The success rate of non-surgical expansion in adults using MAPE with the FCPC protocol was almost 84.4%, and is similar to those obtained in the recent literature [10, 21,22,23]. In contrast to the literature, the mean age of the patients in the present study was higher (29.1 years) than similar studies using MARPE for non-surgical expansion of mostly young adult patients (mean age from 17.1 to 23.3 years) [8, 10, 16, 24,25,26,27] or include subjects younger than 18 years old [8, 16, 24, 25, 28]. Moreover, successful expansion in the present study was achieved in patients up to 49 years. This evidence suggests that non-surgical palatal expansion, assisted by miniscrews or implants, is achievable even for older patients. This can be explained by various predictors for midpalatal suture expansion, such as patient´s individual anatomy, midpalatal suture maturation stage or density ratio , design, stability and location of the expansion appliance used, and the activation protocol applied for the expansion process [8, 21, 23, 27, 30]. In order to evaluate the success of non-surgical expansion, some studies have stated that the ossification of the midpalatal suture of each individual should be assessed by CBCT prior to treatment [29, 30]. The results of a recent study showed that the age differed significantly across midpalatal suture maturation stages, and correlated significantly with the midpalatal suture opening ratio . Similarly, in the present study we found a significant association of age with both unsuccessful expansion and complications. This might be due to an increase of interdigitation occurring in the midpalatal and circummaxillary sutures in late adolescence, becoming more rigid as age progresses, mainly around 30 years of age . A comparison of the results of this novel expansion technique with the staging classification method of Angelieri and coworkers will follow in a separate study including more subjects and to determine the intra- and interobserver reliability of midpalatal suture classification.
A relevant difference in our study that might explain the high success rate in older patients was the rigidity of the expander, the location of the inserted miniscrews, and the novel 2-stage protocol . The MICRO-4 appliance was introduced by Winsauer et al.  which due to its rigidity can establish a more direct transfer of the expansion force to the hard palate. This aspect is especially interesting in older patients, since age-related changes of the suture may require more expansive force on the suture . However, even with more rigid screws and abutments used some minor deformations were observed in our study as described in Table 2. Therefore, the needed force for expansion of a mature palate should never be underestimated.
Secondly, a further difference in our study was the location of the inserted miniscrews in the anterior palate. In general, placement of the miniscrews for the MARPE appliances is mostly in the middle or posterior part of the palate with less bone height, close to the midpalatal suture since these appliances are tooth-and-bone-borne with an additional fixation in the area of the first upper molar . Because of the greater bone heights between 6 and 10 millimeters in the anterior part of the palate, this position is preferred for the fixation of the MICRO-4 device in our study . The more stable anchorage of the miniscrews in this area may explain the high success rate. On the other hand, this location could also be the explanation of the anterior V-shaped expansion pattern in our study, resulting in 54% less expansion around the posterior palate as measured on the CBCT. Similar results with a V-shaped expansion of the dental arch in an antero-posterior plane were also reported with SARPE protocols when using bone-borne devices [9, 31]. However, the pterygomaxillary junction seems to be the point with the highest resistance as studied in a finite element study by Holberg et al.  and might explain the expansion pattern of the non-surgical cohort in the present study.
Thirdly, the second stage of the expansion protocol, the polycyclic closing and opening of the appliance, mimics oscillatory tensile and compressive strains, which are potent stimuli for modulating sutural growth by stimulating both osteogenesis and osteoclastogenesis . Vij & Mao  stated that a cyclic loading protocol may have clinical implications as novel mechanical stimuli for modulating craniofacial growth in patients suffering from craniofacial anomalies and dentofacial deformities. This effect was used in our study and seems to weaken the circummaxillary sutures thus enabling successful expansion even in older patients. Based on the literature and the author´s experience, the protocol has been adapted in a pilot study to avoid technical or clinical complications such as loosening or deformation of screws and expander under rapid and continuous activation . As mentioned above, a 43-year-old patient experienced an asymmetric expansion of the nasomaxillary complex with dislocation of the nasal bone. Possible factors that might lead to this complication were different bone density on both sides or asymmetric expansion force due to asymmetric screw position in the maxilla. Furthermore, this patient did not follow the polycyclic expansion protocol (way too high force applied) and as a consequence this subject was excluded in our study.
Usually, expansion protocols for MARPE (range 0.2 - 0.4 mm per day)  and for SARPE (0.5 - 1.0 mm per day) [7, 31] are much faster than the protocol used in our study. Therefore, the term “rapid” (as used in MARPE) is misleading and should not be used for non-surgical expansion in adult patients. However, some of the unsuccessful expansions observed in the present study were even younger than in the successful treatment group. The regression model showed an increase of almost 18% per year to experience unsuccessful expansion in our study. Although, a recommending baseline could not be found, this protocol can be used in all adult patients with the need for maxillary expansion. Due to the higher morbidity rate of SARPE procedure with the need for general anesthesia most patients prefer an orthodontic procedure rather than a surgical. Starting with MAPE utilizing the FCPC protocol, the SARPE procedure can be followed in case of missing diastema during the activation or early expansion period without any pre-treatment or changes in the hardware as described in our study. Interestingly, in the surgeon´s experience the bone between the central incisors was easy to separate using only a surgical blade as a chisel. The polycyclic activation seems to weaken the midline of the alveolar process and this positive side effect may reduce the risk of tooth damage even in cases with narrow space between the central incisors.
Expansion in adult patients might be associated with various complications and the prevalence of complications in patients undergoing SARPE is up to 34% . Due to greater resistance in maturing maxillary bone, classical tooth-borne expanders cause a strong increase of dental side effects after attempting the expansion . Some studies reported that bone-borne devices are associated with a risk of root lesions or infections, asymmetric maxillary expansion, periodontal damage, or loss of the distractor components [7, 31]. The complication rate of non-surgical expansion using MAPE was 18.5% in the present study. Although no severe complications of MARPE have been reported in the literature, the reduced elasticity of the bony structures in adults, might lead to microfractures with injury of nervous and vascular structures of the mid- or skull base . The most frequent complication observed using MARPE were inflammation and hyperplasia of the mucosa around the miniimplant/screw or loosening or deformation of the screws used. A decrease in bone level and thickness at first molars was observed in 41% and undesirable effects like ulcerations, oedema of the palatal mucosa were observed in 22% of cases . In a retrospective study on 69 patients, 5% of the miniscrews dislodged during expansion and 13% showed clinically visible mobility . The most frequent cause for complications in our study was appliance-related. Generally, difficulties related to non-surgical expansion are associated with the device or with the expansion protocol itself, with the soft tissue around the anchorage of the device, the teeth, and the movement of the maxillary halves.
Using the regression model in our study the risk of complications increased by almost 10% per year above the baseline of 18 years. In other words, patients with 30 years had 1.2 folds higher risk for complication than compared with patients of 18 years old. However, problems with the appliance itself occurred also in two younger patients (23.5 and 25.6 years). Nevertheless, all appliance-related problems were without clinical relevance and a dislodge of miniscrews was not observed. The 12-weeks latency period after insertion of the miniscrews seems to allow osseointegration to enable sufficient force for maxillary expansion. As a consequence of the overload of the midface with asymmetric expansion in our study, all patients had to report the activation and progress with a written protocol daily during the first two weeks until the diastema was clearly visible.
Finally, the results of the present study are quite encouraging, showing that the protocol introduced, along with the MAPE appliance is forcing the expansion of the maxilla with an acceptable complication rate in adults. In case of unsuccessful expansion SARPE can be followed with the same appliance in place. The ease of surgery after pre-treatment justifies the protocol even in older patients.
Since this study had a small sample size it was not possible to evaluate different age groups (e.g. 20-30, 30-40, 40 and older) with respect to success and complications. Therefore, it is considered as a preliminary clinical study of lower evidence and in order to deal with the issue of small sample sizes, it would be more reliable to observe prospective cohort groups and to compare different activation protocols (gradual expansion versus forced controlled polycyclic expansion). A biased selection of subjects (i.e. gender, age, palatal vault) may have occurred in the course of clinical contingencies. The subjects were not randomly selected which limits the ability to generalize the results and could introduce bias. If CBCT is available for future studies a combination of midpalatal suture maturation staging or including density ratio as described by Angelieri et al.  should be implemented in the study design. However, further RCTs are needed to compare different activation protocols and to demonstrate how the patient´s age may influence treatment outcome in a larger number of patients and on long-term evaluation. Another question that raised during the present study is the influence of the pre-treatment on the amount of surgery during SARPE in cases of failed expansion. Is it still necessary to perform a complete osteotomy or can a minimal invasive surgery without splitting of the midpalatal suture, as demonstrated in our study, lead to the same results as a complete osteotomy and reduce morbidity of the patients?