1 Introduction top
Severe injuries which result in disruption of greater than 25% of the posterior table should be considered for cranialization. This involves exposure of the entire sinus, meticulous removal of all sinus mucosa, and removal of the posterior table bone. The anterior table bone is replaced to reconstitute the forehead contour.
2 Exposure topenlarge
While elevating the coronal flap, it is critical to maintain the integrity of the pericranial flap. It may be necessary for repair of dural lacerations.
Pearl: identification of explanted bone fragments
Final orientation of any removed bone fragments can be challenging. Placing the explanted segments on a rough sketch of the skull made with a sterile pen will maintain orientation until re-implantation. Outline each segment and the overall shape of the defect.
The bone fragments should be kept moist until they are re-placed.
3 Defining the sinus margin topenlarge
The margins of the frontal sinus are irregular and may not be visible through the fracture.
It is however critical to determine the precise margins of the sinus to allow for an accurate osteotomy and complete exposure of the sinus. There are several ways to accomplish this:
- Bayonet forceps
- 6-foot (1.83 m) Caldwell x-ray with coin reference
- Intraoperative navigation
One tine of the bayonet forceps (or bipolar cautery) can be inserted into the defect. The other tine spans the anterior table. The internal tine is then “walked” around the periphery of the sinus. The external tine is then used to guide markings which will outline the periphery of the sinus on the outer table bone. The outline can be marked with ink or electrocautery (the electrocautery should be placed at a low setting to coagulate overlying blood and avoid excessive bone injury).
Alternatively, a light source (such as an endoscope) can be inserted through the anterior table bone defect. The light will transilluminate the sinus and delineate its margins.
Beware of pitfalls such as soft-tissue and blood present in the sinus.
6-foot (1.83 m) Caldwell x-ray with coin reference
A 6-foot (1.83 m) Caldwell x-ray with (anterior-posterior Caldwell x-ray with the patient placed 6 feet from the x-ray tube) can be used to delineate the margins of the sinus. The 6-foot penny Caldwell generates a “life-size” representation of the sinus cavity. It is imperative that the orientation (ie, right and left) is clearly documented on the x-ray.
Scissors are then used to cut along the margins of the sinus. Lateral “wings” that project along the orbital rims are also cut out to help with orientation. A second copy of the sinus template is generated from the first in case one is contaminated during the procedure. An “R” is scratched into the right side of both templates to record orientation. Both copies are sterilized and brought onto the surgical field.
The template is then placed over the sinus using the orbital rim “wings” to help with orientation. The template is held in place. The sinus periphery can then be outlined using ink or electrocautery as previously described.
Intraoperative navigation can be used to outline the periphery of the sinuses using the preoperative CT scan. A reference array must be fixed to the skull (or Mayfield head holder) to allow for accurate navigation.
The navigation system is used to guide the probe along the periphery of the sinus.
Ink or electrocautery can be used to mark the outline.
4 Osteotomy topenlarge
After the proposed osteotomy has been marked at the periphery of the sinus, thin plates are applied spanning the sinus margin. An adequate number of plates should be applied to provide stability when the anterior table segment is osteotomized. The plates should be pre-applied prior to the osteotomy. This allows for accurate repositioning of the anterior table bone.
Each plate should be rotated away from the proposed osteotomy line. This can be accomplished by removal all but one screw located on stable bone outside the sinus. The plate can then be rotated away from the sinus.
Perforating the anterior table
A sagittal saw can be used to perform the osteotomy. However, a side-cutting burr (as illustrated) is more controlled and accurate. The bit is used to drill sequential holes along the superior border of the sinus. Individual holes are separated by several millimeters.
The handpiece should be angled approximately 45° towards the sinus and away from the cranial vault to avoid violation of the posterior table.
Completion of superior osteotomy
The holes are then connected to create a single osteotomy on the superior margin of the sinus.
Osteotomizing the orbital rims/glabella
Next, the drill is used to osteotomize the orbital rims and glabella in a similar fashion. Care should be taken to protect the orbital contents and supratrochlear/supraorbital neurovascular pedicles.
Intersinus septum osteotomy
If the intersinus septum is intact, it may be necessary to insert a curved osteotome through the superior osteotomy site and fracture the intersinus septum just deep to the anterior table bone. Care must be used to avoid injuring the posterior table.
A curved osteotome is then inserted through the superior osteotomy to cantilever the anterior table and generate a controlled fracture of any remaining attached bone. A clamp should be used to control the anterior bone fragment as the osteotomy fracture is completed.
The sinus cavity is then suctioned free of any blood or mucous. An elevator and/or forceps are used to remove any bone or mucosa that has been displaced into the sinus cavity. Meticulous dissection technique should be used to avoid iatrogenic injury of the dura.
These injuries can result in severe disruption or even loss of anterior table bone. Therefore, larger bone fragments from the posterior table should be maintained for possible use with reconstruction of the anterior table or obliteration of the frontal recess.
5 Posterior table removal topenlarge
An elevator is used to separate the dura from the posterior table along the entire margin of the defect.
The dura should be elevated from the posterior sinus wall prior to bone removal.
A Kerrison rongeur is then used to initiate the removal of the posterior wall.
The process of dural elevation and bone removal should then be repeated. As the defect gets larger, it is possible to use a double action rongeur to remove the remainder of the posterior wall.
A diamond drill should be used to make a smooth contour between the sinus and intracranial cavities. A malleable retractor is used to retract and protect the brain while drilling. When drilling is complete, there should be a smooth contour between the cranial vault and the sinus.
Particular attention must be paid to the scalloped areas at the periphery of the sinus.
6 Mucosa removal topenlarge
A clamp is used to stabilize the free anterior bone segment(s) that were previously removed. A large diamond burr is then used to remove the mucosa from the inner surface. The mucosa should also be stripped of any posterior table bone fragments to be used for reconstruction of the anterior wall.
7 Closure of the recess topenlarge
An elevator is then used to circumferentially elevate the mucosa in the frontal recess bilaterally. The mucosa is then inverted and pushed inferiorly to obstruct the outflow tract. Free fascia is used for obliteration of the outflow tract.
Alternatively, a sharp 1-2 cm straight osteotome can be used to harvest a thin layer of outer layer calvarial bone. If the graft can be harvested from a region with Intact periosteum, this will help maintain the integrity of the graft. If posterior table bone fragments are available, these can also be used to plug the outflow tract.
8 Closure of sinus ostia topenlarge
Each bone graft is trimmed to fit into the frontal sinus infundibulum using a fine bone rongeur.
The graft is then wedged into place to obstruct the sinus outflow tract.