Soft tissue stability after segmental distraction of the anterior mandibular alveolar process: a 5.5 year follow-up

Abstract

Soft tissue changes were analysed retrospectively in 17 patients following distraction osteogenesis (DO) of the mandibular anterior alveolar process. Lateral cephalograms were traced by hand, digitized, superimposed, and evaluated at T1 (17.0 days), after DO at T2 (mean 6.5 days), at T3 (mean 24.4 days), at T4 (mean 2.0 years), and at T5 (mean 5.5 years). Statistical analysis was carried out using Kolmogorov–Smirnov test, paired t -test, Pearson’s correlation coefficient, and linear backward regression analysis. 5.5 years postoperatively, the net effect for the soft tissue at point B′ was 88% of the advancement at point B while the lower lip (labrale inferior) followed the advancement of incision inferior to 24%. Increased preoperative age was correlated ( p < 0.05) with more horizontal backward movement (T5–T3) for labrale inferior and pogonion′. Higher NL/ML′ angles were significantly correlated ( p < 0.05) to smaller horizontal soft tissue change at labrale inferior (T5–T3). The amount of advancement at point B was significantly correlated with an upward movement (T5–T3) of labrale inferior ( p < 0.01) and stomion inferior ( p < 0.05). It can be concluded that further change in soft tissues occurred between 2.0 and 5.5 years postoperatively. The physiological process of ageing and loss of soft tissue elasticity should be considered as possible reasons.

The combination of orthodontic treatment and maxillofacial surgery aims to provide optimal function and the best aesthetic results for the patient. The clinician needs precise information to increase his ability to predict the surgical effect of skeletal displacement on the patient’s overlying soft tissue profile. Commonly, in a two-dimensional analysis the amount of change necessary to provide appropriate soft tissue profile change by maxillofacial surgery is determined by the use of ratios between the soft tissues and the underlying skeletal and dental base.

Little is known about the effect of mandibular DO on the change in shape and position of the soft tissue profile when compared with bilateral sagittal split osteotomy (BSSO) for mandibular advancement. Commonly used lateral cephalograms can only reproduce a two-dimensional pre- and postoperative situation whereas in recent years there has been a trend in quantifying soft tissue profile changes using three-dimensional evaluation (i.e. optical laser surface scanners, stereophotogrammetry with cameras, or computed tomography assisted imaging ).

Recently, skeletal and soft tissue changes 2 years after DO of the anterior mandibular alveolar segment have been examined. The net effect of the soft tissue at point B′ was 100% of the advancement at point B while the lower lip (labrale inferior) followed the advancement of incision inferior to 46% examined 2.0 years postoperatively. Skeletally, 5.5 years after DO the horizontal backward relapse measured −0.3 mm or 8.3% at point B and −1.8 mm or 29.0% at incision inferior. To the authors’ knowledge, evaluation of the soft tissue profile and its change in the long-term is lacking. The aim of the present study was to evaluate soft tissue changes 5 years after treatment in adult patients treated with DO of the anterior mandibular alveolar process and to relate it to different parameters.

Materials and methods

The study represents a follow-up of an initial sample of 33 patients published previously. The initial patient sample consisted of 33 Caucasian patients (27 females and six males) aged 16.5–56.0 years (mean age 30.3 years, SD 10.7). Of these 33 patients, 17 patients could be re-examined. The follow-up group (T1) consisted of 17 Caucasian patients (14 females and three males); aged 16.5–56.0 years (mean age 29.8 years, SD 11.9). Ethical approval was obtained from the ethic committee of the Kanton Zürich, Switzerland (number 593). All subjects gave written, informed consent.

All patients were treated orthodontically by one orthodontist (MA) and underwent DO of the anterior mandibular alveolar process to correct a skeletal Class II and large overjet with or without incisor crowding at the Pyramide Clinic in Zürich, Switzerland in the years 1998–2004. The female patients in the follow-up group had a mean age of 31.7 years (17.1–56.0 years, SD 12.0 years) and the male patients 21.5 years (16.5–31.4 years, SD 8.6 years) at T1. The surgical procedure was performed by one experienced maxillofacial surgeon (AT); the technique has been published previously. Patients receiving other surgical procedures simultaneously on the mandible and maxilla, such as genioplasty, BSSO, and Le Fort, were excluded. Syndromic or medically compromised patients were excluded. Five cephalograms were taken: the first on average 17.0 days before surgery (T1); the second (T2) between 0 and 12 days (mean 6.5 days) after the osteotomy and before any distraction was carried out; the third (T3) between 13 and 92 days (mean 24.4 days); the fourth (T4) between 0.9 and 3.7 years (mean 2.0 years), and the fifth (T5) between 2.7 and 8.3 years (mean 5.5 years) after distraction of the anterior mandibular alveolar process. The distraction was completed at T3 and the orthodontic treatment at T4. The position of the lower incisors was retained with a bonded only on canine to canine retainer. The DO procedure has been described previously.

Cephalometric analysis

Soft tissue changes were evaluated on profile cephalograms taken with the teeth in the intercuspal position, and including a linear enlargement of 1.2%. The cephalograms were taken with the subject standing upright in the natural head position and with relaxed lips. The same X-ray machine and the same settings were used to obtain all cephalograms.

The lateral cephalograms were scanned and evaluated with the Viewbox 3.1 ® program (dHal software, Kifissia, Greece). The conventional cephalometric analysis for T1, T2, T3, T4, and T5 was carried out by one author (CUJ) and included the reference points and lines shown in Fig. 1 . Horizontal ( x -values) and vertical ( y -values) linear measurements were obtained by superimposing the tracings of the different stages (T2, T3, T4 and T5) on the first radiograph (T1), and the reference lines were transferred to each consecutive tracing. During superimposition, particular attention was given to fitting the tracings of the cribriform plate and the anterior wall of the sella turcica which undergo minimal remodelling. A template of the outline of the mandible of the preoperative cephalogram (T1) was made to minimize errors for superimposing on subsequent radiographs.

Fig. 1
Reference points and lines used in the cephalometric analysis. The coordinate system had its origin at point S (sella), and its x -axis formed an angle of 7° with the reference line NSL. G, glabella; S, sella; NSL, nasion-sella-line; N, nasion; x , horizontal reference plane; NL, nasal line; Cm, columella; Sn, subnasale; ILs, upper incisal line; Ans, anterior nasal spine; Pns, posterior nasal spine; As, apex superior; point A; point A′, soft tissue point A; Ls, Labrale superior; Ss, stomion superior; Ii, incision inferior; Is, incision superior; Si, stomion inferior; Li, labrale inferior; Go, gonion; ML′, mandibular line prime; Ai, apex inferior; point B; point B′, soft tissue point B; Pg, pogonion; Pg′, soft tissue pogonion; Me, menton; Me′, soft tissue menton; S-Line; and y , vertical reference plane.

Conventional cephalometric variables as well as the coordinates of the reference points were calculated by the computer program. The coordinate system had its origin at point S (Sella), and its x -axis formed an angle of 7° with the reference line NSL ( Fig. 1 ). Overjet and overbite were calculated from the coordinates of the points Is (incision superior) and Ii (incision inferior).

The lateral cephalograms of T2 were only used to locate the cephalometric point, called the alveolar surgical anterior base (Asab) before postoperative distraction of the alveolar process was carried out. Asab is the most anterior and inferior point of the lower anterior segment resulting from the surgical osteotomy ( Fig. 2 ). This cephalometric point was introduced to evaluate the movement (rotation vs. translation) of the lower anterior segment base in comparison to the lower incisors as the ratio <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Ii(x-value,T3−T2)Asab(x-value,T3−T2)’>Ii(x-value,T3T2)Asab(x-value,T3T2)Ii(x-value,T3−T2)Asab(x-value,T3−T2)
Ii ( x -value , T 3 − T 2 ) Asab ( x -value , T 3 − T 2 )

Fig. 2
Reference points used in the cephalometric analysis of the lower apical base in DO patients. Ii, incision inferior; point B; Ai, apex inferior; Asab, apical surgical anterior base; Pg, pogonion; and Me, menton. Asab is the most anterior and inferior point of the lower anterior segment resulted by the surgical osteotomy; the reason for its introduction is given in the text.

Error of the method

To determine the error of the method, 21 randomly selected cephalograms were re-traced and re-analysed after a 2 week interval. Horizontal ( x -values) and vertical ( y -values) linear measurements were re-obtained by superimposing the tracings of the different stages (T2, T3, T4 and T5) on the first radiograph (T1). The error of the method (si) was calculated with the formula <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='si=∑d2/2n’>si=d2/2nsi=∑d2/2n
s i = ∑ d 2 / 2 n
where d is the difference between the repeated measurements and n is the number of duplicate determinations.

Statistical analysis

Statistical analyses were conducted using SPSS software (version 19.0, SPSS Inc., Chicago, IL, USA). Normal distribution was confirmed with the Kolmogorov–Smirnov test. The effect of treatment (i.e. the differences between the variables and co-ordinates at T3 and T1, T5 and T1, T5 and T3, T5 and T4) was tested with a paired t -test. The relationships between soft tissue and skeletal variables, age, and gender were analysed with the Pearson’s product moment correlation coefficient and linear backward regression analysis. The drop-out analysis included the unpaired t -test to compare drop-outs with the remaining patients for age and cephalometric features at T1, T2, T3 and T4, and the χ 2 test for gender and age.

Results

Error of the method and drop-out analysis

The random errors are presented in Table 1 . The measurement of the nasiolabial angle (Cm–Sn–Ls) and menton′ ( x -value) were excluded due to the increased random error. No systematic errors were found when the values were evaluated with a paired t test.

Table 1
Random errors (Si) of the cephalometric landmarks and variables.
Variable Si Variable Si Reference point Si (mm)
X Y
SNA (°) 1.14 Overjet (mm) 0.36 Incision sup. 0.48 0.21
SNB (°) 0.82 Overbite (mm) 0.53 Incision inf. 0.58 0.55
ANB (°) 0.48 Cm–Sn–Ls (°) 3.32 Point B 0.28 0.45
NSL/NL (°) 0.86 G–Sn–Pg′ (°) 1.14 Asab 0.35 0.25
NSL/ML′ (°) 1.01 Ls/Cm–Pg′ (mm) 0.67 Pogonion 0.37 1.19
NL/ML′ (°) 0.84 Li/Cm–Pg′ (mm) 0.49 Menton 0.89 0.45
IsL/NSL (°) 1.52 Labrale sup. 0.78 1.30
IsL/NL (°) 1.31 Stomion sup. 1.68 0.99
IiL/ML′ (°) 1.39 Labrale inf. 1.07 1.01
IsL/IiL (°) 1.63 Stomion inf. 1.15 0.85
Point B′ 1.20 1.10
Pogonion′ 1.19 1.15
Menton′ 3.07 1.21
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Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Soft tissue stability after segmental distraction of the anterior mandibular alveolar process: a 5.5 year follow-up
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