Stress and displacement patterns during orthodontic intervention in the maxilla of patients with cleft palate analyzed by finite element analysis: a systematic review

Cleft lip and palate (CLP) is known to be the most common among other congenital disorders in the craniofacial region, with an estimated incidence of approximately 1: 1000 live births [1]. Usually, the medical staff involved in the treatment process is organized into a cleft team consisting of many specialists. Orthodontic treatment in patients with cleft palate involves various types of therapies/applications, can begin nearly after birth with naso-alveolar moulding (NAM) and can be performed until the end of growth (potential orthognathic surgery procedures) [2, 3]. The potential possibility of accurate simulation of the movement of the clefted parts of the maxilla with the use of different orthodontic appliances might be an important point in treatment planning.

One of the methods that can predict the rehabilitation carried out is the finite element method (FEM) [4]. Finite element analysis (FEA) is a modern computer simulation method that has found application in orthodontics in recent years as a simulation method of the force applied to bone / teeth and prediction of its displacement [5, 6]. In FEA analysis, based on patient CT scans, virtual models of anatomical structures are created. The most significant is to evaluate the characteristics of tissues (cartilage, bone, soft tissues, teeth) [5]. In the FEA approach, a single 3D model undergoes simulation to point out elements of mechanics in orthodontic treatment, particular distribution of stress in TADS and archwires, distribution of strain and stress in biological tissues (bones, teeth, PDL), conditions of resorption of roots, directions and the size of displacement for positioning of orthodontic appliance [7]. In FEA an important issue concerns individual variations and response to the applied forces, e.g. different rate, amount and directions of canine distal movement.

The approach in FEA orthodontic studies includes mostly biomechanics of the skull, assessment of (bio)mechanical loads of the skull, and mechanism of tooth movement. There are different aspects of using FEA in dentistry, for example, data on displacement, tension during chewing, distribution of mechanical forces on craniofacial forces [8]. FEA makes it possible to model and reproduce the anatomy of the craniofacial skeleton that helps to understand the parameters that influence bone remodelling [9].

FEA aims to simulate the conditions under which analysis would not be possible by clinical means. However, there are some disadvantages of this technique: complexity of craniofacial structures and their relationship, different information of tissue characteristics, limitation of anatomic details in reproduced models.

Geometric methods of analysis have limitations due to the nonregular and complex structure and shape of human bone. 3-D FEA enables investigating, e.g. reconstruction of alveolar bone and modifications of soft tissue making it useful and the errors of measurements can be reduced. However, it is important to link the model, prototype, and clinical treatment implemented.

The era of modern orthodontics began when Edward H. Angle published a book in 1900 entitled "Treatment of Malocclusion of the Teeth and Fractures of the Maxillae: Angle's System" creating the foundations for modern diagnostics and orthodontic treatment [20]. He is considered the father of orthodontics. Today, innovative technologies and intelligent materials have become part of the tools used by the orthodontist: digital technologies, transparent wires and brackets, aligners, robots, and computer modelling.

The driving force seems to be also Artificial Intelligence (AI) that is used in diagnostics, planning, and monitoring of treatment [21]. Digital techniques such as intraoral scanning are becoming increasingly important in orthodontic treatment. Currently, works are underway on a comprehensive digitization of the orthodontic treatment process in terms of time, cost and patient comfort. To shorten and simplify the treatment process, comprehensive practice management software (PMS) systems have been developed to plan appointments, visibility of patient information, and optimized patient communication. Digital methods allow for more innovative, patient-friendly, and time-saving treatment and are based on the direct-to-consumer business model. Although everything has recently moved in the direction of virtuality, clinical examination remains an indispensable element of therapy [22]. Therefore, it is anticipated that traditional orthodontic treatment will continue to be practiced.

Due to the use of FEM in orthodontics, any material structure (wires, brackets, rings) or maxillofacial structures (bone structures, ligaments) can be analyzed. The main assumption of FEM is the division of a larger (complex) structure into smaller sections with strictly defined physical properties (ligament, different types of bones, enamel, dentine). In this way, the response of the entire structure to the applied force is generated, e.g. orthodontic force [23].

FEM is a theoretical technique useful in the analysis of the biomechanics of maxillary protraction in UCLP patients. The maxillary hypoplasia is 3D and involves sagittal, vertical, and transverse directions with the aim of combining maxillary protraction with maxillary expansion. The comparison of the biomechanics of maxillary protraction with/without maxillary expansion was investigated. There is not much information about the biomechanical response of the maxillary complex if loaded with the protraction force of UCLP on the cleft side that was greater than that on the non-cleft side. Consequently, the existing fissure of the dental arch is enlarged (14). There are only few reports on the biomechanical effect of maxillary protraction on the craniofacial skeleton in patients with cleft and it has not been well explained clinically and experimentally using finite element methods (FEM). The basic mechanism of finite element analysis (FEA) in patients with UCLP is still unknown. There is only some information about maxillary protraction in patients with UCLP [14]. In FEA analyzes, the exceptional properties of craniofacial sutures cannot be expressed, since it was assumed that all craniofacial synchondrosis has the same histological and mechanical characteristics as the surrounding bones (19). Two methods of FEM creation are distinguished: 1) tissue-sectioning technique—generates very thin slices, destructive technique, 2) spiral CT—that enforces less scan interval, non-destructive. A skull sample for a cleft palate is usually inaccessible. The computed tomography technique was used to obtain data from a patient with UCLP (19). Because the original image of computed tomography was not enough to obtain a clear skeleton border for the generated FEM, use the window technique to obtain more readable CT images. Data from CT scans were found to be more reliable in the preparation of a digital image for FEM than from the sectioning method [19]. In the model of a cleft lip and palate, the FEA shows biomechanical characteristics of rapid maxillary expansion. It is possible to determine differences in tissue response in patients versus healthy individuals [17].

The results of some studies implied that a patient with unilateral cleft would be expected to have an asymmetric skeletal development between the non-cleft and cleft sides as a consequence of an asymmetric functional loading pattern. Pan et al. [19] investigated physiological changes and the distribution of orthopaedic force stress on the craniofacial structures of the first premolar maxillary and the crown of the first molar. Several clinical implications arise from the conducted research. It seems that asymmetric and nonuniform stress and strain distribution comparing cleft and non-clefted side [10, 18, 19, 23] and non-clefted side with higher stress and strain level [1]. The RPE procedure in UCLP patients reveals a pyramid-shaped displacement of the nasomaxillary complex along with a fan-shaped expansion of the upper dental arch [3]. The model of the patient with UCLP after ABG revealed that the best effect was obtained after loading the maxillary protraction force [14] [15]. Rapid palatal expansion forces are transmitted along three vertical buttresses [11]. In the craniofacial complex with UCLP, missing mid-palate and deformity of maxillary bone, the resistance to maxillary expansion mainly came from the connection between the maxilla and the pterygoid plates of the sphenoid bone [17]. However, when using FEM to assess the displacement/response of specific maxillofacial structures to an applied (orthodontic) force, many additional aspects must be taken into account. In the case of reconstruction (creation of a model) of the jaw or mandible, computed tomography is used. The computed tomography data used should have an appropriate resolution (sections of at least 0.25 mm, DICOM). In the absence of appropriate contrast and resolution, it becomes impossible to determine the boundaries of such structures: enamel, dentin, periodontal ligament. The transformation of a solid model into a model composed of a mesh of constraints and elements is the basis of FEM analysis. During the mesh refinement process, the convergence of the results with the gradual increase of bonds and elements is verified so that the voltage peaks difference between mesh refinements is 5% or less. Specialized software requires the correct representation of the mechanical properties (Young modulus and Poisson's ratio) for each mesh component [24].

Expansion therapy should be personalized according to the patient’s age, the type of cleft present (primary or secondary palate), and the desired expansion area (anterior or posterior).

The results obtained by use of FEM—are based on modeling software—therefore it is extremely important to enter all the structure data—bones, teeth, ligaments, and the applied force and boundary conditions.

Despite the limitations related to the heterogeneity of the studies included in the review, it can be concluded that finite element analysis is an appropriate tool to study and predict the points of force application for better controlled expansion in patients with UCLP.