The influence of disc degeneration on osteoporotic vertebral fractures
P. Pollintine, P. Dolan, J. Tobias, MA Adams
University of Bristol
Introduction:
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.
Methods:
Cadaveric lumbar motion segments (mean age 50+/- 19 yrs, n=33) were subjected to 2kN 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 simulated (s)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 substracted 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.
Results:
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 degeneratrion, 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.
Conclusion:
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 movement often precipitate the injury.
(1) Pollintine P et al (2000). Veretbral body franctures in the elderly: do old vertebral bodies lose strength following stress shielding by the apophyseal joints? SBPR, Manchester.