Temporomandibular joint ankyloses are a fusion of the mandibular condyle to the base of skull. Surgical advances have stemmed from innovation in computer planning, guidance, and intraoperative navigation, allowing surgeons to restore form and function with greater precision, predictability, and safety. Preoperative computer virtual surgical planning used the computed tomography scan data to render a 3-dimensional image that can be used for surgical simulations and fabrication of intraoperative aids. Temporomandibular joint reconstruction should be considered as a predictable option in the management of temporomandibular joint ankylosis. Intraoperative navigation allows for continuous real-time 3-dimensional positioning of instruments.
Preoperative computer-assisted surgical planning with virtual surgical planning and 3-dimensional printing allows for surgical simulation and fabrication of stereolithographic models, occlusal splints, cutting guides, and patient-specific temporomandibular joint replacement.
Temporomandibular joint replacement is considered most predictable option in the management of temporomandibular joint ankylosis and reankylosis, via either a 2-stage or 1-stage approach.
Intraoperative navigation for temporomandibular joint ankylosis allows for continuous real-time 3-dimensional positioning of instruments relative to regional anatomy, affording greater safety and ability to perform a 1-stage surgery.
Background on temporomandibular joint ankylosis
Temporomandibular joint (TMJ) ankyloses can be defined as fusion of the mandibular condyle to the base of skull. This heterogenous disease yields hypomobility and progressive decrease in rotation and translation of the TMJ. Ankylosis can be classified by location (intra-articular or extra-articular), type of tissue involved (fibrous, bony or fibro-osseous), and by the extent of fusion (complete or partial). , There are multiple etiologies that can lead to TMJ ankylosis; the most common in developed countries is trauma, such as intracapsular fractures. Regardless of the etiology or classification of the TMJ ankyloses, they can result in facial asymmetry, a decreased range of motion, malocclusion, and an ipsilateral shortened ramus that can yield an anterior open bite. There are also compensating midface asymmetries at the piriform rim and orbits, speech impairment, airway compromise, pain, impaired mastication, poor oral hygiene, and psychological stressors that compromise the quality of life for the patient.
Although a patient-specific approach to TMJ ankyloses based on etiology and other patient factors is critical, the treatment is almost always surgical. The goal of treatment is restoration of mandibular movement, function, facial harmony, and prevention of re-ankylosis. Surgical options include gap arthroplasty, with or without interpositional grafting, TMJ reconstruction using autogenous grafts, vascularized flaps or TMJ total joint replacement (TJR) with an alloplastic joint device.
The surgical treatment of TMJ ankylosis is complicated by the altered anatomy with risk of injury to structures within the middle cranial fossa that are in close proximity. This treatment can lead to hemorrhage from damage to the internal maxillary artery, dura mater exposure, nerve injuries, and cerebrospinal fluid leak. Moreover, wide bone resection of the ankylosis with a bone gap of 1.5 to 2.5 cm is required to avoid recurrent ankylosis. Normal local bony landmarks are also absent, making the resection technically challenging.
In TMJ ankylosis surgery, several factors can occur leading to suboptimal results. These include a surgeon’s reliance on 2-dimensional imaging, which are limited owing to the presence of superimposed structures, for treatment planning of a 3-dimensional (3D) issue. Poor visualization of deep skeletal contours of the mandibular condyle and the glenoid fossa, variations in jaw and tooth position relative to each other and the skull base, and variations in head position and craniofacial development all pose challenges to optimal surgical outcomes, especially with concomitant orthognathic surgery.
Surgical advances in the treatment of TMJ ankylosis have largely stemmed from innovation in computer planning, guidance, and intraoperative navigation. These advances allow surgeons to restore patients form and function with more precision, predictably, and safety. Computer-assisted navigation technologies are now widely used in the treatment of oral and maxillofacial diseases. , Specifically, surgeons have adopted computer-aided design and computer aided modeling software that was initially engineered for applications in neurosurgery and radiation therapy, to assist in the planning and implementation of complex TMJ surgeries such as TJRs.
Computer-aided design and computer aided modeling software allows a surgeon to import 2-dimensional computed tomography (CT) data in a DICOM format into a computer software and render an accurate 3D representation of both the hard and soft tissue anatomy. Computer-assisted TMJ surgery includes preoperative virtual surgical planning (VSP), fabrication of surgical cutting guides, and intraoperative navigation.
Preoperative computer-assisted surgical planning
The preoperative computer VSP begins with the CT scan data that are then uploaded into planning software, allowing for the 3D-rendered image of the skull to be manipulated for improved visualization of structures, orientation, and diagnosis. The 2-dimensional and 3D linear and volumetric measurements can be made to assess distances from vital structures or planning resection margins. The skeletal representation can be manipulated or surgical simulations can be performed, such as sectioning and moving bone segments, reducing bony interferences, mirroring, resecting pathology, creating and inserting hardware or a prostheses, and highlighting vital structures to preserve during surgery. Moreover, an adequate margin of bone can be preserved over critical areas such as the middle cranial fossa and the bony auditory canal with safe depth osteotomies.
The data from this VSP can be used to aid in further preoperative planning and fabrication of intraoperative aids. In TMJ surgery, these factors include patient-specific cutting guides, which allow for precise osteotomies during surgery, planning osteotomies in concomitant orthognathic surgery, fabrication of stereolithographic models, custom patient-specific hardware such as alloplastic TMJ TJRs, and occlusal splints for verification of intraoperative and postoperative occlusion. The combination of VSP and 3D printing allows for patient specific cutting guides that can be fixated intraoperatively with screws according to the virtual plan, providing precise transfer of the virtual resection plan into the surgical field and an accurate result in TMJ prosthesis placement ( Figs. 1–4 ).
Financial constraints may be a limiting factor in some centers and parts of the world. Computer planning introduces an additional cost for processing the CT data, completing the virtual surgical plan, fabricating models, cutting guides, splints, and customized patient-specific hardware. The plan relies on a high-quality CT scan; the CT scan must be in 0.5 mm to 1.0 mm cuts, as well as a proper quality DICOM dataset for accurate models and surgical guides. The patient must be scanned in a stable final occlusion, which will represent the postoperative occlusion after a total joint prosthesis is placed. This may, in some instances, require maxillomandibular fixation. If the patient has a poor occlusion that needs correction, an open bite, or asymmetry, the patient should be scanned in an open mouth position. The final planned occlusion is then established and set on either stone models or printed models from intraoral scans. The models are then scanned separately and also in the final occlusion. The data from the scans of the models in final occlusion is then incorporated in the VSP. This process is the same as that commonly used in VSP for orthognathic surgery.
The CT dataset that was used for preoperative surgical planning in TMJ ankylosis cases can be used to construct a stereolithographic model that can aid in evaluating and treatment planning cases. Specifically, the condyle and glenoid fossa anatomy, including their points of fusion and skeletal contours, are easily visible and tangible to the surgeon ( Fig. 5 ). This technique was popularized in the later part of the twentieth century. With improvements in the quality of CT imaging, stereolithographic modeling has also improved the accuracy of representation of patient skeletal anatomy. These models are an invaluable planning aid that provide the surgeon with a 3D representation of the patients’ facial skeleton to visualize complex anatomy, assess the need for bony reductions, plan resection margins and osteotomy designs, and fabricate patient-specific TMJ prostheses. The limitation of the stereolithographic models are the poor representation of the teeth, especially occlusal surfaces owing to the limited resolution of the CT scan. Therefore, dental impressions (often scanned into a cone beam CT) or intraoral scanning of the dentition is necessary to be incorporated into the computer aided surgical plan to set the occlusion properly when concomitant orthognathic surgery is performed.