Computer-Assisted Design and Manufacturing in Combined Orthognathic and Temporomandibular Joint Surgery

Key points

  • Conditions of the temporomandibular joint (TMJ) can either cause or result in malocclusion and other dentofacial deformities. Thus, it is often advantageous to simultaneously correct the deformity and address TMJ pathology, as stability of orthognathic surgery depends on a healthy TMJ.

  • Use of virtual planning has allowed for more efficient and accurate workflow: diagnosis, treatment planning, length of treatment, and outcomes.

  • Of the many conditions involving the TMJ, the authors’ focus is on excessive growth conditions and resorption of the TMJ and how they should be taken into account when doing orthognathic surgery.

Introduction

The advent of virtual surgical planning (VSP) in oral and maxillofacial surgery has aided surgeons and patients by reducing preoperative treatment planning and improving accuracy of surgery. It has become a “game changer” for providers and changed the way surgery is practiced, especially for the asymmetric patient. In this article, the authors focus on the use of VSP for cases involving concomitant orthognathic and temporomandibular joint (TMJ) surgery. Although there are a range of conditions affecting the TMJ in which computer-aided treatment planning can be advantageous, the authors focus their discussion on conditions of the TMJ that can cause resorption or malformation, needing operative intervention and correction, specifically (1) excessive growth conditions and (2) TMJ resorption needing alloplastic reconstruction. The craniofacial congenital deformities will be covered in a different chapter. The authors discuss the key considerations for the surgeon during diagnosis, treatment planning, and execution. These considerations are highlighted in the context of patient care.

General principles of virtual surgical planning

The steps and workflow involved in virtually planning orthognathic surgery are well known. If TMJ surgery is planned concomitantly, especially when using a total joint prosthesis system, there are some minor but important differences one must consider. Cone beam computed tomography (CBCT) is first used to help virtually construct the patient’s anatomic model. However, when fabricating a custom TMJ device, a medical grade CT offers better definition that translates into better adaptation of the prosthesis and continues to be the gold standard. Using the individual cuts from the CBCT or CT dataset, segmentation is done to help outline structures from one another, such as separating the maxilla and mandible, outlining soft tissues compared with bony structures, etc. One must consider the head position of the patient and correct it accordingly. This is the first step one should always do because it is critical and a potential source of errors that is often overlooked ( Fig. 1 ).

Fig. 1
A 31-year-old woman, skeletal class II due to mandibular hypoplasia and microgenia. ( A ) Patient posturing. ( B ) shows correction to have Frankfurt horizontal plane parallel to floor before cephalometric analysis and further treatment planning.

The authors like to use the patient’s clinical analysis (clinical cant and yaw) and maxillary midline position in conjunction with anatomic three-dimensional (3D) lines to align the head. As an example, in severe skeletal class II patients with breathing difficulties due to an obstructed airway, the tendency is for these patients to roll their head slightly forward and up in order to open the airway. One must account for this and using virtual planning, correct the head position in order to provide a more accurate outcome.

After 3D models have been generated and segmented, the models can be imported into an appropriate software where virtual dental models from an intraoral scanner (usually in an STL format) are merged, allowing the surgeon to then plan any necessary osteotomies. The planning phase is not different with virtual technology. One must account for the position of the maxillary incisor in all 3 planes of space and the degree of occlusal plane rotation and transverse changes based on the clinical and cephalometric analysis. Once the necessary movements have been performed virtually, a 3D model is made and used by the manufacturer to fabricate the custom device. The surgical splints can also be 3D printed ( Fig. 2 ).

Fig. 2
( A ) 3D-printed STL model with wax-up of total joint prosthesis. ( B ) Intermediate and final splints virtually constructed to be printed and used the day of surgery.

If the mandible is moved and an intermediate splint is necessary, it is critical not to print the splints until close to the surgery day to account for any possible tooth movements that may have occurred while waiting for the fabrication of the custom device. It is recommended that a second set of dental models is uploaded and checked against the first model closer to the surgery day, to allow for an accurate fitting of the splint during surgery.

Another advantage of using virtual planning when doing concomitant TMJ Surgery, especially a total joint reconstruction (TJR), is the possibility of planning for any bone remodeling and recontouring before surgery, including the placement of the condylectomy and if indicated, the corresponding coronoidectomy. One can then request custom cutting guides that would aid in the accuracy and later placement of the device during surgery ( Fig. 3 ).

Fig. 3
( A-C ) Use of custom-made cutting guides for bilateral condylectomies in the setting of a virtually planned bilateral TJR.

This is not always necessary but very helpful in cases where bone recontouring or removal is planned such as in a large bony ankylosis. Positioning of the screws away from anatomic structures, depicting potential interferences, or perforation in the glenoid fossa are just a few of the advantages of using virtual planning. In the case of condylar hyperplasia (CH), it is possible to virtually plan for the condylectomy, predicting the rotation of the mandible, and plan accordingly.

Although being able to predict soft tissue changes based on the movement of the bones is done at every planning session, advances in technology are now more accurately predicting soft tissue changes with osteotomies but still are lacking validation. Perhaps the greatest advantage of virtual planning versus the traditional method for a TJR fabrication in concomitant orthognathic surgery is the accuracy between the model surgery and device/splint fabrication, because it is all done in the same model. In the conventional method, 2 separate model surgeries were needed, thus creating inaccuracies that affected the fitting of the device and ultimately the outcome for the patient ( Fig. 4 ).

Fig. 4
A 15-year-old woman with PMH of juvenile idiopathic arthritis with progressive degeneration of bilateral TMJ. Virtual planning showing planned movements for 3-piece Lefort 1 osteotomy, bilateral sagittal split osteotomy, and TJR. PMH, past medical history.

Condylar hyperplasia

Growth disorders of the condyle can lead to a dentofacial deformity due to alteration of occlusion, morphology of the mandible, and its potential effects on the maxilla. They tend to be classified as horizontal, vertical, or mixed. It is important to determine if the condition is ongoing or arrested and at what age it began. In general, CH that affects growing patients will affect the maxilla if left untreated, but CH that presents in adults tends to spare the maxilla, unless there has been compensatory orthodontic or dentoalveolar movements.

As proposed by Wolford and colleagues, one can use a classification system to determine if one or both of the condyles are resulting in the dentofacial deformity and thus guide the surgeon on how to correct the problem. In the authors’ experience most of these patient suffer from a unilateral deformity and asymmetry. When attempting to correct a facial asymmetry caused by unilateral CH, a surgeon mainly considers 3 treatment options: waiting for the overactive condyle to cease growth and treat the deformity later, performing a condylectomy (high or low) to arrest unilateral growth before performing orthognathic surgery (if indicated), or performing concurrent condylectomy and orthognathic surgery. Others such as Nitzan have proposed to use condylectomy only as the sole treatment of CH with good outcomes.

There are several questions that must be answered before treating a patient with CH:

Aug 5, 2020 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Computer-Assisted Design and Manufacturing in Combined Orthognathic and Temporomandibular Joint Surgery
Premium Wordpress Themes by UFO Themes