materials

Article
Fatigue of Narrow Dental Implants: Influence of the
Hardening Method
R.A. Pérez 1 , J. Gargallo 2 , P. Altuna 2 , M. Herrero-Climent 3 and F.J. Gil 1,2, *
1 Bioengineering Institute of Technology, Universitat Internacional de Catalunya. C/ Josep Trueta s/n. Sant
Cugat del Valles, 08195 Barcelona, Spain; rperezan@uic.es
2 Faculty of Dentistry, Universitat Internacional de Catalunya, C/ Josep Trueta s/n. Sant Cugat del Valles,
08195 Barcelona, Spain; jgargallo@uic.es (J.G.); paltuna@uic.cat (P.A.)
3 Master Periodoncia, School of Dentistry, University of Seville, 41009 Seville, Spain;
marianoherrero@herrerocliment.com
* Correspondence: xavier.gil@uic.cat; Tel.: +34-936-021-910




Received: 7 February 2020; Accepted: 20 March 2020; Published: 20 March 2020


Abstract: The use of narrow titanium dental implants (NDI) for small ridges, reduced interdental
space, or missing lateral incisors can be a viable option when compared to the conventional wider
dental implants. Furthermore, in many cases, standard diameter implant placement may not be
possible without grafting procedures, which increases the healing time, cost, and morbidity. The
aim of this study was to analyze the mechanical viability of the current narrow implants and how
narrow implants can be improved. Different commercially available implants (n = 150) were tested to
determine maximum strength, strain to fracture, microhardness, residual stress, and fatigue obtaining
the stress–number of cycles to fracture (SN) curve. Fractography was studied by scanning electron
microscopy. The results showed that when the titanium was hardened by the addition of 15% of Zr
or 12% cold worked, the fatigue limit was higher than the commercially pure grade 4 Ti without
hardening treatment. Grade 4 titanium without hardening treatment in narrow dental implants can
present fractures by fatigue. These narrow implants are subjected to high mechanical stresses and the
mechanical properties of titanium do not meet the minimal requirements, which lead to frequent
fractures. New hardening treatments allow for the mechanical limitations of conventional narrow
implants to be overcome in dynamic conditions. These hardening treatments allow for the design of
narrow dental implants with enhanced fatigue life and long-term behavior.

Keywords: narrow dental implants; mechanical properties; fatigue; titanium alloy; Ti–Zr alloy;
hardening; fracture; hardness; commercially pure titanium; strain

1. Introduction
Severe alveolar ridge reduction caused by periodontal diseases, trauma, or tooth loss can result in a
reduced amount of bone in which to place regular diameter implants. In these cases, bone regenerative
techniques have been highly described to increase the bone tissue volume simultaneously or before
implant placement [1,2]. Despite this, reconstructive techniques are not exempt of limitations such as
increasing healing time, cost, and patient morbidity [1].
Different studies indicate the need for a minimum bone space of 1.5 to 2 mm between a tooth and
the implant and 3 mm between implants due to the tissues that should be accommodated, producing a
correct function and esthetics [3].
The mechanical strength of commercially pure titanium is sometimes insufficient for the long
lasting integrity of titanium implants [4], and consequently, the use of narrow dental implants represents
a major risk of fractures [5–7]. Clinicians should be aware of the mechanical problems of dental

Materials 2020, 13, 1429; doi:10.3390/ma13061429 www.mdpi.com/journal/materials

Materials 2020, 13, 1429 2 of 10

implants, especially when the small diameters are used in zones subjected to high occlusal forces.
Consequently, manufacturers have developed implants by increasing the strength in relation to the
commercially pure Ti by two possibilities: (1) alloying the titanium with other biocompatible metals
such as zirconium or niobium; and (2) straining the original commercially pure Ti by cold work.
It is well-known that the mechanical strength decreases very significantly with the diameter of
the implant. A 3.5 mm diameter implant was 5.1 times weaker than a 5 mm diameter implant and
6.8 times weaker than a 6 mm diameter implant [4,8,9].
The design of dental implants always has to consider the cyclic loading during the service life of
the implant, and therefore the fatigue endurance of the materials used play a very important role when
trying to estimate the long term behavior of the device. The crack initiation by fatigue is on the surface
of the dental implant. The crack growths into the dental implants were produced by the cyclic loads to
fracture. The surface roughness produced by shot blasting and the compressive residual stress can
favor the fatigue life of the dental implant. However, the connection surface where the load is applied
and the strength of the material used are key parameters for the long-term mechanical properties of
the dental implant [10–12]. Thus, the assessment of the fatigue behavior of implantable alloys has
been taking greater importance. The materials used, fabrication process, and effective connection
surface can be optimized in order to obtain a narrow dental implant with excellent static and dynamic
mechanical properties. There is very little known about the fatigue of these narrow implants, and the
durability and success rate have been described by only a limited number of clinical reports [10,13,14].
The objective of this study was to analyze the mechanical viability of current narrow implants and
how narrow implants can be improved. This contribution presents a null hypothesis that the hardening
treatments of the titanium do not have an influence on its mechanical behavior in the long-term.

2. Materials and Methods

2.1. Dental Implants.

Different commercially available narrow dental implants were used (n = 150) in this study and
distributed in three groups according to the type of titanium: Group 1, commercially pure grade 4
titanium; Group 2, titanium alloyed with 15% Zr; and Group 3, commercially pure grade 4 titanium
hardened by 12% cold worked. The dental implants used are summarized in Table 1 and illustrated in
Figure 1.

Table 1. Implants used distributed by group [15–18].

Group 3
Group 1 Group 2 Commercially Pure-Grade 4
Group
Commercially Pure-Grade 4 Ti Ti Alloyed with 15% Zr Titanium Hardened by 12%
Cold Worked
Bone level SLA (3.3 mm, h = 8 mm)
(Straumann AGR, Basel,
Switzerland) (n = 25) KL (3.3 mm, h = 8 mm) with
Bone level Osseospeed TX Yellow regular hexagon external
Bone level Roxolid
(3.0 mm, h =11 mm) (Astra Tech, connection (Klockner, Madrid.
Implant (3.3 mm, h = 8 mm)
Dentsply, Charlotte, NC, USA) Spain) (n = 25).
Type (Straumann AGR, Basel,
(n = 25) Bone Level Vega (3.5 mm, h = 8
Switzerland) (n = 25)
Bone level Osseospeed TX Aqua mm) (Klockner, Madrid. Spain)
(5 mm, h = 8 mm) (Astra Tech, (n = 25).
Dentsply, Charlotte, North Carolina,
US) (n = 25)
Connection
Conical internal Cross-fit internal Hexagon external/internal
Type
 Materials 2020, 13, 1429 3 of 10
Materials 2020, 13, 1429 3 of 10

Figure 1. Narrow Dental Implants used in this in vitro study.

2.2. Mechanical Properties

Figure 1. Narrow Dental Implants used in this in vitro study.
Initially, static tension tests were conducted to determine the yield strength of the material, the
2.2.
ultimateMechanical
strength, Properties
and the strain to fracture. The hardness of the specimens was measured using a
Vickers microhardness testertests
Initially, static tension (Akashi,
wereMatsusawa,
conducted to Japan) with athe
determine load of 100
yield gf andof15the
strength s ofmaterial,
indentation.
the
Following the static tests, a fatigue test at various percentages of the obtained
ultimate strength, and the strain to fracture. The hardness of the specimens was measured using yield strength wasa
performed, which allowedtester
Vickers microhardness for the numberMatsusawa,
(Akashi, of cycles before fracture
Japan) withtoa be determined.
load of 100 gfThe andaim15was
s ofto
find the stress
indentation. value at which the sample supported a total of ten million cycles, which is considered to
be theFollowing
fatigue limit. The assays were performed with a servo-hydraulic testing machine
the static tests, a fatigue test at various percentages of the obtained yield strength was (MTS Bionix
858, Minneapolis,
performed, whichMN, USA).for
allowed This
themachine
number was equipped
of cycles beforewith a load
fracture tocell MTS of 25 KN.
be determined. TheTheaimimplants
was to
were loaded with a sinusoidal function of fatigue at a frequency of 15 Hz and
find the stress value at which the sample supported a total of ten million cycles, which is considered 10% stress variation.
The implants
to be were limit.
the fatigue fixed with an inclination
The assays of 30◦ with
were performed the aaxis
with z of the tensile-compression
servo-hydraulic testing machine machine
(MTS
(Figure
Bionix 2).858,The data are represented
Minneapolis, MN, USA).as themachine
This number was of cycles reached
equipped withatafracture
load cellfor
MTSdifferent
of 25 KN.applied
The
stress. The deformed and fractured specimens were observed by means of scanning
implants were loaded with a sinusoidal function of fatigue at a frequency of 15 Hz and 10% stress electron microcopy
(JSM 6400,
variation.
Materials Jeol,
The
2020, Japan). were fixed with an inclination of 30° with the axis z of the tensile-compression
13, implants
1429 4 of 10
machine (Figure 2). The data are represented as the number of cycles reached at fracture for different
applied stress. The deformed and fractured specimens were observed by means of scanning electron
microcopy (JSM 6400, Jeol, Japan).

2.3. Residual Stress

Residual stresses were measured with a diffractometer incorporating a Bragg–Bentano
configuration (D500, Siemens, Germany). The measurements were performed for the family of planes
(213), which diffracted at 2θ = 139.5°. The elastic constants of Ti at the direction of this family of planes
were EC = (E/1+ ) (213) = 90.3 GPa (1.4). The residual stress was designated as:  = EC (1/d0) A; where
d0 is the interplanar distance for the measuring angle  = 0°.

Figure 2. Clamp of the fatigue test machine.

Figure 2. Clamp of the fatigue test machine.

2.4. Statistical Analysis

Statistically significant differences among the test groups for mechanical evaluation were
assessed using statistical software (MinitabTM 13.1, Minitab Inc., New York, USA). Analysis of
Materials 2020, 13, 1429 4 of 10

2.3. Residual Stress

Residual stresses were measured with a diffractometer incorporating a Bragg–Bentano
configuration (D500, Siemens, Germany). The measurements were performed for the family of
planes (213), which diffracted at 2θ = 139.5◦ . The elastic constants of Ti at the direction of this family of
planes were EC = (E/1+ υ) (213) = 90.3 GPa (1.4). The residual stress was designated as: σ = EC (1/d0 ) A;
where d0 is the interplanar distance for the measuring angle Ψ = 0◦ .

2.4. Statistical Analysis

Statistically significant differences among the test groups for mechanical evaluation were assessed
using statistical software (MinitabTM 13.1, Minitab Inc., New York, USA). Analysis of Variance (ANOVA)
tables with a multiple comparison Fisher test were calculated. The level of significance was established
at a p-value < 0.005. Surgeons should be aware of the mechanical problems of dental implants,
especially when small diameters are used in zones subjected to high occlusal forces

3. Results
The mechanical properties of the tested implants are shown in Table 2. The yield strength for the
Grade 4 Ti did not differ among them, despite the difference in diameter. Nevertheless, their values
became significantly higher when the implants were composed of Ti-15Zr or presented 12% cold work.
A similar trend was observed for the maximum strength. On the other hand, the strain to fracture was
lower for the Ti alloy and cold worked Ti. The Ti-15Zr presented a significantly higher value of strain
to the cold worked Ti. The hardness was shown to be significantly higher for the cold worked Ti than
the other conditions. Ti-15Zr also presented significantly higher hardness values than the Ti grade 4.
Finally, the residual stresses were one order of magnitude higher for the cold work implants when
compared to the other conditions.

Table 2. Mechanical properties of the dental implants studied include maximum strength and yield
stress at 0.2%, expressed in megapascal (MPa), ductility is in percentages, the hardness is expressed in
Vickers hardness number (HV) and residual stress in megapascal (MPa). The negative values of the
residual stress represent the compressive nature of the stress. Standard deviation between parenthesis.

Maximum Yield Stress Ductility Hardness Residual

Implant
Strength (MPa) 0.2% (MPa) (%) (HV) Stress (MPa)
SLA 520 (21) 443 (23) 16 (7) 109 (10) −70 (6)
Group 1 Yellow 470 (40) 375 (12) 16 (5) 102 (10) −54 (13)
Aqua 480 (39) 368 (25) 19 (4) 103 (11) −45 (12)
Group 2 Roxolid 887 (34) 689 (23) 24 (3) 197 (13) −55 (17)
KL 1032 (41) 783 (15) 6 (2) 356 (22) −375 (43)
Group 3
Vega 1090 (37) 750 (21) 7 (3) 378 (26) −398 (34)

The differences between the maximum strength of KL and Vega and the other implants were
statistically significant (p < 0.005). Roxolid presented statistical differences with SLA, Yellow, and
Aqua (p < 0.005). For the yield stress, the differences of the cold worked implants (KL and Vega) and
the other implants were also statistically significant (p < 0.005). The same occurred with the hardness
(p < 0.005) and the residual stress (p < 0.005) with the cold worked implants. However, the ductility
was higher for SLA, Yellow, Aqua, and Roxolid, with differences statistically significant in relation to
the cold worked dental implants.
Figure 3 shows the S–N curve for the different dental implants. The dental implants are submitted
at different forces (Y-axis) cycling from the compressive to unloaded. The number of the cycle when
the dental implant is fractured is the value of the x-axis. The results show that the implants alloyed
(p < 0.005) and the residual stress (p < 0.005) with the cold worked implants. However, the ductility
was higher for SLA, Yellow, Aqua, and Roxolid, with differences statistically significant in relation
to the cold worked dental implants.
Figure 3 shows the S–N curve for the different dental implants. The dental implants are
submitted at different forces (Y-axis) cycling from the compressive to unloaded. The number of the
Materials 2020, 13, 1429 5 of 10
cycle when the dental implant is fractured is the value of the x-axis. The results show that the implants
alloyed with zirconium and the grade 4 titanium submitted to 12% strained presented more fatigue
life with
thanzirconium
the titanium anddental implants
the grade (grade
4 titanium 4) without
submitted hardening
to 12% strainedtreatment.
presented Furthermore, as athan
more fatigue life
general rule, it can
the titanium be observed
dental implants that bigger
(grade diameters
4) without tendedtreatment.
hardening to present Furthermore,
a longer fatigue
as alife.
general rule, it
can be observed that bigger diameters tended to present a longer fatigue life.

Figure 3. Stress–number of cycles to fracture (S–N) curve of the different narrow dental implants.

Figure 3. Stress–number
Fracture of cycles
surfaces observed bytomeans
fracture
of (S–N) curve
scanning of the different
electron narrow
microscopy dental implants.
determined that the fracture
was, in all cases, in the connection with the screw since it corresponded to the narrower part of the
Fracture
implant surfaces
(Figure 4). observed by means of scanning electron microscopy determined that the
Materials
fracture was,2020,
in 13,
all1429 6 of 10
cases, in the connection with the screw since it corresponded to the narrower part
of the implant (Figure 4).

Figure 4.
Figure 4. Crack
Crack nucleation
nucleation in the connection
in the connection place
place (dental
(dental implants
implants with
with the
the abutment).
abutment). InIn this
this zone,
zone,
the mechanical load was the highest and the width of the dental implant was the lowest.
the mechanical load was the highest and the width of the dental implant was the lowest.

In Figure 5, the crack propagation can be observed that (arrows indicate the direction of the
crack). The striated microstructure demonstrates the fatigue mechanism of the fracture. Furthermore,
from Figure 5, secondary cracks can be observed, which were perpendicular to the direction of the
propagation.
 Materials 2020, 13, 1429 6 of 10
Figure 4. Crack nucleation in the connection place (dental implants with the abutment). In this zone,
the mechanical load was the highest and the width of the dental implant was the lowest.
In Figure 5, the crack propagation can be observed that (arrows indicate the direction of the
In Figure
crack). The5, striated
the crack propagation demonstrates
microstructure can be observed
the that (arrows
fatigue indicate
mechanism the fracture.
of the directionFurthermore,
of the
crack).from
The striated microstructure demonstrates the fatigue mechanism of the fracture. Furthermore,
Figure 5, secondary cracks can be observed, which were perpendicular to the direction of
from Figure 5, secondary cracks can be observed, which were perpendicular to the direction of the
the propagation.
propagation.

Secondary crack

Figure 5. Crack propagation.

Figure 5. Crack propagation.

The strained implants

The strained presented
implants more more
presented percentage of theof
percentage brittle fracture,
the brittle and the
fracture, andother fractures
the other fractures
were were
mainly ductile, corresponding to the grade 4 dental implants including the dental implants
mainly ductile, corresponding to the grade 4 dental implants including the dental implants made
madewith
withTi15Zr
Ti15Zr(Figure
(Figure6).6).The
The narrow
narrow dental
dental implants
implants treated
treated by cold
by cold work
work had had
lowerlower ductility
ductility due to the
due todifficulty
the difficulty
of theofdislocation
the dislocation movement.
movement. ThisThis produced
produced lowerlower damage
damage of fracture
of the the fracture surface
surface as can be
as canobserved
be observed in Figure 6. Ti-15Zr presented excellent mechanical properties (static and
in Figure 6. Ti-15Zr presented excellent mechanical properties (static and cyclic) withoutcyclic)
without losing
Materials
losing ductility.
2020, 13, 1429
ductility. 7 of 10

Figure 6. Fractography of the narrow dental implants.

Figure 6. Fractography of the narrow dental implants.
4. Discussion
4. Discussion
In clinical scenarios where insufficient bone or limited space is present, narrow sized implants may
In clinical scenarios where insufficient bone or limited space is present, narrow sized implants
be required to replace the tooth lost. Narrow dental implants (3.3. to 3.5 mm) are well documented in
may be required to replace the tooth lost. Narrow dental implants (3.3. to 3.5 mm) are well
all indications including load-bearing posterior regions. Smaller implants of 3.0 to 3.25 mm in diameter
documented in all indications including load-bearing posterior regions. Smaller implants of 3.0 to
are mm
3.25 well in
documented
diameter areonly
wellfor single-tooth
documented non-load-bearing
only regions. <3.0
regions. Mini-implants
for single-tooth non-load-bearing Mini-mm in
diameter<3.0
implants are mm
onlyindocumented
diameter arefor thedocumented
only edentulous arch andedentulous
for the single-tooth non-load-bearing
arch and single-toothregions,
non- and
load-bearing regions, and success rates are not available [4,5,13,14]. Nevertheless, these implants may and
success rates are not available [4,5,13,14]. Nevertheless, these implants may have severe medium-
long-term
have severecomplications
medium- andarising fromcomplications
long-term their mechanical properties
arising andmechanical
from their lower resistance to fatigue
properties and [3,19].
lower resistance to fatigue [3,19]. Therefore, the mechanical properties of commonly used narrow
implants, together with the masticatory process intrinsically bounded with continuous compressive
loads, are important to consider when improving the current limitations of narrow dental implants,
producing titanium alloys, or submitting Ti to cold work.
The grade 4 titanium narrow dental implants presented strength, yield stress, and hardness
Materials 2020, 13, 1429 7 of 10

Therefore, the mechanical properties of commonly used narrow implants, together with the masticatory
process intrinsically bounded with continuous compressive loads, are important to consider when
improving the current limitations of narrow dental implants, producing titanium alloys, or submitting
Ti to cold work.
The grade 4 titanium narrow dental implants presented strength, yield stress, and hardness values
lower than the strained or alloyed with 15% Zr dental implants, and only the ductility was higher than
the treated implants.
Therefore, for high mechanical requirements, narrow dental implants cannot give a reliable
response in their structural integrity.
The fatigue behavior of the narrow implants submitted to cold work was better due to the
compressive effect of the residual stresses on the surface, which makes crack nucleation difficult.
The cold work creates an important number of dislocations in the titanium, producing an increase
in the hardness, compressive residual stress, and mechanical strength [20,21]. Similar values were
obtained by the Ti-15Zr alloy due to the increase in mechanical strength [22–24]. The reason for this
improvement was the distortion created in the crystalline structure by the substitution of titanium
by zirconium atoms, producing difficulties for the movement of dislocations. These are the two
mechanisms for increasing the mechanical properties of the narrow dental implants, as can be observed
in the results in Table 2, where implants with the same diameter (Roxolid (TiZr alloy) and KL) present
an important increase in the mechanical strength. It is also worth highlighting that KL present a
hexagonal connection, which has been generally associated with low fatigue properties, compared to
the internal conical connections [25].
Binon et al. [26] indicated tolerances of manufacturing as a reason for the described loose-fit of
the prefabricated parts and requested the manufacturer to improve the fit of implant components. In
loose-fit situations, the possibility of horizontal movement and micro rotation between the implant
and abutment screw and lower the forces to tighten it, micromovements could lead to a progressive
unscrewing of the abutment screw under dynamic loading conditions. The main cause of the high
fatigue life of the external connection is due to the size of the resistant section; the external system
presents a higher value of the area than the internal. This fact produces a better load distribution
of the load and this is a main factor that explains the differences in the mechanical properties. The
tolerances in the internal connections are better and favor the fatigue behavior of the internal connection
system. However, this factor is not sufficient to improve the fatigue response in relation to the external
connections. Raoofi et al. [15] used finite element analysis to ascertain that the stress concentration
decreased when the internal surface area increased. In general, the place of fracture in the external
connection is in the screw.
The results of Osseospeed Yellow presented the lowest limit of fatigue due to the narrowest dental
implant diameter (3.0 mm) in comparison to the diameters of the Straumann (3.3 mm) and Aqua
Astra-Tech (3.5 mm). The higher values of the Vega Klockner implant may be due to the increase in the
diameter from 3.3 mm (Straumann-Roxolid) to 3.5 mm (Klockner-Vega). However, the mechanism of
alloying produces an increase in the static mechanical properties as cold work, but with high values of
strain to fracture, producing more toughness in the narrow dental implants [27,28].
Fracture was localized in all cases in the connection because it has a less effective diameter.
Fractography revealed how crack propagation was very similar for the grade 4 and Ti-15Zr implants,
where the crack propagation in the fracture surface presented ductility and the grooves of each cycle
could be seen. The strained implants presented a fractography with brittle places. Consequently,
cold work treatment produces an increase in the surface hardness, as shown in the results of the
microhardness and residual compressive stress tests. This fact suggests that the crack nucleation site
changes from the specimen’s surface (for the as-machined metal) to the specimen’s interior (for the
strained metal). This behavior is also shown in grit blasting dental implants, which also improves
the osseointegration [19,29–31]. This change is postulated to result in a significant modification of the
fatigue properties of dental implants made of commercially pure Ti.
Materials 2020, 13, 1429 8 of 10

A limitation of this study is that the dental implants studied had slightly different designs and
connections. We studied the commercial dental implants more widely used in the market within
narrow dental implants. In the same way, we think that the most important factors in this study to
determine fatigue life are the material and surface residual stresses of each dental implant.
This work will help clinicians make a more informed choice when choosing a small-diameter
implant system. Narrow dental implants increase their fracture risk due to their smaller diameter,
which might compromise the prosthetic components and also lead to bone overloading [31].
Abutment fracture is the primary prosthetic failure for two-piece narrow dental implants [30].
The narrower the implant diameter, the smaller the stress distribution area, which could contribute to
the implant itself being more prone to damage accumulation [30]. It has been demonstrated that from
narrow to standard and large diameter implants, an increasing probability of survival is observed with
significant differences favoring cemented when compared to screw-retained prostheses [8,12].

5. Conclusions
Two methods for hardening the commercially pure titanium, cold working (12%) or alloying with
15% of zirconium, improved the mechanical properties, particularly the fatigue response of narrow
implants. Implants with larger diameters showed higher limits of fatigue than narrower-implants.
Commercially pure-Ti grade 4 should be studied by clinicians in order for the long-term success of
the treatment. Narrow diameter implants, hardened with titanium–zirconium alloys or cold working,
resulted in mechanical properties adequate for long-term behavior.

Author Contributions: R.A.P. collected data; J.G. and P.A. undertook analysis of the data; M.H.-C. conceived
the ideas and performed analysis of the data. F.J.G. conceived the ideas, collected data, and led the writing.
All authors have read and agreed to the published version of the manuscript.
Funding: The work was supported by the Spanish government. Ministerio Economía y Competitividad and
FEDER (UE) under the research project number RTI2018-098075-B-C22.
Acknowledgments: The authors are grateful to the Spanish Government and European Union FEDER by the
concession of project RTI2018-098075-B-C22
Conflicts of Interest: The authors declare no conflicts of interest.
Ethical approval: This article does not contain any studies with human participants or animals performed by any
of the authors.

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