Transverse mandibular discrepancy is characterized by unilateral or bilateral cross-bite, a tapered mandibular arch, crowded misaligned anterior teeth and a narrow inter-canine width [1-3]. Minor transverse mandibular discrepancies are generally corrected by orthodontic dental compensation, dental extraction, dental arch expansion or inter-approximal tooth stripping, whereas severe mandibular transverse discrepancies necessitates surgical intervention due to an early fusion of the symphysis and lack of a mandibular midsagittal suture [1,3,4]. Transverse mandibular widening following a midline osteotomy and lateral rotation of the two hemi-mandible segments with an interpositional grafting material is a safe and predictable treatment modality for correction of moderate transverse mandibular discrepancies [5]. However, this surgical intervention is rarely used in patients with severe mandibular transverse discrepancies due to an increased risk of periodontal problems and relapse [1,2,6].

Mandibular midline distraction osteogenesis (MMDO) is a surgical technique to widen the mandible by incremental traction and gradually separating the mandibular symphysis in patients with severe mandibular transverse discrepancies [1,2]. MMDO was introduced by Rosenthal in 1951 [7] and modified by Guerrero et al. [8] in the 1990s. MMDO can be achieved by the use of different types of distraction appliance (bone-borne, tooth-borne or hybrid distraction appliance) [1,2,9-14]. Bone-borne distraction appliance has been recommended since the mechanical forces are delivered directly towards the bone facilitating a parallel basal mandibular bone widening with increased skeletal stability [2,10,11]. However, bone-borne distraction appliance is associated with increased cost, extended duration of surgery, trans-mucosal hardware emergence and the need of a second operation to remove the distraction appliance [1,10]. Tooth-borne distraction appliance apply their vector on the dentoalveolar level and generally facilitate a disproportionate transverse expansion pattern with a larger alveolar bone widening than the basal mandibular bone as well as dental-tipping [2,12,13]. However, tooth-borne distraction appliance is cheaper, cemented preoperatively and provides better aesthetic and patient comfort especially when a lingual device is used [1,12,13]. Hybrid distraction appliance combines advantages of bone-borne and tooth-borne appliance, since the appliance is attached to both the bone and teeth [13,14]. Previous published biomechanical, experimental and human studies reveal transverse skeletal and dental arch expansion after MMDO with the different types of distraction appliance [1,2,8,15-19]. However, the transverse skeletal and dental arch expansion pattern and relapse following MMDO with a bone-borne, tooth-borne or hybrid distraction appliance have not yet been assessed specifically in a systematic review. Therefore, the objective of the present systematic review was to assess the transverse skeletal and dental arch expansion and relapse of the mandible after MMDO with a bone-borne, tooth-borne or hybrid distraction appliance.

Protocol and registration

The methods of the analysis and inclusion criteria were specified in advance and documented in a protocol. The review was registered in PROSPERO, an international prospective register of systematic reviews.

The protocol can be accessed at: https://www.crd.york.ac.uk/prospero/.

Registration number: CRD42018103295.

The present systematic review was conducted in accordance with the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement for reporting systematic reviews [20].

Types of publications

The present systematic review included studies on humans.

Types of studies

Randomized controlled trials, controlled trials, case series and retrospective studies.

Types of outcome measures

Information sources

The search strategy incorporated examinations of electronic databases, supplemented by a thorough hand-search page by page of relevant journals including “American Journal of Orthodontics”, “American Journal of Orthodontics and Dentofacial Orthopedics”, “British Journal of Oral and Maxillofacial Surgery”, “European Journal of Orthodontics”, “International Journal of Oral and Maxillofacial Surgery”, “Journal of Craniofacial Surgery”, “Journal of Cranio-Maxillo-Facial Surgery”, “Journal of Oral & Maxillofacial Research”, “Journal of Oral and Maxillofacial Surgery”, “Oral and Maxillofacial Surgery”, “Oral Surgery Oral Medicine Oral Pathology Oral Radiology” and “The Angle Orthodontist”. The manual search also included the bibliographies of all articles selected for full-text screening as well as previously published reviews relevant for the present systematic review. The search was performed by two of the authors (TSJ and TLB). Any disagreements were resolved by consensus between the two observers.

Search

A MEDLINE (PubMed), Embase, and Cochrane Library search was conducted. Human studies published in English until the 3^rd of July, 2018 were included. The search strategy was performed in collaboration with a librarian and utilized a combination of Medical subject heading (MeSH) and free text terms. The search strategy is outlined in Appendix 1 - 3.

Selection of studies

The titles of the identified reports were initially screened. The abstract was assessed when the title indicated that the study was relevant. Full-text analysis was obtained for those with apparent relevance or when the abstract was unavailable. The references of the identified papers were cross-checked for unidentified articles. The search was performed by two of the authors (TSJ and TLB).

Any disagreements were resolved by consensus between the two observers.

Study eligibility

The inclusion criteria were developed using the PICOS guidelines (Table 1).

Inclusion criteria

The review exclusively focused on studies with an observation period of minimum three months after the end of the distraction period. The transverse skeletal and dental arch expansion or relapse after MMDO with bone-borne, tooth-borne or hybrid distraction appliance should be reported. In addition, at least five patients should be included in the study and the surgical technique, as well as the used distraction appliance must be clearly specified.

Exclusion criteria

Studies with insufficient description of the performed numbers of surgical procedures, significant dissimilarities in demographic data, lack of information on length of observation period and studies involving syndromic patients were excluded. Moreover, letters, editorials, PhD theses, letters to the editor, case reports, abstracts, technical reports, conference proceedings, animal or in vitro studies and literature review papers were also excluded.

Data extraction

Data were extracted by one reviewer (TSJ) according to a data-collection form ensuring systematic recording of the outcome measures. In addition, relevant characteristics of the study were recorded. The corresponding author was contacted by e-mail in the absence of important information or uncertainties.

Data items

The following items were collected from the included articles and arranged in the following fields: study, year of publication, patients, transverse deficiency, distraction appliance, evaluation methods, follow-up, transverse skeletal expansion, transverse skeletal relapse, dental arch expansion, dental arch relapse, frequency of complications and patient reported outcome measures.

Assessment of methodological quality

The quality assessment of the included studies was undertaken as part of the data extraction process. A methodological quality rating system was used and the classification of the risk of bias potential for each study was based on the following five criteria:

The studies were grouped according to:

Statistical analysis

Meta-analyses were to be conducted only if there were studies of similar comparison, reporting identical outcome measures. However, the studies included revealed considerable variations in study design, i.e. different latency period, distraction rates, length of consolidation period and follow-up, type of outcome measures as well as dissimilar evaluation methods. Therefore, a well-defined meta-analysis was not applicable. Parametric data were expressed as mean and standard deviation (M [SD]). Statistical significance level was defined at P = 0.05.

Study selection

Article review and data extraction were performed according to the PRISMA flow diagram (Figure 1). A total of 607 titles were identified and 56 abstracts were reviewed. Full-text analysis included 23 articles and nine studies were finally included in the present systematic review [2,21-28]. No articles were included as the result of hand-searching.

Exclusion of studies

The reasons for excluding studies after full-text assessment were as follows: the transverse skeletal and dental arch expansion or relapse of the mandible after MMDD were not reported [10,29-31], tooth-borne appliance and hybrid appliance was used without differentiating the transverse skeletal and dental arch expansion or relapse between the two treatment modalities [32], the length of the observation period was not specified [11,13,33,34]. Finally, five studies [14,35-38] were excluded because the same patient sample was reported in other publications included in the present systematic review [2,28].

Study characteristics

The included studies consisted of two comparative studies using a retrospective study design [2,21], four non-comparative case series [22,23,26,28] and three non-comparative retrospective studies [24,25,27]. Bone-borne (Surgi-Tec NV, Brugge, Belgium or Modus, Medartis^®, Basel, Switzerland), tooth-borne (Hyrax) and hybrid (Hyrax) appliance were compared in one study [2], while tooth-borne (Hyrax) and hybrid (Hyrax) appliance were compared in another study [21]. Non-comparative studies involved bone-borne (Modus by Medartis, Surgi-Tec and TMD-Flex Rotterdam) appliance in three studies [22-24], tooth-borne (Hyrax, custom-made or not specified) appliance in three studies [25-27], and hybrid (custom-made) appliance in one study [28]. A rigid intraoral distraction appliances were used in all the included studies [2,21-28], apart from one study, in which some patients were treated with the flexible TMD-Flex Rotterdam distraction appliance [24]. No estimate of sample size or power calculation were conducted in any of the included studies. Moreover, detailed information about the transverse mandibular discrepancy, surgical intervention, retention period, blind assessment, assessor training, pre-distraction orthodontic expansion or relapse was infrequently specified. The number and skills of the surgeons involved in the surgical procedure were described in four studies [2,21,22,25]. The transverse mandibular deficit was reported in three studies [2,22,27]. MMDO was performed in local anaesthesia [2,23], intravenous sedation [25,26,28] or general anaesthesia [22]. An active screw mechanism were used in all the included studies [2,21-28]. The distraction appliance was activated after five days [22,26], five to seven days [2], seven days [23,27,28] and eight days [25]. Distraction rate was 0.4 mm per day [27], 0.6 mm per day [22,26], and 1 mm per day [2,23,25,28]. Consolidation period was four weeks [23], six weeks [26], and three months [2,22,25,27,28]. Frontal and lateral cephalograms involving dental measurements, ramal angle, bigonial and biantegonial distance was used to estimate the transverse skeletal expansion [21,23,25,28], dental arch expansion [25] as well as skeletal relapse [21,24,25,28]. Dental cast measurements involving measurements at tooth and bone level was used to estimate the transverse skeletal expansion [26], dental arch expansion [21,23-26,28] as well as dental relapse [21,24,28]. Computed tomography (CT) scan was used to estimate the transverse skeletal expansion [27] and dental arch expansion [22,27]. Frequency of complications was reported in five studies [2,22,23,26,27]. Patient-reported outcome measures were not reported in any of the included studies.

Outcome measures

The result of each outcome measure is presented first and then a short summary is finally provided. The results of the outcome measures are outlined in Table 2 - 6.

Transverse skeletal expansion

Comparative studies

The transverse skeletal expansion was 4.6 (0.9) mm with bone-borne appliance, 3.7 (1.1) mm with tooth-borne appliance and 4.6 (0.9) mm with hybrid appliance (Table 2) [2]. No statistically analysis was conducted and the method used for evaluating the transverse skeletal expansion was not described [2].

The immediate post-distraction transverse skeletal expansion between bone markers placed on either side of the symphysis was 2.3 (1.3) mm with a tooth-borne appliance compared to 5.3 (1.4) mm with a hybrid appliance, as evaluated by linear measurements on posterior-anterior cephalograms (Table 3) [21]. The inter-incisor apices width increased by 2.8 (2.1) mm with a tooth-borne appliance compared to 5.1 (1.7) mm with a hybrid appliance. Bigonial and biantegonial width decreased by -1.9 (6.2) mm and -1.3 (5.1) mm with a tooth-borne appliance compared to -0.4 (6.3) mm and 0.7 (4.4) mm with a hybrid appliance. Statistical analysis was not conducted [21].

Non-comparative studies

The immediate post-distraction distance between the bigonial and biantegonial significantly increased by 2.4 (0.5) mm (P < 0.0001) and 2.3 (0.5) mm (P < 0.0001) with a bone-borne appliance compared to preoperative measurements, as evaluated by linear measurements on posterior-anterior cephalograms [23].

The immediate post-distraction ramal angle significantly decreased by -1.6 (1.3) degrees (P = 0.016) [23] and -1.1 (0.7) degrees (P = 0.001) with a bone-borne distraction appliance [24].

The immediate post-distraction distance between the bigonial and biantegonial non-significantly increased by 0.7 (4.4) mm and 1.2 (4.4) mm with a tooth-borne appliance, as evaluated by linear measurements on posterior-anterior cephalograms (Table 4) [25].

The 3-months transverse skeletal expansion at the bone level between the canines, second premolar and second molar was 3.4 (2) mm, 3.3 (1.4) mm and 2.2 (1.8) mm with a tooth-borne appliance, as evaluated by linear measurements on dental casts [26]. Statistical analysis was not conducted [26].

The 3-months transverse skeletal expansion between the mental foramen and the roots of pre-molars and molars was significantly increased by 2.7 (1.2) mm (P < 0.05), 2.9 (1.8) mm (P < 0.05), and 2.6 (2.1) mm (P < 0.05) with a tooth-borne appliance, as evaluated by linear measurements on CT-scans [27].

The immediate post-distraction bigonial distance decreased non-significantly by 0.3 (6.3) mm with a hybrid appliance, as evaluated by linear measurements on posterior-anterior cephalograms (Table 5) [28]. The ramal angle decreased non-significantly by 0.1 (5.9) degrees [28].

Summary

Comparative studies demonstrated transverse skeletal expansion with the three treatment modalities. Hybrid appliance created more immediate post-distraction transverse skeletal expansion compared to tooth-borne appliance, as evaluated by symphyseal bone markers measurements and inter-incisor apical width, whereas the bigonial and biantegonial width decreased with tooth-borne appliance compared to hybrid appliance. Non-comparative studies demonstrated a significant increase in the bigonial and biantegonial distance with a bone-borne appliance, whereas non-significant differences were disclosed with tooth-borne and hybrid appliance.

Transverse skeletal relapse

Comparative studies

The transverse skeletal relapse was estimated by comparing the immediate post-distraction measurements to measurements obtained after the end of the active orthodontic treatment and follow-up examination (Table 2) [21]. Linear measurements on posterior-anterior cephalograms of symphysis bone markers revealed a relapse of 0.8 (1.7) mm at the end of the active orthodontic treatment and 0.7 (2) mm after 5.1 years with a tooth-borne appliance compared to 0.6 (1.5) mm and 0.2 (1.9) mm with a hybrid appliance, after 6.1 years. The inter-incisor apices distance decreased by 7.4 (1.3) mm and 6.2 (0.7) mm with a tooth-borne appliance compared to 7.5 (1.1) mm and 7 (1.1) mm with a hybrid appliance. The bigonial width increased by 0.1 (8.2) mm and decreased by 0.5 (7.7) mm with a tooth-borne appliance compared to a decrease of 0.6 (6.2) mm and 0.9 (7.3) mm with a hybrid appliance. The biantegonial width decreased by 0.3 (5.5) mm and 0.5 (5.2) mm with a tooth-borne appliance compared to a decrease of 0.4 (4.3) mm and an increase of 0.3 (5.3) mm with a hybrid appliance. Statistical analysis was not conducted [21].

Non-comparative studies

The ramal angle decreased non-significantly by -1.7 (0.7) degrees (P = 0.15) after one year compared to the immediate post-distraction measurements with a bone-borne appliance (Table 3) [24]. The ramal angle was non-significantly changed by 0 (1.1) degrees (P = 1) after 6.5 years compared to preoperative measurements [24].

The bigonial and biantegonial width increased by 0.7 (4.7) mm and 1.2 (4.1) mm after 1.3 years compared to immediate post-distraction measurements with a tooth-borne appliance, as evaluated by linear measurements on posterior-anterior cephalograms (Table 4) [25]. The inter-molar distance decreased with 0.4 (3) mm, whereas the inter-canine distance increased with 0.5 (3.3) mm. Statistical analysis was not conducted [25].

The bigonial width increased non-significant by 0.1 (6.2) mm after 24.1 months compared to immediate post-distraction measurements with a hybrid appliance, as evaluated by linear measurements on posterior-anterior cephalograms [28]. The ramal angle difference was non-significantly changed by 0 (5.3) degrees (Table 5) [28].

Summary

Comparative studies revealed a similar long-term transverse skeletal relapse pattern with tooth-borne and hybrid appliance. Non-comparative studies disclosed non-significant differences in the transverse skeletal relapse with bone-borne and hybrid appliance compared to preoperative and immediate post-distraction measurements.

Transverse dental expansion

Comparative studies

The transverse dental expansion was 5 mm (0.8) with a bone-borne appliance, 4.9 mm (0.9) with a tooth-borne appliance, and 5 (0.9) mm with a hybrid appliance (Table 2) [2]. The method used for measurement of the transverse dental expansion was not described and statistical analysis was not conducted [2].

The immediate post-distraction transverse dental expansion between the canines, second premolars and second molars was 5.1 (1.6) mm, 3.8 (1.3) mm, and 1.1 (2.1) mm with a tooth-borne appliance compared to 5.6 (2.1) mm, 4.4 (2.2) mm, and 1.8 (2.4) mm with a hybrid appliance, as evaluated by linear dental cast measurements [21]. Statistical analysis was not conducted [21].

Non-comparative studies

The 3-months transverse dental expansion between the canines was 3.8 (1.8) mm (P = 0.004) compared to preoperative measurements with a bone-borne appliance, as evaluated by linear measurements on CT-scans (Table 3) [22].

The immediate post-distraction transverse dental expansion between the canines, second premolars and second molars was 3.9 (1.9) mm (P < 0.002), 5.1 (1.5) mm (P < 0.0001) and 2.5 (1.1) mm (P < 0.001) compared to preoperative measurements with a bone-borne appliance, as evaluated by linear dental cast measurements [23].

The immediate post-distraction transverse dental expansion between the canines, first premolars and first molars was 4.4 (0.6) mm (P < 0.001), 4.9 (0.8) mm (P < 0.001) and 2.4 (0.4) mm (P < 0.001) compared to preoperative measurements with a bone-borne appliance, as evaluated by linear dental cast measurements [24].

The immediate post-distraction transverse dental expansion between the canines and molars was 3.2 (3.3) mm (P < 0.05) and 2.2 (4.2) mm (P < 0.05) compared to preoperative measurements, with a tooth-borne appliance, as evaluated by linear measurements on posterior-anterior cephalograms (Table 4) [25]. The 1.3-year transverse dental expansion between the canines, second premolar and second molar was 2.4 (1.9) mm (P < 0.01), 4.9 (3.9) mm (P < 0.01) and 4.1 (3.7) mm (P < 0.05) compared to preoperative measurements with a tooth-borne appliance, as evaluated by linear dental cast measurements [25].

The 3-months transverse dental expansion between the canines, second premolars and second molars was 4.2 (1.8) mm, 4.7 (2) mm and 3.6 (1.3) mm compared to preoperative measurements with a tooth-borne appliance, as evaluated by linear dental cast measurements Statistical analysis was not conducted [26].

The 3-months transverse dental expansion between the premolars and molars was 4.8 (1.6) mm (P < 0.05) and 4.9 (1.3) mm (P < 0.05) compared to preoperative measurements with a tooth-borne appliance, as evaluated by linear measurements on CT-scans [27].

The immediate post-distraction transverse dental expansion between the canines, second premolars and second molars was 7.3 (2.1) mm (P < 0.001), 4.8 (3.1) mm (P < 0.001) and 1.4 (1.9) mm (P < 0.001) compared to preoperative measurements with a hybrid appliance, as evaluated by linear dental cast measurements (Table 5) [28]. Corresponding measurements were 4.8 (1.5) mm (P < 0.001), 4.8 (3) mm (P < 0.001) and 1.9 (3.4) mm (P < 0.05), after 24.1-months [28].

Summary

Comparative studies revealed transverse dental expansion with the three treatment modalities. Non-comparative studies demonstrated a statistically significant immediate post-distraction transverse dental expansion with the three treatment modalities compared to preoperative measurements.

Transverse dental relapse

Comparative studies

Transverse dental relapse was estimated by comparing post-distraction linear dental cast measurements to measurements obtained after the end of the active orthodontic treatment and the follow-up examination (Table 2) [21]. The relapse at the end of orthodontic treatment between the canines, second premolars and second molars was 2.5 (1.5) mm, 1.8 (1.4) mm and -1.4 (2.3) mm with a tooth-borne appliance compared to 4.7 (1) mm, 3.3 (1.5) mm, and 0.2 (1.5) mm with a hybrid appliance. The relapse at the follow-up examination between the canines, second premolars and second molars was 3.8 (0.9) mm, 2.2 (1.8) mm, and -1.4 (2.5) mm with a tooth-borne appliance compared to 5.4 (1.3) mm, 3.4 (1.6) mm, and -0.3 (1.3) mm with a hybrid appliance. Statistical analysis was not conducted [21].

Non-comparative studies

The width distance between the canines, premolars and molars was non-significantly changed by -0.9 (0.6) mm (P = 1), -1 (0.7) mm (P = 0.96) and 0.9 (0.4) mm (P = 0.32) after 6.5 years compared to measurements after one-year with a bone-borne appliance, as evaluated by linear dental cast measurements (Table 3) [24]. Corresponding measurements was -1.5 (0.6) mm (P = 0.16), 0.3 (0.7) mm (P = 1) and 0.4 (0.3) mm (P = 1) after one-year compared to immediate post-distraction measurements [24].

The width distance was significantly changed by -2.5 (1.5) mm (P < 0.001) at the canines and -1.2 (2.2) mm (P < 0.01) at the first pre-molars and non-significantly changed by 0 (3) mm at the second pre-molar, 0.4 (3.3) mm at the first molars, and 0.5 (3.4) at the second molars after 24.1 months compared to immediate post-distraction measurements with a hybrid appliance, as evaluated by linear dental cast measurements (Table 5) [28].

Summary

Postsurgical orthodontic alignment is often initiated, three to six months after MMDO and the transverse dental relapse is commonly evaluated by linear dental cast measurements. Consequently, the definite long-term transverse dental relapse after MMDO is influenced by the postsurgical orthodontic treatment and is challenging to estimate. The transverse dental relapse was reported in a comparative study disclosing increased transverse dental relapse with a hybrid appliance compared to a tooth-borne appliance. A non-comparative study disclosed a non-significant long-term difference in the transverse dental relapse compared to measurements after one-year with a bone-borne appliance.

Frequency of complications

Comparative studies

The frequency of complications was higher with bone-borne appliance compared to tooth-borne or hybrid appliance (Table 6) [2]. Breakage of distractor, ecchymosis and secondary infection were the most commonly observed complications after MMDO with bone-borne appliance [2].

Non-comparative studies

Damage to the central incisor during the vertical osteotomy, gingivitis, wound dehiscence and temporomandibular joint pain have been reported with bone-borne appliance (Table 6) [23]. Delayed response to cold testing of the central incisor, temporomandibular joint pain and click has been reported with tooth-borne appliance [26], whereas no complications has been reported in non-comparative studies with the use of hybrid appliance.

Summary

The frequency of complications was higher with bone-borne appliance compared to tooth-borne and hybrid appliance in comparative and non-comparative studies.

Quality assessment

The quality of the included studies is summarized in Table 7. All the included studies were considered with a high risk of bias [2,21-28].

The objective of the present systematic review was to assess the transverse skeletal and dental arch expansion and relapse of the mandible after MMDO with a bone-borne, tooth-borne or hybrid distraction appliance. Two comparative and seven non-comparative studies with high risk of bias fulfilled the inclusion criteria [2,21-28]. Transverse skeletal and dental arch expansion of the mandible was achieved with the three treatment modalities [2,21-28]. Bone-borne and hybrid distraction appliance seem to facilitate more skeletal expansion compared with tooth-borne appliance, whereas no difference in dental arch expansion was reported [2,21]. Frequency of complications was higher with bone-borne appliance compared with tooth-borne and hybrid appliance. However, no randomized controlled trials were included in the present systematic review. Moreover, different latency periods, distraction rates, distraction vector, length of consolidation period and follow-up, type of outcome measures, dissimilar evaluation methods as well as various methodological confounding factors posed serious restrictions to review the literature in a quantitative systematic manner. Hence, the conclusions drawn from the results of the present systematic review should be interpreted with caution and well-designed long-term randomized controlled trials applying three-dimensional technology, an economic perspective as well as patient-related outcome measures with the three treatment modalities are needed before definite conclusions can be provided.

High quality randomized controlled trials with low risk of bias provide the highest level of evidence for ascertaining the safety and efficacy of a specific surgical intervention. Previous published studies assessing MMDO with bone-borne, tooth-borne or hybrid appliance involves solely non-randomized trials, case series, retrospective studies and several case reports [1,2,21-38]. Consequently, the current level of evidence is inadequate to propose implication for evidence based clinical guidelines. Thus, the treatment of choice for MMDO with the different types of distraction appliance should be case specific, less invasive and achieve the optimal treatment goal in the shortest period of time with less risk of biological and technical complications.

Dental and skeletal structural changes after MMDO with bone-borne, tooth-borne or hybrid distraction appliances have previously been assessed in a systematic review revealing a horizontal V-shaped opening with a larger anterior transverse mandibular expansion declining progressively towards the posterior part of the mandible [1,2], which is in accordance with the results of the present systematic review. Moreover, a tooth-borne appliance seems to exhibit a vertical V-shaped widening of the mandible with larger transverse expansion at the dentoalveolar level compared to the basal bone level, whereas bone-borne and hybrid appliance facilitates a more symmetrical vertical widening [1,2]. However, previous published studies assessing the transverse mandibular expansion pattern after MMDO are largely based on dental cast measurements and two-dimensional radiographs [2,21,23-26,28]. These evaluation methods may be imprecise for measurement of the accurate basal skeletal expansion and relapse, due to superimposition of anatomic structures, difficulties to determine landmarks with high accuracy and postoperative orthodontic realignment of the teeth [39,40]. Cone beam CT and CT-scan demonstrate a high degree of reproducibility including three-dimensional quantitation of bone changes and tooth inclination, which appears to be higher compared to frontal and lateral cephalometric radiographs [41,42]. The transverse mandibular expansion pattern after MMDO with a tooth-borne appliance has previously been assessed using CT disclosing a larger dental arch expansion compared to skeletal expansion [27]. Consequently, the transverse mandibular expansion pattern after MMDO with bone-borne, tooth-borne or hybrid appliance is dissimilar. Thus, the magnitude of the transverse mandibular deficiency and patient´s preference must be taken into account, before selecting the type of distraction appliance for MMDO.

Patient-reported outcome measures are essentially reports of patients’ perceptions of their oral health status and its impact on their daily life or quality of life. However, none of the included studies of the present systematic review assessed patient-reported outcome measures after MMDO. A previous published study assessing patient’s point of view, ease of use and overall patient satisfaction after surgical assisted rapid maxillary expansion with a bone-borne distraction appliance compared with a tooth-borne appliance demonstrated an overall satisfaction rate over 90% for both distraction appliances [43]. However, bone-borne appliance was statistically significant easy to use compared with a tooth-borne appliance [43]. MMDO with a lingually placed tooth-borne appliance has been advocated, since it is minimally invasive and more comfortable for the patient [2]. Moreover, the use of a bone-borne and hybrid appliance increases the length of surgery and necessitates a second operation to remove the hardware [44]. Consequently, further studies assessing MMDO with a bone-borne, tooth-borne or hybrid appliance should include patient-reported outcome measures to establish benefits for the patients and provide information for an assessment of the optimal treatment goal in the shortest period of time with less risk of biological and technical complications.

The most commonly reported complications after MMDO with bone-borne, tooth-borne or hybrid appliance are wound dehiscence, pressure ulcer, distraction appliance-related problems, infection and tooth damage [1,2,22,23,26,27,44,45]. Intra- and postoperative complications were not reported in all the included studies of the present systematic review, but when reported, they were generally low and not serve [2,22,23,26,27]. The frequency of complications was higher with bone-borne appliance compared with tooth-borne and hybrid appliance, which is in accordance with the results of previous published studies [1,2,30,31]. Excessive mobility of the central incisors was reported in one study after MMDO with a bone-borne appliance [2]. Tooth mobility and widening of the periodontal ligament adjacent to the osteotomy has previously been reported after MMDO [45,46], but periodontal and dental morbidity after MMDO with a bone-borne appliance seem to be transient and limited to the distraction and consolidation period [46]. Intraoperative damage to the central incisors during the vertical osteotomy has previously been described in the literature and reported in one of the included studies of the present systematic review [23,30]. A mandibular midline step osteotomy has been suggested in patients with severe crowding to avoid intraoperative damage of the central incisors during the vertical osteotomy [30,47]. MMDO causes rotational movements of the mandibular condyles during the distraction phase [48]. Temporomandibular joint pain and click after MMDO have previously been described in the literature and reported in two of the included studies of the present systematic review [23,26,30]. Permanent temporomandibular joint symptoms after MMDO are uncommon [23] and the temporomandibular pain in the included studies resolved with physiotherapy [23] and removal of the tooth-borne appliance [26].

The present systematic review demonstrates that mandibular midline distraction osteogenesis with a bone-borne, tooth-borne or hybrid distraction appliance is a safe and effective treatment modality to correct transverse mandibular discrepancies. Bone-borne and hybrid appliance facilitate more skeletal expansion compared to tooth-borne appliance, whereas comparable dental arch expansion was observed with the different types of distraction appliance. Limited skeletal and dental arch relapse was observed with the different type of distraction appliances. Frequency of complications was higher with bone-borne appliance compared with tooth-borne or hybrid appliance. However, two comparative and seven non-comparative studies with high risk of bias fulfilled the inclusion criteria of the present systematic review. Moreover, dissimilar evaluation methods, different outcome measures, various methodological confounding factors posed serious restrictions reviewing the literature in a quantitative systematic manner. Hence, the conclusions drawn from the results of this systematic review should be cautiously interpreted and well-designed long-term randomized controlled trials applying three-dimensional technology, an economic perspective as well as patient-related outcome measures with the different types of distraction appliance are needed before definite conclusions can be provided.