Abutment Selection for Anterior Implant-Supported Restorations

With the introduction of dental implants to the market, varying restorative options have been successfully added for restoring the function and esthetics of both completely and partially edentulous patients. Accurate prosthodontic rehabilitation is the key factor for providing the long-term success and the survival of osseointegrated implants. Implantsupported restorations can be fabricated with different techniques. The prefabricated abutments provided by the implant companies are accepted as the gold standard because of their biocompatibility and advanced mechanical properties. However, especially for the anterior restorations, they are increasingly being replaced by custom abutments ideally prepared with CAD/CAM techniques; due to disadvantages of prefabricated abutments such as esthetic flaws, mechanical insufficiency resulting from implant placement, unacceptable emergence profile, and unhygienic regions formed under angled abutments. Currently, custom abutments are reported to have functional and esthetic advantages over prefabricated abutments. In this chapter, indications for proper abutment selection, contemporary production techniques, and different abutment materials will be stated, and the current research on the subject will be discussed.


Introduction
Implant-supported restorations have become a popular treatment choice for the rehabilitation of the partially edentulous patients with missing teeth particularly in the anterior region, where esthetics has an irreplaceable importance in treatment success [1]. Accurate prosthodontic rehabilitation is the key factor for providing the long-term success and the survival of osseointegrated implants [2]. Implant-supported restorations can be fabricated with several Differing properties of the abutments have varying advantages and disadvantages. Occlusal forces with different vectors are significantly lower in the anterior teeth than the posterior teeth because of Class III lever system of the human jaw [23,24]. When compared with the incisors, biting forces are almost two times higher for the premolar teeth and three times higher for the molar teeth. Consequently, the clinical results between anterior and posterior abutments should be importantly different. Furthermore, esthetic parameters related to the selection of an anterior abutment may not be applied to the posterior regions. The readers should be aware that studies investigating the clinical outcomes of prosthetic components are classified as anterior and posterior regions due to significantly different complications and survival rates [22].

Implant-abutment connections
The implant-abutment connections can be classified as either external, which protrudes above the implant platform, or internal, which sets down in the access hole inside the implant body (Figure 1). The external hex design was the first to be used in the manufacturing of dental implants. It was originally 0.7 mm in height and was used to help screw the implant fixture into the prepared osteotomy. It was not used as an antirotational device because the rotation of the

Category Options
Method of connection to restoration -Screw-retained abutment-crown complex -Two-piece design with screw-retained crown over the abutment -Two-piece design with cemented crown over the abutment implant-supported restoration was not an issue. However, when the dental implants began to be used in replacing a single-missing tooth, the external hex with some improved quality was used to prevent rotation of the abutment under loading. The external hex connection is still in use as it is suitable for the two-piece implant placement method, has an antirotational mechanism, and is compatible with different implant systems. The external hex also helps the laboratory technician to achieve the best possible emergence profile because the porcelain can be brought closer to the implant-abutment interface. However, it is not without disadvantages, such as low resistance for rotational and lateral movements owing to its high center of rotation. Furthermore, difficulty in seating the abutments in a deep gingival sulcus, increased screw loosening, and component fractures are problems to be considered when the external hex connectors are used [25].
The internal hex connector is widely used. It is a stable connection with a high resistance to lateral forces because of the lower center of rotation and is also suitable for a one-stage implant placement technique. It is also characterized by a good distribution of imposed force. However, weakening of the lateral wall of the implant at the connecting part and compensation for mismatching in the angle between implant fixtures may cause some problems [25].
There is a general consensus that deep internal attachment in which the screw is exposed to little or no load with an intimate contact between mating surfaces will result in good resistance to micromovement. This movement may be associated with crestal bone loss, as mentioned earlier [26]. In order to simplify the technique of placement of the abutments, an audible and tactile "click" feature was incorporated in the internal connection. Thus, the clinician will be able to tell when the abutment is in its intended position on the implant and the need for a radiograph following placement may be reduced. The internal connection when it is long enough may provide lateral stability for the restorative component from off-axis occlusal forces [25].

Types of abutments
Implant-supported restorations may be classified into two types according to the method by which they are attached to the implant: screw-and cement-retained implant restorations. Screwretained implant restorations enable the direct attachment of the restoration to the implant or the abutment, whereas a cementing medium is used for the retention of the restoration onto the abutment for cement-retained implant restorations [25]. Comparison of the advantages and disadvantages of cement-and screw-retained techniques is shown in Table 2 [27].  [25].
An Update of Dental Implantology and Biomaterial

Abutment selection
For the single-tooth implant-supported restorations, an antirotational abutment mechanism is necessary. Currently, the most widely used mechanism is a hexagon with an internal connection. Because of the anatomical limitations related with the anterior single-tooth implant, the prosthetic abutment should be planned with an antirotational mechanism requiring a two-piece system. It may also be necessary to use angled abutment in order to compensate the implant insertion, which is not within the contours of the final restoration. This also forces the dentist to use at least two pieces: the abutment that engages the hexagon or antirotational design and an abutment screw that connects the abutment to the implant body [28]. Abutments are basically categorized into two types according to the fabrication technique: • Prefabricated abutments

Retrievability Restorations can be removed/replaced without damage or the need for a new restoration
It is possible if weak cements were used, i.e., soft provisional cement, otherwise restorations have to be cut in order to remove them

Interocclusal space/ retention
It can also be used when the interocclusal space is limited, i.e., less than 4 mm Minimum interocclusal space with minimum converges is needed to achieve an optimal retention

Limitation of mouth opening
The mouth opening should be enough for the use of different tools required for screwing and torquing the screws A restricted mouth opening is less problematic than with the use of the screw-retained restorations

Occlusal loading
Unlikely to reduce the occlusal load on the restoration and the implant body The use of soft provisional cement may reduce the occlusal load on the restoration and the implant body

Peri-implant inflammation
The adaptation between the restoration and the underlying implant is significantly better than that in the case of cement-retained counterpart The difficulty of removing the cement and the inferiority of margin adaptation between the restoration and the abutment, when this margin is placed subgingivally can cause peri-implant soft tissue inflammation

Esthetics and occlusion
The implant needs to be placed in its optimal angulation in the anterior zone. The screw hole may interfere with the creation of an ideal occlusal morphology as well as with esthetics.
The screw hole could weaken the porcelain veneer Even if the implant angulation is not optimal, the restoration could still have good esthetics The ideal occlusal morphology can be created in the laboratory in the normal way as in the conventional restorations

Cost
The cost in terms of laboratory time and materials is much more than that for the cement-retained restorations The materials and techniques used for the fabrication of the conventional restorations can be used in this situation Table 2. Comparison of cement-and screw-retained restorations [27].

Prefabricated abutments
The prefabricated abutment is prepared by the manufacturer from different materials and provides a connection between the implant and the restoration, with different platform width, length, and gingival output profile. These abutments can be fabricated from titanium or its alloy, titanium with titanium nitride coating, or ceramic (alumina or zirconia) materials. They may also be angled or straight. The preangled abutment has varied angulations provided by manufacturers, usually 15 and 25 degrees off-axis (Figure 2) [29].

Custom abutments
The custom abutments are preferred for the esthetic and function in the anterior restorations.
In the posterior region, it has been reported that the use of custom abutments is more successful than prefabricated abutments because of the small diameter implants due to insufficient bone mass, chewing forces in these regions are high and gingivae are wider [30].
Titanium abutments are considered as a 'gold standard' because of their clinical success rates and improved physical properties [31]. However, titanium abutments have been reported to cause reflection and grayish coloration of the mucosa around the implant [32]. Abutments made from full ceramics provide optimum esthetics when used in conjunction with full ceramic curtain restorations, reducing shading in soft tissue. Yttrium-stabilized tetragonal zirconia polycrystals (Y-TZPs) have begun to be used as abutment materials due to their durability and biocompatibility in implant-supported fixed dentures. Besides the esthetic qualities of zirconia abutments, they have a high corrosion resistance and a low plaque build-up [33,34]. Tan et al. studied the zirconia's biocompatibility of abutments and did not encounter any complications that occurred in the soft tissue. Zirconia reported that less bacterial involvement occurred on the abutment surface than titanium [35].
Zirconia has been reported in abutments in the implanted platform, in the neck region of the connection, and in titanium abrasions and fractures [36][37][38]. It has been reported that the causes of wear and fracture may be due to differences in the hardness values of titanium and zirconia materials. Due to these observable complications, a system is created in which a zirconia abutment is made up of a titanium neck and a neck of the abutment-implant connection, Figure 2. Different abutment types [29].
and this system is called a "hybrid abutment." With this system, the durability of titanium and the esthetic qualities of zirconia have been used together to become an alternative treatment for individual abutments [36,37,39]. With the understanding that the durability properties of hybrid abutments are higher than that of single-piece zirconia abutments, manufacturers have begun to produce titanium abutments in which they use as "ti-base" for use in accordance with existing implant sizes. Thus, the zirconia abutment on the titanium platform can be prepared and used in one piece, combined with the technique proposed by the firm [39].
Many companies that produce custom abutments for individuals can produce zirconia abutments in various color alternatives prepared beforehand by sintering in different color solutions or in different colors of zirconia blocks [40]. In recent years, monolithic zirconia onto ti-base, which is reported to have a high light transmission, has begun to be used as a hybrid abutment [41]. Patient's smile line can be categorized as low, medium, high, and gummy smile. When the smile line of the patient is low, there is no need to use esthetic abutment in no case. When the patient's smile line is from medium to gummy smile, selection of abutment material depends on the thickness of the buccal peri-implant mucosa. The abutment should be ceramic when the thickness of the buccal gingiva is ≤2 mm regardless of the bone thickness. When the buccal mucosa thickness exceeds 2 mm, there will be no esthetic problems due to titanium abutment use. Previous studies reported that 80% of the patients have buccal gingiva thickness less than 1.5 mm in the anterior region that means that zirconia abutment is necessary for most of the anterior restorations [42].
For anterior single-tooth implant-supported restorations, various options of abutments for cement retention are available:

1.
A two-piece abutment for cement with minimal flare from the implant body

2.
A two-piece abutment wider than the implant body

3.
A two-piece anatomical abutment similar in shape to the cross section of a tooth

4.
A plastic castable UCLA-type coping 5. UCLA-type machined/plastic cast to cylinder and abutment screw 6. Ceramic
Manufacturers usually prefer the fabrication of the abutments wider than the implant body.
An abutment with a wider cervical region enables obtaining an emergence profile for the crown, ensures a greater retention area, and provides a greater premade taper of the abutment. The dentist can customize the abutment preparation, condition, and site for each patient. In addition, the wider abutment and chamfer margin facilitate cement retrieval in subgingival margin applications. This is the most popular abutment used for the direct intraoral technique. The accuracy of the machined implant-abutment interface makes it a popular option. However, the wider abutment design has some disadvantages [29]: 1. The wider abutment is wider all around the implant body. When too close to the adjacent tooth/implant, too buccal, or too lingual, the abutment must be prepared further.

2.
The wider abutment creates an undercut where it tapers down to the implant body, with several inherent problems. The crown margin must be placed above the undercut. If the undercut is more than 1 or 2 mm, long-term soft tissue shrinkage is likely to expose a metal band below the crown margin resulting in compromised esthetics.

3.
If the implant was placed below the crestal bone, the restoring dentist cannot set the abutment on the implant platform without an osteoplasty around the implant, unless Stage I healing screw was of the same dimension as the wider abutment (in which case the osteoplasty would have been performed by the surgeon at implant insertion) [29].
The anatomical abutments (custom-made or premade) present similar advantages and disadvantages as the wider abutment posts. One more advantage is that because anterior teeth are wider faciolingually than mesiodistally, the abutment can reflect the natural tooth cross section [29].
An abutment with minimum flare presents several advantages: • One size of abutment may be used for most of the patients.
• The abutment is seated on the implant platform and engages the hexagon without circumferential hard or soft tissue interference, which is beneficial because the abutment-toimplant connection may be several millimeters below the tissue.
• Minimal preparation is required if the implant is not in ideal position.
• The emergence profile of the crown is used to create the gingival contour and may be customized to the specific requirement of each area.
• The margin of the crown may be a knife-edge, chamfer, or shoulder and may be placed anywhere on the abutment.
• A knife-edge margin may be extended or shortened in the laboratory once the tissue model is fabricated.
• The abutment can be used for direct and indirect crown fabrication techniques [29].
The disadvantages of an abutment of similar diameter as the implant crest module include (1) a less tapered abutment, (2) a thinner outer wall of the abutment, (3) less material to prepare when a chamfer or a shoulder margin is preferred, and (4) no clear marking for the laboratory to determine the desired margin location unless a small chamfer is present on the selected abutment [29].

Clinical cases
Case 1: The extraoral picture taken from a patient with an osseointegrated implant on the left upper lateral region can be seen in Figure 3. Temporary screw-retained crown restoration was prepared and checked for approximately 3 months in order to obtain the recontouring of the emergence profile and interproximal papilla (Figure 4). Optimal emergence profile and soft tissue contouring achieved before fabrication of the final restoration can be seen in Figure 5. In Figure 6, the final crown restoration applied onto custom abutment, which was screw-retained 7 months after the implant surgery can be seen with satisfactory recontoured peri-implanter soft tissue and interproximal papilla. When the distance from the contact point of the crowns to the crest of bone is ≤5 mm, there will be no need for periodontal surgery in order to obtain the interproximal dental papilla (Figure 7) [44].

Case 2:
The extraoral picture taken from a patient with an osseointegrated implant on the left upper central incisor region can be seen in Figures 8 and 9. Temporary screw-retained crown restoration was prepared and checked for approximately 3 months in order to obtain the recontouring of the emergence profile and interproximal papilla (Figures 8 and 9). Following   the impression of the implant site, custom abutment design was achieved with the aid of a special CAD/CAM software (Figures 10-12). In Figure 13, custom designed hybrid abutment can be seen following the milling. Following the intraoral check of the custom abutment (Figure 14), the feldspathic layering porcelain was applied directly onto the hybrid abutment ( Figure 15). In Figures 16 and 17, the final crown restoration applied onto the custom abutment, which was screw-retained 7 months after the implant surgery can be seen with satisfactory recontoured peri-implanter soft tissue and interproximal papilla.
Although zirconia-based abutments have lower fracture strength than titanium abutments, they were reported to be appropriate for use in clinical practice because of having two times higher strength than biting forces [45,51]. A recent study reported that fracture strength of custom abutments was higher than prefabricated abutments, which were fabricated with the    same CAD/CAM system [52]. Zembic et al. evaluated titanium-and zirconia-based custom abutments for the anterior single-tooth implant restorations and reported that biological and technical complication rates for both groups were similar at the end of 5 years usage [50]. Vanlıoğlu et al. compared prefabricated titanium and custom hybrid abutments applied onto    narrow-diameter implants and reported a 100% survival rate at the end of 5 years followup [55]. The authors argue that hybrid abutments can be safely used for narrow-diameter implants as well as implants with standard diameters [55]. Prefabricated zirconia-based abutments showed significantly higher fracture strength than the zirconia-based abutments prepared by milling in the laboratory [46]. In-vitro studies showed that for the prefabricated abutments with zirconia neck, fracture occurs mostly at the neck region with varying rates for both the implants with internal and external connections [46]. A study comparing the fracture strength of prefabricated and hybrid abutments applied onto internal and external connected implants revealed that hybrid abutments with internal connection showed significantly higher strength than prefabricated abutments with internal and external connections [58]. Thulasidas et al. compared the fracture strength of angled prefabricated zirconia and custom hybrid abutments and reported that hybrid abutments showed higher strength than angled prefabricated abutments, and both groups showed lower strength values following artificial aging [53]. Similarly, another study evaluating anterior single-tooth restorations revealed that hybrid abutments prepared with 20° angle showed higher strength than straight prefabricated abutments [54]. It was also reported that fractures occurred mostly at implant-abutment interface [54].

Conclusions
Custom abutments designed individually with CAD/CAM systems provide optimal esthetics and function while preventing the technician to make possible mistakes during fabrication. In the future, developments in implant dentistry may enable fabrication of stronger abutments in less time and cost, and this will make it possible for the clinicians to make more satisfactory restorations with higher survival rates. Recent studies evaluating the fracture strength of custom and prefabricated abutments mostly report that custom abutments have several advantages and superiority compared to prefabricated ones. However, in-vitro study results related with custom abutment usage must be definitely supported with various long-term clinical follow-up studies in near future before recommending to the clinicians.

Author details
Pinar