Overview
Biomechanics of fracture fixation refers to the mechanical principles governing stabilization of fractured bones using internal or external fixation devices. The goal of fracture fixation is to restore anatomical alignment and provide sufficient stability to allow biological healing while permitting early mobilization of the patient.
Orthopaedic implants such as plates, screws, intramedullary nails and external fixators are designed based on biomechanical principles that counteract forces acting across the fracture site. These forces include compression, tension, bending, torsion and shear.
Forces Acting on Fractures
Several mechanical forces act on bone fragments following a fracture. Understanding these forces helps in selecting the appropriate fixation technique.
| Force | Description | Example |
|---|---|---|
| Compression | Forces pushing bone fragments together | Weight bearing across fracture |
| Tension | Forces pulling fragments apart | Muscle contraction |
| Shear | Parallel sliding forces | Oblique fractures |
| Bending | Combination of compression and tension | Long bone loading |
| Torsion | Rotational forces | Twisting injuries |
Principles of Stability
Fracture fixation aims to achieve either absolute stability or relative stability depending on the fracture pattern and fixation method.
| Type of Stability | Movement at Fracture Site | Type of Healing |
|---|---|---|
| Absolute stability | No motion | Primary bone healing |
| Relative stability | Controlled micromotion | Secondary healing with callus |
Strain Theory
Strain is defined as the change in length divided by the original length of the fracture gap. Strain theory explains the biological response of tissues during fracture healing.
| Strain | Tissue Formed |
|---|---|
| >10% | Fibrous tissue |
| 2–10% | Cartilage |
| <2% | Bone |
Load Bearing vs Load Sharing
| Concept | Definition | Example |
|---|---|---|
| Load Bearing | Implant takes majority of mechanical load | Bridge plating |
| Load Sharing | Bone shares load with implant | Intramedullary nail |
Biomechanics of Plates
- Plates function as load-bearing devices
- Placed on tension side of bone
- Convert tensile forces into compression
- Used in compression plating
Types of plating include:
- Compression plating
- Neutralization plating
- Bridge plating
- Buttress plating
Biomechanics of Intramedullary Nails
- Act as load-sharing devices
- Located along mechanical axis
- Provide strong resistance to bending
- Less invasive than plating
Interlocking screws provide rotational stability and prevent shortening.
External Fixation Biomechanics
- Provides relative stability
- Useful in open fractures
- Allows access to soft tissues
- Frame stiffness depends on pin configuration
Key Exam Points
- Absolute stability produces primary bone healing
- Relative stability produces callus formation
- Intramedullary nails are load sharing devices
- Plates often function as load bearing devices
- Strain less than 2% allows bone formation