Access osteotomy – Subcranial access osteotomy

The subcranial access osteotomy gives access to the:

• Medial aspects of the orbits of both sides
• Backportion of the cribriform plate (limited)

The coronal approach is used to expose the forehead.

While elevating the coronal flap, it is critical to maintain the integrity of the pericranial flap.

The osteotomy in the anterior sinus wall may be designed in a linear shape within the boundaries of the sinus outline. Posterior wall removal may be limited to the resection needs, thus facilitating sinus obliteration if necessary.

The margins of the frontal sinus are irregular and may be depicted by transillumination or a template obtained from a plain X-ray.

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:

• 6-feet (1.83 m) Caldwell x-ray with coin reference
• Intraoperative navigation

A 6-feet (1.83 m) Caldwell x-ray (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-feet 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 in the X-ray. 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.

After the proposed osteotomy has been marked, 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 replaced. 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 marked osteotomy line. This can be accomplished by removal all but one screw located on stable bone outside the osteotomy margin..

A sagittal saw or a side-cutting burr can be used to perform the osteotomy.

Next, the saw or drill is used to osteotomize the orbital rims and glabella. Care should be taken to protect the orbital contents and supratrochlear/supraorbital neurovascular pedicles.

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.

Utilizing both cutting and diamond round burrs the posterior sinus table is drilled until the dura is identified. 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.

The drill can be used to take down the necessary amount of the posterior table wall to expose and remove the tumor. In addition, once the dura is exposed Kerrison rongeurs can be used to take down the bone.

As the defect gets larger, it is possible to use a double action rongeur to remove additional 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.

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.

An elevator is then used to circumferentially elevate the mucosa in the frontal recess bilaterally. The mucosa is then removed or inverted and pushed inferiorly to obstruct the outflow tract. It is important that no mucosa remain in the frontal sinus. Free fascia can be 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.

Each bone graft fragments are trimmed to fit into the frontal sinus infundibulum using a fine bone rongeur. As stated earlier, some surgeons prefer to use temporalis fascia or muscle to seal the frontal sinus infundibulum (outflow tract).

The graft is then wedged into place to obstruct the sinus outflow tract.

The anterior table bone fragment is then repositioned and the pre-applied plates are rotated back into position to fixate the fragments.