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

Lamellar bone is common among primates, either in the form of extended plan ar circumferential arrays, or as cylindrically shaped osteons. Osteonal bon e generally replaces circumferential lamellar bone with time, and it is the refore of much interest to compare the mechanical properties and fracture b ehavior of these two forms of lamellar bone. This is, however, difficult as natural specimens of circumferential lamellar bone large enough for standa rd mechanical tests are not available. We found that as a result of treatme nt with large doses of alendronate, the lateral sides of the diaphyses of b aboon 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 elasti c and ultimate properties of almost pure circumferential lamellar bone and osteonal bone in four different orientations relative to the tibia long axi s. After taking into account the differences in porosity and extent of mine ralization of the two bone types, the flexural modulus, bending strength, f racture strain and nominal work-to-fracture properties were similar for the same orientations, with some exceptions. This implies that it is the lamel lar structure itself that is mainly responsible for these mechanical proper ties. The fracture behavior and morphologies of the fracture surfaces varie d significantly with orientation in both types of bone. This is related to the microstructure of lamellar bone. Osteonal bone exhibited quite differen t damage-related behavior during fracture as compared to circumferential la mellar bone. Following fracture the two halves of osteonal bone remained at tached whereas in circumferential lamellar bone they separated. These diffe rences could well provide significant adaptive advantages to osteonal bone function. (C) 2000 Kluwer Academic Publishers.