4.10\ Cranioplasty

4.10.1\ Discussion

Intraoperatively fashioned acrylic cranioplasty is composed of methyl methacrylate resin that can be molded to a desired shape on the surgical field. The material tends to appear heterogeneous on CT and low signal on MRI (Fig. 4.21). The material often contains multiple foci of air due to the exothermic reaction that occurs when formed. The trapped bubbles should not be confused with infection. The methyl methacrylate at times can migrate prior to hardening, the appearance of which can potentially mimic hemorrhage on CT.

Preformed acrylic methyl methacrylate plates are specially molded to fit individual craniectomy defects using a computer-aided design system and 3D CT data. These flaps are usually secured to the adjacent calvarium using titanium plates and are thus continuous with the calvarium. On CT, preformed acrylic plates demonstrate homogeneous intermediate attenuation, around 100 HU (Fig. 4.22). Unlike the acrylic plates prepared intraoperatively, the preformed plates do not contain air bubbles. However, the preformed acrylic plates contain holes that are drilled to promote tissue ingrowth and leave a pathway to prevent accumulation of fluid in the epidural space.

Hydroxyapatite cement paste is sometimes used to close off small gaps in the calvarium. It can be molded to match the particular anatomy and can be applied in conjunction with other materials, such as titanium mesh. The cement

appears as homogeneously hyperattenuating on CT, similar in attenuation as natural bone (Fig. 4.23). On MRI, the hydroxyapatite appears as a signal void.

Titanium mesh is commonly used as a cranioplasty material. These plates generally produce good cosmetic results and minimal discomfort. Titanium produces minimal streak artifact on CT (Fig. 4.24). Solid titanium plates are now infrequently used but may be observed on follow-up imaging (Fig. 4.25).

Porous polyethylene implants are also suitable for covering cranial defects. This type of implant is also custom created for each patient using a computer-aided design system and 3D CT data. The material enables soft tissue and bone ingrowth and displays low attenuation on CT and low signal intensity on T2and T1-weighted MRI sequences (Fig. 4.26).

Synthetic bone grafts that are biocompatible have been developed for cranioplasty. For example, Bioplant HTR Synthetic Bone is a microporous composite of poly(methyl methacrylate) (PMMA), polyhydroxyethylmethacrylate (PHEMA), and calcium hydroxide. On CT, the HTR cranioplasty appears as heterogeneous with an overall attenuation that is greater than soft tissue but lower than bone (Fig. 4.27).

Autologous bone grafts can also be used for cranioplasty. These are often used in the form of split calvarial grafts in which the inner table is separated from the outer table in order to increase surface area for maximal coverage of a craniectomy defect (Fig. 4.28).

Fig. 4.22  Preformed acrylic cranioplasty. Axial CT image (a) demonstrates a high-attenuation left frontal acrylic cranioplasty (arrow), conforming to the natural

Fig. 4.23  Hydroxyapatite cement. Coronal CT image shows the hyperattenuating material (arrow) filling a gap between the craniotomy flap and the adjacent skull

contours of the calvarium. The plate is traversed by numerous holes to allow tissue ingrowth. Photograph of customized acrylic cranioplasty flap without holes (b)

Fig. 4.28  Split-thickness bone graft cranioplasty. Initial 3D CT image (a) shows a right temporal skull defect (*). 3D CT image after cranioplasty (b) shows interval harvesting of bone from the right parietal calvarium and repositioning it into the temporal skull defect (curved

arrow). Corresponding coronal CT image (c) shows the split calvarium at the donor site (arrow) and the repositioned split calvarial graft in the right temporal region (encircled)