Patients with jumping sign exhibit rotational and bony structural abnormalities consistent with high-grade J-sign in recurrent patellar dislocation.

Purpose: To propose a new sign of patellar maltracking in recurrent patellar dislocation (RPD) and compare the differences in lower limb rotational and bony structural abnormalities among the different signs.

Patients and methods: A retrospective study included 279 patients (mean age: 22 years; female: 81%) who underwent primary surgery for RPD over the past 4 years was performed. The patients were grouped based on the characteristics of patellar tracking: low-, moderate- and high-grade J-sign. Patients were further divided into 'jumping' and 'gliding' subgroups based on whether the patella exhibited a 'jumping' sign during flexion-extension motion. All patients received bilateral standard hip-knee-ankle CT scans. The rotational and bony structural parameters of affected knees were measured, and the differences in these variables among the groups were described and analyzed. Reliability analysis was performed to test the consistency of J-sign grading and measurements.

Results: There were 92, 100 and 87 patients in the low-, moderate- and high-grade J-sign, respectively. The overall incidence of the jumping sign is 37%. The incidence of the 'Jumping sign' in the high-grade J-sign was significantly higher than in the other two groups (82% vs. 32% vs. 0, p < 0.001). Compared to those with the gliding sign, patients with the jumping sign have higher proportions of increased femoral anteversion (40.8% vs. 24.4%, p = 0.004), excessive knee torsion (61.2% vs. 15.3%, p < 0.001), trochlear dysplasia (95.1% vs. 69.3%, p < 0.001), pronounced supratrochlear spur (73.1% vs. 32.3%, p < 0.001), increased tibial tuberosity-trochlear groove distance (68% vs. 43.3%, p < 0.001), flatter lateral trochlear inclination (81.3% vs. 27.5%, p < 0.001) and excessive sulcus angle (68.7% vs. 35.3%, p = 0.003). No significant morphological differences were found in various bony structural parameters between knees with jumping sign and high-grade J-sign (all p > 0.05). The inter-observer reliability kappa values were 0.65 for the quadrant classification and 0.83 for the new patellar sign.

Conclusion: In RPD patients, the rotational and bony structural abnormalities in the jumping sign are highly consistent with the high-grade J sign. This finding may assist the surgeon and their patients in making informed decisions about further imaging tests and osteotomy procedures in the treatment of RPD.

Level of evidence: Level IV, cross-sectional study.