Pregnancy-associated changes in material properties of the third metacarpal cortical bone in mares

Objective - To investigate the effect of late gestation, age, and parity on material properties of third metacarpal (MCIII) cortical bone in mares. Animals - 8 healthy mares (treatment group) that died or were euthanatized within 24 hours after parturition because of foaling complications and 6 age- matched, healthy, nonpregnant mares (control group). Procedures - After random assignment for mechanical testing and microradiography, the dorsal half of transverse mid-diaphyseal sections of each MCIII bone was divided into lateral, dorsal, and medial regions. Cylinders of bone from each of the 3 regions were tested in compression in a single cycle to failure, Contact microradiographic views were taken of 100-μm- thick sections prepared from methylmethacrylate-embedded transverse dorsal region specimens, which were further divided into periosteal, intracortical, and endosteal regions for assessment of bone mineral density and porosity. Results - Postpartum mares had lower yield strain in the dorsal and medial regions and failure strain in the medial region. Increasing parity was associated with decreasing elastic modulus in the dorsal region and yield stress and failure stress in the lateral region. Age and parity were positively correlated (r = 0.865; P = 0.0001), but significant correlations were not found between treatment group and age or between treatment group and parity. Increasing age and parity were associated with increasing porosity in the periosteal region. Conclusions and Clinical Relevance - On the basis of the material properties evaluated for MCIII cortical bone in late pregnancy, any increased risk for fractures in mares in late gestation may be related to parity or age or both, and less likely to pregnancy per se.