A preliminary report on the determination of the vertical dimension of occlusion using the principle of the mandibular position in swallowing

N. K. Mohindra, BDS

When constructing replacement complete dentures, trying to achieve the correct occlusion vertical dimension (OVD) can be difficult, particularly in those instances where there is marked over-closure. If the vertical dimension is too great the patient will complain of soreness of the residual ridges, tightness of facial muscles, and clicking of the dentures during speech. An increased OVD is also supposed to induce an increased rate of resorption of the remaining alveolar bone. If the vertical dimension is too small, the patient will look older, and there may be angular cheilitis and temporomandibular joint pain.

In the prosthodontic treatment of the edentulous patient, we are told to use an OVD which is 2-4 mm less than the resting position of the mandible (RVD). However, the RVD is not static and is influenced by a number of factors.¹ It has been shown that the RVD tends to follow the OVD.² As the OVD reduces, due to wear of the artificial teeth and alveolar resorption, the RVD also reduces. Thus new dentures constructed at an OVD 3-4 mm less than the measured RVD may result in an over-closed appearance. If it is proposed to make a large increase in the OVD, then it is recommended that the changes should be tested initially by making gradual additions to the occlusal surfaces of the old dentures.³ In this way the OVD can be built up by 2-3 mm at a time, until an acceptable height is achieved. It is argued that this makes it easier for the patient to adapt to the increase and also allows the dental surgeon to assess exactly what changes the patient can accept.

It can be difficult to obtain a true record of the RVD and, because of this, alternative methods have been used to estimate the OVD for new complete dentures. A screw jack device has been used to allow patients to select their preferred OVD .4 - 7 This method is time consuming and the results vary depending upon whether the estimation commences with the screw jack open or closed.

Biting force has also been used for determining the vertical dimension.8 However, biting force can be modified by touch pressure and sub-threshold pain stimuli arising from forceful biting on the denture bases.9 The use of swallowing is another method that has been used.10,11 During swallowing the mandible is said to relate to the maxilla at the 'correct' vertical relation. This paper describes the use of the swallowing method in establishing the OVD when constructing replacement complete dentures.

Materials and method
Twenty-one people participated in the study (14 female and 7 male). Their ages ranged from 41 years to 91 years with a mean age of 67.0 years. The dentures being replaced had been worn for periods ranging from 3 to 45 years, with a mean of 14 years. Fourteen of the patients had worn the same set of dentures for 10 years or more (Table 1). All the patients attended one general dental practice between February 1993 and June 1993 and were treated by the same general dental practitioner. The sequence of treatment for all patients was similar.

At the first visit a medical and dental history was taken and a dental examination carried out. The proposed treatment was explained to the patient, including the fact that their existing dentures would be modified by adding material to the occlusal surfaces.

At the second visit acrylic resin was added to the lower denture to convert it to a pivot appliance, which provided occlusal contact in the first molar region only The additions were made using light-cured acrylic resin (Triad; Dentsply Ltd). The material was used according to the manufacturer's instructions, apart from the fact that the bonding, agent was applied for 1 minute, instead of the recommended 2 minutes, before curing. This allowed the acrylic resin pads to be removed easily at a later date, if the additions were unsuccessful, so restoring the denture to its original state.

Table I Chart showing age of patients, age of dentures and increase in vertical dimension of occlusion of new dentures

To form the build-up, the lower denture with the uncured acrylic resin additions was seated in the mouth and the patient asked to take a sip of water ,and swallow. It is important to achieve a normal swallowing pattern and this is helped by asking the patient to wet their lips with the tongue and swallow, keeping the lips gently together, The acrylic resin pads were then cured. When hard, the pads were adjusted to remove all indentations of the opposing teeth so providing flat surfaces for the upper teeth to occlude on. If the upper teeth were very worn, the mesiopalatal cusps of the upper first molars were built up so as to obtain point contact between the upper denture and the acrylic resin pads. If necessary a tissue conditioner was added to the fitting surface of the dentures.

At the next visit, the occlusion was checked for any changes that had occurred in the jaw relationship and adjustments made if necessary. Treatment did not proceed until the patient was comfortable and a stable jaw relationship obtained. Once this had been achieved, new dentures were constructed using a copy technique. Whilst constructing the new dentures care was taken to ensure that the OVD was as close as possible to that of the modified dentures. This involved taking impressions in the trial dentures and to avoid an increase in the OVD, the pin was adjusted on the articulator by 2 mm prior to setting up the teeth, to allow for the eventual thickness of the impression material. All measurements of vertical dimension were made using a pair of dividers with dots on the nose and chin, with the patient sitting upright and relaxed. Using this method the following information was recorded:

1. The OVD of the old dentures.
2. The OVD of the old dentures with the occlusal additions.
3. The OVD of the new dentures.

Although at the recall visits the patients all exhibited a speaking space, the RVD was not measured.

Once treatment was completed and the patients were no longer attending the surgery, a questionnaire was sent to them to ascertain their views of the new dentures.

Results
In all cases the swallowing method resulted in the new dentures being constructed with an increase in the occlusal vertical dimension, compared with the old dentures. The mean increase was 9.7 mm. The largest increase was 19 mm, whilst the smallest was 3 mm (fig. 1 and Table I). Subsequent to the fitting of the dentures, it was necessary to reduce the OVD in three people as they were unable to tolerate the increase. In two cases the OVD was reduced by 3 mm and in the other one case by 2 mm. All 21 patients responded to the questionnaire that was sent to them and all of them have continued to wear their new dentures. Their answers are summarised in Table II.

Table II Answers to the questionnaire

Discussion
The use of pivots to modify the occlusal surfaces of complete dentures is well recognised.12,13 The purpose of the pivots is to disclude the worn down artificial teeth so breaking the habituated path of closure. At the same time it allows for the restoration of the vertical dimension of occlusion. It also allows the condyles to descend towards their unstrained vertical position and to assume their unstrained horizontal position. Sears, 12 who did the original work with the pivots, used to position the pivots in the molar region whilst others13 have placed the pivots in the lower premolar region as this is supposed to be the centre of balance. In the technique described in this article, the pivots were placed in the first molar position with the built up mesiopalatal cusps occluding, on the lower flat pads.

McNeill14 has postulated that occlusal schemes that encourage bilateral molar support produce relatively low articular loads in comparison to the effect of anterior tooth contact. This reduction in stress assists in the repositioning of the condyles within the temporomandibular joints. Certainly no patients complained of instability of their dentures when the pivots were added. Indeed a number of them commented that their dentures became more stable.

It is often stated in the dental literature that changes in the OVD should only be done gradually and over a period of time.3,15 Yet the experience gained in using this technique would seem to indicate that many people accept and adapt to larger changes. The people participating in this study were for the most part experienced denture wearers and were reasonably fit. They could therefore be considered to be able to accept change. Even allowing for this fact, if this group of patients is typical of those seen in general dental practice, then it would seem that the policy of increasing, the OVD by only 2-3 millimetres at a time is over cautious. It would be interesting to see the results of treatment in a more frail population.

In the construction of complete dentures, restoration of a 'correct' vertical dimension has always been one of the aims. However it is not always easy to decide what the correct vertical dimension should be. The swallowing method is a functional method of recording the vertical relation without having to worry about trying to achieve a record of the resting position of the mandible, a position which is known to be variable and difficult to achieve. When a person swallows, the teeth come together with light contact at the beginning of the swallowing cycle.16 Thus a record of the relation of the two jaws at this point in the swallowing cycle can be used as the vertical dimension of occlusion.

In the present study this method has resulted, on occasions, in large increases. However these are functional increases, 'selected' by the patients, and overall seem to have been accepted. Further work needs to be undertaken to ascertain the extent to which the bulk of the uncured pivots effect the eventual height produced during swallowing. Would more material produce a greater height or would the subject close through it? The effect of the viscosity of the uncured pivots also needs to be investigated. These projects are difficult to undertake in a busy dental practice but their possible effects should not be overlooked. One other problem with carrying out research in general practice is that it is sometimes difficult to adhere closely to a protocol. General practice is a difficult area in which to control the variables. The need to satisfy the customer (the patient) and the need to run the practice in an economical manner make this difficult.

The use of dots on the nose and chin as markers for measuring the OVD, although very common, is not very accurate.17 Every effort was made to ensure that the muscles were not tensed before taking the readings. This was made very difficult as initially, when the denture with the hardened pivots was placed in the mouth, the brain perceived this as a piece of hard food and the facial muscles tensed up. However, these readings were only ever used to compare and contrast measurements made at the same visit. Although readings were used to compare one set of dentures with another readings were never compared from one visit to the next.

All of the people who replied to the questionnaire stated that they were wearing, their new dentures. When asked to compare their new dentures with the ones they replaced, 18 out of 21 said they were better. This percentage is greater, than that found by Davis and Watson when this question was put to 68 people who had been provided with new complete dentures by undergraduates. They found that only 75%, of the people thought that their new dentures were better. However, their group was larger and they were comparing people who had had dentures constructed using a conventional method with those using, a copy technique. Thus a direct comparison should only be made with care.

Two of the signs of inadequate freeway space are pain in the facial musculature and teeth contacting during speech. Eighteen people replied that they did not have any facial pain whilst 20 people replied that they could speak normally.To the question 'Do you get a feeling, of discomfort under your dentures' four people replied that they got a feeling of discomfort under their dentures and one was uncertain, which could be interpreted as saying yes. The column 'uncertain' was included as some patients are reluctant to criticise their dentists work. Hence it was felt that this column would offer them an alternative to criticising the work outright.

There are many possible causes for pain beneath dentures, and undoubtedly too large an increase in the OVD could be a cause . Discomfort and pain could be the result. If unemployed mucosa is used when fitting the new dentures. The complaint of discomfort could not be correlated with the size of the increase in the OVD.

The author first started using the technique described in this article in May 1992. This article contains the results of those patients treated between February 1993 and June 1993. During the period May 1992 to February 1993 the technique was being developed with both the author and the dental technician becoming familiar with the process. During this time the swallowing technique was used on 10 people.

In this pilot group the OVD was increased by too much in half of the patients. This was caused by not allowing for the thickness of the impression materials and by not rehearsing the patients sufficiently with the swallowing process. The processing of the dentures can cause an increase in vertical dimension of seclusion, particularly if the technician has not taken care to keep the flash to the minimum. Failure to keep the flash to a minimum could have accounted for some increase in OVD. Also, sufficient time was not taken to ensure that a stable jaw relationship had been obtained before constructing the new dentures. Once familiarity with the technique had been achieved, the results obtained were very encouraging.

It is important to realise that this is a small study of only 21 people. All of them were judged as needing new dentures constructed to an increased OVD. More importantly, they were assessed as being able to accept and adapt to this change. They are, therefore, to some extent a pre-selected group of individuals, and caution should be taken in applying these results to all patients requesting new complete dentures. A more frail, less adaptable group of patients may not be able to tolerate such large increases. There is clearly a need for further study into the methods of estimating the correct OVD and into patients' ability to adapt to the change.

Conclusion
The swallowing technique is a functional method of recording the vertical dimension of occlusion. The method has been used to form occlusal pivots on old dentures prior to constructing new ones. The height of the resultant pivots has often been surprisingly large, but the majority of patients have accepted and adapted to this increase, and new dentures have been successfully constructed to this height. This work throws doubt on the traditional method of estimating the vertical dimension of occlusion by first measuring the resting face height and then subtracting 2-4 mm for the freeway space. It also brings into question the merit of routinely limiting increases in the vertical dimension of occlusion to 2-3 mm at a time.

Acknowledgements
I would like to thank all the patients who have participated in this trial and also Dr David Davis of King's College Dental School for his help and advice in the preparation of this paper.

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Reproduced with the permission of the British Dental Journal.