Structure and Function of the Skeletal System

Anatomic and Physiologic Overview

The health and proper functioning of the musculoskeletal system is interdependent with that of the other body systems. The bony structure provides protection for vital organs, including the brain, heart, and lungs. The bony skeleton provides a sturdy framework to support body structures. The bone matrix stores calcium, phos-phorus, magnesium, and fluoride. More than 98% of the total-body calcium is present in bone. In addition, the red bone marrow located within bone cavities produces red and white blood cells in a process called hematopoiesis. Joints hold the bones together and allow the body to move. The muscles attached to the skeleton con-tract, moving the bones and producing heat, which helps to main-tain body temperature.

STRUCTURE AND FUNCTION OF THE SKELETAL SYSTEM

There are 206 bones in the human body, divided into four categories:

·      Long bones (eg, femur)

·       Short bones (eg, metacarpals)

·       Flat bones (eg, sternum)

·        Irregular bones (eg, vertebrae)

The shape and construction of a specific bone are determined by its function and the forces exerted on it. Bones are constructed of cancellous (trabecular) or cortical (compact) bone tissue. Long bones are shaped like rods or shafts with rounded ends (Fig. 66-1). The shaft, known as the diaphysis, is primarily cortical bone. The ends of the long bones, called epiphyses, are primarily can-cellous bone. The epiphyseal plate separates the epiphyses from the diaphysis and is the center for longitudinal growth in chil-dren. In the adult, it is calcified. The ends of long bones are cov-ered at the joints by articular cartilage, which is a tough, elastic, avascular tissue. Long bones are designed for weight bearing and movement. Short bones consist of cancellous bone covered by a layer of compact bone. Flat bones are important sites for hemato-poiesis and frequently provide vital organ protection. They are made of cancellous bone layered between compact bone. Irregu-lar bones have unique shapes related to their functions. Generally, irregular bone structure is similar to that of flat bones.

Bone is composed of cells, protein matrix, and mineral de-posits. The cells are of three basic types—osteoblasts, osteocytes, and osteoclasts. Osteoblasts function in bone formation by se-creting bone matrix. The matrix, which consists of collagen and ground substances (glycoproteins and proteoglycans), provides a framework in which inorganic mineral salts are deposited. Osteo-cytes are mature bone cells involved in bone-maintenance func-tions; they are located in lacunae (bone matrix units). Osteoclasts, located in shallow Howship’s lacunae (small pits in bones), are multinuclear cells involved in destroying, resorbing, and remold-ing bone. The microscopic functioning unit of mature cortical bone is the osteon (Haversian system). The center of the osteon, the Haversian canal, contains a capillary. Around the capillary are circles of mineralized bone matrix called lamellae. Within the lamellae are lacunae containing osteocytes. These are nourished through tiny structures, canaliculi (canals), that communicate with adjacent blood vessels within the Haversian system (see Fig. 66-1).

Lacunae in cancellous bone are layered in an irregular lattice network (trabeculae). Red bone marrow fills the lattice network. Capillaries nourish the osteocytes located in the lacunae.

Covering the bone is a dense, fibrous membrane known as the periosteum. The periosteum nourishes bone and allows for itsgrowth; it also provides for the attachment of tendons and liga-ments. The periosteum contains nerves, blood vessels, and lym-phatics. The layer closest to the bone contains osteoblasts, which are bone-forming cells.

The endosteum is a thin, vascular membrane that covers the marrow cavity of long bones and the spaces in cancellous bone. Osteoclasts, which dissolve bone to maintain the marrow cavity, are located near the endosteum in Howship’s lacunae.

Bone marrow is a vascular tissue located in the medullary (shaft) cavity of long bones and in flat bones. Red bone marrow, located mainly in the sternum, ilium, vertebrae, and ribs in adults, is responsible for producing red and white blood cells. In adults, the long bone is filled with fatty, yellow marrow.

Bone tissue is well vascularized. Cancellous bone receives a rich blood supply through metaphyseal and epiphyseal vessels. Periosteal vessels carry blood to compact bone through minute Volkmann’s canals. In addition, nutrient arteries penetrate the periosteum and enter the medullary cavity through foramina (small openings). Nu-trient arteries supply blood to the marrow and bone. The venous system may accompany arteries or may exit independently.

Bone Formation (Osteogenesis)

Bone begins to form long before birth. Ossification is the process by which the bone matrix (collagen fibers and ground substance) is formed and hardening minerals (eg, calcium salts) are deposited on the collagen fibers. The collagen fibers give tensile strength to the bone, and the calcium provides compressional strength.

There are two basic processes of ossification: endochondral and intramembranous. Most bones in the body are formed by en-dochondral ossification, in which a cartilage-like tissue (osteoid) is formed, resorbed, and replaced by bone. Intramembranous os-sification occurs when bone develops within membrane, as in the bones of the face and skull.

Bone Maintenance

Bone is a dynamic tissue in a constant state of turnover— resorption and formation. The important regulating factorsthat determine the balance between bone formation and bone resorption include local stress, vitamin D, parathyroid hor-mone, calcitonin, and blood supply.

Local stress (weight bearing) acts to simulate bone formation and remodeling. Weight-bearing bones are thick and strong. Without weight-bearing or stress, as in prolonged bed rest, the bone loses calcium (resorption) and becomes osteopenic and weak. The weak bone may fracture easily.

Biologically active vitamin D (calcitriol) functions to increase the amount of calcium in the blood by promoting absorption of calcium from the gastrointestinal tract. It also facilitates mineral-ization of osteoid tissue. A deficiency of vitamin D results in bone mineralization deficit, deformity, and fracture.

Parathyroid hormone and calcitonin are the major hormonal regulators of calcium homeostasis. Parathyroid hormone regulates the concentration of calcium in the blood, in part by promoting movement of calcium from the bone. In response to low cal-cium levels in the blood, increased levels of parathyroid hormone prompt the mobilization of calcium, the demineralization of bone, and the formation of bone cysts. Calcitonin, secreted by the thy-roid gland in response to elevated blood calcium levels, inhibits bone resorption and increases the deposit of calcium in bone.

Blood supply to the bone also affects bone formation. With diminished blood supply or hyperemia (congestion), osteogene-sis (bone formation) and bone density decrease. Bone necrosisoccurs when the bone is deprived of blood.

Bone Healing

Most fractures heal through a combination of intramembranous and endochondral ossification processes. When a bone is frac-tured, the bone fragments are not merely patched together with scar tissue. Instead, the bone regenerates itself.

Fracture healing occurs in four areas, including:

·       Bone marrow, where endothelial cells rapidly undergo trans-formation and become osteoblastic bone-forming cells

·      Bone cortex, where new osteons are formed

·      Periosteum, where a hard callus/bone is formed through intra-membranous ossification peripheral to the fracture, and where a cartilage model is formed through endochondral ossification adjacent to the fracture site

·      External soft tissue, where a bridging callus (fibrous tissue) stabilizes the fracture

Buckwalter (2000) summarized the process of fracture healing into six stages stimulated by the release and activation of biologic regulators and signaling molecules:

·        Hematoma and inflammation: The body’s response is similarto that after injury elsewhere in the body. There is bleeding into the injured tissue and formation of a fracture hematoma. The hematoma is the source of signaling molecules, such as cytokines, transforming growth factor-beta (TGF-β), and platelet-derived growth factor (PDGF), which initiate the fracture healing processes. The fracture fragment ends be-come devitalized because of the interrupted blood supply. The injured area is invaded by macrophages (large white blood cells), which débride the area. Inflammation, swelling, and pain are present. The inflammatory stage lasts several days and resolves with a decrease in pain and swelling.

·    Angiogenesis and cartilage formation: Under the influence ofsignaling molecules, cell proliferation and differentiation occur. Blood vessels and cartilage overlie the fracture.

·    Cartilage calcification: Chondrocytes in the cartilage callusform matrix vesicles, which regulate calcification of the car-tilage. Enzymes within these matrix vesicles prepare the cartilage for calcium release and deposit.

·    Cartilage removal: The calcified cartilage is invaded byblood vessels and becomes resorbed by chondroblasts and osteoclasts. It is replaced by woven bone similar to that of the growth plate.

·    Bone formation: Minerals continue to be deposited untilthe bone is firmly reunited. With major adult long bone fractures, ossification takes 3 to 4 months.

·    Remodeling: The final stage of fracture repair consists ofre-modeling the new bone into its former structural arrange-ment. Remodeling may take months to years, depending on the extent of bone modification needed, the function of the bone, and the functional stresses on the bone. Cancel-lous bone heals and remodels more rapidly than does com-pact cortical bone.

Serial x-ray films are used to monitor the progress of bone healing. The type of bone fractured, the adequacy of blood sup-ply, the surface contact of the fragments, and the general health of the person influence the rate of fracture healing. Adequate immobilization is essential until there is x-ray evidence of bone formation with ossification.

BONE HEALING WITH FRAGMENTS FIRMLY APPROXIMATED

When fractures are treated with open rigid compression plate fixation techniques, the bony fragments can be placed in direct contact. Primary bone healing occurs through cortical bone (Haversian) remodeling. Little or no cartilaginous callus develops. Immature bone develops from the endosteum. There is an inten-sive regeneration of new osteons, which develop in the fracture line by a process similar to normal bone maintenance. Fracture strength is obtained when the new osteons have become established.