Dentures
Dentures are prosthetic devices constructed to replace missing teeth, and which are supported by surrounding soft and hard tissues of the oral cavity. Conventional dentures are removable, however there are many different denture designs, some which rely on bonding or clipping onto teeth or dental implants. There are two main categories of dentures, depending on whether they are used to replace missing teeth on the mandibular arch or the maxillary arch. There are many informal names for dentures such as dental plate, false teeth and falsies.
Advantages
Dentures can help patients in terms of:
1. Masticatory - improving chewing ability by replacing the edentulous area with acrylic teeth. Thus enable better mastication and pleasure to enjoy food.
2. Aesthetic - providing the visual appearance of having natural teeth, providing support for their lips and cheeks, and correcting the collapsed appearance commonly seen between the nose and the chin.
3. Phonetic - Replacing the toothless area, especially the anteriors, to help patients to speak and pronounce certain words properly without air escapes.
Types of dentures
Removable partial dentures
Removable partial dentures are for patients who are missing some of their teeth on a particular arch. Fixed partial dentures, better known as "crown and bridge", are made from crowns that are fitted on the remaining teeth to act as abutments and pontics made from materials to resemble the missing teeth. Fixed bridges are more expensive than removable appliances but are more stable.
Occlusal view of a mandibular partial denure. All seven parts of an RPD are visible on this example.
Same RPD, different view.
Complete dentures
Conversely, complete dentures or full dentures are worn by patients who are missing all their teeth in an arch (i.e the maxillary(upper) or mandibular(lower) arch).
A maxillary denture.
Occlusal view of the same maxillary denture.Problems with complete dentures
Problems with dentures include the fact that patients are not used to having something in their mouth that is not food. The brain senses this appliance as "food" and sends messages to the salivary glands to produce more saliva and to secrete it at a higher rate. New dentures will also be the inevitable cause of sore spots as they rub and press on the mucosa (denture bearing soft tissue). A few denture adjustments for the weeks following insertion of the dentures can take care of this issue. Gagging is another problem encountered by some patients. At times, this may be due to a denture that is too loose fitting, too thick or extended too far posteriorly onto the soft palate. At times, gagging may also be attributed to psychological denial of the denture. Psychological gagging is the most difficult to treat since it is out of the dentist's control. In such cases, an implant supported palateless denture may have to be constructed. Sometimes there could be a gingivitis under the full dentures, which is caused by accumulation of dental plaque.
Another problem with dentures is keeping them in place. There are three rules governing the existence of removable oral appliances: support, stability and retention.
Prosthodontic principles of dentures
Support
Support is the principle that describes how well the underlying mucosa (oral tissues, including gums and the vestibules) keeps the denture from moving vertically towards the arch in question, and thus being excessively depressed and moving deeper into the arch. For the mandibular arch, this function is provided by the gingiva (gums) and the buccal shelf (region extending laterally (beside) from the posterior (back) ridges), whereas in the maxillary arch, the palate joins in to help support the denture. The larger the denture flanges (part of the denture that extends into the vestibule), the better the support.
More recently, there has been a move to increase denture stability with implants. When pressure is applied to alveolar bone bereft of teeth (alveolar bone is the bone in which teeth normally reside), the bone reacts to this pressure by resorbing. After many years of denture wearing, the ridges upon which the dentures rest deteriorate and can easily all but disappear. The insertion of implants into the bone below the dentures can help to seriously combat this unfortunate occurrence. The implants are strategically placed to bear the brunt of the pressure when the denture is used for chewing, keeping the bone from melting away. When implants are integrated into treatment, the denture is now referred to as being an implant supported overdenture and the implants are referred to as overdenture abutments.
Stability
Stability is the principle that describes how well the denture base is prevented from moving in the horizontal plane, and thus from sliding side to side or front and back. The more the denture base (pink material) runs in smooth and continuous contact with the edentulous ridge (the hill upon which the teeth used to reside, but now consists of only residual alveolar bone with overlying mucosa), the better the stability. Of course, the higher and broader the ridge, the better the stability will be, but this is usually just a result of patient anatomy, barring surgical intervention (bone grafts, etc.).
Retention
Retention is the principle that describes how well the denture is prevented from moving vertically in the opposite direction of insertion. The better the topographical mimicry of the intaglio (interior) surface of the denture base to the surface of the underlying mucosa, the better the retention will be (in removable partial dentures, the clasps are a major provider of retention), as surface tension, suction and just plain old friction will aid in keeping the denture base from breaking intimate contact with the mucosal surface. It is important to note that the most critical element in the retentive design of a full maxillary denture is a complete and total border seal (complete peripheral seal) in order to achieve 'suction'. The border seal is composed of the edges of the anterior and lateral aspects AND the posterior palatal seal. The posterior palatal seal design is accomplished by covering the entire hard palate and extending not beyond the soft palate and ending 1-2mm from the vibrating line.
As mentioned above, implant technology can vastly improve the patient's denture-wearing experience by increasing stability and saving his or her bone from wearing away. Implant can also help with the retention factor. Instead of merely placing the implants to serve as blocking mechanism against the denture pushing on the alveolar bone, small retentive appliances can be attached to the implants that can then snap into a modified denture base to allow for tremendously increased retention. Options available include a metal Hader bar or precision balls attachments, among other things.
Complications and recommendations
The maxillary denture (the top denture) is usually relatively straightforward to manufacture so that it is stable without slippage. The lower full denture tends to be the most difficult because there is no "suction" holding it in place. For this reason, dentists in the late 1990s have come to a general conclusion that a lower full denture should or must be supported by 2-4 implants placed in the lower jaw for support. A lower denture supported by 2-4 implants is a far superior product than a lower denture without implants, held in place with weak lower mouth muscles. It is routine to be able to bite into an apple or corn-on-the-cob with a lower denture anchored by implants. Without implants, it is quite difficult or even impossible to do so.
Some patients who believe they have "bad teeth" may think it is in their best interests to have all their teeth extracted and full dentures placed. However, statistics show that the majority of patients who actually receive this treatment wind up regretting they did so. This is because full dentures have only 10% of the chewing power of natural teeth, and it is difficult to get them fitted satisfactorily, particularly in the mandibular arch. Even if a patient retains one tooth, that will contribute to the denture's stability. However, retention of just one or two teeth in the upper jaw does not contribute much to the overall stability of a denture, since a full upper denture tends to be very stable, in contrast to a full lower denture. It is thus advised that patients keep their natural teeth as long as possible, especially their lower teeth.
Conclusion
As can be expected with any removable appliance placed in the mouth, there will be some problems (in respect to the three principles mentioned above) with dentures no matter how well they are made. This is because the best the denturist can do is fabricate the upper denture to work in harmony with the lower denture when the patient is at rest. If the only variables in the equation are the patient's edentulous ridges and the two dentures, the denturist can set the teeth in certain ways to help prevent dislodgement during opening, closing and swallowing. Once food enters into the picture, though, the stability of the denture bases is not impervious to disruption. During chewing, the denture bases will sometimes act as class 1 levers, and when the patient bites down on the anterior, or front, teeth, the posterior, or rear, teeth are bound to move away from the ridge. Although the ideals of denture design will have it that the intaglio surface is in perfect, intimate contact with the ridge and the margins of the denture base will create a perfect suction seal (the seal is actually only on the maxillary denture), ideals are rarely if ever met in this imperfect world, and thus some movement is to be expected. Denture adhesive can then be utilized to compete against the forces trying to pull the denture base away from the mucosa. In a perfect world, a patient with a perfect edentulous ridge with a perfectly fitting denture would require no adhesive, as the actual form of the denture base should work in tandem with the three principles mentioned above, thus precluding movement in any way, shape or form.
From Wikipedia, the free encyclopedia. All text is available under the terms of the GNU Free Documentation License. (original article)