Vertebral endplate and disc changes in the aging sand rat lumbar spine: cross-sectional analyses of a large male and female population

Abstract

Study design: Lumbar vertebral segments from a large cross-sectional population of male and female sand rats were assessed for quantitative determination of lumbar endplate bone mineral density (BMD) and radiologic features.

Objective: To determine radiologic and BMD characteristics in the lumbar spines of males and females ranging in age from 1.5 to 46.0 months.

Summary of background data: The sand rat provides an excellent economical model in which disc degeneration is reliable and well characterized. A major advantage of this model is that disc degeneration is spontaneous and does not need to be induced by stab surgical procedures. Previous work has only examined a small group of male sand rats for radiologic and BMD features of the lumbar spine; more complete data on both genders and a wider age range are lacking.

Methods: BMD data were obtained for 99 male or female sand rats aged 1.5 to 46.0 months using the GE Lunar PIXImus bone densitometer; results were assessed for changes related to age, weight, gender, lumbar vertebral site, and radiologic characteristics of disc degeneration (disc space narrowing, wedging, and endplate calcification). Four age groups were studied: Group 1, 1 to 3.9 months of age; Group 2, 4 to 11.9 months of age; Group 3, 12 to 23.9 months of age, and Group 4, 24 to 46 months of age.

Results: Group 1 females showed a greater incidence of radiologic disc space narrowing and wedging at L1-L2, L2-L3, L3-L4, L4-L5, and of wedging at sites L5-L6, L6-L7, and L7-S. Group 2 females had significantly greater disc space narrowing at L3-L4 and L4-L5. By the time animals reached the age of Groups 3 and 4, males showed more wedging at L1-L2 and L3-L4. BMD data showed that Group 1 females had significantly lower BMD than Groups 2, 3 and 4 at all lumbar levels, and Group 4 had significantly higher BMD than Group 3. For males, there were no differences in younger animals, but older Group 3 and 4 animals had significantly greater BMD than Groups 1 and 2. Caudal BMD values were generally significantly higher than cranial values. Averaged BMD levels correlated significantly with both age and body weight, but the correlation with age at each lumbar level accounted for much more of the variation than did weight. Averaged BMD was significantly greater in disc sites showing disc space narrowing than in sites without narrowing for L1-L2 and L3-L4 through L7-S (P < or = 0.002 for each spinal level).

Conclusion: Results expand our understanding of the association of age and gender to endplate sclerosis patterns in the lumbar spine, and support the hypothesis that endplate sclerosis may play a role in disc degeneration.