Retention of Various Luting Agents Used with Implant-Supported Crowns.

BACKGROUND: Implant supported crowns can either be screw retained or cement retained. Recent advances in implant dentistry has improved the longevity of the implant restoration and has placed importance on esthetics and function. Cement retained restoration has added advantage of superior esthetics and is preferred by the clinician. However, selecting the ideal type of cement for luting implant supported crown has been a topic of debate.
METHODOLOGY: A study was conducted at ESIC Dental College & Hospital, Kalaburagi to determine the retentive ability of three different luting agents. 30 samples were divided into three groups and luted with Zinc phosphate, Zinc Polycarboxylate and Glass ionomer cement. The retentive strength was evaluated using universal testing machine.
RESULTS: Results proved that zinc poly carboxylate cement had the highest retentive value followed by glass ionomer and zinc phosphate. Copyright:

INTRODUCTION

Initially, when dental implants were introduced, screw retention was the major mode of retention of crowns despite compromise in occlusion and esthetics. This was due to the poor success rate of implants during the development era and the need for surgical intervention due to the failure of osseointegration or to repair the prosthetic components in cases of fracture. However, as knowledge improved and techniques advanced, the success rate improved rapidly and the need for retrievability of crowns became less significant. Cement-retained crowns were introduced to replace the missing teeth which had optimal occlusion and superior esthetics when compared to screw-retained prosthesis.

CEMENT RETENTION

Various factors influence the amount of retention in cement-retained restorations. These factors are as follows:

Taper or parallelism

Surface area and height

Surface finish or roughness, and

Type of cement.[12]

Definitive cements are not recommended for implant crown retention because they are too strong for retrievability. The ideal taper and the long wall of abutment dictate the use of provisional cement for retaining implant crowns over a long-term. Further research is required to document cement retention and develop cements specifically designed for implant-supported prostheses.

Aim

The aim of this study was to determine the retentive ability of various luting agents used with implant-supported crowns on titanium abutments.

To accomplish the aim, a study was conducted to determine the tensile strength of three different luting agents. Zinc phosphate, zinc polycarboxylate, and glass ionomer cement (GIC) were used to lute crowns on titanium abutments. The tensile strength was evaluated using universal testing machine and their values were compared.

METHODS

A study was conducted to determine the retentive strength of various definitive cements used to cement implant crowns. Thirty standard abutments (Adin, Israel), 8 mm in height were selected and divided into three groups randomly. The abutments were attached to their respective lab analogs and mounted in block of acrylic resin, 1 mm above the margin. The parallel placement of the lab analogs in the resin was achieved using a dental surveyor. The abutments were tightened to their analogs at 35 N/cm using a torque wrench.

Wax copings were fabricated with blue inlay wax. Wax rings were added to the occlusal portion of the waxed copings to aid in the removal of castings using the universal testing machine. Specimens were invested in phosphate bonded investment material and casted using base metal alloy. The copings were seated on abutments and the fit was visually tested [Figure 1]. In case of any marginal discrepancy or misfit, the casting was repeated. Crowns and abutments were cleaned using steam jet. The copings were randomly divided into three groups (ten in each group). The abutments in Group 1 were luted with zinc phosphate (De Tray, Dentsply), Group 2 with zinc polycarboxylate (Poly F, Dentsply), and Group 3 with GIC (GC corp). The cements were mixed according to the manufacturer's instructions and seated in the crown. The copings were placed on corresponding abutments and were held in place with finger pressure for 10 s. Then they were subjected to 6 kg load for 10 min using the universal testing machine. Excessive cements were removed with the aid of an explorer after setting. The samples were then stored in artificial saliva at 37°C for 24 h. After 24 h, the ultimate tensile strength of each sample was tested in universal testing machine [Figure 2 Instron 5565, Instron Corp].

Figure 1

Abutments with their metal copings

Figure 2

Instron universal testing machine

Each specimen was placed in universal testing machine and the test for tensile strength was performed with a 500Kg load cell at a crosshead speed of 0.5 mm/min. Crowns were pulled from the abutment and the ultimate tensile strength when cement failure occurred was recorded in Newton. Collected data were analyzed using SPSS software version 11 (Chennai, Tamil Nadu).

RESULTS

The results are given in Table 1. Statistical analysis was done using SPSS software version 11.0. The values were analyzed using ANOVA and Tukey's HSD test. The results [Graph 1] show that the mean value of zinc polycarboxylate (623.86 Mpa) was much higher than the mean values of glass ionomer (474.02 Mpa) and zinc phosphate (282.436 Mpa). Two-way analysis of variance Table 2 showed that there was a significant difference between the three different groups (P < 0.005). Post hoc Tukey's test [Table 3] showed statistically significant difference (P < 0.005) when compared between one group and the other two groups. Zinc polycarboxylate required higher tensile force to cause cement failure compared with GIC and zinc phosphate.

Table 1

Descriptive statistics to show the mean value

	n	Mean	SD	SE	95% CI for mean		Minimum	Maximum
					Lower bound	Upper bound
GIC	10	474.020	57.985	18.336	432.539	515.500	410.670	609.470
Zinc poly carboxylate	10	623.806	50.728	16.041	587.517	660.094	561.170	690.680
Zinc phosphate	10	282.436000	42.877	13.559	251.763292	313.108	230.350	340.220

SD: Standard deviation, SE: Standard error, CI: Confidence interval, GIC: Glass ionomer cement

Graph 1

Bar graph showing mean tensile strength of cements

Table 2

Analysis of variance test

	Sum of squares	df	Mean square	F	P
Between groups	585,579.173	2	292,789.586	112.986	0.000
Within groups	69,967.148	27	2591.376
Total	655,546.321	29

Table 3

Post hoc Tukey’s test showing multiple comparison between groups

Grouping (I)	Grouping (J)	Mean difference (I−J)	SE	P	95% CI
					Lower bound	Upper bound
GIC	Zinc poly carboxylate	−149.786	22.765	0.000	−206.231	−93.340
	Zinc phosphate	191.584	22.765	0.000	135.138	248.029
Zinc poly carboxylate	GIC	149.786	22.765	0.000	93.340	206.231
	Zinc phosphate	341.370	22.765	0.000	284.924	397.815
Zinc phosphate	GIC	−191.584	22.765	0.000	−248.029	−135.138
	Zinc poly carboxylate	−341.370	22.765	0.000	−397.815	−284.924

SE: Standard error, CI: Confidence interval, GIC: Glass ionomer cement

DISCUSSION

The null hypothesis that different luting agents did not have any effect on the retention of castings to abutment was rejected (P < 0.05).

In the present study, zinc polycarboxylate (623.80N) showed the highest retention followed by GIC cement (474.02N) and zinc phosphate (282.43N). The tensile retention values between the tested cements were significantly different (P < 0.05).

The high values of zinc polycarboxylate compared to other groups can be explained by its adhesive property. It has been shown that, during the setting of zinc polycarboxylate cement, it can adhere to metal substrates by chelation of metallic ions.[3] Zinc phosphate cement has the lowest retentive values for implant-supported restorations compared to zinc polycarboxylate cement and GIC. This is in confirmation with previous studies conducted by Mansour et al.[4] and Akca et al.[5]

Zinc phosphate cements provide casting retention by micromechanical interlocking into the casting and the abutment surface irregularities.[6] This concept has important implications in cement-retained prostheses. Cements that provide casting retention mainly by mechanical interlocking (like zinc phosphate) will show, for every increase in roughness, a greater percentage increase in retention than adhesive cements.[6]

In our study, GIC had retention value (474.02N) lower than zinc polycarboxylate cement (623.80N). This result is in accordance with the study of Mansour et al.[4] Moreover, consistent with the present study, Akca et al.[5] and Sheets et al.[7] also found polycarboxylate cements had higher retentive strength than the other two cements included in the study.

An important factor in cement selection is the location of cement failure. The mode of cement failure for glass ionomer and zinc phosphate was adhesive and occurred at the cement-abutment interface. The mechanism of retention of zinc phosphate cement is by micromechanical interlocking. As the surface of implant abutment is relatively smooth it caused cement failure at crown-cement interface, as the remaining zinc phosphate cement was found in the inner surface of the copings.

In the present study, no pretreatment was performed on either the copings or the abutments, and thus the surface was inert. GIC does not adhere to an inert surface.[8] Furthermore, the solubility of GIC is more than that of other cements, and it is very susceptible to early water contact and desiccation[9] which can dramatically reduce the mechanical properties of the cement.[10]

Cohesive failure was observed with zinc polycarboxylate. Cement was found to be attached to both abutment and coping indicating that failure happened within the cement. The cohesive failure of polycarboxylate can be explained based on the fact that polycarboxylate has high bond strength with titanium abutments[11] and four times higher adhesion with other dental casting alloys.

As the abutment was not subjected to any modifications, the smooth surface could have decreased cement-abutment micro-mechanical interlocking, resulting in decreased cement retention values. This explains the adhesive failures in GIC and zinc phosphate specimens. However, zinc polycarboxylate can react chemically with inert surface rendered by the abutment and hence cohesive failure occurred.

One of the limitations of this study was that in vivo conditions were not simulated using long-term water storage and thermocycling. Thus, further studies can be focused to determine retention properties of different cements by subjecting the specimens to long-term water storage and thermocycling.

CONCLUSION

Within the limitations of this study, the following conclusion can be drawn:

The tensile strength of titanium abutments with base metal alloys was compared with various luting agents. The tensile strength of zinc polycarboxylate was the highest followed by GIC and zinc phosphate has the least tensile strength among tested cement. There was a statistically significant difference between the three groups of cements

In cases, where the prognosis of implant is questionable, it is advisable to lute with a cement which has low tensile strength for retrieving the crown.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.