4.9\ Craniotomy

4.9.1\ Discussion

Craniotomy consists of opening the cranial vault by removing a bone flap during the course of surgery­ and replacing it at the completion of the procedure, as opposed to craniectomy in which the bone is removed and not replaced. Once the skull is exposed by raising the overlying scalp and pericranial flap, burr holes are drilled, from which the bone flap is created using a saw or drill. The bone flap is set aside during the procedure and replaced upon completion of the surgery. Some of the standard types of craniotomy include the following:

•\ Pterional

•\ Orbitozygomatic

•\ Modified orbitozygomatic •\ Frontal or bifrontal

•\ Parietal or biparietal •\ Subtemporal

•\ Anterior parasagittal •\ Posterior parasagittal •\ Suboccipital

•\ Retrosigmoid •\ Pre-sigmoid •\ Far lateral

•\ Hemicraniotomy

Skin staples are often used to close the scalp flap after surgery and can be present on imaging during the early postoperative period (Fig. 4.14). A variety of devices and methods are available to secure cranial bone flaps following craniotomy. The most commonly used are microfixation plates or clamps (Fig. 4.15). Microfixation plates are often composed of titanium, which produces minimal streak or susceptibility artifact. Complications related to the presence of microfixation plates and screws, such as transcranial migration, are uncommon, except in young children, where absorbable hardware or suture may be used. Stainless steel wires threaded across the craniotomy margin to the bone plates is no longer performed in developed countries, but may still be encountered on imaging (Fig. 4.16).

Hinge craniotomy is an alternative to traditional decompressive craniectomy and allows swollen brain parenchyma to expand extracranially while avoiding some of the complications associated with cranial revision. The bone flap is left attached on one side to the scalp soft tissues, usually the temporalis muscle. This results in the appearance of an outwardly displaced bone flap on CT (Fig. 4.17).

The normal imaging appearance of the craniotomy site can evolve over time. In the early postoperative period, the bone flap margins are sharp and should align precisely with the rest of the skull, unless a craniectomy was done to provide more surgical accessibility. The extent of dural enhancement that normally occurs after craniotomy ranges considerably. Dural enhancement is present in the majority of patients following cranial surgery and can be seen within the first postoperative day (Fig. 4.18). The presence and degree of enhancement are largely dependent on the time elapsed since the time of surgery and the types of substitutes used during closure, which in some cases can persist indefinitely. Dural enhancement tends to be apparent earlier and lasts longer with gadolinium-based contrast than with iodine-based contrast. Granulation tissue also normally forms along the edges of the bone flap, which manifests as linear enhancement that is often visible on MRI and often accompanies dural enhancement. This type of enhancement usually persists up to 1 year following craniotomy.

Pneumocephalus is the presence of intracranial air and is an expected finding after recent cranial surgery. Indeed, virtually all patients exhibit some degree of pneumocephalus during the immediate postoperative period. Pneumocephalus can be identified by air attenuation on CT and signal voids on MRI (Fig. 4.19). Regardless of the location, pneumocephalus normally resolves within 3 weeks of surgery.

During the early postoperative period, typical changes also occur in the soft tissues overlying the craniotomy site, including temporalis muscle swelling (Fig. 4.20), which likely represents edema due to manipulation during surgery.

Fig. 4.15  Microfixation plates. 3D CT image (a) shows a variety of titanium microfixation plates securing the bone

flap. Photographs of a variety of low-profile fixation plates (b–d) (Courtesy of Patricia Smith and Sarah Paengatelli)