Identification of the auditory canal from a cone beam CT
5.1 Craniofacial Imaging
Computed tomography is the preferred method of imaging for head and neck reconstructions. The Hounsfield scale enables identification of soft and hard tissues by their density; this allows for segmentation of the images for reconstruction of 3D models with minimal artifact but at the expense of radiation exposure to the patient (Gordon et al. 2014). In contrast, cone beam computed tomography (CBCT) has become more common in dental and medical practices; their low radiation exposure provides a unique opportunity to capture hard tissue images that have been used for endodontic diagnosis, airway visualization, orthognathic reconstructions, and dental implant planning (Gronet et al. 2003; Vannier 2003; Grant et al. 2013; Estrela et al. 2008). However, CBCT is subject to severe artifact from dental restorations, and lack the contrast to segment soft tissue to complement bone. In addition, due to the inconsistancy of contrast, the Hounsfield Scale does not apply to identify soft versus bone tissues.
Surface scanning has also been used to design and fabricate craniofacial devices. These are noninvasive and have applications in craniofacial planning (Sabol and Grant 2011). These scanning devices use laser, light, or some type of contact scanning technologies employing technologies such as stereo photogrammetry to increase accuracy, and are stationary or handheld (Knoops et al. 2017). The images captured are often used for registration to other medical images to provide more accurate virtual models for virtual planning and to design devices, medical models, and surgical guides. In addition, surface scanning has also been successfully used to fabricate maxillofacial prostheses (Sabol and Grant 2011; Grant et al. 2015).
Cranial defects can be caused by trauma, tumor, or decompressive craniotomy. Historically, the fabrication of a custom cranial implant involved an ambulatory patient, conventional impression techniques, fabricating an indirect stone model of the defect, and fabricating a mold for processing polymethyl methacrylate (PMMA) (Aquilino et al. 1988). Surgical placement involved extensive modification to get an acceptable fit with long hours in the operating room and use of self-curing acrylics to fill in the gaps. The initial use of 3D printing was to print the defect from which a custom wax implant could be fabricated and a mold for PMMA (Fig. 5.2). In this process, the patient does not have to be available to the laboratory, and the process enables more complicated craniofacial implants that fit the defect with minimal modification, cutting the fabrication time down by close to 75%, and operating time nearly in half (Gronet et al. 2003) (Fig. 5.2). This process has now evolved to digital design directly from medical imaging and fabrication of the cranial implant by milling PMMA and Polyetheretherketone (PEEK) implants or 3D-printed titanium and Polyethyl ketone ketone (PEKK) (Fig. 5.3).
The left photo is the SLA skull with a frontal bone and lateral orbit defect. The right is the waxed implant for mold fabrication
Titanium cranial implant manufactured directly from electron beam melting
5.3 Craniofacial Reconstruction
In trauma cases, 3D models may help to recognize the position and the direction of fractures, the number of bone fragments, and the degree of dislocation. Virtual planning can assist in a reconstruction plan with reestablishing contours and fabricating positioning and bending guides for plates and recontour bars. However, there are limitations that can result in surgical delay due to long model production with current additive manufacturing processes (Powers et al. 1998; Holck et al. 1999; McAllister 1998) (Fig. 5.4).
3D rendering of defect of the mandible
Virtual simulation and printed models from medical images provide solid models that simulate osteotomies and grafts, simulate segmental jaw movements, and facilitate preoperative construction of surgical guides, templates, and custom surgical devices (Ander et al. 1994; D’Urso et al. 1999; Kermer et al. 1998).
Guides can be designed and fabricated that allow the prebending of recontouring bars for mandibular stabilization prior to the surgical reduction, positioning guides that reapproximate bone sections for plating, cutting guides to move bone as needed, and customized devices to replace or stabilize sections of the mandible, zygoma, or orbit using biocompatible materials (Singarea et al. 2004). Using virtual surgical techniques, the surgical guides assist the surgeon in osteotomy cuts, implant placement, positioning of bone and soft tissue for reconstruction, and assistance in prebending of reconstruction plates (Fig. 5.5).
Virtual planning for a fibula reconstruction of the maxilla
Recently, the limits of craniofacial reconstruction have been challenged with the success of full total face transplants. The same principles of virtual planning can be very useful in the selection of appropriate anatomical donors to approximate the correct dental occlusion and other anatomical reconstructions. (Murphy et al. 2015a; Sosin et al. 2016) (Fig. 5.6). Cutting guides can be designed to provide an intimate fit of bone margins of the donor anatomy to the recipient site. Current research in this area proposes navigational technologies and mastication simulation (Gordon et al. 2014; Murphy et al. 2015b).
Both the donor and the recipient skulls have been registered and cutting planes established to fabricate cutting guides
In respect to the donors, a facial mask is required after the harvest of the transplant. Conventional fabrication of a total facial prostheses by a maxillofacial prosthodontist or anaplastologist at the time of the surgery can be disruptive and expensive. An alternative technique using 3D printing from medical imaging and photographs have been proposed, as they can be fabricated directly or with a mold, prior to the surgical intervention at a lower cost (Grant et al. 2014) (Fig. 5.7).
Silicone-fabricated donor mask for donor of facial transplant. Fabricated prior to the transplant surgery
5.4 Dental Implant Guides
Dental implant placement is driven by the restorative plan it retains or supports. The purpose of a surgical guide is to assist the surgeon in the location and direction of the osteotomy prior to dental implant placement. The Academy of Prosthodontics defines a surgical template as a guide used to assist in proper surgical placement and angulations of dental implants (The Glossary of Prosthodontic Terms, 2017). Based on the amount of the operative restriction of the drill, the design of the surgical template can be classified as nonrestrictive, partially restrictive, or completely restrictive (Stumpel 2008; Misch and Dietsh-Misch 1999). Historically, surgical guides were fabricated conventionally on dental casts using a variety of techniques and materials including clear vacuum-formed matrix , free-form auto-polymerizing acrylic resin and acrylic resin duplicates of the available prosthesis or diagnostic wax-ups.
Recently, software has become more available that provides dental implant planning from CBCT using digital scans of diagnostic wax-ups or virtual restorations from intraoral scans or diagnostic casts. By registering the images, the restoration can be planned, and a surgical guide can be fabricated to limit the placement of the dental implant to accommodate the restorative plan (Fig. 5.8). In some instances, this workflow allows for “same day” implant retained restorations, even in more complicated cases requiring grafting (Cheng et al. 2008; Stapleton et al. 2014). Once the digital or virtual plans have been designed, the guides and the restorations can be produced with digital manufacturing—either additive or milled.