With a digital motion analyser, the 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. Additionally, with the data obtained from the digital motion analyser, it is possible to determine individual movement trajectories for the programming of semiadjustable or fully adjustable articulators [5, 19].
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 (Figs. 3 and 4). Kinematic facebows allow the true axis of rotation to be determined with greater accuracy but with complexity. These devices, due to their complexity, have not spread but have been superseded by artbitrary facebows. The use of ar arbitrary facebows represents a minimal compromise in the accurate determination of the axis of rotation. Based on dental practice over the past few decades, this discrepancy does not represent a clinically relevant source of error. The system allows motion analysis with an accuracy of one hundredth of a millimetre according to the manufacturer’s data [18]. From the results obtained, it is clear that translation is continuously present during mouth opening from the first mm of the opening starting from the MIP. This contradicts the previously described finding that pure rotational movement occurs in the first 10–20 mm of mouth opening [2, 4].
Right: Trajectories of the Bonwill points during mouth opening and closing and the Bonwill triangle. (KaVo KiD, KaVo Gmbh, Biberach, Germany).
Right: Trajectory of the incisivus point during opening and closing of the mouth, right view, sagittal plane (KaVo KiD, KaVo Gmbh, Biberach, Germany).
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. First, the concept of 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 cases of rotation. By changing the position of the drawing device, a position where only one point appears during opening and closing can be achieved. 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 conducted in 2018 attempted to determine the ability of the pantograph to distinguish between pure rotation and combined rotational and translational movement. The researchers 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 THA displacement of 6.7 mm, and a translation of 2.2 mm can result in THA displacement of up to 13.5 mm [20]. All these outcomes indicate that the results from published mechanical pantograph studies used to determine THA should be interpreted with caution.
Jaw movements have also been investigated using other methods [21]. A method to determine the instantaneous centre of rotation (ICR) was described in the 1970s. Experience in the field of anatomy has shown that bones rarely move along only one axis. In 1973, Grant described a model 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 mouth 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 the ICR using a different method, Chen also concluded that the ICR changes continuously during opening and closing, so it cannot be considered pure rotation. In this in vivo study, the current position of the ICR was calculated from positional changes in four marker points based on photographs of seven individuals. However, 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 centre 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 less than 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 the 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 in the incisivus point and the condyle points were recorded. In addition, the degree of rotation of the mandible around the intercondylar axis was measured. To compare the results, the trajectory of the sagittal projection of the incisivus point to maximum mouth opening was divided into ten equal parts. For each section, the incisivus 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%; consequently, no pure rotation was found. The extent of translation was similar between sexes, but males had longer condylar paths than females regardless of the mandibular size (P < 0.05). A linear correlation between maximum mouth opening and the 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 relation to the arbitrary hinge axis in the temporomandibular joint at the initial stage of mouth opening. Additionally, translation is present from the beginning of movement. This finding is consistent with the most recent research in the literature.
