OSTEONAL STRUCTURE IN THE EQUINE 3RD METACARPUS

In studying the flexural fatigue properties of the equine third metaca rpal (cannon) bone, we previously found that the dorsal region was wea ker monotonically, but more fatigue resistant, than the lateral region , Fatigue resistance was associated with fracture surfaces which demon strated that secondary osteons had ''pulled out'' of the surrounding m atrix; this never happened in lateral specimens, We therefore became i nterested in the osteonal structure of this bone, and began to study i ts birefringence patterns in circularly polarized light, We found that the predominant type of secondary osteon was one in which only the ou termost few lamellae were circumferential, with the inner lamellae bei ng longitudinally oriented, This ''hoop'' pattern had not been describ ed in Ascenzi's classic papers, Using basic fuchsin-stained, undecalci fied cross-sections from the dorsal, medial, and lateral midshaft regi ons of 12 pairs of cannon bones, we classified 360 secondary osteons a ccording to their birefringence patterns, and measured their inner and outer diameters, We found that variants of the hoop category comprise d 60% of all osteons, but were significantly less common in the dorsal region, where the predominant types were Ascenzi's ''longitudinal'' o r ''alternating'' patterns, The dorsal region also had smaller osteons (OD = 156 +/- 19 mu m) than the medial (179 +/- 13 mu m, p = 0.0004) and lateral (182 +/- 13 mu m, p = 0.0001) regions, We postulate that t hese regional variations in osteonal size and structure, which are obv iously produced by regional variations in remodeling, have important m echanical implications.