Shear Strength Specimen Preparation

Anchor-well cylinders were poured with a mixture of orthodontic acrylic at a volumetric 2:1 powder:liquid ratio (polymethyl methacrylate diethyl phthalate polymer #003-07-0358, monomer #871-06-0719, Esschem Co, Linwood, PA, USA). One-inch (25-mm) diameter cylinders approximately 21 mm in height were prepared in a polyoxymethylene (Delrin) 15 hole specimen mold (Part #1599, Ultradent Products, Inc, South Jordan, UT, USA).

To serve as shear testing amalgam specimen condensation molds, fifteen 3.75 mm-thick Delrin disks were machined to 25 mm (1-inch) diameter with 3.5-mm-diameter centered holes, with each disk receiving a bisecting cut to produce two halves for easy removal from the molded specimen (Moran Innovations, Inc, Redlands, CA, USA).

Flat faces of the solid poured-resin cylinders were model-trimmed perpendicular to their outer walls. Holes to serve as cavity wells for the amalgam were drilled 4 mm deep in the center of the face, using a machine lathe with a 4-mm (5/32-inch) diameter drill bit.

Each shear-testing sample group consisted of 15 poured-resin cylinders fully inserted into the 15 holes of the polytetrafluoroethylene molding tray, with 15 Delrin split-ring specimen forming molds (Moran Innovations) snugly seated into the recessed area of the same holes on top of the cylinders, adopted from a previous study testing shear strength of repaired amalgam.¹⁷ Each inserted cylinder top surface was approximately 3-4 mm below the surface of the mold tray, providing space to seat each disk (Figure 3). This tray of specimens was clamped to a flat metal underplate to stabilize the assembly.

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A standard amalgam carrier (stainless steel, double-ended; Pulpdent Corporation, Watertown, MA, USA) was adapted to serve as the calibrated amalgam condenser. To standardize the condensation force, the carrier's spring tension was increased by bending it to a tighter curvature. This tension was calibrated on a digital balancing scale (Nexus NS-2000, Polyproducts Corp, Roseville, MN, USA) to a force of 1 lb (4.45 N). To standardize the condensation force, the carrier's spring tension force of 1 lb (16 oz) was measured at the point when the carrier sleeve was depressed.

A special nib was machined from stainless steel (Moran Innovations) to friction-fit over the large end of the amalgam carrier. This had a 1.5 mm-diameter (1.77-mm²) nib 5 mm long, with a flat, non-serrated tip allowing for condensation of amalgam in the 3.75 mm-deep holes of the split-ring molds. The resulting pressure per condensation stroke with this tip was 2.52 MPa (4.45 N/1.77 mm²). All shear-strength specimens throughout were condensed using this same calibrated condenser (Figure 4).

Each fresh amalgam mass was sectioned into four pieces using a sharp blade. Two of the quarters were used to fill the anchor-well in the acrylic cylinder below, and the remaining two filled the specimen-forming disk fitted on top.

Before any amalgam was introduced into the anchor well, the well was partially filled with the contaminant appropriate for that part of the study. It was subsequently loaded with the first amalgam increment and lightly directed into place using a 2.5 mm-diameter amalgam condenser (SS #1A, American Dental Mfg Co, Missoula, MT, USA) to approximately 1.5 mm below the top of the resin cylinder before condensing with the calibrated 1 lb-force instrument in a set pattern of nine strokes (Figure 5).

After assuring inundation with fluid contaminant, the next increment of amalgam was condensed in the same nine-stroke pattern using the 1 lb-force (0.45-kg) calibrated condenser. Layers of fluid contaminant were continually delivered to the amalgam surface to keep it flooded (Figure 6).

Sufficient extra amalgam was needed to produce a deliberate excess of mercury-rich layer on top, which was pressed down with a spreading or smearing action with less than 1 lb force using the polished flat handle of the cotton pliers to eliminate inadvertent voids. A razor blade at a low angle was used to shear off this soft minimum 1 mm-deep mercury-rich layer flush with the top of the acrylic cylinder. This surface was critical as the primary test plane between the anchor amalgam below and the form-molded cylindrical 3.5 mm-diameter test column or stub standing above.

Each acrylic resin cylindrical test block was transferred to one of the holes in the 15 hole resin-cylinder mold tray, where a split-ring Delrin disk was inserted and pressed tightly against the acrylic cylinder beneath it. These split-ring molds were a snug friction fit, being 1 inch in diameter, the same as the Ultradent mold holes. When secured into place, the central hole of the top mold was filled with fluid contaminant (either water, saliva, blood, or oil) to assure inundation of the fresh razor-cut amalgam surface located at the floor of this chamber. The central holes were slightly smaller in diameter (3.5 mm) than the anchor well (3.9 mm) to allow for complete contact of the amalgam to the anchor-well below in case of slight lateral eccentric discrepancy. It was vital that the contaminant material cover the entire critical surface.

One of the two smaller cut pieces of amalgam was lightly pushed into the split-ring central hole with the 2.5 mm diameter end of the amalgam condenser (amalgam condenser #1A, American Dental Mfg Co) with less than 1 lb-force. The amount of amalgam inserted was about 2 mm deep (approximately half the thickness of the 3.75 mm Delrin ring). After another layer of liquid contaminant inundated the surface, the nine-stroke pattern was used using the 1 lb-force calibrated condenser. The final amalgam increment was similarly contaminant flooded and condensed, with excess amalgam swept away with a burnishing motion using the rounded angular bend of the condenser. The working time for specimens averaged 4.0 minutes ± 30 seconds. Figure 7 demonstrates the overall sequence to form the shear-testing specimens.

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Fresh tubes of blood for this investigation were obtained from a hospital medical laboratory with donor identification undisclosed. Saliva was donated primarily by one of the investigators (BC). Water used in this study was deionized. Handpiece oil was Midwest Plus Handpiece Lubricant (part no. 380130, 2 fl oz [59 mL] RJR5, Dentsply Professional, Des Plaines, IL, USA).

All contaminant fluids were replenished with a minimum of five intervals per specimen at an estimated total of at least 0.25 mL to assure that each increment of amalgam was condensed under flooded or submerged conditions.

After a minimum of 2 hours, the 15 resin cylinders were individually pushed out of the Ultradent mold tray to remove their split-ring Delrin specimen-forming molds. Care was exercised not to disturb the standing fresh amalgam specimen stubs or columns. Samples for 24 hour testing were stored at 37°C under 100% humidity, and samples for 30 day testing were stored at 37°C under deionized water. Any inadvertent excess accumulation of amalgam at the base of the columns was carefully removed with a single-edged razor blade prior to testing.

Compressive Strength Specimen Preparation

The central well of the ADA Specification No. 1 amalgam compression-testing mold device was first loaded partway with the contaminant that was appropriate for each sample. Pre-encapsulated amalgam capsules were mixed using a triturator at 10 seconds per capsule according to the manufacturer's instructions. The resulting bolus of amalgam was sectioned into quarters with a single-edged razor blade.

The initial increment of amalgam was directed into the central well or hole of the compression strength mold device and pushed into place through the fluid contaminant using the beaks of the cotton forceps. After assuring inundation with fluid contaminant, the next increment was added, until all four quarters of fresh amalgam were inserted. Layers of fluid contaminant were delivered by syringe to the surface to keep each increment of amalgam continuously covered. After all the amalgam was inside the device, the plunger rod was inserted and placed under compressive weight according to the ADA amalgam Specification No. 1 protocol¹⁴ for compressive strength specimens (Figure 8). The resulting 4 mm-diameter × 9 ± 1 mm-long cylindrical specimens were carefully ejected from the mold device unaltered and stored in deionized water at 37°C until testing. The sample size of six was adopted from previous published compressive strength reports^9,18 and estimated to give 80% statistical power.

Instron Testing of Samples

Shear strength specimens were mounted into a jig (Moran Innovations), which positioned the specimen stubs horizontally to be tested at a 90° angle using an ElectroPuls E3000 Testing Machine (Instron North America, Norwood, MA, USA) with a semicircular notched blade and vertical crosshead speed of 0.5 mm/min. Values were recorded in megapascals.

Compressive strength specimens were tested standing vertically under compression in an ElectroPuls E10000 Testing Machine (Instron North America) with a crosshead speed of 0.5 mm/min and values similarly recorded in megapascals.

The null hypothesis was evaluated using the Kruskal-Wallis/Bonferroni procedure and tested at an α level of 0.05 with IBM SPSS v25 (Cary, NC, USA). The Kruskal-Wallis procedure, the nonparametric correlate of the one-way analysis of variance, was performed due to the smaller sample size and inability to verify assumptions of normality. The Wilcoxon signed ranks test was used to compare the 1-day and 30-day controls.

Shear Strengths

Mean shear strength reductions for 1-day samples contaminated both at trituration and during condensation (intracapsular+extracapsular) are shown in Table 3 and Figure 9. There was a consistent, although not statistically significant, reduction of strength by each group by 30 days.

Table 3 Shear Strength Test Results (MPa)

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One-day shear strength of the extracapsular saliva-contaminated sample (32.27 MPa) was equivalent to the control (32.23 MPa). This was repeated from a previous run yielding 33.6 MPa, apparently confirming this value.

Thirty-day shear strength p values (p Δ/C) for the intracapsular+extracapsular sample group were water (p=0.076), saliva (p=0.202), blood (p=0.040), and lubricant oil (p=0.042), with the water-, blood-, and lubricant-contaminated groups rejecting the null hypothesis (Table 3).Among the shear strength samples, there was a general tendency for interfacial fractures (across the specific deliberate plane of contamination) in the saliva and blood samples (Table 4). Bulk fractures occurred more often than interfacial fractures, averaging 9 of 15 in saliva and blood extracapsular groups, and for saliva and blood intracapsular groups, the average was 13 of 15. Because this trend was similar throughout all samples, there may have been sufficient weakening from intracapsular contamination saturation at trituration that predisposed specimen fractures through the bulk rather than at the interface.

Table 4 Fracture Sites Examined Under Magnification

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Table 4 Extended

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Any additional effect that intracapsular contamination had on shear strength occurred consistently regardless of contaminant source but not statistically significant (p=0.05, Table 3, Figure 10).

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Compressive Strengths

Mean strengths for 1 day compressive samples contaminated both at trituration and during condensation were lower than the 1 day control but consistently increased after 30 days to the following: control, 447.7 ±76.3 MPa; water, 343.6 ±70.1 MPa; saliva, 307.7 ±24.0 MPa; blood, 281.6 ±35.2 MPa; lubricant oil, 227.8 ±16.9 MPa. The blood-contaminated amalgam value in the intracapsular+extracapsular group was diminished, 157.4 ±39.9, at 1 day but recovered significantly after 30 days (Table 5, Figure 11).

Table 5 Compressive Strength Test Results (MPa)

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Compressive strength values below the selected minimum of 241 MPa, corresponding to the ADA-suggested minimum of 35,000 psi,⁹ were primarily in the 1 day (blood, 157.4 ±39.9 MPa; lubrication oil, 188.4 ±52.5 MPa) samples. At 30 days, only the lubricant oil (227.8 ±16.9 MPa) fell below this hypothetical level (Table 5, Figures 11 and 12).

Compressive Strength Results Relative to Contaminants

Water-contaminated samples complied with the null hypothesis, whereas other contaminated samples were rejected (p>0.05).

After 30 days, only lubricant oil, when permeated throughout the amalgam mass by both trituration and during condensation, had strength at or below the suggested minimum standard of strength (Table 5, Figure 12).

Compressive Strength Results Relative to Shear

In general, all sample mean intracapsular+extracapsular compressive strengths increased between 1 day and 30 days (Table 5, Figure 11). The converse was found for shear strengths with all intracapsular+extracapsular samples diminishing in value but not to a statistically significant extent (Table 3, Figure 9).

Intracapsular Contamination (at Trituration) Results Relative to Extracapsular Contamination (During Condensation)

As a means of introducing contamination, placing a drop inside the amalgam capsule prior to trituration was potentially more thorough than flooding during condensation alone. This was the method chosen by many amalgam investigators (Table 2), and, especially for compressive strengths, its detrimental effect was slightly enhanced beyond extracapsular contamination alone. This effect was dramatized more in the 1 day than in the 30 day amalgam samples. For example, MPa strengths in the saliva contamination group were reduced from 313.2 ± 38.9 to 252.1 ± 31.7 at 1 day and from 326.7 ± 65.9 to 307.7 ± 24.0 at 30 days. In keeping with worst case scenario prospects, long-term conclusions in the present study have been based on intracapsular+extracapsular contamination with 30 day compressive strength results.

What Is the Critical Level of Weakness Relative to Clinical Acceptability?

Compared with other restorative materials such as composite resin or direct gold, amalgam strength is purportedly superior when contaminating conditions cannot be totally controlled. Literature sources, however, were indecisive about what an actual critical threshold of clinical acceptability should be. ADA Specification No. 1 stated only a minimum of 80 MPa tested one hour following ejection from the dental amalgam specimen mold.¹⁴

ADA Specification No. 1 historically had required a minimum compressive strength of 35,000 psi (241 MPa) at 24 hours.⁹ Less strength was conjectured to increase the possibility of fracture under normal biting stress. For durability of the material to resist normally expected clinical biting loads, the degree loss of amalgam strength is not considered as important as maintaining strength above the suggested minimum.⁹

It is not known specifically what the desired minimum strength needs to be for amalgam. When there is adequate bulk, such as in restorations with deeper and wider preparation designs, the inherent alloy strength does not need to be as great.¹⁸ The minimum limit would also vary according to other factors such as stress locations involving central compressive versus edgewise shearing forces.

The period of time when the amalgam is setting is most critical; it has been estimated that 85% of the strength is attained by eight hours after placement.¹⁸ Strength often increases slightly after 24 hours, which has been associated with continued chemical reactions. Phillips suggested that equilibrium of amalgam in the mouth is reached only after an indefinite period of time, if ever.¹⁸

Compressive Strength Results Relative to Other Dental Materials

The Instron E3000 load cell used for amalgam shear strength testing was inadequate for compressive testing, which instead required use of the Instron E10000.

It is useful to compare these values with the normal compressive strength of other resin-based restorative products that might have substituted for amalgam. Compressive strength values of the control and water-, saliva-, and blood-contaminated groups after 30 days was still greater than a typical resin-modified glass ionomer (RMGI) manufacturer's¹⁹ stated value of 230-274 MPa (for Fuji II LC, GC America, Alsip, IL, USA) and that was “indicated for stress-bearing Class II restorations.” Mean contaminated amalgam values were also considerably greater than the 178 MPa for a conventional glass ionomer (GI; Fuji IX, GC America, Alsip, IL, USA) restorative material.²⁰

The mean compressive strength of amalgam 30 days after maximum contamination by saliva, which is the chief contaminant under oral conditions, was 326 ± 65.9 MPa. This value was roughly equivalent to the 200 to 340 MPa textbook range for compressive strength of microhybrid composite and 230-290 MPa for microfilled composite.²¹

Comparable to the present study, a laboratory report by Chen and others²² with a conventional glass ionomer (Fuji IX), a resin-modified glass ionomer (Fuji II LC), and a posterior composite resin (Quixfil, Dentsply Sirona, York, PA, USA) showed compressive strengths of 203, 217, and 300 MPa, respectively. These results²² are particularly relevant in that the GI, RMGI, and posterior composite samples were incubated underwater for 30 days and tested similar to the present protocol (Figure 12).

The 30 day mean compressive strengths of water- and saliva-contaminated amalgam in the present study exceeded the reported values for uncontaminated posterior composite, RMGI, and GI in their study. Also, the blood- and oil-contaminated amalgam strengths were greater than their RMGI and GI values (Figure 12). It should be noted that in actual practice, contamination with either blood or handpiece oil would be highly improbable during the condensation of amalgam.

The maximally contaminated saliva sample after 30 days still retained a compressive strength above the suggested ADA minimum recommendation for amalgam.⁹

Why Test for Shear Strength in Addition to Compressive Strength?

Compressive force, which assesses the overall bulk of amalgam, is the standard method of evaluating amalgam strength. Amalgam, however, can be subject more to tensile than to compressive failure under certain clinical stress conditions. For this reason, shear strength testing was included.

From a previous study, a method of continuously condensing a column of amalgam¹⁷ was adopted to assure not only complete submersion of the amalgam as it was being condensed but also that the contaminating agent was applied directly at the shear-force interface to avoid question whether the actual testing surface had been contaminated. This was considered by the authors to be more specifically focused than other traditional ISO testing methods for estimation of amalgam shear strength under “worst case scenario” conditions, as Nelson and Mahler¹² had described.

The significance of the present study design was that shear testing occurred perpendicular to the discrete plane of known contamination. Although the amalgam had already been condensed while totally submerged in fluid contaminants in addition to contamination at trituration, specific contamination at that particular interface is considered the most crucial. If the inherent interface joint strength was great enough not to cleave at that plane, the amalgam would fracture more angularly into the bulk. In this way, shearing action became a direct test of weakness induced by the contaminant at what might be considered the most vulnerable location.

However, seldom did the fracture occur at the interface. This may indicate that the patterned condensation with the 1.5 mm-diameter 1 lb-force condenser on the plane of specifically known contamination allowed the added increments of amalgam to join effectively despite interference from the contaminants.

Amalgam shear strength after placement was degraded during the following 30 days under water at 37°C.

Compressive forces consist of a component of tensile stress in expansion.^23,24 The compressive test results in the present study were more definitive than the shear strength results. However, in actual clinical cases, it is less frequent that amalgam fails by direct perpendicular compressive force alone. Amalgam failures are generally noted from tensile or shear action in relatively unsupported areas such as in isthmus fractures.¹⁸ In this regard, shear-testing data are considered relevant.

Possible Mechanism of Action for Amalgam Contamination Resistance

Mercury is a metal that uniquely exists in a liquid state at ambient temperature. This molten state allows small particles of other metals to combine and solidify to form an amalgam. The ability of amalgam to flow together and join with itself in a coherent mass owes to the capacity of the mercury to unite with other increments of liquid mercury in the mixture around and through the contaminating fluid. As mercury components coalesce under condensation forces, non-mercuric liquids are apparently expressed outside the amalgam mass. It is postulated that proteinaceous elements of blood and saliva may cling to particles of amalgam alloy and are not as readily expelled. The compressive specimens that had been contaminated with saliva and blood were approximately 0.5-1.0 mm longer than control or water- or oil-contaminated specimens. This was assumed to be due to the residual incorporation of mucin and formed blood elements within the mass, which were unable to be fully squeezed out under condensation pressure.

There may be properties of handpiece oil that likewise have an affinity for solid metallic particles. It is speculated that this factor could cause inhibition of significant chemical actions that ordinarily would have occurred and prevented strengthening of the amalgam during the 30-day incubation storage period.

The remaining amalgam strength of 35,000 psi at 30 days as a suggested minimum value, based on a past ADA recommendation for minimum acceptability,⁹ may still be considered reasonably functional.

Mean compressive values for the water-, saliva- and blood-contaminated samples, in the group including both intracapsular and extracapsular contamination, indicated amalgam strength after 30 days to be at least comparable to published values for intact RMGI. Mean values individually for water-, saliva-, blood-, and oil-contaminated samples after 30 days were superior to the published value¹⁷ of intact uncontaminated GI cement (Figure 12 shows 30-day compressive strengths).

Power and sample size analysis conducted from a prior pilot study anticipated a larger effect size. For data in which the null hypothesis was retained, no post hoc power analysis was conducted.

It is recognized that final conclusions about amalgam strength cannot be determined solely by compressive and shear testing. However, although salivary contamination is recognized as the main challenge clinically, thoroughly condensed amalgam may be capable of providing a functional restoration, even if overwhelmingly challenged by water, saliva, blood, or lubricating oil contaminants.

To conduct studies on actual patients with the amalgam placed totally under these contaminating factors would likely be considered unethical. The closest viable alternative is laboratory testing, from which results the clinician should be able to prognosticate a certain range of outcomes and judge their practicality.

Testing of alternative resin-based restorative products for comparison with an amalgam alloy was desired but not considered feasible due to inherent limitations of the materials. Otherwise, future studies might include testing whether direct gold, glass ionomer, or composite resins can sustain similar contamination without undue debilitation of strength and integrity.

It would be of interest to study whether use of an automatic mechanical condensing device instead of hand condensation would provide greater amalgam strength results under similar conditions. Also, amalgam tensile type testing could be accomplished using both a lower and an upper split-ring mold design to produce specimens for analysis in a three-point stress test, directed across planes of deliberately induced contamination.