Diagnostic Evaluation

Cranial defects may result from trauma, infection, or malignant neoplasms that grow through the skull. When to perform cranioplasty inherently depends on the nature and cause of the skull defect.

One of the most common indications for delayed cranioplasty is after hemicraniectomy for refractory intracranial hypertension (Fig. 32-1). Before the autologous bone flap may be replaced, the overlying scalp must be well healed and vascularized, intracranial hypertension and neurological status should be completely stabilized, and infections (both systemic and cranial) must be fully treated. Although cranioplasty is typically performed approximately 3 months after traumatic brain injury, recent reports indicate that in select, otherwise healthy patients, early cranioplasty after 5 to 8 weeks may aid recovery.^1,2 Other recent literature suggests that the improvement in neurological status after early cranioplasty depends on restoration of normal CSF flow dynamics. Several reports indicate that communicating hydrocephalus occurs at a high incidence after decompressive hemicraniectomy.³ Early cranioplasty combined with implantation of a programmable shunt improved patient outcomes and reduced complications.⁴ On a technical note, early cranioplasty after 5 to 8 weeks may allow easier discrimination of the various tissue layers when the skin flap is reflected. However, onlay synthetic dural substitutes, if used, may not have formed an adherence to the underlying native dura and will often be inadvertently reflected with the skin flap.

FIGURE 32-1 Cranioplasty after left decompressive hemicraniectomy for intractable intracranial hypertension. A, Preoperative computed tomographic scan demonstrating left skull defect. B, Intraoperative photograph of autologous bone flap secured to native skull with plating system. C, Postoperative computed tomographic scan demonstrating cranioplasty.

Patients commonly exhibit hemispheric collapse on the side of the calvarial defect, often referred to as syndrome of the trephined.⁵ Miscellaneous neurological symptoms are attributed to the hemispheric collapse and include headache, dizziness, fatigue, and psychiatric changes.⁶ The most common explanation of this depression of the brain is an alteration of CSF dynamics with resultant neurological compromise.^5,7,8 A report showed that cerebral blood flow, as measured by xenon-enhanced computed tomography (CT), increases after cranioplasty.⁹ However, without some degree of hemispheric collapse, replacement of the bone flap is technically challenging and requires additional CSF drainage at the time of surgery.

Infection after cranial surgery is another clinical scenario in which the native bone sometimes must be discarded, and delayed cranioplasty is preferred. Several reports have indicated that cranioplasty may be avoided after operative débridement of an infected bone flap.^10,11 However, when the infection has devitalized the bone flap or is extensive within overlying scalp or brain parenchyma, discarding the bone flap is required. A standard does not exist for the duration of time required after bone flap infection before cranioplasty may be performed. Conservative estimates have specified delay of reconstructive surgery for 6 months to 1 year.^12–14 Inflammatory markers, such as C-reactive protein and erythrocyte sedimentation rate, as well as serial imaging may assist in the determination of cranioplasty timing.

Immediate cranioplasty is indicated after resection of bone neoplasm, such as a meningioma, that has eroded through the skull (Fig. 32-2). In the adult population, use of synthetic cranioplasty materials such as methyl methacrylate is preferred for large skull defects (>10 cm) because of their ease of use for irregularly shaped defects. For smaller defects, titanium mesh alone is sufficient to achieve an excellent cosmetic result.¹⁴

FIGURE 32-2 Cranioplasty after resection of right frontal meningioma with growth through the skull. A, Intraoperative photograph of skull defect, with DuraGen underlying saline-soaked cotton balls. B, Intraoperative photograph of application of methyl methacrylate sheet over skull defect, applying pressure on native bone edges. C, Intraoperative photograph of methyl methacrylate plated into skull defect. D, Postoperative computed tomographic scan of the cranioplasty.

Preoperative Management

When cranioplasty is being planned, important preoperative studies include CT with bone windows; three-dimensional reconstruction may further guide operative management. Magnetic resonance imaging is occasionally useful if there is a question about the relation of soft tissue structures, such as scalp or dura, to the skull defect. Finally, preoperative management must include a thorough investigation of the patient’s underlying health status and search for any contraindications to cranioplasty. Patients who are hemodynamically unstable, are bacteremic, or have persistent intracranial hypertension may be deferred until a later time. In our practice, we defer cranioplasty if the patient has any active infection, including Clostridium difficile. Although it is unlikely that a gastrointestinal infection would contaminate the cranioplasty, this scenario is difficult to rule out when the patient is postoperative and actively febrile. In general, cranioplasty is an elective procedure and should be undertaken only when these other medical issues have resolved.

In cases of traumatic brain injury or stroke, the patient’s autologous bone flap must be removed from storage before cranioplasty. These bone flaps are usually either placed into deep-freeze preservation or subcutaneously preserved in abdominal fat. Some reports indicate that the preservation in subcutaneous tissue improves the bone viability, thereby lowering cranioplasty revision rate.¹⁵ However, storage at less than −28°C has been shown to be an effective method of preservation and avoids the additional morbidity of an abdominal incision.^16–19 The largest disadvantage of frozen autologous bone graft is a higher rate of reported resorption compared with other cranioplasty materials, especially in children.^20,21

Infection of the autologous bone flap is also a common complication, and sterile technique and care must be taken during the collection and storage preservation of the bone flap at the time of hemicraniectomy. Use of ethylene oxide gas to sterilize autologous bone graft before storage at room temperature has been shown to be an effective alternative to freezing the bone flap.²² Cultures of the bone flap obtained at this time must be reviewed before cranioplasty, as bacterial contamination of the bone flap often occurs in an indolent fashion.²³ Bacterial contamination of an autologous bone flap has been a contraindication for reinsertion during cranioplasty. Although it is not the standard of practice, some reports suggest insertion of infected autograft into the subcutaneous fat to allow the patient’s immune system to clear the infection before cranioplasty is attempted.²⁴

Cranioplasty Material Options

There is a large selection of possible materials for repair of skull defects, which may be categorized into autografts, allografts, xenografts, and bone substitutes. The success and durability of the operation require careful selection of a material tailored to the clinical scenario. The ideal material is malleable, sterilizable, nonmagnetic, radiolucent, lightweight, and able to be easily secured to existing skull (Table 32-1). Although a multitude of different animal bones, metals, and even coconut shell have been used in cranioplasty, this discussion focuses on those materials used in modern neurosurgery.¹⁴

TABLE 32-1 Comparison of Cranioplasty Materials

Methyl methacrylate is polymerized ester of acrylic acid that exists in powdered form and is mixed with liquid monomer, benzoyl peroxide. In an exothermic reaction, methyl methacrylate slowly cools from a paste-like substance into a translucent material with strength comparable to that of native bone.²⁵ During this cooling phase, methyl methacrylate may be shaped to fit any skull defect. Methyl methacrylate may be used for technically challenging areas of the skull, and reconstruction and growth from the native bone edge adjacent to the prosthesis will secure it to the skull. Disadvantages of methyl methacrylate include postoperative infection, at a rate of approximately 5% to 10%, and plate breakdown or fracture.^26,27 A methyl methacrylate prosthesis is at higher risk of infection compared with autologous bone flap because it is not viable, and a fibrous layer will grow around the plate, to which bacteria may adhere. The most common organisms are Staphylococcus aureus and Propionibacterium acnes.²⁸