Novel methodologies and technologies to assess mid-palatal suture maturation: a systematic review

Introduction A reliable method to assess midpalatal suture maturation to drive clinical decision-making, towards non-surgical or surgical expansion, in adolescent and young adult patients is needed. The objectives were to systematically review and evaluate what is known regarding contemporary methodologies capable of assessing midpalatal suture maturation in humans. Methods A computerized database search was conducted using Medline, PubMed, Embase and Scopus to search the literature up until October 5, 2016. A supplemental hand search was completed of references from retrieved articles that met the final inclusion criteria. Results Twenty-nine abstracts met the initial inclusion criteria. Following assessment of full articles, only five met the final inclusion criteria. The number of subjects involved and quality of studies varied, ranging from an in-vitro study using autopsy material to prospective studies with in vivo human patients. Three types of evaluations were identified: quantitative, semi-quantitative and qualitative evaluations. Four of the five studies utilized computed tomography (CT), while the remaining study utilized non-invasive ultrasonography (US). No methodology was validated against a histological-based reference standard. Conclusions Weak limited evidence exists to support the newest technologies and proposed methodologies to assess midpalatal suture maturation. Due to the lack of reference standard validation, it is advised that clinicians still use a multitude of diagnostic criteria to subjectively assess palatal suture maturation and drive clinical decision-making. Electronic supplementary material The online version of this article (doi:10.1186/s13005-017-0144-2) contains supplementary material, which is available to authorized users.


Background
Rapid maxillary expansion (RME) is indicated for a number of clinical situations namely when a posterior crossbite exists (unilateral or bilateral) or limited buccal overjet in patients with constricted maxillary base [1]. Maxillary transverse deficiency may be skeletal, dental or both skeletal and dental in origin [1][2][3]. Expansion in the transverse dimension has not only been used to improve interdigitation of the occlusion and improved function but also to increase arch perimeter to resolve maxillary crowding [2]. Recently contemporary orthodontics has focused on smile esthetics with emphasis on transverse arch dimensions and minimizing buccal corridor visibility [1,4]. Those patients with dentofacial deformity or cleft lip and palate with constricted maxillary segments are candidates for RME or possible surgical expansion [2] dependent upon the time of treatment intervention. Additionally, there has been increased interest in the use of RME to increase nasal airway volume and/or function [1,2].
Treatment options available to clinicians for maxillary expansion include tooth-borne expanders with or without an acrylic support [2,5], bone-borne maxillary expansion devices supported by temporary (skeletal) anchorage devices [5], as well as surgically assisted rapid palatal expansion [1,3]. The treatment of choice is dependent on numerous clinical indications including; the extent of correction required, whether skeletal or dentoalveolar correction is indicated, and perceived efficacy of expansion based on timing of treatment [6].
The amount of skeletal or dentoalveolar effect of the RME is directly correlated with the stage of skeletal maturation of the palatal suture. Treatment timing of transverse deficiencies is recommended relatively early up to peak skeletal growth velocity [6]; however, there is significant variation in the timing of skeletal maturation amongst individuals [2,6] as the palatal suture fusion is poorly correlated with patient age and sex [3]. Failure to properly identify key clinical signs and provide individual assessment to identify a patient's ideal expansion treatment option can lead to iatrogenic side effects and co-morbidities [3,6]. Common side effects of poorly timed and failed conventional RME therapy include acute pain [2], gingival recession, dehiscence formation, palatal mucosa necrosis, buccal dentoalveolar tipping and poor long term expansion stability [3,6]. Conversely prematurely committing a patient to surgically assisted expansion ascribes a patient to a potential significant burden of treatment including increased cost, pain and healing time.
Numerous methodologies have been proposed to discern the architecture and degree of palatal suture fusion including animal and human histologic studies, evaluation of occlusal radiographs, and CT of both autopsy material and animal specimens [3]. Such methodologies presented inherent difficulties in assessing the degree of palatal suture fusion. As defined previously, histological evaluation is the reference standard to evaluate midpalatal suture maturation, unfortunately implementation on active orthodontic patients would require an invasive biopsy, precluding its use [7,8]. Conversely, serial occlusal radiographic assessment is limited in diagnostic quality due to superimposition of nearby anatomical structures [3]. Cone-beam CT (CBCT) allows for 3D rendering of the maxillofacial complex without superimposition of nearby anatomy and delivers a lower absorbed dose of radiation to the patient than medical CT [3]. To date, however, there has been no validated non-ionizing method to assess palatal suture maturation.
The objectives of this systematic review are to thoroughly describe and evaluate the contemporary technologies and methodologies capable of assessing midpalatal suture maturation.

Methods
The Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement checklist was followed; however, several points did not apply to this systematic review. This is a review of both in vitro and in vivo studies rather than solely in vivo studies, convoluting the direct comparison of results amongst these types of studies and their possible clinical inferences. No protocol registration was done.

Eligibility criteria
Both in vitro and in vivo studies will be included to identify all diagnostic modalities of palatal suture maturation. The intervention(s) will be any diagnostic method that is designed to evaluate the degree of ossification and/or interdigitation of the midpalatal suture (the outcome). Comparison will be to other diagnostic interventions designed to evaluate the same outcome variable.
The "participants" will be any human subjects or human specimens being investigated for the degree of midpalatal suture maturation. No animal studies were considered as their applicability in humans would be questionable.

Information sources
A computerized database search was conducted using Medline, PubMed, Embase and Scopus to search the literature ranging from 1980 up until October 5, 2016. A supplemental hand search was completed of references from retrieved articles that met the final inclusion criteria.

Search
Terms and their respective truncations used in the literature search (Appendix 1) were specific to each database. Searches were conducted with the help of a senior librarian who specializes in the health sciences. The selection process was carried out together by two researchers (DAI and HE). All references were managed by reference manager software EndNote to eliminate duplicates.

Study selection
The inclusion criterion "Diagnostic methods to evaluate cranial suture ossification/maturation" was utilized to initially identify possible articles from the published abstract results of the database search. If an abstract was not available, the full text was reviewed for appropriateness of inclusion. Any disagreement on the inclusion of a study was resolved by discussion amongst the reviewers.
Once these abstracts were selected, full articles were retrieved and inclusion in the systematic review was dependent of fulfilling a final inclusion criterion. The final selection criterion was as follows: "In vitro and in vivo human subject studies that describe a novel diagnostic method or technology to assess midpalatal suture maturation/ossification over time". Once more, any disagreement on the inclusion of a study following this final criterion was resolved by discussion amongst the reviewers. The references cited in the finally selected articles were also screened for any applicable references missed in the electronic database search.
Studies describing diagnostic methodologies applied to theoretical models without practical application were excluded. One article was excluded since no German translation was obtained. No other language restrictions were applied.

Data collection process
Data extraction was performed and collected by a researcher (DAI).

Data items
The variables collected included a description of the type of study, type and number of subjects, study objectives, inclusion criteria, imaging modality used, region(s) investigated, and methodology to evaluate degree of ossification/maturation of midpalatal suture (Tables 1 and 2).

Summary measures
The outcome measures included quantitative and/or qualitative results attained with applicable units to describe bone density, ossification or maturation of the palatal suture.

Synthesis of results
As the data was not considered homogeneous enough a meta-analysis was not conducted.

Study selection
Twenty-nine abstracts met the initial inclusion criteria. Following retrieving of the full articles, only five met the final inclusion criteria. Reasons for exclusion due to final inclusion criteria are stated in Additional file 1. A handsearch of the reference lists from the articles that met the final inclusion criteria identified no new articles. Therefore, a total of five articles were finally considered (Fig. 1).

Study characteristics
The methodology of each selected article was summarized in Table 1 and results in Table 2. Study parameters, including the type of study, imaging modality used, methodology to determine the ossification/maturation of the palatal suture and the number of subjects amongst other variables were vastly different amongst the studies meeting the final inclusion criteria.
The studies varied significantly in the number of subjects evaluated and quality of evidence. The studies ranged from having three human subjects in a prospective study [9] to 140 human subjects in a cross-sectional study [3]. The types of studies ranged across the hierarchy of evidence from an in-vitro study [10] to prospective in vivo studies [9,11].
The only study characteristic common to all studies was the region of interest (ROI) investigated, generally speaking, the maxilla. Four of the five studies [3,9,10,12] had a single common ROI which was the palatal suture. One study [11] evaluated four ROIs in the palatal suture and surrounding hard tissue.
All studies but one utilized CT in some form. The types of CT scanners utilized in the four studies included multi-slice low-dose computed tomography (brand information not given) [11], dental CBCT [3] and the extremely high resolution Micro-CT [10]. One study [9] utilized a less invasive modality of US, specifically using color-coded US duplex scanner (Aplio 80, Toshiba, Tokyo, Japan).
To measure the degree of maturation/ossification at the palatal suture, one of three types of evaluations were utilized amongst the five studies: quantitative, semiquantitative and qualitative.
Franchi et al [11] performed a quantitative evaluation of the palate using one blinded operator to calculate the radiodensity (Hounsfield units [HU]) of the ossification at the palatal suture from T0 (pre-expansion) and T2 (at 6 months retention).
Korbmacher et al. [10] also performed a quantitative evaluation of sutural maturation by measuring the maturation of the palate cadaver specimens at one time point. In the coronal plane, an obliteration index (%) and mean obliteration index (%) was calculated by comparison of the total length of the suture to the length that has ossified (evaluated every 370 μm). The degree of interdigitation of the palatal suture in the axial plane was assessed by calculating the interdigitation index, a comparison of the sutural distance (μm) to linear sutural distance (μm).
Angelieri et al [3] developed a novel qualitative methodology for individual evaluation of midpalatal suture maturation. Two evaluators defined the maturational stages (A-E) via comparison of the morphological description of the palatal suture found in previous histologic studies [13][14][15] to the appearance of the suture in the axial plane generated from a standardized CBCT protocol of 140 subjects during initial records [3] To assess the reliability of defining the maturational stages (A-E) a validation study utilizing 30 random axial CBCT cross-sections of the midpalatal suture was performed by three evaluators and weighted kappa coefficients calculated [3].
Kwak et al [12] utilized an objective and quantitative method of fractal analysis, a methodology established previously for the evaluation of mammalian cranial sutures, [16] to be used for the first time in conjunction with CBCT imaging to evaluate the maturity of the midpalatal suture [12]. The cross-sectional study involved 131 subjects (69 men and 62 women) with a mean age Table 1 Summary of articles that met final inclusion criteria Author(s) Franchi et al. [11] Sumer et al. [9] Korbmacher et al. [10] Angelieri et al. [3] Kwak et al. [12] Type of Study  Sumer et al. [9] Korbmacher et al. [10] Angelieri et al. [3] Kwak et al. [12] Result ( 4 months of expansion there was low echogenicity in the suture (US bone fill score = 1) for 2 of 3 subjects. The remaining patient had a bone fill score = 2 at 2 and 4 months respectively. At 6 months post expansion and 2 months after expander removal, 2 of the 3 patients showed a qualitative increase in echogenic material in the suture was seen but less than 100% therefore had a bone fill score = 2, and the remaining patient demonstrated 100% echogenic material, bone fill score = 3. All trends in scores over time were qualitatively confirmed with plain radiographic images.
Frontal plane: No age dependent significance was found for the mean obliteration index (P = 0.244). The mean obliteration index was low, varying in all groups (minimum 0%; maximum 7.3%).
Middle-aged group's mean obliteration index tended to be higher than that of either the younger or older age groups but no significant difference was calculated.  All calculated parameters demonstrated substantial interindividual and intra-sutural variation.
Utilizing CBCT to assess the midpalatal suture avoids any overlapping of soft and hard tissues. Authors note that their proposed methodology may be useful in reliably driving clinical decision making as it relates to pursing a non-surgical (RME) or surgical expansion intervention (SARME).
Adult patients possess a greater proportion of non-fused palatal sutures than what is assumed. Therefore age of the patient should not drive SARME initiation. Authors report a significant correlation between fractal dimension and degree of maturation of the midpalatal suture Determination of the fractal dimension cut-off value could be used as a reference to pursue RME vs. SARME Fractal analysis can be utilized to evaluate the degree of maturation at the palatal suture.
of 24.1 ± 5.9 years. Each subject underwent CBCT imaging, followed by significant image processing to evaluate Cervical Vertebrae Maturation (CVM) stage, palatal stage of maturation (A-E, as defined by Angelieri et al. [3]) and isolation of a ROI for the calculation of the fractal dimension of the palatal suture. To assess the intraand inter-reliability of defining the maturational stages (A-E), 30 random axial CBCT cross-sections of the midpalatal suture were staged by two other evaluators under controlled conditions and weighted kappa coefficients calculated, analogous to the study by Angelieri et al. [3] Statistical analysis included utilizing Scheffe's ANOVA to compare the fractal dimension for each individual maturation stage (A-E) and subsequent Spearman's coefficient calculation to ascertain the correlation between fractal dimension and maturation stage. The generation of a receiver operating characteristic (ROC) curve was used to develop an optimal fractal dimension cut-off value and sensitivity, specificity, false positive rate, false negative rate, positive predictability, and negative predictability calculated. For all statistical analysis, results were considered statistically significant at P < 0.05 [12]. Sumer et al. [9] utilized US to evaluate palatal sutural mineralization in three patients at five different time points; once after the 14 day surgically-assisted RME (SARME) expansion protocol, 2 months post-expansion, 4 months post-expansion, at time of removal of the tooth-borne expander (6 months post-expansion) and 2 months after appliance removal. The authors report that the ultrasound probe was used intra-orally on the skin that overlies the palatal suture, obtaining axial scans with the probe directed perpendicular to the length of  [9]. The authors assigned semi-quantitative bone fill scores (0-3). A bone fill score = 0 was characterized by open suture with clean gap margins and 0% echogenic material. A bone fill score = 1 was characterized by partial ultrasound transmission, localization of gap margins, and reduced echogenic material of ≤50%. A bone fill score = 2 was characterized by partial ultrasound transmission, marginally visible gap margins, and increased echogenic material of >50%. A bone fill score = 3 was characterized by no ultrasound transmission, 100% echogenic readings, and unidentifiable gap margins. The bone filling trends were qualitatively supported by comparison to conventional occlusal radiography [9].

Synthesis of results
Due to high methodological heterogeneity among the included studies a meta-analysis was not supported.

Risk of bias across studies
Each proposed technology or methodology to assess the maturation of the palatal suture lacked validation with a reference standard, namely histological evaluation. There was a lack of homogeneity in the quality of evidence amongst all five studies, ranging from an in-vitro study on human autopsy material [10] to human subject prospective studies [9,11]. Sample sizes across all studies varied greatly, from 3 subjects [9] to a high of 140 subjects in a human subject cross-sectional study [3].

Additional analysis
Not applicable due to lack of meta-analysis.

Discussion
Summary of evidence Modality #1 -Multi-slice low-dose CT and quantitative bone density measurements (HU).
A technique to assess palatal suture maturation includes the use of multi-slice low-dose CT to capture axial slices of the maxilla and quantitatively measure the bone density at a particular ROI in HU [11]. It is known that CT is an excellent modality to evaluate the localized architecture of cancellous and cortical bone of the jaws; [17] however, less is known regarding the quantitative measurement of bone density, the HU scale. Hus were first utilized in dentistry to evaluate the pre-surgical bone density of implant sites [17][18][19]. The HU scale is a linear transformation of tissue attenuation coefficients where air is defined as −1000 HU, distilled water at standardized conditions equal to 0 HU and very dense bone defined as ≥1000 HU [17]. Consequently the authors considered and utilized the calculated Hus as an applicable unit of measurement to quantitatively assess mineralization at the palatal suture [11].
Franchi et al. [11] utilized the Houndsfield quantitative scale to evaluate the radiodensity of four previously mentioned ROI in the maxilla, 2 sutural and 2 bony areas. Pre-expansion (T0) statistical analysis noted a significant difference between the anterior and postural sutural regions (563.3 ± 183.29 HU and 741.7 ± 167.1 HU) and anterior and posterior bony areas (1057.5± 129.4 HU and 1102.8 ± 160.9 HU) (P < 0.05) ( Table 2). Further statistical analysis yielded a significant difference between the anterior sutural and posterior sutural landmarks at T0 (P < 0.05), but no significant differences of these sutural areas at T1 or T2 (P > 0.05, Mann-Whitney). A significant difference between the radiodensity of the anterior and postural sutural ROIs between T0 and immediately post-expansion (T1), but no difference between their radiodensities when comparing pretreatment (T0) and the post-expansion retention phase (T2) readings (P < 0.05) ( Table 2).
Throughout the course of the study, trends in bone density measurements at the suture and its comparison to lateral bony sites followed conventional expectations of successful RME. Pre-expansion the measured HU at the anterior sutural region was significantly smaller than that of the posterior sutural site, and the applied expansion protocol introduced differential sutural opening with greatest opening at the anterior sutural region consistent with the pre-expansion HU scores. Additionally, the results measured at T2 at the end of the 6 month RME retention protocol, were congruent with previous histologic findings, namely post-expansion evidence of reorganization and sutural interdigitation [20].
An inherent advantage of using a low-dose CT protocol, where the voltage was decreased to 80 kV (KV), is subjecting the patient to a lower absorbed dose required for children undergoing radiologic evaluation [21]. Additionally, when the kilovoltage is reduced, image contrast of anatomical structures increases while still acceptable for assessing bone quality via this protocol [21]. Future areas of interest relating to the findings and protocol of this study would include further studies to define an anterior sutural HU: postural sutural HU ratio that best predicts the success of RME treatment. Conversely, further studies could elucidate specific ratios comparing sutural radiodensity to maxillary bony radiodensity that may predict an improved expansion prognosis.
It has to be noted that the reliability of using HU between subjects and within the same subjects the same day has not been demonstrated. Therefore some variation could be due to such factors. Also, not all studies specified patient orientation when taking the images thus effect of patient positioning on the image and HU or grey values is another aspect that should be tested.
Modality #2 -Micro-CT quantification of 3D palatal suture in the frontal and axial planes.
Korbmacher et al. [10] proposed assessing palatal suture maturation via micro-CT scanning and calculation of a number of developed indices, namely the obliteration index (%) and mean obliteration index (%) in the frontal plane, as well as, suture length [μm], linear sutural distance [μm] and interdigitation index in the axial plane.
Korbmacher et al. [10] evaluated 28 human palate specimens in the frontal and axial planes. In the frontal plane there was no demonstrated age dependent difference in the mean obliteration index between specimens (P = 0.244). The specimens were classified into one of three age groups (<25 years of age (yo), ≥25 to <30 yo and ≥30 yo) and results demonstrate that the frontal plane obliteration index varied across age groups between a minimum index of 0% to a maximum interdigitation of 7.3% (44 yo patient) ( Table 2). Although the ≥25 to <30 yo age group consistently had a higher obliteration index in the frontal plane compared to other age groups, the results were not significant. Across all age groups, each subject had at least one frontal sutural cross-section that was devoid of interdigitation (mean obliteration index of 0%), with the oldest patient exhibiting a frontal plane mean obliteration index of 0% being a 71yo female. Investigation into the degree of interdigitation in the axial plane demonstrated no significant age-dependent differences in the calculated interdigitation index (P = 0.633). The authors did report a large standard deviation in the interdigitation index in the axial plane in the youngest and oldest age groups, and considerably less variation in the calculated index in the middle (<25 yo group and >30 yo) group [10] (Table 2).
Results indicated a generally low obliteration index amongst all subjects as well as an age-independent degree of interdigitation in the axial plane; however, across all measured indices there was significant intra-sutural and inter-subject variation [10]. This was the first time micro-CT was used on human samples and although this methodology was not implemented as part of an active expansion study, its principles can still be important to evaluate the pre-expansion maturity of the palatal suture. Additionally, it could be applied during midexpansion protocol to evaluate the efficacy of treatment via calculation of the above noted indices and evaluation of the sutural architecture.
A limiting feature of the Korbmacher et al. [10] modality is the fact cadaver specimens were used, making direct translation of this study's findings poorly applicable to clinical practice [15]. Considering the limitations of the gantry size of the micro-CT unit, and maximum scanning time used (200 min), micro-CT is best used on ex-vivo samples, and very small in-vivo samples to avoid an excessive absorbed dose emitted to patients [22].
Consequently, the use of micro-CT for in-vivo radiologic evaluation of the palate is impractical at this time. Therefore continued improvements to micro-CT technology including decreasing the emitted radiation while maintaining superior resolution, is necessary prior to implementation of such a technique on active RME patients.
An area of interest is the development of a CT-based strain assessment of peri-sutural and maxillary tissues; the development of which the authors believe will help facilitate predicting the success of RME treatment [10].
Sumer et al. [9] utilized US to evaluate sutural mineralization at five time points during the SARME and retention protocol for three patients, scoring each patient's palatal suture calcification via assignment of semiquantitative bone fill scores (0-3).
US findings in the Sumer et al. [9] study demonstrated that immediately post-expansion all subjects had a bone fill score = 0. (Table 2) Two of the three subjects at 2 and 4 months post-expansion were identified as having a bone score = 1, while the remaining subject was determined to have a bone fill score = 2 for these same time periods. Following the removal of the tooth-borne appliance at 6 months and 2 months subsequent to that during continued fixed appliance therapy, the bone scores for two of the subjects demonstrated increased mineralization and identification of echogenic material, having bone fill scores =2. The remaining patient received a bone fill score = 3 due to incomplete transmission of the waves and 100% echogenicity measured at these respective time points [9]. (Table 2) It should be noted that no statistics were reported by the authors.
The results of this study follow those of a similar animal study, [23] such that there was a statistically significant increase in bone fill scores that were directly related to the length of time the patient has been in retention post expansion. A major advantage to US is its low cost and non-invasiveness, [9,23,24] as well as improved usability compared to other methodologies, with the ability to perform real-time chair side evaluations with smaller hand held units. Additionally, US is a reliable method to image early bone formation as demonstrated by previous studies involving distraction osteogenesis [9,23,24]. A study comparing US to normal panoramic radiography, demonstrated that the efficacy of US to measure an osteotomy gap during distraction osteogenesis is equal to that of conventional radiography [9,25]. US also demonstrated increased reliability compared to panoramic radiography to evaluate the maturation of early bone formation [9,25] in the distraction gap. A disadvantage to US is its inability to penetrate cortical bone [9] However, following SARME or successful RME the osteotomy gap and its margins are easily visualized [9]. An area of significant future interest is to ascertain whether this technology can penetrate an immature midpalatal suture prior to the start of RME treatment, and allow the clinician to perform a chair side subjective evaluation of the bone maturity and interdigitation along the whole length of the suture. Limitations to this study included a very small sample size of three patients and lack of a gold standard (histology) or CT to validate the findings. Consequently, an area of future research is the use of this technology and bone fills scores in a similar larger sample size study in conjunction with a gold standard methodology to support the findings [9].
Angelieri et al. [3] utilized a standardized methodology to capture axial CBCT cross-sections of the palatal suture to provide individual staging of midpalatal suture maturation from the authors' proposed maturation stages (A-E).
Results of the validation study demonstrated "almost perfect" inter-examiner reliability with the "ground truth", however, the authors did not report appropriate P-values with their statistics. As was mentioned before, there was no reference standard utilized during the validation study, but rather utilized what the authors termed the "ground truth", [3] the professional opinion of the principal investigator when utilizing their own proposed maturation stages to classify each patient's sutural maturation. Due to the lack of a gold standard, nor listed P-values, the results of the validation should be interpreted with caution. An additional limitation of this methodology is the proposed novel palatal suture maturation classification system itself. The authors developed the stages (A-E) based on comparison of CBCT axial cross-sections of the palatal suture to the perceived likeness of this radiographic morphology to the histological morphology of the suture as determined by previous studies [13][14][15]. Theoretically direct comparison of the histological morphology to the CBCT morphology of the suture is incompatible due to the histological assessment being on the microscopic scale as compared to the macro or eye level scale of sutures depicted in the CBCT axial slices. Consequently, any inference or direct translation of the sutural histological appearance and subsequent development of CBCT based sutural maturation stages is not possible. Therefore, the findings and developed maturational stages should be used with caution, and should not drive clinical decision making. Rather, at best, this maturational staging may be used as part of an extended protocol to subjectively assess palatal suture maturity during the treatment planning process. Future studies to thoroughly validate the proposed maturation stages to an available gold standard are advised.
Modality #5 -CBCT and fractal analysis to quantitatively ascertain degree of sutural maturation per proposed maturation stages of Anglieri et al. [3] Kwak et al. [12] utilized CBCT imaging in conjunction with quantitative fractal analysis to ascertain if this analysis can be correlated to the maturational stage of each subjects palatal suture. Conceptually fractal analysis is based on the observation that cranial sutures can be visualized as a fractal pattern, [16] the dimensions of which are directly related to localized stresses experienced [12]. Additionally, the closer the approximation of two articulating bones, the more complex sutural morphology [12] suggestive of a more mature suture. Conceptually sound, fractal analysis has demonstrated its applicability in various areas dental research [26].
Fractal dimension intra-and inter-reliability results from the Kwak et al. [12] study demonstrated agreement with calculated weighted kappa coefficients of 0.84 (95% CI 0.74-0.93) and 0.67 (95% CI 0.38-0.95) to 0.72 (95% CI 0.48-0.97), respectively ( Table 2). The CVM index inter-and intra-examiner reliability demonstrated agreement with weighted kappa coefficients from 0.69 (95% CI 0.53-0.86) and 0.71 (95% CI 0.56-0.86), respectively. The authors reported that none of the patients investigated possessed a CVM 1-IV nor was any subject classified as having palatal suture maturational stage A. It was found that 13 of 21 subjects with CVM V were classified as having maturational stage B or C (61.9%; males 77.8%, females 50.0%). Additionally, 42 of 110 subjects with CVM VI were classified as having maturational stage B or C (38.2%; males 41.6%, females 34.0%). Post-hoc analysis demonstrated that maturational stages B, C, D and E were related to differences in mean fractal dimension (P < 0.05). A negative correlation existed between fractal dimension and maturation stage (−0.623, P < 0.001). Male and female correlation coefficients were determined to be −0.649 (P < 0.001) and −0.569 (P < 0.001) respectively. A ROC curve was generated and determined the boundary between dichotomous maturation stages A-C and D or E, allowing for fractal dimension to be used to identify midpalatal suture fusion. Predictive statistical analysis noted that fractal dimension is a statistically significant indicator capable of predicting dichotomous maturation stages ((A, B, & C) vs. (D or E) (area under ROC curve [AUC] = 0.794, P < 0.001) [12] ( Table 2).
The study notes a significant correlation between fractal patterning and degree of maturation of the midpalatal suture, and consequently the authors feel that fractal analysis can provide an objective and quantitative methodology to assess palatal suture maturity [12].
Disadvantages of this methodology include requiring significant training and proficiency in classifying the maturation stage of palatal sutures as proposed by Angelieri et al. [3]. Another disadvantage is requiring the clinician to have significant familiarity with image processing and possessing necessary software. Consequently, the time, cost and resources to do so may be prohibitive to clinicians. Additionally, this modality relies on complex statistical analyses to determine the variable (optimal cut-off value) to predict the dichotomous maturation stage of the patient's palatal suture. Kwak et al. [12] argue that if an individual's fractal dimensions can be compared, it may provide a straightforward and clinically viable method to assess the maturation of the palatal suture and aid in clinical decision making as it relates to the modality of expansion at the diagnostic record visit [12]. Conversely, the authors do note a variety of methods to calculate fractal dimensions and the fact these varying techniques produce different fractal dimension values. Consequently, Kwak et al. [12] argue for a more agreed upon method for its calculation to be utilized clinically.
Performing and interpreting these analyses requires significant advanced knowledge of statistics. Ultimately it is the view of the authors that this methodology is impractical in terms of time, cost, resources and knowledge required to complete this methodology for each patient as part of their diagnostic work up in day-to-day clinical practice.
Furthermore, as was stated previously in the discussion, utilization of the crudely proposed maturational staging as defined by Angelieri et al. [3] should be used with caution and lacks validation to a reference standard as does this study as mentioned by Kwak et al. [12]. Further areas of interest include the development of a ratio comparing the fractal dimensions of a mature coronal suture to that of the midpalatal suture [12]. Additionally, improvement in the accuracy of the methodology may be gained by refinement and minimization of the number of actions needed to determine fractal dimensions [12].

Limitations
As mentioned before significant methodological differences were identified (sample size, in vitro vs. in vivo, imaging technique used, lack of adequate reference standard). The results were nonhomogenous consequently a meta-analysis could not be performed, nor direct comparison of the studies possible, limiting any major conclusions regarding these newer contemporary methodologies to assess midpalatal sutural maturation. Overall, these studies did not present solid evidence of their validity for the accurate determination of the maturation of the palatal suture. As a consequence of this weak body of evidence, it is of utmost importance that clinicians use a multitude of diagnostic criteria to properly direct clinical decision making as it pertains to the maturity of the mid palatal suture and appropriate modality of expansion, namely RME or SARME. It is worth noting that expansion does not solely involve the palatal suture but also the circummaxillary sutures, this would also be a limitation present.

Conclusions
Only a weak limited body of evidence exists to support the newest technologies and proposed methodologies that evaluate the extent of mid palatal suture maturation. All discussed novel methodologies lack validation with histological reference/gold standard. Consequently, it is still advised that clinicians use a multitude of diagnostic criteria to subjectively assess palatal suture maturation and drive clinical decision-making as it relates to the appropriate treatment of maxillary skeletal transverse deficiency in late adolescents and young adults (RME vs. SARME). Future considerations in the imaging and assessment of the midpalatal sutural maturation will likely include some form of invasive CT technology, and proposed methodologies should follow appropriate ALARA radiation safety protocols. Non-invasive imaging technologies such as ultrasound present a promising and biologically safer alternative to assess midpalatal sutural ossification. Pubmed (("palate") AND "cranial sutures") AND (("maturation" OR "interdigitation" OR "ossification")) 31 Embase (1)