Identifying thoracic compensation and predicting reciprocal thoracic kyphosis and proximal junctional kyphosis in adult spinal deformity surgery

Study Design. Retrospective analysis. Objective. To define thoracic compensation and investigate its association with postoperative reciprocal thoracic kyphosis and proximal junctional kyphosis (PJK) Summary of Background Data. Adult spinal deformity (ASD) patients recruit compensatory mechanisms like pelvic retroversion and knee flexion. However, thoracic hypokyphosis is a less recognized compensatory mechanism. Methods. Patients enrolled in a multicenter ASD registry undergoing fusions to the pelvis with upper instrumented vertebra (UIV) between T9 and L1 were included. Patients were divided into those with postoperative reciprocal thoracic kyphosis (reciprocal kyphosis [RK]: change in unfused thoracic kyphosis [TK] 158) with and without PJK and those who maintained thoracic alignment (MT). Thoracic compensation was defined as expected thoracic kyphosis (eTK) minus preoperative TK. Results. For RK (n ¼ 117), the mean change in unfused TK was 21.78 versus 6.18 for MT (n ¼ 102) and the mean PJK angle change was 17.68 versus 5.78 for MT (all P < 0.001). RK and MT were similar in age, body mass index (BMI), sex, and comorbidities. RK had larger preoperative PI–LL mismatch (30.7 vs. 23.6, P ¼ 0.008) and less preoperative TK (22.3 vs. 30.6, P < 0.001), otherwise sagittal vertical axis (SVA), pelvic tilt (PT), and T1 pelvic angle (TPA) were similar. RK patients had more preoperative thoracic compensation (29.9 vs. 20.0, P < 0.001), more PI–LL correction (29.8 vs. 17.3, P < 0.001), and higher rates of PJK (66% vs. 19%, P < 0.001). There were no differences in preoperative health-related quality of life (HRQOL) except reciprocal kyphosis (RK) had worse Scoliosis Research Society questionnaire (SRS) appearance (2.2 vs. 2.5, P ¼ 0.005). Using a logistic regression model, the only predictor for postoperative reciprocal thoracic kyphosis was more preoperative thoracic compensation. Postoperatively the RK and MT groups were well aligned. Both younger and older (>65 yr) RK patients had greater thoracic compensation than MT counterparts. The eTK was not significantly different from the postoperative TK for the RK group without PJK (P ¼ 0.566). Conclusion. The presence of thoracic compensation in adult spinal deformity is the primary determinant of postoperative reciprocal thoracic kyphosis and these patients have higher rates of proximal junctional kyphosis.