Stability of palatal rugae after orthodontic/orthopaedic expansion: a scoping review

Objective: The palatal rugae are connective tissue located in the anterior third of the hard palate and present asymmetrically on each side of the palatine raphe. The stability and individualistic features of the palatal rugae have been suggested as an alternative human identification method in forensic medicine. However, there are different views about the tissue’s stability and reliability in individuals undergoing orthodontic expansion, as the palatal mucosa is stretched which likely alters palatal rugal morphology. The present review aims to summarise the available evidence regarding the stability of the palatal rugae after orthodontic expansion. Method: Following the PRISMA-Scr guidelines, an extensive search was conducted using three databases (PubMed, Web of Science, and Scopus). Only studies that had control groups were included to allow for comparison. Results: A total of six studies were identified. One report did not mention the type of expansion used, another used slow maxillary expansion (SME), and the other four used rapid maxillary expansion (RME). The results related to rugal length, morphological shape, and root mean square were inconclusive, likely because of data heterogeneity in expansion mechanics, the age of inclusion, and measurement outcomes. Conclusion: More high-quality research is needed to substantiate the importance of palatal rugae in forensic medicine.


Introduction
The palatal rugae, also known as plicae palatinae transversae or rugae palatine, are located in the anterior third of the hard palate and arranged transversely on each side of the palatine raphe. Their number, shape, length, width, prominence, and orientation vary on each side of the midline in each individual, even in identical twins. 1 The growth and development of the palatal rugae are controlled by epithelial-mesenchymal interactions, similar to the teeth. 2 The rugae are accepted as a stable structure because of the protection afforded by the surrounding soft and hard tissues such as lips, bones, tongue, teeth and cheeks. 3 Previously published papers theorised that, following rugae formation, they remain stable and can resist morphological changes induced by tooth extraction or orthodontic treatment. 4,5 In addition, they are also able to withstand degenerative changes up to seven days after death. 6 The palatal rugae have also been widely studied in a forensic context to determine individual identification as an alternative to fingerprinting. 5,7 Different population groups have demonstrated varying patterns of palatal rugal anatomy. 8 Therefore, the structure can be used to identify population groups in the event of mass casualties. The differences in morphology have been attributed to the genes that control the orientation of the rugae during development. 9   also claimed that similarities in the palatal rugae were present in close sibling groups, which suggested heritability traits. 10 The stability of the palatal rugae has also been utilised in orthodontic treatment as a stable landmark to measure the movement of teeth. Generally, orthodontists have used lateral cephalograms to measure skeletal, dental, and soft tissue changes as well as the inclination and anteroposterior movement of teeth. However, radiographs raise concerns about radiation exposure and the difficulty in finding a stable reference plane or point to perform superimposition. This is usually attributed to the vagary of images and anatomical structures. 11 Therefore, Allen (1889) suggested the use of the palatal rugae as an alternative comparison method, commonly incorporating the entire rugal area for superimposition. [12][13][14] Past studies have postulated that the third medial rugae are stable reference points to quantify tooth movement after orthodontic treatment that involves extraction. 5,[15][16][17][18] However, there are conflicting results regarding the stability of the palatal rugae after orthodontic treatment involving maxillary expansion required in dental transverse discrepancy cases. 19,20 The palatal rugae that are thin and those near the teeth tend to follow tooth movement more easily as they are stretched. 21 If the palatal rugae are altered due to orthodontic expansion, they may no longer be used to identify individuals or as a reference landmark for orthodontic purposes. Therefore, the aim of the current review was to assess the available research regarding the stability of the palatal rugae after orthodontic expansion. It was expected that this would provide confirmation regarding the reliability of the palatal rugae for forensic identification and to investigate whether the rugae are a stable landmark for those who have received orthodontic expansion for superimposition purposes.

Materials and methods
The present review utilised the methodological framework based on the Preferred Reporting Items for Systemic Reviews and Meta-Analysis extension for scoping reviews (PRISMA-Scr) guidelines. The research question for this review was: "Do the palatal rugae remain stable after orthodontic expansion"?
The published literature until July 2022 was searched using three databases, PubMed, Web of Science, and Scopus, without language restriction. The following search terms were applied: (palatal rugae) OR (rugae) AND (palatal expansion) OR (expansion) OR (rapid palatal expansion) OR (slow palatal expansion) OR (orthodontic expansion) OR (maxillary expansion) OR (rapid maxillary expansion) OR (slow maxillary expansion). The protocol was registered in the Open Science Framework for scoping review protocol registration at the following link: https://osf.io/8dfj3

Criteria for study selection
The inclusion criteria for published article selection were defined according to the participants, concept, and context domains (PCC). Participants (P): Patients who had undergone orthodontic expansion. Concept (C): All types of orthodontic expansion. Context (C): Change of palatal rugae and Types of sources: All descriptive, prospective, and retrospective studies except case reports were included. In addition, only studies that had a control group were included.

Data extraction
A total of 188 articles were initially identified. After the screening of titles and abstracts by two independent reviewers (SNT and YML), 165 irrelevant articles and 13 duplicates were excluded. Disagreements between reviewers were resolved by a third reviewer (AMFSM). According to the inclusion and exclusion criteria, ten potentially eligible full-text articles were considered for screening. After assessment, four articles were excluded because they did not satisfy the inclusion criteria (no control group). A total of six articles were therefore included in this review. A summary of the screening process is shown in Figure 1.

Study characteristics
After evaluating their titles, abstracts, and full text, the main characteristics of the included studies are described in Table I. The papers were grouped based on study design (sample size and age group, materials used, inclusion and exclusion criteria), patient intervention, methods of analysis, and primary outcomes regarding changes in the palatal rugae region after orthodontic treatment.

Location, sample size, and age
The studies were carried out in different world regions, but mainly Asia, 19,22,23 Europe 25,26 and America. 24 The number of patients varied widely between the studies. Two studies had a sample size of 137 and 168 patients, 22,23 two studies each recruited 60 subjects, 25,26 whereas the remaining two had an average of 34 patients per study. 19,24 Two studies did not specify an age group, 22,23 whereas one study examined patients over the age of 18 years. 19 The other studies included patients aged between eight to twelve. [24][25][26]

Inclusion and exclusion criteria
One study did not specify its inclusion and exclusion criteria. 22 A second study did not mention its inclusion criteria, 23 whereas another did not specify its exclusion criteria. 24 The remaining three studies identified their inclusion and exclusion criteria. 19,25,26 Three studies excluded patients with a cleft lip and/or palate, 19,23,26 whereas two studies excluded patients with previous orthodontic/prosthodontic treatment. 23,26 Additionally, two studies excluded patients with a history of maxillofacial trauma or surgery, 19,23 whereas two studies excluded patients with temporomandibular joint dysfunction or hypodontia. 25,26

Instruments used
Two studies used dental casts, which were then classified according to the Thomas and Kotze classification to assess palatal rugae patterns. 22   Two studies used photographs of dental casts. 19,24 However, one study 19 used the Santos classification system, whereas another study 24 used the Lysell classification system to assess palatal rugae pattern. Two studies used a 3D model in which an automatic best-fit superimposition algorithm and distance were expressed using Root Mean Square (RMS) to evaluate the rugal pattern. 25,26

Patient intervention
One study had groups of patients who had undergone palatal expansion involving either an extraction or non-extraction approach. 22 Two studies similarly had groups of patients who had undergone extraction and non-extraction; however, an extra group comprised patients who had undergone palatal expansion following a nonextraction approach 23 and patients who had not undergo palatal expansion nor extractions. 19 In addition, two studies assessed patients treated with rapid maxillary expansion (RME) versus no treatment. 25,26 In contrast, one study only looked at patients treated with RME versus fixed appliance treatment. 24

Outcomes -changes in palatal rugae morphology (including length, shape, and root mean square)
Two studies concluded that there were insignificant or no changes in the morphological shape of the rugae post-treatment. 22,23 However, one study noted significant changes in the expansion and extraction group but not in the non-extraction group. 19 One study concluded that there were insignificant changes in the length of the palatal rugae. 22 In contrast, another study concluded that there were significant changes post-treatment. 23 One study identified a significant change in the transverse measurement between the medial aspect of the rugae but no change in anteroposterior measurement in patients who had undergone an RME compared to a control group. 24 In the studies that used RME to assess palatal rugae pattern, one study concluded that there were no significant changes in patients who had undergone RME versus a control group, 25 whereas another study concluded that there were statistically significant changes in patients who had undergone RME. 26

Discussion
Plaster study models are an essential diagnostic tool in dentistry. Although they pose problems with casting and storage, they remain a gold standard for measurement due to the dimensionally stable impression material and gypsum products used for fabrication. 27 Digital study models were introduced in the 1990s, and their reproducibility and validity have made them popular and widely used. [28][29][30] The studies included in this review used plaster models except for two, 25,26 whereas all the included studies were the same regarding the diagnostic tools used for examination.
Patients who had a cleft lip and palate, craniofacial syndrome, trauma, or periodontal surgery that affected the hard palate region or had undergone previous orthodontic treatment were excluded from this review as these environmental influences may have altered palatal rugae morphology and therefore acted as a confounding factor.
Rapid maxillary expansion (RME), slow maxillary expansion (SME), and surgical rapid maxillary expansion (SARME) are three treatment modalities used to treat transverse discrepancy malocclusions, identified as a constricted maxillary arch or posterior crossbite. All of the studies included in this review utilised an RME to perform maxillary arch expansion, except for Lanteri et al. (2020), who used an SME. 25 An RME can produce forces of up to 100N over 2 to 3 weeks, whereas SME produces nearly 10N of force over four months. According to Zhou et al. (2014), an RME produced significant expansion in maxillary intermolar width compared to SME but no significant differences in maxillary interpremolar and intercanine width were identified. 31 An RME gives rise to an increase that favours bodily tooth movement compared to tipping movements. 32,33 Therefore, RME is almost exclusively used in paediatric patients aged 15 years or younger due to a patent palatine suture. 34 However, SARME is indicated when considerable maxillary expansion is needed in non-growing patients.
Unlike RME, recent studies claim that SMEs mainly produce dentoalveolar tooth movement and no skeletal expansion. 35,36 In contrast, it is further claimed that SMEs produce light, continuous physiological forces to increase the deposition of bone in the circum-maxillary suture system and produce the same effect as an RME. [37][38][39] This allows SMEs to produce greater stability during the reorganisation process of the maxillary complex. 33 This could also be attributed to more tolerant physiological forces exerted on the palatal arch, which allows the palatal rugae to slowly adapt, and hence produce no significant changes in the rugae in this expansion group, as reported by Lanteri et al. (2016). 37 This result does not agree with Ugolini et al. (2021). 26 However, both studies used the same method to examine whether expansion causes changes to palatal rugae compared to a nontreatment group. The contrary result could be due to differences in the expansion appliance utilised, as Ugolini et al. (2021) used an RME while Lanteri et al. (2020) used a leaf expander, which is a form of SME. 26,37 Furthermore, both studies captured the digital model at pre-treatment and appliance removal. This may have led to assessor bias as the expander's imprint on the palate was visible on postexpansion models. Ali et al. (2016) 23 showed that the length of the third palatal rugae increased in the expansion and extraction groups compared to the nonextraction groups, which is consistent with previous studies. 19,20,40 The change could also be due to the expansion forces mainly applied to the premolar and molar regions. Hence, the third palatal rugae are affected the most. This supports the theory that "the closer the rugae are to the teeth, the more likely they are to stretch in the direction of the associated teeth". 21,41 Furthermore, Damstra et al. (2015) also found transverse changes between the medial aspects of the rugae in the expansion group compared to a non-extraction group. 24 The intermedial distance of rugae resembles an 'A' pattern because the third rugae is the largest, whereas the distance between the first rugae is the smallest. This separation pattern is not in accordance with the palatine suture, which has a 'V-shaped' pattern, suggesting that this may be due to the overlying palatal mucosa not following the separation of the palatine bone. 42 Damstra et al. (2009) 24 noted no transverse nor anteroposterior changes in the non-extraction group, which supported the result of other studies. 16,24,43 Although there were changes in the linear measurement, the palatal rugae maintained their shape after expansion. 22,23,44 Therefore, it can be theorised that palatal rugae morphology can potentially be used for forensic identification.  44 Therefore, the level of confidence that can be drawn from the conclusions of these articles is low.
The presence of growth factor and its impact on palatal rugae morphology should also be considered as some studies recruited growing patients. 25,26 However, Lysell et al. (1955) showed that the palatal rugae remains unaltered after the 12th week of intrauterine life. 41 Age may make the palatal rugae less defined, but the configuration of the rugae remains unchanged. 45,46 Furthermore, Almeida et al. (1995) and Kim et al. (2012) argued that the changes in palatal rugae due to growth are not clinically significant. 16,47 Therefore, age is not a confounding factor.
The periodontal apparatus consists of periodontal, supracrestal, and supraosseous fibres, which take one to one and a half years to reorganise after orthodontic tooth movement. 48 Ideally, measurement or rugoscopy should be undertaken after the soft tissue is relatively stable and more reliable. 44 All studies included in this review measured their outcome immediately after treatment intervention; therefore, this may be a confounding factor, and confirmatory longitudinal studies should be carried out. However, this may lead to attrition bias and affect the compliance of retainer wear post-orthodontic treatment, and herald relapse.

Limitations
The present scoping review descriptively summarised the data rather than provide an analytical assessment. There was heterogenicity of the data of the studies related to the expansion mechanisms, age and gender of the patients and outcome measurement.

Suggestions
In order to have a level of confidence in the conclusions drawn from studies regarding the stability of palatal rugae after expansion, a sound study design is required. The authors recommend the following: 1. Participants: Increase sample size to increase the power of the study and follow up with the patients at least until after growth has slowed to the adult level to minimise a possible growth factor effect on the palatal rugae. 49 In addition, previous studies have been carried out in different regions, therefore, the results must be viewed with caution and cannot be generalised to other populations. 2. Study design: Longitudinal study with a control group is recommended. 3. Intervention: Consider the use of different types of expansion mechanisms, especially SME, because of a lack of studies using this expander. 4. Outcome: The use of 3D superimposition to measure three-dimensional changes of palatal rugae to provide robust results. 49 Also, using digital models may avoid artifacts of plaster models such as bubbles or fractures.

Conclusion
A low level of evidence shows that orthodontic expansion makes the palatal rugae unstable; therefore, utilising the palatal rugae for forensic identification or as a stable landmark for orthodontic superimposition in patients following maxillary expansion should be used with caution.