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Dental Tribune Middle East & Africa Edition

A n increased uniform amount of coronal dentin significantly amplifies the fracture resistance of endodon- tically treated teeth, regardless of the post system used or the choice of material for the full- coverage restoration45 . A recent article by Coppede et al dem- onstrated that friction-locking mechanics and the solid design of internal conical abutments provided greater resistance to deformation and fracture un- der oblique compressive loading when compared to internal hex abutments46 . These two “seem- ingly” disparate observations define the inherent continuum between natural tooth engineer- ing and the principles of engi- neering necessary to orthobio- logically replicate the native state. The use of a ferrule or collet and a bonded or intimately fit post-core to restore function and form to an endodontically treated tooth is analogous to the use of a long, tapered friction fit interface with a retaining screw (Morse ta- per), to secure an abutment to a fixture. In both cases, the role of contact pressure between mat- ing surfaces to generate frictional resistance provides a locked con- nection. This has been shown to affect the long-term stability of crestal bone support for the over- lying gingival tissues and main- tains a healthy protective and esthetic periodontal attachment apparatus47 . Human symmetry The Roman architect Vitruvius’ (Marcus Vitruvius Pollio) descrip- tion of the perfect human form in geometrical terms was a source of inspiration for Leonardo da Vinci, who successfully illus- trated the proportions outlined in Vitruvius’ work ‘De Architectura’. The result, the Vitruvian man, is one of the most recognised draw- ings in the world and is accepted as the standard of human physi- cal beauty. Vitruvius theorised that the essential symmetry of the human body, with arms and legs extended, should fit into the per- fect geometric forms; the circle and the square. However, Leon- ardo Da Vinci recognised that the circle and the square were only tangent at one place, the base. Observe the insert in Fig 8. The stabilising platform for the hu- man outlined form begins at that tangent; the intersection is graph- ically analogous to the structural configuration of platform switch- ing. In geometry, an oval is a curve resembling an egg or an ellipse. Architects and engineers have used smooth ovate curves to sup- port the weight of structures over an open space literally since the second millennium BC. These arches, vaults and domes can be seen in buildings and bridges all over the world; the most perva- sive example being the keystone arches used by the Romans for aqueducts and mills. An arch directs pressure along its form so that it compresses the building material from which it is constructed. Even a concrete block is readily broken if you hit it on the side with a sledge. But under compression forces from above, the block is incredibly strong and unyielding. Many will remember the weight bear- ing tripod experiments from grade school where an egg acts as one of three supporting legs of a square section of wood bearing books as the load. The structure could support over sixty books, almost twenty pounds, before breaking the supporting egg. One need only look at the root trunk and coronal tooth structure of a multi-rooted teeth and it becomes apparent that strength of the tooth form is dependent upon an arch form for its integrity (Figs 8 & 9). Optimal engineering Is it possible for this natural feat of engineering to be biomimetically replicated to the design parame- ters of osseo-integrated implants? There are a number of paradigms that continue to fuel debate in the dental clinical and scientific communities pertaining to the optimal engineering predicates for implant design. These in- clude smooth vs. rough surfaces, submerged vs. non-submerged installation techniques, mixed tooth-implant vs. solely implant- supported reconstructions, Morse taper abutment fixation vs. a butt- joint interface and titanium abut- ments vs. esthetic abutments in clinical situations where esthetics is of primary concern. The cone-screw abutment has been shown to diminish micro- movement by reducing the bur- den of component loosening and fracture. This enables the identifi- cation of the effects of the param- eters such as friction, geometric properties of the screw, the taper angle, and the elastic properties of the materials on the mechanics of the system. In particular, a relation be- tween the tightening torque and the screw pretension is identified. It was shown that the loosening torque is smaller than the tight- ening torque for typical values of the parameters. Most of the tightening load is carried by the tapered section of the abutment, and in certain combinations of the parameters, the pretension in the screw may become zero. This enables the identifica- tion of the effects of the param- eters such as friction, geometric properties of the screw, the taper angle, and the elastic properties of the materials on the mechan- ics of the system. In particular, a relation between the tightening torque and the screw pretension is identified. It was shown that the loosening torque is smaller than the tightening torque for typical values of the parameters. Most of the tightening load is carried by the tapered section of the abutment, and in certain combinations of the parameters the pretension in the screw may become zero. This tapered abut- ment connection provides high resistance to bending and rota- tional torque during clinical func- tion, which significantly reduces the possibilities of screw fracture or loosening. Biomechanics ‘The seed of a tree has the nature of a branch or twig or bud. It is a part of the tree, but if separated and set in the earth to be better nourished, the embryo or young tree contained in it takes root and grows into a new tree,’ Isaac New- ton. Pressure on the cervical corti- cal plate, micro-movement of the fixture-abutment interface (FAI) as well as microflora leakage and colonisation at and within the FAI are some of the pathologic vectors associated with osseous remod- eling, both crestal and peripheral to dental implants 48 . Occlusal considerations engi- neered into fixture design should enable optimum load distribution for permanent load stability dur- ing functional loading, reduce functional stress transfer to the interfacial tissues and enhance the biologic reaction of interfacial tissues to occlusally generated stress transfer conditions 49 . Future modifications to im- plant biomechanics should focus on designs wherein the osseous trabecular framework retain- ing the fixture will adapt to the amount and the direction of ap- plied mechanical forces, cope with off-axis loading, compen- sate for occlusal plane to implant height ratios differences as well as adjusting to mandibular flex- ion and torsion50 . In this new era of implant driven treatment planning, fix- tures should be engineered to support single crowns with canti- levers instead of implant/implant or implant/teeth connections for a span of any degree. These en- gineering design iterations will minimise high-stress torque load at the implant abutment interface and obviate areas with degrees of bone insufficiency. The goal should be to biomi- metically replicate the natural state to the greatest degree (Fig- ures 10a and 10b) in regard to load bearing capacity. Measuring success Stable crestal bone levels are the yardstick by which treatment success and health are meas- ured in the orofacial ecosystem, whether it relates to natural tooth retention or restorative and/or replacement rehabilitation. It is therefore surprising that the treatment outcome standards for An Evidence-Based Endodontic Implant Algorithm:Back to the Egg; Concluding Part Kenneth S. Serota, DDS, MMSc Fig 8 Fig 9 Fig 10a switched and subcrestally po- sitioned design demonstrated better stress based perform- ance and lower risk of bone overload than the other im- plant systems evaluated. Essential features Platform switching, together with a stable implant-abutment connection are increasingly ac- cepted essential implant design features required to reduce or eliminate early crestal bone loss. A bacteria-proof seal, a lack of micro-movement due to a long friction grip tapered channel and minimally invasive second-stage surgery without any major trauma for the perio- steal tissues are also important factors in preventing cervical bone loss. Media CME DENTALTRIBUNE Middle East & Africa Edition6 (mCME articles in Dental Tribune (always page 6) has been approved by HAAD as having educa- tional content acceptable for (Category 1) CME credit hours. Term of approval covers issues pub- lished within one year from the distribution date (September, 2010). This (Volume/Issue) has been approved by HAAD for 2 CME credit hours. 2 Hours