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Definition : That quality of maintaining a constant character or position in the presence of forces that threaten to disturb it. Or The quality of a prosthesis to be firm, steady or constant, to resist displacement by functional horizontal or rotational stresses. Denture Stability : The resistance of a denture to movement on its tissue foundation, especially to lateral (horizontal) forces as opposed to vertical displacement (termed denture retention) INTRODUCTION: Stability ensures the physiological comfort of the patient. Denture instability adversely affects support and retention. A denture that shifts easily in response to laterally applied forces that are generated during most of the functions of the mouth, causes a disruption in the border seal and prevents the denture base from correctly relating to the supporting tissues. This results in deleterious forces on the edentulous ridges during function. Factors contributing to stability : - Fish in 1948 described a denture as having 3 surfaces : 1. The impression surface. 2. The occlusal surface. 3. The polished surface. Though all these three surfaces are developed independently, they are integrated to create a stable, functional and esthetic result. - Thus the factors contributing to stability can be categorised under the following headings : 1. The relationship of the denture base to the underlying tissues. 2. The relationship of the external surface and border to the surrounding oro-facial musculature. 3. The relationship of the opposing occlusal surfaces.3

. . . . . . .JPD 99, GPT 7

4. Education of the patient. IRELATIONSHIP OF THE DENTURE BASE TO THE UNDERLYING TISSUES: This will be discussed under the following headings : 1. Residual ridge anatomy. 2. Denture base adaptation. 3. The mandibular lingual flange. 1. Residual ridge anatomy : Residual ridge height and conformation are limiting factors for stability. a) Large, square, broad ridges offer a greater resistance to lateral forces than do small, narrow and tapered ridges. (Fig. 1) b) Small and rounded irregularities also contribute favourably to stability. Alveoloplasty should therefore be limited to removal of bone that would prevent the fabrication of a successful prosthesis. Ex in case of sharp spicules, severe undercuts and insufficient inter-arch space. But removal of all irregularities to create a smooth, even ridge would diminish the potential stability. c) The arch form: Square or tapered arches tend to resist rotation of the prosthesis better than the ovoid arches. d) Shape of the palatal vault : Stability is limited by the length and angulation of the palatal ridge slopes. A steep or high arched palate enhances stability by providing greater surface area of contact and long inclines approaching at right angles to the direction of force. But vertical forces tend to unseat the denture easily. According to Burns DR et al, (JPD 95, Vol.73 (4)) Treatment alternatives to increase stability including - Ridge augmentation or vestibular augmentation (Fig. 2)


- Dental implants to provide anchorage for an all implant supported prosthesis. - Mucosa and implant supported overdentures were assessed. The study indicated superior statistics with respect to implant overdentures and a slight but significant improvement in the soft tissue response. According to W. Kalk et al. (IJP 1992 No. 3 Vol. 5) Stability was assessed in 3 groups : 1. Who needed preprosthetic surgery but which was contraindicated, had received new dentures. 2. Same as 1 but were treated with vestibuloplasty and lowering off the floor of the mouth before denture fabrication. 3. Control groups without residual ridge related problem who were treated with new complete dentures. The least displacement of the mandibular dentures occurred in groups 2 & 3, greatest in 1. Loosening caused by tipping of the mandibular denture was least in group 2 because of elimination of the muscle attachments and increase in the extensions. 2. Denture base adaptation : - The relationship of the intaglio of the denture base to the underlying tissues is dependent on the impression procedures of the clinician. - Health of the tissues at the time of impression making is important. Stability is compromised in the following cases : Inflamed mucosa. Distorted or displaced tissues



Hyperplastic tissue.

- Denture base adaptation may be improved by the use of tissue liners, adhesives,and fixatives. - Mucostatic impression techniques increase stability because of less displacement of tissues during impression making and hence less rebounding of the displaced tissues. 1. The mandibular lingual flange : A properly formed denture base outline develops a seal than can be maintained during most of the normal oral functions. The labial and buccal flanges have well defined landmarks that can be visually evaluated. The distolingual extension of the lingual flange is developed arbitrarily. The lingual slope of the mandible is at 90 degrees to the occlusal plane which is a desirable feature. The posterior lingual flange can be extended more inferiorly than anterior lingual flange, although posterior fibres of mylohyoid muscle attach more superiorly on the mandible; they descend vertically to attach to the hyoid bone. When contracted the muscle fibres extend medio-inferiorly allowing the posterior flange to extend to/or beyond the mylohyoid ridge. Anteriorly the muscle fibres are directed more horizontally to communicate with fibres of the opposite side. When contracted the fibres tense the floor of the mouth and limit the extension of the anterior lingual flange.(Fig.3&4) - Any flange extensions beyond the mylohyoid ridge must incline medially away from the mandible to allow for the mandibular mylohyoid muscle contraction. The degree of positive contact of firm ridge to flange may be compromised by the presence of a thin mucosa overlying the bony ridge slopes that dont tolerate the stresses effectively and may require relief.3

According to C. H. Jooste and C.J.Thomas, (IJP 1992 Vol. 5 No.1) Analysis of cineradiographic tracings of movements of marker placed in the mandibular dentures with and without denture extensions during chewing exercises revealed that an extension had a stabilizing effect on the mandibular complete denture. Sublingual Crescent Area : Definition : The crescent shaped area on the anterior floor of the mouth formed by the lingual wall of the mandible and the adjacent sublingual fold. It is the area of the anterior alveolingual sulcus. . GPT 7 Extension of the denture over the resting tissues of the sublingual crescent area completes the border seal and increase the covering surface of the dentures resulting in : 1. Increased retention by allowing the tongue to aid in holding the dentures in place. swallowing, speaking and eating. IIRELATIONSHIP TO OF THE EXTERNAL THE SURROUNDING SURFACE AND OROFACIAL PERIPHERY (Fig.5) 2. Denture is more stable during normal tongue movements such as

MUSCULATURE : Actions of the musculature on the denture base generally result in lateral and vertical dislodging forces. Factors involving musculature and the polished surface of the denture can facilitate stability if : 1. Action of certain groups are permitted to occur without interference by the denture base so that they wont dislodge the prosthesis during function. 2. Dentist recognizes that normal functioning of some muscle groups external can be used to enhance stability ie. alterations in3

contours can lead to dynamic seating and stabilizing action directed towards the prosthesis. (Fig.6&7) The action of Levator Anguli Oris, Incisivus, Depressor Anguli Oris, Mentalis, Mylohyoid and Genioglosus can dislodge the denture base if they are not allowed to function freely. Proper border or muscle molding ensures optimal border extensions. The following factors will be considered : 1. The external surface of the denture 2. Influence of oro-facial musculature 3. Modiolus and associated musculature 4. The neutral zone 1. The external surface of the denture : The location and form of the polished surfaces given by wax to obtain convexity or concavity facially and lingually contribute to functional stability of dentures. Fish believed that the contours of the polished surface provided the principal factor governing complete denture stability. He wrote The shape of the buccal, labial and lingual surfaces can wreck stability as completely as a bad impression or a wrong bite. Thus the horizontal forces exerted by the tongue and cheek can act either as a placing or displacing agent. The lingual and buccal borders of mandibular denture and the buccal borders of maxillary denture can be made concave so that the tongue and cheek will grip and seat the denture.(Fig.8 to 13) 2. Influence of Oro-Facial Musculature : The basic geometric design of denture bases should be triangular ie. in a frontal cross-section the maxillary and mandibular dentures should appear as 2 triangles whose apexes correspond to the occlusal surface.(Fig.14)3

- Maxillary buccal flange should incline laterally and superiorly. - Mandibular laterally and inferiorly and its lingual flange medially and inferiorly. Such inclination provides favourable vertical component to any horizontally directed forces. The tongue should rest against the lingual flange which is inclined medially away from the mandible and concave. A normal tongue position has the following features : a. It should completely fill the floor of the mouth. b. The lateral borders should rest over the ridge on the occlusal surface of the teeth. c. The tip or apex rests on or is just to the lingual side of the lower anterior ridge. According to PAJ. Culver and I. Watt (BDJ 1973 Vol. 135) Although it is recognized that the tongue and the oral musculature in general play a large part in stabilizing the upper denture, great emphasis is still placed in the