Courtesy: Amr Abdelgawad, Maimonaides Medical Centre, NY, USA

Types of Bone

Organized (Lamellar) Bone

• Mature, well-organized bone

• Provides strength and structural stability

---

Cortical Bone

Location

• Forms the outer cortex of long bones

• Predominantly seen in the diaphysis (shaft)

Characteristics

• Dense with low porosity

• Low surface area

• High strength and stiffness

• Lower metabolic activity

---

Cancellous (Trabecular) Bone

Location

• Found mainly in the metaphysis and epiphysis

Characteristics

• Highly porous

• Large surface area

• Less stiff than cortical bone

• Higher metabolic activity

---

Woven Bone

Features

• Disorganized collagen structure

• Mechanically weaker

Clinical Relevance

• Seen in:

  • Early fracture healing

  • Callus formation

---

Bone Structure and Composition

Inorganic Component (Mineral Phase)

• Constitutes 60–70% of bone

• Composed mainly of:

  • Calcium

  • Phosphate

Form

• Present as hydroxyapatite crystals

Chemical Formula

• Ca??(PO?)?(OH)?

---

Organic Matrix

• Accounts for ~30% of bone

• Mainly Type I collagen (~90%)

---

Water Content

• Approximately 5–8% of bone

---

Bone Cells

Osteoclast

Origin

• Derived from hematopoietic (monocyte–macrophage) lineage

Function

• Bone resorption

Features

• Multinucleated giant cells

• Ruffled border and sealing zone

Mechanism

• Carbonic anhydrase — hydrogen ions — mineral dissolution

• Cathepsins and MMPs — organic matrix breakdown

---

RANK–RANKL–OPG Pathway

• RANK — present on osteoclast precursors

• RANKL – produced by osteoblasts ? promotes osteoclast formation

• Osteoprotegerin (OPG) – inhibits RANKL

Clinical Note

• Denosumab blocks RANKL – reduces bone resorption

---

Factors Increasing RANKL

• Interleukin-1

• Corticosteroids

• Vitamin D

• Continuous hyperparathyroidism

---

Osteoblast

Origin

• Mesenchymal stem cells

Function

• Bone formation and matrix production

Key Marker

• Osteocalcin

  • Vitamin K dependent

  • Produced by mature osteoblasts

  • Marker of bone formation

---

Important Gene

• RUNX2 (CBFA1)

  • Controls osteoblast differentiation

  • Mutation – cleidocranial dysplasia

---

Osteocyte

Origin

• Derived from osteoblasts

Function

• Mechanosensation and regulation of remodeling

Features

• Communicate via canaliculi

• Less metabolically active

---

Growth Plate (Physis)

Function

• Responsible for longitudinal growth of long bones

• Present only in growing skeleton

---

Zones

Resting Zone

• Small chondrocytes

• Low metabolic activity

• Abundant matrix

---

Proliferative Zone

• Rapid cell division

• Columnar arrangement

---

Hypertrophic Zone

• Enlarged chondrocytes

Subdivisions

• Maturation

• Degeneration

• Provisional calcification

  Most physeal fractures occur in the zone of provisional calcification

---

Supporting Structures

Perichondral Ring of LaCroix

• Provides mechanical support

Groove of Ranvier

• Responsible for appositional growth

---

Collagen

Structure

• Triple helix with three alpha chains

• Repeating sequence: Gly–X–Y

  • X and Y commonly proline or hydroxyproline

---

Types

• Type I – Bone, skin, annulus fibrosus

• Type II – Cartilage

• Type IX & XI – Minor cartilage types

• Type X – Hypertrophic chondrocytes

---

Articular Cartilage

Composition

Cells

• Chondrocytes (~2%)

Extracellular Matrix (~98%)

• Water

• Type II collagen

• Proteoglycans

---

Layers

Superficial Zone

• 10–15% thickness

• Collagen parallel

• Highest tensile strength

• High water, low proteoglycan

---

Middle Zone

• Transitional

---

Deep Zone

• Collagen perpendicular

• High proteoglycan

• Columnar cells

---

Calcified Zone

• Anchors cartilage to bone

• Separated by tidemark

---

Glycosaminoglycans & Proteoglycans

Glycosaminoglycans

• Linear polysaccharides

Examples

• Chondroitin sulfate

• Keratan sulfate

---

Proteoglycans

Monomer

• Core protein + GAG chains

Aggregate

• Multiple monomers attached to hyaluronic acid

---

Function

• Bind water

• Provide resistance to compression

---

Cartilage Metabolism

Chondroprotective Factors

• Transforming growth factor-beta

• BMP-2, BMP-7

• Insulin-like growth factor

• Glucosamine

Effect

• Increase collagen and proteoglycan synthesis

---

Cartilage-Degrading Factors

• Interleukin-1

• Tumor necrosis factor

• Matrix metalloproteinases

• Cyclooxygenase enzymes

Effect

• Increase cartilage breakdown

• Decrease proteoglycan synthesis

---

Aging vs Osteoarthritis

---

Synovial Joint

Synovium

• Intimal layer

• Subintimal connective tissue

---

Cell Types

Type A Cells

• Macrophage-like

• Phagocytic

Type B Cells

• Fibroblast-like

• Produce synovial fluid

---

Synovial Fluid

• Contains hyaluronic acid

• Low protein content

• Non-Newtonian fluid

---

Joint Lubrication

Elastohydrodynamic Lubrication

• Fluid film with surface deformation

---

Boundary Lubrication

• Occurs when fluid layer is minimal

• Depends on surface molecules

---

Lubricin

• Produced by superficial chondrocytes

• Works with hyaluronic acid

• Reduces friction

---

Key Takeaways

• Cortical bone – strong, low metabolic activity

• Cancellous bone – porous, high metabolic activity

• Woven bone – early healing indicator

• RANK–RANKL–OPG regulates bone resorption

• Growth plate zones are crucial in pediatric injuries

• Proteoglycans provide compressive strength

• Cartilage health depends on balance of synthesis and degradation

• Synovial lubrication is essential for joint function