In actively growing, or remodeling, trabecular bone, the direction of deposition can be determined by a row of osteoblasts on one border of the trabecula (see Plate 2-23). The deposition of new bone by these osteoblasts is often counterbalanced by the removal of bone by osteoclasts from the opposite surface of the trabecula. By this means of coordinated resorption and deposition, the position of a trabecula can shift within a bone while still maintaining relative mass homeostasis.

With age, the balance between the rate of bone formation and the rate of bone resorption changes, leading to a progressive decrease in bone mass. Any number of combinations can cause this effect. Evidence from kinetic studies indicates that after age 40, bone formation remains constant whereas bone resorption increases. Over several decades, through age-related bone loss (men and women) and postmenopausal bone loss (women), the skeletal mass may be reduced to 50% of what it was at age 30. If the bone density and structure becomes so depleted that the skeleton can no longer withstand the mechanical stresses of everyday life, pathologic fractures also known as fragility fractures may result. The compressive strength of bone is proportional to the square of its apparent density; thus, if its density decreases by a factor of 2, its compressive strength decreases by a factor of 4. Many other variables determine the fracture threshold, not the least of which is peak bone density at the time of skeletal maturity.

The biochemical composition and microscopic physical properties are comparable in both cortical and trabecular bone. However, the macroscopic structure of bone produces markedly different physical properties that have broad variations in strength and stiffness to suit local physical requirements. Thus, the thin cortical shell supported by trabecular bone at the ends of long bones is well suited to distribute the concentrated loads in the joints, whereas the tubular cortical midshaft is better suited to support the large torsional and bending loads applied to this area.

All normal adult bone is lamellar bone, whether it has a cortical or a trabecular structure. In adults, immature woven bone, or fiber bone, is seen only in normal fracture healing or in pathologic conditions such as hyperparathyroidism or Paget disease.