AIS is present in 3–5% of the general population. Large curves are associated with increased pain and reduced quality of life. However, no information is available on the impact of smaller curves, many of which do not reach secondary care. The objective of this project was to identify whether or not there is any hidden burden of disease associated with smaller spinal curves.

The Avon Longitudinal Study of Parents and Children (ALSPAC) is a population-based birth cohort that recruited over 14,000 pregnant women from the Bristol area between 1991–1992 and has followed up their offspring regularly. At aged 15 presence or absence of spinal curvature ≥6degrees was identified using the validated DXA Scoliosis Measure in 5299 participants. At aged 18 a structured pain questionnaire was administered to 4083 participants. Chi-squared was used to investigate any association between presence of a spinal curve at aged 15 and self-reported pain at aged 18 years. Sensitivity analyses were performed by rerunning analyses after excluding those who were told at aged 13 they had a spinal curve (n=27), and using a higher spinal curve cut-off of ≥10degrees.

Full data was available for 3184 participants. Of these, 56.8% were female, and 4.2% non-white reflecting the local population. 202 (6.3%) had a spinal curve ≥6degrees and 125 (3.9%) had a curve ≥10degrees. The mean curve size was 12degrees. 140/202 (69.3%) had single curves, and 57.4% of these were to the right. In total 46.3% of the 3184 participants reported aches and pains that lasted for a day or longer in the previous month, consistent with previous literature. 16.3% reported back pain. Those with spinal curves ≥6degrees were 42% more likely to report back pain than those without (OR 1.42, 95%CI 1.00 to 2.02, P=0.047). In addition, those with spinal curves had more days off school, were more likely to avoid activities that caused their pain, were more likely to think that something harmful is happening when they get the pain, and were more afraid of the pain than people without spinal curves (P<0.05). Sensitivity analyses did not change results.

We present the first results from a population-based study of the impact of small spinal curves and identify an important hidden burden of disease. Our results highlight that small scoliotic curves that may not present to secondary care are nonetheless associated with increased pain, more days off school and avoidance of activities.

Introduction: Age-related hormonal changes and inactivity lead to systemic bone loss and osteoporotic fractures. However, it is not clear why the vertebral body should be affected so often, or why its anterior region should characteristically sustain a “wedge” deformity. We hypothesise that intervertebral disc degeneration in elderly spines leads to altered spinal load-sharing in such a manner that the anterior region of the vertebral body becomes vulnerable to injury.

Methods: Forty thoraco-lumbar “motion segments”, consisting of two vertebrae and the intervening disc and ligaments, were obtained from cadaver spines aged 62–94 yrs. Volumetric bone mineral density (BMD) was measured for various regions of each vertebra using a Lunar Piximus DXA scanner. The distribution of the applied compressive force (1.5 kN) between the anterior and posterior halves of the vertebral body was calculated by pulling a needle-mounted pressure transducer along the sagittal midline of the adjacent disc. Pressure measurements were integrated over area to give force. Anterior and posterior disc forces were subtracted from the applied 1.5 kN to indicate loading of the neural arch. Measurements were repeated with the specimens positioned to simulate various postures in life. The strength of each motion segment was determined by compressing it to failure while positioned in a forward stooped posture. Disc and vertebral morphology were assessed from radiographs, and from digital photographs of tissue sections.

Results: Load-bearing by the neural arch in erect posture increased in the presence of intervertebral disc degeneration, and was inversely proportional to the average height of the disc (P< 0.01). High neural arch load-bearing was associated with relatively low BMD in the anterior vertebral body (P< 0.01), and with low compressive strength (P< 0.0001). BMD in the anterior region of the vertebral body was the best univariate predictor of compressive strength (R² = 0.78). Stepwise multiple linear regression showed that 86% of the variance in compressive strength could be explained by the following: anterior vertebral body BMD, vertebral body X-sectional area, and neural arch load-bearing (% of applied load). Forcing age, gender and spinal level into the model did little to improve the prediction.

Discussion and Conclusions: Results strongly support our hypothesis. Evidently, intervertebral disc degeneration and narrowing cause the neural arch to “stress shield” the anterior vertebral body whenever the spine is held erect. This leads to reduced BMD in the anterior vertebral body, weakening the spine when it is loaded in a stooped posture. The small age-dependence of results can be attributed to the relatively narrow age range of specimens tested. Vertebral fracture risk can best be assessed from BMD measured in the anterior half of the vertebral body.

Introduction: Osteoporotic fractures affect certain bones more than others, suggesting that systemic bone loss is not the only underlying cause. We have shown that age-related intervertebral disc degeneration causes the anterior vertebral body (VB) to be stress-shielded in erect postures, and yet severely loaded when the spine is flexed (1). We hypothesise that this unequal loading causes exaggerated bone loss from the anterior vertebral body, making it vulnerable to fracture when the spine is heavily loaded in a forward stooping (flexed) posture.

Materials and Methods: Regional volumetric bone mineral density (BMD) was measured in 35 thoracolumbar motion segments (aged 64–92 yrs) using dual-energy x-ray absorptiometry. The distribution of compressive stress was measured along the mid-sagittal diameter of each intervertebral disc using a miniature pressure transducer. Stresses were integrated over area to give the compressive force acting on the anterior and posterior halves of the VB (1). Motion segment compressive strength was measured in moderate flexion.

Results: BMD of the anterior half of the VB was 26% (STD 13%) lower than that of the posterior half (p< 0.0001), was correlated with % load on the anterior VB in erect posture (r²=0.48, p< 0.0001), and was a better predictor of motion segment compressive strength (in flexion) than was BMD of the whole vertebral body (r² = 0.79 compared to r² = 0.59).

Conclusion: These results clearly support our hypothesis. It appears that intervertebral disc degeneration leads to exaggerated bone loss from the anterior VB, leaving it more vulnerable to fracture when the spine is flexed. Future work aims to confirm this important result on a larger number of specimens, and to compare the relative importance of disc degeneration and overall bone loss on vertebral compressive strength.

Pollintine P et al (2001). SBPR Annual Meeting, Bristol. Backcare Research Award 2002.

Introduction. : Measurements of overall vertebral bone mineral density (BMD_v) do not adequately explain the observed patterns of osteoporotic vertebral fracture. Perhaps bone loss from specific regions of the vertebra has a more important effect on vertebral strength, and risk of fracture, than overall bone loss? We hypothesise that ‘stress shielding’ of the anterior vertebral body by the neural arch in erect standing postures can reduce BMD_v in the anterior vertebral body and thereby reduce vertebral compressive strength.

Materials and Methods: A compressive force of 1.5kN was applied to lumbar ‘motion segments’. positioned to simulate erect standing posture. Compressive stresses within the intervertebral disc were measured by pulling a miniature pressure transducer through it. ‘Stress profiles’ were integrated over area to calculate the total compressive force on the disc¹. This was subtracted from the 1.5kN to calculate the force resisted by the neural arch. Motion segments were then compressed to failure in moderate flexion (to simulate heavy lifting) and their compressive strength obtained. After disarticulation, the BMD_v, of the whole and the anterior half of each vertebral body was measured by dual energy x-ray absorptiometry (DXA). We report preliminary results from 9 specimens, aged 72–92 yrs.

Results: Vertebral strength (in flexion) was inversely related to load-bearing by the neural arch in erect posture (r²=0.42, p=0.05). Strength was directly related to the BMD_v of the whole (r²=0.65, p=0.06) and the anterior (r²=0.8, p=0.005) vertebral body.

Conclusions: These results suggest that habitual load-bearing by the neural arch in erect postures can lead to stress shielding of the anterior vertebral body so that the latter losesBMD_v, and the vertebra is weakened in the anterior vertebral body appears to be a BMD_v better predictor of vertebral strength than BMD_v, of the whole vertebra.

Osteoporotic fractures are associated with bone loss following hormonal changes and reduced physical activity in middle age. But these systemic changes do not explain why the anterior vertebral body should be such a common site of fracture. We hypothesise that age-related degenerative changes in the intervertebral discs can lead to abnormal load-bearing by the anterior vertebral body.

Cadaveric lumbar motion segments (mean age 50 ± 19 yrs, n = 33) were subjected to 2 kN of compressive loading while the distribution of compressive stress was measured along the antero-posterior diameter of the intervertebral disc, using a miniature pressure-transducer. “Stress profiles” were obtained with each motion segment positioned to simulate a) the erect standing posture, and b) a forward stooping posture. Stress measurements were effectively integrated over area in order to calculate the force acting on the anterior and posterior halves of the disc ( 1). These two forces were subtracted from the applied 2 kN to determine the compressive force resisted by the neural arch. Discs were sectioned and their degree of disc degeneration assessed visually on a scale of 1–4.

In motion segments with non-degenerated (grade 1) discs, less than 5% of the compressive force was resisted by the neural arch, and forces on the disc were distributed evenly in both postures. However, in the presence of severe disc degeneration, neural arch load-bearing increased to 40% in the erect posture, and the compressive force exerted by the disc on the vertebral body was concentrated anteriorly in flexion, and posteriorly in erect posture. In severely degenerated discs, the proportion of the 2 kN resisted by the anterior disc increased from 18% in the erect posture to 58% in the forward stooped posture.

Disc degeneration causes the disc to lose height, so that in erect postures, substantial compressive force is transferred to the neural arch. In addition, the disc loses its ability to distribute stress evenly on the vertebral body, so that the anterior vertebral body is heavily loaded in flexion. These two effects combine to ensure that the anterior vertebral body is stress-shielded in erect postures, and yet severely loaded in flexed postures. This could explain why anterior vertebral body fractures are so common in elderly people with degenerated discs, and why forward bending movements often precipitate the injury.

Osteoporotic vertebral fractures are normally attributed to weakening of the vertebral body. However, the compressive strength of the spine also depends on the manner in which the intervertebral disc presses on the vertebral body, and on load-bearing by the neural arch. We present preliminary results from a large-scale investigation into the relative importance of these three influences on vertebral compressive strength.

Lumbar motion segments from elderly cadavers were subjected to 1.5 kN of compressive loading while the distribution of compressive stress was measured along the antero-posterior diameter of the intervertebral disc, using a miniature pressure-transducer. The overall compressive force on the disc, obtained by integrating the stress profile ( 1), was subtracted from the 1.5 kN applied load to give the force resisted by the neural arch. Stress profilometry was performed with each motion segment positioned to simulate the erect standing posture, and a forward stooping posture. Vertebral strength was measured by compressing the motion segments to failure in the forward stooping posture. In life, the spine is usually compressed most severely in this posture.

A univariate analysis of results from the first 9 motion segments (aged 72–92 yrs) showed that vertebral strength increased from 2.0 kN to 4.6 kN as the compressive force resisted by the neural arch in erect postures decreased from 1.1 kN to 0.4 kN (r² = 0.42, p = 0.05). Updated results from this on-going study will be presented at the meeting.

Preliminary results suggest that habitual load-bearing by the neural arch in erect postures can lead to progressive weakening of the vertebral body, which is effectively “stress-shielded” by the neural arch. This weakening is exposed when the spine is loaded severely in a forward stooped posture, when it has a reduced compressive strength. This mechanism could explain some features of osteoporotic vertebral fractures in old people.