Application of Semipermanent Cements and Conventional Cement with Modified Cementing Technique in Dental Implantology.

OBJECTIVES: The aim of this study was to evaluate the influence of artificial ageing on the retention force of original semipermanent cements, as well as the possibility of using conventional cements for semipermanent cementation with adequate modification of the cementing protocol.
MATERIALS AND METHODS: Forty CoCrMo alloy crowns were divided in four groups (each group n=10) and fixed with two semipermanent cements (resin-based and glass ionomer-based cements) and one conventional (zinc phosphate), using conventional and modified cementation techniques on titanium abutments. The samples were stored in humid conditions for 24 hours at 37°C and subjected to thermocycling (500 cycles) and mechanical cyclic loading (7 days, 3, 6, 9 and 12 months function simulation). The cast crowns were removed and the retention force was recorded.
RESULTS: The highest initial retention force measured was for zinc-phosphate cement - conventional cementing (198,00±61,90 N), followed in descending order by zinc-phosphate cement - modified cementing technique (152,00±45,42 N), long term temporary cement - GC Fuji Temp LT (57,70±20,40 N), and semipermanent cement - Telio CS Cem Implant (56,10±18,68 N). After 12 months, the highest retention force measured was for zinc-phosphate cement - conventional cementing (88, 90±14, 45 N), followed by zinc-phosphate cement - modified cementing (48, 15±14,41N), semipermanent cement GC Fuji Temp LT (16,55±3,88 N) and Telio CS Cem Implant (15,55±5,52 N).
CONCLUSIONS: Zinc-phosphate cement - modified cementing technique and original semipermanent cements can be recommended for conditional permanent cementing of implant supported crowns. CLINICAL RELEVANCE: The use of semipermanenet cements and zinc-phosphate cement - modified cementing technique provides a predictable retrievability of implant-supported crowns.

Introduction

The long-term success of implantoprosthetic therapy depends on several factors: good osseointegration of the implant, the quality of the prosthetic reconstruction, and the connection between the implant and a crown. Due to the specific connection between the implant and bone in implantology, there is a specific transfer of loading. This connection is rigid, with no shock absorber mechanism for masticatory force. This has to be taken into consideration throughout the entire prosthetic planning and treatment.

The most commonly used retention modes are cementation and screw-retention. The professional community is divided regarding the use of cement or fixation screws for implant supported crown fixation. In fact, there has been controversy about whether to use cement-retained or screws-retained implant supported crowns, mostly based on retrievability versus esthetics (-).

Passivity is another extremely important factor when analyzing the features of implant-supported crowns. The analysis of the currently available scientific studies showed that, complete passivity of screw-retained implant supported crowns is still a challenge and it should be a goal of modern implantology (, ). On the other hand, dental cements allow the crown to sit passively on implant abutment, filling the space between abutment and crown, hence enabling the compensation of small non-concurrences in a crown fitting (-).

A wide variety of dental cements, with different properties, are commercially available to retain an implant-supported prosthesis. Retention and esthetics are the two main factors to consider when choosing which cement selection guidelines are best for individual patients ().

The retrievability of cemented crowns is highly limited. Removing permanently cemented crowns from abutment requires significant force. This might result in permanent damage of the crown, abutment and even the implant itself. On the other hand, temporary cements might allow easy crown removal, but, taking into consideration their low retention level, marginal leakage, and dissolubility in oral fluids, these cements cannot provide adequate retention and long-term stability of a crown. Ideally, cement should be strong enough to retain the crown, and at the same time, sufficiently weak to allow the dentist to remove the crown when needed. The level of force required to remove the crown should not cause any implant trauma and damage to the crown and the abutment ().

Compared to natural teeth and their adaptation mechanisms, implants have no periodontal ligament support (), thereby lacking adequate shock absorption and adaptability to the high short-term forces which are required during crown removal. Thus, compared to natural teeth, crown removal might represent an implant overloading ().

There is a need for a solution that would enable adequately retained crown and bridges with the simultaneous possibility of retrievability when necessary. The first attempts to weaken retention and try to increase the ease of retrievability were by using petroleum jelly in addition to permanent cements ("Hand-made cements") ().

However, due to the arbitrary retention obtained, this method was soon abandoned. Another way to reach an ideal balance is to apply conventional cement (zinc-phosphate or conventional glass ionomer cement) only on the crown's edges. In this case, the unfavorable characteristic of marginal dissolubility and the level of discrepancy of conventional cements can result in a decrease of cement retention after a certain period of time (, , ).

There are also specifically designed cements for long-term temporary cementing which enable adequate crown retention for the period of 6 months to one year. This semipermanent cementation model and retrievability modalities have been reviewed in some studies (-).

In vitro conditions may be used to simulate some influential factors of the oral environment and material's potential performance in vivo. One of the conventionally used systems of artificial ageing is thermal cycling which includes subjecting of samples to repeated cycle of hot and cold temperatures, in order to reproduce thermal changes occurring in the oral cavity (). Thermal changes induce stress in dental materials; therefore the properties of materials could be changed.

Compressive cyclic loading represents a method of simulating occlusal stresses encountered in the oral environment. The mentioned method of masticatory cycle’s simulation was used in the observation of dental cements retention force in different time points (, ).

Hence, the aim of this study was to test the influence of artificial ageing on the retention force of semipermanent cements, as well as the possibility of using conventional cements for semipermanent cementing with a specific modification of the cementing protocol.

The following hypothesis has been tested:

There is no difference in the retention provided by all tested cements - GC Fuji Temp LT, Telio CS Cem Implant and Harvard Cement (standard and modified cementing technique) at retention measurement time-points.

There is no influence of thermal cycling and compressive cyclic loading on the physical retention of tested cements at measurement time points.

Material and methods

The working model was 40 titanium abutments (Easy abutment NP 0,75, Nobel Biocare, Sweden), conical by 8º, height 5,5 mm, 40 implant replicas (Implant Replica, Nobel Biocare, Sweden. Abutments were screwed into implant replicas with 35 N/cm of torque. The crowns made from CoCrMo alloy (Bond NF – Nickel free, Interdent, Slovenia) were used (Figure 1, 2).

Figure 1

Titanium implant replica and abutment with crown

Figure 2

Titanium implant replica and abutment connected with crown

The abutment access channel was closed with a temporary light polymerizing composite (Temp it, Spident, South Korea). In this study, the crowns were made based on a silicon mold with occlusal two-thirds of the crown of acrylate Frasaco tooth - second lower premolar. The inner side of the crown and the thickness of the cemented space were standardized by using 7 mm plastic molding caps (Plastic Coping Easy Abutment Engaging NP 2 pkg). After the casting, the outer surface of the crown was highly-polished, whereas the inner surface was sandblasted with 50 micron aluminum-oxide particles (Figure 3). The crown fitting check was completed using silicon material (Fit Checker, GC Co, Tokyo, Japan). The samples were cleaned in an ultrasound bath and with hydrofluoric acid to avoid contamination of the binding surfaces.

Figure 3

Samples prepared for artificial ageing

The 40 samples were then divided into four groups with 10 samples each. The study evaluated three commercially available cements: semipermanent cement (Telio CS Cem Implant, Ivoclar Vivadent, Liechtenstein), semipermanent cement (GC Fuji Temp LT, GC, Japan) and conventional zinc-phosphate cement for permanent cementation (Harvard Cement, Harvard, Germany) (standard and modified cementing technique) (Table 1).

Table 1

Type of cements and testing groups

Group	Number of samples	Type of cement	Packing mode
1	10	Telio CS Cem, IvoclarVivadent, Liechtenstein	Dual-curing resin semipermanent cement comes as two pastes in one syringe.
2	10	Fuji TEMP LT, GC	Glass ionomer semipermanent cement comes as two pastes in one syringe.
3	10	Zinc-phosphate cement, Harvard, Germany (conventional cementing)	Conventional permanent cement comes as powder and liquid.
4	10	Zinc-phosphate cement, Harvard Germany (modified cementing technique)	Conventional permanent cement, comes powder and liquid.

Semipermanent cements Telio CS Cem Implant and GC Fuji Temp LT, and conventional zinc phosphate Harvard Cement (standard technique) were prepared according to the manufacturer’s instructions and applied to the complete inner walls of the crown. For modified cementing technique, zinc phosphate Harvard cements were used according to the manufacturer’s instructions, and were applied with a small brush in a thin film band of 1mm to the cervical margin of the inner surface of the copings.

The crowns were carefully placed on the abutments and a 5 kg-controlled force was applied using a hydraulic, digital controlled press. The samples cemented with Telio CS Cem Implant were lightened initially for 3 seconds to obtain a rubbery consistency and for easier removal of excess material. The excess material of the samples with GC Fuji Temp LT and Harvard Cement, Harvard, Germany, was also removed once the cements reached a rubbery consistency. All samples were subjected to controlled pressure for 10 minutes to complete the process of chemical polymerization and set the cement. The samples were stored for the next 24 hours in artificial saliva at 37°C temperature, and subjected to a thermocycling process which consisted of 500 cycles of temperature fluctuations (5-55°C). After that, the samples were fixed in special modules in base of self-curing acrylate material which is similar to the human bone by its modulus of elasticity (Technovit 9100, Heraeus Kultzer, Hanau, Germany).

Chewing cycle’s simulation was performed in masticatory cycle simulating machine (Chewing simulator CS-4.2 economy line (SD Mechatronik, Germany)) in an artificial saliva environment. with a predetermined schedule in term of simulation of mouth function periods (7 days, 3, 6, 9 and 12 months function simulation) (Figure 3, Table 2).

Table 2

Testing periods of artificial aging with mechanical loading cycles

Testing rounds	Testing periods	Number of masticatory cycles (mechanical loading cycles)
1	7 days	192
2	3 months	2 500
3	6 months	5 000
4	9 months	7 500
5	12 months	10 000

Once the artificial ageing process of the material was complete, the retention force of the samples was measured with a Universal testing machine (Instron 1122) (Figure 4). Uniaxial tensile force with 1 millimeter per minute speed was applied and the results were recorded on/ the testing machine graph. The samples were cleaned under the same protocol between the individual testing rounds - crowns were subjected to 220 °C temperature (2 cycles with 12 minutes duration) at the sterilization device to enhance degradation of cement residues and to make its removal easier. The samples were then cooled at the room temperature. Cement excess was at first removed by hand tools, while the rest of the cement from rough inner surface of the crown was removed by sand blasting.

Figure 4

Sample fixed in Universal testing machine

After sterilization, the inner surface of crown copings were cleaned by sandblasting with aluminum oxide 110 μm particles under the pressure of 2,5 barometers and dried using compressed air. Cement residues from the abutment surface were hand-removed with plastic instrument and surface of abutment was polished with paste (CleanJoy, Voco, Germany). Abutments and crowns were cleaned by ultrasound in the final phase of the cleaning protocol. After cleaning, the samples were inspected under ten times magnification to ensure that the surfaces of the samples were free of residual cement.

Statistical analysis

The statistical analyses were performed using the SPSS software (IBM SPSS statistics 24.0, IBM Corporation, New York, United States) at a 5% significance level. The Anova test and post-hoc tests, the Tukey and Tamhane`s T2, were applied to quantitative and continuous variables.

Results

The highest initial retention force was recorded for zinc-phosphate cement – conventional cementing (198.00±61.90 N) followed by (in descending order) zinc-phosphate cement - modified cementing technique (152.00±45.42 N), cement for temporary long term cementation – Fuji Temp LT (57,70±20,40 N) and semipermanent cement – Telio CS Cem Implant (56.10±18.68 N) (Table 3).

Table 3

Retention force of semipermanent cements and zinc phosphate cemenet (conventional and modified cementing techniques) recorded in N

Type of cement	Mean±SD	p
Initial measurement
semipermanent cements		1.000
Telio CS	56.10±18.68
GC Fuji Temp LT	57.70±20.40
zinc phosphate cement		0.074
conventional cementing technique	198.00±61.90
modified cementing technique	152.00±45.42
7 days
semipermanent cements		0.988
Telio CS	33.50±12.71
GC Fuji Temp LT	38.35±14.41
zinc phosphate cement		0,019
conventional cementing technique	179,30±53,78
modified cementing technique	132.65±35.49
3 months
semipermanent cements		1.000
Telio CS	32.10±16.82
GC Fuji Temp LT	31.90±12.96
zinc phosphate cement		<0.001
conventional cementing technique	153.85±47.73
modified cementing technique	87.80±35.70
6 months
semipermanent cements		0.994
Telio CS	25.30±15.11
GC Fuji Temp LT	28.70±13.57
zinc phosphate cement		0.005
conventional cementing technique	131.7±41.26
modified cementing technique	84.05±36.68
9 months
semipermanent cements		0.892
Telio CS	18.80±7,18
GC Fuji Temp LT	22.80±6.96
zinc phosphate cement		<0.001
conventional cementing technique	99.00±14.10
modified cementing technique	62.20±18.29
12 months
semipermanent cements		0. 997
Telio CS	15.55±5.52
GC Fuji Temp LT	16.55±3,88
zinc phosphate cement		<0.001
conventional cementing technique	88.90±14.45
modified cementing technique	48.15±14.41
Bold indicates p< .05

the p values corresponds to statistically difference between 2 semipermanent cements and 2 different cementing techniques with zinc phosphate cement

After exposing the samples to artificial aging, a decrease in retention force was recorded for both cementing techniques. After 12 months, the retention value in those samples that were cemented using a conventional technique was 88. 9±14.45 N. This value was lower (48.15±14.41 N) for the samples cemented by the modified techniques.

Retention values of samples cemented with both techniques (conventional and modified) decreased gradually during all tested periods. A significant difference was not recorded during the initial measurement (p=0,074) but it was found on the 7th day (p=0,019), 3rd month (p<0,001), 6th month (p=0,005), 9th month (p<0,001) and 12th month (p<0,001) (Table 3).

Semipermanent cements Telio CS Cem Implant and long term temporary cement GC Fuji Temp LT also had a similar initial retention force (56,10±18,68 N and 57,70±20,40 N) without a significant difference (p=1,000).

Subsequent testing rounds recorded a continuous decrease in retention force for both semipermanent cements, but without any statistically significant difference at all tested periods: 7th day (p=0,988), 3rd month (p=1,000), 6th month (p=0,994), 9th month (p=0,892) and 12th month (p=0,997) (Table 3).

Discussion

The factors that determine the retention of the crown on the implant abutment are numerous: height and shape of abutment, crown/abutment fit, surface roughness of crown and abutment, functional ageing, cement film thickness, cement type, and the cementation technique used (, -). The present study evaluated two of these factors: cement type and ageing.

Initially, the measurements of retention force of samples that were not exposed to artificial ageing showed that the highest retention force was zinc-phosphate cement Harvard, conventional cementing technique (198.00±61.90 N) followed by zinc-phosphate cement Harvard - modified cementing technique (152.00±45.42 N), temporary cement for long-term temporary cementation – Fuji TEMP LT (57.70±20.40 N) and semipermanent cement – Telio CS Cem Implant (56,10±18,68 N). Our results are in lines with a study by Lugas et al (), which evaluated the degree of retrievability of three different cements, ranked from temporary to definitive cementation use.

Nowadays, zinc-phosphate cement is still used in many studies in order to compare conventional and specially developed cements (, , -). All these studies, including this one, recorded high values of retention for zinc-phosphate cement immediately after cementing.

The high retention force of permanent cements requires the application of significant force during the crown removal from the implant abutment. Due to the need to reduce the load on the implant/abutment/crown complex during the restoration removal and securing the retrievability of the implant-supported crowns, the idea of examining the effects of modifying the cementation technique of conventional cements emerged.

Our study found the difference between the initial retention of unloaded samples, cemented with conventional technique by applying cement to the entire inner surface of the crown (198,00±61,90 N) and with the modified technique of applying the cement layer only along the crown's edge (152,00±45,42 N). Despite the fall of the retention value of cement during the mouth function, the retention force value still remained high after one year when the conventional technique was utilized (88.9±14.45 N). This value was significantly lower for samples cemented using the modified technique (48.15±14.41 N).

Although showing extremely high initial retention values, the main deficiency of zinc-phosphate cement appeared - the dissolution of the cement layer on the marginal parts as well as marginal discrepancy, which has been described in previous studies (, , ).

Considering that the retention surface is directly proportional to the cement retention value, the modified cementing technique used was applying a 2 mm wide cement layer only along the crown edges. During the time of the study, our measurements showed that the cement dissolved at the crown edges, which resulted in the retention value weakening gradually. There were significant differences between the retention values of conventional and modified cementing techniques during all the testing intervals of our study (p=0,019, p <0,001, p=0,005, p<0,001, p<0,001) except the initial measurement (p=0,074). These findings are in contrast to the results of Mehl et al. and Wolfart et al. studies (, ).

However, it should be mentioned that the methodology of these studies did not include artificial ageing of the samples. The reason behind the initial retention force value recorded for zinc-phosphate cement used with metal components of implantoprosthetic reconstructions is its proven high bond with crowns, made of both precious and non-precious alloys. Ergin et al. documented the stronger retention force of zinc-phosphate cement on the surface of a non-precious alloy crown (). For the purpose of this study, the crowns were manufactured of CoCrMo alloy as one of the most frequently used non-precious alloys in modern dentistry, which continues to be the subject of new research on its additional performance which is important in designing and manufacturing materials with optimal characteristic (). On the other hand, implant abutments are made of titanium, which accounts for the high initial retention value of zinc phosphate cement immediately after cementing.

The results of this study showed that semipermanent cements Telio CS (56.10±18.68 N) and GC Fuji Temp LT (57.70±20.40N) also had a similar initial retention force. The retention force decreased gradually along with all mechanical loading cycles’ levels. Comparing the retention force of unloaded samples and retention values measured after 12 months, decreases in retention were recorded (15.55±5.52N and 16.55±3.88 N). This still represented a value high enough for enabling crown stability in function, and low enough to allow for the prosthetics reconstruction to be removed from the abutment without trauma. Hence, these cements fulfil the retrievability condition which was the basic idea in their technological development. Our study showed that the decementation forces of both the semipermanent cements which we evaluated depend significantly on ageing levels. These findings were confirmed in the Kappel et al study ().

The studies conducted by Mundt et al and Alvarez-Arenal et al also concluded that the semipermanent model of cementing provides crown retrievability (, ). A retrospective study conducted by Schwarz et al. found a high survival rate for both, semipermanent and permanent cementing techniques on implant abutments ().

The cements examined in our study, Telio CS Implant (dual-curing resin cement) and GC Fuji Temp LT (glass ionomer self-cure cement), are originally made for long-term, temporary cementing of crowns on implant abutments. These cements are highly sensitive to the presence of humidity during the polymerization process. The fact that our study was conducted in a laboratory, where it was possible to maintain absolutely controlled dry conditions, could have significant impact on the retention values we recorded. It indicates the significance of having an adequate and effective cementing protocol in oral mouth conditions where it is difficult to work in the above mentioned conditions. Maintaining the retention value at a level sufficient for enabling crown stability in function, one year after cementation could be explained with its specific glass-ionomer formulation. Conventional glass-ionomer material shows the material expansion during the initial setting time, which reduces the net amount of curing shrinkage, lower degree of shrinkage comparing to resin modified glass-ionomer () and tooth like coefficient of thermal expansion. These factors could maintain the marginal seal and slow down the dissolution of cement.

Bearing in mind the fact that the retrievability of implant-supported crowns is an essential requirement of modern cement systems in implantoprosthetics, it is clear why the semipermanent cements were the subject of this study. Semipermanent cements and the modality of semipermanent cementing are still insufficiently tested. Since there are a relatively small number of studies dealing with this issue on the global level, this study contributes significantly to improving knowledge and understanding in this relatively new field of fixed prosthetics.

The limitations of this study should be mentioned. To the best of authors knowledge, there is a lack of studies concerning semipermanent cements. Retention for semipermanent cements was tested only up to 1 year of function where the last measured level showed clinically acceptable retention. Therefore, further research should focus on increasing the quantitative level of artificial ageing to test the ultimate functional limits of these cements. Since the results were obtained in an in vitro study, clinical research is needed to confirm these findings.

Thermal cycling is the most widely used testing procedure of arteficial ageing of dental materials. A wide variation in thermal cycling parameters applied in experimental studies has been identified and there is an apparent lack of a standardized protocol evident from comparison across different studies (). In the current study, thermal cycling procedures were performed in accordance to ISO standard ISO/TR 11405:1994(E). An identical, or similar protocol, has been used in other studies (, , , ).

A great variation in the numbers of cycles that equate to average human daily, weekly and yearly masticatory function could be registered. Due to heterogeneity of data regarding loading conditions - number of cycles, force intensity and testing chamber medium condition, it is difficult to make a comparation between diferent studies (, , , ). Cyclic loading tests require more standardized guidelines for testing and reporting.

As reference conditions for compressive cycling loading, the current study used the values from the study by Dudley et al., whose authors based their reference values on a large study conducted by Graf et al. (, , ).

In an effort to create conditions as close as possible to those in the oral cavity, the test was conducted in an artificial saliva environment. The saliva composition used in this study was used according to the study of Huang at all. ().

Since most of the residual cement remained on the inner surface of the copings, due to its roughness, the cleaning of the copings was more complicated since it additionally included sandblasting. Furthermore, it showed individual characteristics due to different cements that were used in this study. As expected, more residual cement was registered in the samples cemented with Harvard cement - conventional cementation compared to the modified technique, while the residual particles amount of both semipermanent cements after the cleaning procedure was negligible. In order to ensure the samples surface were free of residual cement, the samples were inspected under ten times magnification.

Numerous studies indicate that the value of cement retention increases with increasing surface roughness of the restoration or abutment (-). Therefore, in further research, the potential increase in roughness of the bonding surfaces caused by samples sandblasting cleaning procedure and its influence on the retention value of the tested cements must be taken into account.

Conclusions

Within the limitations of the present study we can conclude the following: There is a difference in the retention provided by GC Fuji Temp LT, Telio CS Cem Implant and Harvard Cement (standard and modified cementing technique). Thermal cycling and compressive cyclic loading affects the physical retention of tested cements at measurement time points.

The results suggest that the modified cementing technique for zinc-phosphate cement and original semipermanent cements can be recommended for conditional permanent cementing of implant-supported crowns since it still allows retrievability. Retrievability is in direct relation with cement type.