Comparison of bonding mechanism of a self etching primer and phosphoric acid self etching to enamel by scanning electron microscope-–An in vitro study

Introduction

Direct bonding has revolutionised and improved the clinical practice of orthodontic procedure.¹ A bio-material that provides freedom of isolation, rinsing, reduction of number of steps and good bond strength forms the criterion for ideal orthodontic bonding material. The advent of self etching primer expedites the bonding procedure by combining etching and priming into single step.² Debonding brackets after self etch primer application is also easier and quicker compared with acid etch. As maintaining a sound, unblemished enamel surface after debonding is a primary concern, self etching primers have an added advantage. The incorporation of self etching primers in orthodontics has led to considerable differences of opinion regarding their method of usage, their bond strength, cost and wastage. While there are several controlled studies published to evaluate the shear bond strength of different self etch primers used in orthodontics,^3-6 studies on the bonding mechanism of self etch primer on enamel is limited. Hence the present study was conducted to determine and compare the etching pattern of self etch primer and conventional phosphoric acid etch primer on enamel of human permanent teeth by scanning electron microscopy and also to quantitatively analyse the shear bond strengths.

Material and Method

An in vitro study was conducted at the department of orthodontics and Dentofacial orthopaedics, Tamil Nadu Government Dental College and Hospital, Chennai.

Eighty sound non-carious premolar teeth which were extracted for orthodontic purposes were collected. The teeth were washed in water and stored in 0.1% (weight per volume) thymol solution for four weeks before use. The buccal  surfaces were cleaned and polished with a rubber cup and pumice slurry followed by rinsing with water spray and drying with compressed air. The teeth were then embedded in rectangular shaped blocks made of self cure acrylic such that the crowns were fully exposed and roots completely embedded in the blocks. These acrylic blocks were again colour coded into group I (Navy blue) and group II (red) (Fig I ). A total of 80 brackets (stainless steel, gemini series, 3M unitek) of 0.022” (roth) slot size were used. The two different enamel conditioners (etching) used were 37% orthophosphoric acid     (3 M ESPE) and Transbond plus self etch primer (3M unitek) . Light cure tranbond XT adhesive (3M unitek) was used for both phosphoric acid etching and self etching primer bonding procedure.

In group I (navy blue acrylic block) forty teeth were etched with 37% phosphoric acid gel for 15 seconds. The teeth were thoroughly washed and dried, sealant was applied and brackets were bonded with Transbond XT ( 3M unitek) and light cured (Ortho lax XT,3M unitek) for 20 seconds in accordance with the manufacturer’s  instructions.

In group II (red colour) of remaining forty teeth ( in the red acrylic blocks), Transbond plus self etching primer was used. The primer was applied on the buccal surface as a thin film, left for 15 seconds as per the manufacturer’s instructions. For activation the two components were squeezed together  and resulting mix was applied directly to the tooth surface. Transbond XT(3M unitek) paste was applied on the bracket base and the bracket was then positioned on the tooth and pressed firmly along the long axis of the crown to expel any excess adhesive which was subsequently removed. Light curing was done for 20 seconds.

Each group was divided into 2 sub-groups of 20 teeth each. The first subgroup  was evaluated for the amount of demineralisation and penetration of the adhesive in the enamel surface by scanning electron microscope. The second subgroup was tested for the shear bond  strength of conventional etch and the self etch primer.

Scanning electron microscopy

The etch pattern of the adhesive infiltration in the enamel  was evaluated by  Scanning Electron Microscope (JEOL JSM- 6360 , Japan Electronics Tokyo, Japan) at Anna University, Chennai.(Fig. 2)

The crowns were sectioned from the roots with a diamond disc at the cementolabial enamel junction, and  each crown was cut longitudinally in a mesiodistal direction. All samples were stored for one week in distilled water at room temperature.

The brackets with the remnant tooth substance was submitted for demineralization cycles which promoted complete dissolution of the dental fragments. On an average, five consecutive cycles were carried out, which were composed of 10% chloridric acid solution (32%HCl :Fisher Scientific, Qualigens Fine Chemicals, India.) for five hours at room temperature and 5% sodium hypochlorite solution(NaClO :Fisher Scientific, Qualigens Fine Chemicals,India.) for one hour at room temperature, followed  with a five–minute distilled water rinse .

After complete dissolution of the dental tissues, the bracket bases were  sputter-coated with gold (JEOL, JFC 1600 Japan Electronics) (Fig.3) and evaluated under a scanning electron microscope. To standardize the microscopic observations, the microphotographs of bracket base  were made at 850X,  1000X  and 3500X  obtained from  the center of the samples.

Bond strength testing    

The shear bond strength was tested using Llyods Universal Testing machine(Llyod Instruments Limited, Segensworth, England) Model 100K at CIPET, Chennai.(Fig. 4) It was set at a cross head level of 1 mm per minute  to debond the brackets .

The acrylic blocks  mounted with the  specimens were secured to the lower grip of the machine (fixed head) and a custom made  blade was fixed in the upper  grip (movable head) that was  connected to a  load cell. The bevelled  blade was positioned in such a way that it touched the bracket (Fig.4 ).An occlusogingival  load was applied to the bracket which produced shear force at the bracket  tooth interface. The cross head speed was adjusted to 1mm/minute and was measured in Newtons . The force  at which the bracket debonded  was recorded from the digital display .

The bond strength was calculated in Megapascals by using the following formula (Brantely)⁷

Bond strength in (Mpa)= Force in (Newtons) / Surface area of the bracket in mm²

The readings of the following were recorded:

• The etch pattern of conventional and self etch primer after demineralization  on scanning electron microscope.
• The debonding force of conventional and self etch primer at 24 hours  of time.
• The shear bond strength of conventional and self etch primer at 24 hours  of time. 

Results

Student t Test

Student’s t Test was employed to compare the values between the two groups. The following formula was used:

T = │X₁ –X₂ │ / √S² (1/n₁ +1/n₂ )

Where

S2= (n₁ -1) S₁² + (₂ -1) S₂² / (n₁ -1) +(n2-1)

X₁ - X₂         = Sample means

S₁² and S₂² = Sample variance

n₁ and n₂ = Sample sizes

The descriptive statistics for the shear bond strengths of the 2 groups are given in the tables I and II above. The results of the student t test ( t=6.54  ) indicated that the shear bond strengths were significantly different(p=<.001 ).The mean debonding force  of Group I is 125.70N and of Group II is 108.94N. The conventional adhesive primer system had a mean shear bond strength of   11.85 MPa, and the self etch primer had a shear bond strength of 10.27 MPa. The values indicate that it is statically significant.

Discussion

Conventional etching with phosphoric acid etching causes a selective solubility of enamel and roughens the surface with deeply penetrating micro pores. The composite resin adhesives are viscous and they cannot penetrate the acid etched micropores. A sealant or primer or a bonding agent is applied after etching which on polymerization provides the mechanical retention to the adhesive by flowing into the etched undercuts to aid in retention. Conventional etching with 37% orthophosphoric acid is the most commonly used technique for bonding brackets to the enamel surface as it provides adequate bond strength. 

The drawbacks of acid etching are it causes loss of enamel during etching, fracture and cracking of enamel upon debonding, retaining of resin tags in the enamel following debonding which results in the discolouration of the teeth. This technique requires rinsing and drying the tooth surface after application of the etching agents as it is hydrophobic and is efficient only in a dry environment causing an increase in chair time. 

Acid etching removes about 10-20µm of enamel which renders the  enamel to become  porous making it susceptible to stains. Debonding technique causes fracture of small enamel fragments and cracks resulting in  an additional 6-50 µm   of enamel loss.  Wide variations in enamel surface loss from as little as 10 µm to 30 µm to as much as 170 µm have been reported in the studies of Burapavong⁸. Wayne Barkmeier⁹  has stated that  the enamel lost during acid etching has been found to depend on the acid. The most commonly used acid being   37% ortho phosphoric acid, with an etch time of 15 to 30 seconds per tooth. 

According to Gorelick^10, phosphoric acid technique has been reported to be associated with decalcification and the development of white spot lesions around bonded orthodontic appliances. Studies by Barkmeier⁹, Phillip Campell¹¹, Gorelick¹⁰, Deidrich¹² had reported incidences of enamel fracture on debonding following the use of phosphoric acid as an enamel conditioner. The need to minimize the significant enamel loss during orthodontic treatment had led to the development of newer bonding techniques. 10% Maleic acid was proven to have decreased bond strength. Other alternatives such as air abrasion and laser etching proved to be of less damage to the enamel surface but invariably produced lesser bond strength and were cumbersome.

Self-etching primer is one such innovation that simultaneously acts as an etchant and primer. In a self-etching primer, the active ingredient is a methacrylated phosphoric acid ester. The phosphoric acid and the methacrylate group are combined into a molecule that etches and primes at the same time. The phosphate group on the methacrylated phosphoric acid ester dissolves the calcium and removes it from the hydroxyapatite. As it is not rinsed away, the calcium chealates with the phosphate group to form a complex and then gets incorporated into the network when the primer polymerizes. Three mechanisms act to stop the etching process. First, the acid groups attached to the etching monomer are neutralized as in phosphoric acid, by forming a complex with the calcium from the hydroxyapatite. Second, as the solvent is driven from the primer during the airburst step, the viscosity rises, slowing the transport of acid groups to the enamel interface.

Finally, as the primer is light cured and the primer monomers are polymerized, transport of acid groups to the interface is stopped.  According to Fox et al¹³   extracted premolars form the basis of most bonding tests due to the ease in obtaining test specimens following therapeutic orthodontic extractions. In the present study pre-adjusted brackets of 3M Unitek of Gemini series with fine mesh base were used in accordance with studies of Maijer and Smith¹⁴ as the fine mesh base provides best resin penetration and bond strength. For the study of etching pattern by scanning electron microscope the photomicrographs obtained were analysed using a four point scale of Silverstone 15.

Type A - well developed conventional etch pattern with well defined prisms.

Type B - prisms apparent but poorly defined.

Type C - no prism definition , but surface roughening has occured.

Type D - no discernible changes in enamel surface ie, apparently no etching has occured.

Phosphoric acid etching;

The photomicrographs of conventional primer on scanning electron microscope revealed the  type A etch pattern ( Fig. 5-7).

Self etching primer:

The photomicrographs of the self eching primer revealed the type C etch pattern ( Fig.  8-10).

According to Hobson16, the development of a well defined etch pattern is not necessarily a pre requisite, as high bond strength itself was not necessarily the major factor in determining bond strength. Scanning electron microscopy study done by Zafer Cehreli17 showed that shorter etching time produced a smoother enamel surface compared with longer treatment indicating decrease in absolute enamel loss. Davari and Daneshkazemi18 compared self-etching primers with conventional primers and concluded that the enamel-etching pattern was different with self-etching primers demonstrating a shallower etch pattern. According to Dunn 19, calcium and phosphorous ions released from the dissolution of the hydroxyapatite crystals are suspended in the primer solution itself as the acidic primer is not rinsed off during application. This high concentration of calcium and phosphorous ions will limit further dissolution of the apatite crystals resulting in the reduction in the depth of enamel demineralization.

The result of this study showed that the shear bond strength in group I (phosphoric acid etching) to be 11.85 MPa which was significantly higher than group II 10.27 MPa  (self etching primer) which is within the range quoted by Reynolds1. The values obtained for self etching primer  in this study (10.27 ± 0.65 MPa) were similar in range obtained by Brian Trites6, Bishara20, Davari18, Arnold2 .The topography of etched surface enamel, the duration of etching and concentration of the etchant could also be important factors influencing bond strength13.

A gentler etch pattern of self etching primer on the enamel surface was observed and it could be used for minimal intervention in the orthodontic procedures21. The findings of the current study suggest that the development of a well defined etch pattern is not necessarily a pre requisite for high bond strength and that the relationship between the etch pattern and bond strength is complex. This is similar to the study by Hobson22 . Hence the use of self etching primer would be advantageous and desirable as it overcomes the limitations of phosphoric acid etching by reducing the enamel damage during debonding and the clinical steps thereby saving patient chair time and yet providing clinically acceptable bond strength.  

Future studies

This was an in vitro study and care should be observed in comparing the results with those that might be obtained in vivo. In addition, more research, is needed to determine the shear bond strength of the self etch primer over a longer time period  maybe a week or a month after bonding and after thermo cycling procedures.

Conclusion          

In the present study the etching pattern obtained with self etch primer  was compared to that of conventional phosphoric acid on enamel surface by scanning electronic microscope and the shear bond strength in both the conditions was also quantitatively analysed. The etch pattern of self etch primer was found to be shallower when compared to conventional phosphoric acid etching indicating minimal damage of enamel. Though the shear bond strength of self etch primer was lower than that of conventional phosphoric acid, it was within the acceptable range. Hence a well defined etch pattern might not be a prerequisite for high bond strength inferring that high bond strength values may be obtained even with lesser penetration.