Aging and accumulation of microdamage in canine bone.

Fractures associated with minimal trauma are common in aged human beings. However, bone safety margins are better preserved in aged dogs, which are rarely affected with minimal trauma fractures. Although the hierarchical architecture of canine and human compact bone is similar, the precise reasons for this species difference are unclear. Cyclic loading of bone during normal daily activity leads to the formation of microcracks within the tissue matrix of compact bone. Using a standard bulk-staining technique with basic fuchsin, we examined calcified transverse sections of the mid-diaphysis of the canine humerus from dogs of varying ages. We found that the amount of microdamage and porosity increased exponentially with aging, although the increases were relatively small, compared with human bone. Gender (female, ovariohysterectomized female, male, castrated male) did not have a significant effect on the amount of microdamage or porosity in bone. Alterations to the local material properties of bone tissue, or alterations to the local tissue repair responses, may play a role in the accumulation of microdamage in bone with aging. Determination of osteocyte lacunar density (number of osteocyte lacunae per bone area) and activation frequency (number of actively remodeling osteons per bone area per year) indicated that these variables declined exponentially with aging. There also was a trend for bone from dogs with low osteocyte lacunar density to have a higher microcrack density, but not higher porosity. Furthermore, bones with a high activation frequency did not accumulate microcracks or porosity. Taken together, these data suggest that, in canine bone, although a certain minimum number of osteocytes may be essential for an operational network that forms part of the signaling pathways that orchestrate repair of bone microdamage, increases in porosity with aging may not be directly associated with impaired function of the osteocyte network within bone.

Microcrack density and length in the proximal and distal metaphyses of the humerus and radius in dogs.

OBJECTIVE: To compare microcrack density and length in the proximal and distal metaphyses of the humerus and radius in dogs. SAMPLE POPULATION: Left humerus and radius from each of 10 dogs of medium to large size. PROCEDURE: Metaphyseal specimens were bulk stained in 1% basic fuchsin in graded alcohols and embedded in methylmethacrylate. For quantification of fatigue-induced microscopic damage, transverse sections were prepared from proximal and distal metaphyseal regions, and length and density of microcracks were determined, using light microscopy. RESULTS: Bone region, age, and body weight were not significantly associated with microcrack density or length. CONCLUSIONS AND CLINICAL RELEVANCE: The hypothesis that fatigue-induced injury (increased microcrack density and length) caused by cyclic loading associated with daily activity is greater in bone regions prone to development of osteosarcoma was not supported by data from this study.

In vivo matrix microdamage in a naturally occurring canine fatigue fracture.

Greyhound central tarsal bone (CTB) from animals with (n = 11) and without CTB fatigue fracture (n = 15) was examined histologically for the presence, numerical density, and morphology of in vivo microdamage. Complete fracture of the right CTB is a common occurrence during dog racing, because this is the outside limb when running counterclockwise on a circular or oval track. The CTB consisted of both remodeled cortical bone and inner trabecular bone. Thickening and coalescence of trabeculae were observed, particularly dorsally and medially, causing reduction or elimination of the marrow void spaces. A band of tightly packed transverse osteons was also observed adjacent to the concave proximal joint surface. Typical linear microcracks were most often seen in remodeled cortical and trabecular bone and were often observed adjacent to vascular channels. In contrast, ultra-microcracking, represented by diffuse staining with basic fuchsin, was consistently observed in the plantar process around the attachment site for the plantar ligament complex. Dog status (fractured or intact) and side (left or right) both had a significant effect on microcrack density and microcrack surface density (p < 0.05). Microcrack density and microcrack surface density were increased in the right (fractured) CTB from greyhounds with CTB fracture. There was also a trend for side to have a significant effect on microcrack length, with microcrack lengths being higher in the right CTB of both intact and fractured dogs. These data support the general hypothesis that fatigue fracture occurs because of ongoing cyclic stresses after induction of reparative remodeling. Development of methods for biomechanical testing of small cuboidal bones should allow investigation of relationships between accumulation of loading cycles and bone weakening because of microdamage.