High pressures and asymmetrical stresses in the scoliotic disc in the absence of muscle loading

Stress profilometry was used to measure horizontal and vertical stresses at 5 mm intervals across 25 intervertebral discs of 7 scoliotic patients during anterior reconstructive surgery. A state of hydrostatic pressure was defined by identical horizontal and vertical stresses for at least two consecutive readings. Results were compared with similar stress profiles measured during surgery across 10 discs of 4 spines with no lateral curvature and with data from the literature.Profiles across scoliotic discs were very different from those of normal, young, healthy discs of equivalent age previously presented in the literature. Hydrostatic pressure regions were only seen in 14/25 discs, extended only over a short distance. Non-scoliotic discs of equivalent age would be expected to show large centrally placed hydrostatic nuclear regions in all discs. Mean pressures were significantly greater (0.25 MPa) than those measured in other anaesthetised patients (<0.07 MPa). A stress peak was seen in the concave annulus in 13/25 discs. Stresses in the concave annulus were greater than in the convex annulus indicating asymmetric loading in these anaesthetised, recumbent patients.Intradiscal pressures and stresses in scoliotic discs are abnormal, asymmetrical and high in magnitude even in the absence of significant applied muscle loading. The origin of these abnormal stresses is unclear.The aim of this study was to obtain measurements of pressures and stresses in the intervertebral discs of scoliotic patients in order to try to prove the hypothesis that asymmetrical loading was present in the scoliotic spine and hence could realistically be involved in curve progression. We were also very interested in the stress environment of the intervertebral disc in scoliosis in general. We considered that recording from discs prior to excision during anterior scoliosis reconstructive surgery involved minimal additional patient risk. However, in anaesthetised patients with additional muscle rel