Keywords
abutment - screw loosening - InOcta abutment - SynOcta abutment
Introduction
Abutment screw loosening is a critical concern in dental prosthetics. It significantly
affects the stability and useful life of prostheses.[1] The entry of body fluids into the dental implant assembly lowers the preload value,
causing the tightening torque of the dental implant abutment to be lost.[2] Internal hexagonal abutment connections and CFR-PEEK composite materials are exceptionally
good at preventing screw loosening.[3] Addressing this issue through proper torque management, anti-rotation features,
and retightening protocols is crucial for the longevity and success of implants.[4] Torque loss in abutment screws varies, with the MTC model consistently experiencing
high torque loss, while the IHC model shows minimal loss.[5] Decreasing the abutment screw pitch can also be an effective method to increase
resistance to screw loosening.[6] InOcta and SynOcta abutments, two common types of abutments in dental prostheses,
have different characteristics. The InOcta system is designed with a smooth surface
for immediate or early loading, promoting quicker healing. The SynOcta system features
a unique octagonal connection, which enhances stability and facilitates a precise
fit between the implant and the abutment. This study compares screw loosening in InOcta
and SynOcta abutments through in vitro testing. This study can help to improve the performance and quality of dental prostheses
and provide useful guidelines for selecting the appropriate abutment in dental prostheses.[7]
[8] A dental implant is an alloplastic object, material, or other tissue partially or
completely placed or transplanted into the body for therapeutic, diagnostic, prosthetic,
or clinical testing purposes.[9]
Dental implants are known as an appropriate alternative treatment for tooth loss,
although this treatment can also be associated with problems.[10] Many studies have recently reported the occurrence of many problems and complications
after implant treatment. For example, potential complications such as failure of osseointegration,
surgical complications, marginal bone loss, inflammation around the implant site,
mechanical complications, and cosmetic problems have been observed.[11] Additionally, mechanical complications have been reported, including screw loosening,
screw fractures, implant fractures, veneer fractures, and reduced attachment in implant-supported
dentures.[12]
Screw loosening is one of the common complications in implant treatment.[13]
[14]
[15] A study showed that screw loosening was the most common problem that occurred during
the first year of treatment in 107 cases of one-unit implant restorations using the
Per-Ingvar Brånemark system.[13] Another study examined patients with implant treatment for 5 years and showed that
screw loosening occurred in 7.6% of cases.[14] The screw connects the abutment to the implant. When a screw is tightened, a rotational
force is applied to it, while a tensile force is produced by stretching. This creates
a “clamping force” that maintains the implant–abutment connection (preload).[16] However, the screw can loosen in the presence of a load higher than the clamping
force or when preload is lost.[17] When this happens, the abutment and implant can become mobile, affecting the surrounding
soft tissue and the implant structure, and potentially leading to local inflammation.
Additionally, when stress is concentrated, it may result in screw fracture, abutment
fracture, or even implant fracture.[18]
[19]
[20] Various factors including the implant type and design can affect this component.[21]
[22] The Dentis implant system is one of the most common implant systems used in Iran.[23] In this system, two types of abutments SynOcta and InOcta are made (these abutments
were chosen because they are widely used), which are different from each other in
internal geometry. Thus, this study investigates the effect of these two types of
abutments on the rate of screw loosening.
Materials and Methods
This study was conducted in 2024 in the Faculty of Dentistry of the University of
Medical Sciences. The data were collected in the form of laboratory samples. In this
laboratory study, donated filled fixtures were used, which were fully evaluated in
terms of chemical corrosion, mechanical wear, and fracture before starting the experiment
to ensure the validity of the research results. Specifically, 20 Dentis brand titanium-filled
fixtures were selected and divided into two groups including 10 fixtures for SynOcta
abutments and 10 fixtures for InOcta abutments. So, the sample size was 10.[24] In each fixture, SynOcta and InOcta abutments were mounted respectively.
For each implant, conical tissue level fixtures and titanium screws were used. The
investigated implant systems were mounted on acrylic blocks with dimensions of 20 × 6 × 10 mm
perpendicular to the surface (20 = height,10 = length, 6 = width). The vertical position
of the implant inside the resin blocks was fixed by the supervisor, and the abutments
were mounted on the fixture. After mounting the abutments in their respective fixtures,
the screws were tightened to a torque of 30 N/cm using a Cedar DID-4 digital torque
driver (made in Japan), following the manufacturer's instructions. After 10 minutes,
the screws were re-tightened to the same torque due to torque loss occurring after
the initial tightening.[25]
[26] Then, the samples and resin blocks were transferred to the Chewing Simulator Machine.
In this step, a compressive force equivalent to 90 N was applied to the samples and
inserted into the center of each sample for 10,000 cycles with a frequency of 75 rpm
(which is equivalent to the number of times a person chews in a year). After the end
of the loading period, the screws were loosened once more, and the torque required
to loosen them was measured. The rate of loosening was calculated. After measuring
and recording the screw loosening forces using t-test analysis, the results were compared at the error level of 0.05 using SPSS26
software. Since the study was not conducted on humans, there were no special ethical
considerations required for the implementation of the project.
Results
A comparison was made using the t-test after ensuring the normality of the data in two groups using the Kolmogorov–Smirnov
test (with a significance level of 5%). The results revealed that the mean de-torque
score of the abutment screw in the SynOcta tissue level group was significantly lower
than that in the InOcta tissue level group (p < 0.001). Based on [Table 1], among 10 tissue abutment samples of InOcta, the rate of de-torque was measured
in the range of 25.1 to 26.4 N/cm. The mean de-torque in this group was 25.75 N/cm
and the median was 25.8 N/cm. The standard deviation of de-torque was 0.46 N/cm and
the 95% confidence interval was calculated between 26.07 and 25.42 N/cm ([Table 1]).
Table 1
Investigating the amount of de-torque in N/cm in two groups of InOcta and SynOcta
tissue level abutment groups
|
Abutment type
|
Min
|
|
Max
|
Mean
|
Median
|
SD
|
Confidence interval
|
t
|
df
|
Sig.
|
|
InOcta tissue level abutment
|
25.0
|
26.4
|
25.75
|
25.8
|
0.46
|
25.42
26.07
|
25.1
|
11.52
|
18
|
>0.001
|
|
SynOcta tissue level abutment
|
20.4
|
23.7
|
21.98
|
22.05
|
0.93
|
21.31
22.64
|
20.4
|
For SynOcta group abutments, the amount of de-torque was recorded from 20.4 to 23.7 N/cm.
The mean de-torque was 21.98 N/cm and the median was 22.05 N/cm. The standard deviation
of de-torque was 0.93 N/cm and the 95% confidence interval for its mean was between
21.31 and 22.64 N/cm. The results of the t-test comparing the mean de-torque between the two groups indicated that the T-statistic
value was 11.52, with 18 degrees of freedom (df).
The significance level (p-value) was less than 0.001, indicating a statistically significant difference between
the two groups ([Fig. 1]).
Fig. 1 Comparison of the mean de-torque in the two groups of InOcta and SynOcta tissue level
abutment in N/cm.
Discussion
The abutment screw is a crucial component in the connection between the implant and
the abutment in most implant systems. During tightening, the screw experiences elastic
deformation and stretching, which generates a tensile force known as preload.[17]
[24] In other words, the screw acts as a tension spring and creates a force that firmly
connects the implant and the abutment and keeps these two parts together.[27] Bickford and Oliver have divided the screw-loosening process into two distinct stages.[28] First, the initial tensile deformation of the screw under the influence of the external
force is reduced and leads to the reduction of the clamping force. Then, in the second
stage, the micro-movement at the junction of the implant and the abutment increases
with a further reduction of the clamping force, and this instability leads to screw
loosening.[29] Preload reduction caused by external forces is the primary reason for abutment screw
loosening. The stability of the connection between the implant and the abutment largely
depends on the type of connection used. This issue has been supported by numerous
scientific studies.[30]
A study by Ahn et al confirms similar results. This study indicates that in external
connections, uneven distribution of stress can lead to problems such as breaking screws
and reducing the stability of the connection.[31] Internal connection refers to the design through which the abutment is attached
to the implant, typically with a structure of 4 to 6 mm. This design effectively increases
the contact surface area between the abutment and the implant, improving the distribution
of stresses during loading. Consequently, the change in stress distribution enhances
the stability and efficiency of the connection.[32] Kofron et al investigated the internal connections of implants and observed that
well-constructed internal designs can provide a greater contact surface between the
abutment and the implant, leading to more optimal stress distribution and reduced
screw tension. This helps to reduce problems such as screw fracture and improve the
stability of the connection.[33]
Sakamoto et al conducted a study on the implant–abutment connection and found that
internal connections have significant advantages over external connections in maintaining
stability. The study also indicates that internal connections provide greater resistance
to torque reduction and screw loosening.[34] Another study by Segundo et al demonstrated that internal connections significantly
improve load distribution and reduce stress compared with external designs. The study
specifically emphasized that a more effective design of internal connections offers
greater resistance to screw loosening and decreased torque.[35] Given the important role of the abutment in connecting the implant to the restoration,
differences in the design of abutments can significantly affect connection stability.
Various geometric designs of abutments are likely to alter the stress distribution
at the implant–abutment connection point and may also influence the level of torque
reduction.[36]
Yenigun et al examined the impact of different abutment designs on stress distribution
and mechanical stability of the implant–abutment connection. The results revealed
that geometric differences in abutment design can significantly change the stress
distribution, and affect the stability of the connection, and the level of torque
reduction. Different abutment designs have different impacts on the overall stability
of the connection by changing the load and stress distribution.[37] In addition to the effect of the geometry of the upper part of the abutment connected
to the restoration, the geometric design of the lower part of the abutment that is
in contact with the internal implant has a significant impact on reducing the torque
and stability of the connection. The rotational freedom between the implant and the
abutment is considered a key factor in maintaining the stability of the implant–abutment
connection. In the condition that the rotational freedom is less than 2 degrees, the
implant–abutment connection will be more stable. However, when the rotational freedom
reaches more than 5 degrees, the level of torque reduction increases significantly
and this can lead to instability and screw loosening.
These results emphasize the importance of accurate abutment design and limited control
of rotational freedom. They also indicate that optimal abutment designs can help to
maintain the stability and optimal performance of the connection. The study by Bédouin
et al indicated the effect of different abutment designs and rotational freedom on
the mechanical stability of the implant–abutment connection. The results indicate
that the geometric design of the abutment and the degree of rotational freedom significantly
affect the connection stability. In particular, the torque reduction increases dramatically
and may lead to instability and screw loosening when the rotational freedom exceeds
the desired limit. This study emphasizes the importance of careful abutment design
and limiting rotational freedom to maintain optimal performance and connection stability.[38]
In vitro studies indicate that abutments with conical design show higher resistance to torque
loss than other types of abutments, both before and after cyclic loading. This conical
design specifically creates a better fit between the abutment and the implant. The
result of this improved fit is the reduction of micro-gaps and micro-movements in
the implant–abutment connection point, leading to a significant reduction in torque
loss. In other words, the conical design has better stability and durability by reducing
gaps and unwanted movements in the connection.[39]
Sammour et al examined the effect of different abutment designs, including the conical
design, on the resistance to torque loss. The results indicated that, unlike other
studies, the conical design has a similar performance in torque reduction compared
with other designs. This study also provided different results compared with previous
studies and indicated that the conical design may not be as effective in reducing
micro-gaps and increasing the stability of the connection as some other designs.[40] The results of this study revealed that the InOcta tissue-level abutments show higher
stability against screw loosening due to the design and materials used. The independent
t-test indicated that the mean de-torque score of the SynOcta tissue-level abutment
screw was significantly lower than the InOcta tissue-level abutment screw. These results
indicate that InOcta tissue-level abutments cause less screw loosening, which can
be due to the combination of materials used and their more optimal design.
Additionally, InOcta tissue-level abutments require more attention and care during
the treatment to prevent screw loosening, despite their aesthetic benefits. This difference
may be due to the differences in the structure and design of these two types of abutments,
which can affect the level of stress on the screws. Generally, the results indicate
that InOcta tissue-level abutments have a better performance in reducing screw loosening
than SynOcta tissue-level abutments due to the design and materials used. This confirms
the effect of abutment design on the stability and performance of implants and shows
that the correct selection of abutment can play a vital role in the success of dental
implant treatments.[41] The study by Pardal-Peláez et al is among the studies conducted in this field. This
study examined the impact of abutment design on screw stability and loosening. The
results revealed that abutments with proper design and the use of materials with superior
mechanical properties significantly reduce screw loosening and provide higher stability
under different loading conditions. These results are consistent with those of the
present study and emphasize that the design of abutments plays a key role in reducing
screw loosening problems and positively affects the overall performance of dental
implant treatments.[42]
Conclusion
Based on the experiments, it is concluded that the de-torque of the abutment screw
in the SynOcta tissue level group was significantly lower than that in the InOcta
group under laboratory conditions (p < 0.001).
Limitation
The limitations were the small sample size and the period of the cycles during tests
was short (∼1 year). More studies should be conducted in different cycles and different
implant systems to confirm the results.
