Different Arm Positions and the Shape of the Thoracic Spine Can Explain Contradictory Results in the Literature About Spinal Loads for Sitting and Standing

Study Design. Loads acting in vivo on a vertebral body replacement (VBR) and the shape of the back were measured. Objective. To find an explanation for the contradictory results in literature regarding spinal loads for sitting and standing. Summary of Background Data. In several in vivo studies, the intradiscal pressure was shown to be higher for sitting than for standing. However, stadiometric measurements, load measurements on internal spinal fixators, and 1 study on intradiscal pressure have shown contradictory results. It therefore remains unknown whether sitting or standing causes greater loading on the spine. Methods. Telemeterized VBR was implanted into 5 patients. Implant loads were measured in several sessions during standing and during relaxed sitting on a stool. In the sitting position, the subjects' arms were either hanging at their sides or placed on their thighs. The shape of the back during sitting and standing was additionally determined by rasterstereography. Results. When sitting with their arms hanging, the loads for the 5 patients ranged from 107% to 228% of the values for standing. A relationship was found between this sit-to-stand load ratio and both the kyphosis angles, which increased from 41 to 67, and the distance between vertebra prominens and lordosis apex related to the body height, which increased from 0.21 to 0.26. By placing the arms on the thighs, the force on the VBR was reduced by an average of 13% (2%-41%), when compared to sitting with the arms hanging at the sides. Conclusion. The spinal load differences between sitting and standing depend on several key factors, most notably arm position and individual spinal shape. These 2 parameters, however, varied between studies in the literature and may therefore account for the continued contradictory discussion. Patients can reduce their spinal load during sitting by supporting the upper body by the arms.