Introduction
Osseointegrated dental implants have been proven to be an ideal treatment modality
in restoring the oral function and esthetic of missing teeth because of their clinical
survival rates.[1]
[2]
[3] The prosthetic components of dental implants have been developed dramatically to
secure biocompatibility, harmonize the adjacent soft and hard tissues, and improve
the esthetic and biomechanical merits.[4] Implant abutments are used to connect the implant body with implant-supported restorations.
Numerous materials and techniques have been conducted to fabricate implant abutments
based on different clinical situations.[5]
[6]
[7]
Prefabricated titanium abutments are the most common type used because they have a
simple technique and are inexpensive compared with other types.[8]
[9] However, these abutments may only be applicable to cement-retained restorations,
cases with ideal implant placement, and cases that suit the depth, emergence profile,
and diameter of the restored edentulous area.[10]
[11] Custom abutments have been suggested to overcome the disadvantages of prefabricated
abutments, particularly in off-axial implants in which screw access emerges buccally.
Custom abutments can either be cast using metal alloys or milled by computer-aided
design/computer-aided manufacturing (CAD/CAM) technology. They provide high strength,
long durability, and either cement- or screw-retained prosthesis, and they allow the
fabrication of a fixed prosthesis with proper thickness.[12] Despite their advantageous properties, these abutments have limited application
due to their fabrication sensitivity, high price, and inappropriate esthetic appearance.[13]
[14]
Dentists’ attention has turned toward ceramic abutments to fill the need for suitable
abutments in the esthetic zone. Owing to their adequate biomechanical and optical
properties, zirconia abutments have been commonly used in either cement- or screw-retained
implant-supported prostheses.[15]
[16]
[17]
[18] These abutments can be offered in a one-piece design made of zirconium oxide, including
the abutment and the internal connection part, or a two-piece design in which a metallic
insert is included as an internal connection.[19]
[20] In a 10-year randomized prospective study, Amorfini et al[15] investigated the clinical outcomes of one-piece and two-piece zirconia abutments
and found that the overall prosthetic success rate was 85% and that the observed prosthetic
complications included abutment fracture, porcelain chipping, screw loosening, and
loss of retention. A 12-year retrospective study reported similar complications related
to zirconia abutments, such as abutment fracture occurring at the implant neck and
along the abutment walls adjacent to the screw access hole.[21] Stimmelmayr et al[22] investigated the wear at the abutment implant interface with zirconia and titanium
abutments and found that a significant higher wear of titanium implants was noticed
when connected to one-piece zirconia abutments.
Recently, the use of a digital workflow through CAD/CAM systems has been developed
in implant dentistry to allow the precise machining of implant-supported prostheses
in a shorter duration.[23] Thus, titanium base abutments were introduced to allow for a strong link between
the implant and the ceramic abutment/crown and to provide a favorable esthetic outcome.[24] This review aimed to focus on the technical and clinical applications of titanium
base abutments in implant prosthodontics. Particular attention was given to the titanium
base abutment design, surface treatment and retention of the superstructure, fracture
strength and failure mode, misfit and torque loss, and clinical performance of titanium
base abutments.
Results
Titanium Base Design
Titanium base abutments have a specific geometry that is saved in the CAD/CAM system
to allow for the fast fabrication of restorations. Once the restoration is milled
and has undergone sintering or the crystallization cycle, it is cemented or bonded
to the titanium base extraorally and then inserted into the dental implant.[11]
Two techniques are used to fabricate implant-supported restorations using titanium
base abutments.[11]
[25]
[26]
[27] The first technique is to design and mill the crown and abutment as one piece using
CAD/CAM ceramic restorations or create a wax up using a plastic sleeve and fabricate
the restoration using the pressable ceramic materials. After that, the crown can be
bonded to the titanium base abutment. The advantage of this technique is that it removes
excess cement extraorally before the abutment is screwed into the implant.[11]
[25]
[28] The second technique involves designing and milling, or pressing the abutment and
the crown separately, followed by bonding the abutment to the titanium base. The abutment
is then screwed into the dental implant, followed by crown cementation on the abutment.[26]
[27] Nouh et al[26] assessed the fracture resistance of these two techniques using zirconia and lithium
disilicate restorative materials and found that the abutment bonded to the titanium
base with a separate zirconia crown had the highest fracture resistance (3,730 N),
followed by the one-piece zirconia abutment and crown bonded to the titanium base
(3,400 N), with no significant difference between both techniques.
Recently, titanium base abutments with the concept of angled screw channel have been
manufactured to compensate for the buccal/labial angulated implant position.[29]
[30]
[31] The benefit of this concept is to allow for fabrication of screw-retained restorations
by redirecting the screw access channel to the lingual aspect. The corrected angulation
of these abutments ranges between 0 and 30 degrees to the long axis of the implants.[29] A specific hexalobular head design of the abutment screws has been fabricated to
allow engaging of a specific screwdriver to tightening and torquing the screw.
The height of titanium base abutments varies based on the available restorative space.[25]
[26] Silva et al[25] evaluated the effect of two different heights of titanium base abutments (4 and
2.5 mm) on the retention of zirconia crowns using the pull-out test in a universal
testing machine. They reported no significant effect of the abutment height on the
retention of the crown.
Surface Treatment and Retention of Superstructure
Different cement materials, cementation techniques, and surface treatment procedures
have been investigated in in vitro models to assess the pull-out retention strength between the titanium base abutments
and the superstructures of either abutments or crowns.[25]
[32]
[33]
[34]
[35] Three types of cements, including temporary cement, resin cement, and glass ionomer
cement, have been tested for the tensile bond strength test between titanium base
abutments and zirconia copings. Resin cement presented a significant increase in retention
values compared with temporary cement and glass ionomer cement.[25] In this study, both the titanium base and zirconia superstructure were treated with
an adhesive system, and no mechanical surface treatment methods were used.[25] In another in vitro study, temporary cement and self-adhesive resin cement were used to evaluate the
retention of four superstructure materials to titanium base abutments.[35] A substantial difference in retention values was reported between the two cements,
with resin cement having the highest retention mean value.
Gehrke et al[34] examined the effectiveness of three resin cements in retaining zirconia copings
to titanium base abutments. All titanium base abutments and zirconia copings were
subjected to air abrasion using 50 µm aluminum oxide particles and 15,000 cycles of
thermocycling. Although the retention values of the three cements were high enough
to provide stable retention, the difference between the cements was not significant.[34] In another study, three different resin cements were used to evaluate the retention
of zirconia and lithium disilicate copings to titanium base abutments.[32] Different mechanical and chemical surface treatments, such as sandblasting with
50 µm aluminum oxide particles and bonding agents, were applied to the surface of
titanium base abutments and the inner surface of ceramic copings. The results showed
that the combination of chemical and mechanical surface treatments significantly enhanced
the retention of lithium disilicate and zirconia copings, regardless of the cement
type.[32]
Therefore, it is recommended to modify the surfaces of titanium base abutments and
superstructure materials with chemical and mechanical surface treatments to improve
joint retention. Resin cement is the preferred luting agent to cement the two components
together.
Fracture Strength and Failure Mode
Although the fabrication of abutments completely using zirconia has improved the esthetic
outcomes, particularly in the esthetic zone, these abutments demonstrate a weak connection
and are vulnerable to fracture.[36]
[37]
[38] One of the main advantages of using titanium base abutments is the improved fracture
resistance of the ceramic abutments and crowns, thus overcoming the brittle nature
of ceramic abutments.[24]
Several studies investigated the effect of introducing titanium base abutments into
implant-supported restorations.[24]
[27]
[39]
[40]
[41]
[42]
[43]
[44]
[45] Different designs of zirconia abutments, including one-piece anatomic contour zirconia
abutments and zirconia abutments with titanium inserts, have been examined for fracture
strength tests after screwing them to titanium alloy implants with a regular diameter
(4.1 mm).[24] Zirconia abutments with titanium inserts were found to have a remarkable increase
in fracture resistance compared with the one-piece zirconia. The fracture of one-piece
anatomic contour zirconia abutments occurred either at the coronal part of the abutments
or at the hexagon connection part. By contrast, neither the zirconia abutments nor
the titanium inserts had fracture in the zirconia abutments with titanium inserts;
the fracture occurred only in the abutment screws.[24] However, the one-piece zirconia abutments should be used with caution in the posterior
segments, as the average recorded value of occlusal forces posteriorly could increase
to 720 N.[24]
[46]
Elsayed et al[27] compared the fracture strength of different types of abutments, including titanium,
zirconia, zirconia with titanium inserts, lithium disilicate abutments with titanium
inserts, and combined lithium disilicate abutments and crowns with titanium inserts.
All abutments were restored with lithium disilicate crowns and screwed to titanium
implants with a regular diameter. The authors reported that the lowest fracture resistance
value was found in the one-piece zirconia abutments, with the fracture occurring at
or above the implant shoulder level. The other abutment types with titanium inserts
had significantly higher fracture resistance values, and failure occurred because
of the bending of the titanium inserts and screws.[27]
Regarding screw-retained implant-supported restorations, a recent study evaluated
the fracture strength of partially stabilized and fully stabilized monolithic zirconia
crowns screwed directly to implants or cemented to titanium base abutments.[40] The results showed that the screw-retained monolithic zirconia crowns with titanium
base abutments either partially stabilized or fully stabilized were significantly
stronger than the screw-retained zirconia crowns without a titanium base.[40] In another study, lithium disilicate, zirconia, and polyetheretherketone materials
were employed to fabricate screw-retained implant-supported single crowns (combination
of abutments and crowns) using titanium base abutments, and their fracture resistance
was investigated. Zirconia crowns with titanium base were found to have higher fracture
resistance than other materials, and they could be used in the premolar area.[45] Adolfi et al[44] assessed the fracture resistance of two different designs of assembling screw-retained
zirconia crowns to titanium bases. In the first design, the titanium bases were cemented
to the zirconia crowns using resin cement; in the second design, the zirconia crowns
were fixed to titanium bases through a hexagonal connection notched in both the crowns
and titanium bases. The authors reported that the group with titanium bases cemented
to zirconia crowns had a significantly greater fracture load than the notched restorations.
They concluded that the resin cement applied between the restoration and the titanium
base could have the potential to improve fracture resistance.[44]
Based on the results of previous studies, implant-supported ceramic restorations should
be braced using titanium base abutments to withstand occlusal forces due to high bending
moments.[24]
Misfit and Torque Loss
One of the main requirements to achieve a successful implant-supported restoration
is for the implant to passively fit.[47]
[48] The misfit can induce stresses to the implant–bone interface and create biological
and mechanical complications, such as torque loss and screw loosening, fracture of
abutment screw, marginal bone loss around the implant neck, and loss of implant osseointegration
in advance cases.[49]
[50] Previous studies have suggested that a 150-µm gap can be considered a clinically
acceptable misfit value.[51]
[52]
Many attempts have been conducted to explore the effect of using titanium base abutments
on the misfit of implant-supported restorations.[40]
[44]
[53]
[54]
[55] Ramalho et al[54]assessed the internal fit of implant-supported single crowns fabricated from different
designs, including three screw-retained restorations (milled one-piece abutment/crown,
milled crown cemented to a titanium base, and milled crown cemented to custom abutments)
and three cement-retained restorations (milled two-piece abutment and crown, milled
crown cemented to a titanium base, and milled crown cemented to custom abutments).
They found that restorations with a titanium base and custom abutments had significantly
lower misfit values than digitally milled restorations. Similarly, in another study,
fully digital, titanium base, and custom abutments were fabricated and assessed for
internal fit in different regions of the implant abutment connection (marginal, top,
and middle of the connection) using the silicon replica technique and microcomputed
tomography.[55] Titanium base and custom abutments were found to have a better internal fit than
digitally milled abutments.[55]
A recent study assessed the misfit of screw-retained single-unit restorations constructed
by milling, titanium base, casting, overcasting, and laser sintering processing methods.[53] Titanium base abutments were found to have a significantly better marginal fit than
the casting and laser sintering techniques and a lower fit than the milling process
method. All fabrication techniques showed a misfit of restorations less than 150 µm.[53]
Regarding torque loss, Adolfi et al[44] compared the amount of torque loss, vertical misfit, and stress concentration between
zirconia restorations after being cemented to titanium base abutments using resin
cement or notched to a titanium base using the hexagon shape of the inner surface
of zirconia crowns and the outer surface of the titanium base. The authors reported
that the amount of torque loss, stress concentration, and vertical misfit decreased
significantly in the cement-retained restorations compared with the notched-retained
restorations.[44] In a recent study, the amount of torque loss of titanium bases was evaluated after
being bonded to zirconia, lithium disilicate, or polyetheretherketone restorations,[45] and the material of the superstructure was found to have no significant effect on
the amount of torque loss.[45]
Based on the aforementioned studies, the internal and marginal fit of titanium base
abutments had comparable outcomes with other fabrication techniques. However, the
cement-retained restorations using titanium base abutments could have a better fit
and less generated stress than the screw-retained restorations.
Clinical Performance
The marginal bone loss around dental implants has been proven to be one of the biological
complications that can lead to implant failure. Excess cement has been suggested to
have a remarkable effect on marginal bone loss.[56] One of the advantages of using titanium bases is their ability to cement the superstructure
materials to themselves extraorally and to remove excess cement, thus aiding in the
stabilization of the marginal bone level and reduction of the biological complications.
In addition, titanium bases, as previously discussed, can withstand high occlusal
forces because of their high bending moments. Thus, they can be a viable option for
clinical application.
Owing to the recent introduction of titanium base abutments, few clinical studies
have been conducted to assess their performance with regard to the survival and failure
rates, technical and biological complications, and peri-implant soft tissue response.[57]
[58]
[59]
[60]
[61]
[62]
[63] In a prospective clinical trial, Joda et al[57] restored 44 subjects in two visits each with 50 screw-retained monolithic lithium
disilicate crowns cemented extraorally to titanium bases. Most of the restorations
were placed in the premolar and molar areas in both the maxillary and mandibular arches.
A 2-year follow-up period revealed that the survival rate was 100% for all implants
and that no biological or technical complications were recorded.[57] In a retrospective study, 42 two-piece zirconia abutments were fabricated for 27
subjects and bonded to titanium inserts.[61] All abutments were restored with final restorations, including crowns, splinted
crowns, and fixed partial dentures. After 6.6 years of follow-up, seven zirconia abutments
failed, mainly in the molar area, thus suggesting that zirconia abutments bonded to
titanium inserts could be limited to the anterior and premolar areas.[61] A clinical report assessed the clinical performance of 24 two-piece veneered zirconia
restorations cemented to titanium bases for a period of 1 year.[62] An insignificant effect was observed regarding the crestal bone level, whereas pocket
depth and bleeding on probing changed significantly. A 95.8% survival rate was recorded
because of the loss of one implant. Four technical complications occurred, including
ceramic chipping and screw loosening, thus resulting in an 83.3% success rate of the
restorations.[62]
In a prospective clinical trial, Pamato et al[58] compared two groups of implant-supported crowns delivered to 21 subjects. The tested
group included implants restored with 28 titanium base abutments, while the control
group included implants restored with 24 cement-retained abutments. No significant
difference was found between the two groups regarding bleeding on probing, pocket
depth, and the mesial and distal crestal bone levels at 6-month and 1-year evaluations.
The study showed that both clinical techniques were comparable, as they did not have
a negative effect on the peri-implant soft and hard tissue parameters.[58] Linkevicius et al[63] assessed the level of marginal bone loss in three groups, including 2 mm or less,
2.5 mm, and 3 mm or more of vertical mucosal thicknesses. A total of 55 regular diameter
implants were placed in 55 subjects and restored with monolithic lithium disilicate
crowns using titanium bases. A 1-year follow-up showed that a significant marginal
bone loss was recorded in the 2 mm (1.25 ± 0.8 mm) and 2.5 mm (0.98 ± 0.06 mm) mucosal
thickness groups compared with the 3 mm (0.43 ± 0.37 mm) group, indicating that the
vertical mucosal thickness greatly affected the marginal bone level.[63]
In a recent study, the infiltration of immune cells to the peri-implant soft tissue
was examined after loading implants with different types of abutments, including gold
alloy, titanium, zirconia, and titanium base.[60] A total of 17 patients received 20 implants in the posterior segments of the maxillary
and mandibular arches. Eight weeks later, the abutments with 1 mm peri-implant soft
tissues were removed and examined. The results showed that gold alloy abutments had
a significant increase in infiltration of inflammatory cells, such as macrophages,
T-cells, and B-cells, whereas other abutments, including titanium base, presented
insignificant changes in the inflammatory cell count.[60]
Some manufacturers provide titanium base abutments with different sulcular heights
to compensate for implant placement in different depth levels and variation of soft
tissue heights. Multiple clinical reports have demonstrated the ability to design
and fabricate ceramic abutments and crowns using titanium base to achieve the optimum
emergence profile and improve the esthetic outcomes.[64]
[65]
[66] Martínez-Rus et al[65] assessed clinically the impact of different abutments and soft tissue thickness
on the optical properties of lithium disilicate implant single crowns. Twenty patients
were recruited in this study where 17 had thin (≤ 2 mm) and 3 had thick (> 2 mm) soft
tissue thickness. Zirconia cemented to titanium base, pink-anodized titanium, gold-anodized
titanium, and titanium abutments were customized using CAD/CAM technology to replicate
the emergence profile of all abutments. Color change measurements were obtained 1
mm apical to the gingival margin and at the middle third of the crowns and compared
with the contralateral natural tooth. They found that zirconia abutments cemented
to titanium base had the lowest color change values at the measurement areas and the
gingival biotype had insignificant impact on the color change of the peri-implant
soft tissue with zirconia and gold-anodized abutments only.[65]
Although the number of clinical studies assessing the clinical performance of titanium
base abutments is limited, the use of these abutments can be considered a feasible
treatment option. However, long-term clinical studies are recommended.
