Assessment of mandibular advancement surgery with 3D CBCT models superimposition

Abstract

OBJECTIVES: To assess surgery and short-term post-surgery changes in the position of the condyles, rami and chin after mandibular advancement. METHODOLOGY: Pre-surgery (T1), 1 week post-surgery (T2), and 6 week post-surgery (T3) CBCT scans were acquired for 20 retrognathic patients with short or normal face height. 3D models were built and superimposed through a fully automated voxel-wise method using the cranial base of the pre-surgery scan as reference. Anatomic regions of interest were selected and analyzed separately. Within-subject surface distances between T1-T2, T2-T3, and T1-T3 were computed. Color-coded maps and semi-transparent display of overlaid structures allowed the evaluation of displacement directions. RESULTS: After an antero-inferior chin displacement with surgery in all cases (>4 mm in 87.5%), 25% of the patients showed some kind of posterior movement (< 3 mm), and 69% showed an antero-superior movement after splint removal. Comparing T1-T3, an antero-inferior (87.5% of the cases) or only inferior (12.5%) displacement was observed (>4 mm in 80%). Considering all directions of displacement, the surface distance differences for the condyles and rami were small: 77.5% of the condyles moved <2 mm with surgery (T1-T2), and 90% moved <2 mm in the short-term (T2-T3) and in the total evaluation (T1-T3), while the rami showed a <3 mm change with surgery in 72.5% of the cases, and a <2 mm change in 87.5% (T2-T3) and in 82% (T1-T3). CONCLUSIONS: Expected displacements with surgery were observed and post-surgery changes suggested a short-term adaptive response toward recovery of condyle and ramus displacements. The changes on the chin following splint removal suggested an acceptable adaptation, but with considerable individual variability.

FIGURE 1

Visualization of a right condyle displaced posterior-superior-medially between T1 and T2; Left: Color coded maps indicate outward displacements in red and inward displacements in blue. Right: Semi-transparencies with T1 in solid white and T2 in transparent red (A = anterior; P = posterior).

FIGURE 2

Superimpositions between pre-surgery to immediately post-surgery with splint in place (A), immediately post-surgery to splint removal (B), and pre-surgery to splint removal (C). Some posterior (inward) movement of the chin is noted in B, shown by the blue color code. It can also be noted by the comparison between different area and density of red surfaces representing the anterior (outward) displacement in A and C. Still, the resultant superimposition in C shows an acceptable maxillomandibular relation at splint removal, considered a short-term stability. The right ramus shows a slight lateral movement in A (outward), a recovery tendency in B (inward), and green surfaces in C confirming the adaptive response.

FIGURE 3

Above: Color maps of surface distances between T1 and T2 models for 4 different patients showing an antero-inferior displacement of the chin post-surgery with the splint in place. Patients A and C showed prevalence of vertical displacement as shown in red at the anterior alveolar process and inferior surface of the symphysis. Patients B and D showed a vertical change with a remarkable chin advancement, shown by red mandibular anterior surfaces. Patient D also showed backward ramus displacement. Below: Semi-transparencies of these cases with superimposed T1 (solid white) and T2 (transparent red).

FIGURE 4

Short-term follow-up (T2–T3) of cases shown in the previous figure. After splint removal, an antero-superior tendency of movement was noted in most of the cases in the sample. Additional anterior mandibular displacement can be seen in color maps red surfaces and in semi-transparencies differences between white (T2) and red (T3) models. A rami comparison between Figures 3 and 4 (T1–T2 × T2–T3) suggests a recovery of surgery displacements after six weeks, with a medio-lateral movement in cases B and C, and antero-posterior movement in case D.

GRAPH 1

Direction of displacement of the chin between T1–T2, T2–T3, and T1–T3, expressed in number of subjects (n = 16).

GRAPH 2

Categorization of the amount of displacement of the chin between T1–T2, T2–T3 e T1–T3, expressed in number of subjects (n = 16).

GRAPH 3

Direction of displacement of the left condyle between T1–T2, T2–T3, and T1–T3, expressed in number of subjects (n = 20).

GRAPH 4

Direction of displacement of the right condyle between T1–T2, T2–T3, and T1–T3, expressed in number of subjects (n = 20).

GRAPH 5

Categorization of the amount of displacement of the left condyle between T1–T2, T2–T3 and T1–T3, expressed in number of subjects (n = 20).

GRAPH 6

Categorization of the amount of displacement of the right condyle between T1–T2, T2–T3 and T1–T3, expressed in number of subjects (n = 20).

GRAPH 7

Direction of displacement of the left ramus between T1–T2, T2–T3, and T1–T3, expressed in number of subjects (n = 20).

GRAPH 8

Direction of displacement of the right ramus between T1–T2, T2–T3, and T1–T3, expressed in number of subjects (n = 20).

GRAPH 9

Categorization of the amount of displacement of the left ramus between T1–T2, T2–T3 and T1–T3, expressed in number of subjects (n = 20).

GRAPH 10

Categorization of the amount of displacement of the right ramus between T1–T2, T2–T3 and T1–T3, expressed in number of subjects (n = 20).