Recurrent patellar dislocation patients with high‐grade J‐sign have multiple structural bone abnormalities in the lower limbs

Abstract Purpose To explore the relationship between preoperative J‐sign grading and structural bone abnormalities in patients with recurrent patellar dislocation (RPD). Methods A retrospective study was conducted on RPD patients over 5 years. Patients were categorised based on J‐sign grade into low (J‐ and J1+), moderate (J2+) and high groups (J3+). Trochlear dysplasia (TD) and osseous structures (femoral anteversion angle [FAA], knee torsion, tibial tuberosity‐trochlear groove [TT‐TG] distance, Caton–Deschamps index) were assessed and grouped according to risk factor thresholds. The χ 2 test was used to compare composition ratio differences of structural bone abnormalities among the groups. Results A total of 256 patients were included, with 206 (80.5%) females. The distribution of J‐sign grade was as follows: 89 knees (34.8%) of low grade, 86 moderate (33.6%) and 81 high (31.6%). Among the five structural bone abnormalities, TD was the most common with a prevalence of 78.5%, followed by increased TT‐TG at 47.4%. Excessive tibiofemoral rotation had the lowest occurrence at 28.9%. There were 173 (67.6%) patients who had two or more abnormalities, while 45 (17.6%) had four to five bony abnormalities. Among patients with any bony abnormality, the proportion of high‐grade J‐sign surpassed 40%. Patients with moderate and high‐grade J‐sign had more increased FAA and more pronounced patella alta ( all p < 0.001). The proportion of excessive knee torsion and TD increased with increasing each J‐sign grade, with the more notable tendency in knee torsion (high vs. moderate vs. low‐grade: 61% vs. 22% vs 7%, p < 0.001). Furthermore, the higher J‐sign grade was also associated with more combined bony abnormalities ( p < 0.001). In the high‐grade J‐sign group, 90.2% of the knees had two or more bony risk factors and 40.7% had four or more, which were significantly higher than the moderate and low‐grade J‐sign groups (40.7% vs. 11.6% vs. 2.2%, p < 0.001). Conclusion In patients with a high‐grade J‐sign, over 90% of the lower limbs had two or more structural bone risk factors, and more than 40% had four or more. These proportions were significantly higher compared to knees with low‐grade and moderate J‐sign. In clinical practice, when treating high‐grade patellar mal‐tracking, it is important to focus on and correct these strongly correlated abnormal bone structures. Level of Evidence Level III.