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.
Lag screws should always be placed perpendicular to the bevel of the fracture to prevent displacement of the fragments when the screws are tightened and the bones are compressed.
In general, the lag screw is placed prior to the miniplate because the lag screw will compress the fracture, providing better reduction.
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 a lag screws and a plate 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.
2 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.
3 Lag screw insertion topenlarge
Sagittal fracture lines
Depending upon the bevel of the fracture, 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) provides extremely stable fixation.
Note that the screws and the resultant compression is directed perpendicular to the bevel of the fracture.
Oblique fracture lines
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.
Lag screw application
Click here for a detailed demonstration of lag screw technique.
4 Plate application topenlarge
Apply a 4-hole miniplate as far away from the lag screw as possible without damaging pertinent anatomy (root apices).
Contour the plate using bending pliers.
Position the plate in the desired location. Because miniplate fixation is adaptation osteosynthesis and does not compress the fracture, the plate can be placed in a direction other than perpendicular to the fracture line.
Drill first screw hole
Use a 1.5 mm drill bit with 6 mm stop to drill monocortically through the plate hole next to the fracture.
Insert a 2.0 mm screw, 6 mm in length. Do not fully tighten it until the final reduction and plate position are confirmed.
Insert second screw
Insert a second screw on the other side of the fracture in the same way.
Tighten both screws.
Additional screw placement
Fill the remaining plate holes with screws.
5 Confirmation of reduction top
Confirm adequate reduction. There should be no gap at the lingual aspect. A gap would lead to occlusal disturbance and mandibular widening.
MMF is released and the occlusion checked.
Because two points of fixation have been applied (one lag screw, one plate), it is not essential that the arch bars remain in position and they can be removed.
6 Case example topenlarge
Symphysis fracture of the mandible
X-ray shows fracture through the mandiblar symphysis.
Note the metal tongue piercing.
The fracture is reduced after exposure.
After putting the patient into MMF, a lag screw is placed in the most favorable area to accommodate it. In this case, a lag screw was placed at the inferior border of the mandible.
Because of the lack of curvature in the middle of the mandible, a second lag screw was thought not possible. Instead, a miniplate was used as a second point of fixation.
X-rays show the completed osteosynthesis.
6 weeks postoperatively, the patient had a good occlusion and
... rehabilitation of mandibular opening.