Measurement Methodology for Tempomandibular Joint Displacement Based on Focus Mutual Information Alignment of CBCT Images

摘要

Maxillofacial surgery can lead to displacement of the mandibular joint, bone resorption and finally articulatory dysfunctions. Bimaxillary surgical correction of a high angle absolute mandibular retrognathism case may provoke condy-lar resorption. An in vivo measurement method is developed to evaluate mandibular joint displacement aiming at correlating it with possible long-term clinical symptoms. For 6 patients two consecutive Cone Beam Computed Tomography (CBCT) images are obtained before and after functional craniofacial surgery. The images are superimposed using a fuzzy 3D alignment criterion: Focus Mutual Information (FMI), in which the operator only needs to indicate approximately the (anatomic) structure in the pre-image. Changes in the left mandibular joint are studied from the alignment of the images focussing on the left mandibular ra-mus. An analogous alingnment procedure is applied to the right mandibular ramus. The average age of the patients was 30 years, ranging from 16 to 48 years, 1 male and 5 female. The subtraction images reveal clear change in geometry in 7 of the 12 joints. The rotation in the horizontal plane ranges from -11.2° to 8,8°, with p. = -0.9°, and o = 4.9°, the rotation in the frontal plane ranges from -6.8° to 9.5°, with n = 0.0°, and a = 5.1°. The translational displacement of the caput with respect to the fossa ranges from 0.2mm to 3.1mm in the horizontal plane, with u = 1.0mm, and a = 0.9mm. Minimal distances lower than 1mm have been observed. The semi-automatic alignment and measurement method based on FMI allows for superimposition of 3D CBCT images and makes it possible to accurately measure distortion of the mandibular joint during or immediately after orthodontic surgery allowing for instantaneous corrections.