Key points

Introduction

Skull defects and craniofacial bone abnormalities that require reconstruction are common in a variety of neurosurgical procedures. From the patient’s perspective, the primary reason for repair of these defects may be cosmetic. However, cranial bone provides important support and restores normal cerebrospinal fluid (CSF) flow dynamics, reducing the formation of pseudomeningoceles and protecting vital structures. Craniofacial reconstruction and cranioplasty have a long history, but new surgical techniques and a multitude of material options have recently fueled advancement in this area. This article describes the clinical indications for cranioplasty, preoperative management and timing of reconstruction, materials, and operative techniques.

Introduction

Skull defects and craniofacial bone abnormalities that require reconstruction are common in a variety of neurosurgical procedures. From the patient’s perspective, the primary reason for repair of these defects may be cosmetic. However, cranial bone provides important support and restores normal cerebrospinal fluid (CSF) flow dynamics, reducing the formation of pseudomeningoceles and protecting vital structures. Craniofacial reconstruction and cranioplasty have a long history, but new surgical techniques and a multitude of material options have recently fueled advancement in this area. This article describes the clinical indications for cranioplasty, preoperative management and timing of reconstruction, materials, and operative techniques.

Clinical indications for cranioplasty

Although largely an elective procedure, cranioplasty has several important indications and can improve quality of life for postcraniectomy patients. Following craniectomy, patients can develop skin depression and a sunken flap that can lead to an asymmetric appearance of the head. Although seemingly innocuous, this abnormal appearance can have major negative implications on the psychological well-being of the patient as well as how the patient is perceived by other people. Restoring the normal architecture of the skull can have significant psychosocial benefits to the patient as well as reestablishing the protective barrier of the skull.

Craniectomy essentially nullifies the Monroe-Kellie doctrine that governs intracranial pressure, CSF dynamics, and ultimately cerebral blood flow and can give rise to several complications, including extra-axial fluid collections; hydrocephalus; and sunken flap syndrome, also known as syndrome of the trephined. Sunken flap syndrome results from a combination of receding brain as swelling resolves, disturbances in CSF dynamics, and effects of atmospheric pressure. Miscellaneous neurologic symptoms are attributed to the hemispheric collapse and include headache, dizziness, fatigue, and psychiatric changes. Replacement of bone flap has been shown to lead to neurologic improvement, mostly in motor function, in small case series. Transcranial Doppler ultrasonography has shown improvement in cerebral blood flow following cranioplasty. Larger, controlled studies are needed to better understand the relationship between cranioplasty, cerebral hemodynamics, and clinical outcome.

Timing of cranioplasty

Timing of cranioplasty depends largely on the indication for craniectomy. Immediate cranioplasty has rare indications and may be performed for craniectomy for neoplastic invasion of cranium. Delayed cranioplasty is usually indicated for removal of bone flap for intracranial infection or medically refractory intracranial hypertension.

In cases of intracranial infection with suspected involvement and devitalization of bone, craniectomy is commonly performed. Although recent, small case series have shown the feasibility and safety of immediate titanium cranioplasty after bone flap infection, usually time intervals between craniectomy and cranioplasty between 6 weeks and 1 year have been identified. Ultimately the timing of cranioplasty is patient tailored and sufficient time must pass for adequate treatment and clearance of cranial (as well as any systemic) infection. The previous incision must be well healed and surrounding tissues must be vascularized. Inflammatory markers, such as C-reactive protein and erythrocyte sedimentation rate, as well as serial imaging, may assist in the determination of cranioplasty timing.

In patients who undergo decompressive craniectomy for intracranial hypertension ( Fig. 1 ) in the setting of traumatic brain injury or stroke, the patient’s neurologic status and intracranial pressure must have stabilized and the patient should be free of both systemic and cranial infection. As in cases of cranioplasty after intracranial infection, the patient’s incision should be healed completely. Traditionally, cranioplasty after decompressive craniectomy is performed at approximately 3 months, allowing sufficient time for neurologic and medical recovery, but the optimal timing remains controversial. Some practitioners have argued that early cranioplasty may improve CSF dynamics and lead to better neurologic recovery, although conflicting data in the literature suggest that larger prospective studies of the relationship between timing of cranioplasty and neurologic outcome are needed.

Cranioplasty after left decompressive hemicraniectomy for intractable intracranial hypertension. ( A ) Preoperative computed tomographic scan showing left skull defect. ( B ) Intraoperative view of autologous bone flap secured to native skull with plating system. ( C ) Postoperative computed tomographic scan showing cranioplasty.

( From Piazza M, Sean Grady M. Cranioplasty. In: Winn HR, ed. Youmans and Winn neurologic surgery. 7th edition. Philadelphia: Elsevier, 2017; with permission.)

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 are often inadvertently reflected with the skin flap.

Preoperative management

Once the decision to perform cranioplasty is made, important preoperative studies include computed tomography with bone windows; three-dimensional reconstruction may further guide operative management. MRI is occasionally useful if there is a question about the relation of soft tissue structures, such as scalp or dura, to the skull defect. In addition, 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. Autologous 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. 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. The largest disadvantage of frozen autologous bone graft is a higher rate of reported resorption compared with other cranioplasty materials, especially in children. Autologous bone flaps placed in deep-freeze preservation may be removed from storage on the morning of surgery.

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. Cultures of the bone flap obtained at this time must be reviewed before cranioplasty, because bacterial contamination of the bone flap often occurs in an indolent fashion. The most common isolated organisms are Propionibacterium acnes , Staphylococcus aureus , and coagulase-negative Staphylococcus . Traditionally, bacterial contamination of an autologous bone flap has been a contraindication for reinsertion during cranioplasty, although recent literation suggests that reimplanting bone flaps with positive culture swabs does not increase the risk of postoperative infection.

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 1 ).

Methyl methacrylate is polymerized ester of acrylic acid that exists in powdered form and is mixed with a liquid monomer, benzoyl peroxide. In an exothermic reaction, methyl methacrylate slowly cools from a pastelike substance into a translucent material with strength comparable with that of native bone. 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 secures it to the skull. Disadvantages of methyl methacrylate include postoperative infection, at a rate of approximately 5% to 10%, and plate breakdown or fracture. A methyl methacrylate prosthesis is at higher risk of infection compared with autologous bone flap because it is not viable, and a fibrous layer grows around the plate, to which bacteria may adhere. The most common organisms are S aureus and P acnes . Deep wound infection may be latent and not become clinically apparent for several years. Liquid methyl methacrylate may be absorbed by tissues and has been reported to cause acute hypotension and hypersensitivity. Different types of methyl methacrylate are commercially available. It is a composite material of polymethyl methacrylate and barium sulfate, creating a radiopaque bone cement.

Another option of synthetic prostheses is calcium phosphate bone cement, which, like methyl methacrylate, exists as a powder and forms a malleable substance when it is mixed with liquid sodium phosphate. When it is fully cured, the calcium phosphate prosthesis approximates the mineral phase of bone and is integrated into the native skull and remodeled over time to fit the defect. The most commonly used calcium phosphate material is hydroxyapatite, shown to be ideally suited for small craniofacial defects. When it is used directly against exposed dura, titanium mesh is recommended as an underlay to prevent small fractures in the hydroxyapatite plate from dural pulsations. In contrast with methyl methacrylate, which does not allow further expansion of a growing skull, hydroxyapatite bone cement is often used for skull defects in the pediatric population. Certain types of calcium phosphate prostheses, including hydroxyapatite, have the additional advantage of being osteoconductive, so they serve as scaffolding for growth of new bone.

Titanium mesh, either alone or in combination with methyl methacrylate, is another useful material for cranioplasty. Titanium is nonferromagnetic and noncorrosive, and it does not elicit an inflammatory reaction. Several series have reported a low incidence of infection while still achieving excellent cosmetic results. Most commonly, titanium exists as a metallic alloy with other metals to improve its strength and malleability. Titanium is also used to preform prostheses using three-dimensional computed tomographic reconstructions of the skull base defect.

Computer-designed implants from computed tomographic reconstructions are expensive but effective for complex skull defects. Anatomic models may be formed by polymerization of ultraviolet light–sensitive liquid resin with use of a laser, based on computed tomographic data. These stereolithographic models are then used to manufacture customized titanium plates, hydroxyapatite implants, or methyl methacrylate prostheses. Costs for these prefabricated prostheses may be as high as $4000; however, the precision has been reported to be 0.25 mm for implants as large as 18 cm.

New biocompatible materials and composite implants have recently been used for cranioplasty with excellent results. Porous polyethylene implants are composed of high-density polyethylene microspheres that create interconnected pores, allowing ingrowth of native bone. This unique implant structure rapidly incorporates fibrovascular tissue from the patient and decreases the infection rate of the implant. Porous polyethylene implants may be shaped to cover a large variety of skull defects and secured with titanium screws to native bone. A distinct advantage of this material compared with titanium is that it does not produce artifact on postoperative computed tomographic scans and magnetic resonance images. In a study of 611 cranioplasty procedures using porous polyethylene, all patients achieved excellent cosmetic results with no postoperative infections.

Further efforts to decrease cranioplasty implant infection rates have focused on antibiotic elution from hydroxyapatite cement materials. Hydroxyapatite cement is able to be impregnated with a variety of antibiotics intraoperatively. Tobramycin, a broad-spectrum aminoglycoside with activity against S aureus , gram-negative bacteria, and gentamicin-resistant pseudomonal species, has shown promise in cranioplasty materials. Studies have shown a predictable concentration and sustained release of tobramycin from hydroxyapatite cement for approximately 10 days.

Because each cranioplasty material has its own advantages and disadvantages, studies have examined hydroxyapatite–polymethyl methacrylate composites. Hydroxyapatite has good osteoconductivity but is fragile and cracks easily. In contrast, methyl methacrylate is easier to shape and is stronger, but it has relatively poor osteoconductivity. A composite of both materials using two-thirds hydroxyapatite and one-third methyl methacrylate showed almost the same osteoconductivity as hydroxyapatite alone at the surface of the implant, but it did not penetrate inside the composite. Various formulas of composites show excellent promise in cranioplasty because of the different properties of each substance.