Virtual surgical planning and three-dimensional (3D) printing have broadened the horizons of oral and maxillofacial surgery, including orthognathic surgery. 3D-printed personalized surgical guide and patient-specific implant (PSI) not only serve to guide accurate osteotomies and as a good fitting of osteosynthesis plate, but more importantly define a revolutionary waferless approach concept that is totally different from the conventional wafer-guided jaw fixation technique. This review discusses the limitations of the conventional orthognathic approach, and how PSI may overcome these limitations, improve accuracy, and bring additional benefits in the execution of orthognathic surgery.
Key points
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Conventional orthognathic surgery using a wafer-based approach has limitations and potential errors in various steps of the procedure.
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Virtual surgical planning and three-dimensional printing allow the concept of a waferless approach by patient-specific implant (PSI).
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PSI has proved to be accurate in the execution of the planned movement of the orthognathic procedures.
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There are pros and cons of using PSI as a routine in orthognathic surgery.
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Further development and improvement of PSI as well as an increase in popularity of its use in the near future are expected.
Introduction
When asked what is the breakthrough of the decade in orthognathic surgery, perhaps most surgeons would agree that it is the development of virtual surgical planning (VSP) combined with personalized cutting guides and fixation plates, known in the field as patient-specific implants (PSI). Although the importance of chairside clinical assessment remains unchanged in the presurgical workup, the use of three-dimensional (3D) imaging, surgical simulation software, and 3D printing technology has been rapidly implemented in the past decade to enhance certainty in surgical planning, precision in operative execution, and predictability of the outcome. The popularity of PSI in orthognathic surgery has continued to increase in recent years, because of its reported accuracy and ability to overcome many of the limitations of the conventional approaches, such as reliance on the rotational path of the temporomandibular joint (TMJ) and opposing occlusion. In this article, we discuss the development from conventional orthognathic surgery to VSP, translating the virtual plan to the patient from a wafer-based- to a waferless approach using PSI, technical aspects of PSI design and fabrication, accuracy and pros and cons of PSI, and future directions in the field.
Conventional orthognathic approach
During conventional orthognathic planning, decisions on the movements of surgical jaw repositioning are made based on 2D radiographic examination, as well as dental model analysis, after clinical assessment. Soft tissue changes in 2D after the planned bony repositioning are predicted either by manual cephalometric tracing or by computer programs that merge tracings of cephalograms to preoperative photographs of the patients. After this initial preparation stage, the next step would be to utilize a system that reliably translates surgical planning to the actual surgery.
The development of the method for translating surgical planning to the patient was adopted from our dental background, as the conventional approach relies on the facebow/articulator system. Stone dental casts are mounted in a 3D position replicating their position in relation to the rest of the craniofacial complex, after which mock-model surgeries are performed on these dental casts. Surgical wafers corresponding to the interim and final occlusions based on the model surgery are fabricated and brought to the operating theater, wherein the surgical repositioning of the first jaw (be it the maxilla or mandible) is determined by an interim occlusion in relation to the second, unoperated jaw, guided by these prefabricated surgical wafers (and an assumed reproducible centric relation of the mandible, in the case that the maxilla is the first operated jaw). The surgical movement of the second jaw would then follow the new position of the first jaw and the final occlusion, guided by the final surgical wafer.
Limitations of Conventional Approach
Several potential limitations are associated with the conventional approach to orthognathic surgery. These limitations arise in various steps during the surgical planning phase that translates to errors intraoperatively. Inaccuracies can occur when assessing the severity of dentofacial deformity by clinical and radiographic assessment, replicating the jaw relationship by facebow transfer and dental casts, errors in model surgery, and errors in the surgical wafers.
Errors in clinical and radiographic assessment
Decisions on surgical movement in conventional orthognathic surgical planning rely on a clinical and radiographic assessment of the patient. Although assessment in the sagittal plane, such as antero-posterior positions of the jaws, as well as the pitch of the maxillomandibular position, is often straightforward, assessment of the jaw and roll of the maxillomandibular position is often difficult. Conventional orthognathic surgical planning often focuses on changes in the sagittal plane based on prediction tracings of lateral cephalograms, whereas the planning for changes in the coronal plane is often overlooked. Moreover, if the shape of the bone—especially in the mandible—is not the same on both sides, , then this leads to problems because obvious asymmetry may still be present even after the clinician has moved the maxillomandibular complex to the mid-sagittal plane to the best of his/her efforts.
Errors in facebow transfer
It has been shown that inherent errors exist in facebow transfer. , These errors can occur in the sagittal plane when the horizontal bar of the facebow fails to correspond to the designated horizontal reference plane that is usually the Frankfort horizontal plane. Also, errors can exist in the frontal plane, failing to capture the true clinical canting, or in the axial plane, inaccurately replicating the true jaw of the dental arch. The possible errors from this method could be inaccuracies in positioning the facebow, mobility of the facebow on the patient, soft tissue hindrance, the flexibility of the metal material of the facebow, and operator errors. These errors in facebow transfer are often compounded, resulting in failure to accurately capture the 3D position of the jaws in relation to the craniofacial complex.
Errors in mounting of dental casts
Imprecisions in facebow-transfer, as well as other inherent errors, such as dimensional changes of gypsum on setting, result in an incorrect 3D position of the mounted maxillary dental cast on the articulator. However, the mounting of the mandibular dental cast utilizes a bite registration in the centric-relation position that is assumed to be reproducible theoretically. However, errors often exist when obtaining a bite registration, because of patients’ muscle memory, the accuracy of impression material, and operator technique. This occurs especially in skeletal class II patients who may have a usual bite that is very different from that in centric relation. Moreover, the centric relation position on the operating table may be different than that captured preoperatively, because of positioning as well as the effects of muscle relaxants. Errors in the mounting of dental casts would result in errors in the planning of surgical movements. For example, if the true occlusal canting is not captured by the mounted models, the resulting correction of canting on the patient may be inadequate. Also, in bimaxillary surgery, because the neoposition of the first operated jaw is based on an intermediate wafer in relation to the opposing jaw, errors in the mounting of the opposing jaw would result in an incorrect neoposition of the first operated jaw. This would, in turn, lead to mistakes in the second operated jaw.
Virtual surgical planning: a new era
The introduction of computed tomography (CT) in the latter part of the 20th century has enhanced medical and surgical treatment in that anatomy can be assessed in 3D. Since then, computer programs have been developed for surgical simulation, in which CT data is converted to digital imaging and communications in medicine (DICOM) files to construct virtual reconstructions of objects, such as the maxillofacial skeleton. Software programs for VSP, such as PROPLAN CMF by Materialize (Leuven, Belgium), allow simulated osteotomy and manipulation of the virtual reconstruction of the craniofacial skeleton. This enables clinicians and technicians to plan for the desired surgical movements while looking at the 3D virtual skull and allows visualization of the feasibility of planned bony movements, such as whether there is any bone contact or bony interferences ( Fig. 1 ). Also, when matched with 3D photographic data that is also converted to DICOM files and reconstructed virtually, VSP allows simulation of esthetic results when matched with 3D photography.
Translating Virtual Surgical Planning to Operating Room
VSP is without meaning if the planned movements are not translated to the actual surgery. Without 3D printing technology, such translation is possible by mimicking the digitally planned movement of the stone model surgery on the articulator. Laboratory-made surgical wafers are fabricated based on the stone model surgery as in the conventional approach. However, accuracy of this method is poor.
The advent of 3D printing has made an accurate translation from VSP to the operating room possible. Currently, there are two main methods for this translation: (1). 3D printed surgical wafers based on the virtual occlusion and (2). 3D printed personalized cutting guides and PSI. Surgical planning software allows the fabrication of surgical wafers via a 3D printer, using the virtual intermediate and final occlusion that is based on the planned jaw movement. These wafers are brought to the operating room and the surgery is performed as in the conventional approach. However, a major limitation of this approach is that the positioning of the maxilla still depends on the centric relation position of the mandible, in which accuracy may be questionable because of various reasons discussed earlier. Another limitation is that the vertical positioning of the maxilla cannot be translated from the VSP to the patient. A true waferless approach using PSI, in which patient-specific cutting guides and fixation plates are designed based on the virtual surgery simulation, is a more direct approach and has been shown to be superior in accuracy.
Technical aspects of patient-specific implants
Collaboration with Commercial Venders
Orthognathic VSP and fabrication of PSI can be performed in collaboration with commercial vendors. The surgeon sends the patients’ CT scans, dental models, or scanned dentition by 3D laser topography and clinical photographs to the engineer. Virtual reconstruction of the craniofacial skeleton by segmentation of CT data and cephalometric analysis can be done by the engineer. The treatment planning for surgical movements and occlusion can be done via video conferencing between the surgeon and the engineer, and the PSI would be designed, printed, and mailed to the surgeon ( Figs. 2 and 3 ).
Surgeon-Oriented Approach
Alternatively, VSP and fabrication of PSI can be performed in a surgeon-oriented approach. The main advantages of this approach are that the surgeon would have total control of the whole process and that the time from surgical planning to having the PSI ready for surgery is much faster, because the whole process from planning to product delivery would only take 5–7 days from our experience. , First of all, the surgeon performs the basic VSP and surgical simulation with virtual planning software. The maxillofacial region of the patient is scanned with spiral CT with a slice thickness of less than 1.0 mm, or high-resolution cone-beam CT. This imaging data is exported in DICOM format and can then be processed using surgical simulation software. The patient’s skull model is virtually reconstructed using the threshold segmentation method that separates the high-density bone tissues from other tissues after choosing a suitable threshold value. For a composite skull model with accurate dentition, stereolithography (STL) files of the dentition obtained by laser topographic scanning are fused with the virtual models of the maxilla and mandible. Osteotomy planes are then designed that correspond to how the cuts would be carried out intraoperatively. The osteotomized bones are then moved to the desired position based on clinical and radiographic findings, as well as determined by measurements on the composite virtual skull model ( Fig. 4 ). In practice, the desired final occlusion is often achieved first that can be done based only on the virtual final occlusion, or matched to the scan of a final occlusion achieved on stone dental casts. After establishing the desired final occlusion, the maxillomandibular complex is then repositioned together as one unit. The movement for the genioplasty segment is performed last. With the incorporation of 3D photography, a prediction of the final esthetic outcome can be performed by the soft tissue simulation function built into the program.
