A functional perspective on oral implants – state‐of‐the‐science and future recommendations*

The introduction of oral implants is quite rightly considered by many to represent one of the major improvements in the long history of oral rehabilitation. Oral implants integrate remarkably well into bone, and can also be effectively integrated into the treatment options that can be offered to patients with tooth or other tissue loss. Over the last decades, significant research efforts have been devoted to biomaterials and technical aspects in addition to the biology of osseointegration. Notwithstanding the immense benefit and importance of such findings for modern implants in oral rehabilitation, it also seems timely to carefully and critically evaluate the basic and clinical knowledge related to the functional aspects of the osseointegrated implants in the oro-facial region. In the series of reviews presented in this issue of Journal of Oral Rehabilitation (JOR), different functional aspects of oral implants are described. The following statements are intended to capture the state-of-the-science and to identify new avenues to further the advance of our understanding of why and how oral implants integrate not only into the jaw bone but also into the delicate function of the oro-facial system. Considerable information about the somatosensory processes in peripheral tissues, brainstem and higher brain centres such as the thalamus and the cerebral cortex, including those related to oro-facial mechanosensory and nociceptive mechanisms in animals is now available (1). Some of the circuits and processes involved in oro-facial reflexes and motor control have also been defined in animal experiments. Recent findings of neuroplasticity of central somatosensory and motor processes are applicable to oro-facial pain, to motor control and motor learning, and to behavioural adaptation to changes in the oral sensory environment such as may occur with the placement of oral implants. The types, locations and properties of receptors that are responsible for sensory and motor responses and sensorimotor control, and that are activated by stimulation of implant-supported prostheses require elucidation. Likewise, the central neural pathways and properties of neurones in these pathways that contribute to implant-induced sensory and motor responses, and sensorimotor control are largely unexplored. Tissue damage and inflammation may produce peripheral sensitization as well as central sensitization that appear to be involved in the acute or persistent pain that can develop at an injury site. Studies are needed to determine whether the placement of implants in patients also produces such sensitization phenomena that could account for the neurosensory changes, including neuropathic pain, that may often develop after implant placement. Recent findings of neuroplasticity induced in cortical sensory and motor areas by alterations in sensory inputs or learning in animals and humans underline the need for studies to clarify the cortical mechanisms associated with changes in the oral sensory environment. Such knowledge is very limited for oro-facial sensorimotor functions, but is fundamental to understanding how a person learns a new oro-facial skill or adapts or not to an altered oral environment, and how clinical approaches (e.g. bridges, dentures, implants) aimed at restoring oro-facial functions may produce their rehabilitative effect in patients. Periodontal mechanoreceptors innervate natural teeth and signal information to the brain about temporal, spatial and intensive aspects of tooth loads (2). During the last decade, it has become clear that this information is not primarily used for protective purposes, but is used by the human brain to improve the control of oral motor behaviours (2). It has been demonstrated that periodontal receptors are involved in the fine motor control of biting and chewing, and are particularly important for the precise control of low forces used during intraoral manipulation of food. When natural teeth are replaced with oral implants, the periodontal mechanoreceptors are lost. It is clear from studies of patients with oral implants that important sensorimotor functions are impaired or lost when periodontal receptors are removed during the extraction of teeth. This knowledge emphasizes the importance of maintaining natural teeth with healthy periodontal function whenever possible. For the future, it is important to consider the clinical consequences of the degraded jaw motor control in patients with oral implants. The number of research reports related to the reflex function evoked by stimulation of oral implants, both in animals and man, is very limited (3). However, it is known that some brain stem reflexes, for example, the so-called ‘silent period’ reflex cannot be elicited by stimulating implants unless periodontal ligament receptors are present somewhere in the oral cavity. Thus, although at the histological level the innervation at the implant-bone interface rapidly increases after implant insertion, these free nerve endings do not trigger a reflex input when activated by low-threshold mechanical stimuli. Transcortical loops limiting voluntary jaw-closing force development may, however, be functional. Comparative prospective and case-controlled studies enrolling dentate and edentulous oral implant rehabilitated patients will pinpoint differences in the chewing pattern and muscle activity, and coordination. Such studies will clarify the interactions between periodontal ligament receptors, muscle and joint proprioceptors, and inner ear inputs on the brain stem reflexes. These findings will demonstrate how (ir)relevant these peripheral inputs are towards the central pattern generator function in habitual chewing. Osseointegrated implants are routinely used to rehabilitate limb amputation or edentulism. To enable a satisfactory clinical function with such a bone-anchored prosthesis, the oral implant should be physiologically and psychologically integrated. Amputated patients rehabilitated with a lower limb prosthesis with implant-to-bone anchorage, have reported an ability to sense the type of soil they are walking on. Clinical observations on patients with oral implants, have confirmed a special sensory perception skill, coined osseoperception (4). Histological, neurophysiological and psychophysical evidence of osseoperception is present, making the assumption more likely that the peripheral feedback pathway can be (partly) restored for rehabilitations with osseointegrated implants. This implant-mediated sensorimotor control may have important clinical implications because a more natural functioning with implant-supported prostheses might be attempted. The underlying mechanism of this so-called ‘osseoperception’ phenomenon remains a matter of debate. Furthermore, the term should be used with caution because there is no strong evidence that the perception originates from receptors located in the bone, and the term may not adequately encapsulate both the sensory and motor aspects related to the function of an osseointegrated implant. Extraction of teeth involves elimination of the extremely sensitive periodontal ligaments while functional reinnervation around implants is uncertain. Longitudinal studies are needed to monitor changes that occur after limb amputation or tooth extraction. Functional magnetic resonance imaging studies on human subjects undergoing extraction and subsequent oral implant placement might be ideal to study the effect of such interventions at both peripheral and central levels. Furthermore, continued focus should be put on the physiological and psychological integration to find out whether both aspects can be realized facilitating the trajectory towards total-body integration. Finally, further research is required to make practical use of osseoperception in the design of novel bone-anchored prosthetic appliances and bionic limbs. The aetiology of bruxism has a multifactorial nature, and is possibly different for sleep-related and awake bruxism (5). While peripheral afferent inputs and morphological factors (e.g. occlusal discrepancies, deviations in the anatomy of the oro-facial region) are thought to play only a small role, if at all, psychosocial factors (e.g. stress, educational level) and pathophysiological factors (e.g. sleep arousal, disturbances in the central neurotransmission) are considered to be involved in the aetiology of bruxism. However, although the body of evidence for a role of these factors is growing, it is not yet conclusive. Further, there is still insufficient evidence to support or refute a cause-and-effect relationship between sleep-related and/or awake bruxism on the one hand and failure of oral implants on the other. There is a need to improve the diagnosis and quantification of sleep-related and awake bruxism, thereby respecting the fact that bruxism is not an ‘all-or-nothing’ phenomenon but can rather have different severities, both between and within individuals. A critical question will be to study the differences and similarities between sleep-related bruxism and awake bruxism in their respective clinical consequences for oral implants. Finally, it will be useful to perform studies that are designed to specifically address the purported cause-and-effect relationship between sleep-related and/or awake bruxism on the one hand and failure of oral implants on the other. The most appropriate outcomes to measure chewing ability in patients with oral implants involve valid patient-based measures (6). These may be in the form of scales used to rate levels of satisfaction or direct ratings of chewing difficulty. In addition, oral health-related quality of life instruments can be used as an indicator of how chewing ability (or a deficiency in chewing ability) impacts on one's life. Researchers who wish to measure chewing ability in patients with oral implants or other forms of rehabilitation should use reliable and valid patient-based questionnaires to gather the most appropriate data. Oral implants provide the patient and health professionals with an invaluable treatment option when normal teeth and/or oral tissues are missing or lost. The reviews by leading scientists and clinicians presented in this issue of JOR in addition to the summaries of the state-of-the-science and suggestions for recommendations have outlined what we know today about the underlying physiology and functional implications. Despite the significant advancement of our knowledge, there are clearly also gaps, which need to be filled in order to reliably predict treatment outcome based on assessment of both local and systemic risk factors for oral function.

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