Mineralization of resin using experimental adhesives containing hydroxyapatite

Objectives: Incomplete resin infiltration into acid-etched dentin leaves water-rich, resin-poor regions in hybrid layers that may cause degradation of resins and collagen fibrils.  Incorporation of hydroxyapatite (HAp) into resins may provide a reservoir of calcium phosphate that might “backfill” water-filled voids in hybrid layers and resins.  The purpose of this study was to test the hypothesis that incorporation of HAp into experimental resins can fill in microvoids with calcium phosphate.

Methods:   Using Clearfil SE-Bond adhesive, three experimental adhesives were created by adding 0, 5 or 10wt% HAp.  Resin disks were made from the experimental adhesives, 6 mm in diameter x 0.5 mm thick.  After light-curing, the disks were placed in saline or simulated body fluid (SBF) for up to 8 wks.  After rinsing and drying, the disks were observed by SEM.  We also bonded normal or demineralized dentin with Clearfil SE-Bond primer (20s) followed by covering the primed dentin with adhesives containing 0, 5 or 10wt% HAp.  Microtensile bond strengths were measured after 24 hrs in water and expressed in MPa.

Results: Hydroxyapatite-like crystals were seen on the surface of resin disks containing 10% HAp for 1 wk or 5% HAp for 8 wks in SBF but not in NaCl.  Microtensile bond strengths of all experimental resins were high and similar (66 MPa) to mineralized dentin and low and similar (16 MPa) to demineralized dentin. 

Table 1:  Microtensile bond strength (MPa)
Dentin substrate	0% HAp	5% HAp	10% HAp
Sound dentin	66.8 ± 7.2a	66.8 ± 4.0a	64.5 ± 4.0a
Demineralized dentin	16.5 ± 2.8b	16.6 ± 2.9b	16.1 ± 1.6b

Values are mean±SD, N=8.  Values identified by different superscripts are significantly different (p<0.01).

Conclusions: HAp had no effect on immediate bond strength.  Future experiments will determine if it improves the durability of bond strength over 6-12 months.