Calvarial Reconstruction With Split Calvarial Grafts

David A. Staffenberg

Gerald J. Cho

ANATOMY

The calvarium (skull) consists of separate bone plates encasing the brain. Each named bone plate adjoins the adjacent bone plates at the cranial sutures.
- Paired frontal bones
- Paired parietal bones
- Occipital bone (occasionally with an accessory suture or mendosal suture)
The skull consists of three layers of variable thickness:
- Outer table (cortical bone)
- Middle layer (cancellous bone)
- Inner table (cortical bone)
These three layers are not present at the cranial sutures.
The entire calvarium can serve as a donor site for split calvarial grafts. There are two main danger zone, however:
- The midline, where the sagittal sinus resides subcortically
- Below the temporal crest, where the bone tends to be thinner, increasing the risk for inadvertent intracranial entry
Frontal bone can also be used when this is the site of surgery (eg, a frontal craniotomy is performed by the neurosurgeons for tumor access.
- The craniotomy bone flap can be split, during the neurosurgical portion of the procedure, if additional bone will be required for repair).
The thickest bone resides in the parietal and occipital bones.¹ Parietal bone is the most common donor site because it is the thickest.
Transcortical emissary veins can cause brisk bleeding during the osteotomy. This can usually be controlled with bone wax.
- Subcortical vessels can cause thinning, and thus, weaken the inner cortex.
The split calvarial graft may be a superior alternative to other bone graft donor sites.
- It offers a hidden scar if placed within the hair-bearing scalp.
- It is frequently a painless donor site, particularly when compared with iliac crest and rib.
Split calvarium will typically provide cortical bone grafts that have the following advantages over iliac crest and rib:
- Smooth surface
- Able to hold screws
- Able to bear load
- Warp resistant
In experienced hands, a large quantity of cortical bone can be harvested.
Splitting the calvarium is easiest when the diploe is well-developed.

PATIENT HISTORY AND PHYSICAL FINDINGS

When a donor site is being selected, note the patient’s handedness. This is to decrease potential morbidity in case there is inadvertent intracranial entry and damage to the motor cortex.
- A left-handed patient will have a left side donor site.
- A right-handed patient will have a right side donor site.
When there has been previous surgery, the surgeon must take note of:
- Previous scars, their condition and location
- Mobility of the scalp: An adherent scalp may indicate loss of the intermediate layers of the scalp and therefore the potential for subsequent wound healing problems.
- Palpable bony defects or hardware

IMAGING

Imaging may be useful to determine the characteristics of available sites.
CT scanning with bone windows is currently the most useful imaging modality.
- CT is particularly helpful when there are palpable bony defects or hardware present.
- The thickness of the inner and outer tables of the calvarium, as well as the thickness of the diploe, should be noted by the surgeon.
Imaging is recommended until the surgeon has gained suitable experience.

SURGICAL MANAGEMENT

Preoperative Planning

If coronal incision is being used, the scalp flaps can be retracted to expose the graft donor site.
- The coronal flap is elevated in a subperiosteal plane if a full outer cortical graft is being harvested down to diploe.
- If a split-thickness (partial thickness outer table) calvarial graft is being harvested, a subgaleal plane is elevated to preserve the periosteum.
If a coronal incision is not being used, we recommend a sigmoid-shaped incision over the donor site for wide exposure.
- The flaps are elevated in a subperiosteal plane if a full outer cortical graft is being harvested down to diploe.
- If a split-thickness (partial thickness outer table) calvarial graft is being harvested, a subgaleal plane is elevated to preserve periosteum on the intended bone graft.