Reconstruction of cranial defects after tumor resection includes the recreation of the skull and the overlying soft tissues.
Cranioplasty refers to the procedure of recreating the calvarium to protect the brain from injury. It is also important to give the patient a normal appearance. It is indicated for the repair of defects in the cranial vault. Small defects are non-problematic and are generally covered with burr hole cover plates. However, resection may result in significant loss of bone requiring more elaborate methods of reconstruction.
Incision around tumor
Incisions are marked out at least 1.5 cm around all visible and palpable tumor in the skin.
Dissection is carried through the skin incisions marked around the tumor and continued through the subcutaneous tissues until the cranial bone is encountered.
The periosteum is incised along the margins of the resection.
A burr hole is first placed on the edge of the planned resection line and the neurosurgeon then completes the osteotomies using a craniotome.
Removal of the bone segment and tumor
The tumor is then released and submitted en bloc for permanent pathological examination.
Soft tissue surgical margins are now checked with frozen section to ensure the adequacy of the tumor resection.
3. Reconstruction of the bony defect
Various materials are available for performing cranioplasty.
While autogenous bone is an excellent match and is the only material that will be revascularized over time. It will however require a donor site. Harvesting bone from a donor site adds time and potential morbidity to the procedure. There is also a risk of resorption of the grafted bone.
Donor site options include split calvarium from another portion of the skull. Some surgeons have used split rib because of its natural curvature and the ability to shape it.
When bone is used in cranioplasty, it must be rigidly fixed to minimize resorption. The junction between the native bone and the graft requires a non load-bearing repair, therefore, small thin plates and short self-drilling screws can be used to avoid the risk of penetrating the dura. Depending on the geometry of the defect, typically 3 small plates will stabilize the reconstruction, though sometimes more may be necessary.
Numerous materials can be used to repair skull defects.
With the development of sophisticated modern technology, it is now possible to utilize the data from high resolution CT scans to precisely recreate the defect that we wish to repair on a computer generated model. This allows for creation of a custom-made implant to precisely fit the defect. Depending upon the system used, a variety of implant materials may be utilized. These implants will have small overhangs that can be fixed to the surrounding bone...
...or they may just lay into the defect requiring stabilization with plates.
Note: some implant materials such as titanium can not be adjusted in situ while others such as high density porous polyethylene and polymethylmethacrylate can be altered with a drill at the time of implantation.
Polymethylmethacrylate cement (PMMA, mixed soft and molded)
The powder and the liquid components are mixed into a thick paste which is laid over the dura with a spatula and then molded to precisely fit the defect. The surface is molded to match the contour of the normal skull.
Note: The setting of the cement is an extremely exothermic reaction, and the cement can actually become too hot. Neurosurgeons worry about the potential damage to the underlying brain parenchyma leading to seizures and prefer to remove the implant and to allow it to set outside of the patient. Care should be exercised to minimize inhalation of the fumes by the surgical team.
Depending upon the size of the defect titanium mesh may be used alone as the sole method of repair or it may be combined with a hydroxyapatite cement to create a better contour.
When used alone, the appropriate mesh is selected and cut so that it will extend at least 1 cm beyond the actual defect on all sides. It is then fixed in place circumferentially with enough screws to ensure stability.
For use with hydroxyapatite cement see below.
One of the disadvantages of using titanium is the artefact it causes if MRI or CT scans are obtained in the future.
Hydroxyapatite (HA) cement
Some surgeons will use HA cement alone for repair of small defects. The use of HA cement to reconstruct large calvarial defects is ill advised. HA cement placed over these defects can fragment over time due to the pulsations of the dura and can result in late postoperative infection.
Others prefer to use the HA cement in combination with the titanium mesh that is placed into the defect so that it is at the level of the inner table. The residual defect is then filled with HA cement so that it conforms to the shape of the outer table. To perform this, a malleable mesh (3-D mesh) is placed across the defect and fixed with screws around the periphery.
The central portion of the mesh is then pressed into the defect so that its shape corresponds to the natural inner-table position. (If the mesh used does not stretch, then it must be contoured before it is fixed.)
The residual defect can then be filled with HA cement. The cement is mixed until it has a putty-like consistency and then it can be applied over the mesh with a spatula and smoothed to match the contour of the natural skull.
The flap is harvested larger than what is necessary to close the cutaneous defect as it will contract after harvest and the surrounding scalp can not be stretched compensate for the contracture
The long pedicle length of this flap allows the use of the ipsilateral neck for recipient vessels without vein grafting.
Insetting of flap and closure
The flap is placed into its preplanned position on the scalp with care not to create undue torsion or tension within the pedicle.
The pedicle passed from the scalp defect into the neck or face through a prepared subcutaneous tunnel or it can be placed into an incision made from the defect into the neck or face, for subsequent anastomosis with recipient vessels.
The skin closure is carried out in layered fashion using absorbable sutures and nylon sutures for the skin..
5. Revascularization of flap
The detailed procedure for the revascularization is outside the scope of this surgery reference. However, in short the procedure consists of the following steps:
Appropriate recipient vessels are selected in the neck or face, and dissected so as to be available for anastomosis
The recipient and the donor vessels adventitia are cleaned under a microscope
Appropriate vessel geometry is assured and the vessels are placed into a microvascular clamp and anastomosis carried out using 9-0 nylon sutures
Revascularization is restored after both arterial and venous anastomoses are completed
6. Aftercare following cranial vault reconstruction
General postoperative care
Intensive care 24 hours
Hospitalization 3-7 days until stable with no CSF leak
The use of broad-spectrum antibiotics in the immediate perioperative period is recommended.
Radiologic control examinations are performed routinely the next day after leaving the intensive care unit.
Patient follow-up after discharge. The patient is seen 7-14 days postop, then monthly until completely healed.