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

Abstract

Background: Loads acting on scoliotic spines are thought to be asymmetric and involved in progression of the scoliotic deformity; abnormal loading patterns lead to changes in bone and disc cell activity and hence to vertebral body and disc wedging. At present however there are no direct measurements of intradiscal stresses or pressures in scoliotic spines. The aim of this study was to obtain quantitative measurements of the intradiscal stress environment in scoliotic intervertebral discs and to determine if loads acting across the scoliotic spine are asymmetric. We performed in vivo measurements of stresses across the intervertebral disc in patients with scoliosis, both parallel (termed horizontal) and perpendicular (termed vertical) to the end plate, using a side mounted pressure transducer (stress profilometry)

Methods: 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.

Results: 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.

Conclusion: 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.

Figure 1

Photograph of intraoperative measurement set up. Rostral is to the left, caudal to the right. Three electrodes can be seen inserted through the convex annulus of three adjacent discs. The pressure transducer is seen inserted into the middle disc.

Figure 2

Part of intraoperative representative radiograph showing electrodes and pressure transducer inserted from the convex side of the scoliotic curve (same patient as figure 1).

Figure 3

Diagram of position of needle mounted pressure transducer in scoliotic disc during intraoperative pressure measurements. Detail of pressure transducer tip included to show plane of transducer membrane parallel to axis of introducer needle.

Figure 4

Typical profile of measurements across a single disc with a hydrostatic region. These results showing the typical features of a vertical stress peak in the concave annulus and a hydrostatic region towards the convexity were recorded in the L1/L2 disc of a 13 year old neuromuscular scoliotic patient (Patient 3).

Figure 5

Typical profile of measurements across a single disc with no hydrostatic region. These results showing a vertical stress peak at the concavity but no hydrostatic region were recorded in the L1/L2 disc of a 17 year old idiopathic scoliotic patient (Patient 2).

Figure 6

Hydrostatic pressure levels measured in all scoliotic discs where a hydrostatic pressure region was recorded. Results are shown versus disc level relative to the apical disc.

Figure 7

Variation of the mean hydrostatic pressure level with absolute disc level. Results shown for all scoliotic discs where a hydrostatic pressure region was recorded.

Figure 8

The maximum vertical stress measured in the concave annulus in scoliotic discs. Results are shown for all discs in which a stress peak was recorded versus disc level relative to the apical disc.

Figure 9

Difference in vertical stress between the concave and convex annulus in scoliotic discs. Results are shown for all discs where recordings were made in both the concave and convex annulus versus disc level relative to the apical disc.

Figure 10

Difference in vertical stress between the anterior and posterior annulus in non-scoliotic patients. Results are shown versus absolute disc level for each disc.

Figure 11

Mean hydrostatic pressure measured during surgery in scoliotic discs compared to other conditions. Details of the scoliotic and study back-pain and kyphotic patients are given in tables 1 and 2 respectively. The other results of disc pressures in patients with back pain are from the study of Yonezawa et al [29].