AO Principles & Goals - Fixation Foundation
- Core Aim: Achieve prompt, full function of injured limb.
- AO Principles (📌 ARS M): Foundation for internal fixation.
⭐ The four AO principles are:
- Anatomic reduction of fracture fragments, particularly in articular fractures.
- Stable internal fixation appropriate to the fracture pattern and biology.
- Preservation of blood supply to soft tissues and bone by careful handling and atraumatic surgical technique.
- Early, active, pain-free Mobilization of muscles and joints adjacent to the fracture, preventing fracture disease.
- Goals (derived from principles):
- Anatomical restoration.
- Stable healing environment.
- Vascularity preservation.
- Rapid functional recovery.
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Implant Types - Hardware Heroes
Materials: Stainless steel, titanium alloys (strength, biocompatibility).
Screws:
| Type | Feature | Sizes (mm) |
|---|---|---|
| Cortical | Fine threads, dense bone | 2.7, 3.5, 4.5 |
| Cancellous | Coarse threads, spongy bone | 4.0, 6.5 |
| Locking | Threaded head locks to plate (fixed-angle) | System-specific |
| Cannulated | Hollow for guidewire, precise placement | Varies |
| Lag (Technique) | Threads far cortex only → compression | Any screw |
| Type/Function | Description | |
| ------------------ | ----------------------------------------------------------------- | |
| Compression | Compresses fracture (DCP: eccentric holes) | |
| Neutralization | Protects lag screws from torsion/bending | |
| Buttress | Supports articular fragments, resists axial load | |
| Bridging | Spans comminution, maintains length/alignment | |
| Locking (LCP) | Angular stability (internal fixator), for poor bone quality | Other Implants: |
- K-wires: 0.6-3.0mm; fixation, guides.
- IM Nails: Load-sharing; long bone diaphyseal #.
- Cerclage Wires: Encircle fragments.
⭐ A lag screw, whether a cortical or cancellous screw, achieves interfragmentary compression when its threads engage only the far fragment, pulling it towards the near fragment where the screw shank glides.
Biomechanics & Stability - Balancing Act
- Strain & Healing: Bone healing depends on strain at fracture site.
- < 2% strain: Primary healing (direct osteonal).
- 2-10% strain: Secondary healing (callus).
-
10% strain: Non-union.
| Feature | Absolute Stability | Relative Stability |
|---|---|---|
| Goal | Rigid fixation, anatomical reduction | Alignment, controlled micromotion |
| Fracture Gap | < 1 mm | > 1 mm / comminuted |
| Methods | Lag screws, Compression plates | IM nails, Bridge plates, Ex-fix, Casts |
| Indications | Simple intra-articular/diaphyseal fx | Comminuted fx, Long diaphyseal/metaphyseal fx |
| Healing | Primary (direct) | Secondary (callus) |
| Callus | Minimal/None | Abundant |
Complications & Considerations - Pitfall Patrol
- Infection: Superficial/deep; prompt antibiotics, possible debridement.
- Implant Failure: Breakage, loosening, migration; revision surgery often needed.
- Union Problems: Non-union, malunion, delayed union impacting function.
- Neurovascular Injury: Iatrogenic risk; meticulous surgical technique crucial.
- Compartment Syndrome: ⚠️ Acute; high risk tibia/forearm; urgent fasciotomy.
- Hardware Irritation: Pain/bursitis over implant; removal post-union if symptomatic.
- Refracture: Post-removal (stress protection) or peri-implant.
- Thromboembolism (DVT/PE): Prophylaxis vital in high-risk patients.
⭐ Stress shielding occurs when a very rigid implant bears most of the physiological load, leading to bone resorption (osteopenia) beneath the plate and increasing the risk of refracture after implant removal.
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High‑Yield Points - ⚡ Biggest Takeaways
- AO Principles: Anatomic reduction, stable fixation, preserve blood supply, early mobilization.
- Absolute stability (articular fractures) for primary bone healing (no callus); relative stability (diaphyseal) for secondary healing (callus).
- Lag screw: Provides interfragmentary compression; threads engage far cortex only.
- Plate functions: Compression, neutralization, buttress, tension band, bridging.
- Stress shielding: Rigid fixation may cause osteopenia under the plate.
- Biological fixation: Emphasizes minimal soft tissue stripping to preserve blood supply.
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