Bone status of senescent male rats: chemical, morphometric, and mechanical analysis.

The bone status of male rats 6, 12, and 24 months of age (n = 10) was examined. Femur calcium (Ca), phosphorus (P), and osteocalcin contents; serum chemistry; and mechanical properties of the bone were measured and correlated. Diaphyseal Ca, P, and osteocalcin contents were not different in animals 6 and 12 months of age but decreased significantly at 24 months: -7.4% for Ca, -4.2% for P, and -24% for osteocalcin compared to 12 months. Femurs from 24-month-old (senescent) rats were characterized by a scalloped appearance of the midfemoral endosteal surface and by cortical porosities. These age-associated changes coincided with nearly two-fold increases in serum immunoreactive parathyroid hormone (PTH) and osteocalcin. Serum Ca did not change with age, whereas serum P decreased (-14.8%) from 6 to 24 months. Maximum breaking force required to fracture femurs at midshaft did not change with age. Hence, the strength of the femur as an intact organ was not compromised with age despite the loss of diaphyseal Ca and P in the senescent animal. However, ultimate stress, a parameter that normalizes for differences in bone geometry and size, decreased 35% in femurs from 12- compared with 24-month-old animals. These mechanical results might be explained by the morphometric finding that, in contrast to the small but progressive age-associated increases in femur weight and length, the cortical and medullary areas increased at least two-fold. Therefore, the strength of the intact femur was maintained by architectural compensations, even though normalized tissue strength decreased with age. These findings suggest that bone status was compromised in the aged male rat.