Читать книгу Fundamentals of Fixed Prosthodontics - James C. Kessler - Страница 121

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 frequently misused.

In the routine three-unit fixed partial denture, force that is applied to the pontic is distributed equally to the abutment teeth (Fig 7-34). If there is only one pontic and it is near the interabutment axis line, less leverage is applied to the abutment teeth and 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 (Fig 7-35).

Prospective abutment teeth for cantilever fixed partial dentures should be evaluated with an eye toward lengthy roots with a favorable configuration, long clinical crowns, good crown-root ratios, and healthy periodontium.³⁴ Generally, cantilever fixed partial dentures should replace only one tooth and have at least two abutments.^35,36

A cantilever can be used for replacing a maxillary lateral incisor (Fig 7-36). There should be no occlusal contact on the pontic in either centric or lateral excursions.³⁷ 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 preparation in an inlay or other metallic restoration on the distal of the central incisor to prevent rotation of the pontic and abutment. The mesial aspect of the pontic can be slightly wrapped around the distal portion of the uninvolved central incisor to stabilize the pontic faciolingually.³⁷ The root configuration of a central incisor makes it an undesirable cantilever abutment.

Fig 7-34 Forces applied to the pontic of a routine fixed partial denture are transmitted to both abutment teeth.

Fig 7-35 Forces on the pontic of a cantilever fixed partial denture tend to tip the fixed partial denture or the abutment tooth.

Fig 7-36 Cantilever fixed partial denture replacing a maxillary lateral incisor, using the canine as the abutment.

Fig 7-37 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.

A cantilever pontic can also be used to replace a missing first premolar (Fig 7-37). This scheme will work best 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 attractive if the canine is unmarred and if a full veneer restoration is required for the first molar under any circumstances.

Cantilever fixed partial dentures 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.³⁴ Most commonly, this type of fixed partial denture is used to replace a first molar, although occasionally it is used to replace a second molar to prevent supereruption of opposing teeth.

When the pontic is loaded occlusally, the adjacent abutment tends to act as a fulcrum, with a lifting tendency on the farthest retainer³⁸ (Fig 7-38). To minimize the leverage effect, the pontic should be kept as small as possible, more nearly representing a premolar than a molar (Fig 7-39). There should be 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 retainer.³⁹ 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 in part to the fact that periodontally involved abutments do have extremely long clinical crowns. While cantilever fixed partial dentures appear to be a conservative restoration, the potential for damage to the abutment teeth requires that they be used sparingly.

Fig 7-38 Forces on a full-size molar cantilever pontic place great stress on the mesial abutment.

Fig 7-39 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.