Gross and Microscopic Anatomy

Individual bones are classified according to their size and shape. There are bones that are flat (bilaminar plates), those that are cuboid, and the most common group are bones that are tubular, both long and short. Tubular bones are further subdivided anatomically along their long axis into the epiphysis (extends from the base of the articular surface to the region of the growth plate), the metaphysis (extends from the region of the growth plate to where the diameter of the bone becomes significantly narrow), and the diaphysis or shaft (extends from the base of one metaphysis to the base of the opposing metaphysis). In immature or growing bones, the metaphysis is separated from the epiphysis by a cartilaginous growth plate, or physis. Apophyses, such as the greater and lesser trochanters of the femur, are protuberances that form at large tendoligamentous insertion sites. The medical and forensic determination of skeletal age and growth utilizes the amount and localization of bone ossification, the formulation and size of the secondary ossification centers, and the degree and amount of remodeling .

Despite their differences in size and shape, all bones are of similar composition and generally have a periosteum, cortex, and medullary canal that contains variable amounts of cortical (compact) and cancellous bone, fatty and hematopoietic marrow, blood vessels, and nerves. For any given bone, the quantity and arrangement of cortical or cancellous bone is directly related to the biomechanical requirements (Wolff's law). For instance, bones exposed to the largest torsional forces are usually long bones, and they are composed roughly of 80% cortical bone and 20% cancellous bone. In contrast, bones that predominately transmit weight-bearing forces, such as the vertebral bodies, consist of 80% cancellous bone and 20% cortical bone.

Woven and Lamellar Bone

Histologically, bone tissue, regardless of whether it is cortical or cancellous, normal or part of a pathologic process, is categorized into woven and lamellar types based on the organization of its type I collagen fibers. In woven bone, the collagen fibers are arranged in an irregular feltwork, while in lamellar bone they are deposited in parallel arrays.

Woven bone is fabricated during periods of rapid bone growth; it composes the developing bony skeleton during embryogenesis and portions of bones in the growing infant and adolescent. It may also be the predominant type of bone that is formed in a variety of reactive (fracture-callus, infection-involucrum) and neoplastic (Codman's triangle, matrix of bone forming neoplasms) conditions. Woven bone is hypercellular, and the osteocytes and their lacunae are large and appear to be distributed in a haphazard fashion as the long axes of the cells parallel the feltlike direction of the neighboring collagen fibers. The mineral content of woven bone is higher than that of lamellar bone, and more than 50% of it is deposited outside of the collagen fibers. Overall, this structural organization enables woven bone to resist forces equally in all directions and facilitates rapid formation, mineralization, and resorption. These factors explain why woven bone is weaker, less rigid, and more flexibile than lamellar bone.

Normally, the entire mature skeleton is composed solely of lamellar bone. Lamellar bone, in contrast to woven bone, is synthesized more slowly, is less cellular, and the osteocytes and their lacunae are smaller and distributed in a more organized fashion along the more regular collagen lamellae. Additionally, the process of mineralization of lamellar bone differs from that of woven bone in that it occurs more slowly and continues long after the organic matrix is initially deposited. Subsequently, the mineral content increases as a result of enlargement and increase in the number of the apatite crystals. Microradiographs of undemineralized sections reveal varying densities, with the oldest bone being most heavily mineralized. Since the mineral and collagen fibers are well-organized and intimately bound to one another, lamellar bone has greater rigidity and tensile strength and less elasticity than woven bone.

Both lamellar and woven bone are made by osteoblasts in discrete quantities or units, which are fastened to one another by cement or reversal lines. Cement lines are thin and intensely basophilic on conventional histologic slides, and comparatively little is known about them. Recent studies, however, have shown that they are collagen poor, have less mineral, an increased calcium-to-phosphorus ratio compared to hydroxyapatite, and are richer in sulfur than is the surrounding bone matrix. Some investigators have suggested that cement lines represent a residuum of mineralized around substance that is secreted during the initial reversal phase in the formation of new bone.