Computer-assisted surgical planning in orthognathic surgery: A practical workflow

Introduction

Orthognathic surgery outcomes depend not only on surgical technique, but also on appropriate diagnosis and treatment planning. The adoption of computer-assisted design/computed-assisted manufacturing (CAD/CAM) technology, computer-assisted surgical planning (CaSP), and three-dimensional (3D) printing has created a paradigm shift for patients with maxillofacial deformities and for the surgeons treating them. The advent of noninvasive 3D visualization of the maxillofacial skeleton has allowed for the conversion of a preoperative plan to the manufacture of patient-specific guides and implants. These applications of CAD/CAM technology often share a common workflow, which entails the following: (1) performing a clinical exam, paying particular attention to patient anthropometrics in the frontal and profile views (in addition to obtaining a global appraisal to determine areas of imbalance, weakness, and asymmetry); (2) obtaining patient photographs, both facial and intraoral (preoperatively, photographs can be used with 3D imaging software to simulate treatment outcomes while intraoperatively, photographs can serve as a point of reference); (3) obtaining dental records, either in the form of stone models or digitally fabricated dental models; (4) obtaining conventional or cone-beam computed tomographic data (these images will be used for cephalometric analysis and for 3D planning); (5) conversion of CT data to digital imaging and communications in medicine (DICOM) format for digital image manipulation and 3D reconstruction by a bioengineering company; (6) setting idealized occlusion, either virtually or by hand articulation of dental models; (7) video teleconference between surgeons and biomedical engineers to virtually perform surgery and create planned osteotomies; and lastly, (8) fabrication of patient-specific implants and cutting guides. ^,

There are many advantages to utilizing 3D CaSP. Several treatment options can be considered simultaneously, allowing the ability to determine the best approach. Planned bony movements may alter soft tissue position, which may be more easily evaluated preoperatively with the CaSP software. Location and extent of dysmorphology and asymmetry can be better appreciated. The degree to which occlusal changes affect bony relationships can be evaluated, and the need for adjunctive procedures can be determined. 3D surgical planning allows assessment of actual patient anatomy, correction of deformities in all three planes of space, evaluation of soft tissue, as well as comparison of actual results with planned movements. Being able to perform all these analyses and visualize surgical skeletal movements has provided surgeons with a wealth of information that conventional orthognathic surgery preoperative planning could not provide.

There are no known absolute contraindications to CaSP, and the disadvantages are usually related to cost and logistics. The ability of an office or institution to purchase the necessary software and hardware may preclude CaSP. A recent and accurate CT data set is required by the bioengineering team in order to fabricate precise surgical guides or patient-specific implants. Collaborative planning with the orthodontist to ensure the appropriate orthodontic brackets, surgical wires, and hooks prior to obtaining final dental models requires advanced preparation and can present logistic setbacks if not anticipated. With the advent of onsite 3D printing capability, current challenges will likely be less relevant in the future.

In this chapter, the authors presents their preferred CAD/CAM workflow to perform orthognathic surgery. This approach has been validated by numerous studies performed at our institution by the senior author (D.S.). This chapter will focus on this approach and provide information to surgeons who hope to utilize CAD/CAM-based orthognathic surgery.

Clinical considerations

Dentofacial deformities present a challenge for reconstructive surgeons, who must approach the repair of bony architecture and overlying soft tissue with the goal of restoring function while optimizing aesthetics. Of paramount importance is the proper means of comprehensively assessing the orthognathic patient. A problem list must be generated and diagnoses itemized in order for the surgical plan to meet all functional and aesthetic objectives. The clinical exam must be methodical and organized, looking at static and dynamic relationships. A series of measurements is then taken—both cephalometric and anthropometric—and compared to “standards” to help guide diagnosis and treatment planning.

The clinical examination should begin with a global assessment to determine areas of asymmetry, lack of support, and disharmony. Certain reproducible hard and soft tissue landmarks must be evaluated to determine what is normal and what is abnormal. It is essential that the surgeon identify soft tissue discrepancies that, by virtue of the planned movements, will correct after surgery and those that will not. From the frontal view, the face should be divided into vertical fifths and horizontal thirds. With the patient in normal head position, the profile view is used to assess the anteroposterior dimension for skeletal relationship and its effect on other facial features. Various aesthetic subunits deserve specific consideration. The external nose, ears, eyes, cheeks, and lips display a wide range of variation, and correction of the presenting incongruity may improve or worsen harmony between various facial subunits. A complete examination of the oral cavity and occlusion must not be overlooked, as facial disharmony may be caused by subpar dentoalveolar relationships. Whether performing mandible- or maxilla-first surgery, the planned position of the maxilla should be based on maxillary incisal show at rest, which is generally accepted to be 2 to 4 mm. Orthognathic surgery occasionally affects the temporomandibular joint, and those with temporomandibular joint dysfunction (TMD) should be identified and evaluated. While there is disagreement among clinicians as to when to treat TMD, there is a consensus that orthognathic surgery may improve or aggravate symptoms of TMD.

Patient facial and intraoral photographs are an integral piece of treatment planning for orthognathic surgery. Photographs aid with documentation and authorization for insurance purposes, may be combined with 3D surgical planning systems to simulate potential results, and can be used to evaluate the degree to which various objectives were met. Dental models are obtained to evaluate tooth structure, positioning of teeth, and interocclusal relationships in order to determine the need and/or duration of presurgical orthodontics. Radiographic data is essential to 3D planning for orthognathic surgery. Conventionally, a panorex, lateral, and PA cephalograms were obtained to assess the teeth and facial skeleton. Today, cone-beam computed tomography (CBCT) is utilized to capture, with minimal radiation, skeletal anatomy of the maxillomandibular complex in the sagittal, vertical, and transverse dimensions. Of note, when treating individuals with dentofacial deformity, it is imperative that the surgeon develop a systematic approach for collecting and interpreting data based on patient history and physical examination. This will aid in arriving at the proper diagnosis and formulating an appropriate treatment plan that addresses both form and function.

Workflow for computer-assisted design/computer-assisted manufacturing

The workflow for CAD/CAM-based orthognathic surgery is presented in Fig. 5.1 .

Computer-assisted design

The senior author has utilized CAD planning for all orthognathic procedures for over a decade. Although the use of CAD for orthognathic planning can vary by surgeon, there are several benefits to its use. First, it offers the unique opportunity to integrate clinical data, patient photos, and three-dimensional skeletal data concurrently. Moreover, it offers the opportunity to ensure centric occlusion/centric relation virtually without the use of a facebow. Third, using CAD facilitates three-dimensional visualization of the planned skeletal movements highlighting any potential flaws in the operative plan such as skeletal or dental interferences. Lastly, CAD facilitates the ability to customize hardware. In the senior author’s practice, custom-designed genioplasty cutting guides and custom titanium plates are used to ensure high fidelity and accuracy in the genio movements. The following figures 5.2-5.9 provide a broad overview of the senior authors preferred CaSP work-flow (see Figs. 5.2–5.9 ).