TOSU Addends Maintain Mechanical Properties while Decreasing Polymerization Stress

Objective: The prevalence of composite-resin restorations continues to increase due to their aesthetic appeal. However, their functional life is significantly less than their metal counterparts, potentially due to the effect of polymerization stress on marginal integrity.  Tetraorthospiroundecanes (TOSUs) are expanding monomers that have been found to reduce polymerization stress in silorane resins and composites. The objective of this study was to determine if TOSU addends (Addend1 or Addend2) maintain mechanical properties of BisGMA/TEGDMA (BT) while decreasing polymerization stress.  

Method: Flexural strength and elastic modulus were measured using beam specimens (2x2x25mm) of neat photoinitiated BT (control) along with samples containing 1, 5, 10, or 20wt% of either TOSU Addend1 or Addend2.  Beams were irradiated with a dental lamp for 4min (2min top, 2min bottom), stored 24hrs prior to testing on a mechanical tester (4-pt bend, loading rate 3.7mm/min). Polymerization stress of samples containing 50wt% unmodified glass filler or 50wt% filler plus 5wt% of either TOSU Addend1 or Addend2 was measured between two abraded glass rods (5mm diameter) coated with cyanoacrylate-glue held 1mm apart by a mechanical tester. The composites were irradiated using three dental curing lamps (80s). Maximum load (recorded for 30mins post irradiation) was divided by cross-sectional area to calculate polymerization stress.

Result: The addition of up to 5wt% of TOSU addends to BT (Addend1: strength [95.53±2.48MPa], modulus [2.47±0.17GPa]; Addend2: strength [103.64±10.88MPa], modulus [2.51±0.09GPa]) did not significantly decrease mechanical strength (p<0.05, n=8) from the neat BT control (strength [103.81±10.59], modulus [2.72±0.09GPa]).  The addition of 5wt% addends to BT composite filled with 50wt% unmodified glass significantly decreased polymerization stress (p<0.05, n=6) from control [5.15±0.70MPa] to Addend1 [3.45±0.34MPa] and Addend2 [3.98±0.63MPa].

Conclusion: The addition of TOSU addends significantly lowers polymerization stress in methacrylate composites while maintaining good mechanical properties. Supported in part by: NIH-NIDCR Grant T32 DE07294 and NIH-NIDCR Grant R21 DE018336.