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Factors playing role in denture retention

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  • 10/07/2011 at 4:29 am #12286
    Anonymous
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    Surface tension
    One of the consequences of the surface tension of liquids is the tendency to minimize the area of the free surface, generating the familiar curved surfaces of raindrops and menisci. The mere existence of a curved surface generates a pressure difference across that surface. If the surface is convex (which is described as a positive total curvature) the pressure is higher within the drop than without – therefore, positive pressure. If the total curvature is negative, such as for the ‘waisted’ shape of a drop held between one’s fingertips, the pressure is negative. This is the crucial point: that negative pressure exerts a force tending to draw the fingertips together. This is the force that retains two wet microscope slides together against a straight pull (not a sliding action). At the edge is a very thin film of water, with a large negative curvature because the separation of the slides is small, thus the force is great. Notice that if the bead of liquid at the edge were bulging out, the force would be positive – tending to separate the slides – and some movement would be easy. However, the moment the liquid surface is withdrawn inside the boundary of the slides it becomes negatively curved. This is because the edge of the liquid (the ‘contact line’) is pinned in position on the edge of the slide when the glass is wetted by it. This force can be seen to be the familiar one of ‘capillarity’. The tendency to advance a wetting liquid into narrow spaces – maximizing the wetted area – is caused by the force acting at the contact line, drawing it over the surface. There is necessarily a lowered pressure behind the meniscus, which is negatively curved.

    Thus, on the assumption that the denture base is wetted by saliva an attempt to withdraw the denture generates along its periphery a narrow, highly negatively-curved saliva surface. There is therefore a lowered pressure in the liquid-filled space and a retentive force is experienced. Atmospheric pressure is not involved: only the generated surface tension-mediated pressure difference is effective. However, the existence of this effect is contingent on the wetting of the denture base by saliva, and to this extent only the issue of wettability can be reinstated as a factor of importance.

    Even so, the question remains as to whether such a situation can arise in the mouth. If we consider the peripheral conditions we can see that the only extended location where this can be directly relevant is the posterior border of the palate of an upper complete denture. The remainder of the margin tends to be enveloped in soft tissue such that withdrawal of the denture results in a sliding action rather than straight separation. Thus, for separation to occur, ie, a space develops between the tissue and acrylic, flow of saliva must occur, either from somewhere else to fill that space, or at least as the meniscus is drawn back over the opposing surfaces.

    Viscosity
    A major consideration is the rheology of the saliva and where its viscosity is located. Simply put, this is the rate of separation of the two surfaces under a given applied force and it depends inversely on the viscosity. However, the viscosity of the wearer’s saliva is not readily controllable, although there is some variation from time to time for a variety of reasons. Thus the use of more viscous media as denture retention aids would seem logical, but flushing and solubility would mean a limited time of efficacy. As the viscosity of saliva is many times that of air separation is therefore much more difficult when this fluid fills the space.

    Time
    It is worth noting that flow is a time-dependent phenomenon.That is, the amount of separation of denture and mucosa that can occur depends on the duration of the application of any force. If a reseating force is applied before detachment has occurred, such as in chewing, the displacement will only be transitory and may never reach the point of collapse. Equally, a long period unsupported may in theory see an upper denture fall away simply because enough time has been allowed for sufficient flow to occur. (A patient may be expected to reseat the denture long before this happens.) Thus care is needed in judging retention because it is a dynamic affair: so-called static test results may not offer very helpful comparisons because there is always some time-scale for the test, and the results can only be interpreted on that time-scale.

    Base adaptation
    In plain terms, how well the denture fits is singularly important. This is so because the measure of the fit is the size of the gap between the fitting surface and the mucosa, since it is this that controls the flow occurring there. For a fully immersed system (that is, no air being admitted), the force required for separation at a given rate depends inversely on the cube of separation. Once air is admitted at the edges, the force depends inversely on the fifth power of separation, ie collapses more readily but still implying the benefits of close adaption (this is because, as indicated above, the flow of the air is so much easier that it provides no appreciable resistance to separation in comparison with the effect of the saliva). These relationships also show that the fit must be uniformly good over the entire tissue surface: the viscous retardation contribution from a region of even slightly greater separation will be substantially less than that from a closer fitting area, perhaps even negligible. A secondary feature to note is that the narrowness of the gap contributes a retentive force through the effects of surface tension, via the curvature that results in the liquid surface. The deduction from this is that the retention of dentures against a tipping action will be less effective than against a straight pull.

    Border seal
    Attention was drawn under ‘Surface Tension’ to the fact that along most of the border of a denture there is double contact of acrylic and soft tissue such that displacing the denture in the separation sense does not open a gap along that border. There are two effects arising from this. Firstly, the cross-section through which saliva must flow in order to fill the space is small, and the viscous retardation of displacement correspondingly large. Secondly, the compliance of the buccal tissues in particular means that the lowered pressure beneath the denture caused by that displacement would tend to hold them in place in close approximation to the acrylic, maintaining the seal. It is therefore apparent that the design of the denture should take this into account in terms of extension into the buccal sulcus and in ensuring a smooth enough, grooveless surface so that no leaks occurred.

    Seating force
    It has been suggested that when a denture is put in place a firm seating force be applied as this aids retention.Certainly, the immediate effect will be to ensure the thinnest possible saliva film and so the best effect is caused by the viscous retardation of displacement. However, this must also be achieved at the expense of some displacement of the supporting soft tissue, and if this created a better fit, it would not last long as that tissue rebounded elastically. The continued secretion from mucosal glands would also offset any immediate benefit. It may, however, be useful that the deliberate seating force would tend to expel air which, as noted above, would not contribute to retention. But one imagines that the first displacement (which must be considered inevitable at some point) would reintroduce such bubbles, thereby reducing retentiveness.

    Soft tissue
    Denture retention is therefore a dynamic issue as it mostly depends on factors controlling the flow of the interposed fluid. The better the fit to the tissue, and the better the linear extent of the seal at the border, the better the denture will resist short term displacing forces. Brill’s analogy of a piston in a cylinder of water49 offers a partial description of the fluid dynamics of the border seal but without alluding to the compliant behaviour of the soft tissues (ie when the pressure in the denture-mucosa space drops), which is relevant at least when the denture is first fitted. In the medium term soft tissue remodelling can be expected to maintain mucosal contact on both the tissue surface and at the borders. But, in the longer term, resorption and remodelling of the hard tissue may exceed the adaptive capacity of overlying soft tissues and retention may eventually be lost. However, the patient will have learned progressively to use the dentures as the fit changes and have developed the manipulative skill and control required to compensate for that deterioration. It is therefore in this later-stage context that the so-called ‘muscular control’ becomes particularly important, and also therefore the design of the polished surface to facilitate this.

            

    10/07/2011 at 9:01 am #17472
    sushantpatel_docsushantpatel_doc
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    Denture Valves may provide an affordable option of choice to those patients that are not suitable candidates for implants, or cannot tolerate the limitations of denture adhesives.

    The objective of this article is to demonstrate the rationale for denture valves in complete denture retention:

    1) Short term lower pressure, generated by external means beneath the dentures, tends to hold them in close proximity to the tissues, thus maintaining a seal at the borders.

    2) The role that denture valves may play in the context of patient adjustment and acceptance of the dentures.

    Denture retention is by definition, resistance of a denture to vertical movement in the opposite direction, away from the tissues.

    Psychological acceptance, adhesion, cohesion, viscosity, gravity, oral and facial musculature, vacuum and atmospheric pressure have all been mentioned at one time or another, as major or minor contributory factors. There has been published detailed analysis of the underlying principles of denture design and the contributory factors in complete denture retention.

    The understanding exists and there is general acceptance among clinicians that denture retention is dependent on the control of the flow of interposed fluid, its viscosity and film thickness. Interfacial surface tension contributes to retention, but the most important are good base adaptation and border seal. However at the first displacement, which is inevitable at some point, a gap opens along the border seal, consequently reducing the resistance to vertical movement and subsequently lifting up the denture.

    In practice, complete denture retention remains a perplexing subject. The failure rate remains high. Its logical solution exists but is often obscured by erroneous beliefs.

    Complete denture retention is in fact a dynamic issue. First there is a need to achieve an accurate fit of the denture to the tissues, so that the space between the two is as small as possible. Secondly, there needs to be a border seal. Thirdly, there is a need for a pulling force in the direction of the path of insertion to resist the dislodging forces. Other factors may only contribute to retention if the fundamental principles were achieved in full.

    Psychological Acceptance

    It is important that patients perceive their dentures as stable during function and their aesthetic appearance meet the psychodynamics required by the patient.

    Wearing dentures for the first time can be as hard as learning how to swim.
    Developing wrong habits in the early stages of denture wearing are the major reasons for malnutrition, resulting from not being able to chew and swallow properly. Furthermore, there is nothing to stimulate the desire to eat and socialise when dentures are unstable.

    Adhesion

    Adhesion has been claimed to be instrumental to denture retention. There have been numerous theories to prove that adhesion of saliva to the mucous membrane and the denture base is achieved through ionic forces between charged salivary glycoprotein and surface epithelium or acrylic resin. There has been no known ability to identify a specific mechanism for a direct acrylic-mucosa reaction that would achieve this. The concept of physical attraction of unlike molecules for each other is unimaginable in the denture field.

    Cohesion

    Physical attraction of like molecules for each other creates retentive force and usually occurs with saliva that is present between the denture base and the mucosa. Normal saliva is not very cohesive, and unless the interposed saliva is modified with the use of denture adhesive, retentive force cannot be achieved.

    Viscosity

    This is the rate of separation of two surfaces under an applied force, best described in the context of surface tension and interfacial viscous tension. The force holding two wet glass planes together against a straight pull, or the force holding two parallel plates together are due to the viscosity of interposed liquid.

    Stefan’s law describes that the viscous force increases proportionally to increases in the viscosity of the interposed fluid. When the equation is applied to denture retention, it demonstrates the need for a good base adaptation to the tissues and the importance of taking full advantage of the surface area covered by the denture. This may be relevant to the maxillary denture. However, if the two plates with interposed fluid are immersed in the same fluid, there will be no resistance to pulling them apart. Since the borders of the mandibular denture are bathed in saliva, surface tension, viscosity and film thickness may not play a role in lower denture retention.

    Gravity

    The weight of a lower prosthesis constitutes a negligible gravitational force and is insignificant in comparison with the other forces acting on a denture. Anecdotal or trivial as it may seem, evidence suggests that this may be beneficial in cases where other retentive forces and factors are marginal.

    Oral and facial musculature.

    Muscular control is an important aspect of successful complete denture therapy. Although this may supply additional retentive forces, provided that the polished surfaces are properly shaped, the teeth are positioned in the neutral zone and the denture bases are properly extended to cover the maximum area possible, retention is a quality of the denture rather than that of the patient. Therefore, musculature is relevant only in the context of ‘manipulative skill’ of the patient, rather than in retention in the strictest sense.

    Atmospheric pressure – Vacuum

    Atmospheric pressure has been claimed to be an important factor in complete denture retention. For atmospheric pressure to be effective, it must operate under condition of a pressure difference – (de) pressure. There must be a lower pressure beneath the dentures and only if vacuum were there the full effect could be felt.

    Atmospheric pressure can act to resist dislodging forces, if the dentures have an effective seal around their borders. This is called ‘suction’ because it is the resistance to removal in a direction opposite to that of insertion. But there is no suction or negative pressure, except when another force is applied.

    Under the assumption that vacuum could be generated by exerting a pull that tended to increase the volume beneath the base of the denture and the tissue, the lower pressure would have to be generated by external means and a perfect seal created and maintained around their entire borders for the lower pressure to be sustained.

    This can be achieved by taking full advantage of the mechanism principle of denture valves.

    The benefits to patients in terms of function, successful outcome of denture retention and quality of life that denture valves may offer, outweigh by far the possible side effects- tissue reaction, namely soft tissue proliferation which under proper management can be kept to a minimum.

    The philosophy of ‘best practice’ should include denture valves as an option in complete denture therapy, especially for those patients that are willing to make informed and consensual decisions- but that is another issue altogether.

            

    10/07/2011 at 9:05 am #17473
    sushantpatel_docsushantpatel_doc
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    Upper Dentures are held in place by interfacial surface tension, generally referred to as "suction" or "vacuum" and other important factors such as good base adaptation and border seal.

    The same applies for Lower Dentures, but the small support surface area and the pronounced musculature in the lower jaw, reduce the secure grip of dentures, causing displacement of the denture.

    Wearing dentures for a number of years causes the underlying surface to resorb, resulting in "shrinkage" of the gum. In worst cases, the "shrinkage" is so severe as to make virtually impossible the construction of stable, satisfactory dentures.

            

    11/07/2011 at 2:56 pm #17476
    Anonymous

    Upper Dentures are held in place by interfacial surface tension, generally referred to as "suction" or "vacuum" and other important factors such as good base adaptation and border seal.

    The same applies for Lower Dentures, but the small support surface area and the pronounced musculature in the lower jaw, reduce the secure grip of dentures, causing displacement of the denture.

    Wearing dentures for a number of years causes the underlying surface to resorb, resulting in "shrinkage" of the gum. In worst cases, the "shrinkage" is so severe as to make virtually impossible the construction of stable, satisfactory dentures.

    : One hundred nineteen patients with a total of 258 implants participated in the study. They had been monitored regularly during an observation period of 5 to 15 years (mean 9.3 yr). Seventy-five patients had a resilient retention device (ball anchors or a round clip bar); 44 patients had a rigid bar with or without distal extensions. The incidence and rate of complications were calculated for the overall- and for the 2- and 5-year observation periods. Comparisons were made between the three categories of maintenance and the two types of retention. A Kaplan-Meier analysis was applied for calculations of changes of the retention mechanism.

    Results: The mean number of complications per overdenture during the entire observation period was 3.5; this did not differ statistically between the two retention groups. Some significant differences were found only for the 2- and 5- year period. Broken, loose, or lost female parts were more frequently observed with resilient devices, as were repairs and relining of the resin denture base, whereas tightening of bar retainers was more typical with rigid bars. A change from a resilient retention device to a rigid bar was performed more often than vice versa but not at a statistically significant level.

    Conclusion: Although these long-term results do not indicate a significant difference between the retention groups, a slight superiority of the rigid bar is suggested

            

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