33 patients (27 females; 6 males) were retrospectively analysed for skeletal and dental relapse before distraction osteogenesis (DOG) of the mandibular anterior alveolar process at T1 (17.0 days), after DOG at T2 (mean 6.5 days), at T3 (mean 24.4 days), and at T4 (mean 2.0 years). Lateral cephalograms were traced by hand, digitized, superimposed, and evaluated. Skeletal correction (T3 − T1) was mainly achieved through the distraction of the anterior alveolar segment in a rotational manner where the incisors were more proclined. The horizontal backward relapse (T4 − T3) measured −0.8 mm or 19.0% at point B ( p < 0.001) and −1.6 mm or 25.0% at incision inferior ( p < 0.001). Age, gender, amount and type (rotational versus translational) of advancement were not correlated with the amount of relapse. High angle patients (NL/ML′; p < 0.01) and patients with large gonial angle ( p < 0.05) showed significantly smaller relapse rates at point B. Overcorrection of the overjet achieved by the distraction was seen in a third of the patients and could be a reason for relapse. Considering the amount of skeletal relapse the DOG could be an alternative to bilateral sagittal split osteotomy for mandibular advancement in selected cases.
Since the clinical introduction of distraction osteogenesis (DOG) in the field of maxillofacial surgery by M c C arthy et al. the indications for use in the craniofacial area have significantly increased. The applications comprise mandibular lengthening or widening , reconstruction of the alveolar process for implant placement , DOG for bone transport after trauma or tumour resection for reconstruction of segmental defects or a neocondyle , maxillary DOG for unilateral and bilateral cleft patients , and midfacial or cranial DOG for different types of craniosynostosis .
The main applications of mandibular distraction were in congenital micrognathia , such as hemifacial microsomia , and different syndromes, such as Treacher-Collins, Pierre Robin, Nager, and Goldenhar. A review by S wennen et al. showed that less frequent indications of mandibular DOG were in acquired micrognathia (trauma, temporomandibular joint ankylosis), and that almost no patient data are available for mandibular retrognathia in non-syndromic adult patients, and there is a lack of appropriate data on long-term results with skeletal relapse rates in DOG.
DOG of the lower alveolar segment was introduced by T riaca et al. , and allows the creation of space to align teeth and/or implant placement in patients with increased overjet and retruded alveolar process. The extraction of lower premolars for tooth alignment can thus be eliminated. It is possible to achieve overjet reduction by moving the mandibular anterior alveolar process in a more translational or rotational manner. It is still not clear how translational and rotational movements of the lower alveolar segment influence the skeletal stability of DOG.
The aim of the present study was to evaluate the immediate skeletal and dental effect as well as the amount of skeletal relapse and dental changes 2 years after treatment in patients treated with DOG of the mandibular anterior alveolar process, and to identify factors related to skeletal and dental stability.
Materials and methods
The patient sample consisted of 33 Caucasians (27 females; 6 males), aged 16.5–56.0 years (mean age 30.3 years, SD 10.7). They were treated orthodontically by one orthodontist (MA) and underwent DOG of the mandibular anterior alveolar process to correct a skeletal Class II and large overjet with or without incisor crowding from 1998 to 2004. The female patients had a mean age of 30.8 years (16.8–56.0 years, SD 10.9 years) and the male patients 28.3 years (16.5–43.7 years, SD 10.5 years). The surgical procedure was performed by one experienced maxillofacial surgeon (AT); the technique has been published . Patients simultaneously receiving other surgical procedures on the mandible and maxilla, such as genioplasty and bilateral sagittal split osteotomy (BSSO) were excluded. Syndromic or medically compromised patients were excluded.
Ethical approval was admitted by the Ethic Committee of the Kanton Zürich, Switzerland, number 593. All subjects signed a written, informed consent.
Four cephalograms were taken: the first on average 17.0 days before surgery (T1), the second (T2) between days 0 and 12 (mean 6.5 days) after the osteotomy and before any distraction was carried out. The third (T3) cephalogram was taken between days 13 and 92 (mean 24.4 days), and the fourth (T4) between 0.9 and 3.7 years (mean 2.0 years) after distraction of the mandibular anterior alveolar process. The distraction was completed at T3 and the orthodontic treatment at T4. The retention of the lower incisors was achieved with a bonded canine-to-canine retainer. The DOG procedure has been described earlier .
The skeletal 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 of each patient were scanned and evaluated with the program Viewbox 3.1 ® (dHal software, Kifissia, Greece). The conventional cephalometric analysis for T1, T2, T3, and T4 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, and T4) 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.
Conventional cephalometric variables and the coordinates of the reference points ( Table 1 ) 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).
|Variable||si||Variable||si||Reference point||si (mm)|
|SNA (°)||1.14||IiL–N–point B (°)||1.14||Incision sup.||0.48||0.21|
|SNB (°)||0.82||IiL–N–point B (mm)||0.24||Incision inf.||0.58||0.55|
|ANB (°)||0.48||IiL–A–Pg (°)||1.29||Apex inf.||0.54||0.18|
|NSL/NL (°)||0.86||IiL–A–Pg (mm)||0.49||Point B||0.28||0.45|
|NL/ML′ (°)||0.84||IsL/IiL (°)||1.63||Pogonion||0.37||1.19|