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 (load-sharing osteosynthesis).
Lag screws should always be placed perpendicular to the fracture plane to
prevent displacement of the fragments when the screws are tightened and the
bones are compressed.
Click here for a detailed description of the lag screw technique and its biomechanical principles
Biomechanics of the symphysis
The mandibular symphysis undergoes torsional forces (twisting) during function. Therefore, fixation strategies must take this into account. When using anything less stable than a reconstruction plate, two points of fixation should be applied.
In general, the further apart the points of fixation, the more stable the construct. For symphysis fractures, when two lag screws are applied, they should be separated as much as possible without injuring vital structures.
Illustration shows Champy’s ideal lines of osteosynthesis for symphysis fractures.
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.
AO Teaching video on lag screw insertion
3 Reduction topenlarge
Drilling monocortical holes
It is necessary to predrill two monocortical holes below the apices of the teeth on either side of the fracture to help place the reduction forceps. Manipulate the mandible fragments until anatomic reduction is achieved. Apply the reduction forceps and then place the patient into occlusion and secure with MMF.
Some surgeons prefer to place the patient into occlusion and apply MMF before using the reduction forceps.
The clamp has to be placed perpendicular to the line of fracture to prevent fracture displacement when tightening the reduction clamp.
4 Fixation topenlarge
Lag screw insertion
Depending on the fracture plane orientation, lag screw alignment will vary. For sagittal fractures through the anterior mandible, lag screws placed through the outer cortices from one side to the other within the substance of the mandible (buccal cortex to buccal cortex) provide extremely stable fixation.
Note that the screws and the resultant compression are directed perpendicular to the bevel of the fracture.
For fractures that obliquely pass through the mandible, lag screws are placed from the buccal to the lingual cortices.
Note that the screws and the resultant compression are again directed perpendicular to the bevel of the fracture.
Click here for a detailed demonstration of lag screw technique.
Number of screws
In general, a minimum of two lag screws should be used to provide stable
internal fixation of mandibular symphysis fractures.
Because the symphysis undergoes twisting during function, a single lag screw cannot prevent such motion from occurring.
Alternative: screws placed from opposite sides
Occasionally it is more convenient to place screws from opposite sides. From a biomechanical standpoint it is irrelevant.
Confirmation of reduction
Confirm adequate reduction. There should be no gap at the lingual aspect that would lead to occlusal disturbance and mandibular widening.
MMF should be released and the occlusion checked.
Because two points of fixation have been applied (two lag screws), it is not essential that the mandibular arch bar remains in position to function as another point of fixation
X-ray shows the completed osteosynthesis.