Relationship between CT-Derived Bone Mineral Density and UTE-MR-Derived Porosity Index in Equine Third Metacarpal and Metatarsal Bones

SIMPLE SUMMARY: Racehorses have similar health issues to human runners during their careers. The intense exercise regime leads to changes in their bone compositions where their bones become more dense and less porous in order to adapt to the higher-than-usual physical demands. While this is generally beneficial, their bones also become more brittle in the process, and this can result in stress fractures in human athletes and racehorses alike. The balance between beneficial training responses and overtraining is fine, and we are currently not able to use imaging techniques to distinguish reliably between the two of them before injuries occur. Two important markers of bone health are the density of the bone, which is well established, and the number of pores within the bone tissue, and the latter has recently attracted more interest. We aimed to validate a new MRI-based technique for measuring bone porosity in horses. This has the potential to improve our understanding of bony changes associated with training in racehorses and aid in the identification of factors associated with fracture risks. ABSTRACT: Fatigue-related subchondral bone injuries of the third metacarpal/metatarsal (McIII/MtIII) bones are common causes of wastage, and they are welfare concerns in racehorses. A better understanding of bone health and strength would improve animal welfare and be of benefit for the racing industry. The porosity index (PI) is an indirect measure of osseous pore size and number in bones, and it is therefore an interesting indicator of bone strength. MRI of compact bone using traditional methods, even with short echo times, fail to generate enough signal to assess bone architecture as water protons are tightly bound. Ultra-short echo time (UTE) sequences aim to increase the amount of signal detected in equine McIII/MtIII condyles. Cadaver specimens were imaged using a novel dual-echo UTE MRI technique, and PI was calculated and validated against quantitative CT-derived bone mineral density (BMD) measures. BMD and PI are inversely correlated in equine distal Mc/MtIII bone, with a weak mean r value of −0.29. There is a statistically significant difference in r values between the forelimbs and hindlimbs. Further work is needed to assess how correlation patterns behave in different areas of bone and to evaluate PI in horses with and without clinically relevant stress injuries.