The effect of instrument lubricant on the diametral tensile strength and water uptake of posterior composite restorative material

Abstract

Objectives

This in-vitro study investigated the effect of ‘instrument lubricants’ used during placement of composite restorative material, on the diametral tensile strength (DTS) and water uptake of composite specimens.

Methods

300 posterior composite cylindrical specimens were manufactured: 60 with each instrument lubricant (ethanol, 3-step, 2-step and 1-step ‘bonding agent’) and 60 with no lubricant (controls). Each set of 60 specimens was evenly allocated to one of the following test groups (n = 100/group): Group 1 – tested for DTS immediately after manufacture; Groups 2 and 3 – tested for DTS after immersion in phosphate-buffered saline (PBS) for 1 and 12-weeks respectively, using a Universal Instron machine. Water uptake was assessed gravimetrically. Data were statistically analysed with two-way ANOVA and Tukey’s post hoc test (α = 0.05).

Results

The mean DTS and percentage weight change of composite specimens ranged between 32.49–53.14 MPa and 0.51–1.36% and varied with lubricant used and time incubated in PBS.

All control groups exhibited significantly higher DTS (MPa) (groups 1–3: 53.17 ± 1.78; 50.64 ± 1.85; 45.17 ± 1.77) and lower percentage weight change (groups 2–3: 0.51 ± 0.03; 0.61 ± 0.01) than specimens placed with an instrument lubricant, with significant differences between certain lubricant groups.

Conclusion

Data from the present study suggest that the use of instrument lubricant may adversely effect the DTS and water uptake of composite restorative material.

Clinical Significance

The use of instrument lubricants to aid composite placement is widespread however based on the data obtained it is suggested that discontinuing or limiting the use of instrument lubricants, and if necessary using the ‘bonding agent’ from a 3-step adhesive system is recommended as results suggest this has the least deleterious effect upon material properties.​

Introduction

Dental composite materials are widely used in restorative dentistry with an increasing popularity due to a decline in amalgam use and comparative advantages, such as pleasing aesthetic properties and preservation of tooth tissue . However, despite their popularity, dental composite materials also possess numerous disadvantages, one of which relates to difficulty in handling during placement . Indeed, clinicians widely report adherence of the material to the application instrument and ‘pull-back’ during insertion and condensation . There have been many techniques suggested to overcome this handling challenge, including the development of plastic coated and titanium nitride instruments, instrument wetting resins and the use of various ‘lubricants’ to coat instruments .

Whilst the difficulty of dental composite placement may be ameliorated by the use of an instrument lubricant, this technique should not contaminate or interfere with the setting, chemical or physical properties of the material. Previous research has clearly identified numerous contaminants during placement, such as: blood; saliva; astringents; haemostatic agents; dentine desensitisers and even powder from latex gloves, which can negatively impact upon material properties . As such, although the use of instrument lubricants may improve ease of placement, it may be questioned: in search of improved handling, does a clinician unwittingly ‘modify’ or ‘contaminate’ the material by incorporating instrument lubricant into the polymerised composite, and thus affect its material properties?

There is a dearth of research evidence to demonstrate the impact of instrument lubricant use. Previous studies have investigated the effect of various bonding agent lubricants on the strength of dental composite material, however the methods used to construct specimens differed greatly from those used in the clinical environment, data remain contradictory and none investigated the use of newer 7th generation self-etch bonding systems.

Furthermore, there has been no definitive research to date investigating the possible effects of instrument lubricant use on water uptake . Composite materials in the oral cavity undergo interaction with saliva , however none of the previous studies subjected their composite specimens to an immersion period to simulate such an environment. The inclusion of additional resin matrix from use of a lubricant may increase water uptake and result in increased hydrolytic degradation and have significant deleterious clinical consequences .

Study aims and hypothesis

The present study assesses the impact of a range of instrument lubricants including a 7th generation one-step self-etchant adhesive system. It also employs more clinically relevant composite placement methods, in order to address limitations in previous methodologies. In addition, the possible effects of instrument lubricant on the water uptake properties of composites are also investigated. Moreover, a longitudinal approach, involving a 1-week and 12-week immersion period, to simulate the oral cavity environment is utilised. Thus, the formulated aim for the present study was to evaluate whether the use of various ‘instrument lubricants’ during composite placement affects the diametral tensile strength and water uptake of posterior dental composite material. The null hypothesis tested was that ‘instrument lubricants’ do not affect the diametral tensile strength or water uptake of posterior hybrid dental composite restorative material.

Materials and method

300 hybrid posterior composite cylinder specimens (Solitaire 2, Heraeus Kulzer, Frankfurt, Germany) in shade A1 were prepared in polytetrafluoroethylene (PTFE) moulds, each measuring 5 mm in diameter and 8 mm in depth (comprising 4 × 2 mm increments). Four instrument lubricants were selected: ethanol; 3-step adhesive system (Kerr Optibond , use of adhesive bottle only); 2-step adhesive system (Kerr Solo-Plus, use of total-etch bottle only) and 1-step adhesive system (Kerr All-In-One, self-etch bottle). Please see Table 1 for base components of the adhesive systems used. Bonding systems were brand standardised (Kerr UK Ltd, Peterborough, UK) to eliminate the possibility of inter-brand variability. 60 cylinders were prepared with each of the four selected instrument lubricants and 60 cylinders were prepared as a control group without the use of instrument lubricant.

Table 1
Base components of the adhesive systems used as instrument lubricants .
Adhesive System → Optibond FL
Adhesive bottle
(3 step)
Optibond Solo Plus
Total-etch bottle
(2 step)
Optibond All In One
Self-etch bottle
(1 step)
Components ↓
Adhesive monomer 2-hydroxyethyl methacrylate (HEMA)
2-hydroxy-1,3-propanediyl bismethacrylate
2-hydroxyethyl methacrylate (HEMA)
2-hydroxy-1,3-propanediyl bismethacrylate
2-hydroxyethyl methacrylate (HEMA)
2-hydroxy-1,3-propanediyl bismethacrylate
Self-etching adhesive monomer x x Glycerol phosphate dimethacrylate (GPDM)
Filler Alkali fluorosilicates Alkali fluorosilicates Alkali fluorosilicates
Solvents x Water
Ethyl alcohol
Water
Ethyl alcohol
Methyl alcohol
Propanone

Protocol planning included pilot studies, power calculations for number of specimens required and control of specimen size with dimension/weight measures. All specimens were prepared by 2 operators, with both trained and calibrated for their prospective task. Operator 1 placed the composite material and operator 2 light polymerised the cylinders. This protocol was adopted to eliminate inter-operator variability/error and increase reliability.

Operator 1 placed each 2 mm layer of composite with a stainless steel plugger. Within the control group, the composite was placed and contoured with a ‘non-lubricated’ plugger. For each of the four experimental lubricant groups, the plugger was dipped into each respective lubricant for 1 s (up to a 3 mm marked line on the instrument) and left to drip/drain for 2 s prior to use. No mixing of the adhesive lubricants was required prior to use, as only the (single component) adhesive bottle of the 3-step, the total-etch bottle of the 2-step and self-etch 1-step bottle were utilised.

Operator 2 polymerised each incremental layer for 40 s with a halogen light cure lamp (Elipar , 3M ESPE, Minnesota, USA). Each composite layer was 2 mm in height, to allow for optimal polymerisation . A glass slab was placed beneath the mould to support material during placement and above on the final increment, to prevent formation of an oxygen-inhibition layer. The light curing unit was held as close to the specimen as possible and cured at a minimum threshold intensity, as per the manufacturer’s instructions. The output of the halogen lamp was calibrated after completion of every 20 cylinders using a handheld light-meter and the intensity was maintained above 450 mW/cm 2 to ensure optimal polymerisation (range 450 mW/cm 2 –550 mW/cm 2 ).

Following light polymerisation, the specimen cylinders were removed from the PTFE moulds and weighed on an electrical analytical balance (Model SI-403, Denver Instrument, Colorado, USA). Within each lubricant group, specimens were then randomly separated into three experimental groups: ‘0-week/immediate’, ‘1-week’ and ‘12-week’. The 1-week and 12-week specimens were subject to longitudinal analysis following storage in phosphate buffered saline (PBS). PBS was used to maintain a physiologic environment similar to saliva . For the ‘1-week’ and ‘12-week’ analyses each specimen was placed separately into a small glass storage bottle (with a sealed, stoppered lid) containing 10 ml of PBS and were then stored in an incubator at 37 °C, to replicate the temperature of the oral cavity. 100 specimens were incubated for 1 week and another 100 incubated for 12 weeks. Following storage for each time period, the specimens were removed with tweezers from the individual storage bottles and allowed to drip/drain on a rack for 1 min to remove excess PBS, prior to water uptake and strength testing.

Water uptake was assessed gravimetrically, by weighing the specimens at the time of manufacture and again after immersion in the PBS, on an electronic analytical balance. For each specimen, the percentage weight change over time was calculated.

The cylinder specimens were tested for DTS 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 10 mm/min, until the point of failure (machine set to peak hold). This compressive test identified fracture load (recorded in Newtons) and the DTS was calculated from this (in MPa) utilising the following formula :

DTS = 2F/πdh

where:d = specimen diameter (5 mm).h – specimen height (8 mm).π = 3.1416.

Diametral tensile strength was used as an indicator of mechanical strength to the point of fracture, as it closely replicates forces placed on restorative material in a posterior tooth in the oral cavity .

The data obtained for both water uptake and DTS assessments were analysed with descriptive statistics, parametric analysis of variance (two-way ANOVA) and Tukey’s post hoc tests using the IBM SPSS statistics package.

Materials and method

300 hybrid posterior composite cylinder specimens (Solitaire 2, Heraeus Kulzer, Frankfurt, Germany) in shade A1 were prepared in polytetrafluoroethylene (PTFE) moulds, each measuring 5 mm in diameter and 8 mm in depth (comprising 4 × 2 mm increments). Four instrument lubricants were selected: ethanol; 3-step adhesive system (Kerr Optibond , use of adhesive bottle only); 2-step adhesive system (Kerr Solo-Plus, use of total-etch bottle only) and 1-step adhesive system (Kerr All-In-One, self-etch bottle). Please see Table 1 for base components of the adhesive systems used. Bonding systems were brand standardised (Kerr UK Ltd, Peterborough, UK) to eliminate the possibility of inter-brand variability. 60 cylinders were prepared with each of the four selected instrument lubricants and 60 cylinders were prepared as a control group without the use of instrument lubricant.

Table 1
Base components of the adhesive systems used as instrument lubricants .
Adhesive System → Optibond FL
Adhesive bottle
(3 step)
Optibond Solo Plus
Total-etch bottle
(2 step)
Optibond All In One
Self-etch bottle
(1 step)
Components ↓
Adhesive monomer 2-hydroxyethyl methacrylate (HEMA)
2-hydroxy-1,3-propanediyl bismethacrylate
2-hydroxyethyl methacrylate (HEMA)
2-hydroxy-1,3-propanediyl bismethacrylate
2-hydroxyethyl methacrylate (HEMA)
2-hydroxy-1,3-propanediyl bismethacrylate
Self-etching adhesive monomer x x Glycerol phosphate dimethacrylate (GPDM)
Filler Alkali fluorosilicates Alkali fluorosilicates Alkali fluorosilicates
Solvents x Water
Ethyl alcohol
Water
Ethyl alcohol
Methyl alcohol
Propanone

Protocol planning included pilot studies, power calculations for number of specimens required and control of specimen size with dimension/weight measures. All specimens were prepared by 2 operators, with both trained and calibrated for their prospective task. Operator 1 placed the composite material and operator 2 light polymerised the cylinders. This protocol was adopted to eliminate inter-operator variability/error and increase reliability.

Operator 1 placed each 2 mm layer of composite with a stainless steel plugger. Within the control group, the composite was placed and contoured with a ‘non-lubricated’ plugger. For each of the four experimental lubricant groups, the plugger was dipped into each respective lubricant for 1 s (up to a 3 mm marked line on the instrument) and left to drip/drain for 2 s prior to use. No mixing of the adhesive lubricants was required prior to use, as only the (single component) adhesive bottle of the 3-step, the total-etch bottle of the 2-step and self-etch 1-step bottle were utilised.

Operator 2 polymerised each incremental layer for 40 s with a halogen light cure lamp (Elipar , 3M ESPE, Minnesota, USA). Each composite layer was 2 mm in height, to allow for optimal polymerisation . A glass slab was placed beneath the mould to support material during placement and above on the final increment, to prevent formation of an oxygen-inhibition layer. The light curing unit was held as close to the specimen as possible and cured at a minimum threshold intensity, as per the manufacturer’s instructions. The output of the halogen lamp was calibrated after completion of every 20 cylinders using a handheld light-meter and the intensity was maintained above 450 mW/cm 2 to ensure optimal polymerisation (range 450 mW/cm 2 –550 mW/cm 2 ).

Following light polymerisation, the specimen cylinders were removed from the PTFE moulds and weighed on an electrical analytical balance (Model SI-403, Denver Instrument, Colorado, USA). Within each lubricant group, specimens were then randomly separated into three experimental groups: ‘0-week/immediate’, ‘1-week’ and ‘12-week’. The 1-week and 12-week specimens were subject to longitudinal analysis following storage in phosphate buffered saline (PBS). PBS was used to maintain a physiologic environment similar to saliva . For the ‘1-week’ and ‘12-week’ analyses each specimen was placed separately into a small glass storage bottle (with a sealed, stoppered lid) containing 10 ml of PBS and were then stored in an incubator at 37 °C, to replicate the temperature of the oral cavity. 100 specimens were incubated for 1 week and another 100 incubated for 12 weeks. Following storage for each time period, the specimens were removed with tweezers from the individual storage bottles and allowed to drip/drain on a rack for 1 min to remove excess PBS, prior to water uptake and strength testing.

Water uptake was assessed gravimetrically, by weighing the specimens at the time of manufacture and again after immersion in the PBS, on an electronic analytical balance. For each specimen, the percentage weight change over time was calculated.

The cylinder specimens were tested for DTS 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 10 mm/min, until the point of failure (machine set to peak hold). This compressive test identified fracture load (recorded in Newtons) and the DTS was calculated from this (in MPa) utilising the following formula :

DTS = 2F/πdh
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Jun 19, 2018 | Posted by in General Dentistry | Comments Off on The effect of instrument lubricant on the diametral tensile strength and water uptake of posterior composite restorative material
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