Bibliografia

Neuroscience Letters 450 (2009) 221–224

Corinne Tardieu, Michel Dumitrescu, Anne Giraudeau, Jean-Luc Blanc, Francois Cheynet, Liliane Borel

*Faculté d’Odontologie, Université de la Méditerranée, 27, Boulevard Jean Moulin, 13385 Marseille Cedex 5, France

*Laboratoire de Neurosciences Intégratives et Adaptatives, UMR 6149, Université de Provence/CNRS, Pôle 3C, Case B, 3, Place Victor Hugo, 13331 Marseille Cedex 3, France

*Service de Chirurgie Maxillo-faciale, Stomatologie, Hôpital de la Timone, 163 rue Saint Pierre, 13385 Marseille Cedex 5, France

Postural control is usually described as being based on the visual, proprioceptive, and vestibular systems. Hence, mandible position could have an impact on postural control since it affects the head position. Mandibular proprioception, assisted by the trigeminal nerve and provided by the masticatory muscles and the periodontal ligament [22], contributes to head postural control via the sterno- cleido-mastoidian muscle [14].

Several studies have focused on the relation between dental occlusion proprioceptive information and upright stance some of them reporting postural control is influenced by dental occlusion. The laterotrusive occlusal position and the lack of balance between the antagonist left and right masticatory muscles may cause a devi- ation of the cervical spine [23]. This human modeling study was supported by results on the rat [5]. These authors reported verte- brae alignment changed after dental occlusion modification. Also, changes in the mandibular position could influence gait stabiliza- tion [8] and postural stability [3]. Starting from observations of increases of the spinal curve, it was shown that a cervical hyper lordosis is often linked to a class II angle malocclusion and that a scoliosis and a torticolis augment the risk of anterior crossbite [13]. This reciprocal link between postural deficits and dental malocclusion suggests that mandibular position or dental occlusion may influence static and dynamic posture and even cause postural pathologies.

However, other studies reported that dental malocclusion or temporo-mandibular disorders have no influence on postural con- trol [7]. Finally, as shown by a systematic review [11], contradictory results are still reported concerning the influence of dental occlu- sion on posture. We hypothesized that such contradictions could originate in the nature of the postural task (static versus dynamic) and the sensory cues available to stabilize body in space (light ver- sus darkness).

The present study focused on the influence of dental occlusion on postural control in young, healthy adults. The main issue was to specify the weight of dental occlusion on postural control according to the task difficulty (static and dynamic upright stance) and to the presence or absence of visual cues (eyes open, eyes closed).

Fig. 1. Posturographic recording analysis. A: Stabilogram, B: three-dimensional chart obtained by wavelet analysis, C: posturographic signal power versus frequency plot. This example illustrates the recordings from a test in static eyes open condition, and in dental rest position condition (no dental contact).

Ten young healthy subjects were included, six men and four women, aged from 25 to 28 years (mean: 21 +/- 0.73). They were selected after evaluation of their plaster dental cast and ques- tionnaire assessment. The selecting criteria were bilateral angle I of molars and canines; absence of anterior and lateral crossbite; absence of oral pathology (malocclusion or articular disorders), oro- facial pain, temporary restorative dental treatment or periodontal healing tissues; and absence of neuropathology, postural and gait disorders or vestibular disorders. The subjects gave their written

consent to participate in the study. The experimental protocols were approved by the local Ethics Committee and followed the recommendations of the Declaration of Helsinki.

From the 10 subjects, angle I was obtained using an orthodontic treatment for six of them. For the other four, angle I was natural. For each subject, 12 recordings were performed using three dental occlusion conditions and four postural conditions.

The dental occlusion conditions were the following: Maximal Intercuspal Occlusion (IO), i.e., when the maximum number of teeth is in contact with the mouth closed; Rest Position (RP), i.e., when there is no dental contact while the mouth is slightly open after swallowing; Thwarted Laterality Occlusion (TLO) was considered as being the opposite of the spontaneous one for each subject. Sub- jects were required to bite a hard wax (Moyco) up to dento-dental contact. The wax allows for constant dental positioning during the test [16].

The postural recordings were performed using the Multitest Equilibre apparatus (Framiral, Cannes, France), which is a static and dynamic posturography platform. The postural conditions were chosen so as to implicate different sensory information and differ- ent levels of task difficulty: (i) stable platform eyes open (Static EO);

(ii) stable platform eyes closed (Static EC); (iii) unstable platform

and the frequency stack (in our case the 352 frequencies used for wavelet decomposition of the recording). The color is given by the value of the power matrix at a given time for a given frequency, suit- ability coded (“hot” colors for high power values and “cold” colors for low power values). Wavelet analysis also served to compute the mean power for all 352 frequencies over the whole recording time. For a quick evaluation, the mean power curve is then divided into three spectral regions: the first region spans from 0.05 to 0.5 Hz, the second from 0.5 to 1.5 Hz and the third from 1.5 to 10 Hz. Sub- sequently, a mean level is computed for each spectral region, and is referred to as the power index.

The head position and its stabilization were recorded using a movement analysis system (Codamotion, Charnwood Dynamics, UK). Three active markers were placed in forehead and infraorbital positions to provide for correct analysis in all three space coordi- nates.

For each subject, the motion recordings were performed simul- taneously with the postural ones (60 s), using a sampling frequency of 100 Hz. The angular displacement of the head measured in the XoY, XoZ, and YoZ planes was computed from the position of each active marker by the following formulas:

XoZ plane ˛(t) = arctan    . x(t) − xv(t)

                                               z(t) − zv(t)

YoZ plane    ˇ(t)     arctan Σ=    y(t) − yv(t)

                                                       z(t) − zv(t)

XoY plane   μ (t)    arctan   Σ=   x(t) − xv(t)

                                                       y(t) − yv(t)

where x(t), y(t), and z(t) are the coordinates of the forehead marker at instant t and xv(t), yv(t), and zv(t) are the coordinates of the median of the segment defined by the two infraorbital markers.

Using these values, the head position was defined as the average of each angle, and the head stabilization was defined as the standard deviation of each angle.

Statistical analyses were carried out using repeated-measures analysis of variance (ANOVA) with dental occlusion conditions (RP, IO, TLO) and postural conditions (Static EO, Static EC, Dynamic EO, Dynamic EC) as within-subjects factors. A separate ANOVA was per- formed with the sessions and postural conditions in order to assess the session-repeating effect (habituation) over time. Results were considered statistically significant for P < 0.05.

Habituation: our results showed that repeating sessions induced habituation, i.e., the postural parameters progressively decreased over time. This was the case with the powers  of  the  stabilo- metric recordings computed using the wavelet analysis for the 0.05–0.5 Hz  [F(2,  18) = 6.39;  P = 0.008]  and  the  1.5–10 Hz  [F(2,

18) = 7.26; P = 0.005] frequency bandwidths. Similarly, the average speed of the center of foot pressure decreased [F(2, 18) = 10.46;      P = 0.001], as did the sway area [F(2, 18) = 3.81; P = 0.04].

Fig. 2. Effect of dental occlusion on postural control. Stabilometric signal power in the three frequency bandwidths compared for each postural condition (Static Eyes Open, Static Eyes Closed, Dynamic Eyes Open, Dynamic Eyes Closed) and for each dental occlusion condition (Rest Position, Maximal Intercuspal Occlusion, Thwarted Laterality Occlusion). Vertical bars represent the confidence intervals. *P < 0.05.

Note that this habituation appeared only for the dynamic conditions and that it started from the second session. No significant differences in postural parameters were found between the TLO versus the IO conditions or between the RP ver- sus IO conditions, whatever the visual context (eyes open or eyes closed). Therefore, we focused on TLO versus RP conditions. Dental occlusion effect on postural sway—stabilometric signal power: in the first frequency band spanning from 0.05 to 0.5 Hz and corresponding to the slower movements, the power index was almost the same in TLO and RP conditions (Fig. 2A). However, in the second frequency band (0.5–1.5 Hz) the power index was significantly higher in the TLO condition than in the RP condition [F(1, 9) = 6.99; P = 0.026]. The results also showed a sig- nificant  interaction  of  dental  occlusion x postural  conditions [F(3,27) = 8.04; P = 0.0005], thus indicating that the power index was differently affected by the dental occlusion conditions according to the postural conditions. A detailed analysis showed that the dental occlusion modifies the stabilometric signal power only in dynamic conditions (unstable platform) and when there were no visual cues (P = 0.007) (Fig. 2B). Note that this was a very robust effect, since it appeared in spite of the habituation effect formerly described.

In the third frequency band spanning from 1.5 to 10 Hz and corre- sponding to the faster movements, a significant interaction was also found for dental occlusion x postural conditions [F(3, 27) = 4.90;    P = 0.007]. As for the previously described interaction, its origin was the higher energetic cost during dynamic condition without visual cues in the TLO condition (P = 0.047) (Fig. 2C).  Dental occlusion effect on postural sway—average speed of the cen- ter of foot pressure: a significant interaction was found for dental occlusion postural conditions [F(3, 27) = 4.85; P = 0.008]. Detailed analyses indicated that this effect resulted from the increase in speed of the center of foot pressure in the dynamic EC postural condition (Fig. 3).

Dental occlusion effect on postural sway—body sway area: this parameter was less discriminating since the variance analysis showed that the body sway  areas  were  not  significantly  differ- ent from one experimental condition to another, whether dental occlusion or postural condition (Fig. 3).

Dental occlusion effect on head orientation and stabilization: the movement analysis data showed that neither the head position nor the head stabilization significantly differed for the various postural conditions, whatever the dental occlusion condition.(Fig. 4) illus- trates the head stabilization, i.e., the head angular displacement in the three spatial planes (XoY, XoZ, YoZ) in the dynamic postural con- dition EC, for the three dental occlusion conditions. Only dynamic postural EC is shown because it is the only condition for which the other parameters were modified.

Fig. 4. Effect of dental occlusion on head stabilization. Head stabilization in the three spatial planes in the Dynamic Eyes Closed condition compared for the three dental occlusion conditions. Same conventions as inFig.2

The TLO condition, which simulates an anomaly of the dental occlusion, induces changes in the postural control parameters rela- tive to the RP condition. This is true for the average speed parameter and for the power index, but only when the dynamic postural conditions are used, and in absence of visual cues. These param- eters show that the energetic cost for balancing is higher in the TLO condition.

Note that these significant differences persisted in spite of the habituation effect, which is indicated by the lower amplitude of the postural sway from session to session. Such an effect in dynamic postural conditions (EquiTest) has been reported previously [21]. In the present study, to overcome the habituation phenomena, the recordings were performed in a random order for each subject, and thus the affected recording differed for each subject.

The head position and stabilization were not affected by the TLO condition, even though the postural control characteristics differed. Thus the higher energetic cost previously described seems to be in accordance with the main goal of postural control, which is to stabilize the head in space.

No significant effect was found for the sway area parameter. This result confirms that wavelet analysis parameters have a higher sensitivity and are more discriminating than the classical ones, as shown by Dumitrescu and Lacour [6].

In static conditions, the lateral occlusal perturbation induced no change in the postural control parameters. This dental occlusion affected postural control only in difficult postural conditions. Our results highlight the importance of testing postural condition when assessing an occlusal treatment. Our results may also explain why no improvement in postural parameters was detected after tempo- rary correction of an occlusal anomaly [17], since these tests were performed in static conditions. This may also be the case for the lack of evidence of durable efficiency of occlusal treatments [18,19].

Dental occlusion had no influence on postural control when visual cues were present. This is not surprising, since many studies have described the major role of visual cues in postural control, even when drastic sensory perturbations are present, such as in vestibu- lar loss [1,2]. However, our results differ from those of Gangloff et al. [9] who reported significant differences between the centric rela- tion and the intercuspal occlusion for patients tested in eyes open condition. The differences in experimental conditions may explain these contradictory results.

Finally, postural control was the same for IO and RP. These results agree with those of Bracco et al. [4] and Perinetti [20], who showed an absence of correlation between dental occlusion and postural control. All in all, these results indicate dental contacts have only a weak influence on postural control. Thus, it seems that desmod- ontal proprioception is not a major factor of postural stabilization. However, in the TLO condition, the proprioceptive cues from the mandibular musculo-articular system are involved, and we show here that the postural control differed from that in the RP con- dition. These results agree with those of Gangloff and Perrin [10] who reported impaired postural characteristics when the trigemi- nal muscle is unilaterally anesthetized. Thus, we hypothesize that the influence of dental occlusion on postural control depends on the presence of proprioceptive cues coming from the mandibular musculo-articular system.

Our work shows that dental occlusion differentially contributes to postural control, with no effect in static postural conditions but a worsening in dynamic conditions. Even if the weighting of pro- prioceptive cues linked to the dental occlusion seems to be lower than those of the other sensory cues, these results are coherent with the notion of sensory cooperation and substitution reported after impairment of other sensory systems [12,15]. The sensory informa- tion linked to the dental occlusion comes into effect only during difficult postural tasks and its importance grows as the other sen- sory cues become scarce.

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