Radiological analysis of lumbar degenerative kyphosis in relation to pelvic incidence

Background context: Lumbar degenerative kyphosis (LDK) is characterized by sagittal imbalance resulting from a loss of lumbar lordosis (LL). The pelvic incidence (PI) regulates the sagittal alignment of the spine and pelvis.

Purpose: The purpose of this study is to evaluate the spinopelvic parameters in patients with LDK and to compare them with those of a normal population.

Study design/setting: A cross-sectional study.

Patient sample: The selected patients showed characteristic clinical features of LDK. As control group, asymptomatic volunteers without spinal pathology were recruited.

Outcome measures: Full-length radiographs of the spine in the anteroposterior and lateral planes were taken, extending from the base of the skull to the proximal femur. Pelvic incidence, sacral slope (SS), pelvic tilt (PT), main thoracic kyphosis (TK), thoracolumbar junction (TLJ), LL, and sagittal vertical axis (SVA) were evaluated.

Methods: In terms of PI, the patient and control groups were divided into three groups: low (PI≤45°), middle (45°<PI≤60°), and high PI groups (PI>60°). All the spinopelvic parameters were compared between each group and between the patient and control groups in each group. The correlations between each of the parameters were analyzed.

Results: We evaluated 172 patients with symptomatic LDK and 39 healthy volunteers. The number of LDK patients with low, middle, and high PI groups were 44 (25.6%), 72 (44.8%), and 51 (29.6%), respectively. In the control group, the number of low, average, and high PI patients were 18 (46.2%), 15 (38.5%), and 6 (15.4%), respectively. In the control group, PI determined all spinopelvic parameters except SVA. In the LDK group, PI also determined spinopelvic parameters except for TK and SVA. Lumbar degenerative kyphosis with low PI was associated with pronounced kyphosis in LL and TLJ; LDK with a high PI was associated with relatively preserved lordosis in LL with a flat or lordotic TLJ. In terms of pelvic parameters, low PI showed flattened SS and low PT, whereas high PI showed steep SS and high PT.

Conclusions: The results of this study suggest the importance of the key anatomical parameter, PI, in the determination of sagittal contour in symptomatic LDK patients and normal population. Spinopelvic parameters and pelvic compensatory mechanisms in LDK patients differ according to PI. Identifying the spinopelvic parameters is useful when correcting deformities.