|Complete denture Vs natural dentition
Patients with one edentulous arch opposed with a fully or partially dentate arch are quite often in clinical practice. The type of constructed prosthesis used to restore missing teeth in partially dentate arch (crowns, bridges, implant supported prosthesis) is affected when it is opposed by an edentulous arch ( restored with a complete denture)
One of the most difficult challenges in prosthodontics is the fabrication of complete denture opposing a dentate arch. Because we have a problem with the forces applied against the denture by the very much stronger opposing natural dentition. The principle challenge is how to stabilize the denture (to develop bilateral balanced occlusion) because if the denture tips and moves during function and particularly during parafunction rapid bone loss ensues.
Therefore we have a problem in bone resorption underneath the denture, this will compromise the support and tissues tend to become hyper mobile (fibrous tissues along the crest of the ridge) so it causes midline fracture of the denture and no matter how many times the patient will attend the clinic to repair the denture, it will fracture again.
So we need to understand the problems associated with applying a denture that is opposed by natural dentition:
1- The natural teeth are much stronger so they displace the opposing denture causing very severe resorption of the supporting bone leading to denture rocking and recurrent midline fracture.
When a patient comes to the clinic with one edentulous arch carefully examine the dentate arch for malposed, tipped or supraerupted teeth (before making impressions or starting trt) because the problem always starts from the dentate arch.
We don't expect people to extract all their teeth at the same time rather it takes separate extractions over a long period of time. By the time one arch becomes edentulous you see an opposing dentate arch with irregular occlusal plane.
Teeth that lose their opposing neighbors over erupt.
Series of extractions followed by series of overeruptions of the opposing teeth causing irregularity of the occlusal plane is the core of the problem.
* unfortunately clinicians overlook this problem, they just start trt by taking impressions till the delivery of the denture to the patient with its occlusal plane inclined according to the irregularity of the opposing natural teeth.
Forces that are applied to the denture by the irregular occlusal plane of the natural teeth cause resorption of bone, wear of acrylic teeth, midline fracture (single denture syndrome).
Immediately starting from today if you come across any patient with a single denture (they usually come after the loss of the anterior teeth as they are the last teeth to be lost due to aesthetic reasons) you should start by carefully examining the opposing natural dentition for overeruption of teeth, severity of overeruption and tilting in order to avoid recurrent returns of the patient due to midline fracture.
2- natural teeth are much more stronger than denture teeth which will weaken them, besides that the surface area of natural teeth is usually wider.
Acrylic teeth are manufactured in a way to minimize the occlusal force from the opposing teeth by minimizing their occlusal surface area.
** We always start by treating the dentate arch according to the following sequence:
1- Treat overeruption
2- Adjust dimension of the occlusal surfaces (contacting surfaces)
3- Reinforce denture teeth to avoid attrition
* if we overlook one of these problems we end up in having a denture that isnt stable and imbalanced due to occlusal irregularity.
Treatment of overeruption
** determine the severity of overeruption
By early dental clinic attendance
We go for enameloplasty ( reshaping or recontouring the enamel tissue) only for teeth that are mildly overerupted making sure that we dont exceed the thickness of the enamel
* mock adjustment using articulating paper to mark the overerupted teeth, start by doning modification on the cast to evaluate the amount that can be removed to achieve a straight occlusal plane without exceeding the enamel thickness ( dont grind them horizontally, but reshape them maintaining the anatomical morphology)
If we think the amount of tissue removed will exceed the enamel thickness or when the patient has erosion or attrition ( any kind of surface tissue loss due to different factors as bruxism..) we have to think about modifying the teeth that are below the occlusal plane by building them up. This is achieved by overlays which are prosthesis that lay over the occlusal surfaces of teeth and can be made of metal ( co/cr) or reinforced acrylic that match the shade of natural teeth.
Overlays are fixed / removable prosthesis, with the removable (made of PVC poly vinyl fluoride like the one used for bleeching but we need to reline it) being more preferable for better hygiene purposes.
Classified according to their material:
Initially provided with allo wax then replaced with either material.
Overlays are not applicable because we will need very thick overlays, lifting the occlusal plane excessively which isnt tolerated by the patient.
In this case we think of fixed prosthesis to adjust the level of the overerupted teeth by crowns or bridges if an edentulous space is present adjacent to the overerupted teeth.
Bridges in this case will serve 2 advantages:
- replace missing teeth
- solve the problem of overeruption in the abutment
** eight can be extracted if severely overerupted unless its badly needed for a bridge
- either crown it if it is to be included in the complete denture
- or keep it as it is if not to be included in the complete denture
** Usually when upper teeth lose their neighbors or opposing, they dont only overerupt but also rotate ( due to the palatal root that acts as a fulcrum) in this case the amount of reshaping (in mild overeruption) doesn’t only envolve enameloplasty but also recontouring to minimize the effect of rotation. In this case we prefer to crown the tooth, unless it’s a 7 or 8 we extract them.
In moderate overeruption can we crown the infraoccluded teeth?
We usually think of crowning when couple of teeth are overerupted but its not applicable to crown 8 infraoccluded teeth for example.
Do overlays increase the vertical dimension of occlusion?
No, we can control it during bite registration to determine the most comfortable VDO to the patient regarding free way space, age, muscles.
The only problem is when we have some left natural teeth but in this lecture we are concerned with total edentulism.
** you may need to extract severely overerupted posterior teeth provided that we should end up having enough food table for the patient to maintain their masticatory function.
Adjustment of the dimension of occlusal surfaces:
the dimension of teeth determines how much force is applied to the denture by the natural teeth.
Natural teeth withstand force up to 25 kg/ cm2 ( nearly 25 kg for each molar)
* by reducing the area of occlusal contact you reduce the amount of force (stress) applied to the denture by the natural teeth.
How to avoid acrylic teeth wear by the natural teeth?
By achieving discrepancy in dimension, this means constructing narrower occlusal surfaces (as narrow as possible). The smallest set is sometimes still wide so teeth would need further grinding. For further reduction of teeth wear we exclude a premolar to reduce surface contact with natural teeth.
So to reduce the surface area contact with natural teeth :
1- Select smaller teeth
2- Narrow the occlusal plane
3- Leave one tooth out (PM)
** Since the mandible is 1.5 times smaller than the maxilla it is more prone to massive bone resorption ( leading to knife edge ridge and hyper mobile tissues). To reduce this phenomena we bring the occlusal plane closer to the edentulous arch to reduce the force magnitude.
How is that?
By bringing the application point of force closer to the crest of the ridge. Whereas high occlusal plane leads to multiplication of the magnitude of force on the crest of the ridge. ( moment = force *distance)
So the difference will be regarding the distance the force travels (momentum) not the magnitude of force
Reinforcement of denture teeth
Teeth made of acrylic are weaker than natural teeth. If you dont consider this problem the natural teeth will carve their own curves in the opposing acrylic teeth by grinding them away. This will cause again an irregular occlusal plane leading to midline fracture.
We can reduce the amount of denture teeth wear by reinforcing their occlusal surface in order to protect them from getting abraded and avoid loss of VDO which will eventually end with denture fracture.
So we have 3 things to think about:
1- how to reinforce the occlusal surface
2- how to reinforce the base and avoid midline fracture due to imbalance
3- providing the occlusal surfaces of denture teeth with metal or high impact acrylic (laminate acrylic) if aesthetics is of main concern.
So we only reinforce the contacting surfaces of denture teeth by either metal or laminate acrylic.
- box the teeth (PM is removed to reduce surface area) with beeding wax, place the index
- pour it with an investment material
- a mould is produced from the investment
- wax up the occlusal surface (inlay wax)
-sprue it, cast it
- apply it to denture teeth
** Only the occlusal surface is changed to metal ,the body remains acrylic to provide the damping effect (impact absorption) which dissipates energy fallen on it, otherwise all energy will be immediately transferred to ridge causing bone resorption.
** grind the teeth to provide sufficient space for the metal (that's why we took an index)
How to reinforce the base to avoid midline fracture?
After reinforcement of occlusal surface of teeth:
- put a metallic roof (co/cr)
This applies also to partial denture when we need full coverage (in the maxilla)/ we must make sure that the post dam area is covered with acrylic in order to be comfortable to the patient otherwise every time the patient swallows the junction between soft and hard palate will be traumatized due to presence of metal that will act as a knife edge.
Advantages of metal:
- Thermo conductive patients enjoy hot beverages more than those with acrylic base
An alternative to metal (due to its relatively high cost) is high impact acrylic mesh (reinforced acrylic)
Just pick up the size that matches the patients cast
- it will be embedded inside the acrylic base of denture at the time of packing/ curing so it becomes hard integrated part of the base.
- The mesh of reinforced fibers will provide the base with resistance to cracks
- Carbon fibers so far best reinforcement means of denture bases
Disadvantages of carbon fibers:
1- Poor aesthetics due to the black color the fibers impart on dentures
1- Bonds chemically to acrylic
2- Doesn't cause stress concentration or cracks that may develop later to fractures
Done by : Esra'a Al-Otaibi