I’m a orthopaedics resident in 1st year teach me basics of it
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I have rich, comprehensive foundational content from Miller's Review of Orthopaedics. Now let me compile a thorough, well-organized answer for a 1st-year orthopaedics resident.
Orthopaedics Basics for the 1st Year Resident
Welcome to orthopaedics! Here is a structured primer covering the foundational science you need as a base for everything else you will learn.
Source: Miller's Review of Orthopaedics, 9th Edition
1. BONE - The Core Tissue
Types of Bone
Microscopic classification:
| Type | Characteristics | Clinical Example |
|---|---|---|
| Lamellar | Normal bone; stress-oriented; organized; stronger | Femoral shaft (cortical), distal femur (cancellous) |
| Woven | Immature/pathologic; random organization; high turnover; weaker | Fracture callus, embryonic skeleton, osteosarcoma |
Macroscopic classification:
-
Cortical (compact) bone - 80% of skeleton
- Slow turnover; high Young's modulus; stiff
- Made of tightly packed osteons (Haversian systems)
- Haversian canals (longitudinal) connect to Volkmann canals (transverse)
- Cement lines = outer border of each osteon
- Nutrition via intraosseous circulation through canaliculi (osteocyte cell processes)
-
Cancellous (trabecular/spongy) bone - 20% of skeleton
- High surface area; faster turnover; more elastic than cortical
- Found at metaphyses and epiphyses
Bone Cells (the "BLAST-CLAST" concept)
| Cell | Origin | Function |
|---|---|---|
| Osteoblast | Mesenchymal stem cell | Bone formation; produces type I collagen + osteoid |
| Osteocyte | Mature osteoblast embedded in matrix | Mechanosensing; regulates remodeling via lacunocanalicular network |
| Osteoclast | Hematopoietic monocyte lineage | Bone resorption; ruffled border secretes acid + proteases |
Key concept - RANKL/OPG axis:
- RANKL (on osteoblasts) activates osteoclasts
- OPG (osteoprotegerin, from osteoblasts) decoys RANKL and inhibits osteoclastogenesis
- This balance is targeted by denosumab (anti-RANKL antibody used in osteoporosis/bone mets)
2. FRACTURE BIOLOGY - What Happens When Bone Breaks
The 3 Phases of Fracture Healing
Phase 1 - Inflammation (days 1-7)
- Fracture hematoma forms; hematopoietic cells release growth factors (BMP, TGF-β, PDGF, FGF)
- Fibroblasts, mesenchymal cells, and osteoprogenitor cells recruited
- Granulation tissue forms around fracture ends
Phase 2 - Repair (weeks 2-6)
- Primary callus response within 2 weeks
- Bridging (soft) callus forms at bone ends not in continuity - via endochondral ossification (cartilage replaced by woven bone = "hard callus")
- Medullary callus forms later
- Type II collagen expressed early (unstable fractures) → followed by type I collagen
Phase 3 - Remodeling (months to years)
- Begins mid-repair phase, continues up to 7 years
- Woven bone replaced by lamellar bone
- Guided by Wolff's Law - bone remodels along lines of mechanical stress
- Piezoelectric charges from bone deformation also guide remodeling
- Complete when marrow space is repopulated
Fracture Healing by Treatment Method
| Stabilization | Healing Type |
|---|---|
| Cast (closed) | Periosteal bridging callus + endochondral ossification |
| Compression plate | Primary cortical healing (cutting-cone/Haversian remodeling) |
| Intramedullary nail | Early: periosteal bridging callus; Late: medullary callus |
| Inadequate immobilization + good blood supply | Hypertrophic nonunion |
| Inadequate immobilization + poor blood supply | Atrophic nonunion |
Key rule: Amount of callus is inversely proportional to extent of immobilization. Rigid fixation = less callus (primary healing). Flexible fixation = more callus (secondary healing).
Factors That Impair Fracture Healing
- Nicotine/smoking - increases time to healing, increases nonunion risk (up to 500% increase in lumbar pseudarthrosis), weakens callus
- NSAIDs - inhibit COX → impair prostaglandin-mediated bone healing
- Steroids - inhibit osteoblasts
- Poor nutrition, diabetes, peripheral vascular disease, radiation
Key Growth Factors
| Growth Factor | Role |
|---|---|
| BMP (Bone Morphogenetic Protein) | Osteoinductive; converts mesenchymal cells into osteoblasts via SMAD signaling |
| TGF-β | Induces type II collagen and proteoglycans; stimulates osteoblast collagen synthesis |
| PDGF | Chemotactic; stimulates fibroblast proliferation early |
| FGF | Angiogenesis; fibroblast proliferation |
| IGF-1 | Stimulates osteoblast activity and matrix synthesis |
3. ARTICULAR CARTILAGE - The Joint Surface
Key properties:
- Avascular, aneural, alymphatic - nutrients come from synovial fluid and subchondral bone by diffusion
- Coefficient of friction < ice on ice (0.002-0.04 in a healthy joint)
- Withstands impact loads up to 25 N/mm²
- Viscoelastic - properties change with rate of force application
- Anisotropic - properties vary with direction of force
- Heals poorly - this is why articular damage is so significant clinically
Composition of Hyaline Cartilage
| Component | Amount | Notes |
|---|---|---|
| Water | ~75% | Highest at superficial zone (80%); decreases with aging; increases in OA |
| Type II collagen | ~15% dry weight (90-95% of cartilage collagen) | Triple helix from COL2A1 gene; defects cause SED, achondrogenesis |
| Proteoglycans | ~10% dry weight | Aggrecan (main), lubricin (surface friction) |
| Chondrocytes | <5% | Only cells; maintain ECM; no regenerative capacity after damage |
Cartilage Layers (Zones)
- Superficial zone - collagen fibers parallel to surface; highest water content; most cells
- Middle (transitional) zone - oblique collagen; larger cells
- Deep zone - collagen perpendicular to surface; lowest water content; largest proteoglycans
- Calcified zone - separated from deep zone by tidemark; type X collagen; anchors to subchondral bone
4. MUSCLE - The Motor
Structural Hierarchy (outside to inside)
Muscle → Epimysium → Fascicles (surrounded by Perimysium) → Fibers → Endomysium → Myofibrils → Sarcomeres
The Sarcomere (basic contractile unit)
- A-band (dark) = thick filaments (myosin) - does NOT shorten during contraction
- I-band (light) = thin filaments (actin) - shortens during contraction
- H-band = myosin only (between two I-bands)
- Z-line = borders the sarcomere; contains desmin, α-actinin, filamin
- M-line = center of sarcomere
Sliding filament mechanism: Calcium released from sarcoplasmic reticulum → binds troponin → tropomyosin shifts → actin-myosin cross-bridge forms → ATP-driven power stroke → contraction
Muscle Fiber Types
| Type | Speed | Fatigue | Metabolism | Example |
|---|---|---|---|---|
| Type I (slow twitch) | Slow | Fatigue-resistant | Oxidative | Postural muscles (soleus) |
| Type II A | Fast | Intermediate | Oxidative + glycolytic | Mixed use muscles |
| Type II B/X | Fastest | Fatigues quickly | Glycolytic | Sprinting muscles |
Malignant hyperthermia - caused by abnormality of ryanodine receptors (RYR-1) → uncontrolled calcium release → hyperthermia, muscle rigidity. Treat with dantrolene (decreases Ca²⁺ release from SR).
5. TENDON & LIGAMENT
Tendon Composition
- Water: 50-60% total weight
- Type I collagen: 75% of dry weight (85% of the collagen fraction)
- Elastin: 1-2% - responsible for the "toe region" of the stress-strain curve
- Proteoglycans: up to 5% (decorin is the most predominant)
Tenocytes (fibroblasts): synthesize ECM, produce MMPs, detect strain via cell cilia
- In response to rupture: produce type III collagen (weaker → predisposes to re-rupture)
- Greater proportion of type III in Achilles tendon = explains high rupture rate
Structure: Hierarchy
Tropocollagen → Microfibrils → Fibrils → Fascicles (surrounded by endotendon) → Tendon (surrounded by epitendon/paratenon)
Tendon-Bone Insertion (Enthesis)
Four zones: tendon → uncalcified fibrocartilage → calcified fibrocartilage → bone
- Separated by a tidemark (same concept as cartilage)
- This gradual transition reduces stress concentration
6. THE STRESS-STRAIN CURVE - Biomechanics You Must Know
Every orthopaedic tissue has a stress-strain curve with:
- Toe region - collagen fibers uncrimping (elastin dominant)
- Linear (elastic) region - reversible deformation; follows Hooke's law
- Yield point - transition to plastic deformation
- Plastic region - irreversible deformation (microfractures)
- Failure point - complete rupture
Young's modulus (E) = stiffness = slope of linear region
- Cortical bone: high E (very stiff)
- Cancellous bone: lower E (more elastic)
- Cartilage: lowest E (most deformable)
7. NERVE INJURIES - Seddon & Sunderland Classification
| Seddon | Sunderland | What's Injured | Recovery |
|---|---|---|---|
| Neuropraxia | Grade I | Myelin only; axon intact | Complete, weeks-months |
| Axonotmesis | Grades II-IV | Axon disrupted; endoneurium intact (II), perineurium disrupted (III), epineurium intact (IV) | Possible; grows 1 mm/day |
| Neurotmesis | Grade V | Complete nerve division | Poor without surgery |
Nerve regrowth rate: ~1 mm/day or 1 inch/month - use this to predict recovery timelines clinically.
8. BONE METABOLISM - Key Hormones
| Hormone | Effect on Bone | Other Effects |
|---|---|---|
| PTH (excess/chronic) | Resorption (activates osteoclasts) | ↑ Ca²⁺, ↓ PO₄ in blood |
| PTH (pulsatile/low dose) | Anabolic (teriparatide mechanism) | Used in osteoporosis |
| Vitamin D | ↑ Ca²⁺ absorption (gut), ↑ mineralization | Deficiency → rickets/osteomalacia |
| Calcitonin (thyroid C-cells) | Inhibits osteoclasts; ↓ Ca²⁺ | Nasal calcitonin used in Paget's disease |
| Estrogen | Inhibits osteoclasts | Loss at menopause → osteoporosis |
| Cortisol (excess) | Inhibits osteoblasts; ↑ resorption | Steroid-induced osteoporosis |
9. QUICK CLINICAL PEARLS FOR DAILY WARD WORK
-
Compartment syndrome - 5 P's: Pain (worst, especially with passive stretch), Pressure, Pallor, Paresthesia, Paralysis. Treat with emergent fasciotomy when compartment pressure >30 mmHg or within 30 mmHg of diastolic BP.
-
Open fractures - Gustilo-Anderson classification (I, II, IIIA, IIIB, IIIC). IV antibiotics (cephalosporin ± aminoglycoside ± penicillin for soil contamination). Irrigation and debridement within hours.
-
Fat embolism syndrome - triad of respiratory failure, neurological dysfunction, petechial rash. Occurs 24-72 hrs post long bone/pelvic fracture.
-
Avascular necrosis (AVN) - most common after femoral neck fractures and hip dislocations. Hip dislocations must be reduced within 6 hours to minimize AVN risk.
-
Salter-Harris classification - for physeal (growth plate) fractures in children:
- Type I: through physis (Slipped)
- Type II: through physis + metaphysis (Above - most common, best prognosis)
- Type III: through physis + epiphysis (Lower)
- Type IV: through all three (Through)
- Type V: crush injury to physis (ERasure - worst prognosis)
- Mnemonic: SALTR (Slipped, Above, Lower, Through, Rammed/ERasure)
Where to Go From Here
Your foundational reading priorities in order:
- Miller's Review of Orthopaedics (9th Ed) - work through Section 1 (Basic Sciences) systematically
- Campbell's Operative Orthopaedics (15th Ed, 2026) - reference for surgical approaches and techniques
- Rockwood & Green's Fractures in Adults (10th Ed, 2025) - your fracture bible
The basic science you have learned above - bone biology, fracture healing, cartilage, muscle, tendons, biomechanics, and nerve injuries - underpins every clinical decision you will make. When you understand why a fracture fails to heal (smoking, instability, poor blood supply), you make better treatment choices.
- Miller's Review of Orthopaedics, 9th Ed, Section 1 (pp. 20-72)
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