Summary of results
We present new data on intra-disc pressure profiles of patients with thoraco-lumbar scoliosis. The overall hypothesis we were attempting to test in this study was that abnormal, asymmetrical loading was present in the scoliotic spine and contributed to the progression of the scoliotic deformity. However, due to the surgical positioning and anaesthetised state of the scoliotic subjects we were unsure whether any physiologically significant pressure readings would be obtained since the majority of loading across the motion segment is due to body weight and muscle activity [
30‐
32]. We would perhaps have been expected to find pressures some way between 0.05 MPa and 0.14 MPa, the "intrinsic pressure" of cadaveric discs without muscle loading [
33‐
36]. We also expected to find higher pressures, less hydrostatic discs and qualitatively different pressure/stress profiles in the apical disc than in adjacent discs since it is the most deformed and has more pronounced biochemical changes and lower cell viability than adjacent levels. [
37‐
39].
Rather surprisingly, the readings obtained in the scoliotic discs were much higher in magnitude than we expected. The mean pressure reading in those discs with a hydrostatic nucleus (14/25) was 0.25 ± 0.10 MPa which was on average >3 fold greater than in the available control data (Figure
11).
Despite the lack of muscle activity, our results also showed evidence of asymmetrical loading in the scoliotic spine. Stresses were higher in the concave than in the convex annulus in 18/25 discs (Figure
9). We did not however find increased hydrostatic pressure (Figure
6) or peak stress levels (Figure
8) in the apical disc compared with its neighbours and in all but one disc, stress change was lowest in the apical disc compared with adjacent levels.
Hydrostatic pressures
A number of scoliotic and non-scoliotic discs in this study had non-hydrostatic regions, 44% of scoliotic and 50% of non-scoliotic. For the nuclear matrix to behave in a non-hydrostatic manner it must be abnormal in composition or relatively dehydrated. This would be most likely due to glycosaminoglycan loss or potentially, mechanical loss of fluid due to chronic high loading. With regards to the non-scoliotic discs, the back pain patients would be expected to have degenerate discs with glycosaminoglycan loss. Patient 11 was known to have severe degeneration on MRI imaging and hence showed disc pressures close to zero and non-hydrostatic discs. Patient 10 was younger and had less severe degeneration consistent with higher pressures and a hydrostatic disc. For the kyphotic patients, patient 8 was a severely disabled, paraplegic from T4 distally due to invasion of the spinal cord by tumour. She had flexible kyphosis clinically and the T12/L1 level was non-hydrostatic presumably due to secondary degenerative change at that level. Patient 9 had a very stiff curve and almost complete loss of disc height at all levels and severely dehydrated discs. With regards to the scoliotic discs however, non-hydrostatic discs are less easily explainable. Glycosaminoglycan loss in not know to be severe in the scoliotic disc[
37] and although the disc structure is abnormal, it is not classically degenerate [
40‐
42].
The pressures we measured in discs with hydrostatic regions in the scoliotic patients were considerably higher than those measured in patients with kyphosis and with chronic low back pain, which were on average low and of similar magnitude to that reported by Yonezawa[
29]. In addition, the mean intradiscal pressures measured in scoliotic patients (0.25 MPa; Fig
11) were also higher than pressures measured in healthy awake volunteers in similar postures, 0.12 MPa in the L4/5 disc of a healthy orthopaedic surgeon[
43] and a mean of 0.15 MPa in a group of 22–29 y old Japanese volunteers with no disc degeneration[
31]. These pressures indicate overloading of the scoliotic disc and hence if present chronically could cause relative dehydration and also lead to the non-hydrostatic behaviour seen in some discs.
The finding of high pressures and stresses in recumbent, anaesthetised scoliotic patients with minimal loading due to muscle activity was unexpected and its origins are unclear.
The internal mechanical environment of the intervertebral disc is complex. The factors influencing disc pressure/stress at any point in the disc will arise from both intrinsic factors, viz. disc swelling pressure and matrix organisation [
44]and extrinsic factors including muscle action, body weight and ligamentous tethering[
43]. Swelling pressure arises from the balance between tissue composition, particularly glycosaminoglycan concentration, and the opposing tension imposed by the collagen network [
44]and while changes in glycosaminoglycan concentrations have been reported across scoliotic discs[
37,
45], these would tend to rather affect the swelling pressure profile than increase swelling pressure levels.
With regard to externally imposed forces, body weight and muscle forces, though altered in scoliotic patients[
46], should play no role in these anaesthetized recumbent patients. The patients in this study were also all well supported inferiorly by an evacuated bean-bag which should have reduced externally imposed loading to a minimum. However, the surgical positioning of a patient with a curved, axially rotated spine might increase torsion in the deformed segments of the spine. The 7 surgical patients were all in the lateral position and in vitro tests show that imposition of rotation, flexion or extension in axially loaded spines can lead to a pressure rise in the disc [
47,
48].
Ligamentous tethering or changes in annulus organisation[
49]could be another factor pre-stressing the disc and causing higher pressures however we feel that this is unlikely to explain the magnitude of change seen. Recent studies of the lumbar fascia have shown that it can transmit loads and has contractile properties and hence is an intriguing candidate for the origin of these forces[
50,
51]. Further studies of this structure in scoliosis would therefore be of interest.
Pressure profiles
As well as differences in pressure levels, there were also profound qualitative differences between the pressure profiles measured in the study scoliotic discs during surgery (Figure
4 and
5) and those found in previous measurements made in non-scoliotic discs of comparable age[
25] or in healthy animal discs [
28,
52]. In non-degenerate discs, a hydrostatic region of constant pressure, 'the functional nucleus' is found across most of the disc apart from the first few millimetres of the outer annulus where stresses fall steeply[
25]. No such profile was seen in the scoliotic discs despite their young age. Many of the profiles measured had characteristics previously seen only in degenerate [
26]or asymmetrically loaded discs[
53,
54]such as annular stress concentrations or non-hydrostatic nuclei. The non-scoliotic but pathological discs in this study also showed stress peaks and evidence of stress gradients in the sagittal plane (Figure
10).
Stress peaks and asymmetrical loading
The origin of disc stress peaks has been discussed previously in the literature[
25,
27] in non-scoliotic discs. In cadaveric discs, annular stress peaks seen in degenerate discs are thought to be due to depressurisation of the nucleus and increasing compressive loading of the annulus. Flexion or extension of discs often caused stress peaks to develop. Interestingly, some mildly degenerate discs developed annular stress peaks after depressurisation resulting from fluid loss after creep loading (1200 N over 3 hrs); this fluid loss also exacerbated the effects of flexion/extension. In the study scoliotic discs, although depressurisation was not seen in the hydrostatic region, the abnormal profiles suggest that the annulus was relatively dehydrated thus possibly leading to the stress peaks seen. If these pressures are present in daily life, these discs may be subjected to increased levels of creep loading on a daily basis in addition to the scoliotic lateral flexion and rotation deformity. Other extrinsic influences such as imposed torsion during positioning could also lead to the stress peaks observed, since combined flexion and torsion is reported to produce high stresses in the outer regions of postero-lateral annulus[
53,
54] possibly induced by resistance of the annulus fibres to torque[
55]. If the stress profile is indeed affected by resistance of the annulus fibres to torsion or to other imposed deformations because of the abnormal lamellar organisation in scoliotic discs[
40,
56], results on tests from non-scoliotic discs however may not predict how stresses will be altered in scoliosis.
In 18/24 discs, higher stresses were found on the concave side of the curve compared to the convex indicating asymmetrical loading. This is obviously not explainable by asymmetrical muscle loading since the spinal muscles were relaxed in these patients. The simple presence of concave annular stress peaks, due to the possible previously mentioned factors, could lead to a stress gradient across the disc. However, even in some discs without a defined stress peak, asymmetrical loading was present.