Evaluation of articular disc loading in the temporomandibular joints after prosthetic and pharmacological treatment in model studies

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Temporomandibular joint dysfunctions, according to WHO reports, are the third of dental diseases in prevalence following dental caries and periodontal diseases. 1he type of these disorders is determined by genetic, individual, environmental and psycho-emotional factors.It has been observed that an increasing level of stress leads to growth in harmful movement habits within the stomatognathic system, and a rapid increase in muscle tension adversely affects the functioning of the temporomandibular joints.[4][5][6][7][8][9][10][11][12][13][14][15] The objectives of the model tests, using numeric calculations, was to assess the articular disc loads in the temporomandibular joints after prosthetic and pharmacological treatment of functional disorders of the masticatory organs.
The study involved 10 patients, aged 21-48 years, of both sexes, with full dental arches, randomly selected from a group of 120 patients treated with relaxation occlusal splints (60 patients -group I) and intramuscular injection of botulinum toxin type A (60 patients -group II).The group of 10 representative patients were created from 5 patients taken from group I and 5 from group II.
In all the patients, a specialized functional diagnostic procedure showed the presence of temporomandibular joint dysfunction with the dominant muscle component.The effects of excessive tension of the masticatory muscles were observed as symptoms in teeth and periodontium as well.In the representative group of 10 patients, there were no painful forms of dysfunction.The treatment was performed in the Prosthodontics Clinic of the Jagiellonian University Medical College in Kraków.
In all subjects, a specialized functional examination was carried out, according to the application form developed in the Consulting Room of Temporomandibular Joint Dysfunction of the Prosthodontics Department of Jagiellonian University.The form is based on the typical routine requirements of a functional evaluation of the stomatognathic system, such as: a rating of the mandibular range of movement, symmetry of mandible motion, deviation of opening path, occlusion condition, masticatory muscle sensitivity during palpation, pain of their attachments, and the presence of acoustic and pain symptoms in temporomandibular joints occurring during mandibular movement and palpation examination.Additional examination was concerned with an evaluation of pain intensity and imaging diagnosis. 1,2The patients were qualified for 2 prosthetic treatment groups: occlusal splint therapy (group I) and intramuscular injection of botulinum toxin type A with a total amount of 21 MU (mouse units) for each muscle (group II).The injections were done at the 3 points of vertices of a triangle (7 units in each point) along the largest cross-sectional of the muscle, 1 time, using a 40 U insulin syringe.The pharmacological activity of the toxin lasts about 12 weeks. 1,6he functional examination of the masticatory organ, masseter muscles and temporomandibular joint pain intensity and masseter muscle electrical activity were evaluated 4 times: before the prosthetic treatment began, after 10 days of the initiation of the 2 different treatment methods (splint and intramuscular injections) and after 14 and 22 weeks from the second examination.
An assessment of the loads of 4 disc zones of the temporomandibular joints was carried out based on the results of clinical studies (phase I of the project), and numeric model tests (phase II). 1,17,18In the representatives of the study groups (5 patients in each group), measurements of occlusal forces were performed using the method developed by professor W. Chladek as well as an evaluation of the masseter muscle tension and the anterior temporal muscles using a Bio Research EMG I and surface electrodes. 10,17he maximum occlusal forces were evaluated around the central incisors and the first molars of the left and right side, using a special measuring device -developed for the needs of dental research in the Department of Mechanical Technology and Forming of Silesian Technical University in Katowice (patent no.P 334933).In order to calculate the maximum values of occlusal forces using the above method, the Mayer's formula was applied: where: c -a material constant; d -diameter of dents in the aluminum sample; n -Mayer's coefficient -1.706.
The constant "c" and coefficient "n" appearing in the formula were specified for the series of aluminum samples applied in the research by measuring the diameters of the indentations formed during the calibration of the instrument in the universal testing machine.This formula determines the correlation of the resulting indentation size to pressure-gradient force penetrator, and hence the tooth pressure force on the measuring device.To measure the diameter of the dents in the disposable aluminum plates, a stereoscopic measuring microscope was used. 10,11,18ince the direct measurement of loads in the temporomandibular joints in a living human is not possible, a computer model of the mandible, projecting masticatory organ functions, was constructed, which uses the following model assumptions: 1.For numeric calculations, the mandible was considered a non-deformable body, since the intended purpose of the study is not to evaluate the size of deformations of the mandible.
2. The studies assumed the average orientations of muscle forces, due to the fact that the muscle attachments are located in particular areas of the jaw or skull bones and can be "reduced" to the points in the model study, because the results of the clinical studies carried out on various types of face -long and short -have shown the convergence of the lines of action of the muscle force vectors in the case of both types of face.
3. The loads occurring in the temporomandibular joints have been identified on the basis of a spatial model of the human masticatory system according to Margielewicz's concept.
4. Forces exerted on the temporomandibular joints were estimated based on the load acting on the articular discs.In the numeric calculations the replacement stiffness of discs was taken into account as the actual nonlinear displacement-force characteristics make it difficult to perform numeric calculations.
5. The resilient properties of the muscles were projected using mechanical two-terminals, because the data characterizing the volumes of the cross-section of the muscle, its rest length, and the unit capacity are available.
Analytical dependency mapping replacement stiffness of the masticatory muscle fiber groups was calculated according to the following formula: where: k -the coefficient of unit performance of the cross section of the muscle equals 40 [N/cm 2 ]; A -the maximum cross-section of the muscle; L -the rest length of the muscle. 16,17rom the point of view of biomechanics, one of the most important pieces of information about the masticatory organ muscles is the maximum forces generated by individual masseter muscles.They refer to the largest cross-sections of the muscles, therefore an important element for a model study of the stomatognathic system is the so-called coefficient of the unit performance of the cross section of the muscle.The value of this coefficient, according to Chladek, is 40-100 N/cm. 2,10roportions occurring between the maximum (limit) muscle forces and their bioelectric activity were calculated according to the formula: where: EMG MP -bioelectric activity of the surface fibers of the masticatory organ, EMG MG -bioelectric activity of the medial pterygoid, F MP -the maximum force generated by the surface fibers of the masticatory organ, F MG -the maximum force generated by the medial pterygoid.
For the purposes of this study, the results of maximum occlusal forces, the electrical potential values of masseter and temporal muscles were discussed as well as the model tests following the prosthetic treatment carried out with the use of the two compared methods.
In the case of the clinical measures giving constant results, statistical analysis was based on the traditional methods of calculation: mean values, standard deviation, minimal values, maximal values, standard error of the mean, variance analysis for dependent variables and posthoc Tukey test for dependent variables being the statistical significance measure.
To compare the dependencies between the clinical results obtained in consecutive clinical tests, the non-parametric Friedman test, Kendall's W and Wilcoxon signedrank test (comparing 2 related samples) were used.For the statistical studies, special STATISTICA 2010 computer software was used.
Only a few of the clinical studies were used in numeric model tests.The results of the disc loads in the temporomandibular joints in numeric model tests on the representatives of the study groups were statistically analyzed using the parametric Student's t-test and it was assumed that the results were statistically significant if p ≤ 0.05.

Results
In the numeric model test, projecting the stomatognathic system activity, carried out on the representatives of the 2 groups, a significant difference in articular disc loads was observed between patients undergoing the splint therapy and botulinum toxin type A in favor of the second group.
The results of the clinical research of maximal bite forces (5 patients in group I and II) are presented in Fig. 1 and they were the basis for the work on the numeric tests.This was necessary for identification of the loads within the temporomandibular joints.Table 1 shows the results of the electromyographic measurements obtained in patients randomly selected from both groups evaluated for model tests in order to calculate the loads of the temporomandibular joints after the therapy using occlusal splints and after the application of botulinum toxin type A. The average values of the electrical potentials obtained during the clinical study of the right and left masseter and temporalis muscles were important to create the numerical model of a biocybernetic model of the masticatory system.The ranges of these values were 15.71-96.11μV for both types of muscles.In this individual study, the numeric values of the replacement coefficients of articular disc zone rigidity were applied, calculated by Chladek et al. 10,11 The average values of loads (evaluated in newtons) acting on the individual disc zones of the left and right temporomandibular joint are summarized in Tables 2  and 3.The results of the average load values for all the evaluated zones of the right and left articular disc differ in a statistically significant way in favor of group II, with the exception of the external mid part of the discs.In the case of the anterior of the right disc, the load was lower in patients belonging to group I than in those obtained in group II.
It ensures proper distribution of synovial fluid and blood supply, which is necessary within the articular surface. 1,3,4In the course of functional disorders, the masticatory system comes to excessive and highly damaging overloads of the joints and intra-articular discs, which is caused by a significant increase in masseter muscle tension. 1,3,7,10,12,13,18,19cclusal splints, pharmacological treatment in the form of intramuscular injection and muscle exercises are methods that have been shown to have a positive effect on the state of masticatory muscle relaxation, which can be confirmed by objective testing instruments such as electromyography and occlusal evaluation using a computer program, and T-Scan device.Botulinum toxin is produced by Gram-positive Clostridium bacteria.In their natural environment, these bacteria form spores that release the neurotoxin (the strongest known to man) when they germinate.There are 6 serotypes of the toxin: A, B, C, D, E, F, and G. Botulinum toxin type A was first isolated in 1920s.Its therapeutic potential was put to the test for the first time then.All 7 serotypes are large proteins that act on cholinergic neuromuscular junctions to block transmission of synaptic vesicles. 1,15Edward Schantz and Alan Scott were pioneers of the therapeutic applications of the botulinum toxin used from the late 1970s to manage strabismus and hemifacial spasms at first and then eyelid spasm, uterine atony, and laryngological, nephrological, surgical and neurological therapies as well as aesthetic medicine.The medication is currently registered in approx.120 countries. 15n assessment of disc loading within the temporomandibular joints of patients is not possible.][18][19][20][21][22][23] Despite the rapid growth in model testing and biotechnological studies, only a few reports in the literature assess the loads within the temporomandibular joints.The assessment of the relationship between bite forces and occlusion, and the activity of muscle and loads acting on the temporomandibular joints is a difficult issue from a biomechanics perspective. 17,18esults of the research available in the literature are mostly related to the evaluation of mechanical strain or anatomical changes in the properties of porcine discs, which are similar in structure to human discs.Experimental studies conducted on the intra-articular tissues derived from animals (cattle and pigs) for an assessment of the resistance to crushing or tensile strength have provided valuable information about the mechanical proper-

Discussion
The main aim of the treatment of temporomandibular joint dysfunction is the remission of pain of the muscles and joints and restoration of the physiological norm for proper muscle tension and load of the joint.For the proper functioning of the stomatognathic system, it is very important to provide physiological stress on joints.

Fig. 1 .
Fig. 1.Average values of the occlusal forces measured in groups' representatives following the treatment

Table 1 .
Average values of electrical potentials obtained during clinical studies in order to create the numeric model in microvolts[µV]

Table 2 .
Average values of the forces acting on the right articular disc in [N] and statistical significance denoted by letter p

Table 3 .
Average values of the forces acting on the left articular disc in [N] and statistical significance denoted by letter p