Osteoid
Unmineralized, organic framework of bone secreted by osteoblasts.
Predominantly type I collagen + proteoglycan-rich ground substance.
Provides flexibility & tensile strength; later impregnated with minerals → hard bone.
Clinical tie-in: defective mineralization (e.g., rickets, osteomalacia) leaves excess osteoid → soft, bendable bones.
What is a Joint (Articulation)?
Meeting place of ≥2 bones; confers mobility & transmits force.
Fibrous (no cavity, dense C.T.; essentially immovable)
Sutures – interlocking skull seams
Syndesmoses – ligament/IO membrane between \text{radius} & \text{ulna}; \text{tibia} & \text{fibula}
Gomphoses – tooth pegged into alveolar socket (keeps teeth in place)
Cartilaginous
Synchondroses – hyaline cartilage (epiphyseal/growth plates, first rib–sternum)
Symphyses – fibrocartilage pads (pubic symphysis, intervertebral discs)
Synovial – fluid-filled cavity; freely movable (all diarthroses)
Synarthrosis – immovable (e.g., cranial sutures)
Amphiarthrosis – slightly movable (e.g., pubic symphysis)
Diarthrosis – freely movable (all synovial joints)
Basic planes
Flexion – ↓ angle
Extension – ↑ angle
Hyperextension – beyond anatomical position
Abduction – away from midline
Adduction – toward midline
Rotation – bone turns around its long axis (medial/lateral)
Circumduction – flex-abd-ext-add in sequence → cone
Special movements
Jaw: elevation / depression
Thumb: opposition / reposition
Foot: inversion / eversion; dorsiflexion / plantarflexion
Axes of motion
Nonaxial = gliding (plane joints; intercarpal)
Uniaxial = 1 plane (hinge elbow, pivot atlanto-axial)
Biaxial = 2 planes (condylar knuckle, saddle thumb MCP)
Multiaxial = 3 planes (ball-and-socket shoulder, hip)
Essential features
Articular (hyaline) cartilage – shock absorber, low friction
Joint (synovial) cavity – potential space
Synovial fluid – ultrafiltrate of blood + hyaluronic acid; nourishes cartilage
Articular capsule – fibrous outer layer + inner synovial membrane
Reinforcing ligaments – capsular, extracapsular, intracapsular
Rich nerve & blood supply – pain, stretch, proprioception; vascular envelopes nourish periarticular tissues
Representative locations
Spine: facet (zygapophyseal) joints
Head: temporomandibular joint (TMJ)
Upper limb: shoulder, elbow, wrist
Lower limb: hip, knee, ankle
Stability hierarchy: hip > elbow > knee > shoulder (trade-off with mobility)
Stabilizers
Bone shape/depth (acetabulum deeper than glenoid → hip more stable)
Ligaments – dense regular C.T.; the more, the merrier (but may stretch)
Tendons & muscle tone – dynamic stabilizers (rotator cuff around shoulder)
Capsule + labra/menisci – deepen sockets, disperse load
Knee injury mechanisms: twisting with foot planted, lateral blows, hyperextension → damage ACL, MCL, menisci.
Concentric lamellae around central (Haversian) canal containing vessels & nerves.
Osteocytes in lacunae connect via canaliculi → nutrient/waste diffusion.
Interstitial & circumferential lamellae fill spaces; perforating (Volkmann) canals link osteons.
Function: weight-bearing pillar; routes blood to deep compact bone.
Wolff’s Law – bone remodels along lines of mechanical stress; explains trabecular orientation & cortical thickening in dominant limb.
Hematopoiesis – formation of blood cells in red marrow (axial skeleton, proximal femur/humerus in adults).
Long Bone Anatomy
Diaphysis – shaft of compact bone surrounding medullary cavity
Epiphyses – ends; spongy bone with red marrow
Epiphyseal plate (growth plate) – hyaline cartilage during growth, becomes epiphyseal line
Articular cartilage – covers epiphyses
Medullary cavity – yellow marrow (fat) in adults
Compact (Cortical) Bone Elements
Osteons, lamellae, lacunae, canaliculi, central canals (see microanatomy)
Appositional growth – osteoblasts beneath periosteum deposit new bone → ↑ diameter.
Interstitial growth (endochondral) – chondrocytes in epiphyseal plate divide → lengthening.
Hormonal influences
GH – stimulates epiphyseal cartilage & osteoblast activity.
Calcitonin – from thyroid; lowers serum \text{Ca}^{2+} by inhibiting osteoclasts.
PTH – raises serum \text{Ca}^{2+} by stimulating osteoclast resorption & renal reabsorption.
Estrogen/Testosterone – pubertal growth spurt; later close plates.
Intramembranous ossification
Mesenchymal cells → osteoblasts that lay osteoid directly; forms flat bones (skull, clavicle).
Endochondral ossification
Hyaline cartilage template → hypertrophy → calcification → vascular invasion → bone; produces most bones.
Clinical relevance: fractures through growth plate (Salter-Harris) can disturb length.
Inorganic matrix – \sim 65\% of mass; mainly hydroxyapatite crystals (\text{Ca}{10}(\text{PO}4)6(\text{OH})2) confer compression strength.
Organic matrix – \sim 35\%; collagen + proteins for tensile strength.
Essential dietary factors
Calcium & phosphate for mineralization
Vitamin D for \text{Ca}^{2+} absorption
Magnesium as co-factor, protein for collagen synthesis
Nutrient deficiency → osteopenia, osteoporosis, rickets.
Osteoprogenitor cells – stem cells in periosteum/endosteum; proliferate during growth & repair.
Osteoblasts – secrete osteoid & alkaline phosphatase; line bone surfaces; convert to osteocytes when encased.
Osteocytes – mature sensors; regulate local remodeling via mechanotransduction; maintain mineral homeostasis.
Osteoclasts – multinucleated macrophage lineage; acidify & release lysosomal enzymes to resorb bone; crucial for remodeling & \text{Ca}^{2+} balance.
Balance between osteoblast & osteoclast activity determines bone mass; dysregulation → osteoporosis or osteopetrosis.