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- Material and methods
- Conflicts of interest
- Supporting Information
Jaw-closing movements are basic components of physiological motor actions precisely achieving intercuspation without significant interference. The main purpose of this study was to test the hypothesis that, despite an imperfect intercuspal position, the precision of jaw-closing movements fluctuates within the range of physiological closing movements indispensable for meeting intercuspation without significant interference. For 35 healthy subjects, condylar and incisal point positions for fast and slow jaw-closing, interrupted at different jaw gaps by the use of frontal occlusal plateaus, were compared with uninterrupted physiological jaw closing, with identical jaw gaps, using a telemetric system for measuring jaw position. Examiner-guided centric relation served as a clinically relevant reference position. For jaw gaps ≤4 mm, no significant horizontal or vertical displacement differences were observed for the incisal or condylar points among physiological, fast, and slow jaw-closing. However, the jaw positions under these three closing conditions differed significantly from guided centric relation for nearly all experimental jaw gaps. The findings provide evidence of stringent neuromuscular control of jaw-closing movements in the vicinity of intercuspation. These results might be of clinical relevance to occlusal intervention with different objectives.
Jaw-closing movements are basic components of physiological motor actions. In particular, chewing cycles and swallowing are specific closing movements that must achieve a predetermined requirement with high precision. It might be inferred from this physiological significance that jaw closure is controlled by precise sensorimotor programs .
To enable such demanding motor tasks, the motor cortex provides direct and indirect input to jaw muscle neurons [2-7]. Motoneurons of jaw-closing muscles are reached by direct, fast-conducting, mainly crossed, corticobulbar fibres , whereas suprahyoid motoneurons are reached by fast-conducting, but mainly bilateral, corticobulbar fibres . A group of cells in the region of the medial bulbar reticular formation, called the central pattern generator, elicits alternating movements by contraction of jaw-opening and jaw-closing muscles not dissimilar to those of natural chewing cycles . The central pattern generator can be activated by adequate inputs from specific higher centres  and is organized flexibly. Peripheral receptors can monitor progress and modify the commands being sent to the muscles, either by affecting the central pattern generator directly, via descending motor pathways, or by superimposition of jaw reflexes .
In this context, peripheral feedback by periodontal mechanoreceptors, muscle spindles, and temporomandibular joint afferents are assumed to be crucial aspects of the regulation of recurring jaw movements during chewing [13-18]. Because cell bodies of muscle spindles and periodontal mechanoreceptors are located in the trigeminal mesencephalic nucleus, functional linkage of these cell groups has been suggested [19, 20]. In particular, the muscle spindles provide proprioceptive information about jaw position. The continuous change of spindle properties and fusimotor activity during jaw opening and closing requires recalibration of the mechanosensors. Muscle spindles and periodontal mechanoreceptors have direct projections to the cerebellar cortex. On the basis of this convergence, the cerebellum is able to alter fusimotor activity and to regulate the gain of muscle spindles appropriately by comparison of information from muscle spindle afferents and tooth contacts in intercuspation [21-23]. This inter-relationship might also be used for temporal recalibration of jaw muscle spindles , ensuring precise control of jaw motion into intercuspation.
With regard to the control mechanisms discussed, it might be inferred that even during a deliberate single closing motion (beyond the program-controlled movements during chewing and swallowing), programmed synergies of jaw muscles, essential to achieving precise coordinated jaw closing, will be maintained even if intercuspation is prevented. However, it is not known (i) whether the assumed precision definitely exists, (ii) how different jaw gaps affect closure precision, and (iii) what the term ‘precision’, as a numerical measurement, means in this context.
The main purpose of this study was to test the hypothesis that, despite the prevention of intercuspation, the precision of jaw-closing movements fluctuates within a range of physiological closing movements indispensable to achieving intercuspation without significant interference. In addition, we wished to scrutinize the motor-control strategies involved in these motor actions.
- Top of page
- Material and methods
- Conflicts of interest
- Supporting Information
This study was performed to test the hypothesis that, despite prevention of intercuspation, the precision of jaw-closing movements fluctuates within a range of physiological closing movements indispensable for achieving intercuspation without significant interference. The main result of the investigation was that for jaw gaps ≤4 mm, the reproducibility of closing movements was within the range hypothesized; however, this was not so for jaw gaps larger than this value. Thus, for closing movements in the vicinity of intercuspation the hypothesis was confirmed. In addition, the spatial positions of the incisal point and those of the condyles for these jaw gaps were not significantly different among physiological closing, fast closing, and slow closing. Of particular interest was the finding that the precision of jaw-closing movements in the range 0.3–0.4 mm was also found for centric relation recordings.
The small, but significant, differences between condylar and incisal positions for jaw gaps >4 mm among physiological closing, fast closing, and slow closing, in contrast to gaps of ≤4 mm, requires detailed analysis. The differences might, in particular, be the result of a jaw-gap-dependent loss of target position memory, possibly caused by the disorientating procedure in the initial phase of the recordings (protrusive and retrusive movements and centric relation recording) or by the experimental methodology itself. However, this explanation is valid only if, in principle, the neuromuscular system controls closing in two phases with different precision – an initial phase with lower precision at jaw gaps of >4 mm and a second phase with high precision at gaps of ≤4 mm. The first phase might be more sensitive to extrinsic interferences and conscious alterations than the second because the phase close to intercuspation needs precise control of direction of motion and velocity to avoid the damaging impact of the teeth . The decreasing reproducibility of the three closure tasks at increasing distances from intercuspation for physiological closing, slow closing, and fast closing also supports this notion. So, an appropriate explanation of our findings at wider jaw opening might be that during slow closing and fast closing expectation of an impact and/or the disorientating procedure might have recruited motor units with slightly different force and velocity characteristics to stabilize the jaw motion in the closing phase before impact. The recruitment changes developed a resultant force vector displacing the jaw slightly more in a posterior direction than during physiological closing. Close to intercuspation (≤4 mm), however, as a result of the slowing of jaw velocity driven by stringent control strategies, consistency with physiological closing is maintained. An additional aspect must be considered in this context – the decreasing effect of restoring forces released from the strained surrounding tissues during jaw-closing movements. These forces, which are not controlled by the neuromuscular system and depend on the biomechanical characteristics of the actual tissue, naturally require some flexibility in kinematic behavior, in particular for wider jaw gaps where their effect is most pronounced.
The reproducibility of the condylar and incisal positions for gaps >4 mm differed significantly for physiological closing and fast closing, the variability between replicates being greater for the condyles than for the incisal point. As defined by the uncontrolled manifold method, the uncontrolled manifold ratio of >1 for jaw gaps ≤4 mm clearly demonstrates that stabilization of the movement of the incisal point is of primary importance, in particular in the close-up range of intercuspation. The uncontrolled manifold theory predicts that the central nervous system does not restrict the redundant degrees of freedom of the motor system but uses all of them to ensure flexible and stable performance of motor tasks. Functional motor synergies involve organization of elemental variables to stabilize important performance variables. Applied to our experiment this implies that intercuspation is the stabilized performance variable and the central nervous system uses the redundant possibilities of condyle–disc configurations (the three dimentional displacement capacity of the condyle–disc complex can also be inferred from the (partly significant) displacement differences between the three reconstructed closing trajectories illustrated in Fig. 4) in combination with both joints and the variability of intra- and intermuscular co-contraction to enable interference-free impact of the teeth. In addition, the control strategy has long-term robustness, as indicated by the excellent repeatability (ICC > 0.8) within the 2-wk period demonstrated for all closing conditions.
One limitation of the study might be that the results were obtained from a young population only. However, it can be inferred that a programmed basic motor strategy avoiding tooth damage during powerful closing during chewing might not be altered substantially during a lifetime. Another limitation might be the instrumentation used to register jaw position. However, all the tests performed had identical prerequisites, which makes it unlikely that the results might be altered significantly by the instrumentation.
Our findings have clinical implications. In this study, as found also in several previous studies [33-36], examiner-guided centric relation was significantly posterior in comparison with intercuspation for all measured jaw gaps. Although examiner-guided recording techniques are established methods, there are undoubtedly significant differences between intercuspation and centric relation . Considering these data, and regarding changes of jaw-muscle activity provoked by this jaw relationship, previously demonstrated with oral splints and simulated prosthetic reconstructions [25, 37-41], it might be assumed that spatial deviations of the jaw position from intercuspation might require immediate substantial neuromuscular adaptation, in particular if jaw-gap changes are also implemented. It can, however, be inferred from the success of examiner-guided recording techniques over numerous decades that the motor system of asymptomatic subjects compensates for this challenge without any problems. However, for subjects with reduced adaptive capacity, such as asymptomatic patients with temporomandibular disorder history, the experimentally applied jaw-positioning procedure might be a means of recording and/or adjusting centric relation to reduce motor adaptations. In addition, jaw positioning with unrestricted closing movements might be a useful technique for oral splints. This hypothesis is supported by recent findings [42-44] demonstrating motor adaptations on experiencing pain to save the neuromuscular system from further injury, a neurobiological self-care mechanism. This behavior might be facilitated by oral splints fabricated in a jaw relation determined by unrestricted closing movements, presumably implementing the motor adaptation to pain. However, clinical trials are required to support these speculations.
In conclusion, our findings provide evidence of stringent neuromuscular control of jaw-closure movements in the vicinity of intercuspation, but not for wider jaw gaps. Further studies are needed to confirm the possible clinical implications of these findings.