Key points
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Data collection in a digital workflow includes skeletal anatomy obtained from CBCT and occlusal topography through an optical scanner and STL files.
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Setting the occlusion through conventional haptic effort of articulating stone models is being converted to digital positioning through software manipulation.
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Digital planning provides perspective of the occlusal and anatomic correction with preoperative insight that can improve intraoperative efficiency and clinical outcome.
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Occlusal and bone borne guides that conform to regional anatomy allow transfer the digital plan to clinical delivery of maxillary and mandibular customized rigid internal fixation.
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Patient specific Implants applied for the correction of dentofacial deformities continues to evolve through the merger of advances in rigid internal fixation and digital planning.
Introduction
Digital planning is widely regarded as the standard of care for the management of dentofacial deformities. The perspective of facial bony anatomy combined with an appreciation of the malocclusion at the dental level can provide tremendous advantages in both planning and clinical correction of dentofacial deformities. The multidimensional nature of the deformity can be readily appreciated, providing preoperative insight into the anatomic repositioning of teeth and skeleton, while improving the efficiency of the intraoperative experience and aiding in the delivery of a sound clinical outcome for patients. Patient-specific guides, templates, and custom titanium fixation fabricated to accurate anatomic contours continue to expand the armamentarium of maxillofacial surgeons in their care of the disproportionate occlusion and face ( Fig. 1 ).
History
Digital planning has evolved with advances in radiology that allow accurate capture of the anatomy of the head and neck. Three-dimensional imaging of bony anatomy and subsequent development of software that allows manipulation of the rendered anatomy has been successfully applied throughout the scope of oral and maxillofacial surgery, including implant rehabilitation to total joint replacement, as well as, tumor ablation and reconstruction via osteocutaneous flaps. The application of computer-simulated planning to orthognathic surgery has been a revolution similar to rigid internal fixation applied to stabilize bony osteotomies decades prior. , The advances of each discipline have been merged to push the science of customized titanium fixation forward.
Data collection
Planning surgical correction through a digital medium has continued to evolve from its initial origin. Bite jigs for centric relation record, fiducials to aid in merging data, as well as a gyroscope for natural head position have been replaced with clinical photos, local scanning of models with cone beam computed tomography (CBCT), and optical scanning of the occlusion. Data collection requires skeletal information most commonly through CBCT scans housed within the surgical clinic. The accuracy at the occlusal level, however, is not captured via the scans of the maxillomandibular skeleton requiring the addition of dental anatomy. Stone models mailed to a service provider and subsequently laser scanned offsite or scanned with the office CBCT and transferred digitally allow for the occlusal topography to be merged with the skeletal data. A complete digital workflow exists with the use of an optical scanner to record the occlusal topography via an STL file that possesses superior accuracy to stone models in the establishment of postsurgical occlusion and the fabrication of accurate fitting splints ( Fig. 2 ).
The completion of mandibular surgery first can be advantageous, as it eliminates the dependence on accuracy with the centric relation record and therefore, one less opportunity to induce error into planning. The reorientation of the distal mandible to the position of the maxillary arch via an intermediate splint is independent of the proximal segment and the referenced condylar position with planning. An improperly seated condyle during record collection with mandibular surgery first will result in a deviation in measurements over the osteotomy gap, but properly seating the condyle intraoperatively should deliver accuracy clinically despite the disparity to the virtual plan. If surgeon preference is stabilization of the segments via plate fixation, then ensuring varying lengths are available is necessary, as the clinical plan may not be congruent with that illustrated digitally. Variability of condylar position in data collection may negate the ability to use patient-specific implants, as the custom fixation will not take the inaccuracy into account and lead to an improper clinical result ( Fig. 3 ).
Planning
Careful records are needed to ensure natural head position before the initiation of treatment planning. Many surgeons have moved to orienting head position via clinical photos from past iterations of face bows, laser levels, fiducials, and gyroscopes. An error in orienting natural head position can result in an inaccurate perception of the anatomic layout including midline displacement or canting that will subsequently lead to an inappropriate surgical plan ( Fig. 4 ).
The planning session conducted with an experienced engineer allows visualization of the occlusal and skeletal discrepancies through review of linear measurements to quantify midline deviation, skeletal asymmetry, cant as referenced to the orbital rims, and calculation of yaw based on variation from the midsagittal plane. Treatment planning for the correction of dentofacial deformities is focused on establishing a functional occlusion while maximizing smile and facial esthetics.
Setting of the occlusion is accomplished either through conventional haptic efforts with the surgeon articulating stone models or digitally through manipulation of the occlusion with a software program. A digital work flow is greatly enhanced through engineer experience and repetition with a surgeon to establish the desired occlusion based on the existing deformity: setting an open bite deep against the opposing brackets, a class II deep bite via a tripod, or providing some relief in overjet with a class III correction. A review of accuracy and reproducibility of digitally established occlusion through experienced biomedical engineers in contrast to surgeon efforts in the dental laboratory was performed through the author’s practice. The establishment of the desired occlusion was more accurate and repeatable through digital planning with the surgeon and engineer’s guidance than haptic positioning by surgeons with stone models ( Fig. 5 ).
The skeletal correction through digital planning is initiated, as the established occlusion is manipulated from the existing incisor position referenced to natural head position. Multidimensional reorientation of maxillary central incisor position influences the necessary skeletal correction of the bony foundation, as virtual osteotomies of the maxilla and mandible are manipulated.
The correction of the malocclusion through digital manipulation of osteotomies allows tremendous preoperative insight into steps that will be encountered during the surgical intervention. The movement of the occlusion and adjoining bone illustrates interferences or osteotomy gaps at the skeletal level, providing insight into mitering or grafting requirements, respectively, and can aid in providing a smoother intraoperative experience. , , ,
Patient-specific implants
Design of a patient-specific implant is performed after occlusal and skeletal repositioning have been accomplished. The anticipated maxillary and/or mandibular anatomy is reconstructed, which allows the design and contour of titanium fixation spanning the osteotomy gap. The titanium plate design can be guided by the bony osteotomy and an appreciation of the anatomic structures in the vicinity.
The continued evolution of software and corresponding advances in titanium printing has allowed the hardware to expand from that of a classic linear stock plate to now encompassing a wide range of geometric configurations. The size and thickness of the plate and subsequent screws can be specified and modified to provide additional reinforcement as it extends across the osteotomy.
Patient-specific fixation in the maxilla is designed to maximize bone quality and thickness associated with the piriform rim and buttress and can also possess a variety of geometric contours. The quantity and positioning of fixation screws can be oriented throughout the inferior and superior maxilla cognizant of tooth roots and the thin bony wall of the anterior maxilla spanning the sinus ( Fig. 6 ).
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