Tammy M. Cleek

Factors in Daily Physical Activity Related to Calcaneal Mineral Density in Men

To determine the factors in daily physical activity that influence the mineral density of the calcaneus, we recorded walking steps and the type and duration of exercise in 43 healthy 26-to 51-yr-old men. Areal (g.cm-2) calcaneal bone mineral density (CBMD) was measured by single energy x-ray densitometry (SXA, Osteon, Inc., Wahiawa, HI). Subjects walked a mean (+/- SD) of 7902 (+/- 2534) steps per day or approximately 3.9 (+/- 1.2) miles daily. Eight subjects reported no exercise activities. The remaining 35 subjects spent 143 (2-772) (median and range) min.wk-1 exercising. Twenty-eight men engaged in exercise activities that generate single leg peak vertical ground reaction forces (GRFz) of 2 or more body weights (high loaders, HL), and 15 reported exercise or daily activities that typically generate GRFz less than 1.5 body weights (low loaders, LL). CBMD was 12% higher in HL than LL (0.668 +/- 0.074 g.cm-2 vs 0.597 +/- 0.062 g.cm-2, P < 0.004). In the HL group, CBMD correlated to reported minutes of high load exercise (r = 0.41, P < 0.03). CBMD was not related to the number of daily walking steps (N = 43, r = 0.03, NS). The results of this study support the concept that the dominant factor in daily physical activity relating to bone mineral density is the participation in site specific high loading activities, i.e., for the calcaneus, high calcaneal loads.

Cross-sectional structural parameters from densitometry

Bone densitometry has previously been used to obtain cross-sectional properties of bone from a single X-ray projection across the bone width. Using three unique projections, we have extended the method to obtain the principal area moments of inertia and orientations of the principal axes at each scan cross-section along the length of the scan. Various aluminum phantoms were used to examine scanner characteristics to develop the highest accuracy possible for in vitro non-invasive analysis of cross-sectional properties. Factors considered included X-ray photon energy, initial scan orientation, the angle spanned by the three scans (included angle), and I(min)/I(max) ratios. Principal moments of inertia were accurate to within +/-3.1% and principal angles were within +/-1 degrees of the expected value for phantoms scanned with included angles of 60 degrees and 90 degrees at the higher X-ray photon energy (140 kVp). Low standard deviations in the error (0.68-1.84%) also indicate high precision of calculated measurements with these included angles. Accuracy and precision decreased slightly when the included angle was reduced to 30 degrees. The method was then successfully applied to a pair of excised cadaveric tibiae. The accuracy and insensitivity of the algorithms to cross-sectional shape and changing isotropy (I(min)/I(max)) values when various included angles are used make this technique viable for future in vivo studies.

Dietary salt, bone strength, and mineral content in unloaded rat femurs.

INTRODUCTIONnReduced bone mineral and ultimate strength are regular consequences of unloading bone. The aim of this study was to determine if high dietary salt intake would reduce the bone density and strength to a greater extent in rats with unloaded bones compared to ambulatory control rats fed the same dietary calcium and phosphorus.nnnMETHODSnMature male Sprague-Dawley rats were divided into four groups: two exposed to a spaceflight model that unloaded the hind limbs (HU) and two controls (C) with normal ambulation. Half the HU and C rats were fed normal dietary salt (0.26%, NNa) and half high dietary salt (8%, HNa). The calcium (Ca) and phosphorus (P) content of the diets was normal (Ca 0.5% and P 0.6%) in all four groups. After 4 wk of hind limb unloading, the bone mineral content (BMC) of excised femurs was measured by the ash weight and the ultimate torsional strength was determined by a torsional strength test device.nnnRESULTSnFemoral BMC (mg) was lower in HUNNa than C rats fed normal salt diets. Femurs from HU rats fed normal salt diets showed lower (20-26%) torsional strength (Nmm), compared to all other groups.nnnDISCUSSIONnIt appears that high salt diets with normal amounts of calcium and phosphorus may prevent the decrease in bone torsional strength and BMC induced by unloading the femurs in 6-mo-old rats.

Assessment of Long Bone Flexural Properties from Bone Densitometry

While bone densitometry is the accepted non-invasive method of quantifying bone mineral content in bones, its assessment of bone structural properties is less well understood. The objective of our current work is to compare cross-section shape or areal properties of long bones computed from densitometry data with cross-section flexural properties obtained from surface strain measurements.