Functional stability analyses of maxillofacial skeleton bearing cleft deformities

Abstract

The symmetrically stable craniofacial bony structure supports the complex functions and delicate contour of the face. Congenital craniofacial deformities are often accompanied by bony defects and have been repetitively correlated with compromised dento-maxillary stability, but neither the extent nor the pattern of cleft-related maxillary instability has been explored in detail. Furthermore, it is largely unknown if the bony defect and related instability are correlated with secondary maxillary deformity common among patients with orofacial clefts. With the aid of finite element modeling, we studied the detailed relationship between cleft-related bony defect and maxillary stability under occlusal loading. Craniofacial models were generated based on cone-beam computed tomography data and loaded with mimicked bite forces along the axial axis of each tooth. Our data showed that all cleft models exhibited more asymmetrical deformations under mastication compared with the normal. Models with palatal cleft demonstrated greater asymmetry, greater dental arch contraction, and less maxillary protrusion compared to models with alveolar cleft only. For unilateral cleft models, alveolus on non-cleft side tended to be more protruded and lifted than the cleft side. For bilateral cleft models, the most prominent feature was the seriously contracted alveolar arch and curved and pitched premaxillae. These findings indicated cleft type-specific pattern of maxillary instability, which were largely in accordance with dentoalveolar morphological features among patients. Collectively, our study elucidated the detailed relationship between cleft bony defect and the pattern of maxillary instability, and suggested a prototype for studying the abnormal maxillary and dental arch growth among patients with craniofacial deformities.

Figure 1

Construction of analytical finite element models. CT images of normal maxilla were three-dimensionally reconstructed in mimics (A), after meshed, material properties were assigned to elements based on image Hounsfield values (B). Then, meshes (C) and materials (D) were imported to ANSYS Workbench for biomechanical analysis. Cleft models, for example the unilateral cleft of lip and alveolus and palate (UCAP), were constructed based on the normal model by erasing the corresponding lateral incisor (s) (yellow stripe in (E,F)), surrounding bone and palatal bone (red stripe in (F). Other model construction procedures including meshing and material assigning were kept the same with those of normal model. Except the differences in alveolar/palatal cleft types, cleft models hold great consistency with each other and with the normal model (G). Mesh properties including number of nodes and elements are shown in each model pictures (G). UCA: unilateral cleft of lip and alveolus; UCAP: unilateral cleft of lip and alveolus and palate; BCA: bilateral cleft of lip and alveolus; BCAP: bilateral cleft of lip and alveolus and palate. Scale bar: 3 cm.

Figure 2

Analytical settings (A–H) and results of total deformation in each model (I–Y). An occlusal plane coordinate system (CSYS-O) was constructed via three points (A) i.e. mesial contact point of central incisors and mesial-buccal cusp peak of bilateral upper first molar. This CSYS-O was offset 10 mm along the Z-axis to achieve the alveolar crest (B) and was defined as the coordinate system of alveolar plane (CSYS-A). On the alveolar plane defined by the x- and y- axis of CSYS-A, deformation probes were positioned (C) at the central labial/buccal regions of premaxilla (Pmx), canine (L1, R1), first molar (L2, R2) and the third molar (L3, R3) for deformation data collection (D). Local coordinate systems were also constructed at the central of occlusal surface of each tooth and z-axis was adjusted to fit the long axis of each tooth (E). Simulated masticatory forces were therefore loaded via each local coordinate system through the y-axes guided tooth long axes (F). Surfaces along the skull borders were fixed supported for boundary conditions (G). In order to depict the 3D deformations of alveolus, another coordinate system (CSYS-X) was defined (H) to show the three directions of deformations: x-/y-/z- axis would be separately used to show the transversal/forward-backward/up-down deformations (I). X-axis was fit to the line (yellow dotted line and red arrows in H) through the bilateral mesial-buccal cusp peaks of upper first molar. Y-axis was fit to the sagittal plane of the maxilla and cross the occlusal plane. Above settings were kept the same in all models. The alveolar TC gradually increases caudally (J).

Figure 3

Schemes for CBCT morphometric analysis of alveolus in normal people and alveolar cleft patients. Three reference planes (sagittal plane, FH plane and coronal plane) (red arrows in (A) Were respectively defined. Sagittal plane was defined by crista galli, sellaturcica and basion. Frankfort horizontal plane (FH) was demarcated by 4 points (bilateral Orbitales and bilateral Porions). Coronal plane was set at sellaturcica and perpendicular to the former both planes. Reference points were located bilaterally (red dots in (B)) (R: right, L: left) on the mesial-buccal alveolar crest of canine (R1, L1) and second premolar (R2, L2) and the distal-buccal alveolar crest of first molar (R3, L3). Based on each of the three reference planes, transversal, AP (anterior-posterior) and vertical distances of these three group points were measured as the anterior (RI-L1)/middle (R2-L2)/posterior (R3-L3) width (B), AP dislocation (C) and vertical dislocation (D) of the alveolar arch. Distance from the ANS (anterior nasospinale) to the Frankfort plane was measured to scale the pitch of Pmx (E), the shorter this distance, the more Pmx was pitched.

Figure 4

FEA results of total (A–O) and directional (P–T) deformations show unbalanced and distinctive functional movements in each cleft model. Frontal (A–E), sagittal (F–J) and transversal (K–O) views of deformed alveolus in each model were plotted. Gray contour lines in F-J and blue contour line in K-O show the shape of each maxilla before loaded. Under normal mastication, Alveolus in normal showed mild contraction (K,Q), protrusion (K,R) and lifting (F,S), these deformations were quite symmetrical (P). On X direction, palatal cleft models show much more seriously contracted alveolar arch than others (Q and red arrows in B–E, blue arrows in M,O). On Y direction, simple alveolar cleft models were relatively more protruded than palatal cleft models (R and blue arrows in L,N), though protrusions in cleft models were all smaller than those in normal. Protrusion on noncleft side was larger than that on cleft side in UCA and UCAP (R). On Z direction, alveolar liftings in bilateral cleft models were smaller than those in normal, liftings in noncleft side were larger than the cleft side in unilateral cleft models especially UCAP (red dotted circles in S). For Pmx, it was remarkably protruded and lifted (D,E,I,J) with greatly increased total deformations in BCA and BCAP (T). In UCAP, Pmx showed lateral inclination and rotation toward the cleft side (C,T).

Figure 5

Results of CBCT morphometric analysis of four kinds of alveolar cleft patients. (A–D) shows the transversal views of representative patient’s alveolus and the average alveolar width in each cleft type (E). Alveolar width was significantly reduced in palatal cleft patients (blue arrows in B,D). Alveolar AP dislocations were found increased in all cleft type patients, especially in palatal cleft patients (J). Vertical dislocations were also mostly observed in palatal cleft patients particularly in UCAP. Pmx was significantly pitched in all cleft patients (red arrows in H,I,N,O,P). Typical inclination and rotation of alveolus on non-cleft side toward the cleft was observed (curved red arrows in B, G, and in M). ^*P < 0.05, ^**P < 0.01, ^***P < 0.001.