The Effect of Orthognathic Maxillomandibular Advancement Surgery on the Temporomandibular Joint

Dysfunctional congenital or acquired growth disturbances of themaxilla and/or the mandible are treated by orthognathic surgery.Maxillomandibular advancement surgery (MMA) is indicated forpatients who often present with maxillary and mandibularretrusion, an anterior open bite, and a high mandibular plane angle,resulting in jaw dysfunction. A major challenge following MMA isskeletal relapse, which is believed to be due to the surgicalpositional changes of the mandible, subsequently of the mandibularcondyles and of the temporomandibular joint (TMJ).

The positional change of the mandible and that of the mandibularcondyles is believed to induce adaptive remodeling of the TMJ.Progressive condylar resorption (PCR), a late reaction, developswhen the adaptive capacity of the TMJ is exceeded and may resultin a skeletal relapse and an unsuccessful surgical outcome. Theglenoid fossa, the mandibular condyle, and its location in the fossaare believed to be responsible for the position of the mandible. Aholistic approach to the TMJ is therefore indicated. Long-termthree-dimensional (3D) evaluations are lacking to understand theeffect and the postoperative challenges when performing MMA.Hence, the aim of the thesis was to address the changes of the entireTMJ and the prediction factors for skeletal relapse, and for loss ofcondylar volume and height using a new validated 3D assessmentmethod.

Studies I–III were based on a five-year retrospective cohort with50 patients, comprising three sets of CBCT scans acquired presurgery, two weeks, and five years post-surgery.

Using a new and validated semi-automated method, Study Ievaluated the long-term three-dimensional changes of the TMJ,including the mandibular condyle, the glenoid fossa, and jointspace changes. The largest changes of the TMJ were observed inthe mandibular condyles, particularly at the posterior aspect. Only minor changes were observed in the glenoid fossa and the jointspace. The condylar volume change had the highest explainedvariance of 20–22% for postoperative horizontal skeletal relapseand of 71–72% for condylar height decrease. Progressive condylarresorption was identified in five females, with a mean condylarvolume loss of 32%.  

Study II aimed to evaluate patient demographics, clinical andradiographic characteristics, and the surgical factors affecting longterm postoperative skeletal stability, condylar volume and heightloss, and PCR. The magnitude of the mandibular horizontaladvancement and vertical movement, as well as a preoperativeBody Mass Index (BMI) of < 30, were identified as predictionfactors for horizontal skeletal relapse. Prediction factors forlong-term condylar volume loss included a preoperative BMI of< 30, preoperative mandibular plane angle, and magnitude of themandibular horizontal advancement and vertical movement.Regarding condylar height loss, significant identified factors werethe preoperative mandibular plane angle, age at surgery, and themagnitude of mandibular horizontal advancement and verticalmovements. 

Study III applied validated 3D methods to estimate the positions ofthe mandibular condylar and proximal segments immediately andfive years after the MMA, and their effects on long-termpostoperative condylar volume loss and horizontal skeletal relapse.The mandibular condyles were displaced in a postero-mediosuperior direction, while the mandibular proximal segments flaredlaterally, torqued in an outward direction, and rotated in a CCWdirection during MMA. An additional CCW rotation was observedduring the five-year follow-up. Counterclockwise rotation wasidentified as a significant prediction factor for long-term condylarvolume and height loss and horizontal mandibular skeletal relapse. 

In conclusion, five years following the orthognathic MMA,changes of all parts of the TMJ were observed. The mandibularcondyle presented the largest changes and the largest effect onpostoperative skeletal relapse compared to the glenoid fossa andjoint space changes. The proportion of PCR was 10%. Duringsurgery, the mandibular condyle and proximal segments changedposition and rotated, respectively. Multiple prediction factors forlong-term postoperative horizontal skeletal relapse and loss ofmandibular condylar volume and height were identified. Inparticular, these factors included large horizontal mandibularadvancement and CCW rotation. However, there is anintercorrelation between the predictor variables, underlining themultifactorial aspect of PCR.