A comparative study of three different approaches to the comfortable zone

Increased vertical dimension of occlusion for varying periods differentially impairs learning and memory in guinea pigs

There is an increasing number of studies showing that occlusal dysfunction impairs learning and memory. We previously demonstrated that the brain has a mechanism to calibrate between the activities of spindle afferents and periodontal-mechanoreceptor afferents for controlling the chewing movement, and the accurate calibration can be done only at the proper vertical dimension of occlusion (VDO). Then, the chewing at an inappropriate VDO may induce a severe mental stress due to a mal-calibration. However, it is not clear how the impairment of learning/memory progresses over the period of stress induced by occlusal dysfunction. We investigated by passive avoidance test how the behavior and learning/memory are altered in guinea pigs in which the VDO was raised by 2-3 mm over the period up to 8 weeks. We found that the guinea pigs reared under the raised occlusal-condition (ROC) for 1 week showed a very high sensitivity to electrical stimulation whereas this did not cause the memory consolidation in the 1st-day retention trial, suggesting that such hypersensitivity rather hampered the fear learning. In the guinea pigs reared under the ROC for 2 and 8 weeks, the learning ability was not largely affected and memory consolidation occurred similarly whereas the memory retention deteriorated more severely in the latter guinea pigs than in the former ones. In the guinea pigs reared under the ROC for 3 and 4 weeks, learning was severely impaired, and memory consolidation did not occur. These results suggest that the occlusal dysfunction for varying periods differentially impairs learning and memory.

Regulation of mandibular postures: mechanisms and clinical implications

This review argues that (1) the habitual mandibular position is constantly variable and so cannot be considered as a craniomandibular reference point, (2) there is no unique centric relation, (3) mandibular posture greatly depends on head posture, (4) clinical evaluation of the occlusal vertical dimension is mostly empirical, and (5) neither the vertical dimension at rest nor the centric relation can be determined by means of existing instrument-based clinical methods. However, some physiological conditions exist that facilitate the recording of craniomandibular position.

Comfortable zone of the mandible evaluated by the constant stimuli method

For evaluation of the comfortable zone (CZ) and the most comfortable position (MCP) of the occlusal vertical relation of the mandible, the constant stimuli method with three subjective categories of judgements was applied in 10 edentulous subjects. The evaluated CZ was stable and about 3 mm in width. The occlusal vertical dimension (OVD) of the existing dentures was found to be located within the CZ in nine of the 10 subjects.

A comparative study of two methods for the orientation of the occlusal plane and the determination of the vertical dimension of occlusion in edentulous patients

The aim of this study was to compare two methods used to orientate the occlusal plane (OP) and to determine the vertical dimension of occlusion (VDO). In method A the VDO was established by means of the rest position, the minimal speaking distance, and the patient's profile. Method B used a newly developed registration pin assembly. The VDO was registered using a silicone occlusion rim and the swallowing technique. The results were compared to the values of the new dentures. Three standardized lateral radiographs were taken at the VDO obtained with methods A, B, and at that of the final dentures. On each radiograph the orientation of the OP to the Camper plane and the VDO were measured by two investigators independently. The results indicated no statistically significant differences between the mean VDO with method A and B compared with the new dentures (P greater than 0.05). With both methods it was not possible to orientate the OP parallel to the Camper plane. None of the occlusal planes of the new dentures were parallel either. Their OP diverged on average by 7 degrees dorso-caudally. The time spent with method B to orient the OP and to determine the VDO was significantly lower than with method A (17-50 min).

The effects of changes in bite pressure and mucosal stimulation on the perception of mandibular position in man

In edentulous subjects, the mandibular position (MP) was altered by adjustable hydraulic jacks attached to the denture base. The MP which they began to perceive as high (HMP) was shifted toward the closed position when the bite pressure against the incisal region was increased. Thus the perception of MP may be modulated by information concerning the distribution of bite pressure. The HMP also shifted toward the closed position when the incisal palatal region of the denture-supporting mucosa was subjected to increased mechanical stimulation, the distribution of the bite force being constant. Therefore, mechanoreceptors in the mucosa may be involved in the perception of MP by providing precise information as to the distribution of bite pressure.

The effect of body posture on the preferred vertical dimension of occlusion

Thirteen edentulous adults were investigated to see whether their preferred vertical dimension of occlusion (PVDO) was affected by vertical and horizontal body posture. The PVDO was found by the forced choice method using a graduated series of sticks in random sequence. The subjects were tested in both postures on two separate occasions. No significant difference was found between the PVDO's when the subjects were sitting upright and when they were supine. Nor was there any significant difference in the PVDO of the first and second sessions for either position.

Relationship of the preferred vertical dimension of occlusion to the height of the complete dentures in use

The dependence of the preferred vertical dimension of occlusion (PVDO) of edentulous subjects on the height of the complete dentures the subjects were wearing was investigated. Two experiments were carried out. Experiment I tested whether PVDO is influenced by a change in the vertical dimension of occlusion of the dentures. Experiment II compared PVDO values before and after insertion of dentures that had been constructed with an occlusal vertical dimension equal to PVDO. It is concluded that PVDO is independent of the height of the complete dentures the subject is wearing. Clinical implications are discussed.

On the self-perception of jaw positions in man

A longitudinal study shows that, in the short-term, dentate subjects are able to match fairly well the magnitude of jaw separations to any of three types of memorized standards (imagined, verbally imposed and physically imposed) of various dimensions. All subjects showed instability of the comparison mechanism or long-term changes in the engram. About 66 per cent of the matches were performed with the same precision. The relative precision of the matches is almost independent of the size of the standard. The absolute precision of matching of dentate subjects is similar to that of edentulous subjects. In half of the experiments, the subjects showed a match that equals the magnitude of the verbal standard or the real standard. The subjects tend to give more accurate matches when they refer to a verbal standard than when they refer to a real standard, but the long-term instability of matching is much greater when the subjects refer to a verbal standard than when the subjects refer to a real standard.

Perception of vertical dimension

Ten edentulous adults were investigated to learn more about the preferred vertical dimension of occlusion (PVDO) and the comfortable zone. A screw jack device was employed, using the forced-choice and external approach methods. The dimensional discrimination ability of each subject was assessed at the mean PVDO, and at points 3 mm above and below it. For each subject, both the forced-choice and external approach methods gave a mean PVDO value which was reasonably constant. The comfortable zone widths were less consistent. In all but two of the subjects the mean PVDO values appeared consistent with a clinically acceptable occlusal vertical dimension. No relationship was found between the dimensional discrimination ability of the subjects and their definition of PVDO and comfortable zone.

The precision of measurement and the stability of the preferred vertical dimension of occlusion in man

A longitudinal study on the preferred vertical dimension of occlusion (PVDO) established with edentulous subjects shows that these subjects can be divided into two groups, one in which there is significant instability of PVDO, and a second where the instability of PVDO is not greater than can be expected from the imprecision of the PVDO matches. It is concluded that the subjects in the latter group must have referred to a reasonably fixed standard during their PVDO determinations.

Treatment planning for the management of selected dento-facial abnormalities

Existing treatment planning aids for the management of dento-facial abnormalities do not adequately assess the neuromuscular status of the oro-facial soft tissues. Where an element of mandibular overclosure exists the patient assessment technique provides a reproducibile and clinically acceptable occlusal vertical dimension of occlusion. In the three cases treated a functional impression technique for establishing the upper lip support produced a reproducible profile which was both aesthetically and functionally satisfactory.

Horizontal stabilization of upper and lower borders of the comfortable zone

Using a screw jack device the upper and lower borders of the comfortable zone were recorded in a two step procedure designed to counteract bias inherent in previous measurements. In these measurements, successively increasing and decreasing values of zonal borders were recorded corresponding to a fully closed or fully opened starting position of the screw. Both of these positions are outside the comfortable zone. The two step procedure used a starting position which was supposed to lie inside the comfortable zone. However, the procedure did not achieve its aim fully. It succeeded only in stabilizing the lower border values. In the discussion hypothetical considerations are made concerning the neurophysiological mechanism underlying recordings of the borders of the comfortable zone, which might explain the differential behaviour of the zonal borders.