Mechanical behavior of major connectors--part 2: influence of denture teeth height and loading direction.

STATEMENT OF THE PROBLEM: When force is applied on a denture the moments may develop at the denture major connector (MC) interface and thus enabling more framework deformation. PURPOSE: To determine the influence of crown height and loading directions on the stiffness of different MC designs when supported by an oral cavity simulating model. MATERIAL AND METHODS: Oral cavity models were used as support for different major connector's designs. A set of metal tooth casted crowns, 8, 10, and 12 mm in height, was prepared from molar plastic phantom teeth. The crowns were connected to premolar or molar regions of the major connectors and loaded vertically or in 45 to the occlusal plane. The stiffness of the system was measured. RESULTS: Changing tooth-crown height resulted in small changes in stiffness values when loaded vertically to the occlusal plane. Loading in oblique to the occlusal plane, the mean stiffness value decreased 30% when crown height increased from 8 mm to 12 mm. A reduction in stiffness values of 80-90% compared to vertical loading was found. CONCLUSION: Long denture crowns reduce the rigidity of the removable partial dentures. CLINICAL IMPLICATIONS: The removable partial denture should be supported by the maximal oral tissues. This stresses the importance of periodical examinations for any need of denture-base relining to maintain good support.

Mechanical behavior of major connectors--Part 1: Influence of supporting tissues.

STATEMENT OF THE PROBLEM: Mechanical properties of major connectors are crucial for the performance of prosthetics. PURPOSE: To determine the stiffness of systems of major connectors when supported by a model simulating the oral cavity. MATERIAL AND METHODS: Oral cavity models were made of plaster covered by resilient material. The stiffness of five maxillary and five mandibular major connectors were measured while loading vertically to the occlusal plane on two points, premolar or molar, directly on the frame work and when the major connectors were supported by the oral cavity model. RESULTS: The mean stiffness values of the major connectors loaded on the oral models were 35-folds and 60-folds higher compared to non-supported results for the premolar and molar points, respectively, for both jaws. With no support, stiffness values when the major connectors were loaded on molar point were lower compared to premolar point. When major connectors were supported by the oral models, the opposite occurred: the system showed higher stiffness when the major connectors were loaded on the molar point compared to premolar point. CONCLUSION: The influence of supporting tissues on the stiffness of major connector systems should be included in further studies. CLINICAL IMPLICATIONS: It is recommended to end the distal portion of the dentures at the second molar and to use the whole support provided by the oral tissues of the ridge.

The mandibular first premolar as an abutment for distal-extension removable partial dentures: a modified clasp assembly design.

Sound mandibular first premolars can be used as sole abutments for bilateral distal-extension removable partial dentures, if the denture is designed to minimise the torque applied to the abutment teeth. A simplified modification of the conventional torque-releasing clasp-assembly designs is suggested for these teeth. This modification entails a mesial rest on each abutment tooth connected to the distal proximal plate via a lingual bracing arm. A circumferential clasp arm is optional for buccal retention of the removable partial denture. As with conventional designs, the metal framework is designed to permit some rotational tissueward movement of the distal extension bases, yet not compromise the retention and stability of the prosthesis.

Stiffness of different designs and cross-sections of maxillary and mandibular major connectors of removable partial dentures.

STATEMENT OF PROBLEM: Major connectors of removable partial dentures must distribute forces bilaterally without damaging the supporting tissues. PURPOSE: This study investigated which design and cross-sectional shape of major connectors most favorably influence rigidity and flexibility. METHODS AND MATERIAL: Five designs for maxillary removable partial denture major connectors and 5 lingual bar major connectors of different cross-sectional forms were cast in chrome cobalt alloy on a master cast. Points M and P, which represented the position of the first premolar and second molar teeth, were positioned 20 mm apart on the casting. Vertical and horizontal forces were applied to each point while the opposite side was gripped in an Instron testing machine. A force-deflection curve was obtained for each loading point. Mean stiffness values were obtained for loading in compression and torsion. RESULTS: Values for torsional loading simulating vertical forces were lower when compared with values obtained for compression loading that simulated horizontal occlusal forces. Differences in stiffness were greater in mandibular major connectors loaded at M and P. The half pear-shaped cross section was the stiffest. CONCLUSIONS: In the maxillary arch, the most rigid major connector was the anteroposterior palatal bar combination placed on different horizontal and vertical planes. The most flexible was the U-shaped design. In the mandibular arch, the most important factor in achieving rigidity was the cross-sectional shape of the major connector. The half pear-shaped cross section proved to be the most rigid.

Further aspects of design for distal extension removable partial dentures based on the Kennedy classification.

A supplement to the Kennedy classification of partially edentulous arches for restoration with removable partial dentures has been suggested. Factors affecting denture design relating to the position of the abutment teeth, the symmetry of the edentulous distal extensions, the arch form and the cross-sectional shape of the residual ridges have been discussed and classified. An understanding of the movement of the denture bases in relation to the influence of these factors makes for a rational approach to removable partial denture design and the treatment of complications observed clinically in removable partial denture wearers.

Anterior implant-supported overdentures.

Retention of complete mandibular dentures can be successfully achieved by means of an implant-retained or natural tooth-retained bar and clip system in the anterior segment of the mandible. The same design principles hold true for both methods of anchoring the retentive bar. These retentive elements must be constructed to allow some freedom of movement around a fulcrum line designed to be perpendicular to the sagittal plane.

Prosthodontic treatment for the Applegate-Kennedy class V partially edentulous patient.

Concomitant loss of a canine and adjacent teeth usually occurs in the maxilla as a result of trauma in youngsters with well-preserved dentition or as an outcome of surgical excision of a localized pathosis in that area. It is postulated that in Applegate-Kennedy class V partially edentulous situations, the remaining anterior teeth in the maxilla should not be used as abutments for removable partial dentures. The stability of the prosthesis is maintained by counterresisting the occlusal forces exerted on the restored arch-curve by use of a posteriorly directed path of placement for the prosthesis.

Designing clasps for the asymmetric distal extension removable partial denture.

Retentive clasp components can be created to minimize torquing forces on abutment teeth incorporated in the support and retention of bilateral distal extension removable partial dentures. Functional movement of the combined tooth-mucosa-borne prosthesis about the axis of rotation necessitates consideration of retentive element design modifications, particularly as they relate to bilateral asymmetric distal extension situations.

Removable partial denture with a lateral rotational path of insertion.

A removable partial denture with a lateral path of insertion is useful when unsightly facial clasps are objectional to the patient. Rounded rest seat preparations allow extensions of the minor connector to rotate laterally into embrasure undercuts, providing retention. Guiding planes on the contralateral side ensure a different path of withdrawal from the retentive elements. A Kennedy Class IV arch with a long edentulous span is used to illustrate the denture design.

Use of roots to establish favorable removable partial denture design: case reports.

Judicious retention of roots can alter the position of the anterior fulcrum line and so simplify removable partial denture design. Complicated tooth-tissue-supported designs are thus transformed into much simpler tooth-supported removable partial overdentures.

Overcasting to repair a fractured porcelain fixed partial denture.

Long-span, permanently cemented, multiunit prostheses that suffer a fracture of one of the porcelain facings present difficult and often embarassing problems to the clinician. Silane coupling repair systems, although quick, simple, and inexpensive, do not ensure a long-lasting result. An alternative technique involving use of a porcelain-fused-to-metal overcasting and composite resin cement is presented. The overcasting is luted to the original metal casting.

Factors affecting displacement of free-end saddle removable partial dentures.

Tissueward movement of the free-end saddle of a removable partial denture during function takes place around the most distal fulcrum created by the most mesial rests. Displacement because of sticky foods takes place around the fulcrum created by the most distal rests. These and other factors affecting displacement of the free-end saddle removable partial dentures are discussed, and designs intended to increase retention are presented. The forces applied to the most anterior and posterior axes of rotation can be calculated mathematically to reveal the hazards of a too-short lever arm in combination with a strong retentive clasp system.

RLS--the lingually retained clasp assembly for distal extension removable partial dentures.

The design of clasp for a distal extension removable partial denture that helps preserve both the abutment teeth and the tissues of the edentulous ridge is described. The clasp assembly takes advantage of surveyed lingual undercuts to provide support, retention, and stability. It consists of a mesio-occlusal rest, a distolingual L-bar direct retainer that is located on the abutment tooth adjacent to the residual ridge, and a distobuccal stabilizer (RLS).

Rigidity of major connectors when subjected to bending and torsion forces.

Rigidity tests were performed on maxillary major connectors of different designs and mandibular major connectors of the lingual bar-type with different cross-sectional shapes and thicknesses. Each major connector was clamped on one side while the other side was subjected to bending, torsion, and bending under torsion forces. Deformation was measured with a micrometer microscope. The most rigid maxillary major connector was the design incorporating the anterior-posterior palatal bar. The most flexible was the U-shaped palatal bar. The full pear-shaped lingual bar (4 x 4 mm) was the most rigid of the major connectors. Among the more conventional lingual bars, the semielliptical bar (3.75 X 1.75 mm) was the most rigid, and the narrow semielliptical bar (4 x 1.5 mm) was the most flexible.

An analysis of rotational movement of asymmetrical distal-extension removable partial dentures.

An axis of rotation is created through the most distally placed occlusal rests when a distal-extension removable partial denture is loaded. If the residual ridges are of unequal lengths, this axis of rotation may not be perpendicular to the residual ridges. The resultant movement of asymmetric denture bases and their effect on the tissues are discussed. Movement of I-bar retainers located on the mesiobuccal aspect of asymmetrical abutment teeth may torque the abutment teeth as the denture base moves tissueward. Use of L-shaped direct retainers on the distobuccal surveyed undercut will create a more favorable class II lever effect on the abutment tooth.

A modified direct retainer design for distal-extension removable partial dentures.

The rationale for designing a direct retainer for a distal-extension removable partial denture is described. The advantages of an L-bar clasp arm over the I-bar clasp arm are discussed. The retentive surface of an abutment tooth is divided into three zones according to the clasp tip movement and the importance of placing the retentive tip in the zone of vertical movement is emphasized.