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Tasks for independent work during preparation to practical classes.

4.1. Recommendations for students’ independent work and album for independent work of students.

4.2 Theoretical questions to the lesson:

1. The definition of "fixation" of partial removable dentures.

2. The definition of "stabilization" of partial removable dentures.

3. Classification of methods of fixation (Boyanov).

4. Features of dentures fixing with telescopic crowns and clasps. Indications, clinical and laboratory stages of production.

5. Features of dentures fixing with attachments. Indications, clinical and laboratory stages of production.

6. Indications for fixing of removable dentures without clasps.

7. Pelota: indications for the manufactoring of dentures with pelota, features of manufacturing.

8. Conditions that provide maximum stabilization of the prosthesis.

9. Features of stabilizing the dentures on the upper and lower jaws.

10. The value of the correct definition of the limit of partial dentures, and the choice of methods of fixation in clinic of prosthetic dentistry.

4.3. Practical work (task) that are fulfilled in class:

With the help of diagnostic models and micro stands students demonstrativly consider:
1. On phantom micro stand to demonstrate stages of manufacturing of removable partial laminar dentures.

2. Using a phantom micro stand explain the sequence of clinical stages of manufacturing using various methods of fixation of partial removable dentures.

3. Using a phantom micro stand explain the sequence of laboratory stages during manufacturing clasps.

4. Give characteristic of the demonstrated sample of the partial removable denture of what basic parts it consists and explain the fixation method of it.

5. Stages of bent clasp manufacturing.

 

Content of the theme:

Denture retention is the quality of a denture that holds it to the tissue foundation and/or abutment teeth. Retention is the resistance to forces that tend to displace or remove the denture from the mucous membrane especially in a vertical direction.

Bojanov selected such types of retention:

1. Mechanical (by clasps, attachments).

Retainers are divided by direct and undirect. The direct retainers are disposed on a tooth and are prevented a vertical displacement of RPD.

The direct retainers can be intracoronal and extracoronal. Clasps are the extracoronal retainers. The indirect retainers prevent rotation of RPD.

2. Biomechanical retention

A) Neuromuscular control-refers to the functional forces exerted by the muscles of the patient, that can affect retention. Older patients have more difficulty in adjusting to new dentures, due to the progressive atrophy of their neurological systems.

B)Ridge characteristics, Ridge relationship

1) Mandibular lingual flange: The most desirable feature of the lingual slope of the mandible is that it approaches 90 to the occlusal plane. The extent of the contact of the lingual flange with the lingual ridge slope is thereby dictated by the functional mobility of the floor of the mouth. Any flange extension below the mylohyoid ridge must incline medially away from the mandible, to allow for the mylohyoid muscle contraction.
2) Residual ridge anatomy: 1. Residual height and conformation- large , square, broad ridges offer a greater resistance to lateral forces than do small, narrow tapered ridges.
3). Arch form- square or tapered arches tend to resist rotation of the prosthesis better than ovoid arches.
4). Palatal vault- a steep vault may enhance stability by providing greater surface area of contact, and long inclines approaching a right angle to the direction of the force



C)Condition of mucosa and submucosa

1)Soft tissues: Ideally soft tissues should be firmly bound to cortical bone, containing a resilient layer of submucosa, and be covered by keratinized mucosa. Keratin is a scleroprotein present in the stratum corneum and is the end product of epithelial degeneration. Excessive trauma to the mucosa beneath a denture base can lead to abnormal tissue changes such as development of parakeratin, localized hyperkeratosis, and epithelial ulceration or necrosis. The fatty and glandular tissue work as a "hydraulic cushion".

2) Influence of orofacial musculature: The basic geometric design of the denture base should be triangular. To direct a seating action on the mandibular denture, the tongue should rest against a lingual flange, inclined medially away from the mandible and somewhat concave. The degree of inclination depends on the balance of the muscular forces of the tongue as opposed to the mylohyoid and superior constrictor muscles. Generally, the buccal and labial flanges of the maxillary and mandibular dentures should be concave to permit positive seating by the cheeks and lips. The primary muscles of the lips and cheeks are the orbicularis oris and buccinator muscle, respectively.

3. Physical

The denture is retained by the forces of adhesion, cohesion,viscosity, gravity, as well as the plastic molding of the soft tissue around the polished sur­faces of the dentures. Atmospheric pressure is effective primarily as a rescue force when extreme dislodging forces are applied to the denture.

Adhesion: the physical attraction of unlike molecules. Adhesion of saliva to the mucous membrane and the denture base is achieved through ionic forces between charged salivary glycoprotein and surface epithelium or acrylic resin.

Cohesion: the physical attraction between like molecules. Physical attraction of like molecules for each other creates retentive force and usually occurs with saliva that is present between the denture base and the mucosa. Normal saliva is not very cohesive

Interfacial surface tension: resistance to separation by the film of liquid between

the denture base and the supporting tissues (combines cohesion and adhesion).

Viscosity

This is the rate of separation of two surfaces under an applied force, best described in the context of surface tension and interfacial viscous tension. The force holding two wet glass planes together against a straight pull, or the force holding two parallel plates together are due to the viscosity of interposed liquid.

Stefan’s law (24) describes that the viscous force increases proportionally to increases in the viscosity of the interposed fluid. When the equation is applied to denture retention, it demonstrates the need for a good base adaptation to the tissues and the importance of taking full advantage of the surface area covered by the denture. This may be relevant to the maxillary denture. However, if the two plates with interposed fluid are immersed in the same fluid, there will be no resistance to pulling them apart. Since the borders of the mandibular denture are bathed in saliva, surface tension, viscosity and film thickness may not play a role in lower denture retention

Gravity

The weight of a lower prosthesis constitutes a negligible gravitational force and is insignificant in comparison with the other forces acting on a denture.

Oral and facial musculature.

Muscular control is an important aspect of successful complete denture therapy. Although this may supply additional retentive forces, provided that the polished surfaces are properly shaped, the teeth are positioned in the neutral zone and the denture bases are properly extended to cover the maximum area possible, retention is a quality of the denture rather than that of the patient. Therefore, musculature is relevant only in the context of ‘manipulative skill’ of the patient, rather than in retention in the strictest sense.


Date: 2015-12-24; view: 960


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