BONE COMPOSITION AND STRUCTURE
Hierarchical Organization | 65% Inorganic | Type I Collagen | Lamellar Architecture
BONE TYPES
Critical Must-Knows
- Bone is composite material - mineral provides stiffness, collagen provides toughness
- Hydroxyapatite Ca10(PO4)6(OH)2 is the inorganic mineral phase
- Type I collagen (90% of organic matrix) provides tensile strength
- Lamellar bone is organized mature bone, woven bone is immature
- Hierarchical structure: mineral crystals to lamellae to osteons to whole bone
Examiner's Pearls
- "Bone mineral density increases with hydroxyapatite deposition
- "Collagen fibril orientation determines mechanical anisotropy
- "Osteoid is unmineralized organic matrix (14-day lag before mineralization)
- "Cortical bone has lower remodeling rate than cancellous (3-5% vs 20-25% per year)
Clinical Imaging
Imaging Gallery
Critical Bone Structure Exam Points
Composite Nature
Bone is a composite material combining mineral (stiffness) and organic matrix (toughness). Remove mineral and bone bends like rubber. Remove collagen and bone shatters like chalk. Both are essential.
Hydroxyapatite
Ca10(PO4)6(OH)2 is the key mineral. Calcium and phosphate ions substitute (carbonate for PO4, fluoride for OH) affecting bone quality. Fluoride increases density but brittleness.
Type I Collagen
90% of organic matrix is Type I collagen. Triple helix structure with crosslinks provides tensile strength. Collagen diseases (osteogenesis imperfecta) cause bone fragility.
Hierarchical Organization
Seven levels: mineral nanocrystals to collagen fibrils to lamellae to osteons to cortical/trabecular bone to whole bone. Defects at any level compromise strength.
At a Glance
Bone is a composite material comprising 65% inorganic mineral (hydroxyapatite Ca₁₀(PO₄)₆(OH)₂), 25% organic matrix, and 10% water. The mineral phase provides stiffness while Type I collagen (90% of organic matrix) provides tensile strength and toughness—remove mineral and bone bends like rubber; remove collagen and it shatters like chalk. The hierarchical organization spans seven levels from mineral nanocrystals to collagen fibrils to lamellae to osteons to whole bone. Cortical (compact) bone constitutes 80% of skeletal mass with slower remodeling (3-5% per year), while cancellous (trabecular) bone accounts for 80% of bone surface area with faster turnover (20-25% per year). Osteoid is unmineralized matrix with a 14-day lag before mineralization occurs.
BONEBONE - Key Components
Memory Hook:BONE is both mineral and organic working together in organized layers
COLLAGENCOLLAGEN - Organic Matrix
Memory Hook:COLLAGEN provides the organic scaffold with crosslinks for tensile strength
Overview
Bone is a specialized connective tissue that serves multiple functions: structural support, protection of vital organs, mineral homeostasis (calcium and phosphate reservoir), and hematopoiesis (bone marrow).
Why bone structure matters clinically:
Fracture Biomechanics
Understanding bone composition explains fracture patterns. High-energy trauma overcomes both mineral (compression) and collagen (tension) components. Osteoporotic bone has adequate mineral but poor microarchitecture.
Disease States
Osteogenesis imperfecta (collagen defect), osteomalacia (mineralization defect), and Paget disease (abnormal remodeling) all affect bone composition differently.
Composite Material Concept
Bone is nature's composite material combining the stiffness of mineral with the toughness of collagen. Remove the mineral (acid demineralization) and bone becomes flexible like rubber. Remove the collagen (heating) and bone becomes brittle like chalk. Both components are essential for normal mechanical properties.
Principles of Bone Composition
Hydroxyapatite: Ca10(PO4)6(OH)2
The mineral phase comprises 65% of bone weight and provides compressive strength and stiffness.
Crystal Structure
Chemical formula: Ca10(PO4)6(OH)2
Crystal dimensions:
- Length: 50 nm
- Width: 25 nm
- Thickness: 2-3 nm
- Hexagonal crystal structure
Location: Crystals deposit within and between collagen fibrils, aligned with fibril long axis.
Understanding crystal size and orientation is important for bone biomechanics.
Organic Component - Matrix Proteins
The organic matrix comprises 25% of bone weight and provides tensile strength and toughness.
Type I Collagen - 90% of Organic Matrix
Structure:
- Triple helix: two alpha-1(I) chains + one alpha-2(I) chain
- Length: 300 nm (tropocollagen molecule)
- Diameter: 1.5 nm
- Gly-X-Y amino acid repeat (glycine every 3rd residue)
Assembly:
- Intracellular: Procollagen synthesis, hydroxylation (requires Vitamin C)
- Extracellular: Procollagen to tropocollagen (cleavage of N- and C-propeptides)
- Fibril formation: Tropocollagen self-assembles into fibrils (67 nm periodicity)
- Crosslinking: Lysyl oxidase creates covalent crosslinks (pyridinoline, deoxypyridinoline)
Osteogenesis Imperfecta
Osteogenesis imperfecta is caused by mutations in COL1A1 or COL1A2 genes, producing abnormal Type I collagen. This results in brittle bones with multiple fractures, blue sclerae, and hearing loss. The organic scaffold is defective despite normal mineralization.
Collagen provides the organic scaffold upon which mineral deposits.
Hierarchical Structure of Bone
Seven levels of organization: from nanoscale to whole bone level.
Hierarchical Levels of Bone Structure
| Level | Structure | Size Scale | Key Feature |
|---|---|---|---|
| 1. Molecular | Tropocollagen + hydroxyapatite crystals | 1-100 nm | Mineral in collagen gap zones |
| 2. Nanoscale | Mineralized collagen fibrils | 100-1000 nm | 67 nm periodicity (D-band) |
| 3. Submicron | Fibril arrays (lamellae) | 1-10 micrometers | Parallel fibers in each lamella |
| 4. Microstructure | Osteons (Haversian systems) | 100-300 micrometers | Concentric lamellae around central canal |
| 5. Mesostructure | Cortical vs trabecular bone | 0.1-1 mm | Dense vs porous architecture |
| 6. Macrostructure | Whole bone regions | 1-10 mm | Diaphysis, metaphysis, epiphysis |
| 7. Organ | Whole bone | 10-100 mm | Integrated mechanical structure |
Hierarchical Failure
Bone strength depends on integrity at all hierarchical levels. Osteoporosis (trabecular thinning at mesostructure level), osteogenesis imperfecta (collagen defect at molecular level), and osteomalacia (mineralization defect at nanoscale) all compromise bone strength through different mechanisms.
Understanding hierarchical structure explains how different diseases affect bone strength.
Cortical and Trabecular Bone
Cortical (Compact) Bone
Key characteristics:
- 80% of skeletal mass
- 20% of bone surface area
- Porosity: 5-10%
- Remodeling rate: 3-5% per year
- Location: diaphyses of long bones, outer shell of all bones
Structure - Osteons (Haversian systems):
- Central Haversian canal (blood vessels, nerves)
- Concentric lamellae (4-20 layers)
- Osteocyte lacunae and canaliculi
- Cement line boundary
- Diameter: 200-300 micrometers
Secondary osteons result from remodeling, surrounded by cement lines (reversal lines).
Cement Lines
Cement lines mark the boundary of remodeling cycles. They are hypermineralized and weaker than surrounding bone, serving as sites for crack initiation but also deflection (toughening mechanism).
Cortical bone provides mechanical strength and protection.
Lamellar versus Woven Bone
Lamellar vs Woven Bone
| Feature | Lamellar Bone (Mature) | Woven Bone (Immature) |
|---|---|---|
| Collagen organization | Highly organized, parallel fibers | Random, disorganized fibers |
| Formation rate | Slow (1-2 micrometers/day) | Rapid (4-6 micrometers/day) |
| Mechanical strength | High | Low |
| Osteocyte density | Low | High |
| When present | Normal adult bone, remodeling | Fracture callus, fetal bone, Paget disease |
Lamellar bone:
- Organized structure with collagen fibers parallel within each lamella
- Alternating fiber orientation between lamellae (plywood-like)
- Slow deposition allows optimal organization
Woven bone:
- Rapidly formed during fracture healing
- Disorganized collagen orientation
- Weaker and more flexible than lamellar bone
- Remodeled to lamellar bone over months
Paget Disease
Paget disease produces abnormal bone with both woven and lamellar patterns (mosaic pattern). The rapid, disorganized remodeling produces weak bone despite increased density. Jigsaw puzzle appearance on histology.
Understanding lamellar versus woven bone helps interpret fracture healing and bone pathology.
Clinical Relevance and Applications
Understanding bone composition and structure directly informs clinical decision-making in orthopaedic practice.
Clinical Correlations of Bone Structure
| Structural Level | Disease/Condition | Clinical Implication |
|---|---|---|
| Mineral phase | Osteomalacia/Rickets | Vitamin D supplementation, correct underlying cause |
| Collagen (organic) | Osteogenesis imperfecta | Bisphosphonates, fracture prevention, genetic counseling |
| Trabecular microarchitecture | Osteoporosis | Antiresorptive therapy, fracture risk assessment |
| Cortical porosity | Age-related bone loss | Monitoring with DXA, fall prevention |
| Remodeling cycle | Paget disease | Bisphosphonates to normalize remodeling |
Implant Considerations
Bone quality affects implant fixation. Osteoporotic bone has reduced holding power for screws (trabecular loss reduces surface area for fixation). Cortical bone provides better screw purchase than cancellous bone. Consider cement augmentation or alternative fixation strategies in poor quality bone.
Fracture Healing Implications
- Woven bone forms first in fracture callus
- Remodeling to lamellar bone takes 12-18 months
- Smoking impairs collagen synthesis
- Vitamin D deficiency delays mineralization
- NSAIDs may inhibit bone healing
Surgical Planning Applications
- Assess bone quality on preoperative imaging
- Cortical thickness guides plate selection
- Trabecular patterns influence screw trajectory
- Bone density affects implant choice
- Consider bone grafting for defects
Evidence Base
Bone as Composite Material
- Bone mineral provides stiffness and compressive strength
- Collagen provides tensile strength and toughness
- Removing mineral makes bone flexible (elastic modulus drops 95%)
- Removing collagen makes bone brittle (fracture toughness drops 60%)
Hierarchical Structure of Bone
- Seven hierarchical levels from nanoscale to whole bone
- Mechanical properties emerge from organization at each level
- Failure can initiate at any level depending on loading mode
- Toughness mechanisms operate at multiple scales
Hydroxyapatite Crystal Structure in Bone
- Hydroxyapatite crystals are nanoscale (50×25×2-3 nm)
- Crystals deposit in gap zones of collagen fibrils (67 nm periodicity)
- Crystal orientation follows collagen fibril direction
- Mineral-collagen interface is critical for mechanical properties
Basic Science Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Bone Composition (~3 min)
"Describe the composition of bone and explain how each component contributes to its mechanical properties."
Scenario 2: Hierarchical Structure (~4 min)
"Explain the hierarchical organization of bone from the molecular level to whole bone. How does this relate to fracture risk in osteoporosis?"
MCQ Practice Points
Bone Composition
Q: What is the approximate composition of bone by weight?
A: 65% inorganic mineral (hydroxyapatite), 25% organic matrix (90% Type I collagen), and 10% water. The inorganic phase provides stiffness and compressive strength, while the organic matrix provides toughness and tensile strength.
Hydroxyapatite Formula
Q: What is the chemical formula for hydroxyapatite, the primary mineral in bone?
A: Ca₁₀(PO₄)₆(OH)₂ - calcium phosphate hydroxide. Calcium and phosphate ions can be substituted (e.g., carbonate for phosphate, fluoride for hydroxyl) which affects bone quality. Fluoride increases density but also increases brittleness.
Cortical vs Cancellous Bone
Q: What is the key difference between cortical and cancellous bone in terms of remodeling rate?
A: Cortical bone: 3-5% annual turnover rate, comprises 80% of skeletal mass Cancellous bone: 20-25% annual turnover rate, comprises only 20% of mass but 80% of surface area
The higher surface area of cancellous bone explains its faster turnover and greater susceptibility to metabolic bone diseases.
Osteoid Mineralization
Q: What is the typical lag time between osteoid deposition and mineralization?
A: 10-14 days. Osteoid is unmineralized organic matrix secreted by osteoblasts. The mineralization lag time is clinically relevant - increased lag indicates osteomalacia, while decreased lag may indicate impaired matrix maturation.
Australian Context
FRACS Examination Relevance
Basic Science Viva:
- Bone composition is a core basic science topic
- Expect questions on hydroxyapatite formula (Ca₁₀(PO₄)₆(OH)₂)
- Know Type I collagen structure and crosslinks
- Understand composite material concept (mineral vs collagen)
- Hierarchical structure from nanoscale to whole bone
Key Examination Points:
- 65:25:10 ratio (mineral:organic:water)
- Collagen provides toughness, mineral provides stiffness
- Cortical vs trabecular remodeling rates (3-5% vs 20-25%)
- Osteoid mineralization lag time (10-14 days)
- Woven vs lamellar bone characteristics
Common Questions:
- Describe bone composition and mechanical implications
- Explain hierarchical organization of bone
- Compare cortical and cancellous bone properties
- Discuss clinical correlations (OI, osteomalacia, osteoporosis)
Management Algorithm
BONE COMPOSITION AND STRUCTURE
High-Yield Exam Summary
Composition by Weight
- •65% inorganic (hydroxyapatite mineral)
- •25% organic (90% Type I collagen + 10% non-collagenous proteins)
- •10% water
- •Composite material: mineral = stiffness, collagen = toughness
Hydroxyapatite Mineral
- •Ca10(PO4)6(OH)2 chemical formula
- •Crystal size: 50nm × 25nm × 2-3nm (nanoscale)
- •Deposits in collagen gap zones (67 nm periodicity)
- •Ion substitutions: carbonate, fluoride, strontium affect properties
Collagen and Organic Matrix
- •Type I collagen = 90% of organic matrix
- •Triple helix (two alpha-1, one alpha-2 chain), 300 nm length
- •Crosslinks: pyridinoline and deoxypyridinoline (enzymatic)
- •Non-collagenous: osteocalcin, osteopontin, osteonectin, BSP
Hierarchical Levels
- •Level 1-2: Tropocollagen + crystals → mineralized fibrils (67 nm)
- •Level 3-4: Lamellae → osteons (200-300 micrometers)
- •Level 5: Cortical (dense, 80% mass) vs trabecular (porous, 80% surface)
- •Levels 6-7: Whole bone regions and organ
Cortical vs Trabecular
- •Cortical: 80% mass, 5-10% porosity, 3-5% remodeling/year
- •Trabecular: 20% mass, 80% surface, 50-90% porosity, 20-25% remodeling/year
- •Trabecular affected first in osteoporosis (higher surface area)
- •Cortical provides strength, trabecular provides metabolic reserve
Key Clinical Correlations
- •Osteogenesis imperfecta: Type I collagen defect (brittle bones)
- •Osteomalacia: Mineralization defect (soft bones, excess osteoid)
- •Osteoporosis: Multi-level failure (trabecular loss, cortical porosity)
- •Paget disease: Woven bone mosaic pattern (weak despite high density)