selection and choice of abutments / orthodontics training courses

SELECTION AND CHOICE OF ABUTMENT

INDIAN DENTAL ACADEMY

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Abutment EvaluationAbutment 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. However, a tooth that has been endodontically treated and is asymptomatic, with radiographic evidence of good seal and complete obturation of the canal, can be used as an abutment. The tooth must have some sound, surviving coronal tooth structure to insure longevity. However, some compensation can be made through the use of a dowel core, or a pin retained amalgam or a composite core.


Teeth that have been pulp capped in the process of preparing the tooth should not be used as an FPD abutment unless they are endodontically treated. The supporting tissues surrounding the abutment teeth must be healthy and free from inflammation before any prosthesis can be contemplated. Normally, abutment teeth should not exhibit any mobility, since they will be carrying an extra load. The roots and the supporting tissues should be evaluated for three factors:

• Crown-root ratio.

• Root configuration.

• Periodontal ligament area.www.indiandentalacademy.com


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Crown-root ratio

This ratio is a measure of the length of tooth, occlusal to the alveolar crest of bone compared with the length of root embedded in the bone. As the level of alveolar bone moves apically, the lever arm of that portion out of bone increases, and the chances for harmful lateral forces is increased. The optimum crown-root ratio for a tooth to be utilized as a FPD abutment is 2:3. A ratio of 1:1 is the minimum ratio that is acceptable for a prospective abutment under normal circumstances.


However, there are situations in which a crown root ratio greater than 1:1 might be considered adequate. If the occlusion opposing a FPD is composed of artificial teeth, occlusal forces will be diminished, with less stress on the abutment teeth. The occlusal forces against prosthetic appliances have been shown to be considerably less than that against natural teeth: 26 lb for removable partial dentures and 56 lb for fixed partial dentures versus 150 lb for natural teeth.


For the same reasons, an abutment tooth with a less than desirable crown-root ratio is more likely to successfully support FPD if the opposing occlusion is composed of mobile, periodontally involved teeth than if the teeth are periodontally sound.


Root configurationThis is an important point in the assessment of an abutment’s suitability from a periodontal standpoint. Roots that are broader labiolingually than they are mesiodistally are preferable to roots that are round in cross-section.


Multirooted posterior teeth with widely separated roots will offer better periodontal support than roots that converge, fuse, or generally present a conical configuration. The tooth with conical roots can be used as an abutment for a short span FPD if all other factors are optimal. A single rooted tooth with evidence of irregular configuration or with some curvature in the apical third of the root is preferable to the tooth that has a nearly perfect taper.


Periodontal ligament area / Ante’s law

When the normal complement of roots is not available to distribute the forces of mastication, pathologic stress concentrations may result in the periodontal ligament and supporting bone. This condition is the most fundamental problem with which the dentist must contend each time a fixed prosthesis is designed to replace a missing tooth. Here we must take into accounts Ante’s law, which states, “The total periodontal membrane area of the abutment teeth should equal or exceed that of the teeth to be replaced’’.


The essential feature of this clinical guideline is that the actual area of the interface between tooth and supporting structures must be of a certain minimal amount to withstand and resist the forces that will now be transmitted to those supporting structures. Realistic determination of the area of good, healthy, periodontal ligament available on a potential FPD abutment is not an easy matter.


The combined root surface area of the second premolar and the second molar (A2p+A2m) is greater than that of the first molar being replaced (A1m).


The combined root surface area of the first premolar and the second molar abutment (A1p+A2m) is approximately equal to that of the teeth being replaced (A2p+A1m).


The combined root surface area of the canine and the second molar (Ac+A2m) is exceeded by that of the teeth being replaced (A1p+A2p+A1m)


Also of extreme importance is the actual area of contact between the periodontal structures and the root in question as it relates to the normal amount of contact area for that particular tooth in that particular mouth. One can assume that in a given case, without bone loss, the area of this contact is optimal. Therefore, any loss of bone support compared with the optimal situation decreases the chances of this root being an adequate bridge abutment. If one looks at the problem in this manner, it becomes apparent that a root that appears adequate in one situation may be inadequate in another. The planning and design of a restoration of this type must have the benefit of sound clinical judgment and knowledge of basic biomechanical principles.


Maxillary Maxillary Mandibular Mandibular

Tooth Area mm2 Ranking Area mm2 Ranking

Central 139 7 103 8

Lateral 112 8 124 7

Canine 204 3 159 4

First premolar 149 5 135 6Second premolar

140 6 135 5

First molar 335 1 352 1

Second molar 272 2 282 2

Third molar 197 4 190 3

Comparison of root surface areas of 16 teeth


The above table, which compares the root surface areas of 16 teeth, may aid in visualizing root areas. It is interesting to note that the addition of abutment roots will provide a wide variation of additional support. The addition of such support is not necessarily proportional to the number of abutments supporting prosthesis. There fore, the prosthodontist should not expect a proportional increase in stress-bearing ability, particularly when the amount of periodontal ligament is reduced.


The area of the normal periodontal ligament (PDL) for teeth to be replaced by pontics should be less than the actual PDL area of the existing abutment candidates.

The values given in the table are averages for the various teeth in the mouth under ideal conditions. Two problems are evident when one attempts to arrive at useful interpretation of such data.


Degree and nature of bone loss

In clinical practice, the decision making process in which root surface area information is to be used does not always involve ideal situations. More often than not, there has been some bone loss; thus, a tooth with a moderate amount of bone loss may be still a better candidate for use as a FPD abutment than another tooth with no bone loss at all. To determine as to which of the teeth in question has adequate support for the anticipated loads, there are few aids on which the dentist can rely, none of them capable of giving all the information necessary to make a decision. www.indiandentalacademy.com

These aids are • Radiographs • Periodontal probing, and • Mobility tests.

The radiograph is unquestionably the most useful tool at the disposal of the dentist in making a determination of the integrity of the remaining periodontal supporting structures as related to the loads anticipated. The opportunity to make radiographs from different projection angles should not be overlooked, since the primary areas that can be visualized on radiographs are the mesial and distal surfaces. Any chance for seeing even a little of the facial or lingual surfaces should be taken advantage of.


Periodontal probing is the second tool at the disposal at the dentist and should be used extensively when attempting to determine the efficacy of using a given tooth as an abutment. Periodontal probing is a particularly important step as related to the facial and lingual surfaces, since these areas of the tooth to supporting structures interface cannot be viewed adequately on the radiograph.

Finally, standard clinical tests for mobility should be employed. Any degree of mobility outside the normal accepted physiologic range should be suspected. It means either that the loss of supporting structure, whether or not fully appreciated from the radiographs and probing, is severe enough to alter physically the stability of the tooth or that the occlusion has traumatically loosened the tooth.


It is very important to understand which of these processes is at work in a given abutment situations. Occlusal trauma is usually reversible, and given the fact that the prosthodontist is going to construct a restoration on the tooth in question, there is ample opportunity to correct the situation. A periodontal defect, however, is not always reversible and depending on its severity may require a drastic alteration of the plan of treatment for the tooth in question. The important concept to keep in mind is that the prosthodontist must exercise a certain degree of good judgement in the question of interpretation of the adequacy of supporting structures in a given situations. Many aspects of the final restoration and the chances for success are basically technical and depend on the skill of a particular prosthodontist. Not only good judgment but a conservative approach must be hall marks of the thought processes of the dentist in this all important consideration. www.indiandentalacademy.com

Ante’s law says that in a situation where the preceding values are normal, prosthesis to replace the maxillary first molar would need abutment teeth with at least 335 mm2 of periodontal membrane. This requirement is theoretically well satisfied in the case because of the total average area of the second molar and second premolar is 412mm2, however, has there been enough loss of bone on these two teeth to result in, for example, a total of only 300mm2, the law would not be satisfied.


Tilted molar abutment

Titled abutment teeth are a common problem that must be addressed in construction of fixed partial prostheses. The tooth to be replaced by the restoration frequently has been missing for a long time. Therefore, the tooth distal to the missing one often will have tilted into the space. It is impossible to prepare the abutment teeth for a fixed partial denture along the long axis of the respective teeth and achieve a common path of insertion. There is further complication if the third molar is present. It will usually have drifted and tilted with the second molar. www.indiandentalacademy.com

Because the path of insertion of the fixed partial denture will be dictated by the smaller premolar abutment, it is probable that the path of insertion will be nearly parallel to the former long axis of the molar abutment before it tilted mesially. As a result, the mesial surface of the tipped third molar will encroach upon the path of insertion of the fixed partial denture, thereby preventing it from seating completely.


Some of the possible solutions to these problems are:

Preparation modifications: The design of the preparation could be modified to be in harmony with the line of draw requirements of the other abutment and adjacent teeth while at the same time satisfying all other preparation criteria, such as retention and protection of the pulp. A proximal half crown can be used as a retainer on the distal abutment. This retainer can be used only if the distal surface itself is untouched by caries or decalcification and if there is very low incidence of proximal caries throughout the mouth. www.indiandentalacademy.com

If there is a severe marginal ridge height discrepancy between the distal of the second molar and the mesial of the third molar as a result of tipping, the proximal half crown is contraindicated.


Telescopic crown designs: A two piece restorations is constructed whereby the line of the draw of one component (seated on the tipped tooth preparations) is such that it favors the tooth. The line of the draw of the component is then in harmony with the other abutment preparation.


Broken connectors: In these situations it is desirable to connect units of fixed bridges in some manner that will allow the various components of the prosthesis to be seated separately.


Pre-prosthetic orthodontics: Some degree of uprighting the tooth may be accomplished orthodontically.


Composite resin bonded prosthesis

The most recent innovation in multiple unit restorations is the composite resin bonded prosthesis. Utilization and popularization of this technique is based on the ability to etch certain high modulus, non precious alloys. After etching, the metal can be placed after only a minimum of tooth reduction. To accomplish the goals of this conservative restoration, one must make the metal frame work thin and in-conspicuous which has led to FPD’s with minimal structural integrity.


The essential features of this type of restoration have included:

• Minimal axial reduction lingually at the height of contour.

• 1 mm deep occlusal rests inclined toward the center of the abutment teeth.

• 180-degree proximal wraparounds approximately 0.4mm thick.

• A distinct path of insertion.

• For anterior abutments, bonded cingulum rests have been advocated.


When these composite resin bonded prosthesis are subjected to occlusal loadings, very high complex stresses are generated at the connector areas and extend into the high flexure of the wraparound arms. These high flexural stresses are transmitted to the resin adhesive. During function, the bridge is subjected to a large number of chewing cycles, which may be translated into fatigue failure of the adhesive layer


When the thickness is increased, a substantial decrease in the level of stress concentration results. Another means to substantially reduce the level of stresses within the frame work is to include occlusogingival extensions adjacent to the extraction site.The occlusal rests are also important structural elements in the transmitting of forces from the pontic to the abutment teeth. A similar structural support may be obtained by preparing a ledge on which the occlusogingival extension rests. This support is, in essence, a very minor box preparation. There are pros and cons to both approaches, but one of these two rest concepts should be used.


Structural considerations for the success of this technique should include:

• Wraparound arms as thick as possible consistent with reasonable tooth contour.

• Occlusogingival proximal extensions and

• A sound rest, whether it is on the occlusal surface or in the form of a gingival box.


Canine replacement fixed partial dentures

Fixed partial dentures replacing canines can be difficult because the canine often lies outside the interabutment axis. The prospective abutments are the lateral incisors, usually the weakest tooth in the entire arch, and the premolar, the weakest posterior tooth. A fixed partial denture replacing a maxillary canine is subjected to more stresses than that replacing a mandibular canine, since forces are transmitted outward (labially) on the maxillary arch, against the inside of the curve (its weakest point).


On the mandibular canine, the forces are directed inward (lingually), against the outside of the curve (its strongest point).

Any fixed partial denture replacing a canine should be considered a complex a fixed partial denture. No fixed partial denture replacing a canine should replace more than one additional tooth. An edentulous space created by the loss of a canine and any two contiguous teeth is best restored with a removable fixed partial denture.www.indiandentalacademy.com

Cantilever fixed partial denturesA 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.

In a routine three-unit fixed partial denture, force that is applied to the pontic is distributed equally to the abutment teeth. If there is only one pontic and it is near the interabutment axis line, less leverage is applied to the abutment teeth or to the retainers than with a cantilever. When a cantilever pontic is employed to replace a missing tooth, forces applied to the pontic have an entirely different effect on the abutment tooth. The pontic acts as a lever that tends to be depressed under forces with a strong occlusal vector.


Prospective abutment teeth for cantilever fixed partial dentures should be evaluated with an eye towards lengthy roots with a favorable configuration, long clinical crowns, good crown-root ratios, and healthy periodontium. Generally, cantilever fixed partial dentures should replace only one tooth and have at least two abutments.

A cantilever can be used for replacing a maxillary lateral incisor. There should be no occlusal contact in either centric or lateral excursions. The canine must be used as an abutment, and it can serve in the role of solo abutment only if it has a long root and good bone support. There should be a rest on the mesial of the pontic against a rest seat preparation in an inlay or other metallic restoration on the distal of the central incisor to prevent rotation of the pontic and the abutment. www.indiandentalacademy.com

The mesial side of the pontic can be little ‘wrapped around’ the distal portion of the uninvolved central incisor to stabilize the pontic faciolingually. The root configuration of the central incisor does not make it a desirable cantilever abutment.


A cantilever pontic can also be used to replace a missing first premolar. This scheme will best work if occlusal contact is limited to the distal fossa. Full veneer retainers are required on both the second premolar and first molar. These teeth must exhibit excellent bone support. This design is acceptable if the canine is unmarred and if a full veneer restoration is required for the first molar in any event.


Cantilever fixed partial denture can also be used to replace molars when there is no distal abutment present. When used judiciously, it is possible to avoid the insertion of a unilateral removable partial denture. Most commonly, this type of fixed partial denture is used to replace the first molar, although occasionally it is used to replace a second molar to prevent supereruption of opposing teeth. When pontic is loaded occlusally, the adjacent abutment tends to act as a fulcrum, with a lifting tendency on the farthest retainer. To minimize the leverage effect, the pontic should be kept as small as possible, more nearly representing a premolar than a molar. There should be a light occlusal contact with absolutely no contact in any excursion. The pontic should possess maximum occlusogingival height to ensure a rigid prosthesis.


A posterior cantilever pontic places maximum demands on the retentive capacity of the retainers. Its use, therefore, should be reserved for those situations in which there is adequate clinical crown length on the abutment teeth to permit preparations of maximum length and retention. The success of cantilevers in the restoration of the periodontally compromised dentition is probably due, at least by part, to the fact that periodontally involved abutments do have extremely long clinical crowns. While cantilever fixed partial dentures appears to be a conservative restoration, the potential for damage to the abutment teeth requires that they be used sparingly.


Double abutment Many clinical situations require the use of double abutments in the fixed bridges. The term as used here refers to the use of two adjacent teeth at one or both ends of a fixed prosthesis joined by a solid connector. The usual reasons for use of double abutment are:

• Increase retention of the restorations as a whole • Splint and stabilize periodontally compromised teeth and • Increase the area of the supporting PDL and bone.

Improvement of the retentive aspects of the restoration would seem to be a reasonable justification for including an extra abutment. This rationale is not always true. As seen, the second premolar has insufficient coronal dentin to provide the necessary retention for use as an abutment. Th assumption was made that adding the extra premolar abutment would give the bridge adequate retention of the anterior end.


This abutment would allow retention of the second premolar root to reduce future bone loss, which would occur if this tooth were extracted. This latter point would certainly add credibility to the rationale, but at least two other more conservative methods could be considered to render the second premolar a sound abutment. First, a pin retained intra coronal casting or build up could be made for the second premolar if maintenance of the vitality of this tooth is a prime concern. Second, endodontic therapy and a retentive post and core could be done on the second premolar. The latter method would usually be the method of choice in this situation due to the greater chance of long term success compared with the pin buildup. www.indiandentalacademy.com

Either of these options, particularly the post and core, could obviate the need for double abutting this restoration. The reason being that by correcting the problem involving the second premolar, which is lack of retention, the operator has created a typical three unit prosthesis situation. The preceding example considered the use of double abutment strictly on the basis of lack of retention of the primary abutment choice. A discussion of other reasons for the use of multiple abutments follows. However, before proceeding, it is advisable to consider some of the ramifications of using double abutment as a solution for lack of abutment retention.


During function, the case treated as shown, often develops a cement failure at the second premolar because of poor retention characteristics. The other units will often be retained adequately. Breakdown of the cement layer of this abutment tooth leads to slow destruction by action of the saliva and its acidic components. Had this same loss of retention occurred in the case of a single unit restoration, it would have simply come away from the preparation and the patient would have sought treatment for an obvious problem. Dislodgement of the restoration does not occur, however, when other retainers of a multiple unit restoration remain in place on their respective abutment teeth.


This problem is difficult to diagnose because “loose” retainer is still held in its correct position in relation to the abutment tooth, though no longer cemented. The patient complains of pain. Since the retainer is still held in its correct position relative to the abutment tooth preparation, no marginal opening can be detected, nor can any looseness or movement.As can be easily seen, diagnosis of the patient’s complaint can be difficult, if not impossible, without removal of the entire restoration. Due to these problems, it is imperative that precautions be taken in the design and construction of multiple unit restorations to preclude the loss of retention on any abutment. Further it is strongly recommended that the use of double abutments to compensate for lack of retention on one of the abutment teeth of a fixed prosthesis be discouraged.


The procedure may be justified from the view point of maintaining bone, but it is less justifiable when considered in the light of resistance to the forces to which the restoration will ultimately be subjected. The alternative of pins or posts will usually be found to be the treatment of choice to permit saving of the root. Splinting and stabilization of a periodontally compromised tooth can be more valid reasons for the use of double abutments on a fixed bridge. However, a fundamental decision must be made early in the planning of the case; is the mobility the result of a continuing process of periodontitis, or occlusal trauma. If the mobility of the tooth is only the result of occlusal trauma, stabilization of such a tooth in this manner may be perfectly justified, providing that the trauma can be eliminated in the occlusal scheme of the restorations.


When a tooth is subjected to occlusal forces that cannot be controlled, the adjacent tooth might be added to the restoration as a double abutment to provide the needed resistance to lateral forces. A classic example on this situation would be a bridge replacing a missing maxillary canine. In such a case, the lateral occlusal forces generated on the canine pontic are such that the lateral incisor is seldom an adequate abutment due to its short root form. It is then justified to add the central incisor to such design. It has been shown that mobility resulting from occlusal trauma is reversible once the cause for the trauma is removed.


On the other hand, if the lack of bone support is due to periodontal disease, and if the disease is not totally controlled, using this tooth as part of double abutment is contraindicated. In such a situation, bone loss on the affected abutment tooth continues, with the end result being that this tooth eventually becomes simply another pontic in the bridge. Also, pockets become less cleanable after the placement of the restoration due to poorer access, compounding the problem.Finally, the best justification, for using double abutments is to satisfy Ante’s law. If there are not enough periodontal ligaments for a given number of missing teeth, there is no better solution than to add one or more teeth that do have sound support. www.indiandentalacademy.com

When many missing teeth are replaced by a fixed restoration using a limited number of abutments, most of which do not even possess the normal amount of bone support, failure is assured. One must make decision whether the addition of more abutments in the design of the restoration is more important than satisfying the concomitant requirement for conservatism. There may be no choice if the restoration is to be made at all. If it is not possible to satisfy Ante’s law in this regard, a removable partial denture should be considered so that occlusal forces may be distributed cross arch and to the edentulous ridges.


From the viewpoint of mechanical principles, the advantage of adding a second abutment at one end of a fixed prosthesis is that in so doing, we are better able to distribute the forces that would be applied to the prosthesis. Nothing would be gained if a crown were placed on the added abutment were it not connected rigidly to the remainder of the prosthesis. When the added tooth is made an integral part of the prosthesis, its periodontal ligaments provide resistance to forces transmitted by the other abutment at this end of the bridge. This shared load-bearing responsibility is the essence of Ante’s law. An additional abutment tooth, or teeth, is used to replace the missing tooth. Other wise, only two abutment teeth would be performing the function of resisting forces applied to three occlusal surfaces.


There is a common problem in replacing all four maxillary incisors with a fixed partial denture and the problem is more pronounced in the arch that is pointed in the anterior. This occurs because the pontics lie outside the interabutment axis line and thus acts as a lever arm, which can produce a torquing movement. In order to offset the torque, additional retention is obtained in the opposite direction of the lever arm and at a distance from the interabutment axis equal to the length of the lever arm.


The first premolars sometimes are used as secondary abutments for a maxillary four-pontic canine to canine fixed partial denture. Because of the tensile forces that will be applied to the premolar retainers, they must have excellent retention.



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selection and choice of abutments / orthodontics training courses

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