Planning Removable Partial Dentures
Summary: Clasp retained removable partial dentures should not be designed solely for efficiency in mastication. The preservation of these supporting teeth should be our ultimate aim in design. Only the dentist, who manipulates the mounted diagnostic casts, studies the roentgenograms, and examines and questions the patient as to his oral and general health, has adequate knowledge to dictate denture design. He alone can predict the long range response of the patient's tissues to oral rehabilitation.
Conclusion:
1. The biologic response of tissue to stress depends upon the tolerance of the tissue.
2. Partial dentures should be designed for bilateral distribution of stresses.
3. The type of retainer which will function best in all respects is the one which should be employed in any specific location.
4. The location of the teeth remaining and available for support and the way they are used are much more important than the number of teeth remaining.
5. Splinting of teeth by clasping in sequence is effective for adding stability to a restoration and avoiding the overworking of abutment teeth.
6. Modern periodontal procedures which are saving teeth are making partial denture service increasingly necessary.
7. Removable partial dentures can effectively stabilize loose teeth (especially by the use of continuous clasps) so that they will grow firm again.
8. Elastic clasps are more efficient for retention and less traumatizing to abutment teeth than clasps which lack resiliency.
9. Removable partial dentures move slightly under functional loads.
10. Teeth splinted together by fixed restorations provide strong abutments.
The rationale of partial denture choice
A. Factors
1. Bone maintenance potential- a physical examination is necessary to assess systemic conditions that might affect the patient’s capability to replace bone.
2. Previous cervical bone loss- the prosthetic load is affected, since occlusal stresses are magnified because the extra-alveolar segment of the abutment tooth has been lengthened.
3. Abutment root form, length, and stability- the length of the root determines the intra-alveolar support which can counteract the extra-alveolar stress or the work portion of the lever arm. The amount of remaining alveolar wall which supports the abutment load is reduced rapidly when the root is steeply tapered. The splinting of two or more adjacent teeth is the most dependable solution to this problem.
4. Extent of periodontal involvement- the degree of lateral mobility of an abutment is of lesser significance . Firmness recovery depends upon removal of predisposing factors which produce excess mobility.
5. Caries susceptibility- is a factor that determines the choice in between making a complete or partial denture.
6. Size and form of the residual ridge-if ridge form is poor, adequate stress control will be a problem. Example, if it is low and flat, it will reduce the support surface.
7. Location of the remaining teeth- location of the remaining teeth is more important in construction than the number of teeth that are left.
B. Clinical classification of partially edentulous situations
1. Class I- situation where all the remaining teeth are anterior to the bilateral edentulous ridges.
2. Class II- situation in which remaining teeth of either the right or left side are anterior to the unilateral edentulous ridge.
3. Class III- situations are those in which an edentulous space is bounded by teeth both anteriorly and posteriorly. In this situation, one or more boundary teeth are unable to assume the total abutment support of the prosthesis.
4. Class IV- situation in which the edentulous space lies anterior to the remaining teeth which bound it both to the right and left of the median line. This class is frequently exemplified by two or more incisors missing. The loss of one or both cuspids in addition to the incisors is encountered less often.
5. Class V- situation in which a space is bounded by remaining teeth at its posterior and anterior terminals. Example, a space extending from a weak lateral incisor to the mandibular second molar.
6. Class VI- another situation that has boundaries of teeth remaining anteriorly, thus three out of six groups have the same anatomic landmarks. However class VI conditions are such that the complete occlusal load can be tooth borne and, therefore, a fixed or removable unilateral prosthesis is possible.
Conclusion: The Kennedy classification has many excellent features , but it needs to be modified.
Differential diagnosis: Fixed or removable partial dentures
Three main criteria to help in the choice between a fixed or removable partial denture.
- By which method of treatment will the patient be best served?
- Is the prosthetic service recommended the best that dentistry has to offer?If compromises are necessary, are the alternative methods of treatment acceptable by today’s standards?
- Is the dentist willing to assume the professional liability for the accomplishment of the treatment being recommended
Indications for fixed restorations
- Tooth-bounded Edentulous Regions - NOTE : Not to be used when the edentulous span is too long and the abutment teeth cannot withstand the occlusal forces, and also when the abutment teeth are periodontally involved and need cross arch stabilization
- Modification Spaces - Used especially when a lone standing abutment tooth is present (pier abutment situation)
- Anterior Modification Spaces - Exceptions are in areas of excessive ridge resorption
- Nonreplacement of Missing Molars - may be possible to do a cantilever bridge from two bicuspid abutments
Indications for removable partial dentures
- Free-end situations (Kennedy Class I and II)
- After Recent Extractions - interim until site heals
- Long span - too long to support the edentulous space with fixed restorations
- Need for Effect of Bilateral Bracing - the RPD may act as a periodontal splint by providing cross arch bracing
- Esthetics in Anterior Region
- Excessive Loss of Residual Bone - occurs from the labial in the anterior region
- Unusually Sound Abutment Teeth - in selected instances
- Economic Considerations
A clinical view of mandibular premolars in removable partial denture design
Discuss several factors that must be considered when using premolars in partial denture design.
Discussion:
A. Factors
1. Tooth anatomy-coronal form of the tooth-height of contour needs to be surveyed since it can provide surfaces for reciprocation and retention.
2. Tooth anatomy-root- crown-root ratio must exceed 1:1 with minimal tooth mobility, for the tooth to be suitable for an abutment for the RPD. A marginal ratio is not necessarily a reason to abandon a potential abutment as long as the tooth has minimal mobility.
3. Relationship of the tooth to those in the same arch- tipping of teeth due to extraction, leaves the height of contour on the facial surface of the premolars at the level of the free gingival margin (ideal for reciprocation), but places the height of contour near the lingual cusp tip, making retention difficult. Forces that are not directed along the axis of the tooth, can be destructive periodontally.
4. Relationship of the tooth to those in the opposite arch- cusps in hyperocclusion should be eliminated, they might occlude on rest seats and the partial framework might be thin and fracture.
5. Complications provided by an existing restoration- placement of a rest seat, partially in enamel and partially in a restoration, should be avoided. Weakened cusps must be protected, and the restoration of choice is the gold onlay. The placement of a rest seat in porcelain is not advised. If the restoration needs to be replaced great care must be taken to avoid weakening the lingual or the buccal cusps.
6. Restorative material considerations- gold is the best contact and support surface for the cast assembly. Amalgam is good for rest seats, but poor for abrasion resistance: guiding planes, as well as surfaces in contact with clasp arms. For porcelain-fused to metal restorations care must be taken to place rest seat and guide plane all in metal.
7. Esthetics- they can be enhanced by lowering the survey line on the facial aspect as possible, or by using an I-bar. This is because as the patient ages the mandibular teeth tend to show more as the orbicularis oris muscle looses its tone.
8. Major connector- the amount of free gingival margin determines the type of major connector. The choice and design of the major connector decide the choice and design of minor connectors ( rest seats, retentive surfaces, etc.)
Conclusion: The patient’s preexisting RPD provides clues on how to design, the new partial. This is done more for patient acceptance than for tissue preservation.
Stress-equalizing removable partial denture
Advantages and disadvantages of free-end removable partial dentures.
The mandibular free-end distal extension removable partial denture rests on soft tissue. When stress, such as mastication, is applied to the occlusal surface, the free end or ends move down on the underlying tissue.
Mesiodistal stresses are not as harmful as lateral stresses. To lessen the stresses on the abutment teeth of the RPD patient, a hinge has been incorporated into the appliance. The hinge is placed on the distal area of the abutment. When stress is placed on the free-end of the base, the base moves gingivally and there is less stress on the abutment teeth. There are two serious flaws with this hinge system. First, in time the tissue loses resiliency, and the appliance base is not pushed off the tissue. The next stress pushes the appliance further gingivally, and resorption of tissue results. As resorption continues, the base of the RPD no longer is adapted to the underlying tissue and the space continues to get bigger, hence more movement and more resorption of the underlying tissue. Second, the hinge is next to the abutment tooth in the base of the RPD, there is no movement at this area, but the longer (distally) the base, the more movement (gingivally) as the base rotates around the axis of the hinge. The area adjacent to the abutment tooth experiences no movement, while the distal end of the base experiences too much movement (traumatic) and the result is resorption of the tissues. The longer the base area the more movement, hence more resorption.
Precision-retained RPD’s are excellent appliances. The stimulation is within physiologic limits and in the proper direction. Resorption and rebasing are not major factors. There is one flaw to this appliance, the "guideplane effect" is present and if the bases are not properly adapted to the underlying tissues, distogingival stress may be placed on the abutment teeth.
Levitch designed a free-end RPD that overcomes these problems. The RPD uses a broken stress-type connector between the clasp and the base area. The connector is placed on the distolingual area of the abutment tooth. A double lingual bar or a split linguoplate is incorporated on the appliance. The appliance develops a back action movement rather than a direct action movement. Stresses such as chewing, placed on the base area of the partial in a Type I lever will cause the abutment tooth to rotate around the axis of the hinge. This will result in resorption of the hard tissues and eventual loss of the tooth.
Removable Partial Dentures and Oral Health
The objective of treatment with an RPD is the perpetual preservation of what remains rather than the meticulous restoration of what is missing. RPD effects are briefly discussed including overall longevity, periodontal and mobility considerations, caries, edentulous ridge, occlusion and direction of forces. RPD design considerations to eliminate or reduce any hazardous effects are presented, including clasps, major and minor connectors, and distal extension cases. An incorrectly designed RPD may be considered a potentially destructive appliance. But, by having sound design and a motivated patient, the RPD can be a useful, necessary treatment modality.
Removable Partial Overdentures for Special Patients.
Treatment modality which offers the benefits of superior esthetics and function without jeopardizing the already compromised dentitions of patients with acquired or congenital anomalies
.
Rationale and Indications:
1. Reversible procedure
2. Cost effective
3. Minimal or no tooth preparation
4. Ability to restore a significant loss of VDO
5. Decreased clinical chair time
Advantages: (over the complete overdenture):
1. Covers only the teeth which reduces the bulkiness of the prosthesis and allows for a more rapid adaptation
2. Improved phonetics
3. Supported by a metal framework which minimizes fracture potential
Contraindication:
1. The need for palatal coverage - cleft palate with significant hard and soft tissue defects
2. The need for denture base support - absence of a significant number of teeth, especially posterior teeth
General Treatment Planning Considerations-
a) 2-3 mm of interocclusal space is required in the anterior region beyond the patient's VDO
b) Dual or rotary path of insertion
c) (Complications) - facing fractures and dental caries
Summary: The removable partial overdenture is a viable treatment modality for special patients with specific needs.
Periodontal considerations and guidelines for therapy.
A. Pretreatment record
B. Treatment planning
1. Abutment selection
2. Trauma from occlusion- decide the effect , frequency, duration, magnitude of the occlusal forces. Determine if damages are reversible.
3. Mobility and splinting- increased tooth mobility is the single most important clinical sign of trauma from occlusion. Radiographically, it can be seen as a widened PDL. Nonprogressive mobility or stabilizing mobility does not affect prognosis, and the teeth that have decreased mobility are generally not indicated for splinting. Splinting is a recommendation for the RPD’s using intracoronal attachments. When all the remaining abutments are mobile more than one degree, bilateral splinting is indicated to resist stress in all directions. The major objective of splinting is to protect and preserve the periodontium.
C. Sequence of treatment
1. Informed consent
2. Preparatory management-emergency pain relief, oral hygiene instructions, correction of defective restorations, restoration of deep caries, extractions, endodontic therapy, and replacement of teeth with provisional restorations.
3. Management of occlusal disorders- selective teeth grinding, orthodontic treatment, provisional stabilization, and insertion of acrylic bite plane.
4. Re-evaluation
5. Surgical intervention for periodontal defects-eliminate disease processes and stabilize the periodontium.
6. Surgical techniques for hard and soft tissue defects- (1)soft tissues: gingivectomy and flaps, (2) hard tissues: osteotomy, osteoplasty, (3) dental tissues: root resection and odontoplasty. Pedicle and gingival grafts are also used.
7. Management of compromised marginal tissue.
8. Pocket reduction by inducing new attachment.
9. New connective tissue attachment.
D. Post-treatment
1. Evaluation
2. Recall: controlling periodontal disease.
3. Maintenance therapy- update medical records, evaluate oral hygiene, periodontal probings, root planing and gingival curettage , review habit control, polish restorations and crowns, occlusal adjustments and full mouth radiographs.
Cosmetics and Removable Partial Dentures – The Class IV Partially Edentulous Patient.
The Class IV partially edentulous arch presents a biomechanical and cosmetic challenge. Treatment may include specific indications for fixed or removable partial dentures. Indications for removable:
1. long span edentulous areas
2. significant bone loss
3. unfavorable labial morphology
4. cleft palate or other maxillofacial defect
5. young patients, lg. pulps
Potential problems inherent in fixed:
1. cement failure
2. mechanical breakdown
3. gingival recession
4. periodontal breakdown
5. caries
6. pulpal necrosis
7. irreversible
Removable Partial Dentures With Rotational Paths of Insertion.
Principles: Krol described the rotational path prosthesis seats its first segments, rotational centers, then the framework is rotated positioning to the final seat of the prosthesis. Three basic types: anterior-posterior, posterior-anterior, and lateral.
The posterior-anterior (PA) is used to replace bilateral missing posterior teeth using mesial undercuts of the distal abutments. The anterior-posterior (AP) is used to replace anterior teeth and uses mesial undercuts of the anterior abutments. The lateral rotational path uses mesial and distal undercuts of abutments on either side of unilateral edentulous space.
Whichever segment is seated first, use a rigid retainer in the proximal undercut. The second segment uses conventional clasp retention. The PA has a long occlusal rest with the AP using a long cingulum rest. Category Type I: all PA’s and AP’s replacing posterior teeth. Category Type II: rigid retainer seated first then rotated to seat the second segment.
Problems: The fit of the seated RPD framework’s rigid retainer is critical where it contacts the proximal surface of the abutment.
Contour and location of rest seats: Long rests (> ½ the mesial distal width of the abutment) with nearly parallel walls. For AP rests, they must be parallel to allow initial straight path of insertion. The posterior abutment rest seats should be bilaterally parallel to the vertical path of insertion and 1.5 mm thick.
Analyzing undercuts: Undercuts are analyzed by means of a divider. Place the tip of the divider on the cast at the rotational point and the second tip place in the proximal undercuts. The rotational undercut can be changed by altering the length of the rest. If the tip does not freely move, recontouring will need to be done.
Restoration of abutments: Cast restorations are preferred, but amalgam is acceptable.
Centers of rotation should be in the same horizontal plane. The length of the posterior edentulous ridge will affect the arcs of the radii and will change the amount of blockout required.
Anterior-posterior vertical relation of the undercut is affected by the steepness of the curve of Spee. As the curve steepness increases, the effective undercut is decreased.
The shape of the arch can affect the seating. A square arch has bilateral centers of rotation with radii that are parallel bilaterally and pass through all the abutments perpendicularly from an axis of rotation uniting the right and left centers. In a tapered arch, the radii are no longer parallel to each other. Care is needed when the placing the blockout for the relocated centers of rotation. Lingually tilted teeth, projections of soft tissue, and bone may prevent proper path of rotation.
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