Effect of Abutment Height and Cement
Thickness on the Retention of
Cement-Retained Implant-Supported Restorations
Alaa Abou-Obaid1* and Reem Al-Khudairy2
1Department of Prosthetic Dental Sciences, King Saud University, Saudi Arabia
2Prosthodontist, Al Yamamh Hospital, Ministry of Health, Saudi Arabia
Submission: July 02, 2018; Published: August 20, 2018
*Corresponding author: Alaa Abou Obaid, Lecturer, Department of Prosthetic Dental Sciences, College of Dentistry, King Saud University, Saudi Arabia, Riyadh, Email: firstname.lastname@example.org
How to cite this article: Alaa Abou-Obaid, Reem Al-Khudairy. Effect of Abutment Height and Cement Thickness on the Retention of Cement-Retained Implant-
Supported Restorations. Adv Dent & Oral Health. 2018; 9(4): 555767. DOI: 10.19080/ADOH.2018.09.555767
Objective: To evaluate the effect of abutment height and cement film thickness on the retention of cement-retained implant-supported restorations.
Materials and methods: A total of 20 implant assemblies were randomly divided into equal groups of ten each according to the abutment height (5.5mm, 3mm). Thirty CAD/CAM zirconia copings were fabricated for each abutment height. These copings were sub-divided into 3 groups according to the tested cement film thickness (20μm, 35μm, 50μm). Each coping was cemented with a provisional cement (Temp Bond). Twenty-four hours after cementation, the specimens were thermocycled for 500 cycles and subjected to pull-out test. The tensile strength of each specimen was recorded in (N).
Results: The highest mean values of tensile strength for the standard and shortened abutments were (87.83N) and (63.23N) respectively, noticed with the 20μm cement film thickness. While the lowest mean values were detected with the 35μm cement thickness (68.83N) for the standard abutments and (49.59N) with the 50μm for the shortened abutments. For both abutment heights, the 20μm cement film thickness significantly differed than the other groups. However, no significant differences was found between 35μm and 50μm cement film thickness (P>0.05). Standard abutments showed a significantly greater retentive forces compared to shortened abutments with each cement thickness (P<0.05).
Conclusion: Cement thickness of 20μm has a significant effect on the retention of cement-retained single implant restorations with standard or shortened abutment heights. However, the increase in cement thickness from 35μm and 50μm has no effect on the retention of the restoration.
The prosthetic treatment options to replace missing teeth have rapidly changed to include dental implants as a standard of care and the optimal treatment choice in most cases due to their obvious advantage of preservation of the adjacent tooth structure [1,2]. Dental prosthetic reconstructions can be fixed to an implant either by screw-retained or cement-retained. The choice of either retention type depends on various factors such as retrievability, passivity, occlusion, and esthetics. Both retention types have their advantages. A screw-retained dental prostheses is retrievable without restoration being destroyed. This is especially true, for long span restoration with higher risk of complication . On the other hand, cements when used appropriately can also provide retrievability . Cement-retained prostheses have clinical advantages of the compensation of improperly inclined
implants, improved esthetic due to absence of screw access
hole, greater passivity of fit due to the cement layer between the
implant abutment and reconstruction, less incidence of ceramic veneer fracture, intact occlusal table specially for posterior narrow diameter implant, better stress distribution and reduced cost and complexity of laboratory procedures [5-9]. Furthermore, challenges with accessibility placing the screw driver of screw-retained restoration in patient with a limited opening and/or in the posterior area can be eliminated using cement-retained prostheses .
Cement-retained implant-supported prostheses are widely used as a retention type. Abutment preparation designs and cementation techniques can mimic conventional fixed prosthodontic procedures. Same as natural teeth, abutment geometry such as height and taper influence the retention. The retention is proportional to height and inversely proportional to taper [8,11].
Cementation of the implant prostheses allows the clinician
to optimize the fit on the implant abutment. A provisional luting
agent may be used as a final luting agent when retrievability is
a major concern . A weak luting agent has been suggested to
be used at first, progressing to stronger cements until the desired
amount of retention has been achieved . It is widely accepted
that cement thickness is also a factor affecting the durability of
the cement, leading to retention of the restoration . However,
inadequate cement space resulted in crown seating discrepancy
. Ideal taper and long walls of implant abutment increases the
retention of a given restoration . However, there is not enough
evidence support the ideal cement thickness for retrievability and
the anticipated amount of retention when less than ideal situation
is encountered where abutment hight modification was necessary.
The purpose of this study was to investigate the effect of abutment
height and amount of film thickness of luting agent on the retentive
strength of cement-retained implant restorations. The null
hypothesis was that there would be no significant difference in
the retention of cemented crowns on intact implant abutments or
those that have lost length with different amount of film thickness.
A total of twenty implant fixtures measuring 4.1mm×12mm
(Standard plus implant [ITI system, Straumann AG, Basel,
Switzerland]) were mounted with epoxy resin-glass fiber
composite (NEMA Grade G-10 rod, Piedmont Plastics, Charlotte,
NC) in cylinders measuring 25mm in length and 35mm in diameter
using a dental surveyor (J.M. Ney Co., Bloomfield, CT, USA) to
ensure a straight and parallel placement of the implants. The
implant fixtures were randomly divided into two equal groups of
ten each according to the different implant abutment height used.
The control group consisted of implant fixtures with a standard
5.5mm synocta abutments (048.605 ITI system, Straumann AG,
Basel, Switzerland). In the second group, the abutment heights
were shortened up to 3mm.
Fabrication of the copings were done using the computer-aided
design and computer-aided manufacturing (CAD/CAM) technique.
The abutments were scanned with optical scanner (Cercon eye;
Dentsply, Germany) to fabricate thirty zirconia copings (Cercon
Base; Degudent, Dentsply, York, PA) for each abutment height.
Then, the copings were sub-divided into 3 groups according to the
tested cement film thickness (20μm, 35μm, 50μm). The copings
were designed with two wings in the proximal surfaces to secure
the restoration into a customized jig to be used during the pull-out
test (Figure 1). All copings were air-abraded with 50μm alumina
particles at 0.25MPa.
Before cementation, each screw-retained abutment was
tightened to the recommended torque (35Ncm) and then
retightened (to the same torque value) 10 minutes later to
minimize embedment relaxation between the mating threads.
The abutment screw access opening was covered with
vinylpolysiloxane impression material (Virtual, Refill light body,
Regular set wash material, Ivoclar vivadent, Italy) to prevent
the excess of cement from escaping to the screw access hole. A
provisional cement was used to fix the copings with the abutments
(Temp Bond cement, Kerr, Italy) and a calibrated syringe was used
to place a standardized increment of the cement (0.1mL) in each
coping. Then, each coping was placed on the abutment with finger
pressure for 10 seconds. Excess cement was removed with dental
explorer and the specimen was loaded on its long axis with a
2kg weight for 5 minutes. This procedure was carried out by one
Twenty-four hours after cementation, the specimens were
thermocycled between 5 °C and 55 °C with 10-second dwell times
for 500 cycles (Thermocycler THE-1100, SD Mechatronik GMBH,
After thermocycling, each specimen was vertically secured
in the universal testing machine (Instron 8500; Instron Corp,
Norwood, MA) and subjected to a pull-out test at a 0.5mm/min
crosshead speed (Figure 2). The load required for dislodgment of
the coping was recorded in (N).
The statistical tests were performed using the SPSS 16.0
program (SPSS Inc., Chicago, IL, USA). The data was analyzed using
two and one-way analysis of variance (ANOVA) and independent
t-test. All statistical analysis was performed at 0.05 level of
Table 1 shows the mean uniaxial retention force, standard
deviation and standard error for each group. Two-way ANOVA
(Table 2) revealed significant differences in the retention force
with different abutment heights, cement thicknesses and the
interaction between these two factors (P<0.05).
For the standard and shortened abutments, the highest mean
values of tensile strength were (87.83N) and (63.23N) respectively,
noticed with the 20μm cement film thickness. While the lowest
mean values were detected with the 35μm cement thickness (68.83
N) for the standard abutments and with the 50μm (49.59N) for the
shortened abutments (Figure 3). One-way ANOVA demonstrated
statistically significant differences in the tensile strength among
different cement thicknesses in each abutment height (P=0.00).
Tukey Post Hoc test for multiple comparison showed a significant
difference in the mean of the uniaxial tensile strength noticed
between the 20μm cement film thickness compared to the other
groups (P=0.00) for standard and shortened abutments. However,
no significant differences were found between 35μm and 50μm
cement film thickness for both abutment heights (P>0.05).
Independent sample t-test showed that standard abutments had
a significantly greater retentive forces compared to shortened
abutments with each cement thickness (P<0.05).
The retention of cement-retained implant-supported
restoration plays an important role in the success of treatment.
Cemented restorations can compensate for minor fit discrepancies
using a luting agent . Nevertheless, in some clinical studies,
retention loss found to be frequently occurring complication
[17,18]. Therefore, it is clinically important to understand how
to manage abutment geometry in relation to retrievable weaker
luting cement and appropriate cement thickness to provide the
adequate amount of retention in case of a clinically compromised
situation. The results of this study rejected the null hypothesis,
since the retention of cement-retained implant-supported
restorations were significantly affected by the abutment height
and cement space.
During fabrication of implant-supported restoration,
distortion might occur at any stage due to casting and investing
procedures [19,20]. Therefore, CAD/CAM technique was used to
ensure standardization of the fabrication of all coping specimens.
Luting agents’ tensile bond strength may be affected by variation
in mixing. Conventional mixing method may also risk the mixing
ratio. Thus, lowers cement quality. In this study, cement was
applied using syringe dispenser to calibrate the amount of cement
Cement thickness is one of the factors affecting the durability
of the cement. However, when cement space is inadequate, it will
result in crown seating discrepancies. The cement space helps to
improve seating of a crown, reduces the elevation of restorations,
improves the outflow of excess cement, and lowers the seating
forces, resulting in a better fit and retention of the final restoration
[13,21,22]. In this study, three cement space parameters were
used; 20μm, which is consistent with ADA specification #96 for
ideal cement thickness, 50μm according to several investigations
that apply this cement thickness [23,24] and midway between
these was applied (35μm).
The data of this study showed that the retention of the zirconia
coping was influenced by the geometry of the abutments, this
agrees with dental literature [16,25,26]. Cano-Batalla et al. found
a significant difference in crown retentions when comparing the
heights of 4, 5, and 6mm of cylindrical pre-fabricated abutments
. In this study, the height of 5.5mm produced significantly
higher retention when compared with a reduced height of
3mm. Regardless of abutment height, cement space of 20μm
was statistically significant more retentive compared to other
cement spaces. Whereas, cement spaces of 35μm and 50μm had
no significant differences. The finding of this study agreed with
the finding of another study by Gultekin et al. who reported
that increasing the cement space from 20 to 40μm for the lower
strength cements did not have any significant effect on retention
. The result of this study contradicts the findings by El-Anwar
et al. who recommended increasing the cement layer thickness
regardless of cement type for longer life-time crown fixation .
It can be inferred that weaker cements need less cement space gap
as increasing beyond certain number might affect the strength of
restoration without significantly improving marginal fit .
Weak cement was suggested to be effective to enable restoration
retrieval . However, when retention of the abutment is
compromised by loss of height, a stronger luting cement may
be indicated . It is important to weigh the clinical need for
retrieval against retention.
The Limitation of the study was the lack of evaluation of
mechanical cycling that simulate the dynamic loading with a
combination of compressive and tensile stresses on the restoration
during mastication, only a tensile force is applied for restoration
retrievability. Another limitation was the use of one type of luting
cement. Different results could have been obtained if different
cements were used.
Within the limitation of this study, the following conclusion
can be drawn:
Cement thickness of 20μm has a significant effect on the
retention of cement-retained restorations while, the increase
in cement thickness from 35μm and 50μm has no effect on the
retention of single implant restoration with standard or shortened
The authors would like to thank the College of Dentistry
Research Center and Deanship of Scientific Research at King
Saud University, Saudi Arabia for supporting this research project
(FR0380). The authors are also gratefully acknowledging the
help of Mr. Nassr Al Moflehi, Biostatistician Consultant, College of
Dentistry, King Saud University.