Objective:To explore the measurement method and its diagnostic performance based on MRI and CT measurement of vertebral bone density in patients to predict proximal junctional kyphosis after degenerative scoliosis surgery.Methods:Retrospectively included patients who underwent long-segment fixation and fusion surgery at the Department of Orthopedics, West China Hospital of Sichuan University from January 2010 to December 2020 and had complete preoperative whole-spine X-rays, CT, MRI and other imaging examination results, and were followed up on schedule. 68 cases of adult degenerative scoliosis, 16 male, 52 women, aged 66.87±6.65 years (range, 54-80 years). The patients were measured based on preoperative lumbar spine MRI T 1WI bone quality score (vertebral bone quality score, VBQ) and CT-based Hounsfield (HU) value, and the patients were divided into postoperative proximal junction kyphosis group or non-proximal junction kyphosis group based on the results of postoperative imaging examinations. The age, gender, BMI, comorbidities, lumbar spine VBQ score, L 1 CT HU value and various imaging parameters before and after surgery were compared between the two groups of patients, including pelvic incident, lumbar lordosis, sagittal vertical axis, pelvic tilt, sacral slope, pelvic incidence-lumbar lordosis, T 1 pelvic angle, Upper instrumented vertebrae screw angle, etc. The receiver operating characteristic curve (ROC) was drawn to analyze the diagnostic efficacy, sensitivity and specificity of VBQ score, HU value and their combined indicators. Results:Seventeen cases were included in the PJK group and 51 cases in the non-PJK group. The preoperative VBQ and HU values of the PJK group were 3.10±0.43 and 99.76±16.34 respectively, while those of the non-PJK group were 2.62±0.37 and 115.27±13.46 respectively. The differences were statistically significant ( t=3.896, P<0.001; t=4.482, P<0.001). The area under curve (AUC) of VBQ was 0.773 [95% CI(0.633, 0.914)], the sensitivity and specificity are 82.4% and 70.6% respectively, the AUC of HU value was 0.814 [95% CI(0.680, 0.949)], the sensitivity and specificity are 76.5% and 76.5% respectively. The AUC of the two combined indicators was 0.895 [95% CI(0.795, 0.995)], and the sensitivity and specificity were 94.1% and 82.4% respectively. The maximum Youden index and the critical value were respectively, VBQ value 0.530, 2.895, HU value 0.530, 110.0, the combined index 0.765, 0.734. Conclusion:Both VBQ and L 1 HU value can accurately predict proximal junctional kyphosis after degenerative scoliosis surgery. The accuracy of HU value was slightly higher than that of VBQ value. The diagnosis accuracy of the combined index was the highest.