Fixed dentures

Semi-fixed and supporting structures

Various types of prosthetic structures are used to restore dental arch defects. Semi-fixed dentures are indicated for the loss of a single chewing tooth. Their main feature lies in the name: the patient does not need to remove the structure independently. The fixation mechanism is based on attachment to adjacent teeth using metal wings. These elements are bonded to the enamel or securely fixed using modern light-curing cements.

Prosthetic treatment often requires the preparation (grinding down) of preserved teeth, followed by the placement of artificial crowns on them. Crowns act as supporting elements. An alternative that completely eliminates the drawback of tooth preparation is the placement of mini-implants. Fixed supporting elements also include dental bridges, root inlays, and posts. Their primary function is to establish a rational pathway for transmitting masticatory pressure to the periodontal-gingival tissues (prosthetic bed tissues) and to improve the overall retention of the entire prosthetic system.

Removable partial dentures and materials

Fixed supports work in a tight combination with removable partial structures. The latter are indicated when single teeth, groups of teeth, main chewing teeth are missing, or when the defect is extensive.

• Acrylic plate dentures. The simplest and most affordable. Used to restore segments of the arch.
• Immediate dentures. Temporary appliances placed on the jaw immediately after tooth extraction or during the preparation phase for permanent treatment.
• Skeletal (partial cast) dentures. From the German word bügel (arch). Used for almost any defects. This is the most reliable, comfortable, yet expensive design. Its biomechanical difference is that the masticatory load is distributed evenly between the gingiva and the supporting teeth (in plate analogues, the entire load falls on the gingiva).

Modern prostheses are manufactured from acrylic resins using injection molding, as well as hot or cold compression polymerization. These formulations guarantee long-term preservation of color, shape, and strength. Artificial teeth are produced as ready-made sets with the option to select the shape and shade.

Physical methods of retention

Retention of the prosthesis in the oral cavity is ensured by four main factors:

1. Adhesion. The force of attraction between surfaces through a thin layer of fluid (saliva). It is directly proportional to the contact area and inversely proportional to the thickness of the saliva film. It depends on anatomy: on a flat palate, adhesion increases with vertical displacement, while on a high palate, it decreases conversely.
2. Cohesion (stickiness). A form of adhesion based on the physical phenomenon of wetting. A concave meniscus forms at the edges of the mucosa. The force flattening the meniscus is directed outward, which firmly presses the structure in place. The smaller the radius of the meniscus, the stronger this force.
3. Anatomical retention. Utilization of natural structures of the jaw: the palatal vault, alveolar eminences, interdental spaces, and undercut areas of the crowns near the gingiva.
4. Artificial devices. These include clasps, occlusal rests, pelots, and attachments.

Clasp retention system

Clasps (metal hooks) are the most common extracoronal retainers. They reliably hold the structure in place during speech and mastication. The main requirement for a clasp is that it must not exert a harmful effect on the periodontium or interfere with proper occlusion.

The clasp arm embraces the crown. The boundary for its placement is the survey line (equator):

• Incisors: the equator is close to the incisal edge. For esthetics, the arm is positioned as close to the cervical margin as possible (0.5–1 mm offset).
• Canines and premolars: the equator is located in the middle; the arm is also shifted toward the gingival margin.
• Molars: esthetic requirements are absent. The arm is placed strictly below the equator along its entire length for powerful retention.

Clinical rule: the body of the clasp must strictly not be positioned below the equator in the undercut zone. Otherwise, insertion of the prosthesis will become impossible. Furthermore, the arm must closely fit the tooth along its entire length; otherwise, point pressure may provoke enamel necrosis.

Biomechanics and structural elements

During function, three forces act on the prosthesis: gravity, masticatory pressure (displaces the structure in three planes), and pulling force (from sticky foods). The system rotates around an imaginary axis—the clasp line.

The choice of the direction of the clasp line is critical:

• The best conditions are bilateral placement of supports.
• Optimal for the upper jaw is a diagonal direction.
• Optimal for the lower jaw is a transverse direction.
• Least favorable is a unilateral sagittal direction (threatens tooth overload).

For stabilization, leveling elements (bars that evenly distribute the load) and anti-displacement elements are used. Secondary anti-displacement elements include artificial crowns and bridges working indirectly through the clasp.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Topography of the tooth equator when designing retaining elements.
• Types of clasps by shape, function, and arm position.
• Physics of cohesion: calculation of surface tension forces of the saliva meniscus.
• Anchor retention system (button attachments) and the snap-fit principle.
• Preparation of abutment teeth under tight occlusal contact (creation of a seat).

Other topics in the section "Chapter 10. Methods of restoring dental arch defects (orthopedic design)"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.