Clinical and laboratory stages of manufacturing a clasp prosthesis with a telescopic and locking system of fixation.

Lesson 8.

The supporting-fixing devices of removable dentures, including clasp ones, include all kinds of telescopic systems, including various designs of crowns, support beams. This system is characterized by the presence of two structural elements – a supporting (non-removable), fixed on the teeth and a removable prosthesis.

Prostheses with fixation with telescopic crowns are indicated for defects of I, II or III classes according to Kennedy. The abutment teeth, on which telescopic crowns are attached, must be stable, without pathological changes in the periodontal tissues, the axes of the abutment teeth are parallel. In the antagonistic dentition there should not be a pronounced Popov phenomenon. The use of telescopic crowns is considered the most indicated for defects with single-standing teeth that have retained their normal height.

The manufacture of telescopic crowns is contraindicated in the following cases:

1. the presence of pronounced pathological changes in the periodontium of the supporting teeth;

2. a significant inclination of the supporting teeth, which does not allow creating parallelism between them by preparation;

3. the presence of cardiovascular diseases in history, not allowing the preparation of teeth;

4. pathological abrasion of hard tissues of teeth of II and III degrees.

The manufacture of schema prostheses with a telescopic fixation system includes the following clinical and laboratory steps:

1. preparation of abutment teeth for internal crowns;

2. taking casts, obtaining working models;

3. laboratory production of internal crowns;

4. fit and fixation of internal crowns in the patient’s mouth;

5. obtaining working casts for external crowns;

6. laboratory production of external crowns;

7. fitting of external crowns in the patient’s mouth;

8. taking impressions for the manufacture of removable dentures;

9. determination of central occlusion;

10. checking the wax composition of removable dentures with artificial teeth;

11. fitting and application of the finished prosthesis.

The first is that the internal crown is prepared in the form of a “thimble” in a dental laboratory without restoring the anatomical shape of the tooth. Fit in the mouth, fix with cement. After the cement hardens, an impression is taken for the manufacture of a second – external telescopic crown.

Modeling of the outer crown is carried out in such a way that in relation to the inner crown a gap of 0.5 mm is formed from the vestibular, oral and proximal surfaces and 1 mm along the chewing surface. In the cervical region, the outer crown should fit snugly against the inner one (Fig. 391, b).

Telescopic systems should also include beam or rod fixation of removable dentures. Such fixation is most appropriate for large defects of class III. Crowns are made on the abutment teeth, to which the rods are soldered. Weiser (1911) was the first to propose such a construction.

This design includes a non-removable support part in the form of crowns or root caps, between which there is a rod or beam (male); accordingly, in the basis there is a metal counter-bar (matrix), exactly repeating the shape of the bar.

For strengthening in plastic, wire branches are soldered to the cover plate. Foreign firms produce plastic and metal blanks for telescopic rods with a square, elliptical and drop-shaped section (Fig. 391, a). Such rods fix the prosthesis well during all chewing movements and, in addition, provide reliable stabilization of the abutment teeth. Thanks to the bar, the teeth are combined into a single block, which makes them more resistant to chewing pressure.

The best use of these rods is included defects in the lateral sections with a high coronal part of the abutment teeth. With a low crown, there is not enough space for the stem and base.

However, such a system for fixing prostheses has several disadvantages:

  • firstly, such a design is complicated in its implementation, since instead of one removable prosthesis, it is necessary to make two, that is, fixed and removable;
  • secondly, it is always associated with the manufacture of a fixed prosthesis, the indications for which should be very limited due to the inevitability of the preparation of hard dental tissues.

Therefore, rod fastening is indicated mainly for defects complicated by periodontal disease, when it is necessary to stabilize (immobilize) the abutment teeth. The connection is possible in various directions: sagittal, frontal, and even in a circular (Fig. 391, c).

Fixation with locks (attachments).

Attachment – consists of two (sometimes more) parts, a matrix and a male , which together form a high-precision collapsible connection. One of these parts can be connected to the surface of the artificial crown, fixed in the root of the tooth, fixed on the implant, fixed using the adhesive technique to the hard tissues of the tooth crown. The other is integrated into a removable prosthesis and is used to provide a mechanical connection.

Lock fasteners should functionally provide:

  • Support – resistance to the movement of the prosthesis towards the tissues of the prosthetic bed;
  • Retention – resistance to the movement of the prosthesis in the direction from the tissues of the prosthetic bed;
  • Reciprocating motions – counteraction to forces caused by retention elements;
  • Stabilization – counteracting the forces that cause the displacement of the prosthesis during function;
  • Fixation – opposition to the movement of the abutment tooth from the prosthesis and the movement of the prosthesis from the abutment tooth.

Types of locks:

PRECISION LOCK FASTENING (highprecisiondentalattachments):

Precision lock fasteners are highly accurate, factory-made by milling on computer-controlled machines and have limited accuracy tolerances. The permissible inaccuracy in the linear dimensions of such locks is less than 0.01 mm. The composition and strength of the alloys from which precision locks are made are also strictly regulated. Almost all high-precision lock fasteners are installed by welding (soldering) or cast-on technology. The use of prefabricated components makes it relatively easy to repair prostheses.

Semi-precision dental attachments:

Semi-precision lock fasteners are made by direct injection molding on prefabricated or custom-made plastic or wax blanks. Most of the blanks for semi-precision lock fasteners are factory made by injection molding from ashless plastics. Such fasteners are called “semi-precision (semi-precision)” because. the accuracy of their linear dimensions depends on the conditions of the technological process.

The positive aspects of semi-precision lock fasteners include their relatively low cost, the possibility of manufacturing from any available cast alloys, the absence of dissimilar metals in the prosthesis, the absence of the need for soldering/welding parts of the lock fasteners and the prosthesis frame.

Types of retention provided by locking fasteners:

Activated ZK – provide active retention between the matrix and the male, as the prosthesis is used, they can be reactivated. They are the ZK of choice in the manufacture of removable dentures.

Non-activated ZK – provide passive retention between elements, i.e. the retention force between the matrix and the male during the entire period of use of the prosthesis remains unchanged and cannot be increased or decreased. Most often, such SCs are used in the manufacture of collapsible and articulated bridges or removable mini-prostheses.

Rigid and labile locks:

In rigid attachments (solid/rigid attachments), their elements are fixed in relation to each other. ZK of this type is rational to use in the prosthetics of small included defects in the dentition, when the entire masticatory load is transferred to the supporting teeth.

In labile CAs (resilient attachments), the matrix and male are movably connected, which ensures the redistribution of the load between the supporting teeth and the mucous membrane of the prosthetic bed. Labile ZK are used in the prosthetics of the end defects of the dentition.

Classification of lock fasteners according to the type of design features:

In modern dental literature, it is customary to divide all types of PZK into 6 groups:

1. Extracoronal ZK (Extracoronal attachments)

2. Intracoronal attachments

3. Articular connections (Auxillary attachments)

4. Anchor slam-shuts (Anchors)

5. Arcs (Bars)

6. Other types of slam-shut

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