Bone Growth and Healing

Woven Bone: Appears during embryonic development, with randomly arranged collagen fibers. Replaced by lamellar bone.
Lamellar Bone: Adult skeleton bone, divided into compact and spongy (cancellous) bone.
- Compact Bone: Dense outer layer, organized in concentric circles, providing support and protection (80% of skeleton weight).
- Spongy (Cancellous) Bone: Inner layer with trabecular pattern, high surface area, flexible, and highly vascular (contains bone marrow).

Endochondral Ossification: Hyaline cartilage model replaced by bone tissue, forming long bones.
- Chondrocytes hypertrophy, secreting calcified cartilage.
- Primary ossification center forms in the middle of the bone.
- Secondary ossification occurs at the physis.
Intramembranous Ossification: Connective tissue membrane sheets replaced by bone tissue, forming flat bones.
- Mesenchymal cells differentiate into osteoblasts, which secrete osteoid.
- Osteoid calcifies to form bone tissue.
- Osteoblasts mature into osteocytes when surrounded by bone matrix.

Occurs on the surface of bones.
Osteoblasts in the periosteum secrete new bone matrix, increasing bone diameter.
Osteoclasts in the endosteum resorb bone, enlarging the medullary cavity.

Osteoblasts secrete new bone matrix from the periosteum, initially as woven bone, then remodeled into lamellar bone.
Lamellae are concentric layers forming osteons (cylindrical structures).
Each osteon surrounds a Haversian canal containing blood vessels and nerves for nutrient exchange
Bone remodelled, osteoblasts mature into osteoclasts, inner endosteum bone resorbed so bone not too thick

Trabeculae form via osteoblasts in the endosteum secreting bone matrix.
Trabeculae arranged in a network for structural support and flexibility.
Bone marrow spaces between trabeculae are involved in blood cell production.
Adapts to mechanical stress by remodeling trabeculae via osteoblasts and osteoclasts.
Porous bone for blood vessels and bone marrow for nutrient exchange

Reserve Zone: Adjacent to the epiphysis, contains inactive chondrocytes.
Proliferating Zone: Chondrocytes flatten, divide, and align into columns which lengthen the column and epiphysis
Hypertrophic Zone: Chondrocytes near metaphysis enlarge and mature, weakest point of the physis and common area for fractures
Zone of Provisional Ossification: Chondrocytes calcify by osteoblast activity, blood vessels invade from metaphysis, and bone forms along cartilage matrix.

Bone adapts to the loads under which it is placed.
Increased loading leads to stronger bone; decreased loading leads to weaker bone.
$\text{Examples:}$
- Athletes develop denser bones in frequently used limbs.
- Weightlifting increases bone density.
- Astronauts experience bone density loss in microgravity.

Hormones (growth hormone, vitamin D, estrogen, testosterone) regulate bone healing.
Enables calcium to be removed and used for other metabolic processes.
Ensures correct calcium levels avoiding hyper or hypocalcaemia.
Imbalance of hormones can cause reduction of bone matrix.

Clot Formation (Week 1): Bleeding from osteoid tissue, Hematoma forms, fibroblasts differentiate into chondroblasts which differentiate into hyaline cartilage
Callus Formation (Weeks 2-3):
- Soft callus: osteoid tissue and chondroblasts are generated.
- Osteoprogenitor cells create woven bone
Callus Ossification (Weeks 4-16):
- Hard callus forms with calcium and phosphate.
- Woven bone is replaced by lamellar bone.
Bone Remodeling (Weeks 17+): Normal cortical structure is restored.

Non-union: Fracture fails to heal.
Mal-union: Bone heals misaligned.
Factors: Age, health, diet, underlying conditions (osteoporosis - reduced bone mineralisation)(hyperthyroidism - altered level of calcium in blood), pathological fracture from tumours, poor blood supply, premature weight bearing, complex/compound fracture with multiple fragments or avascular necrosis where bone loses blood supply and dies