Bones

Types and Structure

Bone can be classified in two ways—by shape and by structure. Long bones, such as the femur, are cylindric with rounded ends and often bear weight. Short bones, such as the phalanges, are small and bear little or no weight. Flat bones, such as the scapula, protect vital organs and often contain blood-forming cells. Bones that have unique shapes are known as irregular bones. The carpal bones in the wrist and the small bones in the inner ear are examples of irregular bones. The sesamoid bone is the least common type and develops within a tendon; the patella is a typical example.

The second way bone is classified is by structure or composition. As shown in Fig. 52-1, the outer layer of bone, or cortex, is composed of dense, compact bone tissue. The inner layer, in the medulla, contains spongy, cancellous tissue. Almost every bone has both tissue types but in varying quantities. The long bone typically has a shaft, or diaphysis, and two knoblike ends, or epiphyses.

FIG. 52-1 The structure of a typical long bone. The cortex, or outer layer, is composed of dense, compact tissue. The microscopic structure of this compact cortical tissue is the haversian system.

The structural unit of the cortical compact bone is the haversian system, which is detailed in Fig. 52-1. The haversian system is a complex canal network containing microscopic blood vessels that supply nutrients and oxygen to bone, as well as lacunae, which are small cavities that house osteocytes (bone cells). The canals run vertically within the hard cortical bone tissue.

The softer cancellous tissue contains large spaces, or trabeculae, which are filled with red and yellow marrow. Hematopoiesis (production of blood cells) occurs in the red marrow. The yellow marrow contains fat cells, which can be dislodged and enter the bloodstream to cause fat embolism syndrome (FES), a life-threatening complication. Volkmann’s canals connect bone marrow vessels with the haversian system and periosteum, the outermost covering of the bone. In the deepest layer of the periosteum are osteogenic cells, which later differentiate into osteoblasts (bone-forming cells) and osteoclasts (bone-destroying cells).

Bone also contains a matrix, or osteoid, consisting chiefly of collagen, mucopolysaccharides, and lipids. Deposits of inorganic calcium salts (carbonate and phosphate) in the matrix provide the hardness of bone.

Bone is a very vascular tissue. Its estimated total blood flow is between 200 and 400 mL/min. Each bone has a main nutrient artery, which enters near the middle of the shaft and branches into ascending and descending vessels. These vessels supply the cortex, the marrow, and the haversian system. Very few nerve fibers are connected to bone. Sympathetic nerve fibers control dilation of blood vessels. Sensory nerve fibers transmit pain signals experienced by patients who have primary lesions of the bone, like bone tumors.

Joints

A joint is a space in which two or more bones come together. This is also referred to as articulation of the joint. The major function of a joint is to provide movement and flexibility in the body.

There are three types of joints in the body:

Although any of these joints can be affected by disease or injury, the synovial joints are most commonly involved.

The diarthrodial, or synovial, joint is the most common type of joint in the body. Synovial joints are the only type lined with synovium, a membrane that secretes synovial fluid for lubrication and shock absorption. As shown in Fig. 52-2, the synovium lines the internal portion of the joint capsule but does not normally extend onto the surface of the cartilage at the spongy bone ends. Articular cartilage consists of a collagen fiber matrix impregnated with a complex ground substance. Patients with inflammatory types of arthritis often have synovitis (synovial inflammation) and breakdown of the cartilage. Bursae, small sacs lined with synovial membrane, are located at joints and bony prominences to prevent friction between bone and structures adjacent to bone. These structures can also become inflamed, causing bursitis.

FIG. 52-2 The structure of a synovial joint. Synovium lines the joint capsule but does not extend into the articular cartilage.

Synovial joints are described by their anatomic structures. Ball-and-socket joints (shoulder, hip) permit movement in any direction. Hinge joints (elbow) allow motion in one plane—flexion and extension. The knee is often classified as a hinge joint, but it rotates slightly, as well as flexes and extends. It is best described as a condylar type of synovial joint. The gliding movement of the wrist is characteristic of the biaxial joint. Pivot joints permit rotation only, as in the radioulnar area.