Orthonotes
by the.bonestories
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Phases: inflammation → soft callus (cartilage) → hard callus (woven bone) → remodeling (lamellar). Primary (direct) vs secondary (indirect) healing; absolute vs relative stability concepts. Cell sources: periosteum (key), endosteum, marrow, surrounding soft tissues. Mechanical environment (strain theory) dictates tissue type; too much motion → nonunion. Timelines vary by bone/age/blood supply—tibia slower than femur; smokers/NSAIDs may delay.
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Fracture healing is a complex regenerative process unique among adult tissues in its capacity to regenerate without scar formation — the healed fracture is structurally and biomechanically equivalent to the original bone. However, this process is critically dependent on the local mechanical environment and the biological conditions. Two fundamentally distinct types of fracture healing occur depending on the stability and contact at the fracture site: secondary (indirect) healing — the predominant natural healing process involving callus formation — and primary (direct) healing — which occurs only under conditions of absolute stability and compression, as achieved with compression plating.
| Phase | Timing | Biology | Radiological / Mechanical Correlate |
|---|---|---|---|
| Phase 1 — Inflammation / Haematoma | Days 1–7 (peak at 24–48 hours) | Fracture haematoma forms immediately (from ruptured vessels in the periosteum, cortex, medullary canal, and surrounding soft tissues); platelet aggregation → clot formation; platelets and macrophages release inflammatory cytokines (IL-1, IL-6, TNF-α) and growth factors (PDGF, TGF-β); macrophages phagocytose necrotic debris; mesenchymal stem cells (MSCs) are recruited from the periosteum, endosteum, bone marrow, and circulating blood; the haematoma is NOT to be evacuated — it is the essential biological scaffold for subsequent repair; COX-2-dependent prostaglandins (PGE2) are critical mediators of the inflammatory response and MSC recruitment — NSAIDs and COX-2 inhibitors at this stage impair healing | No radiological changes; clinically: pain, swelling, bruising; the fracture is mechanically at its weakest; fracture haematoma formation is the essential first step — surgical disruption of this haematoma (e.g., aggressive irrigation of a closed fracture) reduces healing potential |
| Phase 2 — Soft Callus (Fibroplasia/Chondrogenesis) | Weeks 1–3 | MSCs differentiate into chondroblasts (in the relatively hypoxic peripheral callus) and osteoblasts (in the well-vascularised periosteal callus); a fibrocartilaginous soft callus forms around the fracture; the cartilage provides provisional mechanical stabilisation; the oxygen tension gradient across the fracture gap (low in the central gap, high at the vascularised periphery) determines cell differentiation — low oxygen → chondrogenesis; high oxygen → osteogenesis; angiogenesis (VEGF-driven neovascularisation) begins to invade the callus; BMP-2, BMP-7, TGF-β, FGF are key osteoinductive factors driving MSC differentiation | Early soft callus not visible on X-ray; clinically: decreasing pain; fracture becomes `sticky` (resists displacement); mechanical stiffness begins to increase; the fracture is still vulnerable to re-displacement |
| Phase 3 — Hard Callus (Mineralisation / Enchondral Ossification) | Weeks 3–12 (variable by fracture and fixation) | The fibrocartilaginous callus undergoes enchondral ossification — the same process that occurs in the growth plate; chondrocytes hypertrophy and undergo apoptosis; the cartilage matrix mineralises (calcium phosphate deposition); vascular invasion brings osteoblasts that replace the mineralised cartilage with woven bone; the woven bone (hard callus) is mechanically stronger than the soft callus; the fracture gap progressively fills with mineralised callus; this phase requires adequate blood supply, mechanical stability, and sufficient minerals (calcium, phosphate, vitamin D) | Hard callus becomes visible on X-ray (mineralised callus around the fracture — `haze` of new bone); fracture line becomes less distinct; mechanically: the fracture becomes increasingly rigid; the callus is visible as a cloud of new bone around the fracture on plain X-ray; the fracture line blurs; clinical union (painless weight-bearing) precedes radiological union |
| Phase 4 — Remodelling | Months to years (may take 2–7 years for complete remodelling) | The woven bone of the hard callus is systematically replaced by lamellar bone through coupled osteoclastic resorption and osteoblastic new bone deposition; the bone is remodelled according to Wolff`s law — bone is deposited along lines of mechanical stress and resorbed from areas of low stress; over time, the callus shrinks, the medullary canal is re-established, and the bone returns to its original shape (remodelling potential is greatest in young children — hence the greater acceptance of angular malunion in children vs adults) | Progressive reduction of callus bulk on serial X-rays; fracture line completely obliterated; cortex re-formed; medullary canal re-established; in children: angular deformity may completely remodel through differential periosteal growth (`remodelling correction` of up to 15–20° in young children in the plane of the adjacent joint) |
| Fracture | Clinical Union (weeks) | Radiological Union (weeks) | Return to Activity |
|---|---|---|---|
| Distal radius (Colles) | 6 weeks | 6–8 weeks | 6–8 weeks (light); 3 months (heavy) |
| Clavicle (midshaft) | 6–8 weeks | 8–12 weeks | 6–8 weeks (light); 12 weeks (sports) |
| Humeral shaft | 8–10 weeks | 10–16 weeks | 3–4 months |
| Femoral shaft (IMN) | 10–16 weeks | 16–24 weeks | 4–6 months (full weight-bearing) |
| Tibial shaft (IMN) | 10–16 weeks | 16–24 weeks | 4–6 months |
| Femoral neck (intracapsular) | 12–24 weeks | 24–52 weeks | 6–12 months (if union occurs) |
| Scaphoid (waist) | 8–12 weeks (undisplaced) | 12–20 weeks | 3–4 months; nonunion risk ~10–15% undisplaced |
| 5th metatarsal Jones fracture | 6–10 weeks (immobilisation) | 10–16 weeks | 10–16 weeks; surgical fixation for athletes (faster return) |
| Vertebral compression fracture | 6–12 weeks | 8–16 weeks | 2–3 months light; 3–6 months heavy |
| Paediatric femoral shaft | 4–8 weeks (age-dependent; younger = faster) | 6–12 weeks | 6–12 weeks |
| Factor | Effect on Healing | Mechanism / Notes |
|---|---|---|
| Smoking | Strongly inhibits — the most important modifiable risk factor; doubles the nonunion rate; delays healing by 2–3 months | Nicotine causes vasoconstriction (reduces periosteal blood supply); CO displaces oxygen from haemoglobin (tissue hypoxia); reduces osteoblast activity and MSC differentiation; impairs angiogenesis (VEGF suppression); cessation for 4–6 weeks before elective surgery significantly reduces healing complications |
| NSAIDs / COX-2 inhibitors | Inhibit healing — particularly in the early inflammatory phase | COX-2 produces prostaglandin E2 (PGE2) — critical for MSC recruitment and osteoblast differentiation in the inflammatory phase; NSAIDs and selective COX-2 inhibitors block this pathway; animal data is strong; human clinical data is less clear but NSAIDs should be avoided (or minimised) in the early fracture healing phase; not contraindicated for analgesia if carefully used |
| Corticosteroids | Inhibit healing | Reduce osteoblast activity; increase osteoclast activity; reduce MSC differentiation toward osteoblasts; impair angiogenesis; reduce IGF-1 production; systemic steroids significantly impair fracture healing and increase nonunion risk; corticosteroid-induced osteoporosis reduces bone quality |
| Diabetes mellitus | Inhibit healing | Hyperglycaemia impairs neutrophil function (increased infection risk); reduces MSC migration (AGE — advanced glycation end-products impair cell signalling); impairs angiogenesis; reduces bone mineral density; neuropathy impairs the protective pain response; well-controlled diabetes (HbA1c <7.5%) has less impact; optimise glucose control before elective fracture surgery |
| Vitamin D deficiency | Impairs mineralisation → impaired hard callus formation | 1,25-OH vitamin D is essential for calcium absorption and bone mineralisation; deficiency → inadequate mineralisation of the hard callus → soft callus persists → delayed union or nonunion; correct vitamin D before elective surgery (>50 nmol/L) |
| Age | Increasing age impairs healing rate | Reduced MSC numbers and proliferative capacity with age; reduced growth factor responsiveness; reduced periosteal vascularity; children heal dramatically faster than adults (paediatric femoral shaft fractures unite in 4–6 weeks; adult femoral shaft = 16–24 weeks); children also have greater remodelling potential |
| Mechanical environment | CRITICAL — the most controllable surgical variable | Controlled micro-motion (relative stability — IMN, functional brace) PROMOTES secondary healing and callus formation; absolute rigidity (compression plate — absolute stability) → primary healing without callus; excessive motion → fibrous nonunion (the fracture heals by fibrous tissue rather than bone because MSCs differentiate into fibroblasts rather than osteoblasts/chondroblasts in high-strain environments); strain theory (Perren): strain = change in length / original length; bone tolerates <2% strain; cartilage tolerates 10% strain; fibrous tissue tolerates 100% strain |
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