Analysis of the characteristics of spino-pelvic sagittal parameters in Degenerative Thoracolumbar Kyphosis deformity and the mechanism of sagittal imbalance

BACKGROUND: Degenerative thoracolumbar kyphosis (DTLK) is a subtype of adult spinal deformity. If DTLK progresses further, it can develop into global kyphosis (GK). Although numerous studies have described the sagittal parameters of the spino-pelvic in adult spinal deformities, there is currently no report on the sagittal parameters of DTLK. To date, there have been no reports that elucidate the mechanism of sagittal imbalance by comparing it with sagittal balance, or by examining the sagittal parameters of healthy individuals. To address this gap, the study employed a retrospective analysis, measuring parameters such as the sagittal vertical axis (SVA), thoracic kyphosis (TK), thoracolumbar kyphosis (TLK), lumbar lordosis angle (LL), upper arc of total lumbar lordosis (UALL), lower arc of total lumbar lordosis (LALL), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), spino-sacral angle (SSA), and PI-LL. This study aims to clarify the occurrence and compensatory mechanisms of sagittal balance and sagittal imbalance in patients with DTLK. OBJECTIVE: To investigate the spino-pelvic sagittal parameters in patients with degenerative thoracolumbar kyphosis and the compensatory mechanisms for the occurrence of sagittal imbalance. METHOD: Our research is a cross-sectional study. The imaging data used in this study were extracted on August 15, 2025. A retrospective analysis was conducted on the preoperative imaging data of 339 patients diagnosed with DTLK at Fuyang People's Hospital from 2016 to 2025. The sagittal parameters of the spino-pelvic were measured using Surgimap software and compared to those of healthy individuals as documented in the literature. The data were divided into a balanced group and an imbalanced group based on SVA and the spino-pelvic parameters of healthy individuals. The balanced group was then compared with the imbalanced group. RESULTS: The spino-pelvic sagittal parameters of DTLK patients were significantly different from those of healthy individuals. In the balanced group, TK, TLK, LL, and PT were higher than those of normal individuals, while SS, PI, PI-LL, SSA, and UALL were lower than those of normal individuals. Compared with the imbalanced group, the balanced group had higher TK, TLK, LL, LALL, and SSA and lower SS, PT, PI-LL, and UALL. SVA showed significant correlations with multiple sagittal parameters. CONCLUSIONS: DTLK patients exhibit significant differences from healthy individuals in various sagittal parameters. Balance is primarily maintained through the posterior tilt of the pelvis and a reduction in the lumbar lordosis to compensate. When an imbalance occurs, the LL, LALL, UALL, PT, and PI-LL increase as compensatory mechanisms for the loss of balance. Once the UALL is exhausted, compensation shifts to an anterior convexity of the lower lumbar curve and a posterior rotation of the pelvis.