Pure rotation in the temporomandibular joint during jaw opening? A digital motion analysis.

doi:10.21203/rs.3.rs-3913755/v1

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Pure rotation in the temporomandibular joint during jaw opening? A digital motion analysis.

https://doi.org/10.21203/rs.3.rs-3913755/v1

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published 18 Apr, 2024

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Objective: The mandibular joint is a complex joint despite its relatively small size. Two separate movements of rotation and translation occur in two articular spaces during mouth opening. Over the years, the approach has changed considerably, but it is still subject to controversy as to which movement is involved in the initial stage of mouth opening, and even whether translation is performed. In the present study, the extent of rotation and translation was investigated in the first 5 mm of mouth opening.

Materials and methods: The study was carried out on 46 volunteers. Anamnesis was taken and patients were examined. A digital motion analysis was made, with ultrasonic transducers (KaVo Arcus Digma 2). With this device, various mandibular movements can be analysed with an extremely high accuracy of 50 micrometres. Each participant performed 3 open-close movements in succession, starting from maximum intercuspal position.

Results: Data were statistically processed using cubic spline interpolation. Linear regression was then used. The resulting line is significantly (p<0.0001) different from the horizontal that represents only rotational movement.

Conclusion: The results show that during mouth opening from maximum intercuspal position, translation is all the time present in addition to rotation.

Health sciences/Health care/Dentistry/Dental anthropology

Health sciences/Health care/Dentistry/Orthodontics/Temporomandibular disorders

Health sciences/Anatomy/Oral anatomy/Mandibular muscles

Accurate knowledge of temporomandibular joint (TMJ) movements plays an important role in prosthetic dentistry. In many animal species, a simple rotational movement of the TMJ along the intercondylar axis during jaw opening is pressent. The human TMJ has more complex kinematics. If humans had only rotational movement, the anatomical structures between the mandible and the mastoid process would be compressed by the mandible, and only a reduced mouth opening could occur due to this physical barrier. This theoretical limitation probably also contributes to the fact that this human joint developed simultaneous rotating and sliding movements.¹

The TMJ is a diarthrosis, capable to rotate along multiple axes. These free movements are limited by chewing muscles, teeth contact, articular ligaments and capsule. The articular disc separates the joint into two articular compartments. Translational movements occur in the cranial discotemporal joint, while rotational movements take place in the caudally located discomandibular joint. Rotation and translation occur simultaneously during mouth opening, resulting in a complex movement.

According to the hinge axis theory, the transversal or hinge axis of the mandible is an imaginary line around which the mandible rotates in the sagittal plane at the beginning of opening and at the end of closing movements. Several methods to determine the position of this axis have been developed and described in the literature.^{2, 3, 4} This theory has gained fundamental importance for dental interventions, including prosthodontic rehabilitations. With mean-value and semi-adjustable articulators, if the vertical dimension increases or decreases within a certain limit in the articulator, i.e. in case of a small change of the OVD, we assume that the position of the condyle in the articular fossa remains unchanged.² Scientific research investigating the stomatognathic system was mostly based on pure rotational movement, as this was a widely accepted theory.^{5, 6, 7, 8, 9} The principle of purely rotational movement played an important role not only in prosthodontics, but also in orthodontics and orthognathic surgery.^{10, 11}

Prosthodontic appliances made in articulators with improperly modeled jaw movements might lead to several adverse consequences, such as occlusal disturbances and incorrect articulation tooth wear, periodontal overload, loosening of removable dentures, disturbance of the temporomandibular joint, and discomfort.¹²

In the 1960s, the range of pure rotation was determined up to a mouth opening of 15–20 mm.¹³ The extent of this range has changed along with the development of different gnathological examination procedures. According to the latest research, probably pure rotational movement does not exist.^{14, 15} Based on all of this, it can be seen that an extremely crucial question in dentistry is whether there is a purely rotational movement in the temporomandibular joint.

The present study investigated whether and to what extent there is a pure rotational movement in the temporomandibular joint during the initial phase of mouth opening. The degree of translation in relation to the degree of opening was examined in the first 5 mm of the jaw opening as the literature currently defines the range of possible pure rotational movement within this interval.^{16, 17} The null hypothesis was, that the pure rotational mouth opening does not exist.

The study was conducted with the participation of 46 volunteers. The inclusion criteria for the study were as follows: (1) good general health and absence of developmental disturbances related to the temporomandibular jaws or the teeth, (2) preserved or restored teeth (excluding third molars), (3) in restored dentition, no guiding surfaces of the restoration during articulatory movements, (4) no orthodontic treatment in the anamnesis, and (5) no history of temporomandibular disorders (TMD) and no present TMD signs and symptoms, (6) no possible and no probable awake bruxism or sleep bruxism, no parafunctions based on patient’s history and intraoral examination. No abnormal tooth guidance (e.g. hyperbalances or premature contacts). (7) straight opening pattern.

Our study has the ethical permission of the Regional, Institutional Scientific and Research Ethics Committee (No. 92/2013). The study was carried out between 2014 and 2020. All participants were informed about the study and signed an informed consent form.

A KaVo Arcus Digma 2 digital motion analyser (KaVo Gmbh, Biberach, Germany) was used for the study. This device operates by conducting measurements based on ultrasound signals and comprises two primary components. The lower unit is affixed to the lower teeth's vestibular surface using a cold polymerizing acrylate material (Structur II, Voco GmbH, Cuxhaven, Germany), connected to a paraocclusal clutch, which, in turn, links to four transmitters through magnets. Throughout the measurement process, this unit remains securely attached to the mandible. The upper unit is fastened to a facebow, with the Frankfurt horizontal plane serving as reference plane.

Ultrasound signals, emitted in the form of regular cones, are generated by the transmitters. Each transmitter corresponds to four detectors, resulting in a continuous connection comprising 16 signal pairs. This digital motion analyzer completes a measurement in each two hundredths of a second, so fifty registrations take place in each second. The measurements were carried out using the manufacturer's own software (KaVo KiD, KaVo GmbH, Biberach, Germany). Each measurement has been made by the same trained examiner. Two randomly selected patients were re-examined. The second registrations were a duplicate, which were not involved in the final statistical analysis. Weighted Kappa scores ranged 0.87–0.92 for intra-examiner reliability.

At the beginning of the meassurement, the calibration was done in "arbitrary axis" mode, i.e. to the Frankfort horizontal plane. To determine the intercondylar axis, the lateral poles of the condyles were calibrated as well. The test was performed in the motion analysis mode. In all cases, the starting position of the movement was maximum intercuspal position (MIP). From there, the patients performed opening-closing movements three times in a row. Data were also exported using the factory software (KaVo KiD, KaVo Gmbh, Biberach, Germany). The statistical program GraphPad Prism (GraphPad Software, Boston, USA) was used to process the data, and the statistical analysis was performed using cubic spline interpolation.

During the examination, the software records three-dimensional coordinates of the three Bonwill points, every two hundredths of a second. From the KaVo KiD, it is possible to export the coordinates for each time point. From these coordinates, it was possible to mathematically determine the followed paths, with an accuracy of hundredths of a millimetre. This allowed to visualize the quantity of translational movements at each stage of mouth opening for each patient. The translation of the condyles is plotted as a function of mouth opening, for each examined individual (Fig. 1). Data obtained at the initial 0.2 mm of opening (initial, steeper section in Fig. 1) were not taken into consideration in the evaluation of the results. These sections are probably artificial due to the physical size of the transducers on the paraocclusal clutch of the instrument and its inertia at the start of the movement. After conducting statistical analysis, the gathered data was transformed into a continuous variable. Through repeated sampling, we quantified the relationship between translation and opening using linear regression, as shown in Fig. 2. The best-fitting regression line yielded an impressive coefficient of determination (R²) of 0.9981. It had a slope of 0.3044 (with a 95% confidence interval of 0.3023 to 0.3065) and an intercept at 0.2382 (with a 95% confidence interval of 0.2318 to 0.2445). Notably, the calculated slope was found to be significantly different (p < 0.0001) from a slope of 0, which would correspond to pure rotation, essentially representing a horizontal line.

With a digital motion analyser condylar path during normal mandibular movements can be tracked much more accurately than with previous mechanical motion analysers. ⁵ Digital motion analysers can also support the process of diagnosing intracapsular pathologies of TMD. In certain TMJ pathologies, characteristic movement trajectories can be observed, which is an important aid in differential diagnosis. Also, with the data obtained from the digital motion analyser, it is possible to determine individual movement trajectories for the programming of semi- or fully adjustable articulators.^{5, 18}

After digital data processing, the software displays the displacements of the three Bonwill points in an arbitrary plane, which was the Frankfort horizontal plane in our examination (Fig. 3, Fig. 4). The system allows motion analysis with an accuracy of one hundredth of a millimetre.¹⁹ From the results obtained, it is clear that translation is continuously present during mouth opening from the first mm of the opening started from MIP. This contradicts the previously described results that pure rotational movement occurs in the first 10–20 mm of the opening.^{2, 4}

For almost a century, gnathologists have been working to understand the individual movement pathways of the mandible. Since the beginning, the presence and amount of rotational movement in the first half of the opening has been a matter of discussion. At first, the concept of a rotational (hinge) movement around the intercondylar axis was dominant. Techniques for determining the position of the true individual axis of rotation, i.e., the terminal hinge axis (THA), were developed in the 1960s by McCollum and Lucia.^{2, 3} The recording device they used is called the kinematic pantograph. The principle of its operation is that a drawing device attached to the mandible draws regular circles on the registration surface attached to the maxilla/skull during the opening‒closing movement in case of rotation. By changing the position of the drawing device, a position can be achieved where only one point appears during opening and closing. This indicates the presence of pure rotational movement, and the tip of the drawing tool then points to the THA. If no point can be detected, this indicates the presence of a translation. To date, digital versions of the kinematic pantograph are still available in dental practice and for scientific studies.¹⁷

A study in 2018 attempted to determine the ability of the pantograph to distinguish between pure rotation and combined rotational and translational movement. They also analysed the degree of inaccuracy in the determination of THA caused by the presence of a translational component in the condylar path. The analysis was mathematically derived, and the results were verified by computer simulation. The results showed that these instruments are not suitable for detecting pure rotation: a translation of 1.1 mm can result in a THA displacement of 6.7 mm, and a translation of 2.2 mm can result in a THA displacement of up to 13.5 mm.²⁰ All these outcomes indicate that results reported in the literature for mechanical pantograph studies used to determine THA should be treated with reservations.

Jaw movements have also been investigated using other methods.²¹ A method described in the 1970s attempted to determine the instantaneous center of rotation (ICR). Experience from the field of anatomy has shown that bones rarely move along only one axis. Grant described a model in 1973 based on this theory.²² The authors’ mathematical deduction was that the direction of tension of the muscles explains the translation along a constantly changing axis during opening and closing, and thus, based on the results, they rejected the possibility of pure rotational movement. Keeping Grant's approach but examining and calculating ICR using a different method, Chen also concluded that ICR changes continuously during opening and closing, so it cannot be considered as pure rotation. In this in vivo study, the current position of the ICR was calculated from positional changes of four marker points based on photographs of seven individuals. However, the results mention that the ICRs are closer to the centre of the condyles during the initial stage of opening (a mouth opening of 10 degrees), indicating the dominance of rotation over sliding movement. A limitation of their study is that the method allowed only two-dimensional investigation.²³ Ferrario et al. used a kinesiograph (Sirognathograph) to investigate the location of the ICR and the resulting instantaneous center of curvature (ICC) during opening and closing. Their results showed that pure rotational movement was not present during either opening or closing; thus, the ICR theory was proven to be correct. Furthermore, they found that the speed of movement affected the position of the ICR.¹

Ahn et al. used a digital model to investigate changes in the ICR position during opening and closing of the mouth with a pantograph in virtual space. In a highly accurate study with a resolution of one thousandth of a second (0.001 s), the ICR did not have a constant position. The points recorded during opening and closing did not coincide with each other, presumably as a result of different muscles acting on the mandible during opening and closing movements. A distance of less than 1 mm was detected between the instantaneous centre of rotation measured at the starting position and at the 10 mm mouth opening. Considering the perceptual limitations of the human eye and the inaccuracy of previous mechanical measuring instruments, the earlier results of pure rotation can be explained.²⁴ This result is below the amount/degree of translation recorded in this study, which was close to 2 mm for the first 5 mm of the mouth opening. The reason for the difference between the results of the two studies is not entirely clear but may be related to the fact that some researchers have questioned the relevance of ICR, as its measurement accuracy raises several problems, and it has been found to be insufficiently accurate and reproducible.²⁵

In an in vivo study, Mapelli et al. investigated the relative contribution of rotation and translation to mandibular movements during mouth opening and mouth closing using an optoelectronic instrument that provides 3D images. For each patient, the shifts of the symphysis point and the condyle points were recorded. In addition, the degree of rotation of the mandible around the intercondylar axis was measured. In order to compare the results, the trajectory of the sagittal projection of the symphysis point to maximum mouth opening was divided into ten equal parts. For each section, the symphysis point displacement attributable to rotation was examined. The amount of rotation was more prevalent than the amount of translation in each section, but never approached 100%, so no pure rotation was found. The extent of translation was similar between sexes, but males had longer condylar path than females regardless of mandibular size (P < 0.05). A linear correlation between maximum mouth opening and condylar path during translation was also examined, but no significant association was found.¹⁶

Based on the results of the present study, a new test method could demonstrate that there is no pure rotation in the temporomandibular joint at the initial stage of mouth opening. Additionally, translation is present from the beginning of movement. This is consistent with the most recent research in the literature.

Accurate knowledge of the dynamics of the temporomandibular joint is fundamental not only for the making of dental prostheses but also in other areas of dentistry. Based on the results of this research and similar findings in the literature, it is recommended that the pure rotational approach should be consigned to the past and replaced by a more modern approach. This suggests that during the opening of the mouth from MIP, there is not only rotation but also translation in the TMJ from the beginning.

Declaration of interest

The authors have no conflict of interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Pure rotation in the temporomandibular joint during jaw opening? A digital motion analysis.

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