J. Dent. 1992;

59

20: 59-64

Value of guide planes in partial denture retention I. Ahmad and N. E. Waters Departments of Dental Prosthetics and Dental Materials Science, United Medical and Dental Schools, Guy’s and St Thomas’s

Hospitals,

London,

UK

ABSTRACT The retention of five nominally identical partial denture castings fitted to a master model was measured on an Instron testing machine in the direction of the guide planes and also at angles inclined to them. It was found that: the maximum retention was considerably reduced when the clasps were removed; the guide planes offered some frictional resistance to displacement, the magnitude being critically dependent on the tit; for frameworks with or without clasps a significant increase (P < 0.01) in retention occurred when the pull was at 22” but not at 12” to the guide planes, and that there was no statistically significant difference between the retention in the forward and backward directions for either a 12” or a 22“ tilt of the dislodging force. KEY WORDS: J. Dent. 1992;

Partial dentures, Guide planes, Retention 20: 59-64

(Received 9 May 1991;

reviewed 2 July 1991;

accepted 1 1 September

1991)

Correspondence should be addressed too: Professor N. E. Waters, Department of Dental Materials, UMDS, Guy’s Hospital Medical School, London SE1 9RT. UK.

INTRODUCTION Guide

planes

have been described

(Hendersonetal.,

1985;

Academy of Denture Prosthetics, 1987) as two or more vertical parallel surfaces of abutment teeth oriented so as to direct the path of placement and dislodgement of a removable partial denture. This description seems to suggest that the path of insertion of a removable partial denture should be at right-angles to the occlusal surface alignment. As Nairn (1974) and Walter (1980) rightly point out, the greatest disadvantage of a vertical path of insertion and removal is that this coincides with the path of displacement of the denture when the wearer tackles sticky foods. The latter author goes on to state that the retention of a denture depends entirely on clasps when there is a vertical path of insertion. Both authors agree that if guide planes can be selected or created to give a path of insertion which is at variance with the path of maximum displacement, then other denture components, such as flanges, base or minor connectors, can aid in retention of the denture against vertical movement away from the supporting tissue. Although these ideas appear to have a sound basis in mechanics they do not seem to have been systematically examined in the literature. o (1992) Butterworth-Heinemann 0300-5712/92/010059-06

Ltd.

Other authors (e.g. Brudvick and Morris, 1981; Stewart et al., 1983) have suggested the use of guide planes to provide frictional resistance to contribute to the retention of a partial denture, but again no direct experimental evidence has been given in the literature to support this proposal. Guide planes are usually sought or created on the proximal surfaces of the abutment teeth facing the edentulous ridge occupied by the denture saddles (Henderson et al., 1985; Kratochvil, 1988). It is generally accepted that guide planes should occupy about two thirds of the width between the buccal and lingual cusps. It is also recommended that more effective use of these surfaces in controlling the path of insertion of a denture can be achieved by selecting sites in widely separated positions in the dental arch (Henderson et al., 1985). Smith (1947) Hindels (1957) and Zoeller (1969) have suggested that the formation of lingual guide planes could assist in the reciprocation of the non-vertically exerted forces on the abutment teeth. Lingual preparation of guide planes has also been recommended by Stewart and Rudd (1968) in the special circumstances of stabilizing periodontally weakened teeth. Other benefits suggested were improved gingival

60

J. Dent. 1992;

20: No. 1

health, and avoiding proliferation of the gingival cuff, a feature associated with the use of full length guide planes which allow a properly supported denture base to make contact with the mucosa (Kratochvil, 1971). A number of authorities have stated that a more pleasing appearance may be created by the presence of long guiding planes on the proximal tooth surface of an anterior bounded saddle (La Vera and Freda, 1977; Zarb and Mackay, 1981; Neil1 and Walter, 1983). According to Preiskel (1984) there is also less wear on the internal surface of precision attachments during insertion of the denture when lingual guide planes are used in association with the bracing arms. However, although as noted above various authors have referred to the possible frictional resistance which guide planes may provide when the dislodging forces act on a denture, no laboratory studies of this effect could be found in the literature. The aim of this study, therefore, was to investigate the frictional resistance provided by the guide planes and the effect on the retention when guide planes were selected at an angle different from the normal path of displacement of the partial denture framework.

METHODS

AND MATERIALS

A model of a partially dentate upper dental arch was used from which first molars and second premolars were missing bilaterally. Chromium-cobalt crowns were constructed for the first premolars and second molars. The crown forms were waxed and trimmed on the surveyor to provide the appropriate mesial or distal guide planes on the abutment teeth. The cast crowns were then trimmed, polished, and cemented onto the cast. The guide planes were checked on the surveyor and slight inaccuracies were adjusted by using a milling machine (Metaux Precieux S.S., Neuchatel, Switzerland). Rest seats were on the mesial side of the second molar crowns and on the distal side of the first premolar crowns.

Fig. 7. Experimental specimen prepared in wax.

The crowns were then surveyed using a Nesor surveyor (Nesor Products Ltd, London, UK) and 0.25 mm undercut areas marked on the disto-buccal aspect of the second molar crowns and the mesio-buccal aspect of first premolar crowns. Wax clasp shelves were then positioned using undercut wax (Krupps Widia, Dental Technik, Essen, Germany). The master model was duplicated in agar-agar duplicating material and five refractory casts were then poured using Econo-vest (Chaperlin and Jacobs, Sutton, UK). Each refractory cast was dried in an oven at a temperature of 200°C for 1 h and then dipped into a model hardening compound (Davis-Schottlander-Davis Ltd, London, UK) for 3 s, drained and redipped for a further 3 s and drained. Each denture base pattern was prepared in wax using 0.5 mm casting sheet wax (Krupps Widia, Dental Technik, Essen, Germany). Standardized molar and premolar preformed wax circumferential clasps (Dentaurum, Pforzheim, Germany) were then added to the waxed frameworks (Fig I). Every effort was made to attach the wax clasp patterns to the main framework wax pattern in as similar a way as possible. Round wax (18 gauge) was used to make loops of equal diameters which were placed on top of each occlusal rest. These loops were used as points of application for pulling Vertical Anterior

Posterior 22’

Fig. 2. Diagram to illustrate the different angulations in which retention was measured relative to the vertical (i.e., to the guide planes). (n.b. The connecting ring on the righthand saddle area has been drawn off-centre to enable the multistrand wire cable connections to be seen clearly.)

Ahmad

the frameworks in the experiments. Care was taken to ensure the loops were attached at the same point of every framework. The partial denture frameworks were then sprued using cylindrical sprue wax of 5 mm diameter and attached to a blue casting cylinder preformer. Each framework was carefully divested, sand blasted and trimmed before being fitted to the master model with minimal alteration of the fitting surface. A crossbar assembly was constructed with loops attached (see Fig. 2) which could be secured to any of the castings with the help of cold-cured acrylic resin. The loop on the crossbar assembly allowed the pulling mechanisms to be attached to the partial denture frameworks (Fig. 2). This twin wire system of total length 14 cm was constructed of multistrand wire cable together with swivel attachments of the type used for fishing lines. The retention of the frameworks was measured on an Instron universal testing machine (Instron Corporation, Canton, MA USA). A sheet of poly(methylmethacrylate) (Perspex, ICI plc, London, UK) 26 cm long, 19 cm wide and 16 mm thick formed the base to which the master model was clamped. This was provided with four holes to enable it to be rigidly bolted to the crosshead platform of the Instron machine. Four slots 5 cm long were cut from each hole so that the base could be moved and fixed at any point within this distance. The base was bolted in the centre to the crosshead of the Instron. Aplumbline was used to centre the master model on the sheet so that the partial denture framework was approximately in the centre. The master model was then fixed at this position with screws and additional Perspex strips to the main Perspex base. Thus the master model could be removed and replaced in the same position on the base. Each partial denture framework was first positioned such that its centre was located vertically below the centre of the load cell and was thus subject to a vertical dislodging force. The framework was then moved first 3 cm and then 5 cm away from this central point in both an anterior and a posterior direction. These shifts were made possible by the slots in the Perspex base. For each of these positions of the base, live readings of the maximum retentive force were recorded. The angle (0) at which the pull was made relative to the vertical was calculated from: 0 = sin ml(d/h)

and Waters:

Partial denture

RESULTS The results obtained for the retention to the master model of the frameworks with all clasps present for a vertical pull and then for the pull at an angle of 12’ and 22’ relative to the vertical in both anterior and posterior directions are given in Tables I and III. The retention for the same two angulations of the dislodging force for the frameworks with all the clasps removed is shown in Tables V and VII respectively. It should be noted that repeat measurements on a particular framework were consistent, and no trend due to wear or changes in surface roughness was detected. Two-way analyses of variance were carried out on each set of results (Tables II and IV for the retention with all clasps present and Tables VI and VIII for the retention with the clasps removed). All four analyses showed that there was a highly significant difference between the retention of the nominally identical frameworks (P < 0.001). With all the clasps present, no statistically significant difference was found between the vertical pull along the guide planes (mean 11.22 N), at 12” anteriorly (mean 11.90N), or at 12” posteriorly to the vertical (mean 12.61 N). However, a statistically significant difference (P < 0.01) was observed between the retention with the pull along the guide planes (mean 11.22 N) and that obtained at 22” anteriorly (mean 12.03 N) and at 22” posteriorly to the guide planes (mean 13.18 N). When all the clasps were removed from the partial denture frameworks the maximum retention was considerably reduced and there was a greater disparity between the retention of the individual frameworks. With no clasps, the maximum retention was 7.06 N and the minimum amount found was 1.76 N. The mean retentions along the guide planes and at 12” anterior and posterior to them, were found to be not

Retention Framework

force (N)

I2 ’ anterior

1

10.68

* 0.130

11.72

z 4 5

11.16 10.22 13.26 10.80

? 0.245 Y!C 0.167 + 0.195 of- 0.7 18

10.22 11 .OO + 0.367 0.249 14.96 + 0.182 1 1.60 + 0.418

All results presented

as mean f s.d.

61

of the denture from the central where d is the displacement position and h is the length of the pulling wire. The length of the pulling string was kept constant at 14 cm and the angle of pull was calculated for the displacements of 3 cm and 5 cm to be 12” and 22” respectively (Fig. 2). After these determinations all the clasps were removed from the partial denture frameworks and the retention of the frameworks measured again.

Tab/e 1. Effect of angle of pull (12 “) on retention with all clasps present Vertical pull (along guide planes)

retention

+ 0.035

12 ’ posterior 13.24+0.114 12.14 11.52 f+ 0.313 0.901 15.14*0.397 1 1.02 f 0.460

62

J.

Dent. 1992; 20: No. 1

Table II. Analysis of variance for the pull either vertical or 12’ anterior or posterior to this direction for frameworks with all clasps present Source

Angles Framework Interaction Error

d.f.

: 2 60

S.S.

ms.

F

Significance

145.990 9.493 1 1.732 10.512

36.498 4.747 1.467 0.175

14.39 3.20 8.43

P

Value of guide planes in partial denture retention.

The retention of five nominally identical partial denture casting fitted to a master model was measured on an Instron testing machine in the direction...
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