Masticatory function in people with dental implants
Abstract: In use, since the 1960´s, dental implants have been reported to allow chewing that is as efficient as with natural dentition. However, input from periodontal mechanoreceptors (PMRs) around the tooth is important for sensorimotor regulation of biting and chewing. Since there is no periodontal ligament between osseointegrated implants and the alveolar bone, there are no mechanoreceptors in close proximity and, consequently, for individuals with such implants the sensory information sent to the central nervous system concerning chewing forces should be different than with natural teeth. The present thesis was designed to characterize the regulation of mastication in individuals with bimaxillary implant-supported bridges and to compare this to chewing with natural teeth, in order to gain new insights into the role of PMRs in this connection. In the first study participants with natural dentition or fixed bimaxillary implant-supported prostheses chewed and swallowed two model foods of differing hardness while the electromyographic activity of the jaw-closing muscles and the position of the mandible were monitored. Those with implants exhibited impaired sensory-motor regulation during chewing, with less elevation of jaw muscle activity in response to hard food and attenuated adaptation of this activity as the masticatory sequence progressed. Next, we characterized the temporal profile of masseter muscle activity during natural chewing by young adults and the influence of food hardness on this profile. The excitatory drive of the masseter muscle was found to be biphasic, demonstrating an early component prior to tooth-food contact and a late component during this contact. To test our hypothesis that sensory input from the PMRs is required to achieve this later increase we finally investigated the effects of the absence of sensory input from PMRs in subjects with fixed bimaxillary implant-supported prostheses on activation of the masseter muscle during the jaw-closing phase of a chewing cycle. Dentate participants exhibited a biphasic muscle drive during jaw-closing and throughout the masticatory sequence, with a component that starts just before the jaw-closing phase and is based on information from preceding chewing cycles, with no need for input from PMRs; and a late component that starts upon contact with the food, which is signaled by the PMRs. In contrast, participants with dental implants showed no such biphasic drive in the beginning of the masticatory sequence. Adaptation of muscle activity during jaw-closing by the latter appeared to involve modifying the rate of the early component; while the more pronounced adaptation by dentate individuals seemed to reflect additional modification of the late, post-contact component, presumably in response to signals from the PMRs. However, later during the masticatory sequence, implant participants did show a biphasic drive during jaw-closing, probably achieved by prediction based on the gradually changing properties of the bolus during chewing. Moreover, the temporal profile of muscle activity during chewing was the same regardless of the hardness of the food. In conclusion, sensory information provided by the PMRs appears to be most critical during the beginning of the masticatory sequence, when initial food contact occurs, whereas prediction of food properties based on information from other mechanoreceptors is utilized more effectively later on.
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