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A contemporary review of the factors involved in complete dentures. Part II: Stability T. E. Jacobson, D.D.S.,* and A. J. Krol, D.D.S.** University of California, School of Dentistry, San Francisco, Calif., and Veterans istration Medical Center, San Frankco, Calif.

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omplete denture stability is the resistance to horizontal or rotational forces. It differs from retention in that stability resists forces in the horizontal plane whereas retention is the resistance to vertical dislodging forces. As described in Part I, stability ensures the physiologic comfort of the patient while retention contributes to psychologic comfort. The lack of stability often makes ineffective the factors involved in retention and . A denture that shifts easily in response to laterally applied forces can cause a disruption in the border seal or prevent the denture base from correctly relating to the ing tissues. The factors that contribute to stability include ridge height and conformation, base adaptation, residual ridge relationships, occlusal harmony, and neuromuscular control. These factors can be condensed into the following categories: 1. The relationship of the denture base to the underlying tissues 2. The relationship of the external surface and border to the surrounding orofacial musculature 3. The relationship of the opposing occlusal surfaces

tissues not only allows the establishment of border seal and coverage of maximum ing area but also provides maximum of the denture base with facial and lingual ridge slopes. The nature of the overlying soft tissues determines the potential of a given region in tolerating stress. While the tissues of the maxillary palatal inclines are ideally designed to resist forces of the denture base, thi maxillary facial and mandibular lingual inclines may be less effective due to the thin alveolar mucosal covering. Optimal denture stability requires that those tissues that provide resistance to horizontal forces be properly recorded and related to the denture base. Boucher2 notes that stability is obtained by incorporating the surfaces of the maxillary and mandibular ridges, which are at right an.gle to the occlusal plane. He further states that stability requires “maximum use of all bony foundations where the tissues are firmly and closely attached to bone.“3 Positive and intimate of the denture base with these inclines as limited by the nature of the overlying soft tissues determines the degree of stability attained.

RELATIONSHIP TO TISSUES

MANDIBULAR

OF DENTURE BASE

The relationship of the intaglio of the denture base to the underlying tissues is dependent on the impression procedures of the clinician. The section on demonstrates the significance of this relationship as it pertains to the tissues that provide resistance to vertical forces directed toward the residual ridge. This relationship also contributes to denture stability. Friedman’ describes the ing of the labial and buccal flanges with the labial and buccal ridge slopes as critical factors contributing to stability. Adequate extension of the denture border as limited by movable

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

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LINGUAL

FLANGE

The most desirable feature of the lingual slope of the mandible is that it approaches 90 degrees to the occlusal plane. This enables it to effectively resist horizontal forces. The posterior lingual flange is usually able to be extended inferiorly more than the anterior lingual flange. Although the posterior fibers of the mylohyoid muscle attach more superiorly on the mandible, they descend nearly vertically to attach to the hyoid bone. Even when contracted, the muscle fibers extend medioinferiorly, allowing the posterior flange to extend to or beyond the mylohyoid ridge. Anteriorly, the mylohyoid muscle fibers are directed more horizontally to communicate with fibers of the opposite side along a midline tendinous raphe. When cu>ntracted,the anterior mylohyoid muscle tenses the floor or the mouth and limits the extension of the more anterior

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Fig. 1. Waxed anatomic model, A, 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 anteriorly. Dotted lines represent an activated mylohyoid muscle.

lingual flange (Fig. 1). The extent of of the lingual flange with the lingual ridge slope is thereby dictated by the functional mobility of the floor of the mouth. In addition to determining the inferior extension of the flange, the musculature of the floor of the mouth may also influence the degree of intimate allowed. Any flange extension below the mylohyoid ridge must incline medially away from the mandible to allow for the mylohyoid muscle contraction (Fig. 2). The degree of positive of firm ridge to flange may also be compromised by the presence of a thin mucosa overlying the bony ridge slopes that does not tolerate stresses effectively and, therefore, may require relief.

RESIDUAL RIDGE ANATOMY Fig. 2. 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.

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The development of stability is limited by the anatomic variations of the patient that determine the residual ridge height and conformation. Large, square, broad ridges offer a greater resistance to lateral forces than do small, narrow, tapered ridges (Fig. 3). Small,

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Fig. 3. Tapered, round, and square ridges diagrammatically in cross section. Potential for denture stability increases as a residual ridge becomes more square shaped.

Fig. 5. Inclination and external contour of lingual flange may be designed to harmonize with muscular action of tongue. Fig. 4. Diagrammatic representation of maxillary and mandibular complete dentures that have been designed to enhance seating of prosthesis during function shown in frontal cross section. rounded irregularities of the residual ridge also contribute favorably to stability. Therefore, alveoplasty at the same time as extractions should be limited only to removal of bone that would prevent fabrication of a successful prosthesis (for example, sharp spicules, severe undercuts, and insufficient interarch distance). Removal of all irregularities to create a smooth, even ridge would diminish potential stability. Another factor to be considered in stability is the arch form. Square or tapered arches tend to resist rotation of the prosthesis better than ovoid arches. The shape of the palatal vault contributes to stability as limited by the length and angulation of the palatal ridge slopes. A steep palatal vault may enhance stability by providing greater surface area of and long inclines approaching a right angle to the direction of force.

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RELATIONSHIP OF THE EXTERNAL SURFACE AND PERIPHERY TO SURROUNDING OROFACIAL MUSCULATURE Some important yet easily overlooked determinants of both denture stability and retention involve the relationship of the polished surface of the denture base to the surrounding musculature of the orofacial capsule. Actions of the musculature on the denture base generally result in lateral and vertical dislodging forces. Certain fact’ors involving the musculature and the polished surface of the denture can facilitate stability in two ways. First, the action of certain muscle groups must be permitted to occur without interference by the denture base so that they will not dislodge the prosthesis during function or compromise stability. Second, the dentist must recognize that normal functioning of some muscle groups can be used to enhance stability. Alterations in external denture base contours can lead to a

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Fig. 6. A and B, Concave contours of external surfaces of maxillary and mandibular denture flanges permit musculature of lips, tongue, and cheeks to effect a seating of denture during function.

dynamic seating and stabilizing action directed toward the prosthesis. The denture border must be extended to the movable tissues. Optimal extension also enhances stability and . The actions of the levator anguli oris (caninus), incisivus, depressor anguli oris (triangularis), mentalis, mylohyoid, and genioglossus muscles can lead to dislodging forces if the denture base does not provide freedom for these muscles to function. Proper border or muscle molding prior to the final impression ensures optimal border extension. The external surface should be developed to harmonize with the associated functioning musculature of the tongue, lips, and cheeks. Fish4 believed that the contours of the polished surface provided the principal factor governing complete denture stability. In 1933 he wrote that “it is not so widely understood that the actual shape of the whole of the buccal, labial, and lingual surfaces can wreck the stability of a denture just as completely as a bad impression or wrong bite.” In order to appreciate this principle, it is necessary to understand the anatomy and function of the muscles that comprise the tongue, lips, and cheeks.

INFLUENCE OF OROFACIAL MUSCULATURE The basic geometric design of denture bases should be triangular. In a frontal cross section, the maxillary and mandibular dentures should appear as two triangles whose apexes correspond to the occlusal surface

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(Fig. 4). The maxillary buccal flange should incline laterally and superiorly. The mandibular buccal flange should incline laterally and inferiorly, and its lingual flange should incline medially and inferiorly. Such inclinations will provide a favorable vertical component to any horizontally directed forces. 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 (Fig. 5). The degree of inclination depends on the balance of the muscular forces of the tongue as opposed to the mylohyoid and superior constrictor muscles.5 Some authors recommend posterior extension of the lingual flange to fill the retromylohyoid space to permit the base of the tongue to contribute to the neuromuscular control of the prosthesis.“’ Inclination of the lingual flange must be designed to guide the tongue to rest over the flange and permit any horizontal forces generated against the denture base to be transmitted as seating forces. Generally, the buccal and labial flanges of the maxillary and mandibular dentures should be concave to permit positive seating by the cheeks and lips (Fig. 6). The primary muscles of the lips and cheeks are the orbicularis oris and buccinator muscles, respectively. These muscles are active during speech, mastication, and deglutition. The proper contour of the denture flanges permits the horizontally directed forces that occur during contraction of these muscles to be transmitted as vertical forces tending to seat the prosthesis.

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IMPORTANCE OF THE MODIOLUS ASSOCIATED MUSCULATURE

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Fish4 describes the muscles of the musculi cruculi modioli or modiolus and their actions in detail. The modiolus or tendinous node is an anatomic landmark near the corner of the mouth that is formed by the intersection of several muscles of the cheeks and lips. These include the orbicularis oris, buccinator, caninus, triangularis, and zygomaticus muscles (Fig. 7). Because none of these muscles contains fibers that have more than one bony attachment, they depend on fixation of the modiolus to allow isometric contraction. By studying the diagram of these fan-shaped muscles, one can understand the various interactions that are possible. Contraction of the triangularis, caninus, and zygomaticus muscles fixes the modiolus, allowing the buccinator muscle to contract isometrically. This causes the buccinator muscle to tense, allowing it to control the food bolus on the occlusal table. Isotonic buccinator muscle contraction in the absence of modiolus fixation would pull the corner of the mouth posteriorly. A similar situation can be described for the orbicularis oris muscle and the remaining musculature of the modiolus. It can be fixed more anteriorly as when the word “hoe” is pronounced or posteriorly as with “he.” The denture base must be contoured to permit the modiolus to function freely. In the premolar region the mandibular denture should exhibit both a shortened and narrowed flange to permit the action that draws the vestibule superiorly and the modiolus medially against the dentures. This action is easily demonstrated by drawing the corners of the mouth inward. The buccinator muscle may be divided into superior, middle, and inferior divisions. According to Fish,4 the superior fibers act to seat the maxillary denture, the middle fibers control the bolus of food, and the inferior fibers contribute to mandibular denture stability. While the middle fibers contract, controlling the bolus, the inferior fibers relax to form a pouch capable of storing food until needed to form another bolus. Extension of a concave denture base into this pouch allows the cheek to lie over the flange. A clinical study involving electromyographic analysis of the function of the buccinator muscle by Lundquist’ s this basic theory by Fish. The study further showed that the nature of buccinator muscle contraction was not able to adapt to changes in the contours of the denture base. Because learning and adaptation appear to be limited, the denture contours should be designed to harmonize with existing buccinator muscle function.

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Fig. 7. Intersection of muscles of facial expression that comprise modiolus.

VARIETY OF TECHNIQUES The idea of establishing harmony between the polished surface of the denture and the associated musculature has provided the basis for a number of techniques for complete denture construction. This “neutral zone” (concepthas been described with various modifications by a number of authors.“‘-!’ The theory used to develop the denture base contours is based on the belief that the muscles should functionally mold not only the border but the entire polished surface. Even the teeth are placed within the “neutral zone,” where facial and lingual forces generated by the musculature of the lips, tongue, and cheeks are balanced. This functional rather than anatomic placement of the artificial teeth is believed to further enhance the stability of the dentures by minimizing active forces. The authors do not necessarily recommend such techniques in all cases, but recognjtion of the need for a harmonious relationship between denture base and surrounding musculature is certainly important.

RELATIONSHIP OF OPPOSING OCCLUSAL SURFACES Harmony developed between the opposing occlusal surfaces also contributes to stability. Regardless of the type of posterior tooth form or occlusal scheme used, the dentures must be free of interferences within the functional range of movement of the patient” The functional range of movement refers to the positions through which the lower jaw moves horizontally during normal speech, swallowing, and mastication. During both functional and parafunctional movements, the occlusal surfaces should not strike prematurely in

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Fig. 8. Tendency to lingualize forces transmitted to mandibular denture to minimize dislodging leverages during working-side in absence of a fully balanced occlusion. localized areas. Such s cause uneven stresses to be transmitted to the dentures during function. This results in lateral and torquing forces that adversely affect stability. Bilateral, simultaneous, posterior tooth in centric relation is essential. For many patients the normal range of horizontal movement of the mandible is limited to centric relation. This is particularly true for skeletal Class III or mandibular prognathic patients. Excursive balance may not be necessary in such situations. Patients who exhibit a wider functional range of movement, often seen in skeletal Class II or mandibular retrognathic patients, require consideration of premature occlusal s, which may occur when the mandible does not close in centric relation.

THEORIES OF OCCLUSION Various philosophies have been proposed either to provide for a fully balanced occlusion throughout lateral and protrusive excursive movements or to control the direction of forces experienced during localized occlusal . Setting of anatomic or semianatomic artificial teeth to provide excursive balance is thought to minimize localized stress concentration and lateral dislodging forces by ensuring multiple points of to distribute functional occlusal forces. On closure through a bolus of food, bilateral posterior tooth s within the range of balance ensure a seating of the prosthesis. To minimize dislodging forces the occlusion must be balanced throughout the functional range of movement of the patient. A balanced occlusion is limited by the buccolingual and mesiodistal width of the anatomic cuspal inclines. Patients who exhibit a functional range of movement beyond the limits of balance would benefit less from such an arrangement.

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An anatomic fully balanced occlusion would, therefore, not be indicated for a skeletal Class II mandibular retrognathic patient displaying a functional range of lateral and protrusive excursions of 4 to 6 mm or greater. Some authors recommend occlusal schemes that direct forces to minimize the unseating of the denture during unilateral excursive tooth s. According to certain monoplane occlusal schemes, positioning Odegree teeth slightly lingual to the mandibular ridge crest may enhance denture stability. Zero-degree teeth may reduce horizontal forces by eliminating the inclined planes introduced by the cusp angles of anatomic teeth. The theories of lingualized occlusion provide both a limited range of excursive balance and a directing of forces to the lingual side of the lower ridge during working-side s (Fig. 8). Such concepts may minimize horizontal stress and enhance denture stability by controlling the leverages induced by eccentric tooth s.‘4-‘6 Other philosophies depend on the learned neuromuscular skills of the patient. Some patients are instructed to chew in such a manner that tooth s are limited to centric relation. Horizontal forces can be minimized when the patient learns to place food bilaterally to ensure simultaneous posterior without a balanced occlusion. Woelfel et al.” showed that most functional closures of the complete denture patients occurred in close proximity to centric relation. A study by Frechettels demonstrated even force distribution regardless of tooth position in patients who chewed bilaterally. In another study he further concluded that bilateral chewing contributed more to denture stability than balanced occlusion.18 However, the learning of new neuromuscular patterns is often limited, and not all complete denture patients appear to be capable of masticating bilaterally in centric relation. The selection of anatomic, semianatomic, or nonanatomic artificial teeth depends partially on the chosen occlusal scheme. Still another factor to be considered in selection of cusp form and occlusal scheme involves the quality of the residual ridge in of height and conformation. Unfavorable ridges exhibiting severe resorption patterns may contribute to compromised stability due to a poor denture base-residual ridge relationship. Use of anatomic artificial teeth in such situations may not provide the advantages normally expected. If balanced occlusion is desired throughout a limited functional range of movement for patients with deficient residual ridges, the use of nonanatomic O-

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Fig. 9. A, Mounted casts depict a severe skeletal Class III edentulous patient. B, To minimize anterosuperior shunting of maxillary denture during function, posterior occlusion must be developed beyond anteroposterior midline of denture-ing area to include line drawn across land area of cast. C, Completed wax-up demonstrates necessary occlusal s. degree teeth set on a curve may provide the desired occlusal s while eliminating the interlocking of opposing anatomic teeth. The subject of occlusion is filled with widely divergent opinions. As concluded by the International Prosthodontic Workshop of 1972: “More long-term statistical investigations are necessary to compare the various occlusal designs so that more definable guidelines may evolve.“” Until then, dentists must rely on clinical experience in selection of posterior tooth form and appropriate occlusal schemes. TOOTH

POSITION

AND OCCLUSAL

PLANE

Other considerations relating to the occlusal surface include the position of the teeth and the level of the occlusal plane. Anterior and posterior teeth should be arranged as close as possible to the position once occupied by the natural teeth, with only slight modifications made to improve leverages and esthetics. The superior-inferior position of the occlusal plane is also a factor to be recognized. A mandibular occlusal plane that is too high can result in reduced stability.

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First, lateral tilting forces directed against the teeth are magnified as the plane is raised. Second, the mandibular denture need:sto be controlled by the musculature of the tongue, lips, and cheeks. An elevated occlusal plane prevents the tongue from reaching over the food table into the buccal vestibule. This compromises stability and makes control of the food bolus and denture more difficult. A raised mandibular occlusal plane is usually present when the vertical dimension of occlusion is increased excessively. Various anatomic landmarks, such as Stensen’s duct and retromolar pad, should be used to determine an acceptable level of the occlusal plane. Bisecting the interridge distance improves the mechanical advantage of the mandibular denture; but, if excessive mandibular ridge resorption has occurred, the occlusal plane would be too low since less resorption usually occurs on the maxillae. RIDGE

RELATIONSHIPS

A problem of stability is the offset ridge relations seen in prognathic and retrognathic patients. Normal dental relationships of the artificial teeth set on ridges

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that are in severe posterior crossbite can adversely affect stability. In complete dentures the normal toothto-tooth position may have to be altered to provide a relationship that can enhance the stability. Weinberg2’ recognizes the need to set teeth in crossbite when the ridges are in a severe crossbite relation. The Class III patient frequently displays a lower arch anterior to the upper arch in centric relation. Sufficient mandibular posterior occlusion must be developed so that against the maxillary denture extends posteriorly more than half the distance from the incisive papilla to the hamular notch (Fig. 9). Without this the maxillary denture would tip anterosuperiorly, traumatizing the maxillary anterior ridge and loosening the maxillary denture. The severe retrognathic or prognathous ridge relationship can be remedied only to a limited extent through prosthetic treatment. While some compromises in the ideal tooth-to-ridge and tooth-to-tooth relationships may be made, the range of correction of such skeletal cosmetic deficiencies without surgical interventions is limited. CONCLUSION Both complete denture stability and retention are essential in providing successful prosthetic treatment. The factors that contribute to these properties are highly interrelated, and the constant interaction between stability and retention often makes them indistinguishable. The factor of stability involve the tissue, occlusal, and polished surfaces of the denture. Care must be taken in the development of all three of these surfaces to ensure optimal stability of the final prosthesis. REFERENCES 1. Friedman, S.: Edentulous impression procedures for maximum retention and stability. J PR~~THET DENT 214, 1957. 2. Boucher, C.: Complete denture impressions based on anatomy of the mouth. J Am Dent Assoc 31:124, 1944. 3. Boucher, C. 0.: A critical analysis of mid-century impression techniques for full dentures. J PROSTHET DENT 1:472, 1951.

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Fish, W. E.: Using the muscles to stabilize the full lower denture. J Am Dent Assoc Zlh2163, 1933. 5. Roberts, A. L.: Principles of full denture impression making and their application in practice. J PROSTHET DENT 1:213,

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6. 7.

8. 9.

10. 11. 12.

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14. 15. 16.

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Roberts, A. L.: Effects of outline and form upon dental stability. Dent Clin North Am, July 1960, pp 293-303. Edwards, L. F., and Boucher, C. 0.: Anatomy of the mouth in relation to complete dentures. J Am Dent Assoc 29331, 1942. Shanahan, T. E. J.: Stabilizing lower dentures on unfavorable ridges. J PROSTHET DENT 12~420, 1962. 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. Fry, K.: The retention of complete dentures. Br Dent J 4497, 1923. Schiesser, Jr., F. J.: The neutral zone and polished surfaces in complete dentures. J PROSTHET DENT 14:854, 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. Payne, S. H.: A posterior setup to meet individual requirements. Dent Digest 4220, 1941. Pound, E.: Utilizing speech to simplify a personalized denture service. J PROSTHET DENT 24:586, 1970. Becker, C. M., Swoope, C. C., and Guckes, A. D.: Lingualized occlusion for removable prosthodontics. J PROSTHET DENT 38:601, 1977. Woelfel, J. B., Hickey, J. C., and Allison, M. L.: Effect of posterior tooth form on jaw and denture movement. J PROSTHET DENT 12~922, 1962.

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Frechette, A. R.: Complete denture stability related to tooth position. J PROSTHET DENT 11:1032, 1961. 19. Lang, B. R., and Kelsey, C. C., editors: International Prosthodontic Workshop on Complete Denture Occlusion. Ann Arbor, Mich., 1972, The University of Michigan, p 132. 20. Weinberg, L. A.: Tooth position in relation to the denture base foundation. J PROSTHET DENT 8~398, 1958.

Reprint requeststo: DR. THEODORE E. JACOBSON UNIVERSITY OF CALIFORNIA SCHOOL OF DENTISTRY SAN FRANCISCO, CA 94143

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