A High-Grade J Sign Is More Likely to Yield Higher Postoperative Patellar Laxity and Residual Maltracking in Patients With Recurrent Patellar Dislocation Treated With Derotational Distal Femoral Osteotomy
Authors: Zhang ZhiJun , Zhang Hui , Song GuanYang , Wang XueSong , Zhang Jin , Zheng Tong , Ni QianKun , Feng Hua 
Published: November 2019

Background: It has been speculated that the patellar J sign may have a negative effect on the clinical outcomes of patients with recurrent patellar dislocation (RPD). Purpose: To (1) evaluate clinical outcomes, postoperative patellar stability, and patellar maltracking correction in patients with RPD treated with derotational distal femoral osteotomy (DDFO) and combined procedures and (2) investigate the influence of J sign severity on the clinical outcomes. Study Design: Cohort study; Level of evidence, 3. Methods: Between January 2015 and December 2016, a total of 78 patients (81 knees) with RPD, a positive J sign, and an excessive femoral anteversion angle (FAA; ≥30°) were surgically treated with DDFO and combined procedures. J sign severity was graded according to a previously described classification system (grades 1-3). Routine radiography and computed tomography were performed on every patient to evaluate the patellar height, trochlear dysplasia, genu valgum, tibial tuberosity–trochlear groove distance, patellar lateral tilt angle, and patella–trochlear groove distance. The patellar lateral shift distance during stress radiography was measured preoperatively and postoperatively to quantify medial patellofemoral ligament (MPFL) graft laxity under anesthesia, and “MPFL residual graft laxity” was defined as the patellar ridge surpassing the apex of the lateral femoral trochlea. Patients were evaluated using the Kujala, International Knee Documentation Committee (IKDC), and Lysholm scores preoperatively and postoperatively. Patients were allocated into 3 subgroups in terms of the severity of the J sign: low-grade group 1 (grade 1; n = 19), low-grade group 2 (grade 2; n = 16), and high-grade group (grade 3; n = 12). Subgroup analyses were performed to investigate the influence of a high-grade J sign on the clinical outcomes. Results: Among the 78 patients (81 knees), 47 patients (47 knees) met the inclusion criteria. The mean follow-up time was 26.1 ± 1.7 months. The mean preoperative and postoperative FAAs were 36.2°± 5.3° and 10.0°± 2.1°, respectively, with a mean correction angle of 26.2°± 5.9°. At the final follow-up, all patient-reported outcomes improved significantly, and subgroup analyses showed that the high-grade group had significantly lower Kujala scores (75.6 vs 85.3 for low-grade group 1 [ P < .001] and 83.4 for low-grade group 2 [ P = .001]), Lysholm scores (77.6 vs 84.6 for low-grade group 1 [ P = .003]), and IKDC scores (78.6 vs 87.3 for low-grade group 1 [ P = .001] and 84.3 for low-grade group 2 [ P = .033]) than the low-grade groups. The total rate of MPFL residual graft laxity was 8.5% (4/47), and the prevalence of the postoperative residual J sign was 38.3% (18/47). Subgroup analyses showed significant differences between the high-grade group and the 2 low-grade groups with regard to the MPFL residual graft laxity rate (33.3% vs 0.0% for low-grade group 1 [ P = .016] and 0.0% for low-grade group 2 [ P = .024]), residual J sign rate (91.7% vs 15.8% for low-grade group 1 [ P < .001] and 25.0% for low-grade group 2 [ P < .001]), and patellar lateral shift distance (14.2 vs 8.1 mm for low-grade group 1 [ P = .002] and 8.7 mm for low-grade group 2 [ P = .007]). Conclusion: In a group of patients treated for RPD with a positive preoperative J sign and increased FAA (≥30°), patients with a preoperative high-grade J sign had inferior clinical outcomes, more MPFL residual graft laxity, and greater residual patellar maltracking.