Jacobson a Contemporary Review of the Factors Involved in CD Retentio Stability Support Pt I 1983

REMOVABLE PROSTHODONTICS SECTION EDITORS

LOUIS BLATTERFEIN S. HOWARD PAYNE

A contemporary review of the factors involved in complete denture retention, stability, and support. Part I: Retention

T. E. Jacobson, D.D.S.,* and A. J. Krol, D.D.S.** University of California, School of Dentistry, San Francisco, Calif., and Veterans Administration Medical Center, San Francisco, Calif

lh e recognition, understanding, and incorporation of certain mechanical, biologic, and physical factors are necessary to ensure optimal complete denture treatment. These factors are the determinants that promote the properties of retention, stability, and support in the finished prosthesis through their influence on the relationship between the tissue surface of the denture base and the mucosal surface of the edentulous ridges. There are varied opinions in the prosthodontic literature regarding the roles played by these factors, their relative importance, and their relationship to clinical procedures. Numerous contradictory and controversial articles proposing various impression techniques have been written in an effort to achieve optimal denture retention, stability, and support.

Bohannan’ appropriately noted that “technique itself is merely the practical application of principles, and if the principles are unsound, the most elaborate and painstaking technique certainly is doomed to failure.” It is necessary, therefore, to understand each property and its contributing factors separately and to recognize their interactions to be able to critically analyze and select procedures and techniques that lead to the fabrication of successful complete dentures (Fig. 1).

DEFINITION OF PROPERTIES

Complete denture retention is the resistance to displacement of the denture base away from the ridge. Bouche? describes retention as the most spectacular yet probably the least important of all complete denture objectives. This property may indeed be least important; however, it provides psychologic comfort to the patient. If a denture is easily dislodged during

*Assistant Clinical Professor, Removable Prosthodontics. **Chief of Dental Services, Removable Prosthodontics.

Psychologic Physiologic Comfort

. ...::::::::+

x7

.:.:.::.:.!g ::::::::. ..::

Comfort

i,‘~ J;

ee % . ‘/*

e . . . . . xc:.:.:. ‘$$$$::y 5 . A&..... ‘>;:+$. ‘.:.’

vi9

Support

Success t Longevity

Fig. 1. Retention, stability, and support are important to success.

Retention

t Psychologic

Comfort

Stability

t Physiologic

Comfort

Prosthesis Success * /

Fig. 2. Certain biologic, physical, and mechanical factors provide retention, stability, .and support and thereby contribute to the properties of a successful prosthesis.

speech or eating, the embarrassment experienced can be mentally traumatic. A retentive denture contributes dramatically to patient acceptance of the finished prosthesis.

Stability is the resistance to horizontal and rotational

THE JOURNAL OF PROSTHETIC DENTISTRY 5

JACOBSON AND KROL

forces. This property prevents lateral or anteroposter- ior shunting of the denture base. Stability has been cited as the most significant property in providing for the physiologic comfort of the patient. Denture insta- bility adversely affects support and retention and results in deleterious forces on the edentulous ridges during function.

Support is the resistance to vertical movement of the denture base toward the ridge. This property maintains the occlusal relationships established on the articulator. A complete denture may continue to function ideally only as long as sufficient support is present to resist tissueward movement under loading. Each of these properties will be evaluated separately (Fig. 2).

RETENTION

Many published articles deal with the subject of complete denture retention. Historically prosthodontists have sought to improve the quality of denture treatment through an understanding and application of the factors involved in retention. Despite numerous research efforts devoted to this controversial topic, disagreements regarding the relative importance of the various contributing factors still exist.

History

Fish’ was among the first to discuss the determinants of retention and differentiate between the tissue, polished, and occlusal surfaces of a complete denture. He emphasized that each of the three surfaces plays a role in retention. The proper design of the tissue, polished, and occlusal surfaces of complete dentures permits the dentist to incorporate the mechanical, biologic, and physical factors of denture retention.

Most prosthodontists agree that the polished surface of a complete denture should possess certain contours to maximize the retentive potential of the functioning orofacial musculature.3-14 Craddock’ described the “gripping” action of the buccinator muscle on the buccal flange of a mandibular complete denture. To maximize the role of the polished surface of complete dentures, some authors have recommended that the external base contours and tooth position be function- ally determined. lo-l4 Proper contour and design of the polished surface should harmonize with the function of the tongue, lips, and cheeks to effect a seating of the denture.

The occlusal surfaces are also important in providing a retentive prosthesis. Schlosser9 and Fishls believed that a balanced functional occlusion is critical in promoting denture retention. Regardless of the occlusal scheme chosen, the occlusion must be free of interfer-

ences within the functional range of movement of the patient to avoid dislodging forces. The position of the teeth within each arch and the level and inclination of the occlusal plane are important in maintaining a stable retentive prosthesis.

TISSUE SURFACE

Several biologic and physical factors have been described as determining the relationship of the tissue surface of the denture base to the underlying soft tissues that will provide optimal retention. It is the understanding of these determinants that may ultimately govern success or failure. Although magnetic devices, implants, mechanical attachments, and recon- structive surgical procedures have been suggested and used in !certain situations, they do not substitute for an awareness of the scientific principles involved in denture retention. The most commonly listed factors of retention include adhesion, cohesion, interfacial surface tension, gravity, intimate tissue contact, peripheral (border) seal, atmospheric pressure, and neuromuscular control.

DEFINITION OF FACTORS

Adhesion. Adhesion is the physical force involved in the attraction between unlike molecules. A drop of water introduced on the surface of a solid glass plate will resist movement away from the glass in proportion to the adhesion between the unlike materials.

Cohesion. Cohesion is the physical factor of electro- magnetic force acting between molecules of the same material. A molecule within a fluid has an attraction exerted on it on all sides by neighboring molecules. The same molecule exerts an attractive force on the neighboring molecules equal in magnitude but opposite in direction. Forces of cohesion are responsible for maintaining the continuity of a water droplet when placed in contact with another material.

Interfacial surface tension. This term pervades much of the literature written concerning denture retention obtained through the mucostatic techniques popularized by Page. I63 ” Page describes interfacial surface tension as a phenomenon similar to Wilson’s” “adhesion by contact. ” Both of these terms refer to the forces involved in maintaining the attraction of two opposed ground solid plates with an intervening fluid film that resists displacing forces applied at right angles to the fluid film surface.‘, l9 PageZo states that “interfacial surface tension operates by virtue of a thin fluid film between two intimately contacted objects.” This term is redundant in that such phenomena can be described by the actions and interactions of the remain-

6 JANUARY 1983 VOLUME 49 NUMBER 1

COMPLETE DENTURE RETENTION

ing factors of denture retention.2’ To simplify the following discussion “interfacial surface tension” as a phrase will not be used, although the phenomenon to which it refers will be explained.

Gravity. The definition here is self-explanatory. The physical force primarily concerns the mandibular prosthesis.

Intimate tissue contact. Intimate tissue contact is the biologic factor that refers to the close adaptation of the denture base to the underlying soft tissues. The impression technique will determine the degree of intimate tissue contact obtained with the tissues at rest and during function.

Border seal. Border seal is the biologic factor that involves intimate contact of the denture borders with the surrounding soft tissue. The seal encompasses the circumference of the denture and includes features such as beading and posterior palatal seal to enhance its effectiveness.

Atmospheric pressure. Atmospheric pressure is the physical factor of hydrostatic pressure due to the weight of the atmosphere on the earth’s surface. At sea level this force amounts to 14.7 psi.

Neuromuscular control. Neuromuscular control refers to the functional forces exerted by the musculature of the patient that can affect retention. This is primarily a learned biologic phenomenon. Certain characteristics can be incorporated into the external contours of the denture base to promote neuromuscular control.

Although most authors agree that all these factors contribute to denture retention, there is disagreement regarding the relative importance of each. As early as 1886 Wilson” was describing adhesion as the overrid- ing determinant. Proponents of the mucostatic theory place little or no emphasis on the role of atmospheric pressure or border seal in retention. They attribute denture retention to forces of adhesion and cohesion resulting from the intimate tissue contact of the denture base at rest.13b ‘6x *‘. 22x 23 Other prosthodontists believe that atmospheric pressure together with intimate tissue contact and peripheral seal comprise the most critical retentive factors.24-34 Contrasting research reports have been written in support of many of the factors. It is necessary to critically analyze these widely divergent opinions based on a study of the relevant research in the prosthodontic literature.

LABORATORY MODEL SYSTEMS

Any attempt to explain the physical factors of retention must begin with laboratory bench studies of model systems that represent clinical situations. The

THE JOURNAL OF PROSTHETIC DENTISTRY

Atm. Pmssure

Atm. Pressure I c

‘Meniscus

I PCl

Fig. 3. Atmospheric pressure (Pa) is in equilibrium with fluid pressure exerted on molecules within capillary tube at level of liquid in container. Therefore, pressure on molecules along doffed line (A) is equal to Pa. Fluid pressure exerted on molecules at higher level (B) is less than at level A in proportion to distance between A and B. Because B is less than A, B is less than Pa, which indicates presence of a pressure gradient across meniscus, which is maintained by surface tension.

simplest system involves the attraction of two glass slabs placed in direct apposition with an interposed fluid film. In 1948 Stanitz2’ used this model to explain the part played by the fluid film in denture retention. In review the phenomenon of surface tension is defined as the force that maintains the surface continuity of a fluid. This results from the imbalance in cohesive forces between molecules present at the surface. The cohesive attraction between molecules is balanced in equilibrium within the fluid. At the surface the absence of neighboring molecules creates the one-sided attraction and imbalance that causes a free potential energy called surface tension. It is a relatively small force when considered alone, but by interacting with other physical factors it becomes an important determinant.

When water rises vertically in a column within a capillary tube standing in an open container of water, the fluid pressure within the water at the height of the column is less than that at the base. The pressure at the base of the column is equal to atmospheric pressure, and therefore the pressure at the height of the column is

JACOBSON AND KROL

Meniscus

1 F

Fig. 4. Two glass slabs are separated by a thin fluid film, indicated by shaded area. Top: Pressure exerted on molecules within fluid film is equal to surrounding Pa (atmospheric pressure at equilibrium). Bottom: Effect of a separating force (F) exerted on the two glass slabs. Now fluid pressure within film is less than atmospheric pressure (Pal. Pressure gradient that develops is maintained by surface tension of meniscus.

less than atmospheric pressure. This phenomenon creates a pressure gradient across the meniscus (Fig. 3). Although it is the forces of adhesion and cohesion that cause the water to rise in the tube, it is the forces of surface tension that maintain the difference in pressure across the meniscus.

This same phenomenon explains the force that holds two glass slabs together. Under a dislodging force perpendicular to the fluid film, the pressure within the fluid decreases. Together with surrounding atmospheric pressure, this creates a pressure gradient across the peripheral meniscus that has formed (Fig. 4). The force needed to separate the glass plates is proportional to the degree of pressure gradient that develops multi- plied by the surface area involved. The smaller the film thickness, the greater the pressure difference and therefore the greater the force required to achieve separation. A 0.0005-inch film thickness requires a separating force of 1.68 psi. ” The fluid film can act in a similar manner in complete denture retention.

tension, which drastically reduced the required separating force. If the active forces involved in these phenomena were primarily adhesion and cohesion, the separating forces required would not drastically change on immersion or reduction of atmospheric pressure.

Note that the use of glass slabs that are ground to allow optical contact introduces molecular forces that could never occur in clinical prosthetics. Optical contact implies a separation between two objects that is small compared to a wavelength of light.” The molecular forces here act independently of the factors discussed above, and the system involves an attraction that resembles a single solid piece rather than separated solids. Use of such experimental models may explain some of the research reports that do not recognize the importance of atmospheric pressure in resisting plate separation.

Further studies use more complex experimental models. Skinner30 and Skinner and Chung31 reported results of controlled laboratory studies of baseplates constructed on master casts with a resilient surface layer simulating soft tissue. Testing the effects of post dam (posterior palatal seal), relief chambers, and border seal, they concluded that the posterior palatal seal and border seal enhanced retention of the baseplates, while the use of relief areas reduced this property.

These studies also confirmed the experiments of Ostlund3’ regarding the effect of 1 mm perforations in the base on retention. Except for those at the ridge crest, introduction of small perforations significantly reduced the baseplate retention. These observations support the importance of using intimate tissue contact and border seal to promote atmospheric pressure. If adhesion and cohesion were predominant, the lack of posterior palatal and border seal and the presence of small perforations would not significantly alter the retentive properties.

CLINICAL STUDIES AND OBSERVATIONS

Tysonz9 demonstrated the role of surface tension and atmospheric pressure in denture retention in a series of experiments in 1967. He confirmed the importance of a thin fluid film between two plates in producing a pressure gradient maintained by surface tension. The system further demonstrated the importance of atmospheric pressure through the use of a bell jar, which enabled the experiments to be carried out under varying atmospheric pressures. The separating force proved to be directly proportional to the atmospheric pressure. Immersing the entire system in water elimi- nated the pressure gradient and effect of surface

A conclusive clinical study by Snyder et al.2* in 1945 demonstrated the effect of reduced atmospheric pressure on the retention of maxillary complete dentures constructed for seven patients. Measurements made in a pressure chamber at 4.7 psi simulating a 30,000-foot ascent above the earth demonstrated a decrease in denture retention. With a 70% decrease in atmospheric pressure, a 50% decrease in retention was noted.

At sea level the force of atmospheric pressure acts with approximately 14.7 psi against the external surface of the denture, provided no air or gaseous pressure exists between the denture base and the tissue



surface. In reality some gas always exists due to the partial pressure of gases dissolved in the saliva. The presence of dissolved gases or air inclusions serves to decrease the effectiveness of atmospheric pressure pro- portionately.

Clinical observations of the authors are also in agreement with the research results cited. The introduction of a small palatal perforation or the presence of an inadequate posterior palatal seal markedly reduces the physical retention of most maxillary complete dentures. Such effects would not be observed if the forces of adhesion and cohesion, which depend primarily on the surface area of intimate contact, were the critical retentive factors. Certainly removal of a posterior palatal seal or placement of a 1 mm perforation does not significantly alter the surface area.

THE ROLE OF PHYSICAL FACTORS

The results of these studies and observations clearly explain the physical retention of complete dentures. The surface tension created at the meniscus of the denture border maintains a pressure gradient between the atmospheric pressure and the reduced pressure within the fluid film that occurs during dislodging forces (Fig. 5). To be effective air must be excluded from the intaglio, and the fluid film must be as thin as possible. Intimate tissue contact is the biologic factor that promotes these conditions by eliminating air entrapment. The border seal maintains this relationship by preventing the ingress of air once the denture is seated. Border seal also maintains the thin fluid film at the denture border, allowing a meniscus to develop in response to displacing forces. “Posterior palatal seal” may be defined as the posterior border seal of the prosthesis. Although adhesion and cohesion are second- ary forces that act within the fluid film, their primary contribution involves forming and maintaining the surface tension of the peripheral meniscus.

The physical factor of gravity contributes to mandibular complete denture retention. Although it is some- times difficult to bring the other factors of retention into play when constructing a lower denture, gravity aids in providing the necessary force to maintain the prosthesis in place at rest. Grunewald35 recommended gold-base complete dentures of a weight similar to that of the lost teeth and alveolar tissues. Such a technique would enhance the effectiveness of gravity on the retentive properties of the prosthesis.

NEUROMUSCULAR CONTROL

Every prosthodontist recognizes the ability of certain patients to wear their dentures and function without

Fig. 5. Thin fluid film exists (shaded area) between denture base and tissues of residual ridges. Meniscus that develops at border of denture is similar to that shown in Fig. 4 between two glass slabs. Note that position of meniscus will depend on where soft tissue loses contact with denture border. Draping effect of cheeks may provide a meniscus along polished surface of denture border CA). When cheek is retracted, meniscus is developed at denture border (5).

complaint despite the fact that they may be extremely ill fitting, unstable, or even broken. The author cites a patient who presented on routine examination wearing a mandibular complete denture that had fractured into three pieces. The patient was able to manipulate the fractured denture and use it during mas#tication (Fig. 6). The biologic factor of neuromuscular control grad- ually becomes a major determinant in complete denture retention as experienced patients learn to alter their muscular function to harmonize with thr: prosthesis.

The fields of oral perception, sensation, and proprio- ception are currently being researched. Individuals appear to vary in their ability to develop the motor coordination and conditioned reflexes necessary to manipulate intraoral prostheses. While some patients are able to adapt to restorations that seem to be unacceptable, others appear to have difficulty learning to control any dentures, regardless of the contours, design, or occlusion. It is muscular control that enables patients to function with dentures that rest on basal tissues that have undergone resorptive changes and no longer relate to the intaglio of the denture base.

Studies by Brill et al. 36 demonstrate that older patients have more difficulty adjusting to new complete dentures. This may result from the progressive cerebral


Fig. 6. A, Three fragments of an interim mandibular denture. B, Patient is able to manipulate three fragments through exceptional neuromuscular control.

atrophy that affects related neurologic systems. They also demonstrated the dramatic decrease in mandibular complete denture retention that accompanied local anesthesia of the oral mucosa in experienced denture patients. 37 This was especially marked in those patients with severely compromised residual ridge height and conformation.

Incorporation of certain physical and biologic factors will assist patients during their development of the neuromuscular skills by providing the initial retention that is necessary for the psychologic comfort of the patient and success of the prosthesis. The mechanical factors of the polished and occlusal surfaces, physical and biologic factors of the tissue surface, and biologic factor of neuromuscular control interact to provide retention of complete dentures from the time of delivery until the prosthesis becomes unserviceable.

CLINICAL IMPLICATIONS

Clinical procedures and techniques should be selected so as to incorporate these factors into the finished denture. For example an impression material pith adequate flow properties should be used to avoid uneven pressure during impression procedures that could result in a localized rebounding effect of the compressed tissues under the denture base and/or denture “sore spots.” Either of these conditions could result in uneven seating of the finished denture and loss of ideal intimate tissue contact.

This does not mean that one should strive for a mucostatic or totally pressure-free impression. A slight generalized pressure on the soft tissues is desirable. Use of a moderately viscous light-bodied impression material with sufficient flow, elimination of full-arch relief spacers in the tray, and use of a nonperforated tray are among those modifications in technique that can lead to an impression recording the tissues in a mildly dis- placed form. This ensures close adaptation of the denture base and may compensate to a degree for the

10

dimensional changes in the finished dentures that prevent the intimate tissue contact present in the impression. Lammie34 and others recognize this com- pensation and further recommend that slight pressure be extended peripherally to ensure that the thin fluid film at the denture border provides the necessary formation of a meniscus. Therefore during the border- molding procedure, providing a slight pressure will aid in positive contact of the denture border at delivery.

The impression material should also provide adequate reproduction of surface detail to prevent small irregularities capable of entrapping air. Accurate impressions can only be made of tissues recorded in their healthy, fully recovered state. The reports of Lytle38 emphasize the importance of the recovery of abused oral tissues obtained by not allowing patients to wear their prostheses for a minimum of 48 hours prior to impression procedures.

ANATOMIC INFLUENCES ON MAXILLARY DENTURE RETENTION

Considerations unique to the maxillary complete denture include the incorporation of a posterior palatal seal to complete the border seal. The posterior palatal seal maintains tissue contact during base movement or soft palate function and compensates for processing changes. This critical area extends between the hamu- lar notches along the flexure line of the soft palate. The posterior palatal seal of the denture must extend horizontally beyond the supportive hard palate to include the muscular aponeurosis of the soft palate. This area is not susceptible to pressure atrophy and therefore allows moderate tissue displacement to maintain the thin fluid film. To obtain the proper amount of tissue displacement, the posterior palatal seal must be deeper as the palatal vault becomes steeper to compensate for greater processing error.

Patients exhibiting highly tapered steep palatal vaults present a special problem. The processing error

JANUARY 1983 VOLUME 49 NUMBER 1


Fig. 7. A, Relatively large buccal space. B, Denture must be fabricated to fill space to ensure adequate border seal along distobuccal flange as limited by functional movement of coronoid process. C, Denture is shown filling buccal space.

may be so severe that no amount of posterior palatal seal can compensate for the resulting deficiency in intimate tissue contact. In these situations a metal base or subsequent bench-cure reline procedure would be incorporated into the initial treatment plan.

A region that often causes problems in maintaining border seal is the buccal space or retrozygomatic space (Fig. 7). This varies in size and shape but must be filled to avoid ingress of air beneath the denture base. Care should be taken to fill the entire buccal space during border molding and subsequent impression making as limited by the normal functional range of movement of the coronoid process.

The remaining border of the maxillary denture benefits from the draping effect of the lips and cheek and is not usually a problem in maintaining border seal if overextension is avoided.

ANATOMIC INFLUENCES ON MANDIBULAR DENTURE RETENTION

The mandibular denture generally presents the major problem with regard to retention. Reasons for this include a movable floor of the mouth, which causes difficulty in establishing a lingual border seal, and lack of ideal ridge height and conformation, which mini- mizes denture stability.


Retromolar pad with glands

/

A

Retrarnolor papilla

Fig. 8. Anatomic demarcation between structures that ultimately forms pear-shaped pad and retromolar pad of edentulous mandibular ridges. (From Sicher, H., and DuBrul, E. L.: Oral Anatomy, ed 6. St. Louis, 1975, The C. V. Mosby Co.)

11

JACOBSON AND KROL

Fig. 9. A, Clinical appearance of retromylohyoid curtain. B, Glandular, nervous, and muscular structures that lie deep to mucosa of retromylohyoid curtain.

Fig. 10. Lingual flange of mandibular denture must incline medially to allow for contraction of mylohyoid muscle, which lies beneath mucosa covering lingual slope of residual ridge. Dotted lines represent an activated mylohyoid muscle.

Intimate tissue contact of the mandibular denture can be achieved through sound impression procedures as outlined above. The elimination of dislodging forces by accurate border molding that prevents overextension can also be accomplished. Special attention to the triangularis muscle, which attaches in the region of the mandibular buccal frenum, and the mentalis muscle, which may be active in the region of the labial flange, should accompany any border-molding procedure. The border seal of the entire facial flange of the denture depends on accurate border molding and is enhanced by the draping effect of the lips and cheek.

A slight posterior seal may be necessary on the distal border of the mandibular denture at the point where the cheek no longer provides contact along the denture

12

border. The denture base should cover the posterior extension of the firmly bound, keratinized tissue of the pear-shaped pad. Craddock39 coined the term “pear- shaped pad,” which refers to the area formed by the residual scar of the extracted third molar and the associated retromolar papilla (Fig. 8). Clinically the pear-shaped pad is distinguishable by the lighter color and firmly bound nature of the overlying mucosa. Immediately distal to the area is the less keratinized, more resilient, and more vascular retromolar pad. It contains glandular tissue and a submucous layer that can tolerate a gentle posterior seal. Lammie34 and Krol suggest beading this region at the junction of the pear-shaped and retromolar pad to ensure peripheral seal along the posterior denture border.

LINGUAL MANDIBULAR ANATOMIC INFLUENCES

The border seal along the distal extension of the lingual flange requires an understanding of the anatomy and dynamic muscle physiology of the region. The anatomy of the retromylohyoid space is discussed in detail in several articles in the prosthetic literature.2r 39-42 The posterolateral portion of the retromylohyoid curtain overlies the superior constrictor muscle, and the posteromedial aspect covers the palatoglossus muscle and lateral surface of the tongue. The inferior wall of the retromylohyoid fossa overlies the submandibular gland, which fills the gap between the superior constrictor and the most distal attachment of the mylohyoid muscle. Border molding must allow for the muscular function in this region. It is possible that medial pterygoid contraction could influence the contours of the distolingual flange by causing a bulge in the posterior wall of the retromylohyoid space (Fig. 9). Adequate seal can be obtained by gently compressing the tissues of the lateral wall of the retromylohyoid fossa lingual to the retromolar pad and tucking the distolingual flange laterally against the mucosa overly-

JANUARY 1983 VOLUME 49 NUMBER 1


Fig. 11. A, Waxed anatomic model and diagram, B, illustrate attachments and fiber direction of mylohyoid muscle. C, Cross-sectional diagram of a mandibular denture indicates relationship of lingual flange to underlying mylohyoid muscle. Posteriorly, as a result of the more vertical fiber direction, flange may be extended more inferiorly than in anterior. Dotted lines represent an activated mylohyoid muscle.

ing the superior constrictor muscle superiorly and the loose connective tissue of the mandible inferiorly. Maximum posterior extension into the fossa is not necessary. Once the border seal is established, further posterior extension adds little to the support, stability, or retention.

The contour and inferior extension of the lingual flange are dependent on the action and anatomy of the mylohyoid muscle. The lingual flange slopes medially away from the mandible to allow for the action of the mylohyoid muscle (Fig. 10). This inclination also enhances the ability of the tongue to control the mandibular denture, providing a seating force to the denture.

The mandibular attachment of the mylohyoid muscle extends anteroinferiorly along the mylohyoid ridge from the lingual tuberosity in the molar region to the genial tubercles at the midline. Posterior fibers extend vertically to attach to the hyoid bone, while the anterior fibers extend horizontally to meet the fibers of the contralateral side to form a midline tendinous raphe (Fig. 11). This explains why the lingual flange can be made longer posteriorly despite a more superior mylohyoid muscle attachment.

Certain authors believe that adequate inferior extension of the flange can provide continuous contact regardless of tongue position of mobility of the floor of the mouth.40 However, the inferior extension of the posterior lingual flange is determined by .:he displacea- bility of the soft tissue and underlying mylohyoid muscle when the floor of the mouth is at its most superior position. In addition, the flange is inclined medially so that the tissue surface of the lingual flange is molded by a contracted mylohyoid muscle. At rest the level of the floor of the mouth may be inferior to the lingual flange and the mucosa may drop laterally away from the intaglio as the mylohyoid muscle relaxes. In such situations the border seal occurs at the border of the lingual flange when the mylohyoid muscle is active. When it is inactive, with the tongue retracted or at rest, the seal may occur as high as along the contact of the intaglio with mucosa overlying the mylohyoid ridge. Fortunately, the tongue often occupies the entire space superior to the floor of the mouth at rest. ISy contacting the lingual denture surface, it is able to promote a seal in this region and enhance retention. Accurate border molding and impression procedures ensure adequate border seal.


JACOBSON AND KROL

Genioglossus

Fig. li. Genioglossus muscle in cross section, illus- trating two main divisions. Superior fibers, A, are contracted, depressing central body of tongue .and causing tip to be retracted and floor of mouth to reach its most superior position. B, Inferior fibers of genioglossus.

MANDIBULAR ANTERIOR LINGUAL INFLUENCES

The most difficult region in which to obtain a border seal is the anterior lingual border. The mylohyoid muscle acts anteriorly as well as posteriorly to raise the floor of the mouth, and the genioglossus muscle func- tions in the region underlying the lingual frenum. The superior fibers of the genioglossus muscle attach to the superior genial tubercles and function in depressing the body of the tongue. Activation of the inferior fibers of the genioglossus muscle serves to protract the tongue. According to Lawson43 it is the action of the superior genioglossus muscle that pulls the tip of the tongue posterosuperiorly, depresses the central part of the tongue to form a concavity during bolus formation, and causes the anterior floor of the mouth to reach its most superior position (Fig. 12).

Several methods may be used to establish and maintain border seal throughout the functional range of movement of the anterior floor of the mouth. Some techniques recommend the horizontal extension of the anterior lingual flange sublingually.+’ Here the lingual

Superior most floor of mouth

Rest

Fig. 13. A technique for establishing anterior lingual seal is diagrammed. First, denture must be border- molded to contact superiormost level of floor of mouth. Extension posteriorly to contact sublingual folds should maintain a border seal when floor of mouth is at rest.

Fig. 14. A second technique. to establish anterior lingual seal. A, Denture is extended to most superior level of floor of mouth. B, Slight pressure on mucosa overlying lingual slope of anterior mandible ensures a border seal when tongue is at rest.

flange is extended inferiorly to contact the highest level of the floor of the mouth (Fig. 13). The flange can then be extended posteriorly to contact the sublingual folds and thereby establish a seal when the tongue is at rest and the floor of the mouth drops. Care is taken not to impinge on the submandibular or sublingual gland ducts.

Another technique involves a similar method of border molding to determine the inferior extension of the flange. However, a slight displacement of the mucosa anteriorly can be tolerated and provides a seal when the muscular floor of the mouth is at rest. This is accomplished by adding a slight additional amount of softened border-molding material to the inner surface of the previously molded anterior lingual area and



reseating the custom tray (Fig. 14). Again, the tongue at rest aids in maintaining the border seal by contacting the polished lingual flange as well as the mandibular anterior teeth.

SUMMARY

Establishing optimal complete denture retention requires an understanding of the factors discussed. Incorporation of these determinants into the prosthesis through proper design and technique contributes to the success of complete dentures.

REFERENCES

I.

2.

3.

4.

5.

6.

7.

8.

9.

10.

II.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

Bohannan, H. M.: A critical analysis of the mucostatic principle. J PROSTHET DENT 4:232, 1954.

Boucher, C.: Complete denture impressions based on the anatomy of the mouth. J Am Dent Assoc 31:I 174, 1944. Fish, W.: Principles of Full Denture Prosthesis, ed 6. Spring- field, Ill.. 1964, Charles C Thomas, Publisher, pp 32-66. Craddock, F. W.: Prosthetic Dentistry: A Clinical Outline, ed 2. St. Louis, 1951, The C. V. Mosby Co., pp 212-213. Lundquist, D. 0.: An electromyographic analysis of the function of the buccinator muscle as an aid to denture retention and stabilization. J PROSTHET DENT 9:44, 1959.

Roberts, A. L.: Effects of outline and form upon denture stability and retention. Dent Clin North Am, July 1960, pp 293-303. Strain, J. C.: Establishing stability for the mandibular complete denture. J PROSTHET DENT 21:359, 1969.

Friedman, S.: Edentulous impression procedures for maximum retention and stability. J PROSTHET DENT 214, 1957.

Schlosser, R. 0.: Complete Denture Prosthesis, ed 2. Philadel- phia, 1946, W. B. Saunders Co., pp 58-59. Schiesser, Jr., F. J.: The neutral zone and polished surfaces in complete dentures. J PROSTHET DENT 14854, 1964.

Beresin, V. E., and Schiesser, F. J.: The Neutral Zone in Complete and Partial Dentures, ed 2. St. Louis, 1978, The C. V. Mosby Co. Lott, F., and Levin, B.: Flange technique: An anatomic and physiologic approach to increased retention, function, comfort, and appearance of dentures. J PROSTHET DENT 16:394, 1966. Fry, K.: The retention of complete dentures. Br Dent J 44:97, 1923. Raybin, N. H.: The polished surface of complete dentures. J PROSTHET DENT 13:236, 1963. Swenson, M. G.: Complete Dentures. St. Louis, 1940, The C. V. Mosby Co., p 481. Page, H. L.: Mucostatics-A brief introduction. Tic, July 1949. Page, H. L.: Mucostatics-A practical comparison. Tic, April 1947. Wilson, G. H.: A Manual of Dental Prosthesis, ed 4. Philadel- phia, 1920, Lea & Febiger. Moses, C. H.: Physical considerations in impression making. J PROSTHET DENT 3:449, 1953.

Page, H. L.: Mucostatics-A Principle, Not a Technique. Chicago, 1946, published by the author. Porter, C. G.: “Mucostatics”-Panacea or propaganda? J PROSTHET DENT 3:464, 1953.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

44.

Howland, C. A.: The retention of artificial dentures. Dent Digest 27:159, 1921. Sussman, B. A.: Procedures in complete denture prosthesis. J PROSTHET DENT lO:lOll, 1960. Stamoulis, S.: Physical factors affecting the retention of complete dentures. J PROSTHET DENT 12:857, 1962. Brill, N.: Factors in the mechanism of full centure retention- A discussion of selected papers. Dent Pratt (Bristol) 18:9, 1967. Hardy, I. R., and Kapur, K. K.: Posterior border seal--Its rationale and importance. J PROSTHET DENT 8~386, 1958.

Stanirz. J. D.: An analysis of the part played by the fluid film in denture retention. J Am Dent Assoc 32:445, 1948. Snyder, F. C., Kimball, H. D., Bunch, W. B., and Beaton, J. II.: EHPct of reduced atmospheric pressure upon retention of dentures. J Am Dent Assoc 32445, 1945. Tyson, K. W.: Physical factors in retention of complete upper dentures. J PROSTHET DENT l&90. 1967.

Skinner, E. W.: A clinical study of the forces required to dislodge maxillary denture bases of various designs. J Am Dent Assoc 47~671, 1953. Skinner, E. W., and Chung, P.: The effect of surface contact in the retention of a denture. J PROSTHET DEKT 1:229, 1951. Ostlund, S. L. G.: Some principles in the retention of dentures. Northwest Univ Bul 49:1 I, 1948. Tilton, G. E.: The denture periphery. J PROSTHET DENT

2290, 1952. Lammie, G. A.: The retention of complete dentures. J Am Dent Assoc 55:502, 1957. Grunewald, A. H.: Gold base lower dentures. J PROSTHET

DENT 14:432, 1964.

Brill, N., Tryde, G., and Schiibeler, S.: The role of learning in denture retention. J PROSTHET DENT 10:468, 1960.

Brill, N., Tryde, G., and Schiibeler, S.: The role of exterocep- tars in denture retention. J PROSTHET DENT 9:761, 1959.

Lytle, R. B.: Complete denture construction based on a study of the deformation of the underlying soft tissues. J PROSTHET

DENT 9:539, 1959.

Craddock, F. W.: Retromolar region of the mandible. J Am Dent Assoc 42453, 1953. Edwards, L. F., and Boucher, C. 0.: Anatomy of the mouth in relation to complete dentures. J Am Den: Assoc 29:331, 1942. Barrett, S. G., and Haines, R. W.: Structure of the mouth in the mandibular molar region and its relation to the denture. J PROSTHET DENT 12:835, 1962.

Haines, R. W., and Barrett, S. G.: The structure of the mouth in the mandibular molar region. J PROSTHE:T DENT 9:962, 1959.

Lawson, W. A.: Influence of the sublingual fold on retention of complete lower dentures. J PROSTHET DENT J 1:1038, 1961.

Barone, J. V.: Physiologic complete denture impressions. J PROSTHET DENT 13:800, 1963.

Keprmt requestr to: DR. THEODORE E. JACOBSON

UNIVERSITY OF CALIFORNIA SCHOOL OF DENTISTRY

SAN FRANCISCO, CA 94143


Documents

Jacobson a Contemporary Review of the Factors Involved in CD Retentio Stability Support Pt I 1983