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Dental Tribune Indian Edition

19Dental Tribune Indian Edition - April 2013 Trends & Applications Unfortunately, the patient revisited her denturist and complications arose after an attempted intra­oral relining procedure. On examination, it was de­ termined that the ball abutments were damaged and needed to be replaced. The female housings needed to be re­ placed, as they were no longer seated properly on the ball abutments. The patient was then given the option of having either another ball­ abutment­retained overdenture or a bar­ and clip­retained overdentu­ re instead. The patient opted for the bar and clip overdenture. The first step was to remove the damaged ball abutments and seat the appropria­ te implant adapters on each implant (H1 adapters of 1 mm in length; Figs. 4a & b). The tube bar was then inserted into the cutting tool and cut to correct length using the cutting disc (Figs. 6a–c). The bar assembly was then connected to the implant adap­ ters and torqued into place. The uni­ versal nature of the ball joint allows the tube bar to be located in the hori­ zontal plane in a truly stress­free ali­ gnment (Figs. 2a–c & 7b–c). The implant adapters were cho­ sen so that when the bar is seated it is parallel to the occlusal plane, with at least 1.0 mm clearance between the underside of the bar and the mucosal tissues (Fig. 7b). This allows access for effective oral hygiene procedures around the dental implants and re­ duces the risk of tissue hyperplasia around the bar when the denture is seated. From a surgical perspective, ridge reduction procedures may be required firstly to aid ideal implant placement and secondly to ensure there is enough space to fabricate the final denture to be seated on the bar assembly. If multiple implants are used, adapters with a range of lengths should be used. Multiple implants are more difficult to place parallel to each other, but the ball joints can accommodate up to 15° of implant di­ vergence. Surgical complications are seen more commonly in bar and clip overdentures than stud­attachment overdentures. Clinically, the whole procedure took six minutes, from re­ moving the ball abutments to torquing the bar assembly into place. The ball­abutment retained denture was then hollowed out so that it could be seated over the bar assembly and used as a provisional while the new definitive denture was being fabrica­ ted. A custom tray was used to make a border­moulded final impression with Impregum (3M ESPE), after blocking out the bar assembly (Fig. 10). A wax occlusal rim was then used to deter­ mine the vertical dimension of the oc­ clusion and obtain a CR record. This was followed by a full wax try­in to ensure that all the aesthetic, phonetic and occlusal parameters were correct. At this point, the denture was ready to be processed. The denture is pro­ cessed in one of two ways: • In the laboratory technique, the female part T (made from pure Grade 4 titanium) is integrated into the denture and a complete prosthesis is returned to the clinic. Part T is contra­indicated for use on two implant bars (Figs. 11a & b). • In the chairside technique, the den­ ture is processed and a window is cut in the denture, through which the dentist can pick up the female part E (made from Elitor—68.6 per cent gold alloy), using self­curing acrylic resin in the patient’s mouth after seating the spacer and blo­ cking out all undercuts (Fig. 10). The total width of the bar with the E clip seated is 4.3 mm (Fig. 12) and 3.6 mm with the T clip seated (Fig. 11a). This is relevant for treatment planning, as ridge reduction may be indicated to provide space for the denture. In the laboratory method, the den­ ture is completed with the female part T integrated into the denture. The dentist then chooses the level of retention required by selecting the appropriate plastic inserts and seating them in part T (Fig. 11b). The plastic inserts are designed to compensate for transfer inaccuracies during the im­ pression, master cast fabrication and post­processing stages. The presence of a laboratory technician is recom­ mended for the chairside technique. A spacer is placed on the tube bar prior to seating the E clip to ensure vertical resilience. The spacer ensures a slight gap between the E clip and the tube bar so that when the patient bites down, the E clip does not over­ load or distort the bar as the denture beds into the supporting mucosa. All undercuts around the bar assembly, especially between the bar clip and tissues, were blocked out with a si­ licone material (Fig. 10). A window was then cut into the lingual aspect of the denture to expose the E clip (Fig. 13a). A small bead of cold­cure acrylic resin was then placed on the E clip, covering the retentive element of the clip. The E clip was then attached to the denture with small increments of resin (Fig. 13b). The resin was al­ lowed to cure fully before the dentu­ re with the E clip was removed from the mouth. The remainder of the void was then filled with cold­cure resin and allowed to cure outside the mouth (Figs. 13c & d). Ideally, this process should take place in a pressure pot. A transfer jig that fits into the E clip and is effectively a tube bar re­ plica can be utilised if a large volume of acrylic has been used to house the E clip. The denture with the transfer jig seated in the E clip is bedded into a patty of fast­set plaster, similar to a denture­repair scenario. Once the stone has set, the denture is placed in a pressure pot with warm water and the self­curing resin is allowed to polymerise. Once the acrylic has fully cured, it is separated from the stone base and the transfer jig and all excess acrylic is trimmed. At least 50 percent of the lamellae of the E clip must be clear of resin. Only the superior part of the E clip with the attachment portion and shoul­ der section is locked into acrylic (Fig. 13c). The lamellae must be free to flex over the tube bar during insertion and removal of the denture. If the resin is in direct contact with the lamellae, the denture may not seat, as the E clip can­ not flex. Finally, the definitive prosthe­ sis was seated (Figs. 14a & b). The level of retention of the E clip was adjusted using the activation and deactivation tools provided in the re­ storative kit. The occlusion was che­ cked and adjusted after verifying that the denture had been properly seated, using pressure­indicating paste. The bar assembly is required to retain the denture in the two­implant scenario. Support is derived from the conven­ tional hard­ and soft­tissue load, bea­ ring areas like the residual ridge and the buccal shelf. The patient was then instructed on appropriate care of the implants and the prosthesis, and a routine recall and maintenance pro­ gramme was instituted. Discussion It is imperative that the block­out procedure around the bar assembly is correct. Otherwise acrylic will enter an undercut area and cure, thus lo­ cking the denture to the bar assembly. As a consequence, there would be no option but to cut the denture from the bar to free it. This will not only ruin the denture, but may also damage the bar—a very costly and time­consu­ ming mistake. The E clip is designed for use with the two­implant bar and should be picked up with a self­curing resin as explained. The T clip is for a laboratory­processed denture on four or more implants, as the plastic inserts correct any processing errors. It must not be used in a two­implant situation. Several studies have shown that conventional bar­ and clip­retained overdentures transfer significant stress to the supporting peri­implant tissues (mainly bone).9–11 The key to the SFI­ Bar system is that the bar is assem­ bled in the patient’s mouth without the use of soldering, laser welding or conventional bonding techniques, thus reducing stress transmission to and bone loss around the implants. Studies have demonstrated that any laboratory­based technique that re­ quires a master cast made from a dental impression will result in a bar that is not truly passive.8, 9 As a result, several authors have suggested that the only way to achieve a passive fit would be to assemble the framework intra­orally and then bond the bridge pontic in place.12, 13 This is the method employed with this system. There is no casting, soldering, laser welding or bonding of components when fabricating the definitive bar. This, combined with the universal ball­joint nature of the components, ensures a true passive fit when the bar is assembled. The finite element analysis clearly shows the stress­free nature of the bar when being assem­ bled and when the prosthesis expe­ riences loading (Figs. 2a–c). No laboratory time is required to fabricate the bar and there are no co­ stly implant components or gold­alloy charges. Clinically, there is no need for the bar sections to be soldered in an attempt to achieve passive fit— a step that may need repeating—as with the conventional method. There are no soldered or laser­wel­ ded joints, so the bar assembly has no inherent weak points that may fracture or corrode. The bar is assembled by the clinician, who also attaches the E clip intra­orally. The reduced number of clinical appointments, laboratory time and component costs result in reduced treatment costs for the patient. In the case presented, for example, the bar as­ sembly was completed in only six min­ utes. This is approximately the same time it takes for a polyether impression material (like Impregum) to set! Conclusion The SFI­Bar is relatively inexpen­ sive compared with conventional gold castings and CAD/CAM options. The overall cost of the prosthesis and treatment time are significantly re­ duced compared with conventional and CAD/CAM techniques. Preci­ sion­milled components provide an improved quality of fit. The physical and mechanical properties of the com­ ponent materials can be controlled ac­ curately, which is difficult to achieve with conventional casting methods. The SFI­Bar can be connected to two or more implants to create a full­arch bar if needed, while the SFI­Bar sys­ tem produces a bar assembly that seats passively as demonstrated by finite ele­ ment analysis. The passive­fit bar as­ sembly can result in greatly reduced stress transmission to the supporting implants. Studies have demonstrated that this is also a viable treatment op­ tion for immediate­loading situa­ tions in the mandible, provided that the implants achieved insertion torques exceeding 50 Ncm approximately. Thefiniteelementdataandimageswere kindly provided by Dr Ludger Keilig, Endowed Chair of Oral Technologies, University of Bonn, Germany. Disclaimer: The SFI-Bar, implant adapters and E clips were provided by Cendres+Métaux. The author did not receive any financial inducements to write this article or payment towards laboratory charges, nor was any other kind of payment given or received. DT Fig 9b Fig 8 Fig 9c Fig 10 Fig 9a Fig 12Fig 11b Fig 13cFig 13bFig 13a Fig 11a Fig 13d Fig 14bFig 14a Dr Tussavir Tambra is a prosthodon­ tist from Wolverhampton in the UK. He can be contacted at dr.tambra@ hotmail.co.uk. Contact Info ← DT page 18