1 Principles topenlarge
Temporary versus definitive treatment
Theoretically, the mandibular external fixator could be used to achieve definitive bone healing. Nevertheless, the external fixator does not offer the same degree of stability compared to internal devices (reconstruction plates).
Most often, the application of the external fixator device will be on a temporary basis only, with subsequent replacement by an internal fixation system.
The external pin fixation device gives a high degree of freedom for the frame assembly as the pins can be placed selectively into each segment and connected with short bars to constitute a subunit. Subsequently, the subunits are joined with further connecting elements to make up the complete framework. In this process each subunit can be manipulated into a reduced position until final tightening of the whole construct.
Other limitations of external fixators
Some disadvantages of an external fixator are:
- The extensive amount of hardware necessary to the complex construction
- The quality of the bony reduction is not guaranteed as the fragment ends are not often surgically exposed
Alternative: biphasic pin fixation
An alternative to the modular technique is the biphasic pin fixation (also known as Joe Hall Morris fixation).
Subsequent to the first phase where fracture alignment is achieved with adjustable connecting rods between the pin pairs (not shown in the illustration), is the second phase when the aligned pins are covered with a silicon tube, eg, endotracheal tube, injected with methyl methacrylate resin. Alternatively the pins can be connected with a moldable plastic shield that hardens after application.
Finally the adjustable rods are removed. This procedure is highly flexible and results in a lean construct.
To optimize the framework stability it is recommended to:
- Choose large pin diameters
- Use at least two pins in each fragment
- Keep a large distance between the pin pairs
- Place pins next to fracture line as close as possible to the fracture line but not less than 1 cm
- Place the connecting rods or plastic bar close to the skin surface in order to keep the lever arms short.
2 MMF topenlarge
If the major fragments are able to be aligned using any of the current MMF techniques, these are used for simplification of the overall assembly process.
3 Pin insertion topenlarge
General consideration for pin insertion
The pin insertion is done through the soft-tissue envelope overlaying the safe zones (click here for a detailed description of the safe zones).
A pair of two pins, if possible, is inserted into each major fragment at appropriate distance to each other and the adjacent fracture lines.
The length of the threaded portion of the pins is chosen to attain bicortical engagement.
Make a small stab incision to prepare for pin insertion at the predetermined
screw locations in the posterior mandible.
The stab incision is done with the blade parallel to the RSTL (relaxed skin tension lines).
Soft-tissue dissection and protection
Bluntly dissect a soft-tissue canal onto the bone and pass a trocar for soft-tissue protection through the canal until it contacts the bone.
A self-drilling pin is loaded into a handle. Using the trocar as a guide, the pin is driven into the bone down to its stopping bevel. Then the handle is disengaged from the pin and the trocar is removed.
If a self-drilling pin is not available or not advisable due to fragment instability, make sure to predrill prior to pin insertion.
Clinical photograph shows pin insertion.
4 Frame assembly topenlarge
Creation of subunits
The two pins in each fragment are connected with a rod and two clamps (as illustrated). The example here shows four subunits.
Linking the two posterior subunits
1.) Apply a connecting rod loosely between two subunits using rod-to-rod clamps.
2.) One fracture is manually reduced by manipulating two subunits.
3.) Tighten the rod-to-rod clamps
Linking the anterior subunits
Repeat steps 1), 2), and 3) for the two anterior subunits.
Final frame assembly
Now only one fracture gap is left between two large assembled mandibular portions as a result of connecting the anterior and posterior subunits. This gap is reduced and fixed through a connecting rod.
Illustration shows the final assembly of the external fixator using the modular technique.
MMF is removed after the final external fixator assembly to allow for mandibular function.
Alternative: using a bow
When a large circumference of the mandible requires external fixation, a bow-shaped rod can be directly attached to the pins.