Zum initialen Einfluss reversibel und asymmetrisch veränderter okklusaler Abstützungen der Mandibula gegen die Maxilla auf die Aktivität der Kau-, Mundboden- und Nackenmuskulatur bei willkürlichem Pressen in maximaler Okklusion

The loss of teeth usually results in asymmetrical occlusal support areas arising from the mandible against the maxilla. It is not been unequivocally established to date whether and to what extent the asymmetrical loss of terminal molars and premolars leads to an imbalance in the primary and secondary muscles of mastication, thereby contributing to the development of craniomandibular dysfunctions. The objective here was to test the following hypotheses within the above context: Reversible and brief changes to asymmetric mandible-against-maxilla occlusal support areas during random maximum pressing have no influence on the activity of a.) the adductors (temporalis muscle, masseter muscle) or b.) the suprahyoid and neck muscles. The subject cohort comprised 36 functionally healthy and fully dentulous subjects (19 females, 17 males aged between 20 and 30 years) with neutral occlusion and stable maximum intercuspation. In order to simulate various asymmetrical occlusal situations, a segmented occlusal splint with 5 parts was fabricated from acrylate for each subject. The variable use of the occlusal segments permitted reversible simulation of the different asymmetrical bite situations in the anterior teeth, premolar and molar regions. Muscular activity was registered using the K 7® surface electromyography (EMG) evaluation system (manufacturer: Myotronics). The activities of the following muscles were measured bilaterally during a randomized sequence of asymmetrical occlusions while the subjects repeatedly briefly pressed their teeth together at maximum pressure: - Anterior part of the temporalis muscle - Masseter muscle - Suprahyoid muscles - Neck muscles (semispinalis capitis muscle, trapezius muscle) The statistical workup of the measured data produced the following clinically relevant results: 1. During simulation of asymmetrical bite situations using variously segmented occlusal splints, significantly lower activities were measured for the temporalis and masseter muscles than registered during the reference pressing situation with a non-segmented occlusal splint (reductions ranging between 8.2 and 44 %). The EMG activities of the temporalis and masseter muscles changed markedly as a function of the number of occluded teeth. The temporalis and masseter muscles showed the greatest reduction in activity during pressing against the molar segment on the right side (right temporalis muscle -22.56 %, left temporalis muscle -43.99 %, right masseter muscle -35.83 %, left masseter muscle -40.93 %). Slightly lower activity reductions were found during pressing against bite segments localized in the anterior teeth and right premolar region (right temporalis muscle -20.56 %, left temporalis muscle -26.22 %, right masseter muscle -25.90 %, left masseter muscle -26.64 %). The muscular activity reductions at peak pressing were even lower when the molar segment was additionally inserted into the splint on the right side (right temporalis muscle -11.14 %, left temporalis muscle -21.99 %, right masseter muscle -14.21 %, left masseter muscle -24.74 %). The lowest activity reduction was found when the subjects exerted maximum pressure with contacts in the anterior, right premolar and molars and left molar region (right temporalis muscle -9.17 %, left temporalis muscle -10.98 %, right masseter muscle -8.21 %, left masseter muscle -13.35 %). The side-to-side comparison showed higher activities in the temporalis and masseter muscles on the side of occlusal contacts (right) than on the contralateral side that was in non-occlusion (left). The most marked bilateral difference was found in the temporalis muscle during peak pressure on the right molar segment (p<0.001). Teeth closure onto the anterior teeth and the right premolar and molar segment produced significantly asymmetrical contractions in the temporalis muscle (p=0.035). In this bite situation, the masseter muscle actually showed the greatest asymmetry (p=0.001). 2. During maximum random pressing, the suprahyoid and neck muscles exhibited contractions. The activities of the suprahyoid muscles in the asymmetrical bite situations differed significantly from the measurements with the full occlusal splint (activity declines between 7.8 and 15.8%). However, no significant differences were demonstrated between the right and left sides (p>0.005). In the neck muscles, no significant activity differences were found between the situations without occlusal splint, with full occlusal splint or with individual occlusal splint segments (p>0.05). The measurements on the right and left sides did not differ significantly from one another (p>0.05). 3. Major interindividual differences were noted in the measurements of occlusal muscular activities during maximum random pressing. Nevertheless, the patterns reflecting how the muscles react to occlusal asymmetries were similar. 4. The differences in maximum muscular activity did not differ significantly between the male and female subjects. Based on the findings obtained on the measurement accuracy and sensitivity of the surface EMG, the hypothesis that asymmetrical occlusal situations during random maximum pressing has no influence on the activity of the temporalis muscle, masseter muscle and floor of the mouth muscles was proven false. By contrast, in functionally healthy young adults, the hypothesis was confirmed that simulated asymmetrical occlusal support areas at least initially have no influence on the activity of the neck muscles that is measurable using the registration technique applied. The findings suggest that symmetrical occlusal contacts are required to achieve or ensure muscular balance. The conclusions for clinical practice would be that pronounced asymmetries in posterior support areas should be avoided, irrespective of the question as to how many posterior teeth should be replaced. The present findings clearly demonstrate that unilateral contacts, primarily in the molar region, lead to strong asymmetrical contractions at peak pressing. Nevertheless, missing molars in particular are oftentimes not treated with prosthetics because their replacement is deemed not necessary on the grounds of esthetics, phonetics or masticatory functions. Moreover, patients with a loss of unilateral posterior support zones more likely adapt their masticatory patterns in that they use the fully dentulous side to grind their food and thereby further potentiate any muscular imbalance. The loss of the posterior teeth additionally alters the conduction of masticatory forces from the teeth to the osseous structures of the facial cranium, thereby intensifying the burden on the temporomandibular joint. The muscular and biomechanical imbalance might indeed increase the risk for development of craniomandibular dysfunctions, particularly from a long-term perspective. The present findings for the neck muscles contradict clinical experience with patients who lack sufficient occlusal support in the posterior arches: not only do they suffer from symptoms associated with the occlusal muscles, but also primarily present with myogenic complaints relating to the neck, shoulder and arm muscles. Apparently, the occlusal muscles are capable of compensating for brief occlusal changes without measurable involvement of the neck muscles- at least in functionally healthy young adults. Further studies are needed to investigate the long-term effects of occlusal changes on the muscular activity and the interplay between the muscles.