Definitions
- Strain is change in fracture gap length divided by original gap length
- Stiffness is resistance to deformation under load
- Interfragmentary strain tolerance is about 100% for granulation tissue, 10% for cartilage and 2% for bone
Stability and Healing
| Stability | Strain | Healing | Constructs |
|---|---|---|---|
| Absolute | Under 2% | Primary, no callus | Lag screw, compression plate, tension band |
| Relative | 2 to 10% | Secondary, with callus | Intramedullary nail, bridge plate, external fixator, cast |
- Strain over 10% prevents bone formation (Perren)
- Comminution spreads strain across many gaps, so relative stability suits it
Construct Factors
- Longer plate working length lowers stiffness and spreads strain
- Leaving screw holes empty near the fracture increases working length
- Larger nail diameter increases stiffness
- External fixator stiffness rises with larger pin diameter, more pins, wider pin spread, bar closer to bone and added planes
- Pin bending stiffness rises with the fourth power of radius
- Locking plates act as internal fixators and do not rely on friction between plate and bone
- Far cortical locking and dynamic locking screws reduce stiffness near the plate
Load Sharing
- Nails share load with bone
- Plates bear load until union, raising the risk of fatigue failure
- See Principles of Internal Fixation and Principles of External Fixation
Clinical Relevance
- Simple fracture patterns fixed with bridging constructs can develop high strain and nonunion
- Over stiff locking constructs can cause asymmetric callus and delayed union
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.