Curvature of the spine: Hydrostatic pressure as an indicator of scoliosis

A problem presented at the UK MMSG Nottingham 2001.

Presented by:
Ms Susan Bibby (Physiology Laboratory, University of Oxford)
Dr Jill Urban (Physiology Laboratory, University of Oxford)
Participants:
S Bibby, J Billingham, C Breward, P Howell, J Urban

Problem Description

The problem we are presenting involves scoliosis, lateral curvature and rotation of the spine. The most common form of abnormal spinal curvature, scoliosis is of unknown origin and affects 0.5–2% of the population. It is more common in females, and usually becomes apparent during the adolescent growth spurt. It is currently hypothesised that idiopathic scoliosis occurs due to asymmetrical mechanical loads being placed on the spine, leading to re-modelling of the vertebral body and intervertebral disc and thus a permanent deformity. This theory is supported by data from animal models, in which asymmetrical loading of the spine leads to scoliosis-like changes. Although the changes occurring in the intervertebral disc are thought to be secondary, the major deformity present is the wedging of vertebrae and discs. Thus they play an important role in the progression and permanence of this disorder.

The intervertebral discs form the joints of the spine, allowing movement, and also have a weight-bearing and shock-absorbing role. Each disc consists of two distinct regions. The annulus fibrosus is composed of fibrous lamellae, or rings, made up of bundles of collagen fibres. The nucleus pulposus, in the centre of the disc, is gel-like and more highly hydrated. The disc is composed mostly of water, which along with proteoglycans and collagen comprises 90–95% of the tissue. Collagen forms the strong, fibrous framework of the disc, while the proteoglycans' main role is to draw water into the disc and hold it there. This creates the disc's inner turgor, which enables it to resist the loads placed upon it due to muscles, ligaments and body weight. If the turgor falls, the disc will lose height. The disc is avascular and aneural, and has a low cell density, approximately 1% by volume.

It is not possible to measure the loads placed on the spine, however we are able to use a pressure transducer to measure the hydrostatic pressure present in the disc. Assuming that asymmetrical loading of the spine induces this deformity, we ask the Study Group to investigate whether wedging will lead to a change in pressure in the disc? Further, can we use our measurements of the pressure in scoliotic discs to tell us about the deformity, and possibly even to assess the loads experienced by the disc? The disc undergoes both lateral bending and rotation in scoliosis, either or both of which may contribute to a rise in pressure. The nucleus behaves like a semifluid ball, and is under hydrostatic pressure; however the annulus is non-homogeneous in nature and may have directional stresses.

Study Group Report

In a healthy disc, the pressure is found to be roughly uniform across the disc, except in narrow regions near the edge where it drops rapidly to zero. In a damaged disc, however, the pressure is found to be less isotropic and to vary more across the disc. A mathematical model was developed to describe the deformation of an invertebral disc under an applied load. In particular, the model determines the motion of the porous elastic matrix and the transport of fluid through it as the disc is deformed under an applied load. It also predicts the distribution of stress between the two phases, and how this depends on the material properties of each.

The disc was modelled as a poroelastic matrix that is saturated with a viscous liquid. We model the presence of the proteoglycans as a pressure difference between the two phases. We imposed conservation of mass and momentum to the two phases (namely matrix and fluid). We reduced the stated model to three coupled equations for the matrix velocity, the volume fraction of the fluid, and the pressure in the fluid phase. We simplified the model by assuming that the deformations of the matrix are governed by linear-elasticity theory. We then utilised the slenderness of the disc to further reduce the model. We solved the simplified model for several prescribed displacements of the free surface. The model is a first step towards describing the behaviour of an intervertebral disc. There are numerous extensions to this preliminary investigation that could be undertaken.

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