Sunday, October 10, 2004

Cine MRI as a Diagnostic Predictor of Success of Mandibular Distraction

RA Maercks, MD, XP Reyna Rodriguez, DDS, JJ Marler, MD, Cathy Ebert, MS, LE Landa, MD, DMD, Enrique Ochoa, MD, and CB Gordon, MD.

PURPOSE: To evaluate airway improvement after mandibular distraction in young patients with obstructive sleep apnea (OSA) and micrognathia using dynamic cine magnetic resonance (MR).

MATERIALS AND METHODS: Thirty-three patients with symptomatic micrognathia underwent imaging using fast gradient-echo sequences in the sagittal midline by using a 1.5-T unit to create cine MR images. These were compared with both normal age-matched controls and with postoperative exams after mandibular distraction. Standard rigid and flexible endoscopy and blinded grading of airway was also carried out. The nasopharynx, oropharynx, and hypopharynx were characterized in terms of airway motion and classified as static patent (SP), dynamic patent, intermittent collapsed (IC), or static collapsed (SC); and the maximal diameter and greatest change in diameter (in millimeters) of these airways were calculated. Tonsilloadenoidal volumes were calculated, and 3-d landmarks were plotted in a proprietary software package. Flow predictions were created by converting average airway dimensions to wire-frame models and using The inspiratory flow was studied using a control volume method to solve the fully three-dimensional laminar Navier-Stokes equations. with in-plane and 90 degrees off-plane configurations. The ratio of airflow and the total pressure drop coefficient were analyzed using standard models.

RESULTS: As in previous studies comparing OSA and normal patients, there were statistically significant differences in the following parameters between the two groups: nasopharynx SP (P < .001) and IC (P < .001); hypopharynx SP (P < .005) and IC (P < .005); and mean change in airway diameter of the nasopharynx (P < .001) and hypopharynx (P < .001). Patients with symmetric micrognathia had correction of all of the airway motion description parameters except for SC. (P < .001) Flow and pressure drop were normalized in 24/33 patients when compared with age-matched controls, and correlated to descriptor changes. Volumes of the supraglottic airway between fixed landmarks were increased by nearly 230% in the average patient. Asymmetric micrognathia (n=5), syndromic micrognathia (n=4), and those patients with severe tracheomalacia (n=3) were significantly improved ((P < .001), but not normalized. CONCLUSION: Cine MRI represents an exciting modality to predict airway behavior and likelihood of success in mandibular distraction. There is no radiation risk to the patient, unlike CT scanning. There is excellent correlation between observed airway motion during flexible endoscopy and cine MRI. Advanced dynamic airway modeling may one day obviate the need for invasive airway instrumentation and predict success for airway correction during complex craniofacial surgeries.