Cortical bone is remodeled by on the periosteal, endosteal, and haversian canal surfaces. These surfaces are called bone envelopes, or remodeling bays. The periosteal surface is responsible for the growth in bone width. The endosteum, which lines the medullary cavity of long bones, carries out complex metabolic and structural activities throughout life. These activities include phases of bone resorption alternating with phases of bone formation (see Plate 2-40). Endosteal activity determines the diameter of the medullary canal, while the combined activities of the periosteum and endosteum determine the overall thickness of the bone cortex. The haversian canal surface is important in bone remodeling and is responsible for the density of the cortex.

Arterial blood supply to two thirds of the inner bone cortex is predominantly centrifugal and is carried out by nutrient arteries entering from the medullary canal (see Plate 2-22); periosteal arterioles supply approximately one third of the outer cortex of a long bone. A highly developed anastomotic network connects the centripedal periosteal arterial system with the centrifugal endosteal arterial system. Venous drainage of the cortex is predominantly centripedal via a large plexus of veins in the medullary canal. (See plates 1-8 and 1-9 in Section 1 for a comprehensive discussion of blood supply to bone.)

STRUCTURE OF TRABECULAR BONE

In contrast to the compact structure of cortical bone, trabecular bone is a complex network of intersecting curved plates and tubes (see Plate 2-23). The bone within each trabecula is mature lamellar bone; the osteocytes are concentrically oriented and have a welldeveloped canalicular network.

Trabecular bone is typically located at the ends of a long bone. Here, the well-defined medullary cavity of the shaft gives way to a different organization: bony trabeculae fill the entire cross section of the bone, occupying approximately 20% of its volume. In the proximal end of the femur, the trabeculae are quite regularly arranged, reflecting the direction of the principal mechanical stresses to which this bone is subjected.

Cortical bone accounts for 80% of skeletal bone mass, whereas trabecular bone constitutes the remaining 20%. However, because of the larger surface area of trabecular bone, its surface-to-volume ratio is approximately 10 times that of cortical bone.

The metabolic activity of trabecular bone is nearly eight times greater than that of cortical bone, which may help to explain why disorders of skeletal homeostasis (metabolic bone diseases) have a greater tendency to effect trabecular bone rather than cortical bone. Observations indicate that the rate of remodeling in trabecular bone may vary greatly in different parts of the skeleton. For example, in adults, trabecular bone at the ends of long bones is in contact with a fatty marrow, whereas in vertebrae of the axial skeleton, it is in contact with a more highly cellular hematopoietic marrow. These differences may help to account for the axial distribution of trabecular osteopenia.