In an elderly Chinese population, intervertebral disc narrowing over a four-year period is associated with the presence of osteoporosis, and is greater in women than men. These results, initially published in the journal Spine, were the conclusions of a study aiming to develop a quantitative index for lumbar space narrowing evaluation in elderly patients.
This is the first study to investigate the influence of ageing and osteoporosis on the morphology of both thoracic and lumbar intervertebral discs, comparing quantitative radiographic data for men and women from an elderly population at baseline and at four year follow up. The authors note that there is a paucity of research on quantitative classifications of lumbar disc space narrowing based on disc areal morphometry.
Quantifying lumbar disc space narrowing
Lead author Jùn-Qīng Wáng (Department of Imaging and Interventional Radiology, Faculty of Medicine, The Chinese University of Hong Kong, New Territories, Hong Kong SAR) and colleagues developed a quantitative index for lumbar space narrowing evaluation in elderly patients, called the Disc Area Index for Lumbar Spine (DAIL). DAIL refers to the area of a disc divided by an area formed by mid-height anteroposterior diameters of the two adjacent vertebral bodies. As the anteroposterior diameters of the two adjacent vertebral bodies are usually unaffected by spine degeneration, the narrower the disc space, the smaller the DAIL value.
Using radiograph images, lumbar disc space was visually classified into four categories by experienced radiologists, with the aid of direct measurements for borderline cases: normal (grade-0), mild narrowing (grade-1, characterised by <30% reduction in disc height), moderate narrowing (grade-2, 30–60% reduction in disc height), and severe narrowing (grade-3, >60% reduction in disc height). DAIL threshold criteria for defining the severity of disc space narrowing from grade one to grades two and three were obtained from receiver operating characteristic analysis.
The study authors propose that the DAIL measurement may help to standardise automatic grading, though it may need to be adjusted and validated for populations that are not elderly or Chinese.
The effect of gender on lumbar disc space narrowing
The study utilised the database of osteoporotic fractures in men (Hong Kong) and osteoporotic fractures in women (Hong Kong). These included data from 2,000 Chinese men (mean age 72.39 years) and 2,000 Chinese women (mean age 72.58 years) in Hong Kong aged 65–98 years, recruited from local communities between August 2001 and March 2003. One thousand, five hundred and nineteen men (76%) and 1,546 women (77.3%) attended the four-year follow up study. Of these, data from 500 women and 600 men were randomly selected; after eight men and nine women were excluded from the study due to inferior radiograph quality, 592 elderly Chinese men and 491 elderly Chinese women were included in the final analysis.
Intervertebral disc narrowing over a four-year period is greater in women than men. In women, the proportion of normally spaced discs decreased from 45.1% at baseline to 36.6% at four-year follow-up, while in men the proportion of normally spaced discs decreased from 49.2% to 40.8%.
It has been hypothesised that relative estrogen deficiency may contribute to the accelerated disc degeneration seen in postmenopausal women, which in turn is associated with an increased prevalence of lower back pain. However, more research is needed to confirm this.
The effect of osteoporosis on lumbar disc space narrowing
This study also found that a lower baseline bone mineral density is associated with a greater decrease of lateral disc areas, both for thoracic and lumbar discs across all patients, irrespective of gender.
Osteoporosis can cause endplate thinning and micro-fractures, which in turn lead to compromised endplate healing. The condition also decreases the vascularization in the endplates adjacent to the degenerated discs, subsequently exacerbating the degeneration of associated discs.
For osteoporotic patients in this study, both elderly men and women experienced a similar extent of disc area loss over the four years.
Looking at disc space narrowing across the spine
The extent of disc space narrowing varies within the spine. For both men and women, this study found that there was greater disc area loss in the mid-thoracic region than in the lower thoracic region. The authors suggest that this result may be associated with curvature of the spine.
The parts of the spine with greater curvature, the mid-thoracic region and L4/L5, tend to lose a greater lateral disc area than parts of the spine with less curvature.