3.7 - Bone Remodeling, Homeostasis, and Fracture Repair

Bone remodeling

The ongoing process in which osteoclasts resorb old or damaged bone and osteoblasts lay down new bone on the same surface; maintains homeostatic balance in adults and children.

Osteolysis

The process by which osteoclasts break down and remove an osteon; balanced by osteoblasts forming a new osteon to replace it.

Appositional bone growth

Growth in bone diameter (thickness) that can continue even after longitudinal growth has stopped; driven by osteoblast activity beneath the periosteum.

How much of the adult skeleton is replaced annually?

About 10–20% of the adult skeleton is replaced each year through normal remodeling, even without injury or exercise.

Effect of mechanical stress on bone

Mechanical stress stimulates osteoblasts to deposit mineral salts and collagen in areas of applied force, increasing bone strength; lack of stress causes bones to lose mineral salts, collagen, and strength.

Why does a casted limb thin (atrophy)?

Immobilization removes mechanical stress, so osteoclast activity exceeds osteoblast activity, causing loss of mineral salts and collagen and reducing bone mass.

Effect of resistance training on bone

Resistance training has a greater effect than cardiovascular exercise on stimulating bone mineral deposition and slowing age-related bone loss; regular exercisers have greater bone density than sedentary individuals.

Dietary needs for bone: calcium and phosphate

Required for osteoblast activity in growth and remodeling; must be regularly consumed in the diet.

Vitamin C role in bone

Essential for collagen synthesis enzymes needed to build bone matrix.

Vitamin A role in bone

Stimulates osteoblast activity.

Vitamins K and B12 role in bone

Necessary for the synthesis of bone proteins.

Vitamin D role in bone

Needed to produce calcitriol, the active hormone that stimulates calcium and phosphate absorption from the digestive tract.

Growth hormone (GH)

Secreted by the pituitary gland; triggers chondrocyte proliferation in epiphyseal plates, increases calcium retention, enhances mineralization, and stimulates osteoblast activity to improve bone density.

Thyroxine

Secreted by the thyroid gland; promotes osteoblastic activity and synthesis of bone matrix.

Sex hormones (estrogen/testosterone)

Promote osteoblast activity and bone matrix production; responsible for the adolescent growth spurt; also trigger closure of the epiphyseal plate, ending longitudinal bone growth.

Calcitriol

The active form of vitamin D, produced by the kidneys; stimulates absorption of calcium and phosphate from the digestive tract.

Parathyroid hormone (PTH)

Released by the parathyroid gland when blood calcium falls below 8.5 mg/dL; stimulates osteoclast proliferation and bone resorption, promotes calcium reabsorption by the kidneys, and indirectly increases calcium absorption by the small intestine — all raising blood calcium.

Calcitonin

Released by C cells of the thyroid gland when blood calcium rises above 10.5 mg/dL; inhibits osteoclast activity, stimulates calcium uptake into bone, causes kidneys to excrete more calcium, and decreases intestinal calcium absorption — all lowering blood calcium.

PTH vs. calcitonin

PTH raises blood calcium (via bone resorption, kidney reabsorption, and intestinal absorption); calcitonin lowers blood calcium (by inhibiting osteoclasts and increasing calcium excretion).

Fracture

A broken bone; can heal even with severe damage if the blood supply and periosteal/endosteal cells survive.

Closed reduction

Manipulation and realignment of a broken bone into its natural position without surgery.

Open reduction

Surgical exposure and realignment of a broken fracture.

Displaced fracture

A fracture that produces abnormal bone alignment.

Nondisplaced fracture

A fracture that maintains normal bone alignment.

Transverse fracture

A break that runs straight across the long axis of the bone.

Spiral fracture

A fracture caused by a twisting force, producing a ragged diagonal break; common sports fracture.

Comminuted fracture

Multiple breaks that result in several small bone fragments between two larger segments; often requires surgery; more common in the elderly.

Impacted fracture

One bone fragment is driven into the other, typically from compression (e.g., falling on outstretched arms).

Greenstick fracture

A partial fracture where only one side of the bone breaks; common in children whose bones are more flexible.

Compression fracture

The bone is crushed; common in osteoporotic bones of older individuals.

Depressed fracture

Broken bone fragment is pressed inward; typical of skull fractures.

Open (compound) fracture

At least one end of the broken bone pierces through the skin; carries high risk of infection.

Closed (simple) fracture

A fracture where the skin remains intact over the break.

Four stages of fracture repair

(1) Fracture hematoma forms; (2) fibrocartilage callus (internal and external calli) forms; (3) bony callus replaces cartilage via endochondral ossification; (4) remodeling restores compact bone.

Fracture hematoma

A blood clot that forms 6–8 hours after a fracture as blood from torn vessels pools at the break site; disruption of blood flow causes death of nearby bone cells.

Fibrocartilage callus

Forms ~48 hours after fracture; internal callus (from endosteum chondrocytes) fills between bone ends with fibrocartilaginous matrix, while external callus (from periosteal cells) forms a hyaline cartilage/bone collar around the fracture to stabilize it.

Bony callus

Forms over several weeks as osteoclasts remove dead bone and osteoblasts replace the fibrocartilage callus with trabecular (spongy) bone via endochondral ossification; this is when a cast can typically be removed.

Final stage of fracture repair

Internal and external calli unite, compact bone replaces spongy bone at fracture margins; remodeling may continue 4 months to 1 year post-injury until no external evidence of fracture remains.