Three-dimensional cone-beam computed tomography for assessment of mandibular changes after orthognathic surgery

Abstract

Introduction: The purpose of this study was to assess alterations in the 3-dimensional (3D) position of the mandibular rami and condyles in patients receiving either maxillary advancement and mandibular setback or maxillary surgery only.

Methods: High-resolution cone-beam computed tomography scans were taken of 21 patients before and after orthognathic surgery. Ten patients with various malocclusions underwent maxillary surgery only, and 11 Class III patients received maxillary advancement and mandibular setback. Presurgery and postsurgery 3D models were registered on the surface of the cranial base. A new tool was used for graphical overlay and 3D display with color maps to visually assess the locations and to quantify positional changes in the posterior border of the mandibular rami and condyles between superimposed models.

Results: The average displacements in condylar position were small--0.77 mm (SD, 0.12 mm) and 0.70 mm (SD, 0.08 mm)--for 2-jaw and 1-jaw surgeries, respectively (not significant, P >.05). All 2-jaw surgery patients had backward rotational displacements of the mandibular rami (mean, 1.98 mm; SD, 1.03 mm), with a maximum surface distance change of > or =2 mm in 8 of 11 subjects. For the 1-jaw surgery, all subjects had small backward rotational displacements of the mandibular rami (mean, 0.78 mm; SD, 0.25 mm), with only 1 subject having a maximum surface distance change > or =2 mm. The difference in mean backward rotational displacement was statistically significant (P <.01).

Conclusions: The visualization of 3D model superimposition clearly identified the location, magnitude, and direction of mandibular displacement. The 3D imaging allowed quantification of vertical, transverse, and anteroposterior ramus displacement that accompanied mandibular, but not maxillary only, surgery.

Fig 1

Superimposition of presurgery and postsurgery models of patient treated with maxillary advancement and mandibular setback. A, Right view; B, frontal view; C, left view. Surface of cranial base was used for registration performed with MIRIT. Cranial base color map is green (0 mm surface distance), showing adequate match of before and after models for cranial base structures.

Fig 2

Lateral views of 3D models of patient. A, 3D model constructed from CBCT image acquired 1-2 weeks before surgery. B, 3D model labeled green constructed from CBCT scan 1 week postsurgery. Other anatomic structures are masked for better visualization of changes in mandibular ramus and condyle. C, A and B are combined after superimposition to identify regions of interest in mandibular rami: condyles (red) and posterior border (green).

Fig 3

Average surface distances at condyles for each patient.

Fig 4

Eleven patients were treated with maxillary advancement and mandibular setback surgery. Note variability in rotation of rami after bilateral sagittal split osteotomy, with smaller displacement at condyle levels, but surfaces of displacement ≥2 mm along lateral surface and posterior border of rami for 8 of 11 patients.

Fig 5

Ten subjects were treated with maxillary advancement only. Only 1 subject had maximum surface distance change ≥2 mm (red along posterior border of rami). For all other patients, mandibular surface displacements were minimal.

Fig 6

Average surface distances at posterior border of ramus for each patient.

Fig 7

Overall average surface distances at mandibular rami.