Chapter 19 Retention Stability.pdf

Chapter 19 Retention and stability of complete dentures Chapter 19 RETENTION AND STABILITY OF COMPLETE DENTURES I- RETENTION: Retention of a denture is its resistance to removal in a direction opposite that of its insertion. It is the quality inherent in a denture that resists the force of gravity,...

Chapter 19 Retention and stability of complete dentures Chapter 19 RETENTION AND STABILITY OF COMPLETE DENTURES I- RETENTION: Retention of a denture is its resistance to removal in a direction opposite that of its insertion. It is the quality inherent in a denture that resists the force of gravity, the adhesiveness of food and the forces associated with the opening of the jaws. FACTORS AFFECTING RETENTION OF DENTURES: I. Physical Factors: A number of physical factors combine to retain complete dentures in position. These physical factors include adhesion, cohesion, interfacial surface tension and atmospheric pressure. 1. Adhesion: Adhesion is the physical attraction of unlike molecules to each other. It is also the molecular attraction 179 Chapter 19 Retention and stability of complete dentures between surfaces of unlike bodies in contact. It acts when saliva wets and sticks to the basal surface of the denture and to the mucous membrane of the basal seat. The effectiveness of adhesion depends on: - The close adaptation of the denture base to the supporting tissues. - Fluidity and consistency of saliva: The amount and consistency of saliva affects denture stability and retention. Moderate flow of serous type is quite effective, provided that the denture base can be wetted. Adhesion of saliva to the mucous membrane is not a problem because saliva wets it very effectively. - Thick ropy saliva adheres well to both denture base and, mucosa. Since much of this type of saliva is produced by the palatal glands under the maxillary denture. It builds up and it is sufficient to force the denture out of their correct position. - The amount of retention supplied by adhesion is directly proportional to the area covered by the denture. Maximum extension of the denture base as determined by a functionally trimmed impression is important for denture retention. It is not expected that patients with small jaws (basal seats) exhibit effective retention resulting from adhesion as patient with large jaws. 180 Chapter 19 Retention and stability of complete dentures 2. Cohesion: Cohesion is the physical attraction of like molecules for each other. It occurs in the layer of saliva between the denture base and the mucosa. - It is effective in direct proportion to the area covered by the denture. - Since saliva is a liquid, hence the layer of saliva must be thin in order to be effective. Therefore close adaptation of the denture base to the mucosa must be as close as possible. 3. Interfacial Surface Tension: The interfacial surface tension is the tension or resistance to separation possessed by the film of liquid between two well adapted surfaces. One of its retentive values is minimal distortion or displacement of the soft tissues by the impression and the denture. Also, a perfect fit of the denture base is essential. Interfacial surface tension occurs in the thin film of saliva between the denture base and the mucosa of the basal seat. It is quite similar in its action to cohesion. It is effective in direct proportion to the size of the basal surface of the denture. 181 Chapter 19 Retention and stability of complete dentures 4. Atmospheric Pressure: When an upper denture is inserted in the mouth of the patient, air is expelled out from underneath the denture. If the borders of the denture are properly adapted to the adjacent tissues, no air can get in. Thus the pressure acting on the fitting surface of the denture is less than that acting on the polished surface, the difference causes a positive force holding the denture in place. - The retentive force supplied by atmospheric pressure is directly proportional to the area covered by the denture base, for atmospheric pressure to be effective the denture must have a perfect seal around its border. - The retention achieved from adhesion, cohesion and interfacial surface tension is able to resist only vertical dislodging forces. Horizontal dislodging forces are resisted by the retentive force supplied by atmospheric pressure. - Upper dentures are mainly retained by physical factors. On the other hand, lower dentures are poorly retained by physical factors; they are almost entirely retained by mechanical forces. 182 Chapter 19 Retention and stability of complete dentures II. Intraoral Factors and Biomechanical Considerations: Number of biomechanical, intraoral factors, influence the retentive quality of complete denture such as: 1) Arch size: The size of the maxilla and mandible determines the amount of basal seat available for the denture. The greater the arch size, the more the support and, the larger the retention. 2) Disharmony in jaw sizes: Some patients have a large maxillary arch and small mandibular arch while others have the opposite disharmony. In such cases, a problem with stability and retention of the smaller arch is usually expected. 3) Arch form: The arch may be square, ovoid or tapered and opposing arches may not necessarily have the same form. As square arch form indicates favorable prognosis, while tapered arch is the least favorable. 183 Chapter 19 Retention and stability of complete dentures 4) Ridge form: The ideal form is a high ridge with flat crest and parallel or nearly parallel sides (square ridge). This type of ridge gives the maximum amount of support, retention and stability. The V-shaped ridge provides less retention to complete dentures, as the peripheral seal may be broken in all areas simultaneously. The flat ridge also has poor prognosis because the lack of vertical height affords little resistance to the horizontal dislodging forces. 5) The vault form: The vault form affects the retention of the maxillary denture. A flat vault resists vertical displacement but it provides little resistance to lateral displacement. The narrow V-shaped vault is also unfavorable for the retention of dentures. It is able to resist lateral displacement however; vertical forces tend to unseat the dentures. In most cases with a. V-shaped vault the residual ridges are also V-shaped in cross section, thus the problem is complicated. The solution involves the development of perfect peripheral seal and proper contouring of the denture polished surfaces so that the cheeks and buccinator muscles can mechanically aid in retention. The U-shaped palatal vault is the most favorable for retention and stability of complete dentures. It resists both vertical and lateral dislodging forces. 184 Chapter 19 Retention and stability of complete dentures 6) The soft palate: There are three classifications for the configuration of the soft palate based on the degree of flexure the soft palate makes with the hard palate, class I soft palate has a gentle curvature and demonstrates little muscular movement, hence it is the most favorable, it allows more tissue coverage for producing the palatal seal. Fig.2 Different curvatures of the soft palate 7) Undercuts on the basal seat: The presence of undercut may cause problems of denture retention and insertion. 185 Chapter 19 Retention and stability of complete dentures 8) Interarch distance: A small interarch distance enhances retention of dentures, because the tongue fills the oral cavity more completely thus, contacting the lingual and palatal surfaces of the denture, providing an excellent seal. 9) Polished surface of the denture: The Polished surface of the denture being shaped concave promotes the neuromuscular control to be in favor with denture retention. 10) Gravity: This physical force is primarily concerned with the mandibular prosthesis. 186 Chapter 19 Retention and stability of complete dentures II- STABILITY: Stability is "The quality of prosthesis to be firm steady or constant, in order to resist displacement by functional horizontal or rotational stresses". Denture stability "is the resistance of a denture to movement on the denture foundation area". FACTORS AFFECTING COMPLETE DENTURES: STABILITY OF I- Intra-oral factors: A number of intra-oral factors influence the quality of complete denture stability in relation to the basal seat area these are: 1- Ridge form: Residual ridges with flat crests and parallel or nearly parallel sides resist lateral forces and enhance denture stability, while flat ridges offer minimal resistance to lateral stresses reducing denture stability. 2- Arch form: Square arch form provides the most favorable prognosis for denture stability; on the contrary ovoid arch form is the least favorable condition. 187 Chapter 19 Retention and stability of complete dentures 3- Vault form: The shape of the vault of the palate affects the stability of maxillary denture. The two extremes, very flat, and very high V-shaped palatal vaults present difficulties in denture stability. While a moderately high, broad palate offers better stability. 4- The mucosa supporting the prosthesis: Mucosa of even, medium thickness (2-3 mm) composed of dense fibrous connective tissue and which is firmly attached to the underlying bone offers the most favorable prognosis. This nature of supporting mucosa prevents the denture and mucosa from moving together in relation to the underlying supporting bone and serves as a cushion absorbing the forces created by the denture. If the oral mucosa is thin, ulcerations and sore spots are more likely to occur in the basal seat area. On the other hand, thick flabby tissues, contribute to excessive vertical and horizontal movements, reducing the stability of the denture. In such condition, it is desirable to reduce these tissues surgically to gain a more stable denture bearing area. 188 Chapter 19 Retention and stability of complete dentures 5- Interarch distance: A small interarch distance in contrast to large interarch distance, enhances stability as, the occlusal surfaces of teeth are closer to the ridge, minimizing undesirable leverage action and tongue forces. 6- The tongue: The size, form and function of the tongue influence denture stability. Small narrow tongue or extremely large tongue adversely affects denture stability, while broad thick tongue enhances denture stability. 189 Chapter 19 Retention and stability of complete dentures II- Mechanical factors: Number of mechanical factors influence the quality of complete denture stability, these are: 1- Position of posterior teeth: Posterior teeth are generally placed to enhance stability of the mandibular denture. i- Buccolingually, the teeth should be placed over or slightly lingual to the crest of the mandibular ridge, however the lingual surface of the teeth should not exceed the medial extension of the mylohyoid ridge, to provide enough space for the tongue to move freely. Encroaching upon the tongue space, causes cramping of the tongue, and lateral movement of the denture whenever the tongue moves. Cramped tongue ii- Anteroposteriorly, the teeth should never be positioned on the upward incline of the mandible. Forces directed to an inclined plane are more dislodging than forces directed at right angle to the supporting basal seat. 190 Chapter 19 Retention and stability of complete dentures 2- Lower occlusal plane: i- Height of the lower occlusal plane should coincide with a line extending from or slightly below the corners of mouth anteriorly and the center of retromolar pad posteriorly. This position usually places the occlusal plane below the greatest convexity of the tongue. This position allows the tongue to function in co-ordination with the buccinator muscle to keep the food on the occlusal surface of the teeth, and aid in lower denture stability. Level of occlusal plane ii- Direction of the occlusal plane: the plane of occlusion should be parallel to the main direction of the denture foundation area, thus the masticatory biting forces are perpendicular to the supporting basal seat, minimizing undesirable lateral forces. 3- Shape of the denture polished surface: The denture polished surface should be properly contoured so that, the tongue, cheek and lips seat rather than unseat the dentures. 191 Chapter 19 Retention and stability of complete dentures 4- Proper relief of the upper denture: The tissue covering the median palatine suture is usually thin and non displaceable, acting as shifting fulcrum around which the upper denture can rotate. Sufficient relief of the upper denture over the median palatine raphe, to compensate for different degree of tissue displaceability, improves upper denture stability. 5- Balanced occlusion: Balanced occlusion in dentures means even simultaneous contact between upper and lower teeth in centric and eccentric position that is in harmony with various mandibular movements. Occlusion is said to be balanced when there is at least three widely separated points of occlusal contact in any lateral or protrusive position, such contact stabilizes dentures on their basal seat. 192