surveying removable partial dentures

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Surveying removable partial dentures: the importance of guiding planes and path of insertion for stabilityO. L. Bezzon, DDS, PhD, a M. G. C. Mattos, DDS, a and R. F. Ribero, DDS b

Department of Dental Materials and Prosthodontics, School of Dentistry of Ribeir~o Preto, University of S~o Paulo, Silo Paulo, Brazil

Statement of problem. Although removable partial dentures are a favored option for the restoration of many situations that involve partial tooth loss, some patients are not satisfied with a removable partial denture, especially when it is not stable during mastication. A dental surveyor can be used to prevent countless problems related to the production of removable partial dentures. Many professionals working with oral rehabilitation fail to take advantage of the many uses of a surveyor in planning and designing chromium alloy and other metal removable partial denture frameworks. Purpose. This article uses an academic approach to describe the criteria used to determine the path and removal of a removable partial denture. A fundamental requirement for understanding the correct use of the dental surveyor is to prevent indiscriminate use of a path of insertion perpendicular to the occlusal plane, and extreme inclinations of the cast in the attempt to create undercuts on some teeth. (J Prosthet Dent 1997;78:412-18.)

When performing oral rehabilitation with rcmovable partial dentures (RPDs), the objective o f the dentist should be to make a prosthesis that the patient can easily seat and remove from the mouth and yet, when seated, the prosthesis will resist the dislodgment potential caused by masticatory function, especially mastication o f sticky foods. The problem involves an interaction o f engineering and biologic elements, so a biomechanical approach should be used for its solution. On this basis, it is imperative that diagnostic casts for all RPDs be analyzed with a dental parallelometer (surveyor), the instrument that permits accurate planning o f each structural detail o f the prosthesis.1 A diagnostic cast should be surveyed for three major reasons: (1) determination o f the path o f insertion to obtain efficient and esthetically pleasing retentive clasps; (2) tracing the survey line to enable positioning o f the rigid parts o f the prosthesis, so they will seat without interference; this procedure provides information about the need to recontour the abutments and other teeth and to improve the functioning o f rigid parts and the esthetic aspects related to it; and (3) analysis o f the con~Associate Professor. bAssistant Professor.412

tour o f soft tissues to prevent the occurrence o f lesions that result from seating the prosthesis. 2 The use o f a dental surveyor permits the dentist to plan, study, and design an RPD that will provide adequate retention, support, stability, and esthetic appearance. 3,4 In 1954, Applegatc s commented that the intelligent use o f the dental surveyor is the best way to prevent the occurrence o f countless problems frequently related to oral rehabilitation with RPDs. However, daily contact with professionals in our vicinity shows that there are many dentists who have no dental surveyor and are unaware o f the importance o f its use when they are making RPDs, because they believe the technician may be more experienced and they choose to delegate the responsibility to the technician. 6 The objective o f this study was to use an academic approach to remind dental practitioners o f the understanding of the dynamics involved in determining the path of insertion, a fundamental condition for the inclusion o f the dental surveyor as a tool o f routine use in planning and designing RPDs. PATH OF INSERTION AND REMOVAL

The RPD path o f insertion and withdrawal is the direction in which the prosthesis moves in relation to theVOLUME 78 NUMBER 4

THE JOURNAL OF PROSTHETIC DENTISTRY

BEZZON, MATTOS, AND RIBERO

THE JOURNAL OF PROSTHETIC DENTISTRY

Fig. 2. When functioning, RPD is pulled by sticky food in direction perpendicular to occlusal plane. This direction, defined as potential path of dislodgment (PPD), indicated by arrows in this schematic drawing, does not depend on direction of insertion selected by dentist. Fig. 1. Parallelometer (surveyor).

UNDERCUT

SURFACES

support system when it is seated in or removed from the mouth, guided by the contact of its rigid parts with the a b u t m e n t s / T h u s the contours of the teeth that contact the component parts of the RPD have a decisive influence on the determination of the ideal path of insertion and withdrawal. Because the paths of insertion and withdrawn involve equal movements but in opposite directions, they may be referred to only as the path of insertion. To determine the path of insertion for any RPD, the diagnostic cast should be positioned on the cast holder and analyzed in relation to the vertical rod (surveying stylus) of the parallelometer (surveyor) (Fig. 1). The vertical movement of the surveying stylus represents the path of insertion itself, which changes with each new inclination given to the diagnostic cast on the adjustable cast holder. The object of this process of analysis is to determine the ideal path of insertion. Because surveying is an individual process for each RPD, specific influencing factors must be analyzed and conditions developed by mouth preparation to accommodate the desired path of insertion. These factors include: (1) guiding planes, (2) tooth contours, in general, and more specifically undercuts on the teeth and soft tissues, (3) esthetic appearance, and (4) interferences. 1,3 GUIDING PLANES

The inclination given to the cast in relation to the surveying stylus (path of insertion determined for the prosthesis) should result in undercuts on the abutments measured gingivally from the survey line. The undercuts should be proportionally equal in depth and convergence angle on all abutments and should permit the retentive clasp tips to engage the undercut in the gingival third of the tooth. Abutment contours should also permit contact of the rigid components of the clasp arms in the opposite side of the middle third of the tooth to accommodate. 9 ESTHETIC APPEARANCE

Esthetic appearance can be considerably improved, especially with anterior abutments, by trying different paths of insertion. Small changes in the inclination of the cast can be made to seat the components of the prosthesis in less exposed regions that do not impair the patient's appearance, without jeopardizing the remaining determining factors. The objective should always be to obtain an appearance as natural as possible. 1 INTERFERENCES In the determination of the path of insertion, it is important to detect and resolve the presence of interference from structures other than teeth that will interfere with the placement of the prosthesis, such as exostoses, soft tissue, and undercut ridges. It is often possible to find a path of insertion that will prevent contact of the prosthesis with these anomalies when, for some reason, they cannot be removed surgically or corrected by other means. DETERMINATION INSERTION OF THE PATH OF

The flat axial surfaces in an occlusal-gingival direction on abutments are referred to as guiding planes. They should be prepared to be parallel to one another and to the path of insertion as determined by the surveying stylus. These surfaces very seldom occur naturally and need to be prepared directly on enamel or on cast or composite restorations. 8 During insertion, the rigid parts of the prosthesis contact with these surfaces, which direct the movement of the RPD until it is seated securely in the predetermined terminal position in the mouth.OCTOBER 1997

To determine the path of insertion, the diagnostic cast should first be positioned on the cast-holding table,413

THE JOURNAL OF PROSTHETIC DENTISTRY

BEZZON, MATTOS, A N D RIBERO

Pi

Pi

Pi

Pi

Fig. 3. Path of insertion (PI) perpendicular to occlusal plane

(dotted line) that coincides with potential PPD can be used for RPD if it provides satisfactory undercut surfaces (arrows)on abutments. These undercuts must be well-defined and of maximum depth required for type of clasp arm used.

Fig. 4. Cross-section of retentive clasp arms shown on facial surfaces of abutments and cross-section of reciprocal clasp arms shown on lingual surfaces of abutments. Horizontal marks on distal of abutments indicate position of neutral zone (survey line).

which in turn should be inclined in such a way that the occlusal plane of the cast will be parallel to the table of the surveyor. 4 The principle that governs the function of a dental surveyor (Fig. 1) requires that the surveying rod (stylus) be at a 90 degree angle (perpendicular) to the platform of the surveyor. A line or plane perpendicular to one plane is also perpendicular to any other plane(s) parallel to the first plane. Therefore positioning the cast with the ocdusal plane parallel to the platform of the surveyor establishes a perpendicular relation between the surveying rod and the occlusal plane, which would accommodate a path of insertion perpendicular to the occlusal plane. The selection of a path perpendicular to the occlusal plane is considered to be favorable because, against the advice of their dentist, many patients insert their RPDs with a biting force and because this orientation of the cast facilitates the mouth preparation necessary to make the prosthesis function as it is intended, once it is fabricated. 3 It is important to understand that a path of insertion perpendicular to the occlusal plane, also referred to as path of insertion of zero inclination, is an orientation given to the cast to start the survey analyses. The survey will be conducted in sequence by the analysis of the four factors that infl