How do anterior/posterior translations of the thoracic cage affect the sagittal lumbar spine, pelvic tilt, and thoracic kyphosis?

Anterior and posterior thoracic cage translations in the sagittal plane have not been reported for their range of motion and effects on the lumbar spine and pelvis. Twenty subjects volunteered for full-spine radiography in neutral, anterior, and posterior thoracic cage translation postures in a standing position. While grasping an anterior vertical pole, with hands at elbow level, subjects were instructed on how to translate their thoracic cage without any flexion/extension, utilizing a full-length mirror. On the radiographs, all four vertebral body corners of T1 through S1 and the superior margin of the acetabulum were digitized. Segmental and global angles of thoracic kyphosis, sagittal lumbar curvature, and pelvic flexion/extension in translation postures were compared to alignment in the neutral posture. Using the femur heads as an origin, the mean range of thoracic cage translation, measured as horizontal movement of T12 from neutral posture, was found to be 85.1 mm anterior and 73 mm posterior. In anterior translation, the thoracic kyphosis is hypokyphotic (Cobb T1-T12 reduced by 16 degrees). In posterior translation, the segmental angles at T12-L1 and L1-L2 flexed, creating an "S" shape in the sagittal lumbar spine, while the thoracic kyphosis increased by 10 degrees. Using posterior tangents from L1 to L5 and T12 to S1, and Cobb angles at T12-S1, the lumbar curve reduced slightly (by less than 3.3 degrees for all global angle measurements) in anterior translation and reduced by 7.4 degrees, 5.7 degrees, and 8.1 degrees respectively in posterior thoracic translation. The angle of pelvic tilt (measured as the angle of intersection of a line through posterior-inferior S1 to the superior acetabulum and the horizontal) reduced by a mean of 15.9 degrees, and Ferguson's sacral base angle to horizontal reduced by a mean of 13.1 degrees in posterior translation. In anterior translation, pelvic tilt and Ferguson's sacral base angle increased by 15.1 degrees and 12.8 degrees, respectively. The findings of this study show that thoracic cage anterior/posterior translations cause significant changes in thoracic kyphosis (26 degrees ), lumbar curve, and pelvic tilt. An understanding of this main motion and consequent coupled movements might aid the understanding of spinal injury kinematics and spinal displacement analysis on full spine lateral radiographs of low back pain and spinal disorder populations.