|Year : 2013 | Volume
| Issue : 1 | Page : 12-17
Questionable abutments: General considerations, changing trends in treatment planning and available options
D Krishna Prasad, Chethan Hegde, Anshul Bardia, D Anupama Prasad
Department of Prosthodontics and Crown and Bridge, A. B. Shetty Memorial Institute of Dental Sciences, Nitte University, Deralakatte, Mangalore, Karnataka, India
|Date of Web Publication||25-Oct-2013|
D Krishna Prasad
Department of Prosthodontics and Crown and Bridge, A. B. Shetty Memorial Institute of Dental Sciences, Nitte University, Deralakatte, Mangalore, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Abutment teeth are called upon to withstand the forces normally directed to the missing teeth, in addition to those usually applied to the abutments. Whenever possible, an abutment should be a vital tooth. Normally, teeth with active periodontal problem should not be used as abutment teeth. The use of multiple splinted abutment teeth, non-rigid connectors or intermediate abutments makes the procedure much more difficult and often the result compromises the long-term prognosis. In cases where tooth preparation cannot solve the problem, the use of various attachments and a telescopic retainer must be considered. Understanding the basic concepts of how to retain various restorative components and how to protect the remaining tooth structure, will enable us to answer the numerous questionable situations that arise during the restorative process will be facilitated. Thus, this will result in final restorations that are based on sound design principles.
Clinical Relevance to Interdisciplinary Dentistry
- Fixed prosthetic treatments are always dependent upon the support they receive from abutment teeth. Abutment teeth should have sufficient coronal structure to provide retention to the prosthesis. It might sometimes be necessary to expose or increase the clinical crown by periodontal surgery for support and esthetics. It is also seen that many a times the teeth are supra erupted as a result of absence of opposing dentition which calls for the need of intentional endodontic treatment. By a combination of treatments with interdisciplinary dentistry, we will succeed in providing a functional prosthesis which fulfils esthetic and restorative needs.
Keywords: Abutments, fixed partial dentures, questionable abutments
|How to cite this article:|
Prasad D K, Hegde C, Bardia A, Prasad D A. Questionable abutments: General considerations, changing trends in treatment planning and available options. J Interdiscip Dentistry 2013;3:12-7
|How to cite this URL:|
Prasad D K, Hegde C, Bardia A, Prasad D A. Questionable abutments: General considerations, changing trends in treatment planning and available options. J Interdiscip Dentistry [serial online] 2013 [cited 2019 Nov 13];3:12-7. Available from: http://www.jidonline.com/text.asp?2013/3/1/12/120516
| Introduction|| |
The goal of dental treatment is to provide the optimal oral health, esthetics and function. Therapeutic efforts should produce predictable treatment results that are easily maintainable and reliable over the long-term. This objective applies to each tooth in the dentition and to the dentition as a whole. Partial dentures transmit forces through the abutments to the periodontium.
Failures are due to poor engineering, the use of improper materials, inadequate tooth preparation and faulty fabrication. Of particular concern to dentists is the selection of teeth for abutments. They must recognize the forces developed by the oral mechanism and the resistance of the tooth and it's supporting structures to these forces. Successful selection of abutments for fixed partial dentures requires sensitive diagnostic ability.
Considerable time and expense are spared and loss of a patient's confidence can be avoided, by thoroughly investigating each abutment tooth before proceeding with tooth preparation.
| Questionable Abutment Situations in Fixed Partial Denture Prosthesis|| |
- Extensively damaged teeth
- Periodontaly weakened teeth
- Pier abutments
- Tilted abutments
- Cantilever fixed partial dentures
- Short abutments.
Extensively damaged teeth
The criteria for the use of a cast metal, metal-ceramic or all-ceramic restoration is when tooth has been damaged to the extent that it must be reinforced and protected. It should not be surprising that unmodified classic preparation designs are used infrequently. The amount of tooth structure destroyed is only one factor to consider in selecting a restorative material and designing a preparation. Equally important is the location of the destruction and the amount of the tooth surface involved. Location can be classified as peripheral, occurring on the axial surfaces of the tooth; central, in the center of the tooth; or combined, with destruction in both sites. 
Principle of substitution
When it is necessary to compensate for mutilated or missing cusps, inadequate length and in extreme cases even a missing clinical crown, the principle of substitution is used.
Two rules should be observed to avoid excessive tooth destruction while creating retention in an already weakened tooth:
- The central "core" (the pulp and the 1.0-mm-thick surrounding layer of dentin) must not be invaded in vital teeth. No retentive features should extend farther into the tooth than 1.5 mm at the cervical line or down 1.5 mm from the central fossa. If caries removal results in a deeper cavity, any part lying within the vital core should be filled with glass ionomer cement. Any preparation feature added for mechanical retention is kept in the safe area of the tooth, peripheral to the vital core
- No wall of dentin should be reduced to a thickness less than its height for the sake of retention. This may preclude the use of a full veneer crown, or if one must be used, it might first require the placement of a core or foundation restoration.
Small to moderate interproximal carious lesions or prior restorations can be incorporated the preparation as a box form. This substitute for grooves serves the dual purpose of caries removal and retention form.
Grooves placed in vertical walls of bulk tooth structure must be well-formed, at least 1.0 mm wide and deep and as long as possible to improve retention and resistance. Multiple grooves are as effective as box forms in providing resistance and they can be placed in axial walls without excessive destruction of tooth structure. 
Bases and cores
When the destruction of tooth structure is more extensive, a decision must be made whether to augment the retention and resistance by adding bases or to build up the tooth preparation with a core.
Cement bases are used only to protect the pulp and to eliminate undercuts in defects in tooth structure produced by the removal of caries or old restorations. Glass ionomer and zinc polycarboxylate cements are excellent materials for this purpose. They are non-irritant to the pulp and have some adhesive properties that make them less likely to become dislodged during subsequent preparation of the tooth. Deep areas of the preparation near the pulp may be covered with calcium hydroxide.
Modifications for damaged vital teeth
- Evaluate pulpal health
- Assess the periodontal condition
- Make a preliminary preparation design
- Remove previous restorations and bases, all caries and any unsupported enamel
- Evaluate the strength of the remaining walls: If the thickness to height ratio of a wall lies between 1:1 and 1:2 it should be supported. Any wall with a thickness to height ratio of less than 1:2 is subject to fracture and should be shortened
- Finalize the preparation design.
Periodontaly weakened teeth
Periodontal health is a prerequisite of successful comprehensive dentistry. To achieve the long-term therapeutic targets of comfort, good function, treatment predictability, longevity and ease of restorative and maintenance care, active periodontal infection must be treated and controlled before the initiation of restorative dentistry.
Rationale for therapy
- Periodontal treatment is undertaken to ensure the establishment of stable gingival margins before tooth preparation. Non-inflamed, healthy tissues are less likely to change as a result of subgingival restorative treatment or post-restoration periodontal care 
- Certain periodontal procedures are designed to provide for adequate tooth length for retention, access for tooth preparation, impression making, tooth preparation and finishing of restorative margins in anticipation of restorative dentistry 
- Periodontal therapy should follow restorative care because the resolution of inflammation may result in repositioning of teeth or in soft-tissue and mucosal changes 
- If traumatic forces applied to teeth with ongoing periodontitis, it may result in increased tooth mobility, discomfort and possibly the rate of attachment loss [Figure 1].
Sequence of treatment
In general, preparation of the periodontium for the restorative dentistry can be divided into two phases: (1) Control of periodontal inflammation with non-surgical and surgical approaches and (2) pre-prosthetic periodontal surgery.
Situations in which a tooth has a short clinical crown and is inadequate for the retention of a required cast restoration, it is necessary to increase the size of the clinical crown using periodontal surgical procedures. Surgical crown lengthening procedures are performed to provide retention form to allow for proper tooth preparation, impression procedures and placement of restorative margins and to adjust gingival levels for esthetics.
A pier (intermediate) abutment is a natural tooth located between terminal abutments that serve to support a fixed or removable dental prosthesis. Rigid connectors (e.g., solder joints) between pontics and retainers are the preferred way of fabricating most fixed partial dentures. However, a completely rigid restoration is not indicated for all situations requiring a fixed prosthesis. Physiologic tooth movement, arch position of the abutments and a disparity in the retentive capacity of the retainers can make a rigid 5-unit fixed partial denture a less than ideal plan of treatment.
Studies in periodontometry have shown that the faciolingual movement ranges from 56 to 108 μm and intrusion is 28 μm. Teeth in different segments of the arch move in different directions (because of the curvature of the arch). The faciolingual movement of an anterior tooth occurs at a considerable angle to the faciolingual movement of a molar.
Use of the non-rigid connector is restricted to a short span fixed partial denture replacing one tooth. Prostheses with non-rigid connectors should not be used if prospective abutment teeth exhibit significant mobility. Nearly 98% of posterior teeth tilt mesialy when subjected to occlusal forces. If the keyway of the connector is placed on the distal side of the pier abutment, mesial movement seats the key into the keyway more solidly. Placement of the keyway on the mesial side, however, causes the key to be unseated during its mesial movements. 
Tilted molar abutments
A common problem that occurs with some frequency is the mandibular molar abutment that has tilted mesialy into the space formerly occupied by the lost natural teeth anterior to it [Figure 2]. It is impossible to prepare the abutment teeth for a fixed partial denture along the long axes of the respective teeth and achieve a common path of insertion. There is a further complication if the third molar is present.
- Uprighting of the tilted molar with orthodontic treatment 
- Fixed partial denture using a proximal half-crown as a retainer on a tilted molar abutment
- Fixed partial denture using a telescopic crown and coping as a retainer on a tilted molar abutment
- A non-rigid connector on the distal aspect of the molar retainer compensates for the inclination of the tilted molar.
Cantilever fixed partial dentures
A cantilever fixed partial denture is one that has an abutment or abutments at one end only, with the other end of the pontic remaining unattached. This is a potentially destructive design with the lever arm created by the pontic and it is often misused.
A cantilever fixed partial dentures should replace only one tooth and have at least two abutments. It can be used for replacing a maxillary lateral incisor. There should be no occlusal contact on the pontic in either centric or lateral excursions.
A cantilever pontic can be used to replace a first premolar if full veneer retainers are used on the second premolar and first molar abutments. Cantilever fixed partial denture replacing a mandibular first molar, using both premolars as abutment teeth. To minimize stress on the abutments, the pontic is the size of a premolar rather than a molar. 
| Questionable Abutment Situations in Removable Partial Denture|| |
- Damaged teeth
- Fixed partial dentures serving as abutments
- Isolated abutment
- Anteriors as abutment
- Missing anteriors
- Fabricating restorations to fit existing denture retainers.
Abutment contours should be altered during mouth preparations in the following sequence:
- Proximal surface is prepared parallel to the path of insertion to create guiding plane
- Height of contour on buccal (and lingual) is lowered when necessary to permit clasp placement to be located more favorably (i.e., middle-gingival third)
- Labial reduction demonstrating favorable location of height of contour and mesiobuccal undercut location. Occlusal convergence of height of contour is also evident
- Spoon-shaped disto-occlusal rest preparation that will direct occlusal forces along long axis of tooth should be the final step in mouth preparations.
Fixed partial dentures serving as abutments
One advantage of making cast restorations for abutment teeth is that the mouth preparations conventionally done in the mouth after cementation of the crown can be done on the surveyor with far greater accuracy during the fabrication stage itself [Figure 3]. It is difficult to make several proximal surfaces parallel to one another while preparing them intraorally.
The ideal crown restoration for a removable partial denture abutment is the complete coverage crown, which can be carved, cast and finished to ideally satisfy all requirements for support, stabilization and retention without compromise for cosmetic reasons [Figure 4]. Porcelain veneer crowns can be made equally satisfactory only by the additional of contouring the veneered surface on the surveyor before the final glaze.
|Figure 4: Cast partial denture abutments - Rest seat fabrication on crowns|
Click here to view
Regardless of the type of crown used, the preparation should be made to provide the appropriate depth for the occlusal rest seat. This is best accomplished by altering the axial contours of the tooth to the ideal before preparing the tooth and creating a depression in the prepared tooth at the occlusal rest area.
The average abutment tooth is subjected to some distal tipping, rotation, torqueing and horizontal movement, all which must be held to a minimum by the quality of tissue support and the design of the removable partial denture. The isolated abutment tooth, however, is subjected also to mesial tipping because of lack of proximal contact. Despite indirect retention, some lifting of the distal extension base is inevitable, causing torque to the abutment.
In contrast, an isolated anterior abutment adjacent to a distal extension base usually should be splinted to the nearest tooth by means of a fixed partial denture. The effect is two-fold: (1) The anterior edentulous segment is eliminated, thereby creating an intact dental arch anterior to the edentulous space; and (2) the isolated tooth is splinted to the other abutment of the fixed partial denture, thereby providing multiple abutment support. Splinting should be used here only to gain multiple abutment support and not to support an otherwise weak abutment tooth.
Missing anterior teeth
When a removable partial denture is to replace missing posterior teeth, especially in the absence of distal abutments, any additional missing anterior teeth are best replaced by means of fixed restorations rather than included in the removable partial denture. In any distal extension situation, some anteroposterior rotational action will result from the addition of an anterior segment to the denture. The ideal treatment plan, which would consider the anterior edentulous space separately, may result in conflict with economic and esthetic realities. Each situation must be treated according to its own merits.
Fabricating restorations to fit existing denture retainers
Ideally, all abutment teeth would best be protected with complete crowns before the removable partial denture is fabricated [Figure 5]. Except in the scenario of recurrent caries (due to defective crown margins or gingival recession) abutment teeth so protected may be expected to give many years of satisfactory service in the support, stabilization and retention of the removable partial denture. Economically a policy of insisting on complete coverage for all abutment teeth may well be justified from a long-term viewpoint. It must be recognized, however that in practice complete coverage of all abutment teeth is not always possible at the time of treatment planning. 
| Questionable Abutment Situations in Overdentures|| |
While selecting abutments for overdentures, the position of the tooth in the arch and its position between the buccal and lingual cortical plates should be evaluated carefully. The crown-root ratio is improved considerably by reducing the clinical crown height when preparing the tooth for its role as an abutment [Figure 6]. The angulation of the tooth to be used for an abutment is also considered. For effective distribution of the functional forces applied to the remaining root, the root should be in an axial position perpendicular to the direction of the occlusal forces.
When periodontal disease produces bone loss and recession, the clinical crown may be of much greater length than the anatomic crown. Therefore during abutment selection care should be taken that there is adequate supporting the present, because the crown-root ratio will be improved later. Mobile teeth or teeth with sever osseous defects are poor candidates for abutments. 
| Discussion|| |
Any tooth can be considered as an abutment tooth. But the abutment tooth have to withstand the forces from a different direction than one crowned as an individual tooth. Many clinicians avoid root filled tooth or teeth including post and core crown because of the chances of fracture of the roots. However, this risk depends upon whether the tooth is used as an abutment or as a member of the abutment teeth in a large bridge so that the forces are shared by other abutments.
With the advent of newer materials and advancing technologies, endodontically treated teeth can be used successfully as abutment teeth. Teeth with active periodontal disease are not used as abutments. The tooth/teeth are first treated for the periodontal problem and then considered as an abutment. Various periodontal procedures can be employed to improve the quality of the tooth/teeth before its use as an abutment.
There is often a concern with selecting tilted tooth/teeth as an abutment. These teeth can be modified using the various prosthodontic approaches in combination with orthodontic techniques. Thus, with the combined approaches of endodontics-periodontics-prosthodontics, we can preserve the natural tooth and utilize it as an abutment.
| Conclusion|| |
From the above discussion, we understand that the abutment teeth bear the forces developed by the oral mechanism and transmit these to the underlying periodontium. Thus, the proper selection of abutments influences the prognosis of the treatment. Successful selection abutments would require prompt diagnosis and meticulous treatment planning. Whenever possible, an abutment should be a vital tooth. However, a tooth that has been endodontically treated and is asymptomatic, with radiographic evidence of a good seal and complete obturation of the canal, can also be used as an abutment.
The past few decades have witnessed acceleration in advancements in materials, techniques and concepts that have been well-researched and scientifically accepted. While root canal therapy saves roots, sound post endodontic restoration saves crowns. Combination of these procedures (endodontic-periodontic-prosthodontic) have been able to successfully salvaged more teeth and restored its form and function in recent times.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]