Complete dentures


A complete denture is a removable appliance used when all teeth within a jaw have been lost and need to be prosthetically replaced. In contrast to a partial denture, a complete denture is constructed when there are no more teeth left in an arch, hence it is an exclusively tissue-supported prosthesis. A complete denture can be opposed by natural dentition, a partial or complete denture, fixed appliances or, sometimes, soft tissues.

Epidemiology and causes of tooth loss

There has been a decline in both the prevalence and incidence of tooth loss within the last decades.; people retain their natural dentition for longer. Nonetheless there is still a great demand for complete dentures as more than 10% of adults aged 50–64 are completely edentulous, with age, smoking status and socioeconomic status being significant risk factors. Tooth loss can occur due to many reasons, such as:
Following the loss of teeth, there occurs a resorption of alveolar bone, which continues throughout life. Although the rate of resorption varies, certain factors such as the magnitude of loading applied on the ridge, the technique of extraction and healing potential of the patient seem to affect this. The edentulous ridge can be classified according to the amount of bone in both the vertical and horizontal axes:
Alveolar bone resorption is an important consideration when designing complete dentures. In the absence of natural dentition, such dentures are relying completely on soft tissues for their support. As a consequence, the forces exerted on the mucosa are significant and may, in turn, lead to an increased rate of bone resorption. Therefore, in order to ensure an equal distribution of forces across the mucosa, complete dentures should have maximum extensions.
Facial muscles on the cheeks and lips also lose their support as teeth are lost, contributing to an 'aged' appearance of the individual. Although complete dentures cannot prevent the loss in muscular tone, they can nevertheless provide some artificial support to mask this loss in tone. Furthermore, perhaps the most noticeable effect of tooth loss from a patient perspective is the loss in masticatory efficiency. Teeth function to help with the chewing of food, breaking it down in small pieces that can be swallowed. Denture wearing can bring some masticatory function back to normal. It cannot, however, fully compensate for the efficiency of the natural dentition because dentures are not fixed in place like teeth are and so have to be actively controlled by the muscles and biting forces are greatly reduced as the dentures are impinging on soft tissues.

Principles of complete dentures

Complete dentures are prone to a variety of displacing forces of differing magnitude as they are resting on oral mucosa and are in close proximity with tissues that are constantly changing due to the action of muscles. Consequently, for complete dentures to be retentive and stable, the retentive forces that hold the dentures in place must be greater than the ones aiming to displace it. Obtaining maximum stability and retention is one of the biggest challenges in full denture construction.

Retention

Retention in removable prosthodontics can be defined as the resistance to vertical dislodgment that can arise from either muscular forces or physical forces. It can be gained from three different surfaces of the denture:
  1. Occlusal surface
  2. Polished surface
  3. Impression surface

    Muscular control of the dentures

The peri-oral muscles can cause displacement of the dentures. Patients can, however, learn to control and coordinate their muscles so that the forces exerted are minimised or counter-acted to prevent such displacement. With age, the ability to learn new skills and acquire some level of neuromuscular control declines. Therefore, the "training" time-frame for patients to learn how to successfully use their new complete dentures is expected to be much longer for older patients.

Transition into complete dentures

Many patients find the idea of wearing complete dentures very upsetting. Such psychological effects, together with the challenges that accompany successful prosthetic wear, can make acceptance of treatment difficult. It is, therefore, reasonable to consider different ways of transitioning into the edentate state in patients who have not yet lost all of their teeth but in which complete dentures will be required in the foreseeable future. Certain teeth can be retained in the short to medium-term with partial dentures provided in the interim so that the patient can become accustomed to denture wearing. Alternatively, if the former is not possible, consideration should be given to whether roots of teeth can be retained in strategic locations in the maxilla or mandible to help with the stability of the prostheses.

Transitional partial dentures

Teeth that can be restored despite a poor long-term prognosis may be retained to transition the patient into the edentulous state via a series of transitional partial dentures. It is important that the patient can maintain good plaque control during this period, as progression of periodontal disease will lead to further destruction of bone that will later become the foundation for denture support. Complete dentures require some level of muscular control from the patient and this process of adaptation can last for several weeks or even months. As patients age, the process of learning and memorising new skills as well as neuromuscular control becomes more challenging. Hence transitional partial dentures can provide a practice period for the musculature, before complete dentures are provided.

Overdentures

An overdenture is a prosthesis that fits over retained roots or implants in the jaws. Compared to conventional complete dentures, it provides a greater level of stability and support for the prosthesis. The mandibular jaw has a significantly less surface area compared to the maxillary jaw, hence retention of a lower prosthesis is much more reduced. Consequently, mandibular overdentures are much more commonly prescribed than maxillary ones, where the palate often provides enough support for the plate.

Tooth supported

Retaining two or three natural teeth as retained roots can greatly improve the retention and stability of a complete denture, especially if the roots are fitted with special precision attachments. The process involves decoronation and elective root canal treatment of the overdenture abutments. For matters of simplicity for endodontic treatment provision, single rooted anterior teeth are preferred, with the exception of lower incisors as they lack sufficient root surface area. If plaque control is satisfactory, tooth-supported overdentures can be considered as a long-term treatment option. Alternatively, if treatment fails, the roots can be extracted and the overdenture can easily be converted into a conventional complete denture.
Advantages
Although an implant supported overdenture is not appropriate for the short-term transitioning stage into conventional complete dentures, it is an option that should be considered for the definitive treatment, given the higher stability and retention of such dentures. Despite complications, the success rate of dental implants is well established, with reports exceeding 98% in 20 years for mandibular anterior teeth. The provision of a two-implant supported overdenture in the mandibular edentulous jaw is now considered as the first choice of treatment, with patients reporting to have a significant improvement in quality of life and greater patient satisfaction when compared to conventional removable prostheses.

Immediate dentures

When clearance of the dentition is the only viable treatment option, immediate dentures can be constructed prior to the extractions and fitted once the teeth have been removed, on the same appointment. Such dentures help restore masticatory function and aesthetics whilst at the same time allowing a period for the soft tissues to heal and the bone levels to stabilise before constructing the definitive complete dentures.

Advantages

In many circumstances patients will already have a set of dentures that will require replacing for various reasons. Whether or not they are deemed satisfactory by the wearer or clinician, existing dentures can provide invaluable information for the construction of a new set

Anatomy of the Denture Bearing Areas

Extensions

There are several anatomical structures that have the potential to cause displacement of the complete dentures. These are:

Patient Assessment

Impressions

Similarly to all removable prosthesis, the first step in denture construction is to obtain accurate impressions of the soft tissues. As the height of the ridge will vary throughout the arch, two sets of impressions are taken. The primary impressions, taken using a stock tray and a suitable impression material, are used to construct special trays. Special trays are made in either acrylic or shellac and have a shape that corresponds to the shape of the mucosa of the individual patient. This way, it is ensured that during secondary impressions there will be n uniform thickness of impression material throughout the tray.

Primary (preliminary) impressions

Although stock trays come in different sizes, it is very likely that some parts will be over- or under-extended and therefore have to be modified prior to impression taking to ensure that the entirety of the mucosa is recorded accurately. Greenstick or silicone putty can be used to extend the trays if they are under-extended; this is of vital importance, as any unsupported impression material may distort until the impressions are cast. A suitable material such as alginate can be used for this purpose.

Secondary (master or working) impressions

As described above, special trays ensure that the secondary impressions accurately record the tissues whilst ensuring a uniform thickness of impression material throughout the tray. Different impression materials will have different thickness requirements. Alginate, for example, requires a thickness of at least 3mm to prevent distortion whereas the more elastic silicone materials can be used in thickness of 1–2 mm. Therefore, when special trays are constructed, it is the responsibility of the prescribing clinician to ask for the appropriate level of spacing between the tray and the tissues.
Another feature which should be incorporated into special trays is tissue stops, which can be described as 2-3mm wide extensions on the impression surface of the special tray. Without the incorporation of tissue stops, when the special tray is tried in the mouth to check for the accuracy of extensions, it will appear over-extended as the laboratory has extended the tray in a way that will allow the specified thickness of impression material to be accommodated. Tissue stops allow the clinician to appropriately assess the extensions of the tray.
The impression materials that can be used with special trays are:
  1. Zinc oxide eugenol impression paste
  2. Impression plaster
  3. Addition silicones
  4. Condensation silicones
  5. Polysulphide
  6. Polyether

    Border moulding

Border moulding refers to the functional or manual manipulation of the cheeks and lips in order to mould the borders of the impression to that of the functional depth of the sulcus and floor of mouth. This is rnecessary for ensure stability and adequate retention of the complete dentures. The following steps can be carried out during impression taking:
There are two ways in which the soft tissues can be recorded during impression taking:
  1. Mucostatic impression records the soft tissues in their resting state, thus no or minimal pressure is applied during impression taking. This technique has the advantage of ensuring a close adaptation of the denture base to the entirety of the mucosa and hence enhancing retention. Due to the fact that the mucosa is uneven in compressibility, however, there will inevitably be an uneven distribution of loads during masticatory function. An impression material of low viscosity is selected for this technique.
  2. A mucocompressive impression is obtained by applying some pressure to the soft tissues during impression taking, thus recording the shape of the soft tissues under masticatory loading. Consequently, the mucosa will have an even distribution of loads during function, but the retention of the denture is adversely affected as it inhibits a close adaptation of the denture base to the mucosa in the resting position, which occurs during the majority of time. Such a technique, however, can be considered in patients with a history of mucosal trauma and discomfort. Suitable materials for this purpose include high viscosity silicone impression materials.

    Bite registration

Once the impressions have been cast, a set of models has been produced that provide the clinician and dental technician with a replica of the upper and lower jaws with which to work in order to produce the final complete denture. An integral part to the construction is to record how the patient is or should be biting, as well as recording all the necessary information for the next stage, the wax try-in.

Occlusal vertical dimension, resting vertical dimension and freeway space

When setting up the teeth during construction of complete dentures, the clinician must decide a vertical height on which the patient will be biting upon; this is termed the occlusal vertical dimension. This task is particularly challenging in complete dentures, as there is no existing occlusion to which the clinician can reference to, and as a result, it is the cause of many errors in complete denture construction. The resting vertical dimension may be defined as the vertical dimension between two points, one on the maxilla and one on the mandible, when the patient's muscles are at a relaxed position. The difference between OVD and RVD is termed the Freeway space. This distance should be between 2–4 mm.
RVD - OVD = FWS = 2-4 mm
In an edentate patient, the OVD cannot be measured unless it was recorded prior to clearance of the dentition or pre-existing dentures provide a satisfactory value. In the majority of cases, however, the OVD needs to be calculated by determining the RVD and allowing for adequate FWS. The patient is asked to relax the muscles of the mandible, and the measurement for RVD is taken with a WIllis gauge from a point on the chin and a point underneath the nose.

The record (bite) blocks

Record blocks are made in such a way so that the dental technician is provided with all the information necessary to provide a wax replica of the dentures. They consist of blocks of wax resting on a rigid base that can be made out of shellac, light-cured or heat-cured acrylic. The base can sometimes be made out of wax, however, such a material lacks the rigidity required to ensure accurate measures are taken. Additionally, it may distort during transport and thus damage the validity of the recordings. Acrylic resins demonstrate the best accuracy of fit and are therefore the most retentive, with heat-cured acrylic being superior to light-cured.
The record blocks are inserted in the mouth and the following should be examined and deemed satisfactory prior to proceeding with any adjustments:
Centric occlusion refers to teeth contact when the jaws are in centric relation. It is sometimes referred to as the retruded jaw relationship.

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