Computer-assisted surgical planning in head and neck reconstruction: Cranium

Introduction

The goal of cranioplasty is to restore structure and function of missing cranial bone while providing support to soft tissues. Choice of surgical approach and reconstructive material depends on the nature and size of the defect, the patient’s history, and surgeon preference. Cranioplasty techniques are broadly divided into two categories: alloplastic and autologous. Alloplastic material is useful for very large defects or when the underlying defect is of a complex contour, but it is more prone to foreign body reaction, infection, extrusion, and plate breakdown. Autologous bone graft can reduce the risk of infectious complications and technical failures associated with synthetic material but presents its own challenges. These include difficulty in shaping the bone and donor site morbidity when harvesting grafts for large cranial defects.

Computer-assisted design, computer-assisted manufacturing (CAD/CAM) technology can mitigate the challenges associated with both alloplastic and autologous cranioplasty techniques. In recent years, CAD/CAM has become increasingly popular for the prefabrication of patient-specific implants (PSIs). The ability to construct a well-fitting implant preoperatively may reduce procedure time and infection risk and is particularly important in cases where a large defect spans an area of intricate contour, such as the fronto-orbital region. CAD/CAM technology has been described in the fabrication of custom titanium, hydroxyapatite, ^, polyetheretherketone (PEEK), ^, porous polyethylene (Medpor), ^, and polymethyl methacrylate (PMMA) implants, as well as for production of intraoperative molds to be used with acrylic prostheses. Based on the computed tomography (CT) scan of the patient, an implant or mold is virtually designed using CAD software and then manufactured either through subtraction (high-speed milling) or additive manufacturing methods. Additive manufacturing has grown in popularity as it allows for the rapid production of complex 3D shapes from a CAD file in a layer-by-layer fashion, thus reducing the waste and cost associated with traditional subtractive techniques.

PSIs have been applied in a broad range of cases, including very large calvarial defects, repeatedly reconstructed cases, and pediatric craniectomy defects. However, the fabrication of these implants are associated with significant cost, based on size and complexity. The average cost for custom-made PEEK or titanium implants has been cited anywhere between an estimated $7000 to $15,000 USD, ^, limiting its availability in low-resource settings.

CAD/CAM is also a powerful tool in the realm of autologous cranioplasty, particularly in split calvarial bone grafting. Many of the commonly cited disadvantages of autologous cranioplasty, such as longer operative times, contour unpredictability, and donor-site morbidity, can be addressed by this technology. Through calculated measurements of the calvarium and estimation of three-dimensional constructs, the surgeon can determine the optimal site to harvest a graft in one piece to provide the most natural contour while avoiding critical structures such as dural venous sinuses. ^, Thus, CAD/CAM may improve efficiency and safety in split-calvarial bone grafting while helping the surgeon to avoid an irregular final construct.

Clinical considerations

The surgeon must consider several anatomic and clinical factors to achieve a successful cranial reconstruction with CAD/CAM technology. Defect size and location are primary concerns. The complexity of implant design increases with defect size, crossing of the sagittal midline, and involvement of the orbital rim, as it is difficult to model accurate contour symmetry in these cases. Prior reconstructive history is also important. In repeatedly reconstructed cases where residual material is often present at the defect site, the irregular wound bed surface can be modeled with CAD software to facilitate accurate implant placement intraoperatively. Koper et al. presented a series of 20 patients undergoing repeated reconstruction with PSIs and demonstrated no difference in operative time among those patients with residual reconstruction material still present compared to those without. Additionally, there was no need for intraoperative manipulation of the implants in either patient group.

A number of implant modifications using CAD/CAM technology have been reported to enhance surgical efficiency and safety. Holes may be designed in the implant for fluid drainage, dural suspension sutures, and temporalis muscle resuspension. Fixation points along the perimeter of the implant, including screw holes or indentations for plate placement, may be specified. These points can be angled precisely to facilitate screwdriver or drill access and avoid injury to the superior sagittal sinus for defects near the midline. The edges of the implant can be tapered to overlap the surrounding skull in to decrease strain forces on the implant and limit palpability of the edges. When there is concern about the ability to achieve tension-free closure, such as repeat reconstructions or irradiated fields, the contour of the implant may be slightly decreased to allow easier soft tissue coverage. Implant manipulation via CAD/CAM technology has also been used to correct temporal hollowing caused by bone resorption, temporalis muscle malposition, and/or temporal fat pad atrophy.

Additionally, CAD/CAM facilitates the use of split calvarial bone grafting for the reconstruction of complex defects traditionally considered not amenable to autologous techniques. Preoperative planning can help the surgeon identify the optimal donor site for large or previously reconstructed defects and avoid the need to harvest separate calvarial segments in a “piecemeal” approach. Measurements of the cranial diploe allow for identification of areas with sufficient width for splitting. Dural venous sinuses can be mapped to avoid craniotomy-associated damage during graft harvest. The final result can be visualized and further refinements to shape can be decided in the planning stage. The utility of CAD/CAM was demonstrated in a series of five patients with large cranial defects (mean defect size of 69 cm ² ) undergoing secondary cranioplasty. In this series, there were no dural tears, sagittal sinus bleeds, or other intraoperative complications, and no immediate postoperative complications requiring extended hospital stay or reoperation. All patients achieved successful reconstruction with satisfactory cranial contour on follow-up.

Workflow for computer-assisted design, computer-assisted manufacturing

A 3D CT scan is obtained to characterize the cranial defect. The surgical team and bioengineer meet to plan the operative approach. Depending on the chosen approach and complexity of the case, the design phase may span several days. Once the operative plan is finalized, manufacturing begins. In the case of alloplastic cranioplasty, custom implants and molds are usually produced within 7 to 14 days. They are then shipped to the medical facility and sterilized. In total, alloplastic implants are typically ready for surgery in fewer than 3 weeks. In the case of autologous cranioplasty, osteotomy guides are usually manufactured within 10 days and ready for surgical use within 2 weeks ( Fig. 6.1 ).

Computer-assisted design

Preoperative 1-mm spiral CT scan of the calvarium is obtained in axial, coronal, and sagittal planes. The data is converted to a 3D reconstruction. The area for cranioplasty is then mapped by the operator.

In the case of alloplastic reconstruction, a mirror image of the contralateral calvarium is transposed onto the affected side (if the defect does not cross midline), or an average skull image is superimposed on the patient’s calvarium (if the defect is midline). To model the implant, the defect-filling portion of the template is sequentially “cookie-cut” and thickness is added to create a tapered edge that abuts or overlaps the surrounding calvarium. The surgeon can determine fixation points at this time. In addition to confirming proper fit of the implant, this process ensures that there is no intersection with the adjacent dural sac. Consideration should also be given as to whether there is sufficient scalp to cover the implant or if additional measures (e.g., galeal scoring, tissue expansion) may be needed.

In autologous split calvarial bone cranioplasty, special attention is given to the contour of the remaining skull, diploe thickness, and location of the sagittal sinus to find an appropriate donor site. Using CAD software, a duplicate calvarium is transposed on the original skull and carefully repositioned to find a new region that best replicates the anatomical contour of the defect. Care is taken to select an area of calvarium with adequate diploe thickness for splitting. A template of the defect is generated and positioned over the determined donor site. From this transposition, an osteotomy guide can be manufactured for intraoperative use.

The CAD planning process is demonstrated for a patient undergoing alloplastic reconstruction (patient A; Fig. 6.2 ) and a patient undergoing autologous cranioplasty (patient B; Fig. 6.3 and Video 6.1 ).

Computer-assisted manufacturing

Additive manufacturing has become the preferred method for fabrication of custom cranial implants and intraoperative guides. An implant is built layer by layer from a CAD file, allowing the manufacture of complex contours with predefined porosity. Various techniques, including photopolymerization, laser sintering, and extrusion-based printing, have been used successfully. ^,

Implant modifications can be made to improve surgical accuracy and efficiency, shown for Patient A ( Fig. 6.4 ) and Patient B ( Fig. 6.5 ).

Operative technique