Fracture Biomechanics

Energy transfer:
- Energy transferred to bone is influenced by mass and velocity: - Formula: ½ M x V² - Velocity has a greater impact than mass.

Bone is viscoelastic: - Properties change with the rate of loading: - Rapid loading: Bone absorbs more energy, becomes stiffer, stronger, and more brittle, which leads to fracture.

Factors determining fracture type: - Energy and rate of loading. - Direction of force. - Bone properties (shape, cortical thickness, pathologies). - Soft tissue forces (e.g., avulsion fractures).

Fracture Types

  • Oblique Fracture: Caused by pure shear force or uneven bending, leading to oblique cracking of osteons.
  • Transverse Fracture: Due to pure tensile bending force (2% strain for cortical bone).
    • Results in de-bonding of cement lines and osteon pullout (e.g., patella, olecranon).
  • Butterfly Fracture: Occurs on the compression side when there is combined tension (bending) and compression.
  • Spiral Fracture: Result of torsion, typically at a 45-degree angle to the horizontal.
  • Segmental Fracture: Often caused by 4-point bending (e.g., car bumper vs. tibia).

Primary Bone Healing

  • Requires absolute stability and rigid fixation under compression, creating a low-strain environment.
  • Healing occurs via intra-membranous ossification.

Sequence of Healing: 1. Contact Healing: - Minimal initial activity at contact sites. 2. Gap Healing: - Blood vessels grow into small gaps, bringing cells. - Mesenchymal cells differentiate into osteoblasts, laying down lamellar bone in small gaps or woven bone in larger gaps. 3. Cutting Cones: - Osteoclasts form cutting cones, tunneling across the fracture. - Channels are left for vascular and osteoblast invasion. 4. Remodeling: - Lamellar bone is laid down, leading to bridging callus and new osteon formation. - Remodeling continues for months.

Secondary Bone Healing

  • Requires relative stability and some degree of strain.
  • Healing occurs by two processes:
    1. Periosteal callus (intramembranous ossification).
    2. Endosteal callus (endochondral ossification).

Healing occurs in three locations: 1. Endosteal surface (bridging fibrous callus). 2. Surrounding soft tissues (bridging fibrous callus). 3. Periosteum (bony callus).

Periosteal Callus: - Bone is produced directly without cartilage formation. - Osteoblasts lay down type 1 collagen. - Does not bridge the fracture and won’t form if there’s excessive periosteal stripping.

Bridging Callus: - Forms between bone ends (endosteal) and in soft tissues. - Fibrocartilage forms first, which is then calcified into woven bone (soft callus) and finally consolidated into hard callus.

Five-Stage Process of Bone Healing

  1. Haematoma (hours):
    • Tissue damage causes clot formation, leading to platelet degradation and release of PDGF.
    • PDGF activates the clotting cascade and cytokines like TNF-alpha and IL-1, attracting immune cells and activating bone morphogenetic proteins (BMPs).
  2. Inflammation (up to a week):
    • BMPs stimulate angiogenesis and are osteogenic.
    • Macrophages clean up necrotic debris, and fibroblasts start forming granulation tissue.
  3. Soft Callus (4 weeks):
    • Reduced strain (15%) allows chondroblasts to lay down type 2 collagen, forming a cartilage scaffold (endochondral ossification).
  4. Hard Callus (4-16 weeks):
    • Cartilage is replaced with woven bone by osteoblasts, stabilizing the fracture.
  5. Remodeling (years):
    • Woven bone is remodeled into lamellar bone in response to mechanical stresses (Wolff’s Law).

Wolff’s Law

  • Bone adapts and remodels according to mechanical stresses:
    • Osteoblasts predominate on the concave compression side.
    • Osteoclasts dominate on the convex tension side.

Principles of Bone Healing

  • The goal is always to achieve primary bone healing.
  • In a low-strain environment, primary healing happens immediately.
  • In higher strain environments, sequential tissue types form:
    1. Clot
    2. Granulation tissue
    3. Cartilage
    4. Woven bone
    5. Lamellar bone

Perren’s Strain Theory

  • Tissue formation at the fracture site is dictated by the degree of interfragmentary strain.
  • Strain reduction improves healing:
    • 100% strain forms granulation tissue.
    • 10% strain forms fibrocartilage (soft callus).
    • 5% strain forms woven bone (hard callus).
    • <2% strain allows primary bone healing.

Problems in Healing: - Too little strain or large residual gaps can lead to non-union. - Too much strain can prevent hard callus formation, leading to hypertrophic non-union or pseudoarthrosis.

Adverse Effects on Fracture Healing

  • Blood supply is critical.
  • Smoking (nicotine) and NSAIDs can impair healing.
  • Head injury may increase blood flow, leading to excessive callus formation.
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