Comparative analysis of the application of virtual and mechanical articulators in functional diagnostics

The paper presents the results of examination of patients with articulation disorders of the lower jaw caused by internal pathology of the TMJ. The purpose of the presented work: to study the effectiveness of the use of mechanical and virtual articulators in the functional diagnosis of patients with internal TMJ disorders. All patients underwent comprehensive clinical and instrumental examination including cone-beam computed tomography (CT) and axiographic examination (optical axiograph Dentograf Prosystom, Russia). CBCT was used to assess the state of the TMJ and determine the individual ratio of jaw and joint models. When axiography was recorded and analyzed articular trajectories of the lower jaw. In the first group of patients dynamic occlusion was evaluated using a mechanical articulator, in the second group a virtual articulator was used. It was revealed that the use of mechanical articulators in functional diagnostics to assess dynamic occlusion is limited and does not allow to obtain individualized patient data, their efficiency was 75%. The use of virtual articulators allows to evaluate the dynamic occlusion during opening and closing of the mouth, protrusion and laterotrusion, as well as the continuous movement of the lower jaw with the registration of all possible dental contacts. Due to the combination of CT data of the patient’s head and virtual models, the highest accuracy of placing models in the virtual articulator in accordance with the individual characteristics of patients was achieved.


Introduction
With the introduction of modern technologies into dental practice, the role of functional diagnostics at various diseases of the dental system has increased significantly [1; 2]. The development of high-precision instrumental research methods allows to identify pathological changes at early stages of their formation [3; 4]. Currently, the diagnosis of oral cavity diseases involves a complex, comprehensive, specialized examination involving the use of high-tech equipment [5; 6; 7]. Part of the research was carried out with the direct participation of a group of patients, another part -on models of the dental system with the utilization of articulators and computer technology [8; 9].
The role of functional diagnostics in dental practice increases significantly in the presence of a combined lesion of teeth, temporomandibular joint (TMJ) with mandible dysarthias and involvement of masticatory muscles in the process [10; 11]. Timely functional diagnostics in such cases make it possible not only to identify the causes and nature of the disease, but also to develop an optimal treatment plan and assess its effectiveness.
Currently, when conducting a functional study of the dental system, the following methods are used: panoramic shot; orthopantomogram; computed tomography [12; 13; 14], which enables to get 3D images of the jaws; magnetic resonance imaging (MRI), which makes it possible to assess the state of soft tissues and TMJ [15,16]; electronic axiography, providing the means of registration of articular trajectories and mandible movement trajectories [17; 18]; creation of dentition models of both jaws using articulators [19; 20]; electromyography, in which the analysis of bioelectric activity and symmetry of the masticatory muscles is carried out [21; 22]; ultrasound examination of the TMJ, visualizing its anatomical state [23].
When conducting functional diagnostics, the assessment of patient occlusion [11; 24] is of particular interest. It is conventionally accepted to differentiate dynamic and static occlusion. Dynamic occlusion is the interaction between the teeth when both jaws move [24]. It is generally accepted that with an optimal, "physiological" state of the dental system, an immediate opening or loss of contact between the teeth occurs at the mandible movement out of the usual occlusion. The occurrence of any changes, undesirable occlusal contacts, especially located close to the TMJ, leads to discoordination of the masticatory muscles, the position of the joint and its structures, the teeth, as well as their increased abrasion [24][25][26]. Studying of occlusion using articulatory foil or paper provides limited information [27]. Dynamic occlusion can be most effectively assessed using individually adjusted articulators [20].
The assessment of dynamic occlusion is of particular importance during: orthopedic treatment, as with prosthetics it is usually necessary to fully restore the masticatory function; orthodontic correction, given that its goal is to normalize occlusal relationships; during maxillofacial operations -for assessment of the TMJ functional state and to draw up a treatment plan; in implantology, it is imperative to take into account the distribution of masticatory load when planning the number, location, size and shape of dental implants; in periodontics, as excessive load on the teeth leads to periodontal disease; restoration therapy, due to the fact that it is necessary to control static and dynamic occlusal relationships [26; 28].

The purpose of this work
To assess the effectiveness of using mechanical and virtual articulators during functional diagnostics in patients with TMJ disorders.

Materials and methods
We examined 47 patients with mandible dysarthia due to internal pathology of the TMJ, of which 40 were women and 7 were men. The age of patients ranged from 23 to 39 years. Patients complained of pain in the TMJ (34 out of 47 examined; 72.3%), headaches (19 patients; 40.4%), difficulty in opening the mouth (38 patients; 80.9%). In a clinical examination, articular noises during palpation and mandible movements were found in all patients, an increased limitation in opening the mouth was noted in 45 of 47 patients (95.7%).
All patients underwent cone beam computed tomography (CBCT) for the diagnosis of TMJ. CBCT was also used to individually correlate jaw and joint models.
All patients underwent an axiographic study using an optical axiograph (Dentograf Prosystom, Russia) with registration and analysis of mandible articular trajectories.

Fig. 1 Mechanical articulator Artex CR (Girrbach)
After the two-layer silicone impressions were removed, the models were gypsum plated using a mechanical facial arc, the movement of the mandible in the mechanical articulator was reproduced using its articular mechanisms. The articulator (articular mechanisms and programmable table) was adjusted according to individual data obtained during optical axiography.
In 24 patients (51.1%) of group II, dynamic occlusion was studied using a virtual articulator with virtual models (Fig. 2).

СТОМАТОЛОГИЯ
After removing two-layer silicone impressions and casting the gypsum models, optical scanning of the jaws gypsum models was carried out in the ZIRKONZAHN software. With the help of the electronic front arc and the results of CBCT, the models were placed in the space of the virtual articulator, which made it possible to reproduce in the three-dimensional space of software environment the position of the virtual dentition relative to the hinge axis and the incisal stop. To take into account all the individual parameters during gypsum casting, an additional CT module (Prosystom) was applicable in the virtual articulator, which allowed measuring the individual distance from the maxillary incisors to the TMJ joint heads for subsequent transfer to the articulator. In this case, three main reference points were used: an interdental point in the area of the cutting edge of the maxillary central teeth and a point in the area of the articular heads of the right and left TMJ. With account of the data obtained, CT models were exported to a virtual articulator.
After that, individual trajectories that had been previously recorded using the axiograph were loaded into the program.

Results
In the 47 patients examined by us, the distribution by gender was as follows: women -40 (85.1%), men -7 (14.9%), the sex ratio was 5.7: 1. The age of patients ranged from 23 to 39 years. During the survey, 34 patients (72.3%) complained of pain in the TMJ area, 19 (40.4%) had frequent headaches, 38 patients (80.9%) noted difficulty and discomfort when opening their mouths. Of all the patients examined, 44 (93.6%) drew attention to a click or extraneous noise in the form of crepitus at mandible movements; 3 patients (6.4%) did not feel pathological noises detected during clinical examination (palpation of the TMJ area).
All patients underwent a comparative analysis of dynamic occlusion, in which the mandible movement on the teeth (closing-opening; lateral movements; moving forward with returning to the initial position) was studied. In a mechanical articulator, gypsum models of the jaws of 23 patients (48.9%) were studied, in a virtual articulator -those of 24 patients (51.1%).
In group I patients the Artex FaceBow facial arch (Girrbach) and the Artex CR (Girrbach) mechanical articulator were used. All patients of group I underwent a clinical examination of the mandible articulation using Bausch 20 micron carbon paper and occlusal wax plates OKKLUSAL INDIKATOR WACHS SAM. In all patients of group I there was analyzed jaw closing in the usual bite and the protrusive and laterotrusive mandible movement. When conducting clinical and laboratory studies using a mechanical articulator in patients of group I, their effectiveness in protrusive and laterotrusive mandible movements was of 75%.
With the mandible moving forward, there is no contact with the masticating teeth. Normally, the entire load is distributed between the temporomandibular joints and the front teeth: the main load falls on the structures of the joint, and a small load falls on the incisors.
With lateral mandible movements (laterotrusion), there is no correspondence in movements in the oral cavity and on gypsum models, since there are differences in the structure of the joint mechanisms of the patient and the mechanical articulator.
The mechanical articulator has a fixed structure of articular mechanisms that does not allow individualization of functional diagnostic data for each clinical case.
In group II patients, the electronic facial arc of the Dentograf axiograph (Prosystom) and the Artex virtual articulator in the EXOCAD program were used. A specific feature of the virtual articulator is the utilization of any patients TMJ articular trajectories, registered earlier during axiographic examinations, to accurately reproduce the TMJ function of the patient.
In group II patients, mandible protrusive and laterotrusive movements were reproduced in a virtual articulator. In clinical and laboratory studies using a virtual articulator for protrusive and laterotrusive movements, the employment of a virtual articulator was effective in 90% of cases, where the virtual model of the mandible repeated all the movements as in a patient.

Discussion
It is a general knowledge that the analysis of occlusion is carried out, either directly in the patient's oral cavity, or on its models made of super gypsum.

DENTISTRY
To simulate the movement of the jaws, special devices called articulators are used. The articulator hinge is a mechanical model of a joint. It approximately repeats the spatial orientation of the patient's TMJ, which makes it possible to reproduce the physiological movements of the joint in almost full volume, both in amplitude and in the direction of displacement [29].
Mechanical articulators are employed to reproduce the individual features of dynamic occlusion, in particular, the mandible trajectory along the contact surface of the maxillary dentition. However, the mandible movements do not coincide with the physiological movements as in the patient in every clinical case, which is due to the fixed (unchanged) structure of the articular mechanisms, i. e. they cannot be individualized for each clinical case. This increases the error in the diagnosis of patients with articulation disorders associated with TMJ dysfunctions.
According to V. Khvatova, the incorrect setting of the mechanical articulator in 95% of cases is the cause of all errors in its application [24].
When using a mechanical front arc, errors in the manufacture of models for the articulator occur quite often. Such errors are due to the fact that when casting gypsum models in a mechanical articulator, the reference is its upper frame. However, the distance from the articular mechanisms to the models does not always coincide with the individual patient data. The shift of the models upwards or downwards from the upper edge of the frame leads to the inaccuracy of these functional test data, since the distance between the model and the articular mechanism of the articulator changes [4]. This is of particular importance both for the diagnosis of internal TMJ disorders and the manufacture of therapeutic occlusal splits in these patients, as well as for prosthetics. Of great importance is the fact that mechanical facial arches are usually fixed on the patient, with account of the skin reference points of the Camper or Frankfort planes. Skin and bone reference points have divergences, which also increases the measurement error.
Technological features of work with mechanical articulators significantly affect the accuracy of reproducing the mandible movements at dynamic occlusion [25]. The main reasons for their occurrence include: the presence of gaps at fixing registration prints on a working gypsum model; gypsum expansion; deformation of casts.
The importance of anatomical features of the dentition should be taken into consideration: individual deviations in the structure of chewing muscles; resilience and elasticity of TMJ certain structures; the state of periodontal tissues, individual tooth mobility; the mandible deflection amplitude at functional loads, specific for each patient [20]. The full set of all these factors cannot be taken into account when using mechanical articulators in functional diagnostics.
The use of virtual articulators minimizes errors caused by the anatomical features of the dental system and technological reasons [30].
A special CT module (Prosystom) allows combining virtual models of the jaws and the trajectories of their movements obtained in the axiographic study with visualization in a virtual articulator for functional assessment of articulation disorders.
The utilization of a virtual articulator in the functional diagnosis of dynamic occlusion enables to perform movements that accurately simulate the real movements of the mandible. When using virtual articulator, movements of the mandible can be carried out using articular mechanisms or reproduced along the trajectories previously recorded during axiography. In this case, the upper jow is fixed in accordance with the anatomical features of the skull, and the mandible can move relative to the maxilla. Fixation of the upper jaw in a virtual articulator makes it possible to stimulate a mandible movement in the real time mode.
The employment of mechanical articulators in functional diagnostics for assessing dynamic occlusion is limited. The effectiveness of their use in the study was 75%. The application of mechanical articulators in assessing the dynamic occlusion because of the peculiarities of the articulator technical structure does not allow obtaining individualized patient data.
Dynamic occlusion in a virtual articulator in functional diagnostics can be assessed in several aspects: mouth opening -closing, protrusion, retrusion, laterotrusion to the right and left.
In a virtual articulator, it is possible to reproduce the continuous movement of the mandible with registration of all possible dental contacts of both jaws.

СТОМАТОЛОГИЯ
By combining the patient's head CT data and virtual models, the highest accuracy of placing the models in the virtual articulator in accordance with the individual characteristics of the patients was achieved.