From Volume 39, Issue 2, March 2012 | Pages 118-126
Removable partial dentures (RPDs) remain a mainstay of prosthodontic care for partially dentate patients. Appropriately designed, they can restore masticatory efficiency, improve aesthetics and speech, and help secure overall oral health. However, challenges remain in providing such treatments, including maintaining adequate plaque control, achieving adequate retention, and facilitating patient tolerance. The aim of this paper is to review the successful provision of RPDs.
While a number of treatment options exist for rehabilitation of partially dentate patients, including fixed bridgework and implant-retained prostheses, RPDs offer a predictable and realistic treatment option for a variety of clinical situations.1 Challenges for dental practitioners include providing aesthetic and retentive RPDs that optimize appearance and speech, while avoiding plaque retention. When compared to other forms of prosthodontic care, such as implant-supported restorations or fixed bridges, RPDs are less invasive, less expensive and much more easy to modify following provision. Against this, inappropriately designed RPDs increase the risk of periodontal disease and caries within abutment teeth, while decreasing patient tolerance and acceptance of the prosthesis.2–6 The aim of this paper is to review the successful provision of RPDs.
Provision of RPDs is indicated in the following scenarios:
As with all forms of dental treatment, a careful consideration of the potential risks and benefits should be considered. Clearly, the potential benefits should exceed the potential risks posed by the planned treatment.
The benefits of RPDs include:
However, against this, some of the risks of RPDs include:
While a variety of classification systems have been proposed for RPDs, a pragmatic approach to predicting some of the challenges likely to be experienced by providers and wearers of RPDs can be considered by classifying the RPD according to its planned support. As such, three broad forms of RPD can be considered:
Ultimately, the success of RPDs can depend on how well they have been designed. RPD design is a careful, yet straightforward, step in the provision of RPDs. Where performed correctly, patient tolerance and denture success are increased, while the probability of disease, such as caries or periodontal disease in abutment teeth, is reduced.2–6 RPD design should be carefully executed, with full knowledge of both clinical and technical considerations.9 As such, it is not appropriate that RPD design be devolved to the dental technician. During the 1990s, certain clinical and legal guidelines were introduced that affected the design, prescription and fabrication of RPDs (as well as other forms of removable and fixed prosthodontics).10,11 The EU Medical Devices Directive (Directive 93/42/EEC) places certain requirements on dental practitioners to provide adequate written instructions when a prosthesis is being manufactured, and on dental laboratories to manufacture this prosthesis to the designed specification.10 The BSSPD have produced guidelines relating to the provision of RPDs, recommending ‘… the design of a partial denture is the duty and responsibility of the clinician…’.11 Despite the introduction of these guidelines, investigations demonstrate that the problems of poor and inadequate prescription still persist.12–16 While other areas of prosthodontics feature similar poor practice, RPD prescription tends to fare worst (Figures 4–6).16–19 In addition to these clinical and legal guidelines, there is an ethical responsibility on dental practitioners to design adequately, and to communicate design features for good quality prostheses that will not cause harm to oral structures.
It is important that a systematic approach is adopted to RPD design. The steps to RPD design are outlined in Table 1. RPD design should be undertaken only in full consideration of clinical, anatomical and technical considerations. As such the following information should be available:
| Concepts for Successful Denture Design |
| Outline Saddles |
| Plan Support |
| Direct Retention |
| Indirect Retention |
| Bracing |
| Reciprocation |
| Connectors |
Furthermore, the cast on which the planned prosthesis is to be made should be surveyed first along the ‘path of natural displacement’ (PND, perpendicular to the occlusal plane). A ‘path of insertion’ is then chosen to optimize aesthetics, and by being non-coincident with the PND increases retention of the RPD. The maxillary and mandibular cast should be mounted on a suitable articulator prior to designing the RPD.
The saddle replaces the missing teeth and supporting structures. Decisions need to be made on which teeth should be replaced and consideration given to the occlusal relationship (opposing teeth). The over-eruption of unopposed teeth is unpredictable but, where occurring, causes technical challenges in making an RPD.20
Flange extension can often be an important consideration. For tooth supported/bounded saddles, flange extension is not critical. However, for mucosa-borne, or tooth/mucosa-borne saddles, flange extension can be helpful in improving support, retention and bracing (described below).
Support is another important consideration in RPD design, and can be considered as resistance against movement of the saddle towards the ridge. As mentioned previously, support for a saddle can be:
Tooth support is gained from occlusal or cingulum rests. While these are an integral component of cast metal RPDs, pre-formed (stainless steel) occlusal rests can also be incorporated in mucosa-borne (acrylic) dentures to increase support.
For cast metal RPDs, careful consideration must be given to the positioning of rests. These are usually adjacent to tooth-borne saddles. Within teeth, occlusal rests on molars and premolars are usually in the mesial or distal third of the tooth, and situated in the middle third of the buccolingual dimension.21 Cingulum rests on canines and incisors are usually positioned in the middle (in inciso-gingival terms) of the lingual/palatal surfaces. Cingulum rests supporting adjacent saddles are in either the mesial or distal third of the tooth when supporting the saddle, but in the middle of the tooth in mesio-distal and inciso-gingival terms when used as indirect retainers. Rest seat preparations should remain in enamel.
For mucosa-borne dentures, support from the underlying mucosa may be achieved by covering as much of the available ridge as possible (ie fully extended flanges), and by making a suitable muco-compressive impression. This can be achieved using polyvinylsiloxane. The importance of this is to ensure that occlusal forces are distributed evenly across the underyling ridge, thereby avoiding localized areas of resorption.
Support from combinations of teeth and mucosae (ie free-end saddles) can be challenging as a support differential exists underneath the saddle (between the tooth housed in aveolar bone at one end and displaceable mucosa at the other). As such, careful use of specialized impression techniques (such as the ‘Altered cast technique’) can be considered.
Retention for RPDs can be described as resistance against movement of the saddle away from the ridge. When considering RPDs, two concepts of retention are important:
Direct retention keeps the RPD in place during function and at rest. Gravity (for maxillary RPDs) and mastication (for maxillary and mandibular RPDs) will encourage displacement. Direct retention can be achieved by a well-adapted fitting surface, appropriately contoured polished surfaces, by engaging a ‘path of insertion’ that is not coincident with the ‘path of natural displacement’ and by utilizing clasps.
Clasps are very important for retention of RPDs. Broadly divided into ‘occlusally approaching’ and ‘gingivally approaching’, these should be positioned in undercuts (relative to the path of natural displacement) to enhance retention (Figure 7). For cobalt-chromium clasps, only the terminal one-third of the clasp should be engaged in an undercut – something which will determine if a mesial-facing or distal-facing ‘three-armed’ or ‘ring clasp’, or a gingivally approaching clasp, will be used. The horizontal depth of the undercut must also be considered: engaging an inappropriate metal in too deep an undercut can lead to difficulties removing a clasp, or lead to strain and permanent deformation of a clasp.
Indirect retention is resistance against rotational forces on a saddle. A classic example relates to free-end saddles – if the terminal abutment tooth is clasped, the saddle can rotate around this tooth. Placing an ‘indirect retainer’ anterior to this clasp allows resistance to these rotational forces to develop (Figure 8a). It is important to realize that, in situations where more than one clasp exists, that a clasp axis develops, and indirect retainers should be placed on the opposite side to the saddle rotational forces, and as far away from this axis as possible, to reduce displacement of the denture. It should also be realized that a lingual plate major connector is more efficient at providing indirect retention in the mandible than a lingual bar (Figure 8b).
Bracing resists against horizontal displacing forces on saddles, usually caused by oral musculature (eg lips, cheeks, etc). This is more of an issue for mucosa-borne saddles than tooth-borne saddles, and is a special problem for free-end saddles. This problem can be overcome by correctly extending the saddle flanges.
Reciprocation resists against horizontal displacing forces on abutment teeth caused by clasps as they are removed from undercuts on teeth. These displacing forces, if not limited, can cause periodontal damage to the abutment tooth, and reduce the efficiency of the retentive clasp by ‘tilting’ the tooth and effectively reducing the depth of the undercut. The reciprocating component should be placed on the tooth surface opposite that which is clasped (eg on the lingual surface for a buccal clasp). Critically, the reciprocating component should lie above the survey line (ie not in the undercut), and should remain in contact with the tooth while the retentive component is in contact on the other side.
The major connector design is usually determined by the distribution of the saddles and the patient's preference. As a basic principle, there should be 3 mm clearance between the gingival margins of the teeth and the connector. In the maxilla, plate connectors, anterior palatal bars and ‘ring’ connectors/skeletal designs can be used, with ‘ring connectors’ being the most popular (Figure 9). While many major connector designs exist for the mandible, lingual bars and lingual plates are the most popular. Lingual bars can be considered more preferable as they do not cross the lingual gingival margin (hence reducing plaque accumulation), but they can only be used when a minimum of 7 mm exists between the lingual gingival margin and the functional depth of the sulcus (to allow 3 mm ‘gingival clearance’ and 3.5 mm minimum cross-sectional thickness for the lingual bar connector: this can be measured on a cast produced with an appropriate technique or in the mouth by using a millimetre probe). In situations where 7 mm clearance does not exist, a lingual plate should be used. This crosses the lingual gingival margin, so satisfactory hygiene is required to avoid gingival inflammation and periodontal disease. However, the lingual plate can offer improved indirect retention when compared to the lingual bar.
While it is impossible to cover all aspects of RPD design in an article such as this, a summary of the clinical stages involved in RPD provision is described in Table 2. This table also includes some ‘helpful tips’ to avoid some common pitfalls. While not all steps may be utilized in everyday practice, should you run into difficulties, Table 2 provides some information on how to manage these.
| Stage | Things to do | Tips |
|---|---|---|
| History and Exam and Primary Impressions | Record a detailed history, including details of previous denture-wearing experience, details of when teeth were lost, relevant medical history. | Try to identify any problems with previous dentures to avoid introducing same problems in new RPD. |
| Carry out a full dental and periodontal examination. Consider radiographs of abutment teeth. | Check that abutment teeth are healthy and periodontally sound to avoid damage from RPD components. Also using teeth of poor prognosis as abutment teeth is not advised as, when these teeth are lost, the RPD will require replacement sooner than expected. | |
| Make good quality primary impressions: stock tray and alginate is good if poured quickly, otherwise use stock tray and polyvinylsiloxane. | Good quality primary impressions will reduce time later. Take special care of free-end saddles − use heavy-bodied polyvinylsiloxane, or impression compound with alginate wash, in this area. | |
| If you can record an occlusal record with occlusal registration material (eg polyvinylsiloxane) or a wax wafer, do so now. Otherwise, you will have to utilize the additional visit (below). | You will need information on the occlusal relationships to design the RPD. If you can record the occlusal relationship now, it will save you a visit later (see below). | |
| Ask the dental technician to survey the casts if you do not intend to do this yourself. | Design the RPD before taking the master impression − it will be too late afterwards! | |
| Ask the dental technician to make suitable special trays. Prescribe spacing and perforations as required. | Special trays help improve the quality of the completed RPD. Use of special trays will result in less distortion of the impression material, while correctly recording the flange extension. Remember the need for spacing when planning on using polyvinylsiloxane or alginate. | |
| Occlusal registration for mounting study casts | The purpose of this visit is to record the occlusion so that you can consider the occlusal relationships when designing the denture (Note: this visit is not always necessary). | Only undertake this stage if you absolutely need to. You do not need to do this if you can fit the study casts together by hand (‘best fit’), if the arch you are planning the RPD for is opposed by a complete denture, or if you were able to record an adequate interocclusal record at the first visit above. |
| Tooth preparations and master impressions | Prepare rests seats and guide planes if needed. Make the master impression in a suitable impression material (eg polyvinylsiloxane or alginate). | Always prepare the teeth for rests before making the master impression. Once you make the master impression, you are ‘committed’ to the shapes of the teeth and it is difficult to correct this later. |
| Framework try-in and occlusal registration | Try-in the framework (for metal-based RPDs) or the wax base (for acrylic RPDs) with wax occlusal rims attached. This stage may not be necessary if replacing a small number of teeth where a definite/stable occlusal relationship exists. In such scenarios, one could move straight to the trial insertion stage following selection of appropriate teeth at the impression stage. | If the framework does not fit in the mouth, check the seating, particularly along guide plane, rest seats, etc. Obvious areas can be easily adjusted. |
| Shape the wax blocks, where appropriate, for the buccal and lingual contours. | For the maxillary anterior region, the labial surface and incisal edge of the wax block will guide the technician as to the position of the labial surface and incisal edge of the prosthetic teeth. Care should be taken to ensure that the lip support is correct. |
|
| Record the jaw relationship. |
Do not record the jaw relationship until the fit of the framework has been addressed, as an unstable base will lead to an inaccurate occlusal record. |
|
| Select an appropriate tooth shade and arrangement. | Involving the patient in the tooth selection is a good approach. If the patient was happy with the previous arrangement of anterior teeth and these are not excessively worn, an impression could be taken of this denture (in the mouth) and used to guide the dental technician for the new RPD. | |
| Tooth try-in | Try-in the denture framework and teeth and assess that the: |
If the occlusal relationship is incorrect, then it may be possible to remove some of the offending teeth and re-set them in the wax. Remove the prosthetic teeth one-by-one until the occlusal relationship appears correct. Then either soften the wax in the area from where the teeth were removed or reposition the teeth and ask the patient to occlude (to locate the teeth into the correct position). However, if this cannot be easily done, do not re-set the teeth and instead re-record the occlusal relationship using wax or polyvinylsiloxane. A re-try-in should be considered in this situation. |
| Especially for acrylic RPDs, ask the dental technician to block out conflicting undercuts before processing the denture. | Blocking out conflicting undercuts aids fitting of the denture. If the dental technician does not do this, then a lot of adjusting/grinding will have to be done at the insertion appointment to get the denture to fit. | |
| Denture insertion | Fit the denture and assess that the: |
Check that the processed acrylic is smooth and that no sharp nodules or edges are present. If these are there, smooth off at the chairside. |
| Denture review | Record any history of: |
Complaints of pain may be related to rough acrylic on the fitting surface, over-extension or occlusal error. These should be investigated and adjusted as necessary. |
Further forms of RPDs can be considered for ‘special’ scenarios, these include the following.
These are used where the denture covers remaining/retained roots. Retention of such roots increases proprioceptive feedback and avoids ridge resorption that would have occurred had these teeth been extracted. ‘Simple’ overdentures overlie root faces, while more complex overdentures may be retained using precision attachments or magnets (Figure 10a, b).22
These are provided in certain situations, typically where ‘conventional’ direct retention is poor while sufficient undercut is present on the labial surfaces of remaining anterior teeth.23 This denture design includes an additional labial component that is rotated into position and ‘locked’ in place (Figure 11a, b).
These are provided for patients with remaining teeth of poor prognosis that will probably be lost in the coming months or years. A transitional denture is of particular benefit to patients who have no previous ‘denture-wearing experience’. As such, providing such a patient with a mandibular complete denture could cause significant adaptation challenges. Instead, retention of key strategic teeth, albeit with reduced prognosis, such as mandibular canines and/or premolars and provision of an RPD will allow the patient time to adapt to the denture. At a later stage, these compromised teeth can be removed and additions made to the RPD.
This is a special form of acrylic/mucosa-borne RPD that is suited for patients with a number of bounded saddles. It focuses on maintaining adequate hygiene and optimizing retention.
In an arch where both crowns and an RPD are being provided for a patient, the denture should be designed before the crowns are fabricated. This will allow the dental technician to incorporate appropriate shapes into the planned crown for rest seats, guide planes and undercuts to increase retention of the RPD. It is disheartening, when following cementation of a new crown, to have to then cut a rest seat preparation into its occlusal surface when fabricating an RPD (Figure 12a, b).
As with all forms of prostheses, appropriate care and maintenance is advised to avoid, in particular, biological failures. All RPD patients should attend for 6–12 month check-ups. At these appointments, RPDs should be removed and careful inspection made of all teeth and gingivae to detect the presence of early caries or periodontal disease. Patients should be encouraged to remove RPDs at night and to brush with a fluoridated toothpaste, rinse and floss (where appropriate). Patients should also be encouraged to remove RPDs for cleaning following meals. Effective cleaning of RPDs can be accomplished using a standard toothbrush and warm water, or a suitable ‘non abrasive’ (eg ‘denture toothpaste’). Other ‘over the counter’ commercial products can also be used for denture cleansing, however, care is indicated to avoid products that may cause corrosion of metal components.
RPDs are a useful and successful form of prosthodontic treatment. However, as occurs with many other forms of treatment, their success is very dependent on the quality of their design and fabrication. With careful consideration, RPDs can be successfully employed to meet the requirements of contemporary patients.
