Assessment of cannon bone mineral density by dual X-ray absorptiometry (DXA) in horses?

DXA (dual energy X-ray absorptiometry) is an established method for the measurement of bone mineral density (BMD), calculated bone mineral content (BMC) and whole body composition in human and partly in veterinary medicine. However, there are only a small number of studies that examine optical bone density in horses using DXA. All these studies are based on small sample sizes. Therefore, the objective of this study was to validate the use of DXA for the measurement of BMD and the calculated BMC in the horse. In total the cannons of 103 horses were scanned ex vivo, using the PIXI LUNAR (R) densitometer. The measuring point was the standardized midpoint between basis and caput of the third metacarpal/metatarsal bone. In a second step the DXA measurements were complemented by chemical analysis, using the respective bone slices. In these bone slices bone ash as well as calcium (Ca), phosphorus (P) and magnesium (Mg) content were determined. The average ash content of the cannon bones was 698 +/- 33.2 g/kg dry matter (DM). The average mineral content was measured in the following order: Ca 268 +/- 21.1 g/kg DM, P 124 +/- 8.36 g/kg DM und Mg 2.42 +/- 0.305 g/kg DM. The ratio of Co to P ranged from 2.14 to 2.18. The DXA results are influenced not only by the bone's mineral content, but also by its diameter. Based on this, the results are separated into the results of the forelimb (fl) and the hindlimb (hl) which generated the following results: BMD: fl 3.20 g/cm(2), hl 4.11 g/cm(2); BMC: fl 25.95 g/cm(3), hl 32.75 g/cm(3). Several robustness checks of the measurements were conducted. For the BMD measurements, the range varied by 3.51-4.48 % for measurements with limb repositioning, and by 1.06-1.85 % for measurements without limb repositioning. For the calculated BMC measurements, the range varied from 3.38-3.94 % for measurements with limb repositioning, and by 1.28-2.79 for measurements without limb repositioning. To determine the importance of the exact bone position for follow-up investigations, measurements in a distance of one, two and three centimeters proximal and distal of the original measuring point were performed. The results of these measurements deviated from the result of the central measuring point at the centre of the diaphysis in a range of 5.19 to 11.8 %. Overall, the results of this study indicate that DXA is suitable for determining BMD and calculated BMC at the third metacarpal/metatarsal bone of the horse. The high reproducibility of the results and the distance measurements suggest that DXA is suitable for measurements at the standing, tranquilized horse. However, the cannon bone is a tissue with a high content of cortical bone. This means that the diameter of the bone changes as a result of training or immobilization, while the bone ash content remains unchanged by such influence. This leads to a weak correlation between the results from the DM and chemical analyses like bone ash or Ca, P and Mg. Thus, while these two types of analysis fit well into the context of prior studies, a direct comparison between these measurements is not possible.