GAP Australasian-Dentist-Mar Apr 2019

Category 62 AustrAlAsiAn Dentist Graeme Milicich Introduction Mother nature designed our teeth to primarily function in compression. they behave in a similar way to a masonry compression dome, like the Pantheon in rome. 1 Compression domes (Biodome) are structures that are designed into the enamel and range through several orders of magnitude from the macro down to the nano. When the Biodome is disrupted, the underlying dentin is then exposed to increased tension that increases with greater removal of critical areas of the Biodome, particularly the occlusal enamel. 2,3,4 the most critical zones providing functional stability are associated with occlusal enamel. the more that can be retained, the more biomechanically stable the tooth remains in the long term. Once the underlying dentin becomes exposed to tension, cracks can begin to propagate, eventually leading to failure that can present as teeth with lost cusps and/or pressure and temperature sensitivity due to cyclical flexing of the dentin as the crack propagates. The Clinical Implications When faced with cracked teeth and missing cusps, the restorative options can be quite complex. Based on the concept of the compression dome and an understanding of force distributions, the ideal way to restore a badly damaged tooth is an adhesive ceramic onlay that retains as much of the cervical half of a tooth as possible. By keeping a restoration in the coronal half of the tooth, it is not exposed to radial hoop stress that occurs in the bottomhalf of a compression dome. 5 When the option of placing a crown is discussed, the treatment the patient finally opts for is often tempered by patient economics. Our restorative materials and teeth all cope best in compressive environments, so it makes sense to try and keep restorations out of areas exposed to significant tensile forces. 1 in the emerging field of Biomimetic Dentistry, several techniques are being taught based on these principles of working with Mother nature’s design. new nomenclature is required to describe these functional zones on a tooth. the coronal half of the compression dome is being described as the Biodome, and the cervical half of the tooth that essentially supports the coronal compression dome is being called the Bio-rim. ideally, restorations should avoid disrupting the Bio-rim as much as possible, within the parameters dictated by existing damage from previous restorative efforts, trauma or decay. the goal is to avoid removing the Bio-rim, which is what is required when placing a crown that is relying on cementation and the ferrule effect for retention. retaining the Bio-rim by placing adhesive onlay restorations helps to recreate the Bio-dome and restore biomechanical function. The Basic Compression Dome Restoration Design Adhesive ceramic onlays are effective at re-creating the compression dome and placing the underlying tooth structure back into compression. 1 these restorations are so effective because they are bonded to the tooth with modern bonding systems that are now in the range of 40-50MPa of tensile strength, which is similar to Mother nature’s tensile bond between enamel and dentin which has been measured at 45MPa. 6 Clinically, dentists are faced with the conundrum that the best restorative option, an adhesive ceramic onlay, is often not a financially realistic one for the patient. As a consequence, the dentist is then faced with having to place a large direct adhesive restoration that is technically challenging. the large volumes that can be involved create issues with maintaining a sound bond in an environment that can lead to high polymerization shrinkage stresses, if high C-Factor shrinkage stresses associated with mismanagement of increment size and placement are not addressed. unintentional voids between layers of composite are also a risk. then there are the issues of creating predictably good contact points and recreating the occlusal morphology so the restoration looks like a real tooth. the Biomimetic adhesive onlay, that primarily stays in the coronal half of the tooth, can be made from lithium disilicate, zirconia or one of the new hybrid composite systems. Adhesive bonding is critical for compression dome restorations. Cementation techniques do not provide an adequate retentive tooth/ restoration interface. All the material options are successful because they are all functioning in compression, avoiding the radial tensile forces present in the cervical Bio-rim zone of the tooth. these complex restorative systems are either lab generated or produced using in-house CAD/CAM. However, the dentist is often faced with the issue of cost for the patient, meaning the dentist is then forced to compromise and place a large direct restoration and face all the associated challenges. A totally adaptable, lab generated onlay system, the OVC 3 , that can be either placed as a direct, semi-direct or indirect restoration has been developed by Dr s McDonald (the inventor of the V3 OVC 3 : A New Restorative Option for Damaged Teeth Based on the Compression Dome Concept By Graeme Milicich BDs Fig 1. Increasing radial stress in the Bio-rim of the dental compression dome (5) lInICal