Quintessence International, 09/1993

Dentin consists of several identifiable structures: tubules with cell processes and fluid; highly mineralized peritubular dentin; and intertubular dentin consisting mainly of collaen and deposited apatite. The structural organization and microstructural variations reflect formative influences, such as tooth size and shape, and alterations caused by age, insult, and disease. However, details of structure-chemistry-property relationships for this anisotropic biologic composite are limited. Materials scientists are focusing on an array of new spectroscopic, analytical, and imaging techniques that are yiel ding improved understanding of structural variations and their dependence on specimen preparation, tooth type, location, storage conditions, and chemical and physical modifications. Such studies should lead to the major refinements in structure-properties models for dentin that are required for continued advances in dental composite resins and adhesives.

The permeability of dentin to adhesive agents is of crucial importance in obtaining good dentinal bonding. In those systems that remove the smear layer, the opportunity exists for resin to infiltrate both tubules and intertubular dentin. Resin penetration into tubules can effectively seal the tubules and can contribute to bond strength if the resin bonds to the tubule wall. Resin infiltration into intertubular dentin can only occur if the mineral phase of dentin is removed by acidic conditioners or chelators. This is more easily accomplished in fractured dentin than in smear layer-covered dentin because of the residual collagen debris that remains on the surface following acid etching of smear layers. The channels for resin infiltration are the perifibrillar spaces created around the collagen fibers of dentin following removal of apatite mineral by acids. The diffusion of adhesive resins through these narrow, tortuous, long channels in 1 to 2 minutes offers a number of challenges that require further research.

Much attention has been directed toward producing dentinal adhesives that withstand the forces involved during polymerization shrinkage of composite resins. Studies have shown that an effective dentin-adhesive bond depends on the wetting and penetration characteristics of the dentinal adhesive system and the reactivity of the treated dentinal surface. The structure of the collagen in the demineralized dentinal layer also seems to influence the behavior of the bond. Adhe4sive systems that do not completely denature the fibrous collagen and leave interwoven banded collagen in the demineralized layer produce superior bond strengths (greater than 20 MPa). Other research efforts seek to develop a nonshrinking high-per formance polymer for use as a marix material for dental composite resins. Spiroorthocarbonates that expand during polymerization have been developed. In combination with a three-component epoxy comonomer, matrix resins have been produced that expand during polymerization and whose mechanical properties, water sorption, solubility, and degree of polymerization are acceptable for dental use. Further developments of nonshrinking composite resins and improved dentinal adhesives will greatly increase the longevity of 21st-century composite resin restorations and should significantly ease clinical placement.

Human enamel and dentin should be used as the physiologic standards with which to compare composite resins, especially in the posterior region. The intrinsic surface roughness of composite resins must be equal to or lower than the surface roughness of human enamel on enamel-to-enamel occlusal contact areas (Ra = 0.64 um). Roughness determines the biologic strength of composite resins. The nanoindentation hardness value of the filler particles (2.91 to 8.84 GPa) must not be higher than that of the hydroxyapatite crystals of human enamel (3.39 GPa). Composite resins intended for posterior use should have a Young's modulus at least equal to, and preferably higher than, that of dentin (18.500 MPa). The compressive strength of enamel (384 MPa) and dentin (297 MPa) and the fracture strength of a natural tooth (molar = 305 MPa; premolar = 248 MPa) offer excellent mechanical standards to select the optimal strength for posterior composite resins. The in vivo occlusal contact area wear rate of composite resins must be comparable to the attritional enamel wear rate (about 39 um/y) in molars. Differential wear between enamel and composite resin on the same tooth is a new criterion for visualizing and quantifying the wear resistance of composite resins in a biologic way. Posterior resins must have a radiographic opacity that is slightly in excess of that of human enamel (198% Al). Based on these standard criteria, it can be concluded that in the 21st century the ultrafine compact-filled composite resins may be the materials of choice for restoring posterior cavities.

The clinical behavior of future adhesives and composite materials for dental restorations will be significantly improved from the performance of today's restorative materials. The adhesives and composites available to the practicing dentists in 10 years will be designed and developed as preventive resin restorations. They will consist largely of adhesive-sealant and composite resin combinations. The new restorations will be developed in response to early detection of caries, resulting in smaller lesions, and smaller restored surfaces exposed to the oral cavity. There will be less margin length, less vulnerability to wear, and less microleakage. The materials of the 21st century will be less technique sensitive in the hands of the clinician and therefore will result in fewer clinical failures. The advent of better wetting and better bonding adhesives to both enamel and dentin, combined with nonshrinking dental resins that are durable, esthetic, and biocompatible, will significantly improve available dental health care. The clinical behavior of these 21st century materials should provide longer wearing, nonleaking, esthetic restorations that are easily manipulated and placed by clinicians at an economical cost to dental patients.

The purpose of this study was to determine if the use of a foam liner in a tray during the nightguard vital bleaching technique would cause a difference in the rate of bleaching or the final shade of the bleached teeth. Ten subjects were selected who had previously successfully bleache d their maxillary arch by using a 10% carbamide peroxide solution. Vacuum-formed guards were fabricated for the mandibular arch so that one quadrant was covered with a conventional-style guard, and the other quadrant was covered with a foam-lined guard. All patients were successful in bleaching their mandibular arch. No patient reported any difference between the two quadrants in the rate of bleaching or in the final shade, nor was any difference apparent to the operators clinically or in photographic evaluation. It appears that the addition of a foam insert does not noticeably alter the clinical result of home bleaching. If bleaching is successful in one arch, it can be epxected to be effective in the other arch.

Discoloration of teeth is common sequel of dental trauma and is often taken as a sign of irreversible pulpal degeneration, particularly if the vrown becomes gray or blue-gray in appearance. If a viable blood supply remains or revascularization occurs following trauma to teeth, removal of blood pigments by normal biologic processes can occur. The assessment of blood flow with laser Doppler flowmetry is an additional diagnostic aid following dental injuries. This technology has been used to monitor the clinical progress of two central incisors that had been palatally luxated in a sportinga ccident. Color changes were observed and were correlated with symptomatic and radiographic evaluation, sensibility tests, and laser Doppler readings. The results of these examinations indicated that the blood pigment within a discolored tooth crown interfered with laser light transmission. The limitation of this new technology in the assessment of teeth discolored following trauma is significant. Visual radiographic and symptomatic assessment remain as the principal diagnostic criteria at the present time.