The radiographic characteristics and developmental mechanism of the lumbar degenerative retrolisthesis under a high-grade PI

Background When talking about backward displacement of the vertebra, it generally refers to the retrolisthesis under low pelvic incidence (PI). It is worth to mention that lumbar retrolisthesis could also occur under a high-grade PI. Little knowledge was known about the radiographic characteristics and developmental mechanism of the retrolisthesis under high PI. This study was designed to describe the radiographic features and to explore the developmental mechanism of this type of backward vertebral displacement. Methods A total of 887 consecutive subjects from our database were retrospectively reviewed. Degenerative lumbar retrolisthesis was found in 78 patients, including 54 patients with a relative low-grade PI (Group L) and 24 patients with a relative high-grade PI (Group H). 60 subjects without lumbar spondylolisthesis were randomly selected as the control group. Clinical and radiologic data were collected and compared between different groups. Results 91.4% of patients in Group H had the type 4 sagittal construction in terms of Roussouly classification, while 92.6% of patients in Group L had the type 1 sagittal construction. The distribution of retrolisthesis was found about two vertebrae higher with larger backward slope in Group H than Group L. Compared with the control, patients with retrolisthesis under high PI had significantly greater thoracolumbar kyphosis (TLK), PI, sacral slope, sagittal vertical axis, T1 pelvic angle and severer disc degeneration and facet arthritis. Logistic regression analysis showed TLK was the independent factor predicting the development of retrolisthesis under a high-grade PI. Conclusions Retrolisthesis under a high-grade PI and type 4 sagittal construction had higher location and larger backward slope than retrolisthesis under a low-grade PI. Retrolisthesis under high PI might be primarily associated with the increased backward sliding forces at the hypertilted vertebra in large TLK segment and lumbar instability caused by disc degeneration and facet arthritis.