Lumbar disc degeneration: association between osteophytes, end-plate sclerosis and disc space narrowing
Peripheral joint osteoarthritis is characterised radiologically by the presence of osteophytes, subchondral sclerosis and joint space narrowing. Joint space narrowing is due to cartilage loss, whereas both subchondral sclerosis and osteophytes are hypertrophic responses of bone, thought to arise directly either to cartilage loss or to biomechanical stress. The pathophysiology and hence the inter-relationship of these features are however not well understood. Recently, strong associations between the presence of enthesophytes, osteophytes and bone sclerosis at various joint sites have been shown. Partly on the basis of these observations, it has been suggested that some individuals may be more likely to develop bone formation in response to disease occurrence. We looked at a series of lumbar spine radiographs and characterised the severity of the component radiographic features. Although the pathology of disc degeneration differs from peripheral joint osteoarthritis, we hypothesised that if there was a predisposition to develop new bone formation in the form of osteophyte or sclerosis in response to mechanical stress, there would be a strong association between increasing severity of the features of new bone formation. The aim of this study was to determine the strength of the association between increasing severity of osteophytes and end-plate sclerosis, and the association between both these features and disc space narrowing in the lumbar spine.
In this study, we found that increasing severity of all three radiographic features of lumbar disc degeneration was associated with increasing severity of the other features, although the association was strongest for osteophytes and end-plate sclerosis, and was stronger within than between vertebral levels. There are some methodological limitations that need to be considered when interpreting the findings. The response rate for participation in the study was 61%. Those who attended for screening could have differed with respect to the frequency of disc degeneration than those who did not attend. Given, however, that the analysis of the inter-relationships between radiographic features was based on an internal comparison of responders, non-participation bias is unlikely to have had a major effect on the observed findings.
The semiquantitative approach used here to characterise the individual radiographic features is subject to errors of precision. Formal assessment of intraobserver variability as determined by was good. At the time of assessment, the observer was not aware of any possible difference in pairwise associations between features and any errors in classification of the features likely to have been non-directional, and would tend to reduce the chance of finding true associations.
As with peripheral osteoarthritis, the pathogenesis of intervertebral disc degeneration remains poorly characterised. Degenerative changes within the disc may result in an alteration of its mechanical properties, increased flexibility and decreased disc height, which in turn contribute to changes in the local stress/strain state within the disc. Also, as has been considered for peripheral osteoarthritis, the bone may be the primary trigger responding to lifelong stress with hypertrophy and stiffening, and transmitting increased load to the intervertebral disc. In peripheral osteoarthritis, there is some evidence that increased bone mass is a predictor of radiographic osteoarthritis. It has also been shown that generalised osteoarthritis is associated with increased levels of insulin-like growth factors in extracts of cortical bone from the iliac crest.
If osteophytes and end-plate sclerosis are independent and separately related to narrowing of the disc, then we could hypothesise that as disc space narrowing increased, the severity of these features would increase in parallel. However, we observed that the proliferative features were more likely to be related in terms of severity than either separately with disc space narrowing.
There are few data concerning inter-relationships between component radiographic features of lumbar disc degeneration in the literature. In a Japanese study, there were significant correlations between osteophytes, end-plate sclerosis and disc space narrowing in the lumbar spine, although data relating to these features were pooled rather than considered by vertebral level. As in our study, the association between osteophytes and end-plate sclerosis was stronger than either of the other pairwise correlations. In a study of twins using magnetic resonance imaging, a strong correlation was seen between the same measurements (including disc height and osteophytes) across all vertebral levels within the cervical and lumbar spine, but a weaker correlation when the same features were compared between the cervical and lumbar spine.
In a series of paleopathological specimens, it was observed that individuals who had a tendency to form enthesophytes were more likely to have both osteophytes and sclerosis present at peripheral joint sites. These data have been considered to be consistent with a bone proliferative response to disease occurrence. Although the pathologies of disc degeneration and peripheral osteoarthritis are different, our data in relation to severity of osteophytes and sclerosis would be consistent with this. Further research is necessary to determine the factors affecting the occurrence and severity of these features.
We have previously shown in this population sample an association between radiographic features of disc degeneration and back pain although disc space narrowing was more strongly associated than the other features. All three features increased in frequency with age. We found evidence also of an association between bone mineral density at the femoral neck and the presence of vertebral osteophytes and end-plate sclerosis, although not disc space narrowing. However, further prospective data are required both to confirm our findings and to determine the temporal nature of the observed associations.