Prosthodontics

On the Move

In this article I should like to explore ways of improving the accuracy, first time, when recording jaw relations (taking the bite) for both complete dentures and maxillary dentures opposed by natural mandibular teeth, as this is the most common fault during denture construction.

Complete Dentures

In the effectiveness and efficiency tip, we have explored the concept of opportunity costs when procedures have to be repeated. How often are you having to retake the bite for any case? The usual method is for your technician to make both maxillary and mandibular wax baseplates with wax rims attached. It is impossible for you to hold these firmly in place whilst manouevering the mandible as well to achieve centric jaw relation. It is essential to achieve this sometimes elusive position, yet this is the physiologically reproducible place that the muscles of mastication return the condyles to after every chewing stroke and swallow. If this is not found then the dentures will be unstable.

My method is to make a heat cured maxillary base plate on the second, working impression, to which the wax bite block can be attached. It has the following advantages:

1. It provides a stable maxillary platform, so that just the mandibular wax rim needs holding in place when recording centric jaw relation
2. It enables you to check the retention of the maxillary base plate at an early stage in the denture making and improve if necessary
3. When the base plate is really retentive, your patient can be advised that this is how ‘tight your new denture will be’ – a very useful way of creating confidence!

You can then concentrate on trimming the blocks for lip support and the correct occlusal vertical dimension.

Always insist that the try in stage is returned from the laboratory in its own articulator

1. You can check on the accuracy of the articulator joint
2. It is easier to adjust teeth in wax when they are on an articulator rather than on loose models.
3. It the bite is wrong, retake but don’t just ask for remounting. I have known this to be overlooked in the laboratory and the original set-up returned on more than one occasion. Prevent such problems and your wasted time by process re-engineering. Break off the mounting plaster from one side and throw it away. The laboratory have then got to remount the case to the new bite when they get the case returned ready for your retry stage.
4. Better still buy your own low cost freeplane articulators (about £45 each for example Medesy, Maniago, Italy) for your technician to mount your cases on. This has a further advantage. It is not unknown, even if you have set the incisal pin to zero that, when the case is returned from the laboratory, the occlusal vertical dimension has been opened by 1 or 2 mms.
   
   The usual causes are insufficient tooth reduction or seven denture teeth have been crammed into the arch space. This is not necessary, remember the shortened dental arch, and be prepared to leave off either the second molar or a first premolar.

Technicians seem to think that opening the bite on the pin does not matter. How wrong they are. If the vertical dimension is opened on a freeplane articulator, then it is virtually certain that the occlusion will be incorrect at the try in stage. The reason is clear. Models will have been mounted on this articulator without using afacebow and there can be no record of the hinge axis. If there is no hinge axis record vertical dimensions must not be opened on the incisal pin.

My solution to prevent this problem ever arising is to superglue the incisal pin at the zero position. It then cannot be moved, and if the case is returned with the pin not fully seated on the incisal table, it is immediately obvious where the fault lies.

Maxillary complete dentures opposed by natural teeth

This is an altogether different challenge to obtain and accurate bite. In my experience, even if you follow the technique in (A) above errors are likely, and if you use conventional wax bite blocks, errors are certain. The diagnostic sign is a finished maxillary denture that may have a retentive baseplate, but moves during function and is therefore unstable. Why should this happen, and is there a way to prevent this?

It is common to find that patients who have been provided with complete maxillary dentures and mandibular partial dentures do not wear the latter, or if they do tissue support declines with time and is not noticed. This leads to significant anatomical changes:

1. During function, the maxillary complete denture rotates around an axis between the first premolar positions and this movement over time leads to (ii) to (v) below.
2. There is loss of occlusal vertical dimension
3. There is resorption of the maxillary anterior ridge, which may become flabby and in severe cases may resorb to the level of the anterior nasal spine.
4. There is concomitant enlargement of the maxillary tuberosities.
5. The relationship of the maxillary ridge to the Frankfort Plane changes so the inclination is closer anteriorly rather than posteriorly, as nature intended (Fig 1). This creates an inclined plane which exacerbates displacing forces.

Prevention is based on the recognition of these adverse changes and depends on following the recipe below:

1. Ensure that the mandibular bite block is really retentive and will not move during record taking. If necessary make an autopoymerising resin baseplate and add clasps. Even better if a cobalt-chromium partial denture has been advised, make this first and add the bite block to it.
2. On the maxillary first impression make a closely fitting, correctly extended, special tray and attach an occlusal rim directly to it.
3. Record vertical dimensions in the usual way adding an extra 1mm freeway space, trim the bite blocks and record centric jaw relation, accepting that there may have been slight movement, then separate the blocks.
4. Add a post dam in border moulding compound.
5. Make a small hole on either side the maxillary baseplate in the second premolar region close to the midline.
6. Apply elastomeric impression adhesive to the fit surface of the maxillary special tray.
7. Mix and apply an even layer about 2-3 mm deep of medium flowable elastomeric impression material into the special tray and seat into the mouth. (I always liked to use Impregum)
8. Refit the lower bite block and ensure it and the lower teeth are correctly placed into the registration from 3 above, get the patient to close firmly together until the maxillary tray is fully seated as checked by visual inspection anteriorly. Tell the patient to keep their jaws closed whilst border moulding is carried out. Place your hand under the chin, so you can check that the patient is not opening their mouth whilst the elastomer sets. The two holes allow excess material to escape. This creates a closed mouth impression.

Any baseplate movement that has occurred during the conventional record taking has thus been corrected by the thin layer of impression material. When removed from the mouth, inspection of the fit surface of the impression shows a very thin layer of impression material in the vault of the palate and 1-2mm thick at the posterior border of the tray. This appearance confirms that movement of the rim has occurred during bite taking. The base has rotated across the first premolar axis but this movement has been corrected by the closed mouth impression. The 1mm extra freeway space allows for the thickness of the impression.

The impression can be boxed, poured up and the models articulated ready for the subsequent stages of denture construction.

Whilst the above methods will take slightly more clinical time, in the long run, they will save you time from repeated eases of unsatisfactory dentures and the ever present risk of complaints.