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Anatomy and Physiology of the Musculoskeletal System

8 min read

The musculoskeletal system serves various functions:

  1. provides protection for organs, including the brain, heart, and lungs;
  2. serves as a framework to support body structures;
  3. makes mobility possible through muscles, tendons, bones, and joints, which also produces heat for maintaining body temperature;
  4. aids in venous return of deoxygenated blood by massaging the venous vasculature; and
  5. is a reservoir for immature blood cells and essential minerals including calcium (more than 98% of total body content), phosphorus, magnesium, and fluoride.

Structure and Function of the Skeletal System

The adult human body contains 206 bones of varying classifications according to shape:

  1. Long: mainly found in the extremities like the femur, and are shaped like rods or shafts with rounded ends.
    • The “shaft” is known as the diaphysis and is primarily composed of cortical bone, a compact type of bone tissue.
    • The ends of the bone are called epiphyses and are primarily composed of porous and spongy cancellous (trabecular) bone.
    • In childhood and adolescence, the diaphysis and epiphyses are separated by the growth plate, or the epiphyseal plate. It is composed of cartilage that nurtures and facilitates longitudinal growth before becoming calcified in adults.
    • The ends of long bones are covered at the joints by articular cartilage, a tough, elastic, and avascular tissue.
  2. Short: smaller bones located in the ankle and hand e.g. the metacarpals.
  3. Flat: found in regions essential for protection e.g. the sternum of the ribcage or the skull.
  4. Irregular: any bones not categorized under any of the previous three, like the vertebrae or the bones of the jaw.

Bone Composition

The shape and construction of specific bones are determined by its function and the forces exerted on it. Bones may either be constructed from strong cortical bone or the spongy cancellous bone.

  • Cortical bone exists in areas where support is needed, such as in the long bones used for weight-bearing and movement.
  • Cancellous bone exists where hematopoiesis and bone formation occur. In the flat bones used for protection while also being important sites of hematopoiesis, cancellous bone is layered between compact bone. Short bones are often cancellous bone covered by a layer of cortical bone.
  • Irregular bones are often similar to flat bones in structure.

Bone Composition

Bone is composed of cells, protein matrix, and mineral deposits.

  • Osteoblasts function in bone formation by secreting bone matrix. Bone matrix consists of collagen and ground substances that provide a framework in which inorganic materials (calcium, phosphorus) are deposited.
  • Osteocytes are mature bone cells involved in bone maintenance, and are located in lacunae.
  • Osteoclasts, located in shallow Hollowship’s lacunae (small pits in bones), are multinuclear cells involved in dissolving (bone matrix is dissolved to maintain the marrow cavity) and resorbing bone.
  • Osteons are the microscopic functioning units of mature cortical bone, or the haversian system.
    • In the center of the osteon (the haversian canal) is a capillary. Surrounding these are lamellae: circles of mineralized bone matrix. In the lamellae are the lacunae that contain osteocytes. The osteocytes are nourished through canaliculi (canals) that communicate with adjacent blood vessels.
  • Within cancellous bone, lacunae are layered in irregular lattice networks known as trabeculae. Red bone marrow fills the lattice network.
    • Red bone marrow is a vascular tissue that covers the medullary cavity of long bones and in flat bones. These are mainly located in the sternum, ilium, vertebrae, and ribs. These produce red blood cells, white blood cells, and platelets through the process of hematopoiesis. In adults, the long bones are filled with fatty, yellow marrow.

The bone is covered by a layer of dense fibrous membrane called the periosteum. This contains nerves, blood vessels, and lymphatics, and attachments points for tendons and ligaments. The periosteum nourishes the bone. The endosteum is a thin, vascular membrane that covers the marrow cavity of long bones and the spaces in cancellous bone.

Bone Formation

Osteogenesis begins before birth. Ossification is the process by which bone matrix is formed and hard minerals are bound to collagen fibers. These mineral components give both its characteristic strength, while proteinaceous collagen give bone its resilience.

Bone Maintenance

Bones are dynamic tissues in a constant state of turnover; old bone is removed and new bone is added (bone remodeling).

  • In children, bone is formed more than bone is dissolved. Bones become larger, heavier, and denser and peak in size and density by 20 years of life.
  • Complete bone turnover occurs approximately every 10 years.

Multiple factors affect the balance of bone formation and resorption: physical activity, dietary intake of certain nutrients, and hormones:

  1. Physical Activity: particularly from weight-bearing activities, this stimulates bone formation and remodeling. Bones subjected to weight-bearing tend to be thick and strong; people who are unable to participate in these activities e.g. bed-ridden, disabled, lose bone mass, becoming osteopenic and weak, increasing risk for fractures.
  2. Nutritional Intake: good dietary habits are integral to bone health.
    • 1000 to 1200 mg of Calcium daily is essential for maintaining adult bone mass. Food sources include low-fat milk, yogurt, cheese, and other milk products. Some foods also have added calcium e.g. orange juice, cereals, and bread.
    • 600 IU of Vitamin D daily plays a vital role in calcium absorption and bone health. Individuals 50 years or older require a higher daily intake of 800 to 1000 IU to ensure good bone health. Food sources include vitamin-D fortified milk and cereals, egg yolks, salt-water fish, and liver.
  3. Hormones:
    • Calcitriol, the activated form of Vitamin D, functions to increase the amount of calcium absorbed in the GIT. It also facilitates mineralization of osteoid tissue.
      • Deficiency results in bone mineralization deficit, deformity, and fracture.
    • Parathyroid Hormone and Calcitonin are the major hormonal regulators for calcium homeostasis. PTH mobilizes calcium, promoting demineralization of the bone and the formation of bone cysts. Calcitonin reacts to elevated calcium levels, inhibiting resorption and increasing deposition of calcium into bone.
    • Thyroid Hormone and Cortisol have multisystemic effects with specific effects on bones.
      • Excessive thyroid hormone, e.g. in Graves’ disease, can result in increased bone resorption and decreased bone formation.
      • Increased levels of cortisol have the same effects. Long-term synthetic cortisol or corticosteroid therapy produce an increased risk for steroid-induced osteopenia and fractures.
    • Growth Hormone has direct (stimulates bone growth) and indirect effects on skeletal growth and remodeling. It stimulates the liver, and to a lesser degree, the bones to produce an Insulinlike Growth Factor 1 (IGF-I) that accelerates bone modeling in children and adolescents.
      • It is believed that low levels of both GH and IGF-I related to aging decreases bone formation and results in osteopenia.
    • Sex Hormones (Estrogen, Testosterone) are important to bone remodeling.
      • Estrogen stimulates osteoblasts (formation) and inhibits osteoclasts (resorption).
      • Testosterone has direct and indirect effects on bone growth. It stimulates skeletal growth in adolescence and skeletal muscle growth throughout life. As a result of skeletal muscle growth, bones are subjected to heavier weight-bearing activities, and results in more bone formation.
        • Testosterone converts to bone-preserving estrogen in adipose tissue for aging men.

Bone Healing

Most fractures (breaks in bone) heal through a combination of intermembranous and endochondral ossification processes. When damaged, the bone begins a healing process to reestablish continuity and strength. The bone regenerates, and does not utilize scar tissue.

  • Endothelial cells in the bone marrow rapidly differentiate into osteoblasts
  • Osteons are formed in the bone cortex
  • A hard callus (fibrous tissue) is formed through intramembranous ossification peripheral to the fracture, and is where cartilage is formed through endochondral ossification adjacent to the fracture and soft tissue, where a bridging callus forms, providing stability.

Healing occurs in four stages, similar to injuries in other parts of the body:

  1. Hematoma Formation: during the first 1 to 2 days. Bleeding into the injured tissue and local vasoconstriction occur. A hematoma forms. Cytokines are released, initiating the healing process by stimulating fibroblasts to proliferate, causing angiogenesis. Granulation tissue begins to form in the clot and becomes dense. Simultaneously, degranulated platelets and inflammatory cells release growth factor, which stimulate the generation of osteoclasts and osteoblasts.
  2. Inflammatory Phase: with the formation of granulation tissue, fibroblasts (fibrocartilaginous soft callus bridge connecting bone fragments) and osteoblasts (new bone) migrate into the fractured site and begins reconstruction. Girth may be restored within three weeks, but weight-bearing ability is still impaired.
  3. Reparative Phase: often within three to four weeks, a firm bony union is formed. Mature bone gradually replaces the fibrocartilaginous callus and the excess callus is reabsorbed by the osteoclasts. The fracture site appears immovable. Cast removal may be safe if used. X-rays show alignment.
  4. Remodeling: necrotic bone is removed by the osteoclasts. Compact bone replaces spongy bone around the periphery of the fracture, resulting in a thickened area on the surface of the bone that may remain after healing. This may take months to years, depending on the extent of modification, function of the bone, and functional stresses. Monitoring is done with serial x-rays to check for progress. Various factors can alter progress, including the type of bone, adequacy of blood supply, condition of fragments, and immobility of the fracture site.
    • If external or internal fixation is used, bone fragments can be placed in direct contact and alignment with each other, minimizing cartilaginous callous formation.

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