Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 103

According to their anatomical shape, bones are classified into four general categories: long, short, flat, and irregular bones. We will focus here on long bones. Long bones have two extremities (epiphyses), a cylindrical tube in the middle (diaphysis, also known as shaft of the bone), and a transitional zone between them (metaphyses) (Figure 3.18). The epiphysis is the expanded end of the bone that is covered by articular cartilage. The metaphysis is the junctional region between the epiphysis and the diaphysis and includes the growth plate (physis). The diaphysis is the shaft of long bones and is located in the region between metaphyses. The growth plate is a zone of endochondral ossification (cartilage‐to‐bone conversion) that mediates growth in bone length in an actively growing cartilage‐to‐bone region (Yang, 2010).

Bone tissue can also be classified by texture, matrix arrangement, maturity, or developmental origin (Yang, 2010). There are two main subtypes of bone: cortical and trabecular bone. Both types are chemically identical, but differ in terms of their structure, arrangement, and cell density. Approximately 80% of bone is cortical bone, with the remainder as trabecular bone (Carter & Hayes, 1977).

Trabecular bone (also known as cancellous or spongy bone) has numerous cavities (Figure 3.18). It is found mainly at the ends of many long bones and in areas like the ears and nose (Cooper, Milgram, & Robinson, 1966). Individual trabeculae are extensively connected and are oriented along the lines of mechanical stress on the bone in question. Trabecular bone is more metabolically active than cortical bone because of its much larger surface area for remodeling.

Cortical bone is dense in texture with few or no cavities, although it does contain pores for blood vessels, for example (Figures 3.18 and 3.19). Cortical bone is typically seen as the hard outer shell that surrounds trabecular bone or the centrally located marrow cavity. It is surrounded externally and internally by a periosteum and endosteum, respectively (Yang, 2010). Cortical bone is organized into Haversian systems or osteons (Figure 3.19), in which the lamellae are concentrically organized around a vascular canal, the Haversian canal. The blood supply of cortical bone enters from the periosteum via Volkmann canals, which also connect Haversian canals with each other (Brooks, 1963).

Lamellar bone is mature bone in which collagen fibers are arranged in parallel. It is located in both trabecular bone and cortical bone, the latter concentrically organized around a vascular canal.

Figure 3.18 The organization of long bones. (a) Three‐dimensional micro computed tomographic image of a rat’s radius and ulna bones at the level of the wrist. (b) Staining a section of the radius with safranin O (Saf O) shows the location of cartilage in the epiphyseal plate (growth plate) and trabecular bone). (c) Hematoxylin and eosin staining showing the location of the growth plate proximal to the epiphysis, trabecular bone within the bone marrow region, and denser cortical bone on the outer edge of the bone. (d) Von Kossa staining of calcified trabecular and cortical bone.

Figure 3.19 Osteons in cortical bone. (a) The osteocytes canaliculi are visible in the osteon. (b) Complete osteons (complete circles) and partial osteons left from past remodeling events are shown.