Surveying removable partial dentures

Surveying removable partial dentures: the importance of guiding planes and path of insertion for stability

O. 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.)

W h e n performing oral rehabilitation with rc- movable partial dentures (RPDs), the objective of 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 of sticky foods. The problem involves an interac- tion of engineering and biologic elements, so a biome- chanical 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 of each structural detail of the prosthesis.1

A diagnostic cast should be surveyed for three major reasons: (1) determination of the path of insertion to obtain efficient and esthetically pleasing retentive clasps; (2) tracing the survey line to enable positioning o f the rigid parts of 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 of rigid parts and the esthetic aspects related to it; and (3) analysis of the con-

~Associate Professor. bAssistant Professor.

tour of soft tissues to prevent the occurrence o f lesions that result from seating the prosthesis. 2 The use of 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 of the dental surveyor is the best way to prevent the occurrence of 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 un- aware 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 re- sponsibility to the technician. 6

The objective of this study was to use an academic approach to remind dental practitioners of the understanding of the dynamics involved in determining the path of insertion, a fundamental condition for the in- clusion of the dental surveyor as a tool of routine use in planning and designing RPDs.

P A T H O F I N S E R T I O N A N D R E M O V A L

The RPD path of insertion and withdrawal is the direction in which the prosthesis moves in relation to the

412 THE JOURNAL OF PROSTHETIC DENTISTRY VOLUME 78 NUMBER 4

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).

support system when it is seated in or removed from the mouth, guided by the contact of its rigid parts with the abutments /Thus 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 with- drawn 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 adjust- able 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

G U I D I N G P L A N E S

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.

U N D E R C U T S U R F A C E S

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 con- vergence 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

E S T H E T I C A P P E A R A N C E

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 remain- ing determining factors. The objective should always be to obtain an appearance as natural as possible. 1°

I N T E R F E R E N C E S

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 can- not be removed surgically or corrected by other means.

D E T E R M I N A T I O N O F T H E P A T H O F I N S E R T I O N

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

OCTOBER 1997 413

THE JOURNAL OF PROSTHETIC DENTISTRY BEZZON, MATTOS, AND RIBERO

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.

Pi Pi Pi Pi

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 oc- dusal plane parallel to the platform of the surveyor estab- lishes 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 fabri- cated. 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 influence the path of insertion mentioned earlier. Obviously, the path of insertion that provides the greatest advantage with respect to the influencing factors will be considered the ideal path of insertion for the situations in question.

Understanding the biomechanics associated with the insertion and withdrawal of the RPD is fundamentally important. Regardless of the path of insertion determined for any RPD, there is always a potential path of dislodgment (PPD) resulting from masticatory function that pulls the prosthesis in a direction perpendicular to the occhisal plane time when the patient completes each chewing stroke and begins the next one, after closing his mouth, opens it again in the continuation of the masticatory cycle (Fig. 2). Thus the path of insertion (PI) is of fundamental clinical importance because it predetermines the placement of clasp arms in retentive undercuts, which provides the retention and helps in stabilizing the prosthesis during function (Fig. 3).

Figure 4 represents a clasp design situation that will provide adequate retention and stability because the relatively long guide planes at this PI are parallel to each other and the facial undercuts for clasp retention are approximately equal to each other and are equidistant below the survey line. Understanding the potential paths of dislodgment is fundamental for the professional to design RPDs in which the PI may be slightly off of the zero inclination of the occlusal plane. This may allow a slight reduction in the amount of retentive undercuts required to prevent dislodgment of the RPD during normal mastication because properly designed rigid components (minor connectors) on the framework will bind slightly when they contact the prepared guiding planes on the abutments and prevent easy dislodgment. The inclination of the diagnostic cast grossly away from the

414 VOLUME 78 NUMBER 4


Pi Pi Pi "PI _ Pi Pi Pi

Fig. 5. Path of insertion (PI) perpendicular to occlusal plane reveals discrepant undercut surfaces (large undercut on left and none on right). Survey line on left abutment is about equidistant between occlusal surface and gingiva. Survey line on right abutment is near gingiva.

Fig. 6. When cast table is inclined to right in attempt to dis- tribute undercuts evenly, apparent undercut can be established on right abutment of cast shown in Figure 5. This requires extreme tilt and guiding planes have been ignored.

zero inclination to produce apparent undercuts will not be satisfactory.

Analysis of Figure 5 shows there is a considerable undercut on the facial surface of the abutment on the left side and total absence of an undercut on the abutment on the fight side. A framework designed at this zero inclination would not have adequate retention. Inclining the cast shown in Figure 5 to the right side (Figs. 6 and 7) should result in a better distribution of the undercut surfaces between the two abutments, creating a medium undercut surface on the facial surface of each abutment. Thus the retentive facial clasp arms could be placed in adequate retentive undercuts; however, it would com- pletely eliminate the guiding plane on the lingual surface of the abutment on the fight side and would place the guiding plane so near the occlusal surface of the abutment on the left side that it would be ineffective.

In the patient's mouth, the occlusal plane would again be in a horizontal position, and the dislodging force or sticky food could act in the direction indicated by the PPD illustrated in Figure 8. During function, the dislodging force could cause the RPD to rotate out of place, starting with the clasp on the right abutment, because there would be no real undercut on that abutment to resist displacement. This condition is frequently identi- fied as a problem for many RPD wearers. In this in- stance, the error was the determination for the PI to be based solely on the retention factor, which, although important, should not be considered in an isolated manner because the guiding planes must also be considered.

P Pi Pi Pi

Fig. 7. Cross-section position of retentive clasp arms on facial surfaces of abutments and reciprocal clasp arms on lingual surfaces. Same cast illustrated in Figures 5 and 6.

Clinically, the situation should be resolved by chang- ing the facial contour of the tooth on the right side to create an undercut necessary to facilitate proper positioning o f the clasp arms and the correct functioning of the prosthesis. The surface could be changed by making a surveyed crown; reshaping the enamel, provided it is thick enough; or placing a suitable restoration.

OCTOBER 1997 415


/

Pi Pi Pi [Pi

/

/

~ Pi

Fig. 8. When RPD illustrated in Figure 7 is acted on by dislodging force, PPD pull illustrated will be perpendicular to original position of occlusal plane not in direction of PI; therefore, there will be no retentive effect by clasp on right side and partial will be easily rolled off abutments. RPD will loosen with every masticatory stroke.

/

Pi / Pi Pi Pi

Fig. 9. Different cast than in Figures 5 through 8 with facial surfaces same as those abutments and different inclinations of lingual surfaces.

Figure 9 depicts another situation in which the initial analysis of the cast in the zero inclination position had undercuts on the facial surfaces of the abutments that were similar to the cast in Figure 5. However, the lingual walls did not have the desired relation to the occlusal plane. I f this cast were tilted to the right side (Fig.

Fig. 10. Inclination of cast holder for distribution of undercut surfaces of the abutments on cast shown in Figure 9 and detection of guide planes on lingual surfaces of right and left abutments (arrows).

I

Fig. 11. Drawing of cast shown in Figure 10 shows position of cross-sections of retentive (facial surface) and reciprocal (lingual surface) clasp arms.

10) to parallel the guiding planes with the PI, these guiding planes would direct the movement of insertion and removal of the prosthesis more favorably. The positions of the retentive and reciprocating clasp arms are shown in Figure 11. Thus, if dislodgment occurs along the PPD during function (Fig. 12), the lingual guide plane on

416 VOLUME 78 NUMBER4


the right side, aided by the one on the left side, should bind against the abutment to resist the dislodgment o f the prosthesis from the support system if the direction of the dislodgment is perpendicular to the occlusal plane. This should provide the stability necessary for efficient function.

Obviously, the inclined PI described for the theoreti- cal case in question would still depend on the analysis of esthetic factors such as the occlusal positioning o f the retention clasp on the right side and on the detection of possible anatomic interferences with the correct placement o f the prosthesis. In any situation, it is necessary to understand that the distribution o f the degree of retention is not the only definitive condition for the determination of the PI.

D I S C U S S I O N

Despite the significant evolution of materials and pro- cedures for oral rehabilitation, cobalt-chromium alloys continue to be the alloy of choice for RPDs. The option to restore with an RPD permits the resolution o f com- plex clinical problems, including extensive modification spaces, distal extensions, and so forth, and to do so in a relatively rapid manner and at an operational cost sub- stantially lower than that o f other treatment modalities. However, failures are observed with a number of patients who are dissatisfied with this type o f dental prosthesis.

A critical analysis has revealed that dissatisfaction could be avoided in many instances if the prostheses were made in such a way as to satisfy the basic requirement o f mini- mal dislodgment during mastication or speech, with dissatisfaction about esthetic appearance being relegated to a second plane.

Many RPDs are made without essential elements for correct functioning, with the excuse of producing a better esthetic appearance. Often these prostheses are esthetic but lack stability. Furthermore, unstable prostheses may significantly impair the prognosis o f treatment because o f the irritation they provoke in the support system.

The fabrication o f an RPD as a viable treatment mo- dality for oral rehabilitation must be based on mastering the use o f the dental surveyor, starting with the understanding o f the aspects involved in the dynamics of insertion and withdrawal o f the prosthesis. A correct analysis o f the factors that influence the PI prevents the occurrence of two frequent errors in the construction of these prostheses: (1) the indiscriminate use of the PI with zero inclination, and (2) excessive inclinations of the cast on the assumption that undercut surfaces can be "cre- ated" by tilting the cast.

As a general rule, the PI with zero inclination should be the starting point for the surveying process. This path, which coincides with or slightly deviates from the PPD, may be ideal when it provides a favorable approach to

/

Pi IPi Pi Pi

Fig. 12. When prosthesis illustrated in Figure 11 undergoes action of PPD in direction perpendicular to occlusal plane, guide planes that guide seating of RPD along PI prevent its dislodgment during function by providing necessary retention and stability.

the four factors that influence the PI. The PI inclined in relation to the occlusal plane must necessarily be determined as a function of guiding planes that will impose such inclination on the prosthesis, thus reducing the PPD.

When the ideal PI is determined for a particular cast, the rational use of the surveyor emphasizes the pecu- liarities o f the contours of the abutments and of the re- maining support regions, suggesting the necessary alterations to be made in the contours of the abutments for the fabrication of the prosthesis.

In sequence, fully mastering the activities of surveying and designing the framework, as well as the planning and execution o f the alterations in the support system, results in a rapid and easy, but also sophisticated, construction of RPDs. This will result in malting esthetically pleasing and stable RPDs that will significantly contribute to the health of the stomatognathic system.

S U M M A R Y

1. A PI perpendicular to the occlusal plane (path with zero inclination) should be the starting point for the analyzing process.

2. A PI with zero inclination coincides with a PPD. 3. The ideal PI should provide a favorable approach

to the four influencing factors of the PI. 4. A PI inclined in relation to the occlusal plane must be

directed toward guiding planes that will reduce the PPD.

OCTOBER 1997 417


REFERENCES

1. Miller EL, Grasso JE. Removable partial prosthodontics. 2nd ed. Baltimore: Williams & Wilkins; 1981. p. 103-17.

2. Weinberg LA. Atlas of removable partial denture prosthodontics. St Louis: CV Mosby; 1969. p. 81-4.

3. Lavere AM, Freda AL. A simplified procedure for survey and design of diagnostic casts. J Prosthet Dent 1977;37:680-3.

4. Coy RE, Arnold PD. Survey and design of diagnostic casts for removable partial dentures. J Prosthet Dent 1974;32:103-6.

5. Applegate OC. Essentials of removable partial denture prosthesis. Phila- delphia: WB Saunders; 1954. p. 17-35.

6. Leeper SH. Dentist and laboratory: a "love-hate" relationship. Dent C[in North Am 1979;23:87-99.

7. Wagner AG, Forgue EG. A study of four methods of recording the path of insertion of removable partial dentures. J Prosthet Dent 1976;35:267-72.

8. Boitel RH. The parallelometer. A precision instrument for the prosthetic laboratory. J Prosthet Dent 1962;12:732-6.

9. Stern WJ. Guiding planes in clasp reciprocations and retention. J Prosthet Dent 1975;34:408-14.

10. Applegate OC. Use of the paralleling surveyor in modern partial denture construction. J Am Dent Assoc 1940;27:1397-407.

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Copyright © 1997 by The Editorial Council of The Journal of Prosthetic Den- tistry.

0022-3913/97/$5.00 + O. 10/1/84588

Factors influencing perceived treatment need and the dental attendance patterns o f older adults Tickle M, Worthington HV. Br DentJ 1997,'182:96-100.

Purpose. As more older individuals maintain their dentition longer, the maintenance of these dentitions will make up a larger part of dental practice in the future. The aim of this study was to identify the variables that influence perceived treatment needs of these older individuals and pre- dictive variables for reported dental attendance. Subjects and Materials. A cross-sectional study that used a self-reporting, posted questionnaire was used to sample two cohorts o f older individuals living in two economically different electoral wards (one deprived and one affluent) of Liverpool, England. A random sample of 250 individuals aged 60 to 65 years were drawn from each electoral ward and used for the posted sample questionnaire; 500 questionnaires were distributed. The instrument used was a Subjective Oral Health Status Indicators (SOHSI). This instrument measured the impact of oral conditions in three broad areas: (A) impaired function; (B) experiential effects of oral conditions, and (C) social and psycho- logic effects of oral conditions. In addition, individuals were asked when they last saw a dental practitioner as well as "Do you think that you need dental treatment now?" Data were collected and statistically analyzed with a two-tailed test of significance. Results. Of the 500 questionnaires mailed 342 were returned (68.4%). There was a highly significant correlation between living in a deprived area and reported poor dental health. Recent pain experiences and concern for oral health and appearance were significant predictors for perceived treatment need. The main factor that predicted a subjective need for treatment was a reported history of regular, asymptomatic recall dental visits. These individuals were six times more likely to have a perceived treatment nccd than a poor dental attendee. The edentulous patients in both groups were less likely to be regular and asymptomatic attendees and they are less likely to perceive the need for regular treatment. 22 references. - - R P Renner

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Surveying removable partial dentures