A stable denture is one that is relatively immovable in relation to the underlying mucosa and bone during function. Complete dentures stay in place if the retentive forces acting on the dentures exceed the displacing forces and the dentures have adequate support. This support is determined by the form and consistency of the denture-bearing tissues and the accuracy of fit of the denture.
Retentive forces offer resistance to vertical movement of a denture away from the underlying mucosa and act through the three surfaces of a denture. These surfaces may be defined as follows:
- Occlusal surface: that portion of the surface of a denture which makes contact or with the occlusal surface of the opposing denture or dentition
- Polished surface: that portion of the surface of a denture which extends in an occlusal direction from the border of the denture and which includes the palatal surface. It is that part of the denture base which is usually polished, includes the buccal and lingual surfaces of the teeth, and is in contact with the lips, cheeks and tongue
- Impression surface: that portion of the surface of a denture, the shape of which is determined by the impression. It includes the borders of the denture and extends to the polished surface. The retentive forces that act upon each of these surfaces are of two main types, muscular forces and physical forces.
- Muscular forces – exerted by the muscles of the lips, cheeks and tongue upon the polished surface of the denture and by the muscles of mastication indirectly through the occlusal surface
- Physical forces – they rely on the presence of an intact film of saliva between the denture and mucosa. They act primarily between the impression surface of the denture and the underlying mucosa, and are to a large extent dependent on the maintenance of a seal between the mucosa and the border regions of the denture and upon the accuracy of fit
Patients wear their dentures successfully because they have learnt to control them
with the muscles of their lips, cheeks and tongue. This skill may be developed to such a high degree that a denture that appears loose to the dentist may be perfectly satisfactory from the patient’s point of view. Muscular control is extremely important, particularly in the case of the lower denture where the reduced area of the impression surface and the difficulty of obtaining a border seal reduce the influence of physical retention. The successful muscular control of dentures depends on two factors:
- The design of the dentures
- The ability of the patient to acquire the necessary skill
Design of dentures
During mastication the muscles of the cheeks, lips and tongue control the bolus of food, move it around the oral cavity and place it between the occlusal surfaces of the teeth. In this activity, they press against the polished surfaces of the dentures. If these surfaces are correctly shaped with the buccal and lingual surfaces converging in an occlusal direction, this muscular force will seat the dentures on the underlying mucosa.
In addition to this active muscular fixation of the dentures during function, there will be a certain amount of passive fixation when the muscles are at rest, as the relaxed soft tissues sit on the dentures, maintaining them in position. Conversely an incorrectly shaped denture results in the muscular force dislodging that denture. In short, the muscles can either help or hinder denture stability.
Patient’s skill and adaptability.
The patient’s ability to acquire the necessary skills to control new dentures is related to biological age. In general, the older the patient, the longer the learning period. In the extreme case, the elderly or senile patient may not be able to acquire this skill at all and so new dentures may fail even though they are technically satisfactory. It is for this reason that replacement dentures for an older patient should be constructed in such a way that the patient’s skill in controlling the previous denture shapes can be transferred directly to the replacements.
This is achieved by copying the old dentures as closely as possible. An example of the muscular control of dentures is when a patient incises. The forces tend to tip the upper denture, causing the posterior border to drop. This movement is normally resisted by the dorsum of the tongue, which presses against the denture and reseats it. Patients who complain of difficulty when incising with dentures which otherwise appear to be satisfactory should be examined very carefully to establish whether or not tongue control is present.
The muscles of the lips, cheeks and tongue are also capable of causing denture instability. Displacement will occur if the denture interferes with the habitual posture and functional activity of the surrounding musculature. For example, distal movement of a lower denture may be produced by the lower lip if the anterior teeth are placed too far labially.
The teeth should therefore be placed just far enough lingually to prevent this displacement but not so far as to allow excessive tongue pressure to develop.
It is not uncommon to see lower dentures that are unstable because the posterior teeth have been placed too far lingually; under such conditions of restricted tongue space, movement of the tongue during function will tend to lift the denture.
40.1.3. Retention aids
In exceptional cases, such as surgical or congenital defects of the hard palate, it may not be possible to obtain the required retention by routine clinical techniques. In such circumstances the use of denture fixatives, long-term soft liners or springs may be of Value.
- Denture springs – denture springs are usually of coil type and are attached by pivots to the buccal flanges of upper and lower dentures in the premolar region. The springs are often partially covered by acrylic flanges, to stabilize the springs and reduce irritation of the buccal mucosa.
The springs exert a force which acts to separate the dentures and thus helps to maintain the dentures in contact with the supporting tissues. The continuous mechanical effect of the springs causes rapid reduction and resorption of the edentulous ridges. The brute constant stress in the muscles of mastication can be the reason for muscular pain and disturbances of mastication and speech.
- Incorporating heavy materials, usually metals or alloys, into the lower denture base in an attempt to improve stability. Various materials have been used – lead, tin, silver amalgam, or a cast metal substructure.
The impact on the supporting tissues is similar to that caused by denture springs. Heavy denture base materials should be avoided unless other requirements, such as strength, are of overwhelming importance.
- Long-term soft liners – soft liners enable free, flexible margins to extend into the anatomical defect and engage tissue undercuts. The liner can be constructed as an integral part of the denture base or as a separate obturator section retained on the denture base by rare earth magnets.
- Denture fixative – denture fixative can be an invaluable aid to retention, particularly under difficult anatomical circumstances. Fixatives come in powder, paste or sheet form, the latter having the advantage of staying longest between the denture and mucosa.
- Horizontal and vertical arrangement of teeth – the position of artificial teeth greatly influences the retention and stability of complete dentures. The maxillary and mandibular anterior teeth should be arranged with optimum vertical overbite and horizontal overjet of 0,5 mm to 1,0 mm.
They do not contact each other in centric relation, which compensates for the difference between centric relation and maximum intercuspation – freeway space. Posterior teeth on the mandibular denture should be arranged in a natural alignment, with the cusp tips on the occlusal plane and the central grooves aligned with the crest of the residual ridge. This type of arrangement stabilizes the complete dentures against the vertical forces of mastication.
- Bilaterally ballanced occlusion – this concept is based on the work of von Spee 1890, and Monson 1921. This is a prosthodontic concept which dictates that a maximum number of teeth should contact in all excursive positions of the mandible – on the working and on the nonworking side.
In only 16% of the population is there bilaterally balanced occlusion. Bilaterally balanced occlusion is particularly useful in complete denture construction, in which contact on the nonworking side is important to prevent tipping (turning over) of the denture.
The concept of bilaterally balanced occlusion should not be applied to natural dentition in complete occlusal rehabilitation. Occlusal contacts on the nonworking side are actually occlusal interferences, therefore they are undesirable occlusal contacts. They may produce excessive frictional wear of the teeth and excessive occlusal forces – therefore they are of a particularly destructive nature.
The contribution of physical forces to the retention of a denture is heavily dependent upon the presence of a continuous thin film of saliva between denture and mucosa, which wets both surfaces. The forces of adhesion and cohesion play a part in achieving this condition.
Adhesion is the force of attraction between dissimilar molecules such as saliva and acrylic resin or saliva and mucosa, which promotes the wetting of the denture and mucosal surfaces.
Cohesion is the force of attraction between molecules, which maintains the integrity of the saliva film. These intermolecular forces of adhesion and cohesion may be thought of as forming a chain between the denture and the mucosa.
Surface tension is the result of cohesive forces acting at the surface of a fluid. In the case of saliva, these cohesive forces result in the formation of a concave meniscus at the surface of the saliva in the border region of the denture.
When a fluid film is bounded by a concave meniscus the pressure within the fluid is less than that of the surrounding medium; thus, in the intra-oral situation a pressure differential will exist between the saliva film and the air. The size of this pressure differential is inversely related to the diameter of the meniscus, i.e. the closer the fit of the denture to the tissues the stronger the retentive force attributable to surface tension.
Factors for obtaining optimum bio-physical retention
The aspects of complete dentures that influence the amount of biophysical retention obtained are: border seal, area of impression surface and accuracy of fit.
- Border seal – for optimum retention, the denture border should be shaped so that the channel between it and the sulcus tissues is as small as possible. It is not possible to produce a facial seal along the posterior border of the upper denture as it crosses the palate. In this area, another approach to creating the smallest possible space between denture and mucosa is adopted.
A groove known as a post-dam is cut into the working cast so that the posterior border of the finished denture has a raised lip which becomes embedded a little way into the palatal mucosa. If the post-dam has width as well as depth the basic retention of the denture will be improved.
- Area of impression surface – the degree of biophysical retention is proportional to the area of the impression surface. It is important therefore to ensure maximum denture bearing area coverage.