Co Cr RBD / the path of insertion, block out and relief
2 likes2,230 views
Cobalt-chromium alloys have become popular materials for removable and fixed dental prosthetics since the 1980s due to their strength, corrosion resistance, and lower cost compared to noble metals. Co-Cr alloys have excellent biocompatibility and provide strength and rigidity while reducing the weight of dental restorations. When fabricating Co-Cr removable partial dentures, it is important to establish a path of insertion that avoids interference and maximizes esthetics and retention. The path is determined by factors like the location of connectors and retentive/guiding surfaces, and the master cast is blocked out accordingly to define the path and prevent interference during seating.
Co Cr RBD / the path of insertion, block out and relief
- 2. 1 Introduction The application of predominantly base-metal alloys in removable and fixed prosthodontics has become more popular since the 1980s, due to the increasing cost of noble metals, especially after the global financial crisis of 2008. Cobalt- Chromium (Co-Cr) alloys are among the bestknown base metal alloys in dentistry with various and successful clinical applications. Co-Cr alloys can be generally described as alloys that have high strength, are heat-resistant and non- magnetic, and have favorable resistance to wear, corrosion, and tarnish. 1 They possess excellent biocompatibility1,2 and corrosion and tarnish resistance3,4 while the high modulus of elasticity (E) provides the requisite strength and rigidity without the need for heavy cross-sections, thus reducing the weight of metal substructures. Currently, biomedical applications of Co-Cr alloys are mainly related to the fabrication of orthopedic prostheses for knee, shoulder, and hip replacement as well as for use as fixation devices for fractured bones (joint endoprostheses).5 The first known dental application of Co-Cr alloys (along with Ni-Cr alloys) was in the 1930s, for the fabrication of removable partial denture (RPD) frameworks. Since then, both Co-Cr and Ni-Cr base- metal alloys have become increasingly popular compared with conventional Type IV gold alloys, which were predominant metals previously used for RPD framework fabrication.5 In addition to the excellent mechanical and stain- resistant properties of Co-Cr alloys, they have almost half the density of Au- based alloys, and thus the weight of fabricated dental restorations and frameworks becomes significantly lighter. Path of Insertion It is defined as “the direction in which a prosthesis is placed upon and removed from the abutment teeth, The path of insertion or path of withdrawal is the angle made by the direction of the removable partial denture with the remaining teeth
- 3. 2 during insertion. A simple method to establish an ideal path of insertion is to alter the tilt/angulation of the cast on the surveyor. Changing the tilt will change the guiding plane and the amount of mouth opening needed to seat a denture. Exaggerated tilts (beyond 100 to the horizontal plane) should be avoided to prevent excessive mouth opening during insertion.6 (Fig.1) Clinical Considerations ● Multiple paths of insertion are possible in a class I case. A single path of insertion is obtained by preparing additional guiding planes on the lingual surfaces of remaining teeth. Since the denture is constructed such that it is forced to contact all these guiding planes during insertion it reduces the occurrence of multiple paths of insertion (Fig.2) ● A class II case with modification 1 will have a single path of insertion. This is because the modification space has two guiding planes and the two sides of the arch are connected by a rigid major connector. The side having the modification space governs the path of insertion (Fig.3). ● A single path of insertion is possible in a class III case wherein the edentulous space is bound by teeth anteriorly and posteriorly. The guiding planes formed on the proximal surfaces (mesial and distal) of the abutment teeth, control the path of insertion (Fig.4). ● A class IV case will also usually have a single path of insertion. Fig.1 Fig.2 Fig.3 Fig.4
- 4. 3 Factors That Determine Path of Placement And Removal • Location of vertical minor connector • Retentive areas • Interference • Aesthetics • Guiding planes. • Denture base.7 Retentive Areas Retentive areas must exist for a given path of placement and must be contacted by retentive clasp arms, which are forced to flex over a convex surface during placement and removal. For a clasp to be retentive; its path of escapement must be other than parallel to the path of removal of the denture itself; otherwise, it would not be forced to flex and thereby generate the resistance known as retention. Clasp retention therefore depends on the existence of a definite path of placement and removal.(Fig.5,6) Guiding Planes Guiding planes are parallel surfaces of abutment teeth that direct the insertion and removal of a partial denture. The path of insertion should be parallel to the guiding planes. Proximal tooth surfaces that bear a parallel relationship to one another must either be found or be created to act as guiding planes (Fig.7,8). Fig.5 Fig.6 Fig.8Fig.7 3
- 5. 4 As a result of this contacting between the guiding planes and the component of partial denture a guiding planes act as: 1- Guide the prosthesis in and out of the mouth without strain on the teeth contacted or on the denture itself and without damage to the underlying soft tissue (Assure definite path of insertion). 2- The frictional forces of contact of the prosthesis with the guiding plane wall will contribute to the retention of removable partial denture. 3- Stabilize the prosthesis against lateral forces. 4- Protect weakened teeth. 5- Insure positive clasp action. The components of the denture that contact the guiding planes during placement of removable partial denture are :
- 6. 5 Interference The prosthesis must be designed so that it may be placed and removed without encountering tooth or soft tissue interference. If the interference cannot be avoided by changing the path of insertion, so the interference must be eliminated during mouth preparation (by surgery, extraction, modifying interfering tooth surfaces, or altering tooth contours by restorations) or on master cast by a reasonable amount of block-out. Esthetics To obtain optimum esthetics in removable partial denture therapy: 1- Metal component must be concealed. Less metal will be displayed (most esthetic location of clasps) if the retentive clasp is placed at a more distogingival area of tooth surface made possible either by changing the path of placement or by the contour of the restorations. 2- Esthetics also may dictate the choice of path selected when missing anterior teeth must be properly positioned in the partial denture. In such will have to be modified excessively (Fig.9). A- The first path of insertion (zero tilt), the less esthetic removable partial denture. B- The most esthetic path of insertion, the most esthetic removable partial denture. A B Fig.9
- 7. 6 Blocking out the master cast for CO-CR RPD After the path of placement and the location of undercut areas have been established on the master cast, any undercut areas that will be crossed by rigid parts of the denture (which is every part of the denture framework but the retentive clasp terminals) must be eliminated by blockout. In the broader sense of the term, blockout includes not only the areas crossed by the denture framework during seating and removal but also (1) those areas not involved that are blocked out for convenience; (2) ledges on which clasp patterns are to be placed; (3) relief beneath connectors to avoid tissue impingement; and (4) relief to provide for attachment of the denture base to the framework.7 Type of blockout 1- Parallel blockout Parallel blockout is necessary for areas that are cervical to guiding plane surfaces and over all undercut areas that will be crossed by major or minor connectors. Hard inlay wax may be used as a blockout material. It is easily applied and is easily trimmed with the surveyor wax trimmer. Trimming is facilitated by slightly warming the surveyor blade with an alcohol torch (Fig.10). Site of Parallel blockout : 1. Proximal tooth surfaces to be used as guiding planes. 2. Beneath all minor connectors. 3. Tissue undercuts to be crossed by rigid connectors. 4. Tissue undercuts to be crossed by origin of bar clasps. 5. Beneath bar clasp arms to gingival crevice. Fig.10
- 8. 7 2- Shaped blockout For locating clasp patterns may or may not be used. this should not be confused with the actual blocking out of undercut areas that would offer interference to the placement of the denture framework (Fig.11,12). Site of shaped blockout : ● On buccal and lingual surfaces, to locate the wax patterns for clasp arms. Wax ledge for reciprocal clasp arm as cervical as possible also ledge for location of retentive clasp arm, ledge is applied below the survey line around the abutment teeth. Wax ledges on buccal surfaces of premolar and molar abutments have been duplicated in refractory the cast for exact placement of clasp molar pattern and the premolar wrought wire clasp. 3- Arbitrary blockout Such areas are the labial surfaces and labial undercuts not involved in the denture design and the sublingual and distolingual areas beyond the limits of the denture design. These are blocked out arbitrarily with hard baseplate wax, but because they have no relation to the path of placement, they do not require the use of the surveyor. Arbitrary block out is done to: 1. Prevent distortion of duplicating mold when the master cast is removed. 2. Facilitate removal of refractory cast from impression during duplication. Fig.12Fig.11
- 9. 8 Site of arbitrary blockout 1. All gingival crevices. 2. Gross tissue undercuts situated below areas involved in design of framework. 3. Tissue undercuts distal to cast framework. 4. Labial and buccal tooth and tissue undercuts not involved in denture design. Relieving the master cast for CO-CR RPD Removable partial denture framework usually displays intimate contact with the teeth and soft tissues. However, there are certain areas where contact is not desirable. In these areas, framework is shaped to stand away from the underlying tissues; this is accomplished by relief which is the procedure of placing wax in certain areas on the master cast before duplication, to create a raised area on the refractory cast.7 (Fig.13) Purpose of relieving 1. To prevent tissue impingement resulting from rotation of the denture framework. 2. To prevent abrasion of the cast. 3. To create space for the acrylic resin (beneath the retentive ladder). Sites of relieving 1. Beneath lingual bar connectors or the bar portion of the linguoplates. 2. Areas in which major connectors will contact thin tissue, such as hard areas so frequently found on lingual or mandibular ridges and elevated palatal raphes. 3. Beneath the ladder minor connectors for attachment of resin bases.
- 10. 9 References 1. Evans EJ, Thomas IT. The in vitro toxicity of cobalt-chromemolybdenum alloy and its constituent metals. Biomaterials 1986;7:25-9. 2. Craig RG, Hanks CT. Reaction of fibroblasts to various dental casting alloys. J Oral Pathol 1988;17:341-7. 3. Viennot S, Dalard F, Lissac M, Grosgogeat B. Corrosion resistance of cobalt- chromium and palladium-silver alloys used in fixed prosthetic restorations. Eur J Oral Sci 2005;113:90-5. 4. Mueller H. Tarnish and corrosion of dental alloys. In Davis J, eds. Corrosion. Materials Park OH. ASM International, 1987, p. 1351-2. 5. Marti A. Cobalt-base alloys used in bone surgery. Injury 2000;31:18-21. 6. Deepak Nallaswamy Veeraiyan, Karthikeyan Ramalingam, Vinaya Bhat. Textbook of Prosthodontics. First Edition, Replika Press Pvt Ltd., 310 EPIP, HSIDC, Kundli, Sonepat (Haryana),2003. Chapter 18 p.321-326. 7. Alan B. Carr, David T. Brown. McCracken’s REMOVABLE PARTIAL PROSTHODONTICS. Thirteenth Edition. 3251 Riverport Lane St. Louis, Missouri 63043.2016.p.141-144. Relief prevents tissue impingement (red arrow) resulting from rotation of the denture framework (blue arrows) Fig.13