This article includes updates in the management of mandibular trauma and reconstruction as they relate to maxillomandibular fixation screws, custom hardware, virtual surgical planning, and protocols for use of computer-aided surgery and navigation when managing composite defects from gunshot injuries to the face.
Key points
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Mandibular injuries have been treated effectively for generations using closed reduction and open reduction with internal fixation.
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With advances in computer-aided surgery, complex and difficult surgeries are now possible with the precision and accuracy once achieved by only a select few seasoned surgeons.
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The increasing research and applications of custom hardware, patient-specific planning, and virtual surgery has led to, and will continue to lead to, improved patient function, improved esthetics, decreased operative times, decreased costs, and most importantly beneficial patient outcomes.
Introduction
Mandibular injuries have been treated effectively for generations using closed reduction and open reduction with internal fixation. Recently, there have been several notable advances in surgical management that have added substantially to a facial surgeon’s ability to tackle simple as well as complex mandibular injuries more effectively.
Recent hardware advances in closed reduction include maxillomandibular fixation (MMF) screws in lieu of arch bars and hybrid systems that combine traditional arch bars with screw fixation. Open reduction and fixation has seen some very exciting applications of technology that include prebent and custom patient plates.
As a result of their complexity, treatment planning for facial ballistic injuries has seen an increase in the use of virtual surgery, patient-specific surgical guides, and intraoperative navigation and/or imaging to yield predictable and consistently repeatable results, once only achievable by seasoned surgeons.
The authors briefly review updates in management of mandibular trauma and reconstruction as they relate to MMF screws, custom hardware, virtual surgical planning (VSP), and protocols for use of computer-aided surgery and navigation when managing composite defects from gunshot injuries to the face.
Advances in Closed Reduction
MMF has a long history in the treatment of facial fractures dating back to 460 bc when Hippocrates used gold wire to fixate teeth for a mandible fracture. Over the years there have been many modifications, including Barton bandage, suspension wires, Ivy loops, arch bars, MMF screws, and embrasure loops. Erich arch bars (Karl Leibinger Co, Mulheim, Germany) continue to be the most commonly used technique. MMF screw fixation has the benefit of speedy application, decreased risk of puncture injury to the surgeon, less damage to the periodontium, and simple application and removal. Their use is not without complications. The most commonly reported complications include screw loosening, iatrogenic damage to tooth roots, screw fracture, and ingestion. A combination between MMF screws and arch bars known as hybrid systems are the newest advances to closed reduction. Commonly used systems include the SmartLock System Hybrid MMF (Stryker, Kalamazoo, MI), the MatrixWave (DePuy Synthes West Chester, PA), and the OmniMax MMF System (Zimmer Biomet, Jacksonville, FL). These systems are approved by the Food and Drug Administration for use in adults and children with fully erupted permanent dentition as a temporary means of fixation. These systems allow expeditious placement associated with MMF screws while maintaining lugs at crown level, allowing traction vectors closer to the occlusal table. Potential complications are similar to those of MMF screws. Although the hybrid systems are much costlier than Erich arch bars, Kendrick and colleagues’ cost analysis of the Stryker SmartLock system versus traditional arch bars found no difference when accounting for operating room time, cost, and time saved.
Advances in Open Reduction
Virtual surgical planning/Stereolithography
Among the greatest technological advances in craniomaxillofacial (CMF) surgery is computer-aided CMF surgery. Bell divides computer-aided CMF surgery into 3 main categories: (1) computer-aided presurgical planning, (2) intraoperative navigation, and (3) intraoperative computed tomography (CT)/MRI imaging. Computer-aided presurgical planning in mandibular trauma and reconstruction involves computer-aided design and computer-aided manufacturing (CAD/CAM) technology and VSP, which can then be applied to the fabrication of stereo-lithographic models and custom plates.
Even in basic mandible fractures, intraoperative bending and contouring of reconstruction plates can be time consuming and inaccurate. Complex, multi-segment, and/or avulsed mandibular defects make this task much more difficult and potentially frustrating.
Although originally developed for industry, initial medical applications of CAD/CAM included neurosurgery and radiation therapy. CAD/CAM has since proven indispensable in the reconstruction of complex mandibular trauma and other CMF surgery. CAD/CAM software enables the clinician to import 2-dimensional CT data in Digital Imaging and Communications in Medicine (DICOM) format to a computer workstation and to generate an accurate 3-dimensional (3D) representation of the skeletal and soft tissue anatomy. These digital models can be manipulated by virtual surgery allowing restoration of bony segments to their pretraumatic positions. Stereo-lithographic models of the virtually reduced mandible are then fabricated and can be used to manufacture custom cutting guides, plates, and splints. These models have been reported to be accurate within 1 mm and have shown to decrease operative time and wound exposure time when used in the planning stage.
Despite this degree of precision, there are a few areas where significant inaccuracies can be introduced. One critical area of inaccuracy is in the dental occlusion. CT scans, whether medical grade or cone beam, are unable to accurately capture occlusal anatomy. If accurate occlusal relationships are critical for surgical planning, then separate impressions must be taken from patients. These impressions can be done using traditional analog techniques with alginate/polyvinyl siloxane and stone or using newer digital impression techniques. Analog models are scanned into digital information. The dental and occlusal data can then be fused with the maxillofacial CT. Fusion can be done manually by a computer surgical planning engineer. However, if more precision is necessary, an occlusal fiduciary marker (a marker embedded in a bite registration or a bite registration mounted with a registration device) should be used when obtaining the maxillofacial CT and scanning the models. The fiduciary helps in accurately registering the models to the maxillofacial CT.
A second area of inaccuracy is in plate bending. The inherent rigidity of reconstruction plates makes them difficult to bend accurately, particularly when attempting to match the irregular contours of a mandible. Although smoothing osteoplasty can be performed on a model and subsequently transferred to patients, performing the same osteoplasty on patients potentially introduces another source of error. Repeated plate bending decreases fatigue resistance and increases the risk of plate fracture. By combining the use of VSP, various CAD/CAM techniques, and novel surgical hardware manufacturing, it is now possible to implant custom plates that require little to no bending.
Custom plates
There are 3 main processes of manufacturing custom plates applicable to CMF surgery and mandible reconstruction: prebent plates, milled (subtractive manufacturing) plates, and additive layer manufacturing (ALM).
Prebent plates
In the infancy of stereo lithography (SLA), a well-documented technique involved printing a corrected stereo-lithographic model to which a reconstruction plate would be hand bent. Prebending decreases operative time and improves accuracy yet still introduces areas of fatigue more prone to failure with cyclic loading. Although the cause of plate fracture is multifactorial, limiting the number of bends to the plate should help maintain the original plate strength. The use of custom milled or printed plates substantially reduces the need for bending, often completely.
Subtractive and additive manufactured plates
The creation of milled plates is a subtractive technique that uses high-precision drills to cut a solid titanium/titanium alloy billet into the desired shape. Computer numerical control (CNC) machining dates back to the early 1940s when Parsons first created numerical control for the aerospace industry. CNC is the computer automation of machining tools, which eliminates the need for manual instrumentation. The 1940 CNC prototype engineered by Parsons at the Massachusetts Institute of Technology is the forbearer of this technology today.
ALM, commonly known as 3D printing, is the newest technique used in plate fabrication. There are 2 main divisions within ALM: sintering and melting. Sintering involves heating metal powder to a temperature just before liquefaction, which allows cohesion to occur at the molecular level. This process allows control of porosity, which may later result in bone growth into the final structure. Melting involves the complete liquefaction of the metal within an accurately shaped container that results in a homogenous structure. These fabrication techniques are further divided into SLA, selective laser melting, selective laser sintering, and electron beam melting/direct metal printing. Current materials used in fracture and reconstruction plates include pure titanium and titanium, aluminum vanadium alloy (Ti6Al4V). Stainless steel 316L and cobalt chrome alloys have also been used in other applications.
Regardless of the additive method used, the basic steps are similar. A CT scan is taken after which the DICOM files are imported into a surgical planning software program. After the mandible is virtually reduced or reconstructed, the surgeon and the engineer collaborate, usually in the form of an online meeting, to design the plate. Once the final design is established, a software program slices the plate digitally into multiple horizontal layers. These data are then entered into a production machine that contains metal powder. A computer-guided laser runs over the powder and solidifies it layer by layer, from the bottom up. Any leftover powder can be reused, which is one major difference between ALM and milling. With milling, the shavings can be reused only after specialized processing, which is costly.
In both the subtractive and additive techniques, the plate is fully customizable and is designed over a digital workflow with an engineer and surgeon. Options for customization in both techniques include plate thickness, shape, screw hole position, number of screw holes, and varied thickness within the same reconstruction plate.
Navigation/intraoperative computed tomography
The digital workflow makes it possible to visualize the entire mandibular and facial skeleton, which is somewhat unrealistic. Unfortunately, these conditions do not translate directly to the operating room. Blood, edema, and avulsive soft and hard tissue defects can make it difficult to see appropriate landmarks for repair. A custom-fabricated plate that looks perfect during VSP can result in malocclusion, facial asymmetry, and poor bony adaptation if the implant does not seat in its exact planned position. The use of navigation systems has helped bridge the gap between virtual planning and reality.
As described by Bell, navigation is analogous to global positioning systems (GPSs) in a car. The localizer represents a satellite in space; the instrument/probe represents track waves emitted by the GPS unit in the car; the preloaded CT scan represents the map. Intraoperative navigation systems were initially developed for use in neurosurgery as a rigid system known as framed stereotaxy. Since its development, newer technology allows for navigation without rigid head frames, making the tool more accessible to other applications. Frameless stereotaxy is now used commonly in endoscopic surgery and in CMF surgery. In CMF surgery, navigation is most commonly used in orbital reconstruction and has been reported to be accurate to within 1 to 2 mm. By manipulating the surgical instrument affixed to a probe, one can precisely correlate a position on a computer surgically planned CT scan with patients in coronal, axial, and sagittal views. This ability allows for an intraoperative verification (evaluative phase) after the initial 3 phases of computer-aided CMF surgery have been completed (planning, modeling, surgery). The surgeon has the ability to adjust and verify in real time the positioning of bone and fixation, potentially avoiding an unnecessary return to the operating room if inconsistencies are seen on postoperative imaging.
Complex mandible reconstruction related to ballistic trauma
Gunshot wounds (GSWs) to the craniofacial region result in devastating functional disabilities and esthetic deformities, which are further magnified by the associated psychological trauma. Because most of these patients return to work, return to their preinjury lifestyle, and have a low rate of suicide recidivism, adequate reconstruction is essential to their comprehensive rehabilitation.
Although most GSWs involve injuries to extremities, most self-inflicted GSWs are to the head and neck. The infrequency of these injuries, combined with their enormous complexity, makes their reconstruction a daunting task. The complexity of these injuries prompted Rene Le Fort to exclude them in his classic article, reporting GSWs as “veritable explosions in the face” and “without surgical interest.” The reported mortality is 15%, with complications in those who survive as high as 30%. Survivors have a long road to recovery that ideally involves a large multidisciplinary care team that includes surgeons, medicine, psychiatry, physical therapy, occupational therapy, speech/language pathology, case management, and social work.
In 2014, Shackford and colleagues published an 11-year, 720-patient, multicenter retrospective review of GSW injuries to the face. Of the 720 patients, 20% died within 48 hours. Of those who survived the first 48 hours, 15% were ultimately discharged or transferred. The remaining 85% underwent surgical reconstruction. 41% of these injuries were the result of low-velocity handguns and 40% involved the mandible. Patients with mandibular trauma required an average of 1.7 operations. This finding was consistent with Taher’s review of 1135 facial gunshot injuries requiring an average of 1.5 operations.
The tissue damage caused by high-velocity bullets (>1200 feet per second [fps], military/hunting weapons) results in tremendous soft and hard tissue defects both from immediate damage and progressive die-back phenomenon. Low-velocity bullets (<1200 fps) may not cause the same avulsive defects and rarely result in a significant die-back phenomenon but can result in comminution. Traditionally, external fixation was used in this setting to prevent further devascularization of bone secondary to periosteal stripping and to temporarily maintain large bony defects without soft tissue retraction until definitive repair. This procedure has been largely replaced with rigid internal fixation but is still a useful adjunct in the armamentarium for treatment of complex GSWs to the mandible. The significant defects caused by gunshot injuries to the mandible are not unlike those defects caused by ablative tumor surgery or necrotizing infection. Many of the techniques used in the reconstruction of ablative tumor defects can be applied to gunshot injuries to the mandible.
For those cases in which the soft tissue and hard tissue mandibular defects are amenable to primary repair, local flaps, and/or nonvascularized bone grafts, aided by VSP, can expedite the surgical process. In grossly comminuted fractures or continuity defects, the contralateral mandible can be mirrored to the injured side to approximate the mandible’s pretraumatic form, which can then be used as described earlier to prebend plates or design custom plates.
The workflow starts with establishing stable reference points by placing patients in maxillomandibular fixation and taking a CT scan using the specific VSP protocol (typically 1-mm cuts) ( www.medicalmodeling.com ). As mentioned earlier, stone models of the dentition can be sent to the surgical planning engineers and merged with the CT data for added accuracy. Next, a Web-based planning session is scheduled. The fractured and displaced bony segments are aligned virtually by an engineer guided by a surgeon. If any adjustments need to be made, they are done during the Web meeting. Once the virtual reduction has been completed and confirmed by the surgeon, the placement of the plate is virtually planned. If a custom plate is to be used, the surgeon can decide the shape of the plate, thickness (which can be varied across the length of the plate), and number of holes. Stereo-lithographic models are made to aid in reduction in the operating room. In some instances, positioning guides with predictive holes for the reconstruction plate can help with reduction. In segmental defects, cutting guides are made to precisely freshen the edges of the defects to allow for easy buttressing with reconstructed tissue ( Fig. 1 A–H ).
For complex composite mandibular defects, microvascular reconstruction has become a mainstay. Ever since Hidalgo reported the use of the fibula free flap for mandibular reconstruction, it has become the workhorse for such defects. Other flaps used in mandibular reconstruction include the deep circumflex iliac artery (DCIA) flap, scapula, and osteo-cutaneous radial forearm. For many reasons outside the scope of this article, the fibula flap is the most versatile and frequently used. The DCIA flap, scapula flap, and osteo-cutaneous radial forearm flaps have their specific indications but are used much less frequently in the authors’ practice.
There is some controversy over the method of fixation used for these reconstructions. Some investigators advocate fixation of the neo-mandible with mini-plates, whereas others prefer reconstruction plates. Advocates of mini-plates argue that a stress-shielding phenomenon occurs with load-bearing reconstruction plates that impedes osseous healing. However, reconstruction plates have been shown to have less need for removal, lower infection rates, and greater ability to accurately shape the neo-mandible to mimic the native mandible.
The mandible, when viewed from above, is shaped like an omega, with an irregular contour ( Fig. 2 ). A common problem with adapting the fibula to the shape of the mandible is the discrepancy between the curved mandible and the straight fibula. Traditionally, multiple osteotomies were made to account for this discrepancy. However, recent cadaveric studies suggest that maintaining bone segments 2 cm or greater has a more reliable chance of containing a periosteal vessel then smaller segments (94% vs 65% in 1.0 cm). One method to address the discrepancy, while also limiting the number of fibula osteotomies required, is to take a subunit approach to mandibular reconstruction. The mandibular defect is divided into subunits (body, symphysis, condyle, and ramus). Fibula osteotomies are performed allowing for a straight segment to replace each of these subunits, as necessary. Therefore, a hemi-mandibular defect would only require 2 closing osteotomies and 3 segments, each with sufficient length to maintain vascularity. In addition, this simplifies adaptation of the reconstructive plate to the neo-mandible.
