1 Principles topenlarge
Lag screw fixation
Lag screw fixation uses stabilization by compression that relies on the bony
buttressing of the fracture to help stability.
One reason why the mandibular body is well suited to lag screw fixation is the thickness of the bony cortices which provide extremely secure fixation when the screws are properly inserted, providing interfragmentary compression.
Click here for a detailed description of the lag screw technique and its biomechanical principles.
Zones for screw placement in the mandibular body
There are two intrabone structures that must not be harmed by screw insertion: the mandibular canal and the tooth roots.
Danger zone – “no mans land”
The classical description for screw placement in plate and screw osteosynthesis is depicted for orientation. Monocortical screws can be inserted above the level of the mandibular canal. The use of bicortical screws is restricted to the area below the course of the nerve canal at the level of the lower mandibular border.
Variations of mandibular canal and alveolar nerve
There may be variations of the course of the mandibular canal and the alveolar nerve. The standard situation is depicted in the previous step, but the vertical height of the canal can be located next to lower border of the mandible (as illustrated here).
The alveolar nerve itself can consist of a single bundle inside the canal
but it can also spread in a plexus formation inside the overall bony
cross-section of the mandible. These situations prevent the usual screw
insertion and require an individual screw and/or plate placement.
Click here for further information on principles of the inferior alveolar nerve.
Screw insertion pattern
Screw insertion can be done in a serial pattern at the lower border of the mandible.
Alternatively, screws can be placed in a tripod fashion using the lower and the upper insertion zone. In the upper zone, the anatomic relation between the nerve canal and the tooth apices varies. Sometimes there is no space between the tooth apices and the nerve, which is a clear contraindication to follow this pattern.
Using the lag screws in a tripod fashion provides additional stability. Note how the superior screw avoids both the dental roots and the alveolar nerve in the illustration.
Number of screws
In order to withstand rotational forces a minimum of two screws is required. For additional stability it is recommended to use at least three screws especially if 2.0 mm screws are applied.
Following special considerations may need to be taken into account:
- Multiple fractures
- Edentulous atrophic fractures
- Teeth in the line of fractures
- Involvement of alveolar area
- Infected fracture with or without bone loss
Click on any subject for further detail.
2 Reduction topenlarge
In a first step the patient is placed into MMF. The bony fragments are then reduced manually.
A fixation clamp can be applied to maintain the reduction. Therefore, a towel clamp may be used transcutaneously to keep the reduction at the lower border with the prongs coming in from the medial side and laterally. Usually, there should be no risk of injuring the marginal mandibular branch of the facial nerve. The accessibility to the lower border may be compromised by the towel clamp.
With a large dimension reduction forceps it is possible to prefix the lateral fragment in a sagittal fracture configuration to the contralateral outer cortex. On the contralateral cortex the prong is applied transmucosally.
Another method to maintain the reduction is the use of MMF screws. They must be applied monocortically in the posterior fragment. All screws in the lower jaw must be integrated into an appropriate cerclage with the maxillary counterpart to keep the fragments aligned. The posterior fragment should have traction to the anterior maxilla and vice versa.
In this case an arch bar is not suited for reduction of the fragments. It can fix only one of the fragments via the teeth. This will usually be the larger anterior fragment.
3 Fixation topenlarge
The vertical angulation of the screw should be varied as far as possible to meet the bevel of the fracture at 90° and to provide an uninterrupted approximation of the inner and outer fragment during the compression process.
Alternative: transbuccal system
The use of the transbuccal system may become necessary in the dorsal caudal
region of the mandibular body, which is inaccessible transorally with a screw
Click here for a detailed description of the transbuccal system.
Maximal stability between the fragments is achieved by compressing the rough surfaces of the fracture interface as it yields maximum frictional effect.
A crucial point is the choice of the appropriate screw length so that the far tip of the screw fully engages the far cortex with the screw tip exiting slightly above the bony surface.
Click here for a detailed demonstration of lag screw technique.
The MMF is now released. A control of the final reduction is hardly possible by clinical assessment. Only the anterior outer fracture line can be checked for accurate alignment. The internal bony situation will only be deduced from the general control of the occlusion and articulation. The anatomic reduction can only be assessed postoperatively with 3-D imaging techniques.
Illustration shows the completed osteosynthesis with fully inserted bicortical screws.
X-ray shows completed osteosynthesis
4 Case example topenlarge
In this case the sagittal fracture line runs in transitional zone between the posterior body and the angle.
Lag screw insertion
According to the principle, lag screws are inserted perpendicular to the fracture line. Three screws of different length were inserted to fully engage the far cortex.
The X-ray shows the completed osteosynthesis.