Postoperative changes in mandibular prognathism surgically treated by intraoral vertical ramus osteotomy

Abstract

This study investigated short- and long-term postoperative skeletal changes following intraoral vertical ramus osteotomy (IVRO) for mandibular prognathism, as determined from lateral cephalograms. The subjects were 20 patients with mandibular prognathism who had undergone surgical orthodontic treatment combined with IVRO. Lateral cephalograms were taken at six time points: 1 month before surgery, and 1 day, 3 months, 6 months, 1 year, and approximately 2 years after surgery. Intermaxillary fixation (IMF) with four monocortical screws was maintained for 1 week in all patients. Mean posterior movement of the menton (Me) was 5.9 mm at surgery. 3 months after surgery, the FMA and FH-CorMe angles had increased 6.3 and 6.2 degrees, respectively, indicating clockwise rotation of the distal segment of the mandible. This rotation was observed in all 20 patients, suggesting that postoperative rotation of the mandible in the postoperative short term is likely to occur after IVRO and could be considered an adaptation of the mastication system newly established by surgery. In the long term after IVRO, Me had moved anteriorly by only 0.9 mm and the relapse ratio was 15.3%. These findings suggest the excellent long-term stability of surgical orthodontic treatment combined with IVRO in patients with mandibular prognathism.

Sagittal split ramus osteotomy (SSRO) and intraoral vertical ramus osteotomy (IVRO) are well-established procedures for correcting mandibular prognathism. Both techniques have advantages and disadvantages. The advantages of SSRO include large bony contact between the distal and mesial segments and application for both advancement and retraction and reduction in the duration of intermaxillaly fixation (IMF), with possible complications including inferior alveolar nerve (IAN) injury, unfavourable split, and high blood loss. IVRO affords a lower incidence of IAN injury, technical simplicity, low blood loss, and short duration of surgery. Its disadvantages include application for only retraction of the mandible, less bony contact between the proximal and distal segments, and requiring a relatively long period of IMF. Orthognathic surgeons must weigh up these advantages and disadvantages when deciding which surgical treatment to use in cases of mandibular prognathism.

Another important factor for surgeons to consider is postoperative stability. While the literature contains a number of studies on postoperative changes after SSRO and a review paper by Costa et al., only a few reports concern postoperative stability after IVRO. In addition, the evaluation period covers 2-year periods at most, and only some changes have been examined. The postoperative changes that occur after IVRO remain to be clarified. This study investigated short- and long-term postoperative skeletal changes following IVRO for the correction of mandibular prognathism.

Patients and methods

The patients were 20 adolescents or adults (4 males and 16 females; mean age 20.6 ± 6.0 years; range 16–36 years) who underwent orthognathic surgery by IVRO at a single institution between August 2003 and March 2008. The inclusion criteria were mandibular prognathism without asymmetry, no trauma or recognized syndromes, no additional surgery such as genioplasty before or after IVRO, and no facial growth at the commencement of orthognathic surgery. The study was conducted with institutional review board approval.

Cephalometric analysis

Lateral cephalograms taken at six time points were used to evaluate postoperative stability: 1 month before surgery (T1), 1 day after surgery (T2), 3 months after surgery (T3), 6 months after surgery (T4), 1 year after surgery (T5), and approximately 2 years after surgery (T6). All cephalograms were obtained with vertically adjustable head holders in the intercuspal position and were traced onto acetate paper by the same investigator (J.N.) in order to eliminate inter-examiner variability. Six cephalograms for each subject were superimposed by cranial structure on the S-N line at the sella to reduce measurement error. The mandibular plane on all cephalograms was matched to the preoperative mandibular plane by superimposing the anatomical mandibular structures such as symphysis or mandibular canal ( Fig. 1 ).

Fig. 1
Method for deciding the mandibular plane. The mandibular plane was identified by superimposition of the mandible with anatomical landmarks on all lateral cephalograms taken at the different time points, such as T1–T3. T1, 1 month before surgery; T2, 1 day after surgery; T3, 3 months after surgery.

11 linear and 12 angular measurements were taken to assess the postoperative skeletal changes in each case. Cephalometric landmarks for the angular measurements and linear measurements taken are shown in Figs. 2 and 3 , respectively. X Y coordinates were constructed using a line parallel to the Frankfort horizontal plane (FHP) at the sella for the X coordinate and a perpendicular line drawn intersecting the X line at the sella for the Y coordinate. Six distance measurements on each cephalometric tracing were also made ( Fig. 4 ).

Fig. 2
Cephalometric angular measurements. [1] SNA, [2] SNB, [3] ANB, [4] SNP, [5] FMA, [6] U1-SN, [7] IMPA, [8] FMIA, [9] interincisal angle (I.I), [10] FH-occlusal plane (FH-Occ), [11] gonial angle (Go), [12] FH-CorMe.

Fig. 3
Cephalometric linear measurements.

Fig. 4
Cephalometric distance measurements. (a) U1-Pp, (b) L1-Mp, (c) U6-Pp, (d) L6-Mp, (e) overjet, (f) overbite. Pp, palatal plane; Mp, mandibular plane.

Surgical procedure

All patients received preoperative and postoperative orthodontic treatment with the fixed standard edgewise appliance to create stable occlusion postoperatively. Extraction during presurgical orthodontic treatment was performed in some cases. The surgical procedure used was the bilateral vertical ramus osteotomy reported by Bell. The osteotomy line was designed posterior to the top of a sigmoid curve 7 mm from the posterior ramus border. The inferior site of the proximal segment was trimmed with a bur, as necessary. Postoperative IMF used 0.5 mm stainless steel wire and four monocortical screws placed between the roots of the maxillary and mandibular cuspids and bicuspids ( Fig. 5 ). IMF was maintained for 1 week in all patients. Immediately after IMF removal, vertical elastics were employed in all cases and Class II elastics were employed in most cases during postoperative orthodontic treatment. The edgewise appliance was removed by 1 year after surgery in all patients. None of the patients showed open bite or reversed overjet between 1 day and 2 years after surgery.

Fig. 5
Postoperative IMF.

Landmark localization error

To evaluate errors in landmark localization during tracing, 12 cephalograms randomly selected from 120 were retraced twice by the same investigator (J.N.) with an interval of at least 3 months between tracings. The landmark localization error was assessed with these measurement data using the formula of Dahlberg. The errors for angular cephalometric measurements, linear measurements, and distance measurements were in the range 0.18–0.33 degrees, 0.33–0.88 mm, and 0.16–0.30 mm, respectively.

Statistical analysis

The Wilcoxon signed-rank test was used to compare the mean differences in measurements on the cephalograms at each time period. Data was analyzed using the statistical package JMP (SAS Institute, Release 6.0, Cary, NC, USA). Differences of p < 0.05 were considered significant.

Results

Operative changes

Table 1 shows the mean values of each measurement 1 month before surgery and 1 day after surgery and the differences between them. For the angular measurements, significant differences were seen for SNB, ANB, SNP, Go, FMIA, I.I and FH-Occ. Specifically, SNB (3.1 degrees) and SNP (2.8 degrees) were decreased and ANB (3.4 degrees) was increased, indicating posterior movement of the distal segment of the mandible. The gonial angle was decreased to 5.5 degrees, due to disparate movement of the posterior ramus border and mandibular inferior border with separation of the mandible into proximal and distal segments. Mean changes in FMIA, I.I, and FH-Occ, which served as dental landmarks, were +1.4 degrees, +1.1 degrees and −0.6 degrees, respectively. Regarding the linear measurements, significant differences were found in L1( x , y ), L1R( x , y ), B( x , y ), Pog( x , y ), Me( x , y ), G( x ), and Cor( x , y ). Mean horizontal changes of the mandible were −6.1 mm for B, −6.0 mm for Pog, and −5.9 mm for Me, which were all due to surgical setback of the mandible. Regarding vertical changes of the mandible, L1, L1R, B, Pog, and Me exhibited upward movement of 1.8, 1.4, 1.2, 1.4, and 1.4 mm, respectively. No significant differences were found for landmarks of the maxilla and upper teeth. Distance measurements showed significant increases in overjet and overbite.

Table 1
Mean values of each measurement taken 1 month before surgery (T1) and 1 day after surgery (T2) and differences between them.
T1 T2 T1–T2
Mean SD Mean SD Mean SD
Angular measurements (degree)
SNA 81.8 4.4 81.9 4.4 0.0 0.3 N.S
SNB 84.2 4.5 81.0 4.0 −3.1 1.5 ***
ANB −2.5 2.1 0.8 2.1 3.4 1.5 ***
SNP 84.7 4.7 81.8 4.2 −2.8 1.5 ***
FMA 28.1 4.8 27.5 5.1 −0.6 1.9 N.S
Go 127.3 5.5 121.9 6.1 −5.5 4.3 ***
FMIA 61.0 6.0 62.4 5.8 1.4 2.5 *
U1-SN 112.5 7.0 112.8 7.4 0.3 0.9 N.S
IMPA 91.5 8.2 90.8 7.9 −0.7 1.9 N.S
I.I 120.6 8.8 121.7 9.0 1.1 2.1 *
FH-Occ 9.6 3.7 8.9 4.0 −0.6 1.2 **
FH-CorMe 68.5 3.3 67.9 4.2 −0.6 2.0 N.S
T1 T2 T1–T2
Mean SD Mean SD Mean SD
Linear measurements (mm)
A ( x ) 66.8 3.8 66.8 3.8 0.0 0.3 N.S
( y ) 54.1 4.0 54.1 3.9 0.0 0.3 N.S
U1 ( x ) 74.1 4.3 74.1 4.3 0.0 0.4 N.S
( y ) 78.3 4.9 78.2 4.9 −0.1 0.4 N.S
U1R ( x ) 61.0 3.9 61.0 3.9 0.0 0.4 N.S
( y ) 56.3 4.5 56.3 4.4 0.0 0.3 N.S
L1 ( x ) 77.7 5.7 71.1 4.5 −6.7 2.9 ***
( y ) 78.2 4.0 76.5 4.0 −1.8 1.2 ***
L1R ( x ) 66.6 6.0 60.4 5.0 −6.2 3.0 ***
( y ) 98.2 4.9 96.8 4.7 −1.4 1.0 ***
Mo ( x ) 45.0 3.9 41.9 3.6 −3.1 1.2 ***
( y ) 73.2 4.5 72.5 4.4 −0.7 0.4 ***
B ( x ) 71.3 5.4 65.3 4.7 −6.1 3.0 ***
( y ) 98.7 5.6 97.4 5.5 −1.4 1.2 ***
Pog ( x ) 72.8 7.1 66.9 6.7 −6.0 3.3 ***
( y ) 114.2 6.3 112.8 6.2 −1.4 1.1 ***
Me ( x ) 63.5 7.4 57.6 7.2 −5.9 3.5 ***
( y ) 122.1 6.2 120.7 6.1 −1.4 0.9 ***
G ( x ) −5.9 5.6 −11.0 5.3 −5.1 4.2 ***
( y ) 85.6 7.9 85.6 7.2 0.0 2.4 N.S
Co ( x ) 25.7 2.7 19.0 3.0 −6.8 2.6 ***
( y ) 28.0 5.3 27.2 5.3 −0.8 1.1 **
T1 T2 T1–T2
Mean SD Mean SD Mean SD
Distance measurements (mm)
Overjet −3.6 2.8 3.2 1.3 6.9 2.9 ***
Overbite 0.6 1.9 1.2 1.3 0.7 1.4 *
U1-Pp 30.2 3.6 30.1 3.6 −0.1 0.4 N.S
U6-Pp 25.8 2.0 25.7 2.0 −0.1 0.3 N.S
L1-Mp 45.3 2.9 45.4 3.0 0.1 0.4 N.S
L6-Mp 35.2 3.8 35.3 3.8 0.1 0.4 N.S
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Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Postoperative changes in mandibular prognathism surgically treated by intraoral vertical ramus osteotomy
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