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I 11 special _ endo-implant algorithm I roots2_2010 4)themagnitudeanddirectionofocclusalloads; 5)theamountofavailabletoothstructure;and 6)theanatomyofthetooth. Anycombinationofvectoredstressconcentration and high tensile stresses will predispose these teeth to fracturing without an adequately engineered restorative design. _Re-engineering Re-engineering negative treatment outcomes is a significant part of the contemporary endodontic oeuvre. The presence of apical periodontitis may affecttheoutcomeofinitialendodontictreatment;26 however,thereisgeneralconsensusthatapicalperio- dontitisisthemostimportantvariablethatinfluences a positive outcome with non-surgical and surgical retreatment.27–29 Positive treatment outcomes may be more likely in certain teeth with a combination of both procedures, rather than with one or the other alone (Fig. 6). The premise that non-surgical retreatment im- proves the outcome of peri-apical surgery has been supported by both historical and current studies.30–32 Apical surgical ‘correction’ of intra-canal infections mayisolate,butnoteliminate,theresidualmicroflora oftheroot-canalspace.Itshouldthereforebelimited to situations in which non-surgical retreatment is judged impractical. With the range of sophisticated equipment and material in the conventional endo- dontic armamentarium, this is a remote considera- tionatbest.Whentheaetiologyisindependentofthe root-canal system, surgery is the most beneficial treatment.33 Non-surgical retreatment may still be indicated in these cases, especially when intra-canal infection cannot be ruled out. Time constraints or financialpressuresshouldneverbeafactorinmaking surgery the first treatment choice (Fig. 7). Thereareamyriadofvariablesassociatedwithnon- surgical retreatment, and treatment outcome studies inendodonticshavebeenegregiouslyabusedbythose wishingtodiminishthevalueofre-engineeringnatural teeth.Manystudieshavecategorisedteethwithcaries, fractures, periodontal involvement and poor coronal restorationsasnegativeendodonticoutcomes.34,35 Prior procedural errors,36 occlusal considerations,37 material choice for the restoration38 and design of the full coverage component all suggest that success is a function of comprehensive treatment planning as much as technical expertise. Evidence-based or controlled best evidence studies should conclude that these are non-endodontic causes of failure and that the success of endodontic treatment itself is high and predictable. Kvist and Reit39 have shown that while surgical cases demonstrated higher healing rates than non- surgical retreatment cases initially, four years after treatment there was no difference between the two modalities, owing to ‘late’ surgical failure. The failure rate for surgical therapy appears to be analogous to thefailurerateforretreatmentasafunctionofthesize of the lesion treated.40 Levels of apical resection41 and the type of root-end filling material make a difference tosurgicaltreatmentoutcomesuccess;42 however,the dentine-bonded composite technique and the use of compomer materials has not been widely reported on. As these techniques dome the resected root face, sealing off the cut tubules, they may prove to be the Fig. 9_An arch eliminates tensile stresses in spanning an open space, as all forces are resolved into com- pressive stresses. It requires all of its elements to hold it together, thus making it self-supporting. The incor- poration of platform switching into the design of an implant abutment simulates three oblate spheroid shapes—one vertical, two horizontal. The objective is to ensure that axially vectored compressive stresses are contained within an idealised shape that is structurally enhanced by the useofaprecisefriction-fitconnection. Fig. 10a_Foundational dentistry mandates that the impact of an ortho-biological replacement unit be commensurate with the biological objectives and functional require- ments of the natural tooth. Fig. 10b_As the number of implant-supported single-tooth replacements increases, implant- abutment connection design should ensure that occlusal table replication displays equivalency in both dimen- sion and cuspal inclination with the surrounding natural dentition. Fig. 9 Endo- Restorative Implants BIOLOGY LOAD BEARING CAPACITY LOAD BEARING CAPACITY Mechanical BIOLOGY Mechanical Coronal seal Type of Abutment Joint Bone- implant interface Type of Abutment Joint Compression tension Residual tooth structure Bone quality and quantity Tilted abutment Splinted teeth Length, apical diameter Length, diameter Splinted implants Fig. 10a Fig. 10b