Risk factors for osteoporosis at the knee in the spinal cord injury population

Background: The objective of this study was to determine modifiable and nonmodifiable risk factors for bone loss at the knee in individuals with spinal cord injury (SCI) by examining known risk factors for osteoporosis in the general population and additional, unique nonmodifiable SCI elements including age at injury onset, injury duration, and extent of neurologic injury (level and completeness). Methods: Risk factors were examined by logistic regression in 152 individuals with chronic SCI. Knees were classified as osteoporotic based on whether bone mineral density (BMD) of the knee as assessed by dual-energy x-ray absorptiometry fell within the 95% confidence interval of the BMD of the knee of individuals who had experienced fractures at the knee. Results: Accuracy for predicted membership in the osteoporotic group and nonosteoporotic group were 79.22% and 69.33%, respectively. Of all variables included in the analysis, 3 had a significant effect on predicted group membership: completeness of injury (P < 0.0001), body mass index (BMI) (P = 0.0035), and age (P = 0.0394). Individuals with complete injuries were 6.17 times (617%) more likely to have BMD of the knee low enough to place them in the osteoporotic category. The odds ratio for BMI indicated that every unit increase in BMI lowered the odds of being in the osteoporotic group by 11.29%. The odds ratio for age indicated that every 1-year increase in age increased the odds of being in the osteoporotic group by 3.54%. No other modifiable or nonmodifiable risk factors were significant predictors. Conclusion: Completeness of injury dictates and overrides most modifiable and nonmodifiable risk factors for bone loss at the knee leading to pathologic fractures in SCI. SCI osteoporosis may be classified more appropriately as neurogenic in origin.