Comparison of different maxillary advancement protocols in patients with unilateral cleft lip and palate: a finite element analysis

Finite element analysis is a computer-aided simulation frequently used especially in engineering and health fields [17]. With these simulations, an infinite number of variables are transformed into a predictable finite number of elements. In this way, the data obtained can be converted into predictable results in terms of treatment mechanics [18]. Different methods such as modelling with medical images, anatomical modelling and segmentation have been proposed to obtain the finite element model to be analyzed [17]. In the modelling method which is frequently used in medical images, the tissues such as bone and dental structures should be well-separated and their mechanical properties should be well-defined to the program; because when the effects of the force are examined in the software, the three-dimensional image will react according to the mechanical properties and boundary values [17].

In order to create the most accurate physical features of the patient, the image used in our study was created using cone beam computed tomography data of a patient with Class III skeletal malocclusion and cleft lip and palate. The properties of structures such as enamel, dentin, cementum and periodontal ligament were manually entered to the analysis, where the reference values were applied by taking the values defined in previous studies into account [19, 20].

The acrylic plate, infrazygomatic plate, face mask, menton plate and miniscrews used in our study were modelled in ANSYS Spaceclaim software based on the dimensions in the product catalogue. The acrylic plate connected with a 0.043-inch wire passing through the palatal region was applied to open the occlusion in each model. Eman et al. [21], Yang et al. [17] and Zhang et al. [22] also used bite blocks to open the occlusion in order to correct the cross-closure in the incisor region, in their studies. On the contrary, Parveen et al. [15] did not use any appliance to open the occlusion. Routinely, acrylic plates are not used in all cases where skeletal anchorage is applied. However acrylic plates can be inserted in order to control unwanted movements of the cleft fragment, such as undesirable expansion, and to better control the movement of the alveolar bone on the cleft side.

When the literature about the forces applied for maxillary protraction in skeletal Class III maxillary retrusion patients was examined, it was seen that forces ranging from 150?g to 1200?g were applied [22?24]. Generally, an average force of 500?g was applied for the methods using face masks, while an average force of 150?200?g was applied for BAMP treatments. In this study, the elastic forces were applied 500?g per side for the models with face mask therapy. In the model of the elastic use over infrazygomatic and menton plates, the elastic forces were applied 250?g per side.

Not only the amount, but also the direction of the applied force is important. Parveen et al. [15] investigated the effects of face mask application with different force vectors (+?20?, 0? and ??20?) relative to the occlusal plane, on individuals with unilateral cleft lip and palate in a finite element analysis study. According to their study, it was found that more movement was obtained in all planes in patients with a face mask applied with +?20? (inferior to the occlusal plane) compared to the other groups. Although there is no general consensus in the literature regarding the angle of force application during face mask usage, it is generally preferred that the applied force should pass 20? to 30? below the occlusal plane [22, 25]. In this study, the direction of the elastic force was applied to pass 30? below the occlusal plane.

In the present study, the highest Von Misses stress in the alveolar bone adjacent to the teeth and in the lateral walls of the alveolar processes and pterygomaxillary suture as a result of the application of force was detected with a face mask over the acrylic plate appliance. This result is thought to be due to the fact that some of the force applied in the traditional model is transmitted to the dentition through the acrylic appliance on the maxillary teeth. This finding coincides with the results of previous studies that showed labial inclination of maxillary incisors following face mask treatment [26]. In the maxillary protraction model with a face mask over the infrazygomatic plates, the maximum stress occured in the lateral area of the zygomatic process of the maxillary bone, in the infrazygomatic region and in the pterygomaxillary suture. The stress, which were especially concentrated in the area where the plates were applied, gradually decreased while showing a peripheral distribution. In protraction models applied directly over the infrazygomatic crest, a slight tensile force was observed in the alveolar bone. Since the force was applied directly to the bone region in skeletally supported maxillary protraction models, it is an expected result that the stresses were concentrated in the zygomatic process region of the maxillary bone.

When the displacement values were analyzed, the highest displacement in the sagittal direction occurred in the part of the maxilla where the plates were applied and in the pterygomaxillary suture region in the skeletally supported protraction models, while in the maxillary protraction model with a face mask over acrylic plate the highest displacement was observed in the alveolar bone and surrounding structures surrounding the maxillary posterior teeth. Consistent with our findings, in an overview of systematic reviews [27], it was declared that a better anterior movement of maxilla can be expected with bone-anchored devices, in comparison with dental anchored devices during orthopaedic treatment of maxillary retrusion.

When the amount of displacement in the sagittal direction was compared, it was observed that the skeletal models showed an anterior displacement especially in the maxillary basal bone, while the maxillary protraction model with a face mask over acrylic plate showed an anterior displacement in the dentition. When the amount of displacement was evaluated, it was observed that there was more change in the maxillary protraction model with a face mask over infrazygomatic plates than in the maxillary protraction model with a face mask over an acrylic plate. The least amount of displacement was occurred in the model with menton plate compared to the other models. This is thought to be due to the fact that the angle of the application of force by the elastic is different from the other two models. Kamath et al. [24] investigated maxillary protraction by using a face mask over the infrazygomatic crest, and elastic from the infrazygomatic crest to the menton plate in their bone-assisted maxillary protraction study. They concluded that the most sagittal movement of the maxilla was found in the face mask group over the infrazygomatic crest. In this respect, our findings are in agreement with the findings of Kamath et al. [24].

In the present study, the data of a patient who had not undergone secondary alveolar bone grafting was used. In some studies [14, 22], face mask therapy performed after an alveolar bone graft produced more anterior maxillary migration and less pronounced mandibular clockwise rotation than those in the ungrafted group. It was concluded that the alveolar bone graft seemed to play a crucial role to distribute the stress more evenly between the segments. In the present study, more pronounced movement of cleft side supports the importance of secondary alveolar bone grafting on distribution of stress.

In the vertical direction (Z-axis), the maximum displacement was observed as the movement of the premaxillary region to the superior direction in the model with maxillary protraction with a face mask over infrazygomatic plates. The least movement in the vertical direction was observed in the maxillary protraction model protocol involving the use of elastic over infrazygomatic plates and menton plate. In the study conducted by Kim et al. [28], maxillary protraction model with miniplates resulted in superior movement of the maxilla, comparable with our findings. Zhang et al. [22] performed maxillary protraction with a face mask in patients with cleft lip and palate, and showed a superior movement in the anterior part of the maxilla, comparable with our study.

Although skeletally assisted maxillary protraction therapy with a face mask cannot completely prevent palatal rotation, it uses a downward and forward force. Jahanbin et al. [29] reported that a downward angle of more than 30? is needed to prevent counterclockwise rotation of the palatal plane in patients with cleft lip and palate. They stated that when intermaxillary elastics are used in the treatment of BAMP, counterclockwise rotation occurs with class III elastics. Yan et al. [30] evaluated the three-dimensional movement of the craniomaxillary complex during maxillary protraction with bone anchorage and dental anchorage by finite element analysis. In their study, the craniomaxillary complex in the dental anchorage model was displaced anteriorly with a counterclockwise rotation, and the degree of rotation gradually decreased as the angle between the force vector and the occlusal plane increased between 0? and 30?. Also, in the bone anchorage model, the craniomaxillary complex was displaced anteriorly with counterclockwise rotation and the degree of rotation gradually decreased with increasing angle from 0? to 20?. However the nasomaxillary complex rotated posteriorly with clockwise rotation when a 30? force vector was used. Similarly, in our study, there were differences in terms of elastic application between the models using face masks and the model using intermaxillary elastics. While an angle of 30 degrees was applied in the models using a face mask, an angle of 40 degrees was applied in the model where elastic was applied through miniplates. These differences affected the movements occurring in the maxilla and the degree of rotation in the maxilla. According to our findings, there was a clockwise rotation in the mandible in the models using the face mask, while there was a counterclockwise rotation in the model using elastic over miniplates, comparable with the literature [24, 29, 30]. Therefore, in cleft patients with skeletal Class III malocclusion and maxillary retrusion, lower anterior facial height can be controlled by changing the direction of the applied force, regardless of which anchorage is used.

The compressive stresses on the frontonasal, frontomaxillar and nasomaxillar sutures seem to occur as a result of the tendency for counterclockwise rotation of the nasomaxillary complex.

While the face mask over acrylic plate showed mostly dentoalveolar effect, orthopaedic effect was observed more in skeletal supported models. Especially in the zygomaticomaxillary, zygomaticotemporal and zygomaticofrontal sutures in the midface region, the stress distribution was more intense than in the tooth-supported model, and an anterior displacement was also observed. Therefore, better aesthetic results can be obtained in the mid-face region with skeletally supported models. These findings in our study are comparable with the study of Yang et al. [14].

When the displacement values in the maxillary dentition were analyzed, the findings of our study were associated with more skeletal movement in the skeletally supported maxillary advancement models and more dental movement in the acrylic plate over face mask model, comparable with the findings of Kamath et al. [24].

When the changes occurring in miniscrews with infrazygomatic plates and menton plate were analyzed, it was shown that excessive force accumulation may occur in the fixation screws used in the fixation of mini plates, especially in the screw closest to the arm in which force was applied, this may cause loss of the fixation screw. Another important consideration is the number of fixation screws used. In the study by De Clerk et al. [31], it was emphasised that the number of screws applied to the maxilla should be at least three, and three fixation screws were used in this study. If two fixation screws are used, the distribution of the force between the screws will be more critical and this will affect the effectiveness of the treatment. Additionally, it was observed that the miniplates and screws on the cleft side were subjected to more stress. This finding indicates that the risk of loss of mini screws and mini plates in that area is higher. This finding is in agreement with Jahanbin et al. [29] and Kamath et al. [24]. Kamath et al. [24] stated that the success rate could be increased with minimally invasive surgery, full compliance with postoperative instructions and good follow-up by the orthodontist.

In the light of the findings in our study, it is seen that high compression stresses occur in the glenoid fossa in the models including face mask, while less stress occurs in the maxillary protraction model including the use of elastic over infrazygomatic plates and menton plate. Stress created in the temporamandibular joint area in the early period may trigger joint disorders in the future. Mathew et al. [32] showed that the stress in the glenoid fossa decreases with the increase in the angle of the force applied for maxillary protraction with the occlusal plane in their finite element stress analysis study, which supports the findings of our study. Clinically, BAMP treatment can be preferred over face mask treatment, in patients who are prone to temporomandibular disorders.

There are some limitations of the study. Although the results obtained in this study provide information about the initial stress distribution and displacement pattern in the maxillary protraction of individuals with cleft lip and palate, the actual treatment outcome may be different because the soft tissue and post-surgical scar tissue in the lip and palate were not taken into account during modelling. Therefore, further studies are needed to investigate the effects of soft tissue and postoperative scar tissue on maxillary protraction in patients with cleft lip and palate. Furthermore, a cleft model with various cleft dimensions and maxillary complex ratio in sagittal/vertical dimensions need to be developed to generalize the results in different cleft types.

In the present study, maxillary expansion was not used in none of the maxillary advancement protocols to evaluate the effects of maxillary protraction methods, solely. However, usually maxillary expansion is the first stage before maxillary advancement protocols. In the present study, it was shown that both cleft and non-cleft sides received either expansive or contractive forces regarding the chosen maxillary advancement protocol. For example as both cleft and non-cleft sides received expansive forces during traditional face mask treatment, expanding maxilla before maxillary protraction may cause overexpansion of maxilla in patients without secondary alveolar bone graft. On the other hand, BAMP protocol resulted in narrowing of both cleft and non-cleft sides, therefore expansion can be suggested in all patients to overcome the constriction. To understand the effects of maxillary expansion further, future studies can be conducted regarding maxillary protraction methods including maxillary expansion.