همایون علاقه مند

Ensuring the long-term stability of orthodontic treatments and preventing tooth relapse remains a significant challenge in orthodontics, often addressed through the extended use of retainers. Given the pivotal role of retainers' physical and mechanical properties in this regard, this study aimed to investigate the tensile strength, flexural strength, and deflection of three distinct types of fixed orthodontic retainers.

In this laboratory study, each group comprised 12 samples of twisted, flat, and coaxial retainer wires, each measuring 8 mm in length. These samples were affixed onto the polyethylene blocks using cyanoacrylate adhesive and subsequently subjected to compressive loads using a universal testing machine. The maximum bending strength and deflection rate were recorded from stress-strain diagrams. To evaluate the tensile strength, the identical samples underwent tensile loading. Statistical analysis was performed utilizing SPSS software, employing analysis of variance tests to compare data between groups.

Significant differences were observed in the bending strength (P=0.004), deflection (P<0.001), and tensile strength (P<0.001) among the investigated retainer wires. The flat retainer wire exhibited the highest bending strength, averaging 822.876±136.07 MPa, while the coaxial retainer wire showed the lowest, with an average of 673.65±73.35 MPa. Regarding deflection, the flat retainer wire displayed the highest average at 4.04±0.6 mm, surpassing the other two types of retainers. However, the tensile strength of the twisted retainer wires, averaging 699.7±83.46 MPa, exceeded that of both the flat and coaxial retainer wires.

The flat retainer wires demonstrated the highest bending and deflection strength, while the twisted retainer wires exhibited the highest tensile strength. Overall, the bending strength, deflection, and tensile strength of the 3-strand twisted and flat retainer wires outperformed those of the 5-strand coaxial retainer wires, increased flexural and tensile strengths imply a higher resistance of the wires to the forces induced by harder and more viscous foodstuffs.

The aim of this study was to investigate the effect of different surface treatments on the microtensile bond strength (µTBS) of two types of composite substructures with Vita Mark II ceramics by resin cement.

Sixty-four substructure specimens were molded from two dual-cure composites Core.it and Build-it, equally, and cured by LED light. The specimens of each group were randomly divided into 4 subgroups (n=8) treated by one of HF acid 10%, air abrasion, Er: YAG laser, and one group without any treatment (control group), and then the specimens of each group were bonded to Vita Mark II CAD/CAM ceramic blocks using two Duo-Link and Panavia F 2.0 resin (n=4 and 20 slice in any group). Each final specimen was thermocycled between 5 °C and 55 °C for 2500 cycles and then cut by a slow speed saw to obtain 5 sticks with cross-section dimensions of about 1×1 mm². The µTBS test was done at a speed of 0.5 mm/min by Universal Testing Machine. The fracture pattern was then determined using a stereomicroscope. Statistical differences between groups were determined by one-way ANOVA using Tukeychr('39')s multiple comparison tests.

Among all 16 groups, the highest µTBS was observed in the group with Core.it substructure composite and Duo-link resin cement without any surface treatment and after that in the second step in build-it substructure composite group and Panavia resin cement without surface treatment. The most common fracture pattern in all groups was cohesive in resin cement (P value<0.05).

According to this study, composite substructure surface treatment by hydrofloridric acid, laser and air abrasion reduced µTBS between substructure- ceramic and so  is not recommended.

 Dental ceramics are considered as materials that can restore the appearance of natural teeth. Etching the inner surface of a ceramic restoration with hydrofluoric acid (HF) followed by using a silane coupling agent is a well-known and recommended method to increase the bond strength. The aim of etching on ceramic structure is to enhance the surface roughness (Ra) and energy and to cleanse the bonding area. The aim of this study was to evaluate the effect of different HF concentrations and etching times on the Ra of IPS e.max CADTM and Vita mark IITM.

 Two HF concentrations (5% and 10%) and three etching times (20, 60 and 120 seconds) were evaluated. Etched patterns were observed by scanning electron microscopy (SEM) and Ra was measured using atomic force microscopy (AFM). Surface element analysis was performed using energy dispersive X-ray spectroscopy (EDAX). Data were analyzed on SPSS 20 using ANOVA and T-test.

 The Ra had no significant difference among various Vita mark IITM specimens (P=0.973). Among IPS e.maxTM specimens etched with 5% HF, the AFM results showed that 20-s etching time had the lowest Ra and among those etched with 10% HF and 120-s etching time had the most Ra. In IPS e.maxTM specimens etched with acid for 20 s, a significant difference was observed in Ra of 5% and 10% acid concentrations (5% HF lower than 10% HF) (p=0.012).

  Among IPS e.maxTM specimens etched with 5% and 10% HF, increasing the etching time lead to higher Ra. For both IPS e.maxTM and Vita mark IITM, 20-s etching with 5% HF provides acceptable Ra for the bond.

The purpose of this study was to evaluate the addition of sodium fluoride to fifth-generation adhesive (Solobond M) on the degree of microleakage and type I collagen content of dentin.

The present in vitro study was conducted on 120 orthodontically extracted human premolars devoid of decay and anatomical defects. Two series of 20 premolars were selected to test immunohistochemistry (IHC) and two series of 40 premolars to evaluate microleakage in two time points of 24 hours and 3 months. In both tests and at both time points, the tests were performed on the samples divided  into four groups: 1-control (only Solobond M), 2-bonding group (Solobond M) containing fluoride, 3-bonding group (Solobond M) containing Chlorhexidine (CHX) after acid etching, and 4-bonding group (Solobond M) containing fluoride+CHX after acid etching.

The IHC score at 24 hours and 3 months was significantly higher in the CHX, Fluoride, and Fluoride+CHX groups compared to the control group. The IHC score in the CHX+fluoride group was higher than that in the CHX group (p=0.04). The degree of microleakage at 24 hours and 3 months was significantly lower in the Fluoride+CHX and Fluoride groups compared to the control group. The degree of microleakage in the Fluoride group was lower than in the CHX group. The IHC score and the microleakage degree had no significant difference in 24 hours and 3 months between the Fluoride+CHX, Fluoride and CHX groups.

It seems that the effect of fluoride on non-degradation of collagen is greater than that of the CHX.

Previous researches showed that neutral soaps significantly increased bond strength and decreased microleakage of dentin bonding agents. The aim of this study was histopathological evaluation of the effect of sodium sulfonate application as neutral soap on the cat's dental pulp after an ideal composite restorative treatment.

Ten adult cats with 40 caries-free canine teeth were selected for the study. Class V cavities were prepared in the buccal surfaces of all the teeth. The teeth of the right quadrants were treated with composite, whereas the teeth of the left quadrants were primarily etched, after using of sodium sulfonate solution, they treated with composite. The first five cats were killed after one week and the other five cats after one month. The teeth were fixed, decalcified and after being sliced, paraffined blocks were made and microscopic slides were prepared and histologically evaluated.

The results of the histological evaluation showed that there were no statistically significant differences between the intensity of inflammation in the pulp of the teeth in the right and left quadrants after one week (p=1.000) and one month (p=0.773). However, there was significant difference after one week regarding predentin formation (p=0.017) and in the teeth of the left quadrants, the formation of predentin was more that the right quadrants, but no meaningful difference was observed after one month (p=0.203).

It seems that the sodium sulfonate in composite restorative treatments without fear of damage to the pulp can be used.