Background
Occlusal information from periodontal mechanoreceptors is used in the control of biting behaviors [
1‐
6]. Adjustment for occlusal interference is necessary in patients with occlusal-related crowns and dental filling restorations. Failure to adjust for occlusal interference may result in compromises in tooth structure, oral mechanics, and quality of life [
4‐
6]. However, the ability to properly adjust for occlusal interference requires a high level of skill from the dentist, because there is no objective consensus on an optimal method of adjusting for occlusal interference. The exact adjustment for slight occlusal interference by the objective evaluations is required.
Recently, studies have investigated the hemodynamic responses observed in the human cortex after dental stimulation [
7‐
18]. Advances in functional brain imaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography allow the cortical representation of dental-related movement or perception to be examined in healthy humans, including the movement and perception of the tongue, lips, and teeth [
7‐
11]. Some studies have used functional brain imaging to study tooth perception [
12,
13] and chewing, including parafunction [
14‐
18]; however, to our knowledge, there are no reports on the cortical representation of tooth perception in individuals with occlusal interference. The identification of cortical areas involved in the perception of occlusal interference may offer new methods for occlusion preparation in prosthetic appliances. Therefore, the purpose of this study was to quantify changes in brain activity during experimental occlusal interference.
Discussion
One of the important results of the present study is that the BOLD signal in the left cortical somatosensory region was increased during a molar-tapping task performed with experimental occlusal interference of the right first molar.
Penfield and Rasmussen intraoperatively investigated human sensory somatotopy [
21] and reported that the teeth, gingiva, and jaw were represented in the cortical somatosensory representation. Converging results from magnetoencephalography [
22] and tactile stimulation [
23] studies indicate that the sensory representation of the oral area is located in the primary somatosensory cortex, the so-called ‘sensory homunculus’. The left superior frontal gyrus was activated during unilateral chewing on the right side [
15]. Therefore, we speculated that activation of the left somatosensory cortex would be affected by experimental occlusal interference of the right first molar, and that this would be captured by fMRI. Our results indicate that experimental occlusal interference can be objectively visualized by fMRI.
Activation changed from the contralateral somatosensory cortex to the bilateral somatosensory cortices after the removal of the experimental occlusal interference. This suggests that the legitimacy of adjustment for the occlusal interference using fMRI was elucidated. To our knowledge, this is the first report of the utility of fMRI for assessing the effect of occlusal interference. We expect that, in the future, this method of assessing the presence of occlusal interference, i.e., fMRI during the tapping task, could be applied clinically. In particular, it may be useful to assess the presence of occlusal interference in patients who cannot judge the occlusion themselves.
Another important result of the present study is that Brodmann’s area 46 was activated during experimental occlusal interference of the right first molar. Activation of this area was not present 60 minutes after the removal of experimental occlusal interference. It has been reported that Brodmann’s area 46 as well as in insula controls higher brain functions including the stress-sensitive neuromodulatory systems, which, in turn, control sympathoadrenal and hypothalamic-pituitary-adrenal activity [
24‐
26]. Therefore, the present data suggest that the experimental occlusal interference was an acute stressor. The present results indicate that the disappearance of occlusal interference can be judged by the disappearance of activation of Brodmann’s area 46 in addition to the development of bilateral activation of the cortical somatosensory regions. The insula also controls higher brain functions including the stress-sensitive neuromodulatory systems, but the alterations of BOLD signals could not be caught according to the experimental interferences in the present study. We could not appropriately explain the reason. The possible explanation was that there might be a subtle distinction about response’s phenomenon between the Brodmann’s area 46 and insula.
To our surprise, activation in Brodmann’s area 46 didn’t disappear immediately after the removal of the experimental occlusal interference, and did not disappear until 60 minutes after this time point. The present result suggests that a patient’s adjustment for occlusal interference should make a better result for some time at least over 1 hour. Therefore, our results could recommend the observation i.e. one day, one week etc. after the adjustment of the occlusal interference in dental office.
In the tapping task used in the present study, there was bilateral and uniformly diffuse activation of the bilateral inferior aspect of the primary motor cortex close to the lateral fissure, and the bilateral insula, bilateral thalamus, and bilateral cerebellum (Table
2), in agreement with previous positron emission tomography [
10] and fMRI [
14,
15,
27] findings. These regions are believed to receive sensory information from the mandibles and the temporomandibular joint, and to control masticatory movements and the lingual and facial muscles [
28,
29]. Based on the conformity between our results and previous reports about activation’s areas during the present task, it would be quite appropriately done.
In the present study we used fMRI to investigate the relation between occlusal interference and brain activity because the low spatial and temporal resolution of positron emission tomography makes it difficult to monitor brain activity during tapping tasks. fMRI allows the activity of precise brain regions to be linked to the performance of the tapping task. We selected the BOLD technique because of both the general knowledge and the perfect establishment of its technique. The BOLD technique permits the depiction of slow-flow vessels in T2-weighted images and the depiction of fast-flow vessels by acquiring images with electrocardiogram triggering during the slow-flow cardiac phase [
10‐
13,
22]. With the experimental occlusal interference technique used in this study, the occlusal height of the right first mandibular molar could be raised by up to 0.75 mm using the whole crown covered with resin. For this reason, occlusion between the maxillary and mandibular first molars should be a key role for individuals with normal occlusion. In addition, the maxillary and mandibular first molars tend to be repaired by dental restorations because they have the earliest eruption of all permanent teeth and the longest existence in the oral cavity.
One possible limitation of our present study is the small sample size. In addition, we included only healthy volunteers of a young age. There may be changes in occlusion with age, and it is not clear if our results can be generalized beyond young adults with normal occlusion. Larger and more varied samples should be studied to establish the generalizability of our results to patient populations with a variety of occlusions. In addition, the present study design was that placement of a restoration in hypo-occlusion or for that matter simply placement of a new restoration that changes buccal, lingual morphology or interproximal contact pressure could result in the same types of activations as seen in the hyper-occlusion state. So, the BOLD signal activations that are observed up to 60 min post-removal of the interference is indicative that brain activation patterns may vary as the conditions in the oral cavity change over the short term. Moreover, fMRI cannot be performed on all patients in the clinic because it requires an MRI system. However, we expect that this method of adjusting occlusal interference, combined with fMRI and the tapping task, could be applied clinically in the future. In particular, it may be useful for patients who cannot judge their exact occlusion themselves. For future work, moreover, we also consider the clinical applications of near-infrared spectroscopy (NIRS) methods for adjusting occlusal interference based on our present results. As NIRS is less prone to movement artifact, allows subjects to be in a sitting position, like fMRI, is less expensive and therefore more likely to be the method of choice in a dental setting.
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MO, KY, TT, SK, NW, SM, MK, KM and YM: Conceptualized and designed the study, participated in the performance of the research, participated in data analysis, drafted the initial manuscript, and approved the final manuscript as submitted. SS, EM, IM, SN, MK and KM: Participated in the performance of the research, and data collection. SK, IY, SM and MK: Drafted the initial manuscript, critically reviewed the manuscript, and approved the final manuscript as submitted. All authors read and approved the final manuscript.