Development of a remineralizing calcium phosphate nanoparticle-containing self-etching system for orthodontic bonding

Abstract

Objectives: This study aimed to incorporate hydroxyapatite nanoparticles (nHA) or amorphous calcium phosphate nanoparticles (nACP) into a self-etch primer (SEP) to develop a simplified orthodontic bonding system with remineralizing and enamel preserving properties.

Materials and methods: nHA and nACP were incorporated into a commercial SEP (Transbond™ plus) in 7% weight ratio and compared with the plain SEP as a control. Shear bond strengths (SBS), enamel damage, and adhesive remnant index (ARI) scores were evaluated at 24 h and post 5000 thermocycling. Field-emission scanning electron microscope (FESEM) was used to inspect the distribution of the nanoparticles in the experimental SEPs and evaluate the enamel surface integrity both before bracket bonding and post bracket debonding. Phase determination and remineralizing capability of the modified SEP were characterized by X-ray diffraction and Raman spectroscopy, respectively.

Results: The addition of nHA or nACP to the SEP significantly reduced the SBS, ARI, and enamel damage (p < 0.05) as compared to the control SEP; however, only nHA-SEP survived the thermocycling protocol and yielded acceptable SBS (13.38 MPa). Enamel remineralizing ability of the developed nHA-SEP was confirmed by both FESEM images and Raman phosphate map.

Conclusions: Incorporating nHA into SEP resulted in clinically acceptable bond strengths with remineralizing ability.

Clinical relevance: The newly developed nHA-SEP has unprecedented ability to simultaneously etch, prime, and remineralize the enamel in a single step leaving immaculate enamel surface with the potential of saving cost and time at the post-debonding step.

Keywords: Adhesive residue; Bond Strength; Calcium phosphates; Enamel damage; Enamel remineralization; Self-etch primers.

Fig. 1

A flowchart summarizing the experiments implemented in the study

Fig. 2

FESEM images of debonded enamel surfaces etched with the control SEP. a and b Bracket debonding after 24 h water storage. c and d Bracket debonding post thermocycling. a and c Retention of all or considerable amounts of adhesive. b Chipping of enamel (arrow). d Cracking of enamel (arrow) with adhesive retention

Fig. 3

FESEM images of debonded enamel surfaces etched with nCaP-SEP. Immaculate buccal enamel surfaces with no or minimal adhesive retention following etching with 7% nHA-SEP (a–c) and 7% nACP-SEP (d, f); bracket debonding was conducted after 24 h storage in water (a, d, e) and post thermocycling (b, c, f)

Fig. 4

X-ray diffractograms of control SEP, nHA, nHA-SEP, nACP, and nACP-SEP samples. Peaks denoted with triangles are identified as HA single crystalline phase (JCPDS No. 09–0432)

Fig. 5

FESEM images displaying the distribution of nanoparticles within the polymerized experimental SEPs. a nHA-SEP: spherical or rod-shape nHA are regularly distributed in isolated and agglomerated forms of particle size below 200 nm, and agglomeration size up to 484.17 nm; b nACP-SEP: spherical nACP showing more dispersion with isolated particle size below 150 nm, and agglomerated particles up to 222.87 nm

Fig. 6

FESEM images showing the etch-patterns of premolar buccal enamel surfaces etched with the control (CSEP) (a, b), 7%nACP-SEP (c, d), or 7%nHA-SEP (e, f). Etching with CSEP yielded open micro-pores with etch-patterns varied from type 2 (a) to type 3 (b); etching with 7%nACP-SEP displayed mostly type 3 etch-pattern (c, d) with nCaP precipitates (c′, d′), while the 7%nHA-SEP revealed a milder etching effect presenting smooth surfaces (e, f) with evident nCaP precipitations in the form of small granules (e′) or larger aggregations (f), with partial or complete micro-pores obliteration (e, f)

Fig. 7

Raman spectra of untreated enamel (red curve), enamel treated with 7%nHA-SEP (green curve), and enamel treated with the control SEP (blue curve). The spectra of the three areas depict five characteristic bands: four phosphate ion vibration modes (ʋ1-PO at 960 cm⁻¹, ʋ2-PO at 430 cm⁻¹, ʋ3-PO at 1045 cm⁻¹, and ʋ 4-PO at 580 cm⁻¹), and B-type carbonate ion at 1075 cm⁻¹. Raman peak at 960 cm⁻¹ typifies tetrahedral PO4 group (P–O bond) as the highest phosphate peak within HA; the untreated enamel zone demonstrated the highest intensities at all phosphate peaks, followed by nHA-SEP and CSEP consecutively, signifying the dissolution of HA during enamel treatment