Bending and fracture of compact circumferential and osteonal lamellar bone of the baboon tibia

Lamellar bone is common among primates, either in the form of extended planar circumferential arrays, or as cylindrically shaped osteons. Osteonal bone generally replaces circumferential lamellar bone with time, and it is therefore of much interest to compare the mechanical properties and fracture behavior of these two forms of lamellar bone. This is, however, difficult as natural specimens of circumferential lamellar bone large enough for standard mechanical tests are not available. We found that as a result of treatment with large doses of alendronate, the lateral sides of the diaphyses of baboon tibia contained fairly extensive regions of circumferential lamellar bone, the structure of which appears to be indistinguishable from untreated lamellar bone. Three-point bending tests were used to determine the elastic and ultimate properties of almost pure circumferential lamellar bone and osteonal bone in four different orientations relative to the tibia long axis. After taking into account the differences in porosity and extent of mineralization of the two bone types, the flexural modulus, bending strength, fracture strain and nominal work-to-fracture properties were similar for the same orientations, with some exceptions. This implies that it is the lamellar structure itself that is mainly responsible for these mechanical properties. The fracture behavior and morphologies of the fracture surfaces varied significantly with orientation in both types of bone. This is related to the microstructure of lamellar bone. Osteonal bone exhibited quite different damage-related behavior during fracture as compared to circumferential lamellar bone. Following fracture the two halves of osteonal bone remained attached whereas in circumferential lamellar bone they separated. These differences could well provide significant adaptive advantages to osteonal bone function.