Osteonal structure in the equine third metacarpus

In studying the flexural fatigue properties of the equine third metacarpal (cannon) bone, we previously found that the dorsal region was weaker monotonically, but more fatigue resistant, than the lateral region. Fatigue resistance was associated with fracture surfaces which demonstrated that secondary osteons had “pulled out” of the surrounding matrix; this never happened in lateral specimens. We therefore became interested in the osteonal structure of this bone, and began to study its birefringence patterns in circularly polarized light. We found that the predominant type of secondary osteon was one in which only the outermost few lamellae were circumferential, with the inner lamellae being longitudinally oriented. This “hoop” pattern had not been described in Ascenziʹs classic papers. Using basic fuchsin-stained, undecalcified cross-sections from the dorsal, medial, and lateral midshaft regions of 12 pairs of cannon bones, we classified 360 secondary osteons according to their birefringence patterns, and measured their inner and outer diameters. We found that variants of the hoop category comprised 60% of all osteons, but were significantly less common in the dorsal region, where the predominant types were Ascenziʹs “longitudinal” or “alternating” patterns. The dorsal region also had smaller osteons (OD = 156 ± 19 pm) than the medial (179 ± 13 pm, p = 0.0004) and lateral (182 ± 13 μm, p = 0.0001) regions. We postulate that these regional variations in osteonal size and structure, which are obviously produced by regional variations in remodeling, have important mechanical implications.