Oxygen inhibition layer of composite resins: effects of layer thickness and surface layer treatment on the interlayer bond strength

An oxygen inhibition layer develops on surfaces exposed to air during polymerization of particulate filling composite. This study assessed the thickness of the oxygen inhibition layer of short‐fiber‐reinforced composite in comparison with conventional particulate filling composites. The effect of an oxygen inhibition layer on the shear bond strength of incrementally placed particulate filling composite layers was also evaluated. Four different restorative composites were selected: everX Posterior (a short‐fiber‐reinforced composite), Z250, SupremeXT, and Silorane. All composites were evaluated regarding the thickness of the oxygen inhibition layer and for shear bond strength. An equal amount of each composite was polymerized in air between two glass plates and the thickness of the oxygen inhibition layer was measured using a stereomicroscope. Cylindrical‐shaped specimens were prepared for measurement of shear bond strength by placing incrementally two layers of the same composite material. Before applying the second composite layer, the first increment's bonding site was treated as follows: grinding with 1,000‐grit silicon‐carbide (SiC) abrasive paper, or treatment with ethanol or with water‐spray. The inhibition depth was lowest (11.6 μm) for water‐sprayed Silorane and greatest (22.9 μm) for the water‐sprayed short‐fiber‐reinforced composite. The shear bond strength ranged from 5.8 MPa (ground Silorane) to 36.4 MPa (water‐sprayed SupremeXT). The presence of an oxygen inhibition layer enhanced the interlayer shear bond strength of all investigated materials, but its absence resulted in cohesive and mixed failures only with the short‐fiber‐reinforced composite. Thus, more durable adhesion with short‐fiber‐reinforced composite is expected.     

Skeletal and dental effects of serial extractions performed with or without maxillary expansion—A retrospective controlled study

Objectives

The study aimed to compare severe crowding treatment's skeletal and dental effects by serial extractions or maxillary expansion and serial extractions in the mixed dentition phase.

Setting and Sample Population

The retrospective controlled study included lateral cephalograms of 78 subjects aged 8.5 ± 1.4 years, 52 consecutively treated because of severe crowding, and 26 untreated controls matched for baseline age and observational period.

Methods

Subjects were clustered according to the treatment modality, either serial extraction (EX) or expansion and extraction (EXP-EX) group. Sagittal and vertical skeletal as well as dental cephalometric parameters were assessed at baseline and after the eruption of all permanent posterior teeth, and group comparisons were performed.

Results

Both treatment modalities significantly affected the vertical skeletal parameters in terms of decreasing the mandibular and occlusal plane inclination and increasing the facial height index. A distinct treatment effect on the gonial angle was observed, with a significant decrease in its superior part observed in both extraction groups. The annualized changes in the superior part of the gonial angle significantly differ (P = .036) between the Control (−0.04 ± 0.6), EX (−0.44 ± 0.6) and EXP-EX (−0.34 ± 0.5) groups. Upper and lower incisor inclination did not change significantly in any of the groups; however, the interincisal angle at follow-up was significantly smaller in the Control compared with both treated groups.

Conclusions

Serial extractions and a combination of maxillary expansion and serial extractions have similar significant skeletal effects, mainly affecting vertical cephalometric parameters if performed during the pre-pubertal growth phase.