Anterior thoracic posture increases thoracolumbar disc loading

Abstract

In the absence of external forces, the largest contributor to intervertebral disc (IVD) loads and stresses is trunk muscular activity. The relationship between trunk posture, spine geometry, extensor muscle activity, and the loads and stresses acting on the IVD is not well understood. The objective of this study was to characterize changes in thoracolumbar disc loads and extensor muscle forces following anterior translation of the thoracic spine in the upright posture. Vertebral body geometries (C2 to S1) and the location of the femoral head and acetabulum centroids were obtained by digitizing lateral, full-spine radiographs of 13 men and five women volunteers without previous history of back pain. Two standing, lateral, full-spine radiographic views were obtained for each subject: a neutral-posture lateral radiograph and a radiograph during anterior translation of the thorax relative to the pelvis (while keeping T1 aligned over T12). Extensor muscle loads, and compression and shear stresses acting on the IVDs, were calculated for each posture using a previously validated biomechanical model. Comparing vertebral centroids for the neutral posture to the anterior posture, subjects were able to anterior translate +101.5 mm+/-33.0 mm (C7-hip axis), +81.5 mm+/-39.2 mm (C7-S1) (vertebral centroid of C7 compared with a vertical line through the vertebral centroid of S1), and +58.9 mm+/-19.1 mm (T12-S1). In the anterior translated posture, disc loads and stresses were significantly increased for all levels below T9. Increases in IVD compressive loads and shear loads, and the corresponding stresses, were most marked at the L5-S1 level and L3-L4 level, respectively. The extensor muscle loads required to maintain static equilibrium in the upright posture increased from 147.2 N (mean, neutral posture) to 667.1 N (mean, translated posture) at L5-S1. Compressive loads on the anterior and posterior L5-S1 disc nearly doubled in the anterior translated posture. Anterior translation of the thorax resulted in significantly increased loads and stresses acting on the thoracolumbar spine. This posture is common in lumbar spinal disorders and could contribute to lumbar disc pathologies, progression of L5-S1 spondylolisthesis deformities, and poor outcomes after lumbar spine surgery. In conclusion, anterior trunk translation in the standing subject increases extensor muscle activity and loads and stresses acting on the intervertebral disc in the lower thoracic and lumbar regions.

Fig. 1

Radiographic set-up for comparison of spinal loads and stresses during A anterior translation of the thorax; and B the neutral standing posture

Fig. 2

a Representative quadrilateral-element spine model (C2–S1) illustrating neutral and anterior translated postures (subject 008, 68 kg, 173 cm). The global X,Y coordinate system (digitizing tablet origin) represents the posterior-inferior corner of the S1 vertebral body. Acetabulum and femoral-head centers are indicated by the symbols x and ο, respectively (bold symbols correspond to the anterior translated posture). Intervertebral disc-load magnitudes are in gray-scale, with lower load values shown in darker gray. b Detail of the quadrilateral-element model. α is the angle between the horizontal axis X and the anterior-posterior IVD bisector. The local coordinate system used to determine IVD forces and stresses is represented as x-y. Other parameters defined in text

Fig. 3

Overall averaged sagittal profiles (C2–S1) obtained for 18 subjects. Vertebral body centroids (mean, SD) for neutral and anterior postures are illustrated. Graphic on right shows mean (SD) anterior-posterior (X coordinate) shift in vertebral body centroids

Fig. 4

Mean (SD) IVD compressive-stress distribution for C2–S1 segments of 18 subjects. Statistically significant differences (paired-observation t-test, p<0.05) between neutral and anterior postures were observed for T9–T10 to L5–S1 segments

Fig. 5

Mean (SD) IVD shear-stress distribution for the C2–S1 segments of 18 subjects. Negative values indicate anterior shear. Statistically significant differences (paired-observation t-test, p<0.05) between neutral and anterior postures were observed for T6–T7 to L5–S1 segments

Fig. 6

Mean (SD) A Anterior IVD compressive-load and (B) posterior IVD compressive-load distributions for neutral posture and anterior translated posture. Statistically significant differences (paired-observation t-test, p<0.05) between neutral and anterior postures were observed for T8–T9 to L5–S1 segments (posterior load) and T11–T12 to L5–S1 segments (anterior load)