Deformable templates for preoperative computer-aided design and fabrication of large cranial implants

Abstract Prefabricated, patient-specific cranial implants can be designed entirely on computer, from patient 3D head CT scan source data. Implant computer-aided design (CAD) reduces production cost by 50% or more over the present common manual methods utilizing full skull models. We present an overview of how our top-down CAD system utilizes a patient's polygonal mesh skull-surface image derived from CT. We begin by identifying a crestline on that surface which indicates the maximum extent of the cranial defect margin. Next, a skull template surface image (i.e., either a right-left mirror of the patient's image or an average image) is registered to the patient's skull-surface image via globally distributed anatomical landmarks that drive a first-pass thin plate spline (TPS) warp. Landmarks local to the defect site, including the skull defect margin crestline, drive a second-pass TPS warp. The implant template is also smoothed and possibly modified to insure the desired contact with the skull for fixation, as well non-intersection with adjacent skull or soft-tissue structures. Implants designed for five patients with large (greater than 100 cm 2 ) cranial defects show a result that should be clinically effective.