Dental Technology

Registration rims

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The base of the registration block should normally be made of a rigid material. Close adaptation of the base to the working cast is essential for stability in the mouth and accurate registration of jaw relations.

Registration rims are usually made of wax. The upper block should measure approximately 22 mm in height from the deepest part of the sulcus adjacent to the midline fraenum. The equivalent dimension of the lower block should measure approximately 19 mm anteriorly

Wax rims are positioned bucco-lingually in the same place as the lost teeth, according to the amount of resorption that has taken place. The occlusal surface of the lower rim passes posteriorly from its anterior edge to a point 2/3 up the retromolar pads. The upper rim should be created using an occlusal rim inclinator so that in the mouth it can easily be adjusted to be parallel to the alar-tragal line antero-posteriorly.

The registration rims are mounted on a semi-adjustable or average value articulator (according to clinical requirements), preferably using the split cast technique. After noting the prescription for tooth arrangement, the maxillary anterior teeth are set up in accordance with the marked centre line, always conforming to the contour of the wax rim.

Unless the prescription says otherwise, or a neutral zone technique has been used, the mandibular posterior teeth are placed to conform to the buccal contour of the wax rim. It is wise, particularly with flat lower ridges, to avoid the most posterior tooth being positioned over an inclined plane, and to achieve this, the last tooth should be at the posterior extremity of the horizontal part of the ridge.

The teeth are adjusted to allow balanced articulation in lateral and protrusive excursions. Any part of the try-in base which was removed to facilitate registration is replaced unless this interferes with occlusal balance.

How to make a special tray

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Upper spaced tray

1. Make sure the cast is trimmed close to the sulcus

2. Design the dimension and form of the tray drawing a margin liner around the model approximately one to two mm short of the deepest part of the sulcus

3. Soak the cast in the water for some minutes so that the wax will not stick to the cast

4. Heat, adapt and cut the wax onto the model following the prearranged guide marks

5. A second layer of wax has to be applied to obtain a 2 to 4mm of thickness

6. Using a blade occlusal stops has to be made to provide stability

7. Adapt the light-cured over the wax space

8. Press the palate down first after the ridges and sulcus and carefully around the remaining teeth

9. Remove the light-cured acrylic excess with a knife

10. Using the remaining light-cure produce a straight handle due existence of anterior teeth

11. Fixed the cable on the anterior part of the tray

12. Make holes around the tray with carbide bur to allow impression material’s retention

13. Leave the tray 5 minutes on the light curing box

14. Remove the tray from the model

15. Remove all the wax remaining after curing

16. Trim the sharp edges and irregularities

Apply varnish on the tray and until to set.

Lower close fitting tray

1. Follow the same procedures describe above sequence 1-2-3 and 4

2. For the close fitting apply only one layer of wax

3. For the close fitting tray is not necessary to produce occlusal stops

4. Adapt the light-cured over the wax space pressing around the ridges, buccal and lingual sulcus

Remove the light-cured acrylic excess
5. Remove the light-cured acrylic excess with a knife

6. Using the remaining light-cure to produce stub style handle design for edentulous patients

7. Leave the tray 5 minutes on the light curing box

8. Remove the tray from the model

9. Remove all the wax remaining after curing the tray

10. Trim the sharp edges and any irregularities

Apply varnish on the tray and wait until sets with a knife around the lingual and buccal sulcus

How a poorly designed and/or constructed tray can have a negative effect on the outcome of the master cast.

The custom made impression trays are designed by the dental technician for an individual patient, they are constructed based on the preliminary cast (first impression) and is used no more than one time to take the secondary impression from the patient being discarded.

The material most common used to produce the special trays is the light-cured acrylic due to its price, ease manipulation to be adapted onto the model and working time. There are many advantages to use custom trays:

· The distortion of tissues are controlled

· Control over the flow of the impression material

· Great support of the impression material

· Minimize patient’s discomfort fitting

· Save impression material

· Reduced distortion during pouring of model

· Used for muco-static and muco-compressive impressions

Basic principles are very important to be followed when custom trays are constructed. Custom trays need to be rigid enough to support the impression material, some times it could be too flexible, then the secondary impression will not be accurate due the pressure applied on the tray forcing it to bend causing distortion on the impression to be taken.

The stability of the tray can have a negative effect on the impression taken due to the thickness of the light cure used when constructed. The exact thickness must beapproximately 2mm, and 1mm short to the mucobuccal fold to allow for border moulding. Sharp and rough edges have to be trimmed appropriately for the reason that

may irritate the patient.

The impression tray made with light-cured have to become rigid when goes into the light-cured box. The dental technician as well has to make the tray no too thin, especially where the handle is positioned because it probably will break due the pressure applied on this region.

The custom tray cannot be too large, the tissues located around the edges of the impression will be affected being distorted on the impression, or if the tray is too small the tissues will not receive support, the full depth and width of the anatomical sulcus will certainly not be obtained.

Some special trays are required to be made with holes; it will be requested by the dentist if necessary depending the type of impression taken and the material to be used on. These holes have to be made distributed all over the tray being not too big and not to small, the impression material before set will undergo pressure so parts of it will flow out side these holes. These holes will stop the impression to be distorted then the secondary cast will be more accurate when poured.

The design from the handles is very important when a custom tray is made. The dental technician has to be aware about these 3 basic designs:

Stepped: The handle can protrude from the mouth

Without distorting the lips

Straight: Used for fully dentate patients or where

anterior teeth are standing

Stub: Used for edentulous patients, who require some

lip support

There are many reasons that a handle’s position on the custom tray could affect directly the impression taken. The impression taken from mandible or maxilla requires different support made by dentist’s hands also the condition of the patient’s dentition, oral tissues and ridges may affect the type of the handle to be constructed. Using the right handle’s shape will be possible to achieve:

· Better support of the impression material

· Controlled distortion of tissues

· Reduce the discomfort for the patient

· Obtain accurate impression from denture related surfaces, alveolar ridges and mucosal tissues

· Reduce the impression material used.

The handle should be positioned in the anterior so that it does not interfere with placement of tray or border moulding procedures.

The final outcome cast when the handles are not in the right position will result in a poor representation from the tooth surface, ridges and soft tissues

When a custom made tray is constructed is essential to make stops, however this design does not apply for close fitting trays. The stops are used to hold the inner surface of the tray out of contact with the patient’s tissue when the impression is taken.

The second impression taken using a custom made tray without stops possibly will display problems on the oral impression. Probably, the final outcome for the second cast when poured will be an inadequate representation of the patient’s dentition, as a result the dental technician will not be able to construct indirect dental restorations or removal dental appliances with acceptable accuracy.

How to make a dental cast for dentures

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The procedure of pouring the primary and secondary cats are the same, first the technician must clean and disinfect the impression then apply a debubblizer to reduce the bubble formation, when pouring the cast and a separator to allow the cast to come out as it can be stuck on the impression causing the cast to break.

To pour the cast first the plaster has to be prepared, the mixture used was 50 ml of water and a 50/50 mixture of plaster of paris and kaffir D stone and then steered to a consistent cream, some may prefer other gypsums, often a 50/50 mixture is used to reduce costs.

After the impression is placed on the vibrator, then the plaster can be poured slowly filing all the impression, after the based is poured in flat surface covered with paper, the filled impression is turned over to join the base, the excess of plaster has to be cut of with a plaster knife forming a nice rounded shape around the base.

Then it can be let to set for about 30 to 40 minutes before the cast can be removed and trimmed

How to make a denture

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The manufacture of full and partial dentures is not an easy process. Denyists and technicians are the members of dental team responsible for the construction of prosthodontic appliances. Team approach has to be used to complete the final prosthesis successfully; patients have to collaborate for its completion. Itcan take up to 4 weeks to the denture to be rady

The dentist will take a patient’s impression using a stock tray then sent it to the dental laboratory along with prescription; also the impression must be disinfected and labelled

The dental technician receives the stock tray from surgery. First of all ,the technician has to be able to complete the work required described on prescription, if not the work must be send to another dental laboratory competent to deal with it.

The dental technician must wear PPE (protective personal equipment) when required. Disinfection of impressions also has to be done straight away to avoid risk of cross contamination.

The impression received must be casted by the technician to create a model with 50/50 plater of paris and kaffir D gypsum material.
The next stage, the dental technician will make a custom made (special) tray using the cast already poured in light cured acrylic, it will allows the dentist to take an even more accurate impression from the patient. ( special tray is sent back to the surgery for another impression).

Another impression is taken by clinician, and then sent back to the dental laboratory. The dental technician will pour another cast with more accuracy of details. Bite blocks made of wax are constructed on it (upper and lower) and sent back to the dentist for try in and adjustment in the patients mouth made by the dentist.

The dentist uses the bite block to register the relationship between your upper and lower jaw. He/She should also take measurements to determine the correct positions for the individual teeth to be placed. At this time the tooth colour and size are usually chosen.

The dental laboratory having received all the information from the bite stage will use this information to construct the dentures, using the bite block and the chosen teeth.
mimic the jaw relatiomship on what we call an Articulator. This will enable the dentures to be made in the correct position in regard to each individual’s mouth.

Articulation of wax rims should be constructed with balance occlusion/articulation to reach maximum effectiveness in mastication, before beginning the setting up of the teeth. It is important procedure that the artificial teeth must be set in definite relation to the casts and occlusal plane to create a relationship between both of them.

A try- in stage is required, that is when the deture is first contructed in wax, than sent to the dentist to check if any changes are required. Changes could incvolve the fitting of the base plate of the denture, to teeth positionining, dimensions and size and colour.

When the teeth are settled up they must lay within the boundaries of the muscular neutral zone otherwise the oral musculature will dislodge the denture when in use. Is important that the teeth being placed are over the crest of the alveolar ridge to reduce the probability of tipping forces dislodging the denture as the main bulk of tissue and bone is supporting the business end of the denture.

Before positioning of teeth some dimensions must be established:

Peripheral outline of base plates

It is essential that the base should cover the maximum bearing area to increase the retention and stability of the denture.

A poorly fitting denture would result in minimal surface tension and peripheral seal. The peripheral edge of the denture should be rounded and slightly enlarged to conform to the shape of the sulcus. The roundness of the periphery helps fill the full of the sulcus and reduce the radius of the salivary meniscus so increasing surface tension.

Posterior palatal border

Sited in the area of soft displaceable tissue just posterior to the border of the hard palate this location is ideal for a posterior dam because:

· It will assist in maintaining a peripheral seal, thus contributing to denture retention

· It will prevent food debris from seeping under the denture thus contributing to patient comfort

Occlusal vertical dimension

Vertical dimension is the height of the face between any two arbitrarily selected points that are usually located on the tip of the nose and on the chin.

Rest vertical dimension is the vertical dimension of the face with the mandible in a rest relation

Occlusal vertical dimension is the vertical dimension of the face when the teeth or occlusion rims are in contact in centric occlusion.

The occlusal vertical dimension should provide for most patients a minimum inter-occlusal clearance (freeway space) of 2-4 mm in the premolar region. It is established by adjustment of the lower occlusal rim and verified using various techniques of clinical measurement.

Failure to provide sufficient freeway space may lead to muscular discomfort, pain involving the denture bearing areas, and possible increased bone resorption. Excessive freeway space may lead to cheek biting, angular cheilitis, poor appearance and contribute to discomfort from the temporomandibular joints. Progressive incremental additions of acrylic resin to the occlusal surfaces of existing or diagnostic dentures may be necessary before a satisfactory occlusal vertical dimension can be established.

Centric occlusion and articulation

Centric relation is the most posterior relation of the mandible to the maxilla at a selected dimension. The centric occlusion must be recorded on the wax rim.

The primary reason for articulation is to maintain the correct relationship of the mandible and the maxilla while setting up the teeth to ensure the correct positioning of the teeth.

Positioning of teeth in the dental arch

Other factor that is required to be respected is the mechanical conditions. Dentures have to offer balanced occlusion when at the same time contacts of the occluding surfaces of the teeth on both sides of the opposing arches happens during mastication. The arrangement of the teeth have to enable them to maintain an even sliding contact during mastication without encountering cuspal interference, to maintain the denture stable in the mouth when displacing forces act upon them.

The upper anterior teeth must be set to maintain the fullness of lips and achieve a desirable aesthetic result. The labial surface of the maxillary incisal should be 8-10mm in front of the centre of the incisive papilla. The incisal edge should be on the occlusal plane. The midline should be taken from a line drawn down the middle of the face. The maxillary centrals should be on that (vertical) line. The lower midline should coincide with the upper, however this is not critical. There should be buccal overjet all the way around the arch.

The upper posterior teeth should be set as straight as possible, with a slight curvature of the occlusion surface towards the posterior plane of the denture, the palatal cusps should be positioned directly above the lower ridge.

All lower teeth should be placed directly over the lower residual ridge and within the neutral zone. This will increase the stability and retention of the denture. The lower posterior teeth should be positioned directly over the lower residual ridge and within the neutral zone for maximum stability.

When waxing up the denture the gingival marging and the interdental papilla should be shaped according to the patient’s age and oral health. Waxing up the denture will access on the restoration of the facial contour. The labial flange area is a very important to restore the facial contour, and is essential to restore it to its correct thickness and avoid any unnecessary bulk, however it is necessary to have some bulk on the canine areas to reproduce the canine eminences, and give support to the corners of the lip.

When the denture is completed and ready to finish into acrylic (teeth setting-up/ correct shape of denture flanges/correct level of occlusal plane), then is time to initiate the flasking process:

· Trial wax denture must first be seal it down with wax to prevents plaster getting between the wax denture and model during investment;

· Model can be covered with could mould seal/ soap as separator agent;

· Denture flask is applied Vaseline to the inside surfaces to help in devesting;

· Add plaster on the shallow half of the flask with a mixture of plaster of Paris;

· Smooth the edges of the flask using a plaster knife;

· Allow it to set then separate all the plaster with either could mould seal/soap/Vaseline

· Check the flask will still close fully and denture position is right;

· Spray de-bubbliser on the wax work, and rub the mixture between all the teeth, and

· Place the shallow half onto the other half, and then leave to set.

The mould is then held together under pressure and placed into a curing bath where the process of hardening the plastic takes place, usually over several hours.

When the denture has been processed and has cooled down, the denture is removed from the plaster mould. Any excess material is then trimmed off and the denture is polished.

The removal of the polymerised denture from the flask requires to be undertaken with considerable care to avoid either damaging the teeth or fracturing the base.

After deflasking any residual material is removed by light grinding and a final polish applied to the denture by use of differing grades of abrasives and the application of a polishing agent to give the polished surfaces a surface which is truly reflective and easy to clean.

During the abrasion and polishing process care should be taken to prevent excess wear of the peripheral edge and teeth, also the surfaces and edges of prostheses must be smooth and free of sharp edges which could cause damage to the patient’s mouth when in use. The final prosthesis is sent back to the dentist.

The dentures are then reseated to the master cast and the occlusal balance restored by selective grinding where necessary. The finished denture is then returned to the surgery.

After all this process the clinician must ensure that the prosthesis meets the client’s functional and aesthetic requirements, if not the final adjustaments will be made and the denture returned to the dental laboratory.

Standard Operating Procedure:

1. Articulate the wax rim, according with the recorded dimensions.

2. Set up Teeth

3. Wax up denture

4. Send for trial

5. Correct anything that was required from the trial

6. Final wax up

7. Flask

8. Eliminate the wax

9. Could mould seal the cast mould surface

10. Pack the acrylic

11. Polymerisation (cure).

12. Deflask

13. Trim

14. Polish

Complete Dentures Terminology

Denture Space is that portion of the oral cavity which is may or may be occupied by maxillary and / or mandibular dentures. It is the space between residual ridges which is available for dentures. It is that space in edentulous mouth which was formerly occupied by teeth and the supporting tissues which have since been lost.

Four Essentials for the Efficient Functioning of Complete Dentures

Support, retention, muscle balance and occlusal balance

Support is the foundation on which the denture rests. It consists of the tissues which bear the load of mastication on the dentures.

Retention is the resistance of the denture to removal from the mouth.

Muscle Balance implies that the muscular forces of tongue, lips and cheeks act on the denture in such a way that the denture is not dislodge during functional movements of the mouth, with the teeth out of contact.

Occlusal Balance implies that the forces exerted by one denture on the other act in such a way that the dentures are not dislodge during functional movements of the jaws with the teeth in contact.

Stability is the quality of denture to be firm, steady, constant, and not subject to change of position when forces are applied. It is the quality of denture to resist displacement by functional stresses.

Articulation refers to the static and dynamic contact relationship of maxillary and

mandibular teeth as they move against each other during function.

Occlusal Interference Any tooth contact that inhibits the remaining occluding surfaces from

achieving stable and smooth contacts.

Occlusal Pattern The form or design of the masticatory surfaces of a tooth or teeth based on

natural or modified anatomic or non anatomic teeth

Maximal Intercuspal Position The complete intercuspation of the opposing teeth

independent of the condylar position.

Overbite: Describes the amount of coverage of the lower anterior teeth , by the upper

anterior teeth in a vertical plane, The average amount being 2mm.

Overjet: The distance between the palatal surface of the upper anterior teeth, and the labial

surface of the lower anterior teeth. The average amount being 2mm.