Clinical Sciences, Faculty of Veterinary Medicine, University of Montreal, St-Hyacinthe, QC, Canada
External fixators are used for the immobilization of long bone fractures. Their use requires the use of transcortical fixation pins (minimum of 4) attached to one or more connecting bars. The fastening elements of the pins to the connecting bars are called clamps. Once set up, we call ‹montage› the resultant, the frame fixed in the bone.
There are 3 types of external fixators:
- Linear devices
- Circular devices
- Hybrid devices.
External fixators are versatile. They are used in various scenarios. Their indications are:
- Stable and unstable fractures
- Open fractures
- Gunshot fractures
- Corrective osteotomies for angular limb deformities
- Delayed unions and non-unions
- Arthrodesis of certain joints
- Stabilization of certain joints during ligament or tendon damage
Advantages Are Numerous
- Easy to use
- Preservation of soft tissues
- Remote stabilization of trauma site
- Allows treatment of open wounds
- Complementary to other methods of internal fixation
- Well tolerated
- Easy and simple removal (sometimes only deep sedation is enough)
- Reasonable cost
Disadvantages Are Limited
- Blind insertion of IM pins
- Insertion is dependent of the operator
- Re-evaluations are more frequent (2x)
- Daily fixator care by the owner
- Watch out for furniture and legs at home!
External Fixator Components
The device is created using several components
- Fixation pins
- Connecting bars
Fixation Pins (Fixator Base)
There are 3 types of transcortical fixation rods:
1. Smooth pins:
a. Their insertion is easy
b. Poor retention force
c. Tendency to become loose with time
2. Threaded pins with negative profile
a. Easy insertion
b. Better retention strength in the bone
c. Optimized stability at the rod/bone interface
d. Tend to break at the junction of the smooth part and the threaded part
i. Use Ellis type stems, where the thread is short and the junction is in the medullary cavity
3. Positive profile threaded pins
a. Insert must be preceded by drilling a hole, so more complex
b. Insertion must be unidirectional: avoid inserting and removing the rods otherwise this results in microfractures of the threads
c. The greatest retention force in the bone
d. Greater stability at the rod/bone interface
Clamps are used to fix
- The fixation pins to the connecting bars
- Connecting bars between them
- Intramedullary pins at connecting rods
The clamps can be either single or double. Clamps are usually the weak link in the chain. For several years now, manufacturers have developed and marketed stronger, more versatile and more durable clamps!
The connecting bars bind the fixation pins, thus stabilizing the bones involved in the fracture. The connecting bars determine the rigidity of the frame!
There are different types of connecting bars:
- Stainless steel
- The most common, several sizes, but less rigid and heavier than others
- Rigid, light and radiolucent
- Rigid, light and expensive
Linear external fixators are classified into 3 types according to their configuration:
1. The number of sides (skin surfaces) perforated by fixation pins:
a. Uni or bilateral
b. Type I, type II or type III
2. The number of planes of the fixator:
a. Uni or biplanar
i. Unilateral-uniplanar (type Ia)
ii. Unilateral-biplanar (type Ib)
iii. Bilateral-uniplanar (type II)
iv. Bilateral-biplanar (type III)
Principles of Application
Reduction of the Fracture
- First, proceed to the reduction of the fracture
- Closed: biological approach
- «Open, but do not touch»
- visualization of the fracture, attends reduction, but no debridement
Fixation Pins Insertion
- Number of rods
- Use a minimum of 2 pins per bone fragment
- Maximum of 4 pins
- Pins must engage both cortices
- Size of the pins
- Diameter of 20% of the smallest diameter of the bone
- Distance from the fracture site
- In general, ½ to once the diameter of the bone
- For example, a tibia has a diameter of 18 mm in its mid-diaphyseal region, so the pin closest to the fracture site should be 9–18 mm away from it
- Distance between them
- Insertion technique
- Always use a drill with a maximum speed of 100–150 rpm. Higher speed will result in heat production: necrosis, premature pin loosening and replacement over time
- Use a skin incision large enough to avoid rotational stretching lesions
- Smooth rods
- Parallel placement on the ground for the most distant pins of the fracture site
- Placement at an angle (20° if possible) for the intermediate pins
- Negative threaded pins (Ellis)
- Insert as smooth pins
- Less need to angulate the intermediate pins
- Positive threaded pins
- Drill a guide hole with a drill bit
- The guide hole should be 0–10% smaller than the pin diameter
- 2.5 cm diameter bones, 5 mm (20%) diameter pins are used, a 4.5 mm diameter hole is drilled before insertion of the positively threaded rods
- Slow unidirectional insertion!
- Use the appropriate connecting bars, the most rigid possible
- The connecting bars should be at least one cm (1 cm) from the skin, to avoid contact wounds
- The clamps should be towards the inside of the limb as this provides increased rigidity
- Minimize the amount of soft tissue between bone and skin
- Avoid tendons, ligaments, nerves and vessels
- Make sure to avoid cracks, otherwise the bone will burst!
Let’s Talk Biomechanics!
Or, what are the factors influencing the strength and rigidity of the fixator?
1. Configuration of the external fixator
a. The type of fixator used (type I/II/III) influences the rigidity of the assembly
b. Type I<Type II<Type III
2. Type of pins
a. Positively threaded rods> negatively threaded rods> smooth rods
b. Retention force at tearing and premature release
3. Positioning of the pins
a. Place the pins through the widest part of the bone to maximize grip and avoid cracking the bone
b. In the center of a round bone
c. In the widest part of a triangular bone
d. The pins should be well distributed along the length of the bone to distribute the stress and maintain the maximum strength of the fixator.
4. Inserting the fixing pins
a. The method of insertion of the fixing pins is very important.
b. Limited speed is used and irrigated (sterile saline) to dissipate the heat produced and thus reduce the risk of thermal necrosis of bone.
5. Number of pins
a. The higher the number of pins per bone fragment (up to 4 per fragment), the more rigid the fixator and the greater the stress distribution at each bone/stem interface.
b. On the other hand, too many pins can cause too much stress on the bone and become a source of fracture.
c. In general, 3 pins per fragment are used.
6. Size of the fixation pins
a. Larger pins are more resistant to deformation
b. They are more resistant to tearing and premature loosening
a) Pins approaching but not exceeding 20% of the diameter of the bone are recommended
b) Pins with a large diameter will result in weakening of the bone (fracture)
7. Connecting bars
a. Distance of connecting bars with bone
i. Leave enough space to allow postoperative swelling (1 cm in a 30-kg dog)
ii. Increasing the distance between the bone and the connecting bars decreases the rigidity of the fixator.
8. Connecting bars
a. Stiffness of these
i. The type of connecting bar influences the rigidity of the frame.
ii. Carbon or aluminum bars are stiffer than stainless steel because they are often of larger diameter.
iii. The addition of a juxtaposed connecting bar doubles the rigidity of the frame!
1. Immediately post-op and for the first 3–5 days
a. Use antiseptic/antibiotic ointment and sterile gauze at the skin/pin interface
b. Pack a substantial amount of sterile gauze between the skin and the connecting bars and cover the entire assembly with a sterile bandage
c. Change the bandage every day for 3–5 days
i. Remove the bandage
ii. Clean the skin/pin interfaces with chlorhexidine (0.05 to 0.2%)
iii. Apply ointment and redo bandage
iv. Which will be changed every day for 3–5 days after the surgery
2. After 3–5 days
a. Remove the bandage and change it to a bandage that covers only the fixator, especially the tips that could hurt the patient or his owners
b. Apply a little ointment until the granulation tissue is visible around the fixation pins
i. In general 5–7 days
3. For the duration of the fixator
a. Verification of the frame by the owner every day and removal of crusts that form around the fixation pins and clean the skin/pin interface daily
b. Rechecks every two weeks at the veterinarian for
i. Verification of the frame condition
c. Radiographic reassessment every 4 weeks
1. Discharge at the skin/pin interface
a. This is the most common complication
b. Due in general to soft tissue movement
a. Differentiate between superficial infection vs. osteomyelitis
i. During superficial infection, there is no loss of function
ii. During osteomyelitis, loss of function
iii. Radiographic evaluation justified in case of loss of function or change of weight bearing
3. Premature loosening of one or more fixation pins
a. Appearance discomfort and abundant discharge = loosening of one or more pins
b. Recheck as needed
i. Removal of pins
ii. Replacement of pins - new location
iii. Adding pins - new location
4. Frame failure
a. Inappropriate assembly - not rigid enough
b. Fracture of the bone
i. Pin diameter too large
ii. Too many pins per fragment
c. Too much stress on the frame because the patient was not kept at rest.
d. Consider an alternative method of fixation
Removal of the External Fixation Device
1. The device can be removed when the fracture demonstrates clinical and radiographic evidence of bone union.
2. The frame can also be dynamized, derigidified, for example
a. Type II to Type I
b. Remove a juxtaposed connecting bar
c. Remove a pin per fragment
3. Can be removed under sedation or under anesthesia
a. Threaded pins must be unscrewed!
4. The skin should not be sutured, wounds are cleaned daily until the end of healing by second intention (5–7 days)
5. The activity of the animal must be restricted for 6 weeks
a. Time that the fracture remodels
b. Holes left by the extraction of the pins are filled.
Other Types of External Linear Fixators
There are linear external fixators on the market whose connecting bars and clamps are replaced by tubing and acrylic. The Innovative APEF system is an example: www.innovativeanimal.com/acrylic-pin-external-fixation-system.php
The same principles of application apply, except that the ends of the pins are perforated through a tube of large diameter corrugated plastic, the distal end is plugged and acrylic is poured into the tubing. Liquid acrylic form polymerizes, thus becoming rigid. Polymethyl methacrylate (PMMA) is frequently used. The sterile form is expensive, but there is a non-sterile form used for the treatment of hooves in cattle (Technovit™).