Multi-modality study of the compositional and mechanical implications of hypomineralization in a rabbit model of osteomalacia.
Osteomalacia is characterized by hypomineralization of the bone associated with increased water content. In this work we evaluate the hypotheses that 1) 3D solid-state magnetic resonance imaging (MRI) of (31)P (SSI-PH) and (1)H (SSI-WATER) of cortical bone can quantify the key characteristics of osteomalacia induced by low-phosphate diet; and 2) return to normophosphatemic diet (NO) results in recovery of these indices to normal levels. Twenty female five-week old rabbits were divided into four groups. Five animals were fed a normal diet for 8 weeks (NOI); five a hypophosphatemic diet (0.09%) for the same period to induce osteomalacia (HYI). To examine the effect of recovery from hypophosphatemia an additional five animals received a hypophosphatemic diet for 8 weeks, after which they were returned to a normal diet for 6 weeks (HYII). Finally, five animals received a normal diet for the entire 14 weeks (NOII). The NOI and HYI animals were sacrificed after 8 weeks, the NOII and HYII groups after 14 weeks. Cortical bone was extracted from the left and right tibiae of all the animals. Water content was measured by SSI-WATER and by a previously reported spectroscopic proton-deuteron nuclear magnetic resonance (NMR) exchange technique (NMR-WATER), phosphorus content by SSI-PH. All MRI and NMR experiments were performed on a 9.4 T spectroscopy/micro-imaging system. Degree of mineralization of bone (DMB) was measured by micro-CT and elastic modulus and ultimate strength by 3-point bending. The following parameters were lower in the hypophosphatemic group: phosphorus content measured by SSI-PH (9.5+/-0.4 versus 11.1+/-0.3 wt.%, p<0.0001), ash content (63.9+/-1.7 versus 65.4+/-1.1 wt.%, p=0.05), ultimate strength, (96.3+/-16.0 versus 130.7+/-6.4 N/mm(2), p=0.001), and DMB (1115+/-28 versus 1176+/-24 mg/cm(3), p=0.003); SSI-WATER: 16.1+/-1.5 versus 14.4+/-1.1 wt.%, p=0.04; NMR-WATER: 19.0+/-0.6 versus 17.4+/-1.2 wt.%, p=0.01. Return to a normophosphatemic diet reduced or eliminated these differences (SSI-PH: 9.5+/-0.9 versus 10.6+/-0.8 wt.%, p=0.04; DMB: 1124+/-31 versus 1137+/-10 mg/cm(3), p=0.2; US: 95.6+/-18.6 versus 103.9+/-7.5 N/mm(2), p=0.2; SSI-WATER: 12.4+/-0.6 versus 12.2+/-0.3 wt.%, p=0.3) indicating recovery of the mineral density close to normal levels. Phosphorus content measured by SSI-PH was significantly correlated with DMB measured by micro-CT (r(2)=0.47, p=0.001) as well as with ultimate strength (r(2)=0.54, p=0.0004). The results show that the methods presented have potential for in situ assessment of mineralization and water, both critical to the bone's mechanical behavior.
Laboratory for Structural NMR Imaging, Department of Radiology, University of Pennsylvania Health System, Philadelphia, PA 19104, USA. sailaja.anumula@uphs.upenn.edu
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D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D004195:Disease Models, Animal; D005260:Female; D008279:Magnetic Resonance Imaging; D010018:Osteomalacia / Q000473:pathology*; D011817:Rabbits