In the life of an individual, teeth play an indispensable role. Complete loss of teeth results in loss of aesthetics as well as function. Thus, the replacement of teeth in the form of artificial dentures is an important requisite. Mucosa borne complete denture is the conventional treatment modality adopted most commonly for the edentulous patients. The patient’s acceptance of the finished prosthesis is dependent on various factors, among which retention and stability of the complete denture play a significant role. The loose and unstable denture is a persistent source of annoyance to the patient as well as to the dentist. The mandibular denture is usually more problematic than the maxillary denture owing to the smaller surface area coverage of the foundation tissues. The problem of retention and stability is more pronounced with mandibular denture as compared to the maxillary denture because the covered surface area is approximately half to that in the maxillary arch and presence of palate adding to the area against the mobile tongue on the floor of the mouth in the mandibular arch. A complete mandibular denture is said to be stable if it is entirely and continuously under the patient’s functional control. The patient should be able to eat comfortably and with minimal movement of the denture relative to its foundation area. The retentive dentures can be fabricated in the majority of the patients regardless of the condition of the ridge. It is mainly the stability of the denture that is limited by the ridge conditions. Stability is the quality of a removable prosthesis to be firm, steady, or constant to resist displacement by functional horizontal or rotational stresses.[1][2]

The dentures move relative to the underlying bone during their functioning. Most principles of denture construction have been formulated to minimize the off-vertical forces transmitted to the supporting structure, as the toleration of vertical forces is better. The instability has the potential of being traumatic to the supporting tissues. Thus, with complete denture philosophies and techniques, the achievement of maximum stability and retention appears to be the major objective throughout the clinical procedure. Stability is usually the distinguishing factor between success and failure. Modification on impression surface, occlusal surface, polished surface, and even on the compromised mandibular residual ridge have been devised to improve stability and retention in the mandibular denture.[3][4]

There are a variety of factors on which the stability of mandibular denture depends. These appear under the following categories:

1. The relationship of the denture base to the underlying tissues
2. The relationship of the polished surface and border to the surrounding orofacial musculature
3. The relationship of the opposing occlusal surfaces

1. The relationship of the denture base to the underlying tissues   

A.Fitting Surface/Intaglio surface/Impression surface    

The production of a good fitting surface depends primarily upon the impression. There is general agreement that all complete dentures should cover the maximum area possible along with close adaptation to the underlying surface, but great care must be taken to confine the area of the impression so that it does not impinge into the zones of muscle attachments.

Various modifications in the design of the denture in resorbed ridges:

Lingual flange design (sublingual crescent extension): It involves a lingual wing (horizontal extension of the lingual flange) placed in a biologically acceptable fashion by increasing the area of the denture, which enhances retention and stability.[5][6][7] 

Posterior lingual extension: The posterior part of the alveolingual sulcus (the retromylohyoid fossa) divides into two parts: anteroinferior and posterosuperior. The lingual flange of the mandibular denture should be turned into the anteroinferior part to produce maximum stability for the denture since no muscle lies directly underneath.

B. Residual ridge factor

Height and width: Denture stability is directly proportional to the height and width of the ridge. Resorbed ridges have poor stability.

Ridge shape and size: Denture stability is more in a square, parallel-sided broad ridges than the small narrow tapered-ridges because the former provides greater resistance to the functional horizontal dislodging forces.

Interarch space: The appropriate vertical dimension enhances denture stability. If there is excessive interridge distance, the stability decreases because of increased leverage. In case of excessive resorption, the interridge space increases, putting the occlusal surface of teeth farther from the supporting area, which results in biomechanical disadvantage for the denture due to increased leverage.

Ridge parallelism: Parallelism of the edentulous ridges favors seating of the denture bases during tooth contact because of favorable directions of forces, thus increasing the stability. Deviation from the parallelism of the ridges adversely affects stability.

Ridge relation: Denture stability is a problem in severe prognathic and retrognathic ridge relations.

Ridge quality: Denture stability becomes decreased by the presence of hypermobile or flabby tissue on the ridge surface.

Ridge resorption can result in a prominent and sharp mylohyoid ridge, which limits the denture extension below the mylohyoid area, which can affect denture stability. Mylohyoid ridge reductions can aid in the success of mandibular denture by permitting the increased surface area coverage by the denture. In highly resorbed ridges with shallow sulcus depth, adequate stability is not achievable through non-surgical procedures. Pre-prosthetic surgery measures, including sulcus deepening and/or ridge augmentation procedures, are to be adopted to ensure the stability of the mandibular denture.

2. Relationship of the polished surface and periphery to surrounding orofacial musculature

The polished surface of a mandibular denture extends from the denture border in the vestibular depth upwards to the occlusal surface of the teeth. This surface should be shaped to accommodate the musculature contacting the surface around the denture, which in turn aids in the denture stability. In case the direction of muscle contraction is not respected and is against the shape of the contacting surface, then the muscular force serves as a dislodging factor and results in impaired denture stability.

Seating action on the mandibular denture occurs if the tongue rests against the lingual flange that is inclined away from the mandible medially and is concave. In general, the labial and buccal flanges of the maxillary and mandibular dentures should be concave to permit positive seating by the musculature of cheeks and lips.

Posteriorly, similar conditions govern the shape of the polished surface. When the natural teeth are present, they lie in a neutral zone of muscle activity enclosed by the buccinator and tongue. Following the loss of teeth and resorption, the neutral zone area gets reduced. Accordingly, it is essential to record the neutral zone and use narrow posterior teeth set over the residual ridge in the premolar region, but deviating to little buccal in the molar region.

Role of frenum

The frenum though not a muscular structure, however, should be permitted adequate space at the denture border. Otherwise, it will cause the dislodgement of the denture.

Role of modiolus

The modiolus or tendinous node is an anatomic landmark near the corner of the mouth that is formed by the intersection of various muscles (Orbicularis oris, depressor anguli oris, levator anguli oris, quadratus labii superioris, quadratus labii inferioris, buccinators, risorius, and zygomaticus major) of the cheeks and lips. The mandibular denture base must be contoured to permit the modiolus to function freely. This hub of muscles adds to denture stability.

3. Relationship of opposing occlusal surfaces

A. Occlusal Surface:

Although every aspect of denture construction is important, there can be no doubt that the establishment of the correct occlusal relationship of the opposing teeth is the most important. Even the skilfully made retentive and stable mandibular dentures will become unstable and cause irritation due to interfering occlusal contacts. Any tooth interference during function should be removed to achieve occlusal equilibration. The functional range of movements refers to the positions through which the lower jaw moves horizontally during normal speech, swallowing, and mastication.

Ideally, the dentures should be fabricated so that all the posterior teeth have simultaneous contact in the eccentric positions as well as in centric relation, i.e., dentures should have bilateral balanced occlusion.

In the monoplane occlusal scheme, positioning zero degrees (cuspless/flat) teeth slightly lingual to the mandibular ridge crest enhances the denture stability. The lingualized occlusion limits the range of excursive balance and direct forces to the lingual side of the mandibular ridge during working side contacts. These contacts may reduce the horizontal stresses, thereby enhancing denture stability by controlling the leverages induced by lateral/protrusive (eccentric) tooth contacts.

B. Tooth Position

Artificial teeth arrangement should be as close as possible to the position that was occupied by the natural teeth. Only slight modifications should be made to improve leverages and esthetics. Complete stability is often difficult to achieve due to the yielding nature of the supporting structures. The wider and larger the ridge and closer the teeth are to the ridge, the greater is the lever balance.

More lingual the teeth placed relative to the ridge crest, greater is the balance. The more buccal the placement of teeth, the poorer is the balance. More centered is the force of occlusion anteroposteriorly, greater is the stability of the denture base.

C. Occlusal Plane

There is a general tendency to regard the occlusal plane as being related mainly to esthetic and occlusion. However, the occlusal plane is also an important factor of stability. Ideally, the best stability occurs when the occlusal plane is parallel and anatomically oriented to the ridges. An occlusion plane that is too high forces the tongue into a new position. This can disrupt the normal position of the floor of the mouth resulting in partial loss of the border seal.

If the occlusal plane becomes tipped (anteriorly or posteriorly), there will be a shunting effect and a loss of stability. If the occlusal plane is lower in the molar area, there will be a tendency for the upper denture to be displaced posteriorly and the lower anteriorly. If the occlusal plane is lower in the incisor area, the shunting effect will be the opposite. 

4. Other factors

A. Patient muscle control and coordination

A well-coordinated patient will usually manage even if the dentures are not retentive and stable; these patients are called “oral acrobats,” but this situation should not be taken for granted. Conversely, if the patient has poor muscle control, especially if complicated by conditions of senility, Parkinsonism, etc. even an ideal fit may still leave the patient in a situation where he or she cannot manage the dentures. Thus, patient education and training are vital.

B. Tongue Factor

The position of the tongue is also important in denture stability and retention.   If the tongue retracts upon opening the mouth, it is virtually impossible to obtain a good lingual border seal of the lower denture. To overcome this, a training groove is made just below the central incisors. The patient is instructed to place the tongue on the groove during denture use except when eating and speaking. Most patients can learn to keep their tongue on this correct position in a few weeks. Afterward, the groove can be filled in with auto polymerizing acrylic resin. Adequate tongue support prevents anteroposterior movement of the mandibular denture and hence adding to the stability.[8]

An unstable denture may result in reduced maximum bite forces. Due to disuse atrophy, the jaw muscles may weaken in such cases.[9] This situation is a vicious cycle as the resultant weakening of the jaw muscles may further contribute towards a reduction in maximum bite forces during mastication. This effect is more pronounced in patients with long-term use of conventional dentures and who have unstable lower dentures.[10]  An unstable denture will affect not only the masticatory efficiency but also the psychology of the patient. In patients, particularly in those who are more active socially, unstable dentures may lead to social isolation due to embarrassment; this also affects the diet of the patient and lead to malnutrition.

In cancer patients involving the oral cavity with tongue resection, denture stability is difficult to obtain. Resection of the part of the tongue can impair the movements of tongue musculature required while recording the neutral zone. As a result, the forces of the cheeks and lip musculature are not neutralized, and stability is affected.