EXTERNAL FIXATION PRINCIPLES
Damage Control Orthopaedics | Frame Biomechanics | Pin Site Management
EXTERNAL FIXATION INDICATIONS
Critical Must-Knows
- Safe corridors: Critical for pin placement - avoid neurovascular structures
- Bicortical purchase: Increases construct stiffness 2-3 times vs unicortical
- Near-far-far-near: Pin configuration maximizes frame stability
- 2-week window: Optimal timing for external fixator to IM nail conversion
- Pin spacing: Pins spread along bone segment increase stability (working length concept)
Examiner's Pearls
- "Predrilling reduces thermal necrosis - critical for pin purchase
- "Pin diameter should not exceed one-third of bone diameter (stress riser risk)
- "Frame stiffness increases with the fourth power of pin diameter
- "Soft tissue transfixation with tensioned wires causes less damage than half-pins
Clinical Imaging
Imaging Gallery
Critical External Fixation Exam Points
Safe Corridors
Know the safe zones for pin placement in each anatomical region. The anterolateral tibia and lateral femur are safest. Avoid the posterior tibia (neurovascular bundle), anteromedial proximal tibia (saphenous nerve), and anterior mid-humerus (radial nerve).
Pin-Bone Interface
Thermal necrosis is the enemy of pin fixation. Predrill at low RPM with irrigation, avoid wobble, and use sharp self-drilling pins only in cortical bone. Ring sequestrum around pins indicates thermal damage.
Conversion Timing
Convert to internal fixation within 2 weeks to minimize infection risk. After 2 weeks, pin site colonization significantly increases deep infection rates following IM nailing. Consider staged approach with pin-free interval.
Checketts Classification
Pin site infection grading guides management. Grade 1-3 (minor) respond to local care and oral antibiotics. Grade 4-6 (major) require pin removal, debridement, and IV antibiotics. Grade 6 involves ring sequestrum.
Unilateral vs Circular External Fixators
| Feature | Unilateral Frame | Circular Frame (Ilizarov/TSF) |
|---|---|---|
| Ease of application | Simpler, faster to apply | Complex, steep learning curve |
| Pin type | Half-pins (Schanz screws) | Tensioned wires + half-pins |
| Stability | Good in single plane | Excellent multiplanar stability |
| Adjustability | Limited post-op adjustment | Unlimited 6-axis correction (TSF) |
| Soft tissue transfixion | Minimal (unilateral placement) | Greater (wires cross limb) |
| Ideal use | Damage control, temporary spanning | Definitive fixation, deformity correction |
ALLS SAFESafe Pin Corridors - Tibia
Memory Hook:Anterolateral and Lateral approaches are safest for tibial pins!
PINSExternal Fixator Stiffness Factors
Memory Hook:PINS dictate the stiffness of your external fixator construct!
SCORESChecketts-Otterburn Pin Site Grading
Memory Hook:Pin site problems SCORE from minor to major - escalate treatment accordingly!
Overview and Epidemiology
External fixation is a technique of fracture stabilization that utilizes pins or wires inserted into bone and connected to an external frame, thereby bypassing the soft tissue envelope. First described by Malgaigne in 1840 and refined by Hoffmann, Ilizarov, and others, it remains essential in modern trauma and reconstructive surgery.
Key Applications:
- Damage control orthopaedics (DCO): Polytrauma patients requiring rapid stabilization
- Open fractures: Severe soft tissue injuries requiring wound access (Gustilo IIIB/C)
- Pelvic ring injuries: Hemodynamic stabilization before definitive fixation
- Periarticular fractures: Spanning fixation while soft tissue recovers
- Definitive treatment: Limb lengthening, deformity correction, arthrodesis
Historical Context
Gavriil Ilizarov revolutionized external fixation in the 1950s with tensioned wire circular frames, enabling distraction osteogenesis and complex deformity correction. The Taylor Spatial Frame (TSF), developed in the 1990s, uses hexapod geometry for computer-assisted multiplanar correction.
Advantages of External Fixation:
- Minimal surgical exposure - preserves soft tissue biology
- No implant at fracture site - reduced infection risk in contaminated wounds
- Allows access for wound care and soft tissue procedures
- Adjustable post-operatively
- Can be applied rapidly in damage control setting
Disadvantages:
- Pin site complications (infection, loosening)
- Patient discomfort and inconvenience
- Risk of pin tract infection if converted to internal fixation
- Requires patient compliance with pin care
- Cumbersome for rehabilitation
Pathophysiology
Understanding frame biomechanics is fundamental to successful external fixation application and troubleshooting.
Biomechanical Principles
Load Transfer: External fixators transfer load from bone-to-pin-to-bar-to-pin-to-bone. The stiffness of the construct depends on factors at each interface and within each component.
Pin Factors
Pin diameter:
- Stiffness increases with the fourth power of pin diameter
- Doubling diameter = 16x stiffer construct
- Optimal adult Schanz pin: 5-6mm diameter
- Pin should not exceed one-third of bone diameter (stress riser risk)
Pin number:
- Minimum 2 pins per fragment (preferably 3)
- Additional pins provide redundancy and distribute load
- Diminishing returns beyond 4 pins per segment
Pin spread:
- Wider spacing along bone segment increases construct stiffness
- Converging pin configurations reduce stability
- Ideal: pins at extremes of each fragment
Bicortical vs unicortical:
- Bicortical purchase increases stiffness 2-3 times
- Essential for weight-bearing constructs
- Unicortical acceptable only in tensioned wire systems
The Fourth Power Rule
Pin diameter is the single most influential factor in construct stiffness. A 6mm pin is approximately 5x stiffer than a 4mm pin (6^4 / 4^4 = 1296/256 = 5.06). Always use the largest pin the bone will safely accommodate.
Frame Factors
Bone-bar distance:
- Closer bar to bone = stiffer construct
- Each cm increase in distance significantly reduces stiffness
- Balance against soft tissue swelling and wound access
Bar-bar distance (stacked configurations):
- Double-stacked bars increase bending stiffness
- Delta or triangular configurations provide torsional stability
Bar diameter and material:
- Carbon fiber bars stiffer than stainless steel at equivalent weight
- Larger diameter bars increase construct rigidity
Bone-Pin Interface
Thermal necrosis:
- Generated by drilling without cooling
- Causes ring sequestrum - bone death around pin
- Prevention: Predrill with irrigation, low RPM, sharp drill bits
- Self-drilling pins acceptable only in good cortical bone
Pin insertion principles:
- Incise skin and fascia (cruciate incision)
- Blunt dissection to bone (protect neurovascular structures)
- Predrill with 3.2mm drill and irrigation (or smaller)
- Insert pin perpendicular to bone axis
- Confirm bicortical purchase
- Avoid wobble during insertion
Clinical Presentation
Indications for External Fixation
Trauma Indications:
Open Fracture Management
Gustilo IIIB/IIIC fractures:
- Severe soft tissue injury requiring flap coverage
- External fixation allows wound access and soft tissue resuscitation
- Can span joints to protect vascular repairs
- Bridge to definitive internal fixation when soft tissues allow
Heavily contaminated wounds:
- Farm injuries, blast injuries
- Serial debridement required
- Internal fixation contraindicated initially
- External fixation provides stability without burying implant
Principles:
- Apply frame distant from zone of injury
- Use safe corridors for pin placement
- Configure to allow wound access
- Plan for conversion to internal fixation (if applicable)
Limb Reconstruction Indications
Definitive external fixation:
- Limb lengthening (distraction osteogenesis)
- Complex deformity correction
- Bone transport for segmental defects
- Infected nonunion management
- Ankle/hindfoot arthrodesis with poor soft tissue
Investigations
Preoperative Assessment
Radiographs:
- Standard orthogonal views of injured segment
- Include joints above and below
- Assess bone quality for pin purchase
CT scanning:
- Often performed through external fixator for definitive planning
- 3D reconstructions for articular fractures
- Metal artifact reduction protocols available
Vascular assessment:
- ABI (ankle-brachial index) if pulses diminished
- CT angiography for suspected vascular injury
- Essential before spanning constructs near vessels
Intraoperative Assessment
Image intensifier:
- Confirm pin placement in safe corridors
- Verify bicortical purchase
- Check fracture reduction
- Assess overall frame alignment
Clinical assessment:
- Confirm neurovascular status post pin insertion
- Check soft tissue tension around pins
- Ensure adequate wound access
Imaging Gallery
Management
Safe Pin Corridors
Understanding anatomical safe zones is critical for avoiding neurovascular injury during pin placement.
Safe Corridors by Anatomical Region
| Region | Safe Corridor | Structures at Risk | Notes |
|---|---|---|---|
| Proximal femur | Lateral | Sciatic (posterior) | Subtrochanteric level |
| Femoral shaft | Anterolateral to lateral | Femoral vessels (medial) | Perforators posteriorly |
| Distal femur | Lateral | Popliteal vessels (posterior) | Above adductor hiatus |
| Proximal tibia | Anterolateral | Saphenous (anteromedial), CPN (posterolateral) | Common peroneal at fibular neck |
| Tibial shaft | Anterolateral | Posterior tibial vessels, tibial nerve (posterior) | Anteromedial subcutaneous distally |
| Distal tibia | Anteromedial | Anterior tibial vessels, superficial peroneal | Subcutaneous border safe |
| Calcaneus | Lateral to medial (from lateral side) | Posterior tibial vessels, medial plantar nerve | Insert perpendicular to long axis |
| Iliac crest | ASIS, anterior superior spine | Lateral femoral cutaneous nerve | Stay anterior on ilium |
Pin Insertion Technique
Step-by-step:
- Incision: 1-2cm over planned insertion site
- Soft tissue dissection: Blunt dissection with hemostat to bone
- Tissue protection: Soft tissue protector sleeve around drill/pin
- Predrilling: 3.2mm drill, low RPM, with saline irrigation
- Pin insertion: Self-tapping Schanz pin, avoid wobble
- Confirm purchase: Check bicortical engagement, stability
- Wound management: Cruciate incision relaxes skin tension
Technical pearls:
- Always palpate neurovascular structures before incision
- Drill perpendicular to bone axis
- Predrill 0.5mm smaller than pin for press-fit
- Never force pins - redrill if resistance encountered
Surgical Management
Unilateral External Fixators
Systems:
- Hoffmann, AO, Synthes, Orthofix
- Modular components allow customization
- Single-bar or double-bar configurations
Indications:
- Damage control orthopaedics
- Open fracture temporary stabilization
- Spanning periarticular fractures
- Pediatric fractures (quick, minimally invasive)
Advantages:
- Rapid application
- Simple technique
- Unilateral pin placement (easier soft tissue management)
- Good for temporary fixation
Disadvantages:
- Limited to single-plane stability (unless biplanar)
- Cantilever loading on pins
- May require conversion to alternative fixation
Key technical points:
- Minimum 2 pins per fragment
- Pin spread along bone segment
- Bone-bar distance minimized
- Avoid pin placement through planned incisions
Complications
Pin Site Complications
Checketts-Otterburn Classification:
Pin Site Infection Classification and Management
| Grade | Description | Clinical Features | Management |
|---|---|---|---|
| Grade 1 | Minor - slight redness | Erythema around pin, no discharge | Improve pin site care, observation |
| Grade 2 | Minor - redness + discharge | Serous discharge, local erythema | Oral antibiotics + local care |
| Grade 3 | Minor - heavy discharge | Purulent discharge, soft tissue involvement | Oral antibiotics, intensify care |
| Grade 4 | Major - soft tissue infection | Cellulitis, requires IV antibiotics | IV antibiotics, consider pin removal |
| Grade 5 | Major - osteomyelitis | Bone involvement, pin loosening | Pin removal, IV antibiotics, debridement |
| Grade 6 | Major - ring sequestrum | Bone necrosis around pin tract | Pin removal, sequestrectomy, IV antibiotics |
Pin site infection prevention:
- Meticulous insertion technique (avoid thermal necrosis)
- Daily or twice-daily pin site cleaning
- Dry dressing protocol preferred (vs. wet)
- Early recognition and treatment
- Patient education on pin care
Pin loosening:
- Causes: infection, thermal necrosis, excessive micromotion
- Signs: increased movement, pain with loading
- Management: replace pin in new site if needed, assess for infection
Mechanical Complications
Frame failure:
- Component breakage (rare with modern systems)
- Clamp slippage
- Management: revise construct, add components
Loss of reduction:
- Inadequate initial construct stiffness
- Pin loosening
- Management: revise frame, consider alternative fixation
Malunion/Malalignment:
- Insufficient reduction at application
- Progressive deformity from unstable construct
- Prevention: adequate imaging, appropriate configuration
Neurovascular Complications
Nerve injury:
- Usually from pin placement outside safe corridor
- Prevention: meticulous technique, anatomical knowledge
- Most are transient neuropraxia
Vascular injury:
- Rare with proper technique
- Risk with pelvic C-clamp, posterior pelvic pins
- Requires urgent vascular consultation if suspected
Soft Tissue Complications
Skin tethering and necrosis:
- From pins placed through tense/mobile skin
- Prevention: adequate incision, skin tension release
- Management: cruciate incision extension, pin repositioning
Muscle transfixation:
- Causes pain with joint motion
- More common with wires than half-pins
- Prevention: insert pins with limb in functional position
Compartment Syndrome Risk
External fixation does not eliminate compartment syndrome risk. Fractures stabilized with ex fix still require vigilant monitoring for the first 24-48 hours. Fasciotomy wounds can be managed with external fixation in place.
Evidence Base
Timing of External Fixator to IM Nail Conversion
- Deep infection rate under 2 weeks: 3.6%
- Deep infection rate 2-4 weeks: 4.8%
- Deep infection rate over 4 weeks: significantly increased
- Pin site infection doubles deep infection risk
SPRINT Trial - Open Fracture External Fixation
- No significant difference in reoperation rates for open fractures
- Early conversion to nailing acceptable
- Supports ex fix as temporary stabilization strategy
Pin Site Care Protocols
- Insufficient evidence to recommend specific cleansing regimen
- Dry vs wet dressing controversy unresolved
- Consistency of care may be more important than specific protocol
- Patient education emphasized
Pelvic External Fixation in Hemodynamic Instability
- Pelvic volume reduction decreases bleeding
- Can be applied rapidly in resuscitation setting
- Part of damage control algorithm
- May be combined with angioembolization
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Open Tibial Fracture - Damage Control
"A 28-year-old motorcyclist presents with a Gustilo IIIB open tibial shaft fracture with extensive soft tissue stripping. He is hemodynamically stable after resuscitation. You are asked to stabilize the fracture. Describe your approach to external fixation."
Scenario 2: Spanning Knee External Fixator for Pilon Fracture
"A 45-year-old woman falls from a ladder and sustains a comminuted tibial pilon fracture (AO 43-C3) with significant soft tissue swelling and fracture blisters. CT shows severe articular comminution. Describe your staged management approach."
Scenario 3: Pelvic External Fixation for Hemodynamic Instability
"A 35-year-old male pedestrian struck by a car is brought to the trauma bay. He is hypotensive (BP 75/50) despite 2 units of blood. Pelvic X-ray shows widening of the pubic symphysis (8cm diastasis) and disruption of the left SI joint. The trauma team leader asks you to stabilize the pelvis. Describe your approach."
Australian Context
In Australia, external fixation is widely available and utilized across metropolitan and regional trauma centers. The technique is a core component of damage control orthopaedics protocols and is taught as an essential skill in orthopaedic training programs.
Equipment availability: Most Australian hospitals with orthopaedic services stock modular external fixation systems compatible with damage control applications. Major trauma centers maintain circular fixator systems (Ilizarov and Taylor Spatial Frame) for definitive reconstruction and limb lengthening. Access to specialized limb reconstruction services is available at tertiary centers in each state.
Training and guidelines: The Royal Australasian College of Surgeons (RACS) includes external fixation as a core competency for orthopaedic trainees. Australian trauma guidelines (based on ATLS and local protocols) incorporate pelvic external fixation into major trauma algorithms. The Trauma Verification program for Australian hospitals mandates availability of pelvic stabilization equipment.
Practical considerations: The geographic challenges in Australia mean external fixation skills are particularly valuable for managing trauma in regional and remote settings where prolonged transport to definitive care facilities may be required. External fixation provides stable transport and can be applied with basic equipment. Transfer protocols exist for patients with external fixators requiring specialist limb reconstruction services.
EXTERNAL FIXATION PRINCIPLES
High-Yield Exam Summary
Pin Placement Safe Corridors
- •Tibia: Anterolateral (entire length), anteromedial (distal third)
- •Femur: Lateral (shaft and distal)
- •Calcaneus: Lateral to medial (stop at medial cortex)
- •Pelvis: ASIS or supra-acetabular
Biomechanical Principles
- •Pin diameter: Stiffness proportional to 4th power
- •Bicortical purchase: 2-3x stiffer than unicortical
- •Pin spread: Wider spacing increases stability
- •Bone-bar distance: Closer bar = stiffer construct
Checketts Classification
- •Grade 1-3: Minor (local care, oral antibiotics)
- •Grade 4: Soft tissue infection - IV antibiotics
- •Grade 5: Osteomyelitis - pin removal, IV antibiotics
- •Grade 6: Ring sequestrum - surgical debridement
Pin Insertion Technique
- •Incise skin and fascia - blunt to bone
- •Predrill with irrigation (prevents thermal necrosis)
- •Insert perpendicular to bone axis
- •Confirm bicortical purchase
Conversion Timing
- •Under 2 weeks: Low infection risk - direct conversion
- •2-4 weeks: Intermediate risk - consider staged approach
- •Over 4 weeks: High risk - staged conversion mandatory
- •Pin site infection present: Treat first, then convert
Frame Types
- •Unilateral: Damage control, temporary stabilization
- •Biplanar: Increased rotational stability
- •Circular (Ilizarov/TSF): Definitive, deformity correction
- •Hybrid: Periarticular fractures with metaphyseal extension