Maxillary displacement due to scar tissue influence on quad-helix expansion treatment in unilateral cleft lip and palate patients: Finite element analysis

Introduction

Oro-facial clefts, including cleft lip (CL) with or without a cleft palate (CLP), and a cleft palate without a cleft lip (CP), are common congenital defects. These clefts result from the failure of tissues forming the lip and palate to combine during early embryonic development [1]. Unilateral cleft lip and palate (UCLP) or clefts that occur on one side are more common than other types of clefts [2]. The maxilla in UCLP patients is divided into two asymmetrical parts, including the segment without a cleft, called the major segment, and the segment in the cleft section, called the minor segment [3].
To address aesthetic and functional issues in UCLP patients, primary surgical intervention is required. Cheiloplasty and palatoplasty in CLP cases are considered factors affecting the maxillary growth in three dimensions, i.e., transverse, vertical, and sagittal [4, 5]. Post-surgical wound healing process causes scar tissue formation, characterized by reduced endothelial cells and fibroblasts, lower vascularity, and cell density. The absence of elastin fibers and abundance of collagen fibers cause hardening and stiffening of the scar tissue. This is believed to be a factor inhibiting maxillary growth [6].
The maxillary discrepancy in CLP cases results in severe crowding of the teeth, accompanied by anterior crossbite and/ or posterior crossbite. This condition can be corrected with orthodontic treatment using expansion techniques [5]. Maxillary expansion in CLP patients can be achieved with rapid palatal expansion or slow palatal expansion (SPE). Rapid palatal expansion uses heavy force to separate the inter-maxillary suture, which can be traumatic and uncomfortable, requiring patient and parental cooperation for device activation. Quad-helix is an SPE that is often used for CLP cases [7]. Activation of quad-helix can correct transverse direction disturbances, so that it is possible to obtain a more symmetrical arch [8]. It requires less patient cooperation for activation, offers greater comfort, better adaptation, and provides constant force until the required expansion is achieved [9].
Case reports and studies on the effects of quad-helix utilization in CLP patients are available in large quantities, but these studies are retrospective in nature, and only describe the results of treatment, stability, effectiveness, and efficiency without thoroughly explaining bio-mechanics that occur, especially related to maxillary displacement due to scar tissue influence during quad-helix treatment [10-12]. Finite element analysis (FEA) is a computer simulation technique that uses mathematical matrix analysis to study bio-mechanics of body’s anatomical structure under applied forces [13]. Finite element analysis can simulate forces applied to dento-maxillofacial structures to predict displacement in transverse, vertical, and sagittal directions, so that orthodontists can study the point of force application to obtain more optimal expansion results in UCLP cases [14]. This information is presented through a visual representation, showing the magnitude and direction of displacement in dento-maxillofacial structures [15].
Based on the description that has been explained, researchers are interested in analyzing maxillary displacement due to the influence of scar tissue and maxillary lip tension after surgery in quad-helix treatment in UCLP cases using FEA. FEA parameters observed are displacements in the anterior and posterior regions in the major and minor segments.

Objectives

This study aimed to determine the displacement of the maxilla in UCLP patients due to the influence of forces from the quad-helix, scar tissue, and upper lip using FEA.

Material and methods

This research was approved by the Ethics Committee of the Faculty of Dentistry of UGM, approval number of 249/UN1/KEP/FKG-UGM/EC/2023. Cone beam computed tomography (CBCT) scan images of a 9-year-old male with UCLP were obtained from the Radiology Installation archives at RSGM UGM Prof. Soedomo, Indonesia, and used to create a finite element model. Quad- helix device and CBCT scan results were reconstructed into a 3D model using Autodesk Inventor Professional 2019 software (USA) (Figures 1A-B). Subsequently, 3D quad-helix model was attached to 3D maxillary model (Figure 1C) for further discretization and simulation processes with ANSYS 2022 R2 software (USA) (Figure 1D). The model was given boundary conditions with zero displacement on the outer surface on the superior and posterior sides of the 3D maxillary model [16]. Next, the boundary conditions and material properties were assigned to the model according to previous studies for the next processing stage (Table 1).
Processing stages were carried out in two simulations. In the first simulation, quad-helix expansion force was applied. The second simulation combined quad-helix expansion force, with forces from scar tissue and upper lip. Setting of working conditions are presented in Table 2.
The last step involved the evaluation of displacement pattern. Simulation results were presented in visual and numerical data. In x (transversal), y (vertical), and z (sagit­tal) axes, fourteen reference points on the major and minor segments of the palate and teeth were investigated in this study (Figure 2).

Results

Displacement of the maxilla simulation model for unilateral cleft lip and palate patients, with the application of quad-helix expansion force

Quad-helix expansion force of 2 N was applied to the palatal part of the major and minor segments of the first molar parallel to the transverse plane in the buccal direction. The results of maxillary displacement on the x-axis, y-axis, and z-axis are demonstrated in Table 3.
The simulation results indicated that in the transversal direction (x-axis), there was a buccal displacement at all reference points, with the minor maxillary segment showing greater displacement than the major segment. The highest displacement values for both the major and minor maxillary segments were found in the anterior region, specifically at point T1. The displacement in the vertical direction generally showed the maxillary major segment moving inferiorly slightly more than the minor segment. The highest displacement value in the maxilla in the major and minor segments was located in the posterior region, at point P3. The sagittal direction showed that the displacement of the maxillary minor segment towards the anterior appeared to be greater than that of the major segment. The highest displacement value in both segments was located in the anterior region, at the T1 point.
The average results on the maxillary major segment showed that the highest displacement occurred in the transverse direction, with an average displacement of –0.1311 mm, followed by the sagittal direction, with an average displacement of 0.0755 mm, and the lowest displacement occurred in the vertical direction, with an average displacement of –0.0541 mm, indicating a tendency for the major segment to move buccally, slightly anteriorly, and inferiorly. The average of the maxillary minor segment demonstrated that the highest displacement occurred in the sagittal direction, with a mean value of 0.1781 mm, followed by the transverse direction, with a mean value of 0.1458 mm, and the lowest displacement was recorded in the vertical direction, with a mean of –0.0496 mm. From this value, it can be assumed that the minor segment moved more anteriorly and buccally, with a slight upward displacement.

Displacement of the maxilla simulation model for unilateral cleft lip and palate cases with the application of quad helix expansion force, scar tissue, and upper lip tension

In the second simulation, there was a combination of three forces applied: 1) quad-helix expansion force of two 2 N on the palatal side of the upper first molar of the major and minor segments, with force direction parallel to the transverse plane towards the buccal direction; 2) scar tissue force of 17.52 N in radial direction towards the center of the scar tissue; 3) the force from the upper lip tension was 3.7 N posteriorly on the surface of the alveolar process at the level of the vestibule. The results of maxillary displacement in this simulation are shown in Table 4.
Simulation results in the transverse direction showed that there was greater buccal displacement in the anterior and posterior regions of the maxilla in the major segment compared with the minor segment. The highest displacement value of the maxillary major and minor segments was located at point T3. Results of the vertical direction revealed that the displacement of the maxillary major segments on the side of the cleft (P1, P2, and P3) towards the inferior direction was greater than that of the minor segments. While at point T (T1, T2.1, T2.2, and T3), the displacement of the maxillary minor segment was larger than the major segment. The highest displacement value for the maxillary major and minor segments was located at point P3. The displacement in the sagittal direction showed that the maxillary minor segment moved more anteriorly than the major segment. The highest displacement value of the major segment was recorded at point T2.2.
The average results on the maxillary major segment showed that the highest displacement occurred in the transverse direction, with a mean value of –0.0998 mm, followed by the sagittal direction, with a mean value of 0.0645, and the lowest displacement occurred in the vertical direction, with a mean value of –0.0610 mm. From this value, it can be seen that the major segment moved buccally slightly anteriorly and inferiorly. The mean for the minor segment revealed that the highest displacement occurred in the sagittal direction, with a mean value of 0.0782 mm, followed by the transverse direction, with a mean value of 0.0689 mm, and the lowest displacement occurred in the vertical direction, with a value of -0.0298 mm. This means that the major segment moved anteriorly, buccally, and slightly inferiorly.

Differences in displacement patterns in the first and second simulations

The displacement of the first and second simulations is presented in Figure 3. Based on data results in the three directions of maxilla observation, the major and minor segments in the first and second simulations showed a similar displacement pattern, and at the major segment, it moved towards the buccal direction, and slightly anteriorly and inferiorly. While the minor segment tended to move more anteriorly and buccally, and slightly inferiorly. The difference in the value of maxillary displacement between the second and first simulation is highlighted in Table 5.
Based on the average difference between the first and second simulations, there was a decrease in minor segment displacement in the transverse, sagittal, and vertical directions. A greater decrease in displacement values was seen in the maxillary minor segment than in the major segment. This decrease in displacement was more visible in the anterior region of both maxillary segments. The maxillary major segment also tended to decrease in the second simulation in the transverse and sagittal directions. While in the vertical direction, there was an increase in displacement towards the inferior direction of the reference points in the cleft area as well as a decrease in the displacement of the reference points in the teeth.

Discussion

This research identifies maxillary displacement due to the influence of scar tissue on quad-helix expansion mechanism in UCLP patients using FEA. The FEA technique is a proven and efficient mathematical method for evaluating orthodontic bio-mechanics in a non-invasive manner. The point of application, magnitude, and direction of a force can be adjusted to simulate clinical situation. Using FEA, the amount of displacement experienced at the maxillary major and minor segment reference points can be measured theoretically. A comparison of displacement between the maxillary major and minor segments can be done, so that the optimal point of application of force for treatment can be predicted [17].
The quality of a 3D model greatly influences the results of FEA simulation. CBCT results for UCLP patients and quad-helix device scanner obtained in this study are surface models, so they do not have a representation of material on the inside. Simulation using FEA requires a 3D model with a solid shape; therefore, the maxillary model in this study was created using the NURBS method. In this process, NURBS curves are created by determining the number and position of curves, which correspond to reference points of the original object. This process requires high precision, and it often takes a long time [18]. In recent years, an increase in FEA geometric modeling has been observed. Işeri et al. [19] used a cranial geometric model with a total of 2,349 elements, while a study in 2007 conducted by Holberg utilized a cranial model with a total of 30,000 elements and 50,000 nodes. This research used 154,808 elements with 266,047 nodes to create a more accurate 3D model.
The quad-helix force employed in the current study refers to the activation carried out in clinical conditions. Activation by expanding the posterior part of the quad-helix device to one molar or about 8 mm will produce a force of 3.89 N [20]; therefore, a force of 2 N is applied to each maxillary segment. The scar tissue that forms as a side effect of unilateral cleft lip and palate repair has stiff and hard characteristics, which can affect the dynamics of maxillary displacement during expansion treatment. The amount of scar tissue force used in this study was 17.52 N, with a radial force direction from the edge to the center of scar tissue, based on Wei et al. [21].
While the upper lip tension used was 3.7 N, based on a study by Trotman et al. [22] on lip tension in UCLP patients aged ± 13 years. This age is close to the age of patient who participated in the current study.
Simulations of the application of expansion force from quad-helix without scar tissue and upper lip tension present greater buccal displacement in the maxillary minor segment, with a downward displacement pattern to the posterior. The results of this study correlate with studies conducted by Holberg et al. [23] and Hemanth et al. [17]. In both investigations, the point of maximum buccal displacement was found at the top of minor segment of the maxillary canine. Lee et al. [24] evaluated stress distribution in UCLP expansion cases, and reported asymmetrical stress distribution between the major and minor maxillary segments. This could be caused by differences in the area and supporting structures of the two maxillary segments. The posterior and superior parts of the maxilla provide resistance for both maxillary segments, with greater resistance in the major segment.
When force from scar tissue and upper lip tension are applied, the buccal displacement of minor segment of the maxilla is less than that of the major segment, with a downward displacement pattern from posterior to anterior. The results of maxillary displacement in this study support those of Bell and LeCompte [25], demonstrating an increase in inter canine width by 3.62 mm and inter-molar width by 6.7 mm when using quad-helix device. Additionally, the current study’s results are in line with those of Al-Gunaid et al. [26], who observed that when the expansion force works, there is an opposing force produced by scar tissue and upper lip tension concentrated in the cleft area. This can also be interpreted by referring to a research by Ishikawa et al. [27], who stated that the shape of dental arch in UCLP patients is determined by the location of scar tissue. The location of scar tissue in this study was closer to anterior, so the tension of scar tissue affected the anterior area more.
Displacement in the vertical direction of simulated quad-helix expansion forces without scarring forces or accompanied by scarring forces and upper lip tension, identified similar displacement patterns in the inferior direction. These findings support a research conducted by Ayub et al. [28], reporting a decrease in palatal height in UCLP cases after the expansion procedure was carried out. These findings show consistency in the effects of palatal expansion in UCLP cases, where there are structural changes related to palatal height after expansion. Ribeiro et al. [29] investigated the use of quad-helix in healthy patients, and reported the inferior displacement being greater than the superior. This could happen because the resistance to expansion forces is more in the superior than inferior part, and the expansion forces are located far below the center of resistance. Therefore, the authors assumed that the inferior displacement of the maxilla in the major segment is greater than that in the minor segment, because the expansion force exerted on the minor segment is closer to the center of resistance of the superior portion of the minor segment than the expansion force acting on the major segment.
The results of displacement in the sagittal direction of the first and second simulations demonstrate that the anterior displacement of the maxillary minor segment is greater than that of the major segment. The minor segment in this study also showed that the greatest displacement occurred anteriorly, followed by buccally, and inferiorly. This is in line with a research conducted by Meng et al. [30]. It can be assumed that this may be due to the typical curved shape in UCLP cases, in which the collapse in the anterior region is greater than in the posterior region, so that when receiving an expansion force, the anterior part of the minor segment can produce two forces, i.e., buccal force and anterior rotation force, which expand and rotate the minor segment to form symmetrical curve [26]. The results of this study are slightly different from that of Takahashi et al. [5], who observed palatal length increase after expansion, but it was not significant. This is possible because the measurement of palatal length is carried out by measuring the length of a line drawn from the incisor papilla perpendicular to the line parallel to cervical maxillary molar teeth in the major and minor segments, if it is observed that the line is located in the major segment. This study shows that the displacement of the major segment in the sagittal direction is smaller than that of the minor segment.
This research confirm that scar tissue can provide additional resistance to the expansion forces generated by quad-helix. Scar tissue can reduce the effectiveness of the applied expansion force, resulting in more restricted displacement compared with a situation without scar tissue. Studies using FEA demonstrate that the presence of scar tissue located closer to the anterior region of the maxillary minor segment will further inhibit the expansion effect. Therefore, it is necessary to consider applying expansion forces to the anterior area as well as the application of asymmetric force to the maxillary major and minor segments to obtain a more symmetrical arch.
There are several limitations to be acknowledged. Firstly, limited data related to variations in scar tissue characteristics and upper lip tension limits generalizability of the study’s results. Secondly, this study only relied on one CBCT data converted into a 3D model, which may not adequately represent diversity of UCLP cases. Thirdly, this study did not consider several dento-alveolar and soft tissue components, which have different material properties. These factors are likely to influence the accuracy of simulation results compared with actual clinical conditions. Lastly, the inability to incorporate the time variable in FEA simulations restrains its effectiveness for time-based analysis studies.

Conclusions

Quad-helix expansion force of 2 N, a scar tissue force of 17.52 N, and an upper lip tissue force of 3.7 N produce a maxillary displacement pattern that is similar to the simulation without scar tissue. The major segment moves buccally, slightly anteriorly, and inferiorly, while the minor segment tends to move anteriorly and buccally, and slightly inferiorly. However, there are indications for a large decrease in the minor segment displacement in three observation directions. The greatest decrease in displacement occurs in the anterior region of the minor segment due to additional resistance from scar tissue.

Disclosures

1. Institutional review board statement: The study was approved by the Ethics Committee of the Faculty of Dentistry of UGM with letter number: 249/UN1/KEP/FKG-UGM/EC/2023.
2. Assistance with the article: Data source from Riona Ulfah’s thesis from Faculty of Dentistry, Universitas Gadjah Mada, Indonesia.
3. Financial support and sponsorship: None.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

References