Testing methods and variables Rate of loading

Bond strength is influenced by the speed o f force application. Due to the visco elastic response o f the adhesive the bond strength is dependent on the test rate. The application rate o f the force is generally set by arbitrary agreement at 0.75 (+/- 0.3) mm min'^ (ISO TR 11405), although studies report a range o f rates (see Table 1.2).

Dentine bonding studies have not analysed the relationship between force application speed and bond strength results.

Test geometry

The test methods which have developed over the years have intended to measure shear or tensile bond strengths. A variety o f models have been described for each method. It is well recognised that both the mean bond strength results and the standard deviations o f these types o f experiments vary widely, and it is unlikely that mechanical bond strengths will ever yield consistent data even for one bonding agent under controlled conditions (DeHoff et al 1995).

Shear test

Tests which were designed to apply loads in shear test are relatively easily performed, but there is a tendency to develop a bending moment. Loads can be applied in push mode by a blunt end shear bar or by a knife edge, or in pull mode by a wire loop.

Tensile test

Models intended to test tensile bond strength include a point or uniform application o f load, by virtue o f the geometric configuration o f the sample. Thus the load is applied by incorporating a hook within the restorative material to attach the loading apparatus, but this arrangement causes stress concentration. However even if the whole restorative material is secured in a clamp arrangement the distribution o f the stress along the interface will not be uniform.

Results o f examples o f both shear and tensile bond strength tests for the fourth generation materials used in the in vitro studies in this thesis are included in Table 1.2.

Finite Element Analysis (FEA)

Studies using finite element analysis have demonstrated that the stresses at the interface in either shear or tensile tests are not uniform, but are highly dependent

upon the test geometry and loading configuration (van Noort 1989). Indeed he concluded that the measurement o f bond strength could only give a nominal bond strength value and not the true stress at fracture because o f this non uniform distribution o f stress along the interface. Using a 2D finite element model o f a block o f composite adhering to a flat dentine surface, it was demonstrated that in tensile mode the stress pattern is dependent on the height o f the restoration. The maximum stress appeared within the bulk o f the restoration for composite blocks less than 3 mm, but at the edge o f the block when the height was greater than 3mm. Different stress patterns resulted when using point and uniform tensile loads.

In shear mode the same model demonstrated that the stresses were tensile on the side o f load application and compressive on the far side away from the load. The interfacial stresses increased as the distance between the point o f application and the dentine surface is increased and related to bending moments generated. Planar shear bond tests were further examined by 3D FEA with a cylindrical restoration adherent to a dentine substrate (De H off et al 1995). The patterns o f stress concentration and the underestimation o f the true interfacial bond strength by conventional shear tests were confirmed.

In addition, using the 2D model described above, finite element analysis has been used to demonstrate that high modulus composites have higher stress at the edge o f the restoration(van Noort 1989) and that the extension o f the adhesive in a flash or fillet beyond the interface will result in an artificially high value for dentine bond strength (van Noort et al 1991).

W hilst the FEA studies have highlighted the dangers o f conventional testing techniques and explained some o f the disparity in the results o f published studies, FEA methods themselves provide but a theoretical method o f analysis. The difficulty o f model construction limits the complexity and detail o f the models used. The analysis is further hampered by both the lack o f data on the modulus o f elasticity o f the components o f the interfacial region and the lack o f knowledge o f interfacial stress patterns in vivo. This technique, at present, may therefore be used most

appropriately to confirm and explain failure patterns with differing loads and geometry in vitro, rather than to predict interfacial behaviour under differing conditions in the clinical environment.

Table 1.2 Summary o f bond strength studies.

Examples of methods and results of bond strength studies, using 4* generation dentine bonding systems used in this thesis.

Authors & date Test

Dentine Bonding System

Comments

SBMP OB CFLB2

Barkmeier & Erickson 1994

Shear bond strength 25.5±7.5 MPa.

A ir th in adhesive:

10.1+6.6 MPa

3.66mm diameter cylinder of P50. Stored in water at 37°C for 24hrs. Slightly reduced by extreme air drying of primer, significantly reduced by aggressive thinning of adhesive.

Fortin et al 1994 Shear bond strength 10.5+3.5 MPa 12.9±1.5 MPa _- OB significantly better than SBMP.

No correlation between shear bond strength and microleakage Tam & Pilliar 1994 Fracture toughness

and SEM study _{No figures}

Stored in water at 37°C for 180 days.

Examined morphology of fractured interface;bond failure occurred in the unsupported collagen layer and overlying resin-modified layer. Sano et ai 1994 Microtensile test 38 MPa 55 MPa Stored in water at 37°C for 24 hours, load rate 1mm min'*

Increasing tensile bond strength with decreasing size of the bonded area.

Barkmeier et al 1995 Shear bond strength 19.4+3.1MPa 5 mm diameter sample. Stored in water at 37°C for 24 hours. Load rate 5 mm min*

Bond strength to enamel (28.2 +4.9) significantly better than dentine.

Mason et al 1996 Shear bond strength Inv/frol8.7MPa in vitro 16.5MPa

12.5MPa 12.3MPa

6 mm diameter cylinders, stored for 10 days, thermal cycled. Load rate 0.5 mm min‘*

Significant difference between SBMP and OB in vitro.

Plasmans et al 1996 27.8-12.8.MPa Stored in 30-95% humidity, at 23-27®C for 24 hours. Load rate 2 mm m in’

Shear bond strength dependent on humidity . Yoshyama et al 1996 Microtensile test O cclu sal 17.8MPa

G in g iv al 18.9MPa

16.6 MPa 15.9 MPa

Stored in water at 25®C for 24 hours. Load rate 1 mm min* Gingival and occlusal dentine- no significant difference. Sclerotic and ‘normal’ dentine - significant difference Tam&Yim 1997 Fracture toughness

K .C

0.45 Mhlm'^^ (SD 0.23)

early failure of specimens

- 24 hour storage at 37®C in water. Load rate 0.5mm min'* Bovine teeth, miniature short rod specimen geometry.

Ln

1.7.3 Developments in bond strength testing methods