Fracture Healing

Overview Fracture healing is the process of bone regeneration. Unlike other tissues which heal by scar formation, bone heals by regeneration of normal tissue structure. It is a complex cascade of cellular and biochemical events that restores the integrity of the bone.

Primary (Direct) Healing

• Definition: Bone healing without callus formation.
• Prerequisites: Absolute Stability (Strain less than 2%) + Anatomic Reduction.
• Mechanism (Gap less than 0.01 mm): Cutting Cones (Osteoclasts bore tunnels, Osteoblasts fill with osteoid).
• Gap Mechanism (Gap less than 1 mm): Gap healing (Woven bone to Refined by Cutting Cones).

Secondary (Indirect) Healing

• Definition: The natural form of healing involving callus.
• Prerequisites: Relative Stability (Strain 2-10%) + Vitality.
• Phases:
  1. Inflammation: Hematoma, MSC recruitment.
  2. Soft Callus: Cartilage formation (Enchondral ossification).
  3. Hard Callus: Mineralization (Woven bone).
  4. Remodeling: Wolff's Law (Lamellar bone).

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Biology of Bone Before understanding healing, one must understand the players.

Cellular Components

1. Osteoblasts ("Build"):
   • Secrete Osteoid (Type I Collagen) and regulate mineralization.
   • Express RANK-L to control osteoclasts.
   • Derived from Mesenchymal Stem Cells (MSCs).
2. Osteocytes ("Sense"):
   • Mechanosensors trapped in lacunae deep within the mineralized matrix.
   • Communicate via canaliculi.
   • Secrete Sclerostin (inhibits bone formation). Mechanical loading inhibits Sclerostin (allowing bone formation).
3. Osteoclasts ("Chew"):
   • Multinucleated giant cells derived from Monocyte/Macrophage lineage (Hematopoietic).
   • Resorb bone via acid secretion (HCL) and proteases (Cathepsin K).
   • Activated by RANK-L. Inhibited by OPG.

The Matrix

• Inorganic (65%): Hydroxyapatite (Calcium Phosphate crystals). Provides compressive strength.
• Organic (35%):
  • Collagen Type I (90%): Provides tensile strength. Triple helix structure.
  • Non-Collagenous Proteins: Osteocalcin (marker of turnover), Osteopontin (cell attachment), BMPs (growth factors).

Blood Supply

• Centrifugal Flow (Normal): Marrow → Cortex → Periosteum. (Inside to Outside).
• Centripetal Flow (Post-Fx): Periosteum → Cortex → Marrow. (Outside to Inside). The Periosteal blood supply becomes dominant after medullary disruption (e.g., reaming). This highlights the importance of preserving the soft tissue envelope (periosteum) during surgery.

Biophysics of Stimulation

• Piezoelectricity:
  • Stress on collagen generates electric potentials.
  • Compression side: Electronegative → Stimulates Osteoblasts (Bone Formation).
  • Tension side: Electropositive → Stimulates Osteoclasts (Bone Resorption).
• Streaming Potentials: Fluid flow in canaliculi stimulates osteocytes.

Biomechanics (Strain Theory)

Perren's Strain Theory: Tissues can only exist if they can withstand the strain of the gap.

• Strain Formula: Change in Gap / Original Gap.
• Bone: Ruptures at 2% Strain.
• Cartilage/Granulation: Tolerates greater than 10% Strain.
• Conclusion: To form bone (Primary), strain must be less than 2%. To heal via callus (Secondary), granulation tissue first stiffens the gap, reducing strain to allow cartilage, then bone.

The phases open with "Inflammation: Hematoma, MSC recruitment," but the hematoma is not inert debris - it is the first regenerative signal, and preserving it is a surgical goal rather than an afterthought.

Why the hematoma matters

• The fracture hematoma forms within minutes and provides both a fibrin scaffold and a reservoir of platelet-derived signalling molecules (PDGF, TGF-beta).
• Experimentally, excising or preventing the early hematoma impairs union, whereas a transplanted hematoma can itself induce bone. This is the biological reason to preserve the hematoma with closed/indirect reduction and to avoid over-stripping the fracture ends.

The acute pro-inflammatory burst (first 24-72 hours)

• Neutrophils and then macrophages infiltrate and release a coordinated cytokine wave: TNF-alpha, IL-1, IL-6, IL-11 and IL-18.
• These cytokines are chemotactic for mesenchymal stem cells, promote angiogenesis, and prime the periosteal and marrow progenitors that will build callus.
• Macrophages are obligatory: their depletion in models markedly impairs callus formation, and the switch from pro-inflammatory (M1) to reparative (M2) macrophages marks the transition out of the inflammatory phase.

Angiogenesis precedes osteogenesis

• The injured, hypoxic fracture site stabilises HIF-1-alpha, driving VEGF and new-vessel ingrowth. No vessels, no bone - revascularisation must precede mineralisation, which is why periosteal blood supply and gentle soft-tissue handling dominate the outcome.

The COX-2 link

• Prostaglandins generated by COX-2 during this inflammatory phase are required for progenitor differentiation and the soft-to-hard callus conversion. This is the mechanistic basis for the NSAID caution developed elsewhere in this topic - blunting the inflammatory signal can blunt healing. (The clinical NSAID evidence is covered in the Controversies and Factors sections; the dedicated nsaids-in-orthopaedics topic holds the full drug discussion.)

History

• Pain: Resolution of pain is the first sign of union.
• Function: Ability to weight bear. Return to activities of daily living.
• Risk Factors: Ask about smoking, diabetes, steroid use, NSAIDs.

Physical Exam

• Tenderness: No tenderness at fracture site (Clinical Union).
• Motion: No abnormal mobility (Clinical Union).
• Stress: Painless stressing of the fracture.
• Soft Tissue: Assess status of the envelope (healed wounds, skin grafts).

Radiographic Assessment (RUST Score)

• Radiographic Union Scale for Tibial fractures (RUST).
• Methodology: Review AP and Lateral X-rays. Identify the 4 cortices (Anterior, Posterior, Medial, Lateral).
• Scoring Per Cortex:
  • 1 Point: Fracture line visible, No callus. (Unhealed)
  • 2 Points: Fracture line visible, Callus present. (Healing)
  • 3 Points: Fracture line invisible, Bridging callus. (Healed)
• Interpretation:
  • Minimum Score: 4 (Unhealed).
  • Maximum Score: 12 (Fully United).
  • Clinical Definition of Union: Score of greater than 10 usually correlates with mechanical stability and ability to weight bear without pain.
• General Signs: Bridging bone on 3 of 4 cortices. Blurring of fracture line. Remember: X-rays lag behind clinical signs.

Assessment of Non-Union

• CT Scan: Gold standard to assess bridging. Can differentiate bony union from fibrous non-union.
• Labs: Vitamin D, PTH, Calcium, ESR/CRP (Infection).
• MRI: Assessing infection (osteomyelitis) vs sterile non-union.

Casting Principles

• Three-Point Molding: To maintain reduction, pressure must be applied at the apex of the curve and counter-pressure at the proximal/distal ends.
• Functional Bracing (Sarmiento):
  • Allows joint motion.
  • Hydrostatic containment of soft tissues stabilizes the fracture.
  • Micro-motion stimulates callus (Secondary Healing).
  • Common for: Humeral shaft, Tibial shaft.

When biology is poor (Atrophic Non-union), we augment it.

Bone Grafts

1. Autograft (Self):
   • Source: Iliac Crest (Gold Standard), RIA (Reamer Irrigator Aspirator - Femur).
   • Properties: Osteogenic (Cells) + Osteoinductive (Signals) + Osteoconductive (Scaffold).
   • Cons: Donor site morbidity (pain, infection, nerve injury).
2. Allograft (Cadaver):
   • Source: Bone bank (structural struts or morcellized chips).
   • Properties: Osteoconductive strictly. (No cells, minimal signals).
   • Cons: Disease transmission (rare 1 in 1 million), slower incorporation, immune reaction (minor).
3. Synthetics (Ceramics/TCP):
   • Properties: Osteoconductive (Scaffold).
   • Cons: Brittle, expensive, can cause seroma.

Growth Factors

• BMP-2 (Infuse):
  • Potent Osteoinductor. Indicated for Acute Open Tibia fractures.
  • Mechanism: Recruits MSCs to differentiate into osteoblasts.
  • Risks: Swelling (C-spine airway compromise), Heterotopic ossification, Cost.
• BMP-7 (OP-1): Historic use for non-unions.
• PRP/BMAC: Bone Marrow Aspirate Concentrate. Rich in MSCs and growth factors (PDGF, TGF-beta).

The biology section noted that osteocytes secrete sclerostin (a brake on bone formation) and that mechanical loading switches it off. That makes sclerostin - and the PTH/Wnt axis - a druggable target for accelerating healing, distinct from the local osteoinductive BMP grafts covered under Biologic Adjuvants (and in the dedicated growth-factors-bone-healing topic).

Teriparatide (recombinant PTH 1-34)

• Intermittent (once-daily) PTH is anabolic - it favours osteoblast survival and enhances callus formation, the opposite of the catabolic effect of continuously elevated PTH seen in hyperparathyroidism.
• Used off-label to support healing in selected fragility fractures and established non-unions (for example pelvic and vertebral insufficiency fractures, and atypical femoral fractures related to bisphosphonates).
• Not licensed for fracture healing: supportive evidence exists but is limited and heterogeneous, so it stays an adjunct in high-risk hosts, not routine care. It is avoided where a bone tumour, skeletal metastasis, Paget disease, or prior skeletal irradiation is a concern.

Anti-sclerostin antibody (romosozumab)

• A monoclonal antibody that neutralises sclerostin, giving a dual effect - increased formation and reduced resorption - and licensed for osteoporosis.
• Mechanistically attractive for healing because loading normally suppresses sclerostin, but dedicated fracture-healing acceleration trials (tibial and hip) have not met their primary endpoints, so it is not established for speeding union.

Do not conflate with anti-resorptives

• Bisphosphonates and denosumab are anti-resorptive, not pro-healing; they do not accelerate union, and bisphosphonates are linked to atypical femoral fractures - a separate problem from augmenting repair.

Historical Evolution

1. Ancient Era: Splinting and casting (Egyptians).
2. 1950s (AO Foundation):
   • Founded by Mueller, Allgower, et al.
   • Emphasis on Anatomic Reduction and Rigid Fixation.
   • Result: Primary healing, but high non-union rate due to biological stripping.
3. 1990s (Biological Fixation):
   • Shift to MIPO (Minimally Invasive Plate Osteosynthesis) and IM Nails.
   • Emphasis on Biology (Blood supply) over Anatomy.
   • Acceptance of secondary healing (Callus).
4. Future: Gene therapy (BMP), 3D printed scaffolds, Stem Cells.
   • Gene Therapy: Viral vectors delivering BMP-2 genes to local cells.
   • 3D Printing: Custom scaffolds matching the defect, seeded with MSCs.
   • Biophysical Stimulation:
     • LIPUS: Low Intensity Pulsed Ultrasound. Mechanical vibration stimulates integrins.
     • PEMF: Pulsed Electromagnetic Fields. Induces electrical currents (streaming potentials) to stimulate calcification. NOTE: Contraindicated in patients with pacemakers.

Biomaterials and Healing The stiffness of the implant dictates the strain at the fracture site.

• Stress Shielding: If an implant is too stiff (e.g., thick steel plate), it takes all the load. The bone beneath it senses no load (strain less than 2%) but also no "need" for strength. This causes bone resorption (Wolff's Law in reverse) and porosis under the plate.
• Titanium: Being less stiff (closer to bone), it allows some load transfer, reducing stress shielding.
• Carbon Fiber: Even closer modulus to bone. Used in oncology (radiolucent).

Healing does not always go to plan.

1. Delayed Union

• Definition: Healing that takes longer than expected for the specific fracture and host (typically 3-6 months), but is still progressing.
• Management:
  • Conservative: Functional bracing, LIPUS (Low Intensity Pulsed Ultrasound), Nutrition.
  • Surgical: Dynamization (remove locking screws) to increase load/strain.

2. Non-Union

• Definition: Failure to heal by 6-9 months, or no progression on X-rays for 3 consecutive months (FDA definition).
• Types:
  • Septic Non-Union: Infection until proven otherwise. Check CRP/ESR. MRI.
  • Hypertrophic: Good Biology, Poor Stability. (Needs Stability). Elephant Foot appearance.
  • Atrophic: Poor Biology. (Needs Biology + Stability). Pencil Tip appearance.
  • Oligotrophic: Intermediate.
• Workup:
  • History: Smoking, Diabetes, NSAIDs.
  • Exam: Mobility at fracture site? Sinus?
  • Imaging: CT Scan (Gold Standard for union).
  • Labs: Infection markers, Vitamin D, Ca/PO4, PTH.

3. Malunion

• Definition: Healed but in a non-anatomic position.
• Parameters: Shortening, Rotation, Angulation.
• Tolerance:
  • Humeral Shaft: Tolerates huge deformity (20 deg angulation, 3cm shortening).
  • Forearm: Zero tolerance (loss of pronation/supination).
  • Tibia: Tolerates minimal varus/valgus (less than 5 deg).

4. Synostosis

• Definition: Fusion between two adjacent bones (e.g., Radius/Ulna, Tibia/Fibula).
• Cause: Disruption of Interosseous Membrane (IOM) + High Energy + Same approach (single incision).
• Consequence: Loss of rotation (Forearm).

Early Phase (0-2 weeks)

• Elevation: Reduce edema.
• Motion: Early ROM (if fixation allows) to prevent stiffness and stimulate blood flow.
• Review: Wound check.

Middle Phase (2-6 weeks)

• Loading: Proprioceptive weight bearing. Wolff's law stimulation.
• X-rays: Check alignment and maintenance of reduction.

Late Phase (6-12 weeks)

• Strengthening: Restore muscle mass.
• Full Weight Bearing: When hard callus visible (3/4 cortices).

Time to Union (Average)

• Upper Limb: 6-8 weeks.
• Lower Limb: 12-16 weeks.
• Tibia: Slowest bone to heal (poor blood supply). 16-20 weeks common for open fractures.

Non-Union Rates

• Tibia: 10-15% (especially open). The most common long bone non-union.
• Femur: 1-2% (with IM nail). Success story of modern orthopaedics.
• Clavicle: 5-10% (conservative).
• Scaphoid: High rate due to retrograde blood supply.

Factors Predicting Poor Outcome

• Smoking (Odds ratio greater than 3).
• Open fracture (Gustilo III).
• Infection.
• NSAID use (Prolonged).

Key Differences

1. Speed: Heals twice as fast. High osteogenic potential of thick, vascular periosteum.
2. Remodeling Potential:
   • Can correct Angulation (in plane of motion).
   • Cannot correct Rotation.
   • Overgrowth: Femur fractures stimulate growth (1-2cm leg length discrepancy common) due to hyperemia.
3. Physeal Injuries:
   • Growth arrest (Physeal bar) → Deformity.
   • Salter-Harris Classification.

Children are not just small adults.

Global Epidemiology

• Tibial shaft fracture is the most common long-bone fracture; nonunion affects roughly 5-15% (highest with open injuries and segmental bone loss).
• Smoking, diabetes, NSAID exposure, and high-energy/open injury are consistent risk factors for impaired union worldwide.

Side-by-Side Society Guidance

Registry & Trial Signals

• Large RCTs (SPRINT, TRUST, BESTT) anchor practice: ream closed tibial nails, LIPUS adds no benefit for fresh fractures, BMP-2 reduces reoperation in open tibiae.
• National trauma/arthroplasty registries (NJR, AOANJRR, Swedish/Norwegian) track reoperation and nonunion as quality metrics rather than implant survival for diaphyseal fixation.

High- vs Limited-Resource Practice

• High-resource: routine IM nailing, CT for suspected nonunion, ortho-plastic flap cover, selective BMP/autograft.
• Limited-resource: external fixation and functional bracing (Sarmiento) remain mainstays; autograft (iliac crest) preferred over costly BMP/synthetics; clinical and plain-radiograph union assessment.

Smoking Cessation (Universal Principle)

• Offer behavioural support plus pharmacotherapy (nicotine replacement, varenicline, or bupropion where available).
• For elective osteotomy/fusion, aim for cessation before surgery; cotinine testing can confirm abstinence.

• NSAIDs: Animal data and a meta-analysis (OR ~2.1 overall) suggest impaired union, but the effect is dose- and duration-dependent and absent in children. Short low-dose courses for simple fractures are likely safe; prolonged use is best avoided in high-risk cases. True equipoise persists.
• Bone stimulators (LIPUS/PEMF): The TRUST RCT showed no benefit for fresh tibial fractures. Their role, if any, is debated for established non-union where high-level evidence is lacking.
• BMP use: Reduces reoperation in open tibiae, but cost, swelling, heterotopic ossification and off-label spinal complications have narrowed enthusiasm; reserved for selected high-risk or non-union cases.
• Reaming in open fractures: SPRINT showed benefit only in closed tibial fractures; the closed-vs-open reaming question remains debated, especially in contaminated wounds.
• Optimal definition of union: No universally accepted radiographic threshold exists. RUST improves reliability but CT is often needed; clinical and radiographic union frequently disagree.
• Biologic adjuncts (PRP/BMAC, synthetics): Widely marketed but supported mainly by low-level evidence; routine use is not established.

Planning for Union

• Patient: Stop smoking, Optimize diabetes, Nutrition.
• Implant: Choose load-sharing (Nail) vs load-bearing (Plate) based on fracture pattern and soft tissue.
• Biology: Preserve soft tissue (Open reduction vs Closed).

The Tibial Shaft (The "Unforgiving Bone")

• Anatomy: One third is subcutaneous (anteromedial surface). Poor blood supply.
• Healing: Slow (16-20 weeks).
• Management: IM Nail (standard) vs Plate (distal/proximal).
• Pearl: Reamed nailing creates a "bonfire" of growth factors and autograft.

The Scaphoid (Retrograde Flow)

• Anatomy: Blood enters distal pole. Proximal pole depends on intraosseous flow.
• Risk: Proximal pole fracture → Avascular Necrosis (AVN).
• Healing: Compression screw (Herbert screw) buried in bone.

The Clavicle (S-Shaped Strut)

• Anatomy: Membranous bone formation (unique).
• Malunion: Shortening greater than 2cm affects shoulder biomechanics (scapular dyskinesis).
• Non-Union: Atrophic common in smokers.

The Femoral Neck (Intracapsular)

• Anatomy: Synovial fluid washes away hematoma (no clot = no callus).
• Result: Must heal by Primary Healing (Compression).
• Risk: AVN due to medial circumflex artery damage.

The Humeral Shaft

• Healing: Typically 8-12 weeks.
• Acceptance: Accepts up to 20 degrees anterior angulation and 30 degrees varus/valgus due to shoulder range of motion compensation.
• Nerve: Radial nerve palsy (Holstein-Lewis fracture in distal third). 90% resolve spontaneously. Watch and wait for 3 months if closed injury.

The Distal Radius (Colles)

• Healing: 6 weeks for bony union.
• Remodeling: Limited in adults. Malunion (shortening) leads to ulnar impaction syndrome.
• Management: Volar locking plate allows early motion.

The Ankle (Weber B)

• Mortise: 1mm shift = 42% decrease in contact area.
• Healing: 6 weeks NWB (Syndesmosis) or 2 weeks NWB then boot (stable).
• Risk: Post-traumatic arthritis if articular step greater than 2mm.

The Pelvis (Ring)

• Healing: Cancellous bone heals fast (6-8 weeks).
• Weight Bearing: Depends on posterior ring stability (Sacrum/SIJ).
• Complication: Venous Thromboembolism (highest risk in orthopaedics).

The Fifth Metatarsal (Jones Fracture)

• Zone 2: Metaphyseal-Diaphyseal junction.
• Physics: Adductor longus traction creates tension side failure.
• Healing: Poor blood supply. High non-union rate.
• Management: Screw fixation often required for athletes.

The Next Frontier The future of fracture healing lies in targeted biological intervention.

• Personalized Medicine: Genetic profiling for non-union risk.
• Smart Implants: Sensors embedded in plates to measure strain and notify patients when to weight bear.
• Bio-printing: 3D printed vascularized bone grafts.