Three-Dimensional Computer-Assisted Surgical Planning and Manufacturing in Complex Mandibular Reconstruction

Key points

  • Virtual surgical planning offers potential for increased accuracy and efficiency, while reducing time and money spent in the operating room.

  • The goals when reconstructing complex mandibular defects are to restore proper shape, symmetry, and the relationship between the maxilla and the mandible to allow for mastication, deglutination, articulation, and if appropriate rehabilitation with dental prosthesis.

  • Three-dimensional planning has 3 phases: presurgical planning, modeling and rapid prototyping, and execution of the surgery in the operating room.

  • Three-dimensional planning encompasses a wide variety of concepts, most commonly indicating virtual surgery planning software, stereolithic models, custom cutting guides, endosseous implant guides, prebent plates, or patient-specific implants.

  • Currently, mandibular reconstruction using computer-aided design/computer-aided manufacturing technology focuses on hard tissue only.

Background

The use of 3-dimensional (3D) planning, known also as computer-aided design, computer-aided manufacturing, and virtual surgical planning (VSP) for head and neck reconstructive surgery has been rapidly evolving since the early 1990s, where even at that time the technology was understood to make surgeries more accurate and efficient. Three-dimensional planning has continued to revolutionize how we perform free tissue transfer to reconstruct mandibular defects, because even simple mandibular reconstructions require precision and understanding of the complex spatial and functional relationships of the mandible. There has been a surge in use because this technology develops, and now allows faster turnaround from preplanning, to manufacturing, and finally to the operating room. Three-dimensional planning encompasses many different components, but generally describes the concept of performing preoperative surgical planning via a 3D virtual simulation based on radiologic imaging, and then creating models, guides, and stents to help assist the surgeon while performing the case in the operating room. All of this preplanning allows for better understanding of each case before entering the operating room, and can take the guesswork out of many steps such as resection margins, osteotomies of the free flap, bending of plates, and in-setting of the free flap. This in turn results in more efficiency and less time spent in the operating room. ,

Complex mandibular defects and goals of care

Although there are no accepted criteria indicating when VSP should be used, it is particularly beneficial for reconstructing complex mandibular defects. Again, there is also not a set definition of what makes a mandibular defect complex, and this can be subjective based on surgeon experience. Generally accepted definitions of complex include the following: hemimandibulectomy or a long span defect requiring extensive osteotomies and shaping of the neomandible, any resection involving the condyle, large expansile lesions that do not allow prebending of plates, symphyseal defects, and when there is a plan to place immediate or delayed endosseous implants. , The goals of reconstruction are to restore proper shape, symmetry, and the relationship between the maxilla and the mandible to allow for mastication, deglutination, articulation, and if appropriate rehabilitation with dental prosthesis.

Limitations of the traditional free hand approach

VSP is often discussed in contrast with the traditional free hand approach where many surgical decisions are based on intraoperative findings, and success in complex cases relies heavily on surgeon skill and experience. Every step starting with ablation, which requires precise surgical margins, to shaping and in-setting the neomandible are performed by hand without guides, and based on the synthesis of 2-dimensional information. This results in heterogenous outcomes among surgeons, and for complex cases can also result in long operating times. Increased ischemia time can also result in complications, namely, flap failure. Free hand method is still routinely used for “noncomplex” mandibular reconstruction, that is, defects that can be repaired with 1 segment, and do not require extensive reshaping. Thus, one of the advantages of VSP is to level the playing field among surgeons of a wide skill set, to achieve more standardized results in complex cases.

Computer-aided design and manufacturing for maxillofacial surgery

The process of 3D planning can be divided into 3 phases per Hirsch and colleagues. First, the presurgical planning stage, then the modeling and rapid prototyping stage, and finally the execution of the surgery in the operating room. There are many advantages to incorporating 3D planning into mandibular reconstruction, but the overarching theme is that the use of 3D planning results in being more prepared, and having a deeper understanding of the intricacies of the case before even setting foot into the operating room. , Preoperative models allow for a better understanding of the problem, and virtually performing the surgery allows the surgeon to consider several options before selecting the most ideal plan, regarding type of flap and number of osteotomies, as well as type and arrangement of hardware. Second, surgical margins can be determined more precisely by using the 3D imaging to set an ideal margin and maximize the preservation of normal tissues. Third, the normal contralateral mandibular anatomy can be mirrored to use as a guide, which allows for a more anatomically accurate and functional reconstruction. Much of the guesswork is removed through the use of cutting guides, prebent plates, and/or patient-specific implants. Finally, these models can facilitate communication, both between the surgeon and the patient, as well as between the 2 teams of surgeons if using a 2-team approach. Beyond the osseous reconstruction, VSP allows for planning of both immediate or delayed endosseous implants. This process can be complicated by interferences caused by the reconstruction plate and screws used for the free flap if the implants were not considered in the planning phase. Additionally, proper placement with correct position and angulation of the dental implants is crucial because the future prosthetic rehabilitation is usually of higher complexity because it needs to replace multiple teeth, as well as deficient soft and hard tissues, in a patient with altered occlusion.

Accuracy outcomes

An important and often overlooked step of 3D planning is postoperative analysis. This is generally measured by overlaying the postoperative computed tomography (CT) scan (actual outcome), with the virtual surgery planned image (planned outcome), and then measuring mean error, or the difference between a fixed point, like the condyle. VSP consistently has produced more accurate results over traditional free hand surgery across multiple studies for mandibular reconstruction. , However, as found in recent systematic reviews, there is not a universally accepted means of measuring surgical accuracy; Pucci and colleagues found 12 studies published within the last decade that describe and measure accuracy through a combination of 12 different methods. This stems from the availability of a vast amount of different VSP software programs, as well as many unique in-house systems. This lack of an accepted standard of accuracy leads to the question, how much benefit in accuracy are we really gaining? Therefore, more focus should be placed on a functional and esthetic result, which has generally been under-reported in current studies.

Step by step: planning and execution of complex mandibular reconstruction

The process of computer-aided design/computer-aided manufacturing consists of defined stages of patient evaluation, virtual surgery, reconstruction design, final approval, and components manufacturing and delivery to the operating surgeon. Mandibular reconstruction using computer-aided design/computer-aided manufacturing technology focuses on hard tissue only. Although capabilities allowing 3D soft tissue capturing via synchronized multiple camera photography exist, their use in surgical planning is not widespread yet. Therefore, soft tissue envelope assessment is completed on clinical evaluation by a surgeon and existing or anticipated defects or deficiencies must be taken into account and included in treatment planning.

Patient evaluation is based on a CT or cone beam CT scan data recorded in a DICOM format that is transferred to the manufacturing company’s software. These data can be transferred in the format of a physical disk or pushed through online, depending on the individual company’s capabilities and preferences. Depending on the plan for mandibular reconstruction, maxillofacial CT scan alone or any additional scans of lower extremities, scapula, and pelvis can be submitted. Patient-specific data from a planned flap harvest site is not required because generic anatomic models are widely available and can be used with an acceptable margin of error for surgical planning in most cases. However, in the authors’ practice, when a fibula flap is planned, CT angiography of lower extremities is routinely obtained as part of the preoperative workup thus are used in VSP.

Once data are uploaded to the manufacturer and deemed of acceptable quality, a video conference with a surgeon and a bioengineer is scheduled. Generally acceptable CT scans are those obtained with less than 1.25 mm slice thickness and less than 0.6 mm pixels.

First, virtual surgery addressing the pathology and creation of the final defect are completed. Osteotomies for benign tumors are placed with a 1.0- to 1.5-cm margin and designed with an angle that would be accessible in the operating room. For malignant tumor resection, margin determination should take the timing of the surgery and any potential new tumor growth into account. For correction of mandibular deformities resulting from pathologic fractures owing to osteonecrosis or osteomyelitis margins can be placed within healthy appearing bone on the CT scan. Planning mandibular resection in accordance with the Brown classification with osteotomy placement at the junction of segments, even if with a greater margin than necessary, facilitates reconstructive efforts and should be used. When a planned resection margin leaves inadequate subcondylar bone stock for plate design with at least 3 screw holes ( Fig. 1 ), the entire condyle should be planned for removal. A prosthetic condyle can be incorporated into a custom plate at the time of VSP session ( Fig. 2 ). The remaining mandibular segment then is ideally positioned in occlusion with the opposing arch if adequate remaining dentition is present ( Fig. 3 ). In edentulous patients with segmental defects, standardized mandible models can be used to approximate the ideal position of the remaining deformed and displaced mandibular segment relative to a stable landmark. Midline skull base structures, such as the center of foramen magnum, clivus, or crista galli, may be used as the landmarks to establish symmetry ( Fig. 4 ). Occasionally, in patients with multiple prior surgeries and radiation resulting in soft tissue scarring and contracture, it makes this ideal position inaccessible in a real-life setting. In those instances, coronoidectomy is recommended and the new mandibular position is determined within the clinically achievable limits and the patient is informed of the compromise.

Aug 5, 2020 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Three-Dimensional Computer-Assisted Surgical Planning and Manufacturing in Complex Mandibular Reconstruction
Premium Wordpress Themes by UFO Themes