Stress-reduced direct cuspal-coverage restorations
Structurally compromised teeth are teeth that underwent substantial loss of tooth structure due to previous caries, pre-existing restorations and endodontic procedures. The more structurally compromised is the tooth, the lower is the proportion of the residual DEC (dentin-enamel complex) region in the tooth and the higher is the potential of a catastrophic failure of the residual tooth structure. Cast coverage restorations and large amalgam restorations have been selected for the restoration of endodontically-treated teeth for many years. Metal-based restorations and the residual tooth structure behave as two different entities during function because they are not bonded to the residual tooth structure. They also have a coefficient of thermal expansion different from those of both enamel and dentin. As matter of fact, the residual tooth structure is continuously subjected to both occlusal and thermal stresses. Furthermore, the need for mechanical retention or resistance forms like boxes, grooves, slots, pins and posts creates regions of great stress concentrations which dramatically weakens the residual tooth structure and increase the potential for crack formation.
When clinicians select a composite resin restorative material for the restoration of structurally compromised teeth, they need to keep in mind that composite resin is a rigid material; it does not lack of strength or stiffness but lacks of toughness. Toughness is defined as the resistance of a material to the rapid propagation of cracks. Toughness is an inherent property of the material and can be used to predict structural performance .
When most of the DEC on the lateral and proximal walls of severely structurally compromised teeth is lost, Dr Simone Deliperi and colleagues (Simone Deliperi, David Alleman, David Rudo) have proposed the “wall-papering” of the residual cavity walls with polyethylene fibers in the Operative Dentistry Journal in 2017. Polyethylene fibers (Ribbond Inc, Seattle, WA) help to both emulate the crack shielding mechanism of the DEC (dentin-enamel complex) and absorb the stress from either polymerization shrinkage or occlusal load. The DEC is a functional interphase that provides crack tip shielding; the DEC should be preserved during restorative procedures. Dentists can design the strategic placement of restorative materials into the cavity to both resist the mode of failure and mimic the performance characteristics of the intact natural tooth. The term “wall-papering” describes a concept of covering the cavity walls with overlapping closely adapted pieces of Leno Weaved Ultra High Molecular Weight Polyethylene (LWUHMWPE) ribbons. The key for success is that the ribbons are adapted and polymerized as closely as possible against the contours of residual tooth substrate. The resulting thin bond line between the fibers and the tooth structure creates a “bond zone” that is more resistant to failing due to the intrinsic stress and energy absorbing mechanism of the polyethylene ribbons. The formation of defects and voids, from which crack propagation may start, is also reduced. The fibers’ tight adaptation to tooth structure allows a dramatic decrease of the composite volume between the tooth structure and the fiber thus protecting the residual weakened walls from both the stress from polymerization shrinkage and the occlusal load. By using a similar approach, fiber-reinforced stress-reduced direct composite restorations may be performed in the restoration of structurally compromised vital and devital teeth including the direct coverage of multiple cusps!
Live patient Demo, hands-on courses
At the Sardinia Dental Teaching Center in Cagliari (Italy), Dr Deliperi gives live patient demo, hands-on courses and literature review on stress-reduced direct posterior restorations including fiber-reinforced stress-reduced direct composite restorations. Dr Deliperi also trains clinicians in designing the restoration of structurally compromised teeth through a step-by-step fiber lay down protocol.