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.