Date of Completion
Avinash Bidra, DDS, MS, FACP; Douglas Adams, Ph.D.; Liisa Kuhn, Ph.D.
Field of Study
Master of Dental Science
Statement of the Problem: The increased use of CAD/CAM technology and patient’s high esthetic demands has created an increasing market for a new class of restorative material called interpenetrating phase composites (IPC). This is distinguished from “particle-filled composites” by having both the ceramic phase and resin phase mutually continuous and interconnected three-dimensionally. The structural logic of IPCs is two-fold: 1) with two continuous phases there is an enhanced resistance to crack formation and 2) due to a similar elastic modulus as dentin, IPC crowns may be less likely to fail compared to more traditional ceramic materials. One commercial IPC material, 1.8 g/cc Enamic (Vita Zahnfabrik, Germany), consists of an approximately 86% porous feldspathic porcelain matrix by weight and a copolymer of triethylene glycol dimethacrylate and urethane dimethacrylate. Although this IPC has comparable strength and fracture toughness to other ceramics, an earlier version of Enamic (1.7 g/cc), had significantly higher fatigue durability than lithium disilicate due to its lower elastic modulus compared to that of 1.8 g/cc Enamic. 1.7g/cc Enamic was never marketed, however, because it lacked in other mechanical properties such as wear and polishability. Improvements in either the ceramic or polymer phases, such as increased neck formation between the ceramic matrix particles, may enhance the overall mechanical properties of both 1.7g/cc and 1/8 g/cc Enamic. A chemical or pre-ceramic approach to increasing neck formation was chosen to eliminate the dependency on temperature and particle size for product control and optimization.
Objective: To measure the impact of chemically increasing the neck formation between the preceramic matrix of 1.8 g/cc Enamic through polydimethylsiloxane (PDMS) infiltration and oxidation at low temperature on biaxial flexure stress and abrasion resistance of the final sintered, resin infiltrated product. This may allow the use of 1.7 g/cc Enamic by improving the wear and polishability while maintaining the higher fatigue strength due to a lower modulus of elasticity.
Materials & Methods: Pre-sintered 1.8 g/cc Enamic blocks were sectioned into tabs and infiltrated with 1,000 Cst PDMS under vacuum for a variable number of times: 0x (control), 1x, 3x, 5x. The tabs were subject to low temperature oxidation to form new necks from the PDMS in a Carbolite Type 3600 oven at a rate of 1°C/min until 600°C. Added neck formation was visualized with scanning electron microscopy (JEOL JSM-5900LV; JOEL Ltd.). Samples were sent to VITA headquarters (Bad Säckingen, Germany) for proprietary resin infiltration to finalize the Enamic tabs. Load-to-failure tests were done using a piston-on-three-ball fixture (ISO 6872 – 7.3.3., Fixture No. WTF-SB-211M; Wyoming Test Fixtures) assembled onto a servohydraulic machine (858 Mini BionixII; MTS). The load results were used to calculate bi-axial flexure stresses (ISO 6872 – 126.96.36.199). Abrasion tests were done at VITA headquarters with a toothbrush abrasion testing machine for 32 hours (Vita Zahnfabrik, Germany). The results were analyzed by ANOVA followed by a post-hoc Tukey test (α=0.05).
Results: The mean biaxial flexure stresses for the control and experimental tabs were as follows: Control = 142.06 MPa, 1x = 139.97 MPa, 3x =120.78 MPa, and 5x = 117.56MPa. The control group was significantly stronger than all other infiltrated groups (p =0.007). The 3x infiltrated group was significantly different than the control and 1x infiltrated group (p=0.028 and p=0.044). Mean abrasion weight loss percentages for each Enamic tab group were as follows: Control = 0.43% ± 0.22, 1x = 0.59% ± 0.01, 3x = 0.76% ± 0.01, 5x = 0.45% but were not statistically different ( p = 0.101). Only the control tab was most similar to known abrasion weight loss percentage of VITA Enamic (0.42%). SEM imaging at 5000x magnification revealed that increased infiltration led to increased neck formation.
Conclusions: Our results showed that increasing the neck formation of the pre-ceramic matrix with PDMS in Enamic through low-temperature oxidation has a negative effect on bi-axial flexure stress and abrasion wear. This is in contrast to traditional particle-filled resin-based composites because increasing the volume fraction of ceramic decreased strength and abrasion resistance. Infiltrated Enamic tabs, however, do have less variance in abrasion resistance compared to control tabs.
Hsieh, Jessica K., "Enhanced Ceramic-based Interpenetrating Phase Composites via Pre-ceramic Chemical Necking" (2016). Master's Theses. 969.
J. Robert Kelly, DDS, MS, D.Med.Sc