The purpose of this study was twofold. First, we assessed the change of postural control variables with an occlusion condition in a healthy cohort; second, we calculated the required changes in order to measure potential clinical relevant changes.
The blocked occlusion condition introduces small but significant decreases of the frontal and sagittal sway. The frontal sway decreases by 0.67 mm, the sagittal sway by 0.33 mm. In addition, the pressure distribution moves slightly to the front with the blocked occlusion condition. The relative pressure under both fore feet increase by 0.67%, where the dominant leg seems to be the left fore-foot with an increase of 0.33%. Already the healthy, symptom free population has a small change of the pressure distribution with the blocked dental occlusion condition. This is important to note because of its limitation for the clinical test.
A possibility to gain a brief insight into whether disturbances of the TMS have an effect on postural control is the Meersseman test. In short, the Meersseman test predicts a TMD, if a change of the postural control can be observed. As we have shown, already the healthy, symptom free population does show a change of the measureable variables with the blocked occlusion. For a clinical relevant outcome the observed changes have to be outside the range of the 1st to 3rd quartile (Table 3). For instance, the frontal sway has to be smaller than 3 mm or larger than 1.33 mm with the blocked occlusion condition compared to rest position to be clinical relevant. The minimal changes of Table 3 can be used for both sex and any age decade, as no gender or age specific differences were measured.
While the clinical relevant assessment was done the first time in this study, many researches was conducted to evaluate effects of occlusion conditions on the pressure distribution (13, 17–19). Bracco et al. (17) measured a reduction in body sway with a myocentric jaw position that is comparable with the values presented in this study. The influence of an occlusion condition is by far smaller than other interventions like closed eyes according to Baldini (13). Contrary to our results, Amaricai et al. (18) could not observe any change of the weight distribution between the rest position and any symmetrical occlusion condition. Also a recent study by Michalakis et al. (19) measured significant changes in the lateral direction (sagittal sway) but no changes in the anterior posterior direction. They study involved only 20 subjects (14 m/6f) and they only counted the number of subjects that showed a change.
Several explanations are discussed in the scientific literature to give a theoretical explanation of the observed phenomena. The most common explanations can be categorized in two categories, sensory dependent theory (7, 20) and mechanical dependent theory (21, 22). The sensory dependent theory states, that missing sensory information in the occlusion condition alters brain circuits, that lead subsequently to a change of the motor pathways (7, 20, 23, 24). The mechanic dependent theory states that either a relaxation of muscles of the anterior triangle or a change of the air tunnel lead to a repositioning of the head and therefore leads to a change of body positioning and subsequently to a change of the pressure distribution (21, 22, 25, 26). Unfortunately, to date none of the theories has been investigated more deeply.
Based on the presented results the change between rest position and blocked occlusion is independent of differences between gender and age. It has been reported, that male subjects have a larger postural sway than female subjects do (27–29) and that the postural stability does decrease with age (30, 31). However, this did not affect the influence of the occlusion conditions´ change. Additionally, we have found no difference between any age and gender group for the change of the pressure variables measured. The body mass and body mass index (BMI) of the cohort has been compared with the general German distribution (32, 33). Only marginal differences in any age or gender group could be found. Even though weight might be another cofactor, the results in terms of the clinical relevance match the population and are therefore representative.
There are some limitations of this study. The inclusion criteria are healthy subjects free of TMD symptoms. For the assessment of the TMD a standardized questionnaire was used (15). Qualified orthodontists/dentists evaluated this questionnaire and discussed open questions in an interview. No excessive dental/orthodontic or orthopaedic examination was undertaken. While this is a common procedure to get a first insight in TMD, it might be a source of incorrect diagnosis. In this study only the correlation between the change of the occlusion condition (between rest position and symmetrical blocked occlusion) and spatial pressure parameters was evaluated. This is obviously no investigation of a cause effect relationship, nor does this study address the theoretical background. New studies should be conducted, that investigate more the principal mechanism behind occlusion interventions. Several research reviews and meta-analysis have shown that a change of the occlusion condition can affect the remaining body, however a deeper understanding of the underlying mechanism is required to optimise the interventions (1–7). For instance, a time dependent analysis of the COP path might be useful to determine if the control strategy and therefore the sensory pathway is effected with a change of the occlusion condition (34).
This study shows, that already in the healthy symptom free population a Meersseman test provokes a small but measureable change of the body sway and pressure distribution. A TMD might therefore not be the only source for a change of these parameters. In addition, the changes are general small and might be influenced strongly by other factors as vision and cognitive workload. Therefore, it has to be questioned if the Meersseman test is clinical relevant. Of course, within this study only healthy symptom free subjects were evaluated. Another study has to be conducted with TMD patients in order to proof, if the test can predict any TMD correctly, without having too many false positive. In an individual assessment, the range of the pressure values for the healthy, symptom free population has to be considered.