Orthonotes
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Load sharing vs load bearing; absolute vs relative stability; primary vs secondary healing. Plates: compression (DCP/LCP in compression) vs bridging (relative stability); working length matters. Nails: intramedullary load‑sharing devices; reamed vs unreamed; interlocking controls length/rotation. External fixation: pin density/configuration, frame stiffness; circular frames allow controlled micromotion. Screw biomechanics: lag by technique vs design; pull‑out strength depends on cortical engagement and size.
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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.
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 |
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 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 |
| Concept | Definition | Example |
|---|---|---|
| Load Bearing | Implant takes majority of mechanical load | Bridge plating |
| Load Sharing | Bone shares load with implant | Intramedullary nail |
Types of plating include:
Interlocking screws provide rotational stability and prevent shortening.
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