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

Knee torsion predicts the preoperative J-sign grade in patients with recurrent patellar dislocation : a prospective study

Aims

The aim of this study was to determine the values for knee torsion in healthy knees, and explore predictive thresholds of torsion in grades of the J-sign in patients with recurrent dislocation of the patella.

Methods

A five-year prospective study was conducted involving healthy volunteers and patients with recurrent dislocation of the patella. The knees of the patients were categorized into four groups (J-sign -, J-sign 1+, J-sign 2+, and J-sign 3+) based on visual examination of the patellar maltracking. The torsion of the knee and other bony abnormalities were obtained from routine hip-knee-ankle CT scans. The ranges of the thresholds were determined from the 95% CI of the mean for each group. The predictive thresholds for each grade of J-sign were identified from receiver operating characteristic (ROC) and predictive probability curves.

Results

The study included 44 knees (in 22 healthy volunteers) and 255 knees in 234 patients with recurrent dislocation of the patella. In the J-sign -/1+, J-sign 2+, and J-sign 3+ groups, there were 87 (29.1%), 85 (28.4%), and 83 (27.8%) knees, respectively. Age, sex, and laterality were comparable among the groups. There were significant differences in the mean torsion between the groups: healthy knees 1.4° (-8.8° to 5.4°); J-sign 1+ 6.9° (SD 4.3°); J-sign 2+ 9.2° (SD 4.8°) and J-sign 3+ knees 13.9° (SD 5.4°) (p < 0.001). The ranges of the thresholds for the four groups based on the 95% CI of the means were: < 5.5°, 7.2° to 8.1°, and 10.2° to 12.8°, respectively. Ordered regression revealed a 25.4% increased probability of advancing to a higher J-sign grade for every 1° increase in torsion (adjusted odds ratio (OR) 1.254 (95% CI 1.182 to 1.331); p < 0.001). It was shown in the ROC curves that the presence of torsion effectively differentiated between normal and positive J-sign knees (area under curve (AUC) 0.92; sensitivity 0.76; specificity 1; cut-off 5.5°). Analysis of the ROC and predictive probability curves identified threshold values for the three abnormal J-signs at 7° and 12°, showing an excellent discriminatory performance (pall post-hoc < 0.01).

Conclusion

A threshold of 5° to 6° of torsion in the knee is associated with patellar maltracking in extension. In patients with recurrent dislocation of the patella, there is a one-grade increase in J-sign for approximately every 4° increase in torsion. The 7° and 12° of torsion represent the thresholds for the preoperative J-sign -/1+, J-sign 2+, and J sign 3+, respectively. Routine bilateral lower limb CT scans are therefore required if surgery is to be considered in the knees of patients with recurrent dislocation of the patella who have a positive J-sign.

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.

Non-operative Management of Acute Knee Injuries

PURPOSE OF REVIEW

Acute knee injuries are commonly encountered in both the clinical and sideline setting and may be treated operatively or non-operatively. This article describes an evidence-based approach to non-operative acute knee injury. This includes history, physical exam, imaging, and initial management. In addition, the non-operative management of three such injuries-ligament injury, meniscus injury, and patellar dislocation injury-will be discussed via a case-based practical approach.

RECENT FINDINGS

Aside from grade III ACL tears, most acute knee ligament injuries, especially in the absence of other concurrent injuries, can be treated non-operatively. There is new evidence that acute traumatic meniscus tears in those younger than 40 can be successfully treated non-operatively and can do equally, as well as those that undergo surgery, at 1 year out from injury. Based on the current literature, a short period of knee bracing in extension with progression to weightbearing to tolerance is recommended after initial patellar dislocation. Many of the most common acute knee injuries, including MCL tears, meniscus tears, and patellar dislocations, can be managed non-operatively. A detailed systemic approach to initial evaluation, including pertinent history, physical exam, and appropriate imaging, is essential and complementary to the subsequent non-operative treatment algorithm.

Patients with trochlear dysplasia have dysplastic medial femoral epiphyseal plates

PURPOSE

To investigate the growth of the epiphyseal plate in patients with trochlea dysplasia using a 3D computed tomography (CT)-based reconstruction of the bony structure of the distal femur. The epiphysis plate was divided into a medial part and a lateral part to compare their differences in patients with trochlear dysplasia.

METHODS

This retrospective study included 50 patients with trochlea dysplasia in the study group and 50 age- and sex-matched patients in the control group. Based on the CT images, MIMICS was used to reconstruct the bony structure of the distal femur. Measurements included the surface area and volume of the growth plate (both medial and lateral), the surface area and capacity of the proximal trochlea, trochlea-physis distance (TPD) (both medial and lateral), and height of the medial and lateral condyle.

RESULTS

The surface area of the medial epiphyseal plate (1339.8 ± 202.4 mm2 vs. 1596.6 ± 171.8 mm2), medial TPD (4.9 ± 2.8 mm vs. 10.6 ± 3.0 mm), height of the medial condyle (1.1 ± 2.5 mm vs. 4.9 ± 1.3 mm), and capacity of the proximal trochlear groove (821.7 ± 230.9 mm3 vs. 1520.0 ± 498.0 mm3) was significantly smaller in the study group than in the control group. A significant positive correlation was found among the area of the medial epiphyseal plate, the medial TPD, the height of the medial condyle and the capacity of the proximal trochlear groove (r = 0.502-0.638).

CONCLUSION

The medial epiphyseal plate was dysplastic in patients with trochlea dysplasia. There is a significant positive correlation between the surface area of the medial epiphyseal plate, medial TPD, height of the medial condyle and capacity of the proximal trochlear groove, which can be used to evaluate the developmental stage of the trochlea in clinical practice and to guide targeted treatment of trochlear dysplasia.

LEVEL OF EVIDENCE

III.