Autologous Bone Harvest to Correct Open Skull Defects

DEFINITION

Skull defects can be congenital or acquired.
- Congenital defects can be the result of encephalocele, cutis aplasia, or foramina parietalia permagna.
- Acquired defects can be the result of trauma, tumor or tumor extension, osteomyelitis, bone flap loss, or persistent skull defects following surgery or prior cranioplasty.
Skull defects can result in lack of bony protection to the brain, contour deformities resulting in visible defects, as well as visible pulsatile regions on the face that can cause significant body image concerns for the patients.¹^,²

ANATOMY

Layers of the scalp include the skin, galea aponeurosis, followed by a layer of loose connective areolar tissue, and the pericranium (periosteum).
The mature skull consists of three distinct layers, the outer table, medullary cavity, and inner table.
Deep to the skull is the endosteal layer of the dura mater.

PATIENT HISTORY AND PHYSICAL FINDINGS

Large skull defects are noticeable—particularly on the frontal and anterior temporal regions anterior to the hair line—due to the associated contour deformity. They may be associated with pulsatile movement of the overlying scalp.
Skull defects posterior to the hairline may be palpable on exam, though not visually detectable.
Bulging of soft tissue over a skull defect may be an indication of increased intracranial pressure. This is a particularly important finding in the setting of multisuture cranial synostosis with residual skull defects following prior cranioplasty and indicates the need for cranial vault expansion in addition to reconstruction of the skull defect.
The soft tissue in the region of the bony defect should be evaluated. Evaluation should include assessment of prior surgical scars and evidence of previous radiation injury to the skin.
The volume and contour of the soft tissue envelope over the calvarial defect should be evaluated. This is particularly important if the calvarial vault will undergo significant expansion with bony reconstruction. Expansion may result in inadequate soft tissue coverage over the bone grafts.¹

IMAGING

A CT scan of the head and maxillofacial skeleton with fine cuts should be obtained for preoperative planning purposes.
3D reconstructions of the calvarium are used to visualize the defect and plan for reconstruction (FIG 1).

SURGICAL MANAGEMENT

The goals of calvarial vault reconstruction are to provide adequate protection for intracranial structures and to re-establish an aesthetic cranial contour.

Preoperative Planning

Preoperative CT scan with 3D reconstruction images should be reviewed.¹^,²^,³
Location and size of the bony defect should be assessed.
An appropriate calvarial donor site should be identified. Adequacy is determined by the presence of sufficient bicortical calvarial bone.
If inadequate calvarial bone is available, iliac crest or split rib grafts should be considered.

Positioning

The patient is placed in a supine or prone position, depending on location of the calvarial defect. A gel-padded Mayfield head rest (horseshoe) is positioned based on the location of the defect and bone graft donor site.
- If the patient is positioned prone, careful padding is required to avoid pressure on the globes.
If a posterior calvarial donor site is planned for anterior bone grafting, the patient may require closure of the donor site and repositioning for access to the anterior calvarium.

Approach

Surgical approach to the calvarial vault is typically performed via a bicoronal incision, which provides adequate access to the skull defect as well as to potential bone graft donor sites.
A bicoronal incision can be modified based on prior surgical scars or if a noncalvarial donor site is planned.
Transection of prior scars should be avoided because it compromises blood flow to the scalp.
Iliac donor sites can be approached through either an anterior or a posterior approach.¹
Split rib grafts are approached via a limited thoracotomy incision.⁴

FIG 1 • Preoperative 3D CT scans of a patient with Saethre-Chotzen. A. The AP view demonstrates a right forehead skull defect and a left orbital rim defect. B. Right lateral view demonstrates the right lateral forehead defect. The parietal skull demonstrated adequate thickness for split calvarial bone graft harvest.