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, fullspine radiographic views were obtained for each subject: a neutralposture 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.

Role of pelvic translation and lower-extremity compensation to maintain gravity line position in spinal deformity

“…Since the barycentremetric study by Duval-Beaupere et al, 6 researchers have attempted to further study the impact of sagittal plane alignment on biomechanical loads and degenerative changes of the spine. 5,8,12,[17][18][19] Several methods have been developed to identify the location the center of gravity in the context of spine-related research. In 1992, Duval-Beaupere et al described a method 6 based on a gamma-ray scanner prototype that was combined with lateral radiography.…”

Development and Validation of a Musculoskeletal Model of the Fully Articulated Thoracolumbar Spine and Rib Cage

“…Prior musculoskeletal models have incorporated the thorax as a single rigid segment [2], have neglected the mechanical contribution of the ribs and sternum [2][3][4][5], or have lacked an anatomically realistic model of the rib cage [6], making them unsuitable for predicting thoracic skeletal and muscular loading. A few prior models included an articulated thoracic spine, but not the rib cage or the detailed thoracic musculature [3,5,[7][8][9]. In addition, these prior models were not validated against in vivo measures of spine and trunk muscle loading, and were only used to assess vertebral loading during a neutral standing posture.…”

Selective Activation of Thoracic Extensor Muscles during 3 Different Trunk Extensor Strengthening Exercise

“…Inappropriate or excessive lumbar extension by ICL produces pain due to greater disk loading, compressive force, and shearing force caused by an increased spinal curve in the sagittal plane. 5,9 For safe and effective TES exercise, it is necessary to minimize lumbar extensor muscle (ICL) activity to decrease the stress on the lumbar spine during exercise. Second, the synergistic activity of the erector spinae pars thoracis (ICL and LT) and lumborum (ICL) muscle is assumed to be the main mechanism of trunk extension; these muscles do not form a homogeneous muscle mass, but rather have anatomical and functional differences.…”