Dynamic healing and remodeling of mandibular ramus in segmental mobility and cortical overlap after intraoral vertical osteotomy

The ramus immediately after intraoral vertical ramus osteotomy (IVRO) shows segmental mobility, cortical overlap, and projection, which yield smooth-surfaced continuity and stability after one year. This study aimed to elucidate the three-dimensional (3D) morphological changes driven by regional healing and remodeling 12 months postoperatively (Post) and their relationships with the immediate postoperative (Imm) segmental overlapping pattern. We performed a retrospective study of the morphological change of mandibular ramus in terms of time and amount of cortical overlap after IVRO. 3D computed tomography data of 108 hemi-mandibles were analyzed by independent superimposition of Imm proximal and distal segment to Post mandible. Analysis showed extensive regional bone resorption and apposition resulting in a relatively flat-surfaced and morphologically intact Post ramus. The middle ramal remodeling was significantly associated with Imm overlapping patterns and Post mandibular movement, new bony projections of various shapes being observed at the posterior border of the Post ramus. The ramus after IVRO underwent dynamic morphological changes during healing/remodeling which were partly associated with Imm overlap. Moreover, the Post functional and smooth-surfaced ramus seems to have been facilitated by the close surgical approximation of segments and the postoperative mandibular functional rehabilitation, even with segmental positional changes.


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The intraoral vertical ramus osteotomy (IVRO), sometimes described as a vertical subcondylar or 50 oblique osteotomy, is an orthognathic surgical technique for setting back the mandible [1,2]. It is a 51 common orthognathic procedure [3], along with sagittal split ramus osteotomy (SSRO), for prognathic 52 or asymmetrical mandible. Though IVRO is advantageous as compared with SSRO[4,5], certain factors 53 must be taken into consideration for clinical application such as the induction of bone healing and 54 remodeling [1,6].

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Since IVRO induces the distal segment to move backward, it overlaps with the proximal segment and 56 becomes mobile to some extent without any intersegmental fixation. The bone healing commences in 57 direct contact, with some mobility of cortical bones between two segments. Several studies have been 58 conducted on the potential and outcomes of healing/remodeling, mainly based on histological 59 findings [6][7][8], and on radiographic images [9][10][11][12][13]. However, these studies yielded limited insight into how 60 the preoperative (Pre) ramus can be healed and remodeled to reshape the final postoperative (Post) 61 one. superimposition of the three stages. The Imm osteotomy gap was completely fused by bony healing 75 and smoothened by remodeling on Post ramus. Post ramus also showed decreased ramal height and 76 width, a smaller angular region, and a more upright and shorter condyle than Pre or Imm ramus.

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The dotted line on the 3D model of Imm ramus in Fig

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The primary purpose of this retrospective study was to investigate 3D morphological changes of 86 Imm ramus as characterized by segmental mobility, cortical overlap, discontinuity, and projection after 87 IVRO to stable Post ramus as seen in shape changes and smooth-surfaced continuity after one year.

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We hypothesized that the Imm ramus would be transformed into Post ramus via dynamic remodeling 89 depending on elapsed time and mandibular function after surgery and also on the segmental 90 overlapping pattern of Imm ramus. We also wanted to confirm that a direct model comparison of the 91 Pre, Imm, and Post rami would reveal interval morphological changes occurring in the Imm ramus 92 attesting to regional bony remodeling. We introduced a novel direct segmental overlapping method to 93 avoid potential bias induced by postoperative positional changes of the proximal/distal segments. The 94 specific aims of this study were to: 1) verify the superimposition of fragmented Imm proximal and distal  Outcome Variable 129 The first variable was the type and extent of surface remodeling, which included the bony apposition, 130 resorption, and less marked change. The newly-formed bone surface of Post ramus as compared with 131 that of Imm ramus was categorized as bony apposition and the reduced surface as bony resorption.

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The second outcome variable consisted of morphological changes of the ramus, including new 133 structural formation. superimposed Imm ramus (G). The Imm proximal segment is indicated by a yellow line with black arrowhead, and the Imm distal segment as a blue line with white arrowhead.

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The separated Imm proximal and distal segments in (D) were also superimposed to Pre mandible (E)

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to verify the reliability of superimposition method, as shown in (F, H, and I).
159 Note) proximal segment indicated by black arrowhead; distal segment by white arrowhead 160 161 The Imm mandible was segmented into proximal and distal segments at the IVRO osteotomy line 162 (Fig 2D; white arrowhead for the segmented Imm distal segment; black arrowhead for the segmented 163 proximal segment). The Imm distal segment was superimposed to Post mandible using two sequential Korea) (Fig 2H). The differences between Post and Imm rami were measured at four ramal regions: 170 the mandibular condyle (Fig 2H-a), sigmoid notch ( Fig 2H-b), middle ramal region (Fig 2H-c), and angle 171 (Fig 2H-d).

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The data for the morphological descriptions and the inter-surface measurement values were collected.

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Descriptive statistics were computed for all study variables (R project, www.r-project.org). Fisher's exact 174 test was performed to find potential associations between predictor and outcome variables. In addition,

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The second variable was the intersegmental overlap pattern during the Imm period, which was 212 classified into four categories (Fig 1B, E-H; table 1). The laterally juxtaposed proximal segment in 213 relation to the distal segment, classified as the 'Lateral' type ( Fig 1E), was the most frequently observed 214 pattern (N=69/108, 63.9%).

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The third variable was the functional range of mandibular movement (

Outcome variables 232
The first outcome variable pertained to the type and extent of surface remodeling from Imm to Post 233 ramus. These remodeling patterns were evaluated in four subdivisions of the ramus: the mandibular 234 angle, sigmoid notch, middle ramus, and condylar region (as indicated in Fig 2H). Fisher's statistical 235 analysis was performed to confirm the association between the Imm segmental overlapping type (as a 236 second primary predictor variable) and the ramal remodeling pattern (as an outcome variable) (table 3-237  6). In addition, the possibility of inter-regional correlation between these four subdivision regions were 238 evaluated and rejected by the Chi-squared and Fisher's exact tests (p > 0.05; details not shown). Details

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of remodeling in the four regions were as follows:

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The most prominent bone resorption at the mandibular angle was found around the angular tip of the 241 proximal segment on the lateral surface and the proximal tip of the distal segment on the medial surface 242 (Fig 3E, F, K, L; indicated by black arrows; Table 4). The major bone apposition, on the other hand,

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was found at the angular tip of the distal segment on the lateral surface (Fig 3E, F

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Again, the bony apposition is shown in blue and indicated by white arrows and the resorption in red and 258 indicated by black arrows.

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Note 1) Color coding for E and F; orange and red for 1.5 to 2.5 mm and gray for more than 2.5 mm of 260 bone resorption; light and dark blue for -0.5 to -1.5 mm and in gray for more than -1.5 mm of bone 261 apposition.

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Note 2) Color coding for K and L; in orange and red for 0.9 to 1.5 mm and in gray for more than 1.5mm 263 of bone resorption; in light and dark blue for -0.4 to -1.0 mm and in gray for more than -1.0 mm of bone 264 apposition.  The middle ramal overlapping region between the proximal and distal segments also showed bony 288 apposition and resorption at the lateral and medial surfaces, resulting in complete cortical continuity for 289 all overlap types (Fig 3). The finding of 'pronounced bony apposition/resorption' prevailed in 67 cases

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(62.0%) on the lateral surface and 80 cases (74.1%) on the medial side (Table 5). The remodeling 291 pattern in this region was significantly associated with the segmental overlapping type (Table 5; p < 292 0.001 for both lateral and medial regions). In addition, this remodeling was also significantly associated 293 with Post mandibular movement ranges of protusion and lateral exercusion (Table 3; p < 0.04 and 0.01 294 each).

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The sigmoid notch of Post ramus simultaneously showed both 'moderate apposition and resorption' 296 of more than 1 mm in the majority of cases (88 cases, 81.5%), when compared with the Imm ramus 297 (Fig 4; details not shown). The proximal segment near the osteotomy line mainly showed bone

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The Imm proximal (ivory, indicated by black arrowhead) and distal segments (gray, indicated by white 307 arrowhead) were independently superimposed to Post ramus (light green) to reveal the Post remodeled 308 ramus. The distance between them showed the level of bony apposition (indicated by black arrows) 309 and resorption (indicated by white arrows).