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20 Dental Tribune Middle East & Africa Edition | May-June 2015Implant tribune < Page 19 > Page 21 The luting screw is discarded. The Application The Passive Abutment is intend- ed for the fabrication of implant- supported SCREW RETAINED CASTINGS (e.g. crowns, bridg- es, mezzo-structures, cast bars, custom posts) on one or more implants where excellent pros- thetic fit is desired. The use of a burnout plastic cylinder allows freedom of choice in choosing the casting alloy. The complex- ity of laboratory procedures is greatly reduced when compared to complex casting procedures with gold cylinders. The Passive Abutment System is available for direct connection to all Southern Implants product ranges. Passive Abutments are also available for Compact Conical Abutments. For direct connection to Exter- nally Hexed, IT, Tri-Nex, Deep Cone and Internally Hexed con- nection Implants, both non-engaging and engaging versions are avail- able: 1. Non-hexed or non-engaging versions are indicated for multi implant case (bridges). The non-hexed (non-engaging) interfacial component has an in- ternal taper fit to allow for non- parallelism of implants of up to 14 degrees per abutment i.e. 28 degrees between 2 implants. 2. Hexed or engaging versions are indicated for single implant cases and multi-unit custom abutments cases. Problems of Conventional Cast Structures Frameworks incorporating cast to gold cylinders are very commonly used in implant re- construction, as are castings fabricated using plastic burn- out cylinders. These castings however are subject to signifi- cant difficulties. Significant de- terioration of the fitting surface of the cast structure occurs as a result of laboratory procedures i.e. (Fig. 3) - sandblasting of the casting - the casting is subjected to re- peated high temperature cycles during casting and porcelain fir- ing procedures. This results in oxidation of the fitting surfaces and further deterioration of fit. - the gold fitting surface is dete- riorated by multiple “fittings” on the model, especially if the ana- logues are not kept clean. The larger and more complex the casting, the greater the like- ly degree of discrepancy of fit. Hence, larger castings with fit discrepancies are often cut and soldered, or laser-welded. It is commonly reported that these attempts to improve the fit result in even greater fitting problems and may be amplified by porce- lain firing. Clinical implications of misfit- ting implant structures Discrepancies in fit are ex- tremely difficult to detect clini- cally, if not impossible where the interface in-between implant and superstructure is subgingi- val. Vertical misfits will only be detected on x-ray, if the misfit occurs interproximally and the x-ray beam is oriented perpen- dicular to the interface. If the discrepancy is in the buc- co-lingual plane, it will not be detected on x-ray. Even gross discrepancies may be missed where x-ray tech- niques are not optimal (Fig. 4). Most importantly, poorly fitting prostheses can result in: -bacterial accumulation at the prosthetic/implant interface, which will result in bone loss around the implants (Fig.5) -mechanical strain being ap- plied to the implant, which may result in bone loss -poor preload of retaining screws and thus more frequent screw loosening -fatigue loading of the retaining screws, resulting in screw frac- turing. The Laboratory Procedure 1. Model preparation: The appropriate analogues must be selected and the model pre- pared using a silicon or rubber soft tissue mask. The highly recommended use of a remov- able soft tissue mask will allow easy access to the analogues for further lab procedures and will greatly ease later assembly pro- cedures. 2. Wax-up: The Titanium Ring and Waxing Sleeve are assembled on each implant analogue, using the brass equivalent of the prosthet- ic screw to hold them in place (Fig.6). Do not over tighten, so as to avoid distortion of the plas- tic sleeve. The waxing sleeve can be cut back or added to as needed. The wax-up is completed and sprued before removing the wax-up from the model. 3. Investing and Casting: The retaining screw must be removed to allow the wax-up with plastic cylinders to be lifted from the model, leaving behind the loose titanium interfacial component (Fig.7). Standard procedures are used for invest- ing and casting. An appropriate casting alloy must be chosen, depending on whether a ceram- ic veneered bridge or cast bar is being manufactured. Alloys that are commonly used are: Degunorm, Argipal, Begopal 300, Begocer-G, Pors-on 4, De- gudent G etc. Complete burn-out. The plastic cylinder requires an oven tem- Fig. 6. Waxing onto the plastic cylinder Fig. 7. Removal of the wax up, ready to be casted Fig. 9. Luting screws and interfacial rings Fig. 10. The application of cement onto the titanium ring Fig. 8. Refining the screw seat perature of about 820°C for at least 45 minutes. As with all implant work, it is best to de-invest ultrasonically as opposed to blasting with sand or glass beads. This helps pre- serve the sharp edges and fitting surfaces of the casting. 4. Refining the screw seat: The “screw seat” is the internal ledge in the casting where the head of the screw will seat (en- gage). The cast surface of the screw seat will likely be rough due to the casting procedure and must therefore be refined using special hand-held reamers (Fig. 8). (LT18-2.4, LT18-2.6 or LT18- 2.8) The correct diameter of reamer must be chosen. This is an important step to en- sure proper seating and tighten- ing of the prosthetic screw. 5. Fitting the casting to the mod- el: The titanium interfacial com- ponents are secured to the ana- logues using the small luting screws. Do not over tighten, as this may result in the head of the Peek luting screw breaking off. The casting can then be placed over the secured interfacial components (Fig. 9). The casting can be easily fitted and removed from the model without the need to remove and replace the luting screws. If the prosthesis needs to be screw-retained on the model, then one or more of the small luting screws can be exchanged for a prosthetic screw (the pros- thetic screw secures the pros- thesis to the analogue, while the short luting screw has a smaller head and can only retain the ti- tanium interfacial component to the analogue.) 6. Luting the prosthesis to the titanium interfacial component: After completing the fabrica- tion of the prosthesis, sandblast the fitting surface of the casting and the top surface of the tita- nium ring. The titanium ring is best clamped to an analogue by the short luting screw for ease of handling while sandblasting. This also protects the fitting sur- face of the titanium ring. Avoid sandblasting the polished collar of the titanium ring. After sandblasting, it is very im- portant to disassemble and ul- trasonically clean the following: - the titanium interfacial compo- nents - the short luting screws - the fitting surfaces of the pros- thesis Also clean the analogues (Im- plant Replicas) of the model by brushing with soap and water or steam cleaning to remove any debris, which may interfere with perfect seating of the interfacial components. Luting of the prosthesis to the ti- tanium rings will now take place on the master model. - attach the titanium rings to the model with the short luting screws - apply self cure resin cement or dual cure resin cement to the sandblasted surface of all of the titanium rings. Refrigeration of self-cure resin cements will usu- ally lengthen working time for ease of use on multi-unit struc- tures. Important: Limit the amount of resin cement being applied to the angle between the sand- blasted horizontal plane and vertical plane of the titanium ring (Fig.10). This will avoid ex- cess cement extruding upwards through the screw hole in the casting and so inadvertently locking the luting screw into the cement. Definitely avoid placing any cement in the area immedi- ately around the head of the lut- ing screw. Fit the prosthesis over the titani- um rings and settle the prosthe- sis firmly into place with finger pressure to extrude excess ce- ment. Arch castings can be left seated under their own weight to allow cement to harden. Smaller bridges or single units need to be held lightly in place by using one or more prosthetic screw in place (instead of using a luting screw), to allow cement to harden. (e.g. use the middle screw in a three-unit structure) IMPORTANT: Do not over tight- en the prosthetic screw being used to retain the prosthesis during cement hardening as this may lead in distortion of the multi-unit structure. 7. Finishing & Polishing: Once resin cement has hard- ened, remove all luting screws and then remove any prosthetic retaining screws so that the prosthesis can be lifted from the model (Fig. 11). Attach polishing protectors or implant lab analogs, of cor- rect diameter to each of the fit- ting surfaces of the cemented titanium rings. Remove excess extruded resin cement using a sharp blade, probe or hand scal- er. Polish the remaining cement line using a fine edged, lens shaped rubber wheel and blend the casting into the titanium ring where needed. You will notice that the cement line is often not of constant thickness. This variation is indicative of the extent of casting misfit, which existed and has now been cor- rected by the cement space of the Passive Abutment. Once polishing is completed, remove the protector caps or implant lab analogs (Fig.12) and replace the casting on the cleaned model analogues to in- spect and verify the quality of fit obtained. Resin cement is best cleaned from analogues using a brush with alcohol. The fit would be expected to be excel- lent in all areas. A titanium ring can easily be removed by forcing a sharp blade into the cement line, or by punching out the ring using the shaft of a lab handpiece drill applied through the screw ac- cess hole (place the bridge rings down on a folded towel for pad- ding and give the drill shaft a sharp tap). Important: As this technique re- lies absolutely on the accuracy of the master model to achieve passive fit of the prosthesis, it is vital that accurate impression techniques be used and that the quality and condition of the model and analogues be main- tained at all times. Try-in procedures Should it be necessary to try-in a passive abutment case (i.e. the rings are not yet cemented into the framework) the following method may be followed: a. Remove the temporary abut- ments from the implants. b. Place some petroleum jelly (“Vaseline”) or chlorhexidine gel around the head of each