increasing calcium intake from food sources (from 900 to 1500 mg daily) prevents bone loss from the spine in premenopausal women (Baran et al., 1990)

BONE MINERAL DENSITY

Peak bone mass is most often measured and referred to as bone mineral density (BMD, g/cm2)1 because of the strong correlation between BMD and bone strength (Bouxsein and Marcus, 1994; Moro et al., 1995). Many studies have found associations between low BMD and history of previous fracture (Gluer et al., 1996; Honkanen et al., 1991). However, these studies are difficult to interpret because immobilization and inactivity following a fracture may have contributed to the reduced BMD as noted by several investigators (Henderson et al., 1992; Kannus et al., 1994). Those individuals with a tibial fracture who were immobilized for longer periods (mean of 27 weeks) had a deficiency in bone density at a mean of 9 years following the fracture (Kannus et al., 1994). Thus, it is not clear whether low BMD led to the fracture or resulted from the fracture.

Prospective studies have also found a relationship between increased fracture risk and low BMD (Kelsey et al., 1992). Low bone mineral content (BMC) or BMD as measured by either procedure involving radiation (such as dual energy x-ray absorptiometry [DXA], single photon absorptiometry [SPA], or radiographic absorptiometry) or ultrasound is significantly associated with an increased risk of future fracture in older women as illustrated in a recent meta-analysis (Marshall et al., 1996). The majority of studies of fracture risk and BMC have been completed in older, often postmenopausal, women; few studies have investigated young adults. The number of published reports comparing the relationship of BMD to the incidence of stress fractures is comparatively fewer than for complete fractures. In a 12-mo prospective study, stress fracture occurrence was associated with low BMD in female, but not male, athletes (Bennell et al., 1996). A longitudinal study conducted in 1,319 recruits at the Naval Health Research Center in conjunction with Johns Hopkins University found a lower BMD of the tibia and femur (using DXA) in recruits who sustained stress fractures (n = 59) during basic training compared with those recruits without stress fractures (Beck, 1997; Shaffer, 1997). Bone strength index2 also was found to be lower in recruits with fracture compared with those without fracture. Another important predictor of fracture risk is the rate at which bone is being lost (Beck, 1997). The faster bone is lost, the more likely it is that undue mechanical stress can lead to fracture. The rate at which bone is lost is, in itself, an independent risk for fracture, as documented by studies of states of rapid bone loss such as those that occur after organ transplantation (Shane et al., 1997).

1  

Bone mineral density is bone mineral content, which is the amount of mineral at a particular skeletal site, divided by the area of the scanned region. BMD and bone mineral content are both measured by a variety of related technologies including dual energy x-ray absorptiometry.

2  

Bone strength index is based on the observation that the resistance of a bone to bending and twisting is directly related to the section modulus as measured by DXA and inversely related to bone length (Selker and Carter, 1989).



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