Acrylic resin jigs as an aid to parallel guiding plane preparation

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aPostgraduate prosthodontic student, Division of Restorative Dentistry and Periodontology. bSenior Lecturer, Consultant, Restorative Dentistry (Special Needs), Division of Restorative Dentistry and Periodontology. (J Prosthet Dent 2008;99:162-164) Guiding planes are defined as vertically parallel surfaces on abutment teeth and/or dental implant abutments orientated to contribute to the direction of the path of placement and removal of a removable dental prosthesis.1 The functions of guiding planes include2: (1) providing 1 path of placement/removal of a prosthesis, thereby eliminating excessive stress upon either the restoration or the abutment teeth; (2) ensuring the intended action of various reciprocating, stabilizing, and retentive components; (3) aiding with retention against dislodging forces other than those acting parallel to a given path of insertion and stabilizing against horizontally directed forces, and (4) eliminating troublesome food traps. Miller3 stated that flat surfaces eliminate horizontal forces upon the abutment tooth as the guiding plate or minor connector moves past it. This will not occur when the prosthesis contacts a convex surface. Such contact would tend to deflect the tooth within its alveolar housing, and over a period of time, this repeated trauma may be detrimental to the abutment tooth. The RPI system described by Krol4 attempts to manage the inevitable tissue-ward displacement of a distal extension base under occlusal loading. The distal guide plane is integral to its function. The guiding plane is positioned in the occlusal one third of the proximal surface. When occlusal pressure is exerted on the artificial teeth, the proximal plate moves tissue-ward without torquing the abutment tooth. When a retentive arm flexes over the height of contour, a horizontal force is placed on the abutment tooth.5,6 The length of the guiding plane required to balance this force has been shown to vary with the degree of undercut used,7 and its location should diametrically oppose the retentive element if it also has a reciprocating function. Ideally, the guiding plane should extend one third the buccolingual width of the tooth, and its vertical extension should be from the marginal ridge to the junction of the middle and gingival thirds.2 Different techniques have been described to facilitate the transfer of the orientation of the guiding planes from the surveying instrument intraorally. One technique involves preparing the surfaces and making an impression which is poured into quick setting plaster. The resultant cast can be verified for parallelism on the surveyor prior to making definitive impressions.2 An alternative method is to trial prepare the guiding planes on diagnostic casts and then highlight this area in colored pencil. This can then be brought chairside and used as an aid in tooth preparation.3 A bur set parallel to the path of insertion and fixed within a modelling plastic index fabricated on the surveyor is also of use in transferring this information.8 This article describes a method for accurately transferring guiding planes from a diagnostic cast to the mouth, thus ensuring relative parallelism for a given path of insertion. At the treatment planning stage, jigs are made on the diagnostic casts, and the selected path of insertion is indicated on them. These jigs are then used clinically to ensure the relative parallelism of guiding plane preparation for a selected path of insertion. In the patient scenario presented, guiding planes were prepared on the mesial surface of the mandibular right second molar, the distal surface of the mandibular right first premolar, and the distal surface of the mandibular left first premolar.