Bone Health
Introduction
A significant decline in hip and knee region bone mineral density (BMD) occurs after motor complete spinal cord injury (SCI) which leads to a lifetime increased risk of lower extremity fragility or low trauma fracture. Preserving and maintaining bone mass is crucial to decrease the risk of fragility fractures. Within the first few days following SCI there is an increase in excreted calcium (known as hypercalciuria) that is 2-4 times that of individuals without SCI who are confined to prolonged bedrest (Bauman & Spungen 2001) and reflects excessive bone resorption. Longitudinal studies also highlight a higher rate of hypercalcemia (excessive calcium in the blood) for people after SCI that leads to rapid bone mineral loss in the first 4-6 months that slows for the remaining first year post injury (Hancock et al. 1980; Frey-Rindova et al. 2000). Early studies also suggest that bone mineral density (BMD) stabilizes by 1-2 years after SCI (Griffiths et al. 1976; Hancock et al. 1980; Garland et al.1992) at 25-50% below that of able-bodied peers in the hip and knee region. Other investigations support a continual loss of bone mass with time since injury (Demirel et al. 1998; Bauman et al. 1999; Eser et al. 2005) and suggest that a steady-state of a lower extremity bone mineral homeostasis is not reached.
The immediate and excessive loss of bone mass post SCI is believed to result from a decrease in mechanical loading as a result of reduced or complete loss of muscle function and/or weight-bearing activities. Autoimmune, neural, vascular, hormonal and nutritional changes may also negatively impact bone but, the relative contributions of these factors are unknown (Jiang et al 2006). The reader is referred to two recent review articles which characterize the regional changes in bone density and architecture (Jiang et al. 2006, Craven et al. 2008). Further, an inadequate dietary calcium intake (Tomey et al 2005) or insufficient vitamin D may contribute to the rate and severity of BMD decline (Bauman et al. 1995). Aging and inactivity accentuate bone resorption further, resulting in site-specific decreases in bone mineral content (trabecular>cortical bone). Additionally, women with motor complete SCI experience regional declines in hip and knee region BMD during menopause that are greater than age-matched able-bodied women (Garland et al. 2001). These changes in bone density and bone architecture all contribute to the increased risk for low trauma fractures in people with SCI.
