Biomechanics of Fracture Fixation

Biomechanics of Fracture Fixation - Bone's Big Rules

• Key concepts:
  • Stress ($F/A$): Force/area.
  • Strain ($\Delta L/L$): Relative deformation.
  • Young's Modulus ($E = Stress/Strain$): Stiffness.
• Bone properties:
  • Anisotropic: Direction-dependent properties.
  • Viscoelastic: Rate-dependent response (stronger if rapid).
• Stress-Strain Curve:
  • Elastic deformation: Reversible.
  • Yield Point: Elastic limit, onset of plastic change.
  • Plastic deformation: Permanent.
  • Ultimate Failure Point: Fracture.
• Bone types:
  • Cortical: Dense, high stress resistance, low strain capacity.
  • Cancellous: Spongy, lower stress resistance, high strain capacity.

⭐ Wolff's Law states that bone remodels in response to mechanical stress.

Biomechanics of Fracture Fixation - Stability Secrets

⭐ Perren's Strain Theory: Fracture healing is inversely proportional to interfragmentary strain.

• Stability Goal: Create optimal mechanical environment for bone union.
• Absolute Stability:
  • No motion at fracture site; aims for primary bone healing.
  • Requires anatomical reduction and interfragmentary compression.
  • Interfragmentary strain < 2%.
  • Examples: Lag screws, compression plates.
• Relative Stability:
  • Allows controlled motion; promotes secondary bone healing via callus.
  • Maintains alignment, length, and rotation.
  • Interfragmentary strain 2-10%.
  • Examples: Intramedullary nails, bridge plates, external fixators.

Biomechanics of Fracture Fixation - Hardware Heroes

📌 Screws Squeeze, Plates Protect, Nails Navigate, ExFix Externalize.

Implants achieve fixation by applying biomechanical principles:

• Screws: Convert rotational force (torque) into axial force (compression).
  • Lag Screw: Compresses fragments; near cortex overdrilled, far cortex tapped.
  • Pitch: Thread distance; ↑pitch for cancellous bone. Core diameter dictates strength.

Biomechanics of Fracture Fixation - Fixation's Fate

• Bone Healing & Stability:
  • Primary (Direct) Healing: Requires absolute stability (e.g., compression plates).
    • Contact Healing: No gap, direct osteonal bridging.
    • Gap Healing: Gaps <1mm filled by lamellar bone. Minimal callus.
  • Secondary (Indirect) Healing: Requires relative stability (e.g., IM nails, casts, ex-fix).
    • Forms robust callus via endochondral & intramembranous ossification. More physiological.

vs. secondary bone healing (callus))

• Complications & Failure Pathways:
  • Implant Failure:
    • Fatigue: Cyclic loading leads to material failure.
    • Overload: Single high-energy event exceeds implant strength.
  • Non-Union: Often precedes/causes implant failure. Key factors:
    • Excessive motion at fracture site (inhibits vascularization, disrupts callus).
    • Poor vascularity/blood supply (ischemia).
    • Infection.
    • Large fracture gap.

⭐ Stress shielding beneath a rigid plate can lead to cortical osteopenia and refracture upon implant removal.

High-Yield Points - ⚡ Biggest Takeaways

• Stress shielding from overly rigid plates causes osteopenia under the plate.
• DCPs convert torsional/bending forces to axial compression for primary healing.
• Locking plates offer fixed-angle stability, vital for osteoporotic/comminuted fractures.
• IM nails, as internal splints, share load, ideal for diaphyseal fractures.
• Relative stability (nails, ex-fix) promotes secondary healing (callus); absolute stability (plates) for primary healing.
• Shorter implant working length equals increased stiffness and reduced motion.