic drivers identified in this study should be further evaluated as possible factors associated with suboptimal outcomes after surgical management.

Thin Flap Trochleoplasty With Medial Patellofemoral Ligament Reconstruction for Recurrent Patellofemoral Instability With High-Grade Trochlear Dysplasia: A Series of 63 Consecutive Cases

BACKGROUND

Trochlear dysplasia is present in 68% to 85% of patients with recurrent lateral patellofemoral instability (LPI) compared with 3% to 6% of healthy controls. Trochlear dysplasia has been associated with medial patellofemoral ligament (MPFL) graft failure and lower quality-of-life (QOL) outcome scores. The correction of trochlear dysplasia with trochleoplasty is indicated in patients with recurrent LPI and high-grade trochlear dysplasia with a trochlear bump and a significant J-sign.

PURPOSE

To assess the clinical and patient-reported outcomes after patella-stabilizing surgery, including thin flap sulcus deepening trochleoplasty, and to explore the influence of pathoanatomic variables on postoperative QOL scores in patients who have undergone patellar stabilization, including trochleoplasty.

STUDY DESIGN

Case series; Level of evidence, 4.

METHODS

A total of 63 consecutive knees in 46 patients with recurrent LPI and high-grade trochlear dysplasia who underwent MPFL reconstruction with thin flap trochleoplasty were followed for 2 years postoperatively. Clinical outcomes and Banff Patellofemoral Instability Instrument (BPII) 2.0 scores were collected prospectively. The t test was used to determine changes in QOL. Multiple linear regression was conducted to determine significant pathoanatomic variables that affected outcomes after MPFL reconstruction.

RESULTS

Overall, 63 knees with a mean follow-up of 32.9 ± 17.0 months (range, 12-84 months) were included. All patients had preoperative high-grade trochlear dysplasia (Dejour type B: 46%; Dejour type D: 54%), with a mean trochlear bump height of 6.1 ± 1.8 mm (range, 3.2-10.5 mm). The BPII 2.0 score increased significantly from preoperatively (29.3 ± 12.4) to postoperatively (71.8 ± 17.4) (P < .001), with a large effect size (Cohen d = -2.41). There was 1 redislocation (1.6%) and 3 reoperations (4.8%). Persistent apprehension and the J-sign were present in 8.5% and 13.6% of the knees, respectively. Multiple linear regression analysis demonstrated no statistically significant predictive relationship between the 24-month postoperative BPII 2.0 score and the preoperative BPII 2.0 score, tibiofemoral rotation, the Beighton score, or trochlear bump height (R = 0.47; R2 = 0.22; F = 5.23; P = .13).

CONCLUSION

Thin flap trochleoplasty combined with MPFL reconstruction in patients with recurrent LPI and high-grade trochlear dysplasia resulted in low redislocation and reoperation rates at a mean of 33 months. Disease-specific QOL scores were statistically significantly improved from preoperatively to postoperatively.

Editorial Commentary: Machine Learning and Artificial Intelligence Are Valuable Tools yet Dependent on the Data Input

Machine learning is likely to become one of the most valuable tools for predicting outcomes in patients with patellofemoral instability. Traditional statistical analysis is challenging in this diagnosis as the result of the multitude of risk factors. However, 3 important cautions must be considered. (1) Machine learning is limited by the quality of the data entered. Many of the risk factors for patellofemoral instability rely on classification systems with significant interexaminer variability and patient-reported outcomes used to track changes contain inherent biases, especially with regard to race and gender. Poor data quality will lead to unreliable predictions, or "garbage in equals garbage out." (2) The optimal machine-learning algorithms for addressing specific clinical questions remain uncertain. (3) The question of how much data we really need for accurate analysis is unresolved, which again, is completely dependent on the quality of the data. Machine learning is the future; just beware of what goes into the chicken salad.

Qualitative visual assessment of the J-sign demonstrates high inter-rater reliability

OBJECTIVES

To assess the inter- and intra-rater reliability of the classification of the J-sign as "large" versus "small or none" as compared to another two-level system ("present" versus "absent") and a three-level system ("large," "small," or "none") and to identify anatomical and patient factors associated with the presence of a large J-sign.

METHODS

Forty patients (40 knees) with recurrent patellar instability were prospectively enrolled and recorded on video actively extending their knee while seating. Four raters classified patellar tracking on two separate occasions using three systems: 1) two groups: J-sign versus no J-sign; 2) three groups: large J-sign, small J-sign, or no J-sign; and 3) two groups: large J-sign versus small or no J-sign. The intra- and inter-rater reliability of each system was assessed using kappa statistics. Anatomical (trochlear dysplasia, tibial tubercle-trochlear groove (TT-TG) distance, patellar height) and patient (Beighton score) factors as well as Knee Injury and Osteoarthritis Outcome Score (KOOS) subscales were compared between patients with a large J-sign and patients with a small or no J-sign.

RESULTS

Inter- and intra-rater reliability were found to be highest with the two-level classification system of a large J-sign versus a small or no J-sign (inter-rater kappa ​= ​0.76, intra-rater kappa ​= ​0.75). Patients with a large J-sign had more severe trochlear dysplasia as assessed with the sulcus angle (p ​= ​0.042) and were more likely to have a tight lateral retinaculum (p ​= ​0.032) and an elevated Beighton score (p ​= ​0.009). No significant differences in KOOS subscales were noted based on the presence of a large J-sign versus a small J-sign or no J-sign.

CONCLUSION

Qualitative visual assessment of patellar tracking with the J-sign demonstrates substantial inter- and intra-rater reliability, particularly when utilizing a two-group classification system to identify knees with a large J-sign. Patients with a large J-sign demonstrate an increased incidence of a tight lateral retinaculum, generalized ligamentous laxity, and trochlear dysplasia.

LEVEL OF EVIDENCE

Level III - cross-sectional study.

What is the J-sign and why is it important?

PURPOSE OF REVIEW

Recurrent lateral patellofemoral instability is a complex condition that requires a thorough evaluation to optimize treatment. The J-sign test is classically part of the physical examination, but its significance and importance remain unclear. This review aims to describe how to perform the test and classify the observation as well as to analyze the most recent literature on its clinical applications.

RECENT FINDINGS

The J-sign test has been described as positive (present) or negative (absent), and classified using the quadrant method and the Donnell classification. Suboptimal inter-rater reliability has been shown for both classifications, making comparison between clinicians and studies challenging. The J-sign is most predominantly associated with patella alta, trochlear dysplasia, lateral force vector, and rotational abnormalities. A growing number of studies have shown a correlation between a positive J-sign and lower clinical outcome scores and higher rate of surgical failure.

SUMMARY

The J-sign is an important aspect of the physical examination in patients with recurrent lateral patellofemoral instability. Although there is no consensus on how to perform or classify the test, it can be used as a marker of severity of patellofemoral instability and is one of the tools available to guide the treatment plan.