Skeletal System—Joints & Bone Study Notes

Bone Matrix and Osteoid

• 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.

Classification of Joints

• What is a Joint (Articulation)?
  Meeting place of ≥2 bones; confers mobility & transmits force.

Structural Classification

• 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)

Functional Classification

• Synarthrosis – immovable (e.g., cranial sutures)

• Amphiarthrosis – slightly movable (e.g., pubic symphysis)

• Diarthrosis – freely movable (all synovial joints)

Joint Movements & Axes

• 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)

Synovial Joint Anatomy & Examples

• 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

Joint Stability, Injuries & Protective Factors

• 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.

Bone Microanatomy – The Osteon (Haversian System)

• 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.

Principles of Bone Adaptation & Hematopoiesis

• 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).

Macroscopic Bone Structure

• 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)

Bone Growth Mechanisms & Hormonal Regulation

• 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.

Bone Formation (Ossification) Pathways

• 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.

Bone Chemistry & Nutrition

• 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.

Bone Cells & Their Roles

• 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.