Dental Tribune Indian Edition

18 Dental Tribune Indian Edition - April 2013Trends & Applications Dr Tussavir Tambra United Kingdom The advent of CAD/CAM tech- nology and the more widespread utilisation of implants in modern dentistry have led to an explosion of treatment solutions designed to ad- dress any situation encountered by the general dentist. As patients have become more awa­ re of the benefits of implant therapy, they have begun to demand more immediate restoration of their teeth. The provision of a fixed prosthesis has always been the goal in denti­ stry; however, the cost of such tre­ atment is pricing the vast majority of patients out of the implant market. Immedi ate loading, avoiding con­ ventional grafting techniques by pla­ cing implants at various angulations (All­on­4, Nobel Biocare; Columbus Bridge, BIOMET 3i), has resulted in a significant uptake of treatment by edentulous patients and those with a failing dentition. This is mainly be­ cause a fixed bridge is provided and treatment times are reduced from months to hours, avoiding a conven­ tional denture. Most edentulous patients can tolera­ te a complete maxillary denture with few problems. The vast majority of problems arise in the mandible, whe­ re the underlying supporting tissues are not designed to function under this type of occlusal loading. Even a properly constructed complete lower denture can move as much as 10 mm in function. This continuous move­ ment of the prosthesis results in loss of the supporting bone (or remodelling), further destabilising the denture. Poor ridge form increases denture instabili­ ty and this produces more remodelling. Edentulism fulfils the WHO definition of a physical impairment. Treatment protocol A simple treatment protocol was devised to treat this problem. Ac­ cording to this protocol, two dental implants are placed in the inter­fora­ minal area of the mandible, to which either a bar or stud attachments are connected to retain the lower denture. This treatment greatly improves both masticatory efficiency and function in patients. Over the last two decades, attempts have been made to render the implant­retained overdenture the standard treatment for edentulism,1 as demonstrated most recently by the McGill consensus.2 Prosthetic failure, usually loss of retention, and the technical difficul­ ties encountered when relining or changing stud attachments proved to be major negative factors in dentists’ attitudes towards this treatment mo­ dality. Several attempts were made to redesign and improve the attach­ ments; however, owing to previous negative experiences, most dentists became reluctant to adopt implant­ retained overdentures as a routine treatment option. The push to place more implants in an attempt to im­ prove the situation led to the bar­ and clip­retained overdenture scenario. This technique was more successful but still encountered similar issues to the stud­attachment overdentures.3 Poor stress transmission from the prosthesis to the supporting implants results in bone loss around the im­ plants (especially the most distal im­ plants in the multiple bar scenario), in addition to prosthetic and surgi­ cal complications.4 This resulted in implant companies and clinicians moving away from the two implant­ retained overdenture treatment option in favour of fixed solutions, such as round­house bridges fixed on four or more implants. As a result, the vast majority of patients cannot access implant therapy owing to financial constraints. The McGill consensus brought the implant­retained overden­ ture back into the spotlight as a way of increasing access to implant dentistry and improving patients’ quality of life. Improved component manufactu­ ring techniques, and greater care and attention to both surgical and restora­ tive treatment planning have signifi­ cantly improved treatment outcomes using overdentures.5 Recently Cendres+Métaux intro­ duced the Stress Free Implant Bar, or SFI­Bar, to the dental community. This unique, implant­platform­inde­ pendent restorative bar overdenture solution allows the fabrication of a true passive­fit bar and clip system on two or more implants (Fig. 1). Finite ele­ ment studies and clinical evaluation of the system have found minimal stress transmission from the prosthesis to the implants under loading (Figs. 2a–c), with most stresses being evenly di­ stributed between the supporting im­ plants. Vertical loads are transmitted effectively to the supporting implants, while undesirable lateral stresses are largely eliminated. More recent cli­ nical studies have also shown it to be a viable immediate­loading tre­ atment solution. The technique is in its infancy, so long­term (five years or more) data is not available. The SFI­ Bar is a modular system that connects multiple dental implants with no solde­ red or laser­welded joints. The minimum inter­implant distan­ ce is 8 mm and the maximum is 26 mm. This is an expandable bar system, in which add­on kits (Fig. 3) can be used to incorporate multiple implants to create a round­house bar. Implant adapter abutments are first torqued onto the implants (Figs. 4a & b). They form one half of a universal ball joint—the other half being in­ corporated into the bar element. The bar itself is formed by a hollow tube bar that fits onto the end of each ball joint (Fig. 5). This tube bar is cut to the correct length using a specialised jig and cutting disc (Figs. 6a–c). The jig is designed to mimic a ball joint connection, ensuring a perfect section each time. The jig slides along the tube bar until it reaches the implant adapter, accurately sizing the bar. The tube bar is then locked in place and cut to size with a cutting disc (Fig. 6c). This pro­ cess can be carried out either chair side (two­implant bar) or in the laboratory (four­implant bar or larger). An im­ plant­level master cast will be required for cutting in the laboratory. The cut­ ting of the tube bar must always be car­ ried out extra­orally. Once the tube bar has been cut, the ball joints are inserted into each end of the tube bar prior to seating on the implant adapters (Figs. 7a–d) and tor­ qued into place. The SFI­Bar is now complete and the patient is ready for the retentive element to be housed in the denture. The ball joints can ac­ commodate non­parallel implant pla­ cement up to a maximum of 15° angu­ lation correction. The absence of any soldered or welded joints means that a greater length of the bar can be enga­ ged by the retentive clip. In conventio­ nal techniques, the presence of a weld increases the bar thickness, at that point preventing any retentive clip en­ gaging that area. In the SFI­Bar, the clip engages the full length of the bar between the ball joints (Fig. 8). The bar assembly must be parallel with the occlusal plane; therefore, a selection of implant adapters of varying lengths should be available. Most of the major implant compa­ nies offer CAD/CAM­fabricated bar and clip solutions. However, these bars are relatively expensive and are fabricated through a conventional im­ pression and master cast technique. Studies have shown that 50per cent of all errors during impression making and cast fabrication result in non­pas­ sive fit of bars and frameworks. Thus, any bar fabricated through an impres­ sion or cast technique cannot be truly passive.6–8 A clinical case will be pre­ sented below in order to demonstrate the direct chair­side method and the use of the SFI­Bar on two implants to restore an edentulous mandible. In addition, the main points for use with the indirect method will be outlined. Case presentation In 2006, a 60­year­old female patient initially presented, complaining of an ill­fitting lower denture. The patient had worn a conventional complete man­ dibular denture for over 20 years, op­ posing a metal­based maxillary remo­ vable partial denture. The patient had visited a denturist on several occasions to try to improve the situation. After multiple relining procedures, the patient decided to seek expert help. An OPG radiograph revealed a severely resorbed mandible that clinically presented as a classic bowl­shaped deficiency (Figs. 9a–c). Radiographic examination reve­ aled there was adequate bone volume in the anterior region for the placement of dental implants. However, a fixed solution would only have provided a shortened dental arch, as the mental fo­ ramen had become more mesial owing to bone resorption. Placing implants di­ stal to the mental foramen was not an option, owing to the proximity of the inferior dental nerve and lack of bone height. The patient was not keen to have any nerve repositioning or complex bone grafting. Another important fac­ tor negating the fixed solution was the size of the volume defect. This would have been difficult both to correct and to maintain and would have produced a poor aesthetic result. The additional bulk of denture flanges allowed proper facial support. After discussing all the relevant is­ sues, the patient decided that the re­ movable overdenture retained with two implants was the best and least complicated treatment option for her. The upper denture was not an issue for the patient, as it was retenti­ ve and stable. In order to limit costs, the upper denture was not replaced. A surgical guide was fabricated af­ ter the vertical dimension, aesthetic and phonetic parameters had been corrected in the wax denture try­in. Two 4.1 mm RN connection dental implants (Straumann), each 8 mm in length, were placed in sites #32 and #42 (Figs. 7a & 9b). These were allo­ wed to integrate for three months pri­ or to the provision of a ball­abutment­ retained overdenture. This denture functioned without surgical or pro­ sthetic issues for a five­year period. The implant-retained bar overdenture: The SFI-Bar Fig 1 Fig 2bFig 2a Fig 2c Fig 4aFig 3 Fig 4b Fig 6aFig 5 Fig 6b Fig 7aFig 6c Fig 7b Fig 7dFig 7c → DT page 19