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The effect of instrument lubricant on the diametral tensile strength and water uptake of posterior composite restorative material: an in-vitro study.

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The effect of instrument lubricant on the diametral tensile strength and water uptake of posterior composite restorative material: an in-vitro study.

Mr J Patel, Miss C Granger




Dental composite materials are widely used in dentistry with increasing popularity and improved mechanical properties and aesthetics1. Despite this they also possess many disadvantages including poor handling during placement, which may impact upon the long-term outcome of the restoration2, 3. This handling difficulty is well documented and many techniques have been suggested to ameliorate placement difficulties and minimise adhesion of dental composite material to instruments4, 5.  Some clinicians advocate the use of various instrument lubricants, such as wiping the instrument with ethanol or coating it in dentine bonding agent5. Whilst this may help improve handling, it may conversely interfere with properties of the material. Thus one may question – in search of improved handling, does the clinician unwittingly modify the material by introducing contaminant into the composite, potentially affecting its properties?


This investigation assessed the effects of instrument lubricant on the diametral tensile strength (DTS) and water uptake of composite restorative material.




300 hybrid posterior composite cylinder specimens (Solitaire 2, Heraeus Kulzer, Frankfurt, Germany) in shade A1 were prepared in polytetrafluoroethylene moulds (5mm diameter x 8mm depth, with 4 x 2mm increments).   4 instrument lubricants were selected: ethanol; 3-step adhesive system (Kerr Optibond; bonding agent bottle); 2-step adhesive system (Kerr Solo-Plus; total-etch primer-adhesive bottle) and 1-step adhesive system (Kerr All-In-One; self-etch, primer, adhesive bottle). Bonding systems were brand standardised to eliminate inter-brand variability. 60 cylinders were prepared within each lubricant group and 60 cylinders were prepared as a control group without the use of instrument lubricant.


Protocol planning included pilot studies, power calculations and protocol refinement. Specimens were prepared by 2 operators, trained and calibrated for their prospective tasks. Following construction, specimens were randomly separated into three experimental groups (100 per group) (‘0-week/immediate’, ‘1-week’ and ‘12-week’) for longitudinal analysis following storage in phosphate buffered saline physiologic buffer (PBS) within individual sealed bottles at 37°C.


Water uptake was assessed gravimetrically with an electrical analytical balance at 1-week and 12-weeks. DTS was assessed at all three time-points on a Universal Testing Machine (Model HK5S, Instron Ltd, High Wycombe, UK), with their long axis perpendicular to the applied compressive load, with a 5KN load cell at a constant crosshead speed of 10mm/min, until the point of failure (machine set to peak hold).




Data were analysed with parametric analysis of variance (ANOVA) and Tukey’s HSD tests using the IBM SPSS statistics package.


Results identified statistically significant differences between specimen groups for both DTS and water uptake. Control specimens exhibited a significantly higher DTS and lower water uptake than all specimens placed with the use of an instrument lubricant (p<0.01). All experiment groups (including control) showed a reduction in DTS over time. Control samples matched evidence based values for DTS6, 7. They also exhibited a rapid early water uptake concordant with previous research literature.


Samples placed with ethanol as an instrument lubricant displayed distinctive banding (see figure 3). These sites commonly proved a weak-point for fracture (see figure 4) and further assessment is necessary here. Fracture patterns varied significantly suggesting a high tendency for catastrophic failure in ethanol and 1-step (self etch) groups (see figure 4). Further investigation of this is required and it is postulated that carbon and proton nuclear magnetic resonance (NMR) spectroscopy analysis may be useful here. 



Diametral Tensile Strength


•All composite specimens placed with an instrument lubricant showed statistically significant reduced DTS, than those specimens placed with no lubricant.
•The proposed mechanism for this is related to a less cohesive interphase between increments and increased water uptake of samples due to the increased presence of unfilled resin content and HEMA which swells in the presence of water.
•2-step and 1-step group specimens had a lower DTS compared to control and 3-step specimens suggesting that the acidic monomer in both these systems, when used as an instrument lubricant, may further impair the mechanical properties of the material.
•Samples placed with ethanol displayed a significantly reduced DTS when compared to control samples and fractures commonly occurred at the increment interface.

Water Uptake


•Statistical analyses identified that use of an instrument lubricant during composite placement significantly increases the rate of water uptake, with those placed using bonding systems as lubricants demonstrating the highest water uptake.
•Composites placed with ethanol as an instrument lubricant showed significantly greater water uptake than the control group, but significantly less water uptake than those placed with bonding agent lubricants.
•This increased water absorption may deleteriously affect the physical and mechanical properties of dental composite.

Clinical Significance


•It is promulgated that the use of instrument lubricants should be discontinued. However if considered necessary, the utilisation of ‘bonding agent’ from a 3-step adhesive system as a lubricant is recommended, as results suggest this has the least deleterious effect upon the properties of dental composite restorative material.


1.Burke, F.J.T., McHugh, S., Hall, A.C., Randall, R., Widström, E. & Forss, H. (2003). Amalgam and composite use in UK general dental practice in 2001. British Dental Journal, 194(11), 613–618.
2.Liebenberg, W.H. (1999). Bonding agent as an instrument lubricant: potential effect on marginal integrity. Practical Periodontics and Aesthetic Dentistry, 11(4), 475–478.
3.Priyalakshmi, S. & Ranjan, M. (2014). A Review on Marginal Deterioration of Composite Restoration. Journal of Dental and Medical Sciences, 13(1), 6–9.
4.Powers, J.M. (2002). Composite restorative materials. Restorative Dental Materials, Edition 11. St Louis: Mosby.
5.Tjan, A.H. & Glancy, J. (1988). Effects of four lubricants used during incremental insertion of two types of visible light-activated composites. Journal of Prosthetic Dentistry, 60(2), 189–194.
6.Della Bona, A., Benetti, P., Borba, M. & Cecchetti, D. (2008). Flexural and diametral tensile strength of composite resins. Brazilian Oral Research, 22(1), 84–89.
7.Penn, R.W., Craig, R.G. & Tesk, J.A. (1987). Diametral tensile strength and dental composites. Dental Materials, 3(1), 46–48.
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