Normal Bone Anatomy

Definitions and Synonyms

• Cortex: the dense outer portion of a bone; also referred to as compacta

• Medulla: the inner part of a bone contained inside the cortex. It may be composed of marrow and fat in the midportions of long bones, or marrow, fat, and coarse cancellous bone near the bone ends. Cancellous bone is sometimes called spongiosa.

• Primary spongiosa: cancellous bone consisting of mixed spicules (trabeculae) composed of both bone and calcified cartilage

• Secondary spongiosa: cancellous bone containing no residual cartilage

• Mature bone: bone in which the collagen of the extracellular matrix is arranged in layers, or lamellae (lamellar bone)

• Immature bone: bone in which the collagen of the extracellular matrix is randomly arranged (woven bone)

• Anisotropism: having a different symmetry or grain in different directions. It is usually applied to the way structures behave optically. Anisotropic substances shift the wave vector of polarized light such that they are visible between crossed polarizers. The term birefringence is often used interchangeably for anisotropism, but because birefringence represents only one instance of anisotropism, its use is technically incorrect.

Radiologic Features and Gross Pathology

Bone has evolved in vertebrates primarily to resist gravity and provide structural stability for and protection of the soft tissues. Although bones are synthesized and turned over via a continuous cycle of formation and destruction by cells, the vast majority of their mass consists of extracellular matrix. The properties of this extracellular matrix derive from the fact that it is a composite material consisting of an organic portion, most of which is type I collagen, and an inorganic portion composed of aggregated mineral that, aside from its minor chemical components, is essentially made up of calcium hydroxyapatite crystals. The organic portion of bone, accounting for 25% of bone mass, is responsible for its tensile strength, and the inorganic portion, accounting for 75% of bone mass, is responsible for its stiffness. The resulting composite bears a striking resemblance to reinforced concrete.

Bones have an outer compact cortex and an inner medulla. The cortex is composed of compact bone, which is both dense and solid, consisting of about 90% bone matrix and 10% space. The spaces in compact bone are the vascular canals, the osteocyte lacunae, and the osteocyte canaliculi connecting the lacunae. This structure enables the cortex to resist bending, torsion, and shear forces ( Fig. 1-1 ). The medulla consists of cancellous bone and marrow at the ends of long bone and fatty marrow within the shafts of the long bones. The cancellous bone at the ends of long bones is spongy and is composed of approximately 25% bone and 75% space. Most of the space in cancellous bone is the bone marrow, which may be primarily adipocytic or hematopoietic, depending on bone location and patient age. Cancellous bone is organized into highly perforated vertical plates interconnected by thinner horizontal struts. This arrangement is optimal for resisting loading forces ( Fig. 1-2 ). In short, bone maintains the least amount of mass necessary to provide maximal strength, for in bones, form follows function (Wolff's law).

The shape of a bone defines its anatomical classification. Tubular bones, which are those of the upper and lower extremities as well as in the fingers and toes, may be long or short. A cross section demonstrates that they have a compact cortex while being internally hollow for most of their length ( Fig. 1-3 ). This arrangement matches the stresses these bones experience, with bending, shear, and torsional forces acting strongly at their surfaces, but negligible forces affecting the medullary cavity. At their ends, where loading requires transfer of force from bone to bone, the compact bone is thin and most of the bone mass is cancellous ( Fig. 1-4 ). Flat bones, found in the axial skeleton, are more flat than circular in cross section and have a much higher ratio of surface area to volume than their tubular counterparts. Short bones, sometimes called epiphysioid bones because they develop as secondary ossification centers, have a high ratio of cancellous bone to compact bone and include the carpal and tarsal bones. Irregular bones, such as the vertebral bones, have a combination of characteristics of the other bone types ( Fig. 1-5 ).

Bone develops primarily from mesodermal precursors; the paraxial mesoderm gives rise to the calvarium and axial skeleton, and the lateral plate mesoderm gives rise to the bones of the limbs. A small portion of the midline skull base is ectodermal in origin and develops from the neural crest. Although a thorough discussion of embryology, physiology, and cell signaling are beyond the intentions of practical surgical pathology and must be limited here, it is important to remember some aspects of bone development so as to make more sense of later osseous alterations. Traditionally, skeletal development is divided into intramembranous and endochondral ossification. This division separates bones into those that have never gone through a precartilaginous modeling phase and those that were first formed as cartilage models. This separation is based on a series of observations of static points in embryologic development in the dynamic cascade of anatomic and physiologic events with subsequent reconstruction of the sequence in a linear fashion. This separation is still used for illustration purposes, but it is inherently flawed; most bone mass in the adult skeleton was never preformed in cartilage, for although most bones had cartilage models, their cortices were formed using these models as a surface mold without any cartilage precursors. Additionally, some membranous bones have a transient cartilaginous phase. In the adult skeleton, more than 75% of skeletal mass is compact bone and, by this reasoning, can be said to be membranous in origin, and the remaining 25% of bone is spongy and represents the residual secondary spongiosa derived from endochondral ossification.

Cartilage is the other major connective tissue found in the skeleton. Like bone, cartilage is also a composite material, although its elements are collagen and complex glycosaminoglycans attached to a backbone of hyaluronic acid. The collagen, principally type II, accounts for its structural stability while the glycosaminoglycans provide the resiliency of the cartilage; glycosaminoglycans are extremely hydrophilic, which is why hyaline cartilage, the most common skeletal type of cartilage, is up to 70% water by weight. The direction and concentration of the collagen fibers in articular hyaline cartilage change in different parts of the cartilage and make its chondrocytes appear to conform to different shapes depending on their zonality ( Fig. 1-6 ). Hyaline cartilage is pliable and easily compressed, yet because it easily reabsorbs water after compression, it has a “memory” and returns readily to its predeformational shape. Hence, hyaline cartilage is specialized for the loading forces affecting it and is an effective cushion for the bone articulations. Hyaline cartilage constitutes all of the cartilage in the adult skeleton with the exception of the joint menisci and small portions of the ligaments and tendons and is formed embryologically from mesenchymal stem cells working through the SOX pathway. Mice deficient in SOX-9 fail to produce hyaline cartilage. In adults, hyaline cartilage is found in the articular cartilage of the joints, and in children, it is found there as well as in the growth plates of the epiphyses and apophyses. Because the matrix is soft, pliable, and easily deformed, cartilage is able to add to bone length by a small amount of mitotic activity and a great increase in extracellular matrix. This process happens within the interstitium of the cartilage matrix; therefore, it is termed interstitial growth. Cartilage can also add to its mass on its outer surface. In this setting, the primary importance of growth is more to change the shape than the diameter or length. This is accomplished by differential mitotic activity as well as matrix formation. This type of growth is called appositional growth, and it is responsible for primary modeling of the immature skeleton as well as providing the appropriate complementary convex and concave shapes for joint surfaces ( Fig. 1-7 ).

In adults, articular cartilage has no blood supply; it derives its nutrition indirectly from the synovial fluid. In children, the growth plates derive their nourishment from epiphyseal arteries and become progressively more ischemic as the proliferative zone and epiphyseal blood supply move away from the metaphysis.

Fibrocartilage, which constitutes the joint menisci, annuli fibrosis of the spine, and insertion portion (entheses) of ligaments and tendons, is composed mainly of type I collagen, with a smaller component of glycosaminoglycans than hyaline cartilage and, consequently, less water. Although it resembles other types of dense connective tissue histologically, its cells are rounder than those of other dense connective tissue, tend to retain lacunae, and resemble chondrocytes more than fibroblasts ( Fig. 1-8 ). The outer one third of the meniscus and the annulus fibrosus each has a small amount of vascularity, but most fibrocartilage is avascular.

Elastic cartilage is not found in the skeleton but is found mainly in the epiglottis, the external and middle ear, and the arytenoid cartilages of the larynx. It is similar in histologic appearance to hyaline cartilage, but its perichondrocytic matrix is heavily enveloped by elastic fibers that extend into the less cellular interlacunar matrix ( Fig. 1-9 ).