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INTRODUCTION
Owing to genetic factors and the impact of postoperative scars, patients with cleft lip and palate (CLP) often exhibit insufficient maxillary development, skeletal Class III malocclusion, dental crowding, and velopharyngeal insufficiency.1-5 Patients with unilateral cleft lip and palate (UCLP), who account for approximately 80% of patients with CLP, also experience asymmetric maxillary development.6 Currently, the treatment of asymmetric maxillary hypoplasia and dental crowding secondary to UCLP is challenging.7
Distraction osteogenesis (DO) is a strategic approach to counteract maxillary hypoplasia by applying continuous forces to stimulate bone and soft tissue regeneration.8,9 In comparison with traditional orthognathic surgery, total maxillary DO is more advantageous for CLP because it increases maxillary anterior movement, maintains postoperative stability, and reduces the risk of relapse and speech disorders, especially considering the greater soft tissue tension due to scar tissue from prior repairs.10-12 However, the prolonged use of a visible external device can cause physical and psychological discomfort.9,13 Anterior maxillary segmental distraction osteogenesis (AMSDO), a variant of total maxillary DO, is a less invasive approach that involves osteotomy in the premolar region and the use of intraoral devices to advance the segment. AMSDO is indicated for patients with maxillary sagittal deficiency, especially those with dental crowding or those who need additional space for restorative treatments, since it offers aesthetic benefits and the advantage of bone formation within the dental arch, thereby reducing the likelihood of tooth extraction.14-16
In the AMSDO procedure, conventional distraction osteogenesis (CDO) typically employs screw expanders fixed to teeth or anchored to the maxilla using mini-implants. However, CDO does not allow adjustment of the distraction vector, restricting its effectiveness in correcting maxillary asymmetry.17 Alternative approaches, such as using buccal-side devices separately with varying distraction lengths to rotate the anterior bone segment, are associated with issues such as challenging parallel placement and discomfort.18 Moreover, the current knowledge of AMSDO is limited and predominantly based on retrospective case reports. The stress-distribution and displacement patterns of bone segments and dental arches during the distraction process are not well understood. Finite element analysis (FEA) is a modern computer simulation method that can simulate and predict displacement, internal stress, and strain in vitro.19,20 FEA’s noninvasive simulation of clinical conditions is advantageous, enabling the analysis of various structures such as teeth and bones within a single model.20,21 While FEA has been extensively used in the orthodontic field, its application in DO, particularly AMSDO, has been less explored, indicating a need for further research in these areas.22
In the light of these considerations, the present study introduced an innovative asymmetric tooth-borne distractor that has obtained a relevant patent but has not yet been commercialized. Conceptual FEA was employed to evaluate and compare the displacement and stress distribution in the maxillary segment and teeth when applying the CDO and asymmetric distraction osteogenesis (ADO) modes in AMSDO. These findings will provide guidance for the appropriate design and application of clinical distraction devices and offer more personalized and effective clinical interventions for clinicians addressing asymmetrical maxillary hypoplasia.
MATERIALS AND METHODS
Asymmetric distractor design
The asymmetric distractor consisted of three expansion screws, four orthodontic bands, and metallic arches for connection (Figure 1A). The middle screw (indicated by a yellow arrow) controls the anterior-posterior distraction, with each 1/4 turn of the screw resulting in 0.25-mm opening of the device. The left screw (red arrow) controls the rotation of the anterior maxillary segment. Turning the screw in the direction of the blue arrow causes clockwise rotation of the disc structure in front of the distractor, which in turn rotates the anterior maxillary segment (Figure 1B). Each 1/4 turn generates rotation of approximately 0.5°. Similarly, the right screw (green arrow) controls the rotation of the posterior segment of the maxilla, the effect of which is to rotate the anterior segment in the opposite direction.
Cone-beam computed tomography imaging and finite element modeling
This study was approved by the Independent Ethics Committee of the Shanghai Ninth People’s Hospital, affiliated with the Shanghai Jiao Tong University School of Medicine (Approval No. SH9H-2023-T424-1). Written informed consent was obtained from the parents for cone-beam computed tomography (CBCT) data. A CBCT scan of a 12-year-old female adolescent with a diagnosis of UCLP (right side), skeletal Class III malocclusion, and dental crowding was obtained by using iCAT FLX (KaVo Dental, Biberach, Germany) before orthodontic treatment and saved in the DICOM format. The patient exhibited insufficient sagittal development of the maxilla, bilateral absence of the maxillary lateral incisors, and right dental arch collapse, leading to a shift in the maxillary midline to the right. In a CBCT assessment performed one year after alveolar bone graft surgery, the patient was found to show a bony bridge connecting the cleft defect, measuring approximately 4.3 mm in thickness, 5.6 mm in width, and 10.5 mm in height. After assessment, this case was considered to be suitable for direct FEA model construction without presurgical orthodontic intervention because of the appropriate maxillary width and ample osteotomy space between the premolars. CBCT data were imported into MIMICS (version 21.0; Materialise, Leuven, Belgium) to extract the data for the maxillary bone and teeth separately. Subsequently, the files in the .stl format were processed in Geomagic Wrap 2021 (Raindrop, San Jose, CA, USA) to generate surface meshes. Finally, model construction and assembly were performed using SOLIDWORKS 2021 (Dassault Systèmes, Waltham, MA, USA). The osteotomy lines for the model were designed using the software. A horizontal osteotomy was performed approximately 4–5 mm from the apex of the roots, extending toward the piriform aperture edge, followed by separation of the nasal septum and nasal floor. Vertical osteotomy lines were marked on the buccal aspect of the maxilla between the roots of the premolars on both sides. The osteotomy lines, avoiding contact with the roots, were extended to the palate and connected to separate the anterior maxillary segments (Figure 2A).15 These components were transferred to the ANSYS WORKBENCH (version 17.0; ANSYS, Inc., Canonsburg, PA, USA), and the final three-dimensional mesh consisted of 201,367 elements and 360,166 nodes (Figure 2B).
Finite element simulation
On the basis of previous studies, the mechanical properties of the materials in the model were assigned during finite element simulation using ANSYS 17.0, as shown in Table 1.22-24 The teeth, compact bone, cancellous bone, periodontal ligament, and distractor were all set as homogeneous and isotropic. The coordinate axes were established with X, Y, and Z representing the transverse, sagittal, and vertical orientations, respectively. The Y-axis was defined along the anterior nasal spine (ANS)-posterior nasal spine line, while the X-axis was perpendicular to the Y-axis on the palatal plane. Positive displacements indicated movements toward the non-cleft side and anteriorly for the X- and Y-axes, respectively, and downward for the Z-axis. The distractor was fixed to the teeth using orthodontic bands. For simplification, the CDO load was set to a 0.5-mm opening, while the ADO load was set to a 0.5-mm opening along with 0.5° rotation toward the non-cleft side. These values were used to simulate the average daily distraction in clinical practice.25-27 A spring element with longitudinal stiffness of 2.4 N/mm was added to the palatal osteotomy site to simulate the effects of palatal scarring.28 Stress perpendicular to the buccal surface of the alveolar bone was added to simulate the effect of perioral forces.29 The force values applied to each region are listed in Table 2. For the FEA, the displacement and von Mises stress were evaluated in the marked regions. In this study, the increase in the dental arch length was quantified by the Y-axis displacement between the alveolar bones of the first molars and premolars.
RESULTS
Displacement of the bone segment
Table 3 presents the three-dimensional displacements of bony and tooth landmarks in both anterior and posterior segments. In CDO, the anterior maxillary segment displacement varied from 0.2742 to 0.4207 mm; the greatest movement was observed at the non-cleft first premolar alveolar bone, and it symmetrically reduced toward the anterior and superior regions (Figure 3A). In ADO, the displacement varied from 0.2135 to 0.5461 mm, peaking at the cleft side’s first premolar alveolar bone, and decreased from the cleft to non-cleft side and from the inferior to superior regions, with greater displacement on the cleft side (Figure 3B). Both methods showed minimal posterior maxillary displacement.
On the X-axis, CDO showed a slight anterior segment displacement from –0.0087 to –0.0036 mm (Figure 3C). In contrast, ADO exhibited a wider range of anterior segment displacement from –0.0059 mm to 0.2327 mm, with all landmarks shifting toward the non-cleft side. The maximum displacement was observed at the ANS point and gradually decreased from anterior to posterior and superior to inferior (Figure 3D).
On the Y-axis, CDO resulted in displacement of 0.2688–0.4179 mm of the anterior segment, with the first premolars and prosthion showing the most movement, decreasing from inferior to superior. This led to an increase in the dental arch length of 0.4040 mm on the cleft side and 0.4167 mm on the non-cleft side (Figure 3E). ADO displayed a range of 0.1922–0.5445 mm, with the cleft side’s first premolar alveolar bone peaking, which gradually decreased from the cleft side to the non-cleft side and from inferior to superior, leading to an arch length increase of 0.5492 mm on the cleft side and 0.2666 mm on the non-cleft side. The displacement on the cleft side was greater than that on the non-cleft side, following the same pattern as that of the rotational direction of the distractor (Figure 3F).
The Z-axis revealed the maximum upward displacement in the ANS for both methods, decreasing posteriorly and transitioning to downward displacement at the end of the bone segment. By combining the displacements on the Y-axis, the bone segment exhibited counterclockwise rotation in the sagittal direction. The ADO exhibited lower vertical displacement than the CDO (Figure 3G and 3H).
Displacement of the teeth
The three-dimensional displacements of the central incisors, premolars, and molars are presented in Table 2. For the central incisors and premolars, crown displacements exceeded alveolar bone movements, whereas root displacements were smaller, indicating labial tipping during the distraction process. In the CDO group, the non-cleft side displacements were greater, peaking at the central incisor (Figure 4A). Conversely, the ADO showed greater displacement on the cleft side, with the premolars experiencing the maximum movement (Figure 4B). The molar crowns and roots were displaced more than the alveolar bone, with the posterior maxillary segment displacement occurring mainly due to dental effects.
The displacement trends of the central incisors and premolars in all three axes mirrored those of the anterior maxillary segment. On the X-axis, CDO showed minimal displacement toward the cleft side, while ADO showed a broader range toward the non-cleft side, with the maximum displacement at the central incisor's root apex on the cleft side (Figure 4C and 4D). On the Y-axis, both models showed forward movement with crown displacement exceeding root displacement, indicating tipping movement (Figure 4E and 4F). On the Z-axis, except for the central incisors in the CDO group, all teeth showed extrusion. The displacement of the molars in both models was similar, exhibiting extrusion and distal movements (Figure 4G and 4H).
Stress distribution
Stress distribution was similar for both distraction methods, with higher stress on the cleft side. In the facial skeleton, stress is concentrated in the alveolar bone of the anchorage teeth, particularly along the zygomatic buttress. The maximum stress on the lingual distal alveolar bone on the cleft side of the anchoring molar was 24.286 and 24.783 MPa for CDO and ADO, respectively (Figure 5A and 5B). In the anterior maxillary segment, the maximum stress occurred on the lingual side of the mesial alveolar bone of the anchoring premolar on the cleft side (21.124 MPa in CDO and 22.782 MPa in ADO; Figure 5A and 5B). The stress in the maxillary dentition was mainly concentrated in the molar root bifurcation area, peaking at 69.895 MPa in CDO and 73.987 MPa in ADO on the distobuccal root of the molars on the cleft side (Figure 5C and 5D). In the anterior maxillary teeth, the stress was significantly lower, with the highest stress found in the apical region of the premolars on the cleft side: 3.266 MPa in CDO and 3.300 MPa in ADO (Figure 5C and 5D). This stress-distribution pattern is likely related to resistance during tooth distraction. High-stress areas exhibit greater resistance to movement, whereas low-stress areas exhibit less resistance. The distobuccal root of the first molar bends mesially, and the alveolar bone counteracts the tooth’s propensity for extrusion and distal movement during the distraction process, leading to significant stress generation. In contrast, the anterior segment, which is the distraction region, is less constrained, allowing for greater displacement of the teeth with the bone segment, resulting in a lower stress distribution.
DISCUSSION
Patients with UCLP typically exhibit maxillary retrusion and asymmetrical growth with alveolar clefts potentially causing anterior segment collapse and dental crowding.1-4 AMSDO is a less invasive method that alleviates dental crowding by creating extra space and moving the anterior maxillary segment forward. Research indicates that AMSDO can lead to significant improvements in midfacial skeletal and soft tissues, surpassing traditional orthognathic surgery in terms of postoperative stability.16,30 In comparison with total maxillary DO, AMSDO is less invasive, provides an additional dental arch area, offers aesthetic advantages in comparison with large extraoral devices, and causes less impairment to the functioning of the soft palate.16,25 Nonetheless, AMSDO also carries the risks of postoperative bleeding, transient paresthesia, and anterior open bite with rotation of the palatal plane.16 Moreover, this method requires substantial tooth movement after surgery, necessitating a detailed treatment plan and extensive orthodontic treatment throughout the entire process.30,31 The current understanding of AMSDO remains limited, and caution is advised in interpreting its research findings, especially for patients with UCLP, where biomechanical responses may vary due to palatal discontinuity and surrounding anatomical deformities.16,23 This study aimed to evaluate and compare the stress distribution and displacement of the maxilla and teeth in AMSDO patients using CDO and ADO, factoring in perioral forces and scarring for clinical relevance.
Previous studies positively evaluated the effect of AMSDO on the forward movement of the maxillary segment. Zhang et al.32 used a tooth-borne appliance and achieved a mean ANS advancement of 5.56 mm. Qian et al.27 used a bone-borne appliance and achieved forward movements of 3.98 mm and 4.66 mm for the ANS and point A, respectively. In this study, CDO was performed using a tooth-borne appliance, simulating a distraction of 0.5 mm, resulting in a forward movement of 0.3450 mm for point A. Three-dimensional measurements of the ANS, point A, and prosthion showed counterclockwise rotation of the anterior maxillary segment during CDO. This occurred because the force generated by the distractor was lower than that of the resistance center of the anterior maxillary segment.26,33,34 Some researchers have proposed using intermaxillary elastics after distraction to improve unnecessary rotation.30,32 This study found that the vertical displacement in ADO was less than that in CDO, likely because the horizontal rotational change in ADO altered the rotational center of the anterior maxillary segment, resulting in fewer vertical side effects in comparison with those observed with CDO.
The ratio of the displacement of point A to the incisal edge in the sagittal plane has been used as a standard to evaluate the bony effect, with values ranging from 62% to 70% for tooth-borne appliances and 86% for bone-borne appliances.26,27,34 The bony effect in this study was 74%, and this type of dental effect can be well-controlled during postoperative orthodontic treatment.32 Although implant anchorage boosts this bony effect, it also increases the risks of infection and bleeding. Additionally, in patients with CLP, underdeveloped maxillary and alveolar discontinuities, which are crucial factors influencing the choice of the anchorage method, may restrict the implant space in the palate.15
In individuals with UCLP, inadequate arch length results in anterior dental crowding, which has been historically managed with tooth extraction for alignment.3,4 Meanwhile, these patients often have dental anomalies, congenital tooth agenesis, and impacted teeth requiring space expansion for future implantation and restorative treatments.35 In this study, the arch length increased by an average of 0.4108 mm in the sagittal direction after CDO, with a ratio of 82% of the total distraction. This ratio matches that reported by Ho et al.,26 who used a tooth-borne device. Therefore, AMSDO can expand the arch length through osseous formation at the osteotomy site, thereby potentially decreasing the need for tooth extraction.16,36
In this study, stress distribution during distraction showed higher values on the cleft side, which may be related to the presence of greater perioral forces on the scarred cleft side, suggesting that achieving more forward movement on the cleft side may be more beneficial for correcting the patient’s deformity. The limitations of CDO in altering the direction of distraction may necessitate additional postoperative orthodontic adjustments for asymmetrical development.17 Some researchers have attempted to achieve rotation of the anterior segment by placing distractors on the buccal side of both sides of the osteotomy line at different distraction lengths.18 However, buccal devices have drawbacks, such as difficulty in parallel placement on both sides, inadequate vestibular space leading to discomfort in the buccal soft tissues, and the need for secondary surgery for device removal.37-39
In this study, an asymmetric distractor was placed on the palatal side to achieve rotation of the anterior segment. The results showed that during CDO, the bone segment moved slightly toward the cleft side, potentially exacerbating maxillary asymmetry and complicating subsequent treatment, whereas ADO effectively moved the anterior maxillary segment toward the non-cleft side, with the ANS point shifting by 0.2362 mm on the X-axis in that direction. Assuming the anterior maxilla can achieve the distraction and rotation angles derived from FEA over a 14-day distraction period, the final ANS point could achieve approximately 4.35 mm of forward movement and 3.31 mm of lateral movement toward the non-cleft side for the ADO, which is advantageous for correcting maxillary insufficiency and midline deviation. Dental arch length expansion on the cleft side was also more pronounced in ADO, with a 0.5492-mm increase in comparison with a 0.2666-mm increase on the non-cleft side. Over a 14-day distraction period, the dental arch of the cleft side is expected to expand by approximately 7.69 mm, which is markedly more than the expansion of 3.73 mm on the non-cleft side. Since bone collapse and crowding often occur on the cleft side in patients with UCLP, greater bone growth on this side facilitates the alleviation of dental crowding and correction of abnormal inter-arch relationships.
Our research elucidated the role of biomechanics in AMSDO, with particular emphasis on the application of ADO in treating patients with UCLP. Notably, this method shows some degree of extrapolation potential. Patients with UCLP represent a subset of individuals with asymmetric maxillary hypoplasia, and the experimental results of this study suggest that efficacious treatment modalities for these patients may also benefit patients without cleft conditions who present with similar skeletal developmental patterns. Nevertheless, this inference is being made with caution, acknowledging the need for further research to validate the generalizability of these findings to non-cleft populations that exhibit analogous skeletal discrepancies.
Although this study modeled craniofacial structures and considered the influence of perioral forces and scar tissue to simulate anatomical structures as realistically as possible, it still had certain limitations. First, all structures constructed in this study were assumed to be homogeneous and isotropic, which is not the case for real anatomical structures.40,41 Second, this study only selected a typical case for model development. Because of variations in dental arch conditions and the extent of maxillary bone defects among patients, the experimental results of this study may not represent all clinical scenarios. Importantly, in specific cases, such as those with narrow maxillary arches or severe crowding, presurgical orthodontic treatment is essential.31 In some cases, to accommodate the osteotomy space, premolar extraction may be necessary, potentially necessitating prosthodontic implants. Although presurgical orthodontic treatment was not performed in the case selected for this study because of the favorable anatomical conditions, future studies must consider the impact of presurgical orthodontic treatment to provide a more comprehensive assessment of surgical design efficacy. Therefore, this study provides a reference for the biomechanical mechanisms of applying short-term displacement loads under ideal conditions; future comparisons with clinical cases are necessary to validate the results of this study.
CONCLUSIONS
The anterior maxillary segment showed forward movement postoperatively. Both distraction methods achieved maxillary arch length expansion through osseous formation and minor dental effects, thereby benefiting patients with insufficient maxillary development and dental crowding. However, CDO slightly moved the anterior segment toward the cleft side, potentially worsening maxillary asymmetry and causing greater stress due to the scar tissue.
ADO effectively moves the segment toward the non-cleft side, markedly expanding the dental arch length on the cleft side and facilitating the correction of midline deviation and inter-arch abnormalities, suggesting that ADO may be beneficial in correcting maxillary asymmetrical development.
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