The Advantages of Customized Abutments
The single-tooth implant requires an antirotational abutment feature. Interlocking features to stabilize the implant prosthetic abutment connection have been designed along with a better understanding and application of biomechanical parameters such as torque, preload, clamping, thread mechanics, micromovement, and settling.
Presently, a hexagon with an internal connection remains the most widely used. As a result of the specific anatomical limitations of the single-tooth implant, the prosthetic abutment not only must be designed with antirotational features (which requires a two-piece system) but also may need to be angled to compensate for implant body insertions that are not within the contours of the final restoration. This also requires 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.
Two abutment options are available to those undergoing dental implantation. Prefabricated abutments are pre-manufactured abutments mass-produced to fit an existing implant system. They are the most economical and are typically used for cement retained restorations. They can be both straight and angled. Although such pre-fabricated abutments fit well into the implant, they rarely offer proper shape and form. The results are often over-contoured or under-contoured abutments that do not shape the gum tissue properly and create a very unnatural appearance of the crown.
Abutments may also be custom in design. Three general choices of abutments for cement retention for custom abutments are available such as a plastic castable type coping, a machined coping with a plastic cylinder and a CAD-CAM customized abutment of titanium (alloy) or ceramic.
An abutment that is the same diameter (or more narrow) than the implant crest module has several advantages:
- One size of abutment may be used for almost all 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 to implant connection may be several millimeters below the tissue.
- Minimal preparation is required if the implant is not in ideal position (i.e., too close to a tooth or facial position).
- The emergence profile of the crown is used to create the gingival contour and may be customized to the specific requirement of each patient condition.
- The margin of the crown may be a knife edge and may be placed anywhere on the abutment.
- The abutment can be used for direct and indirect crown fabrication techniques.
- In the indirect (laboratory-assisted) technique, a knife-edge margin may be extended or shortened in the laboratory after the tissue model is fabricated.
- The soft tissue below the crown margin is thicker and less likely to recede.
- The soft tissue is thicker, so a grayish line below the crown is less observed.
The abutment that is similar in diameter or more narrow as in platform switching to the implant body also has disadvantages:
- It is at considerable disadvantage to modify the soft tissue drape. The subgingival contour is only developed by the crown, which is 1.5 mm or less below the FGM. This dimension may be inadequate to fully modify the soft tissue drape. Hence, this abutment design is limited to situations in which the soft tissues are more ideal or minimum space is a variable between the implant and the adjacent tooth.
- The abutment is less tapered because it is narrower at the base. Hence, it is more difficult to modify the path of insertion of the crown.
- There is a thinner outer wall of the abutment, so less material is present to prepare a taper or a margin when a chamfer or shoulder margin is preferred.
- There is no clear marking for the laboratory to determine the desired crown margin location unless a small chamfer is present or prepared on the abutment selected.
An abutment flare 1 to 2 mm wider than the implant body is the most common type provided by manufacturers and is the most popular abutment used for the direct intraoral technique. It is provided in a straight and angled configuration. An abutment with a 1- to 2-mm “flare” positioned 1 to 2 mm above the implant platform presents several advantages. The wider cervical region improves the emergence profile of the soft tissue starting 1 to 2 mm above the bone. The wider abutment also provides a greater surface area for retention and provides a greater premade taper of the abutment. Because the implant is often positioned 1.5 mm more palatal than the emergence of the adjacent teeth, the 1- to 2-mm flare begins the process of developing the ideal emergence profile of the crown 1 to 2 mm above the bone. The dentist can modify the abutment preparation for each patient, condition, and site. Unlike a custom abutment made with a plastic component, the accuracy of fit of the premade implant–abutment interface decreases the force to the abutment screw and reduces the risk of abutment screw loosening. It is a less expensive abutment than one customized and may even be provided at no charge by the manufacturer when the implant body is purchased.
The wider abutment design also has disadvantages:
- The wider abutment is wider all around the implant body. When too close to the adjacent tooth or implant, too facial, or too lingual, the abutment must be modified
- The wider abutment creates a flare with an undercut where it tapers down to the implant body, with several inherent problems. The crown margin must be placed at or above the undercut. Too often, the dentist perceives the abutment flare as a crown margin, which may be several millimeters below the tissue. This makes it most difficult to capture the “flare” in an impression, difficult to seat the crown, and difficult to remove the excess cement. The crown margin should be related to the FGM and should only be 1 mm ± 0.5 mm subgingivally for esthetics, regardless of where the abutment flare is located.
- If the implant was placed below the crestal bone, the restoring dentist cannot seat the abutment on the implant platform without an osteoplasty around the implant. If the stage II PME is of the same dimension as the wider abutment, the osteoplasty would be performed by the surgeon at implant insertion.
- It is more difficult to seat the abutment below the tissue margin because it must push the tissue away from the narrower implant body.
The ceramic premade abutments are usually white in color. Ceramic premade abutments have become popular to prevent metal below the crown margin from causing a grayish hue to be seen through the cervical tissues when there is a thin biotype. The white ceramic abutment below the tissue makes the tissues appear more coral pink in color.
Two types of ceramic abutments are generally found by manufacturers, a milled all-ceramic abutment or a ceramic abutment attached to a milled titanium component. The one-piece milled abutment does not have a metal to ceramic connection and hence is stronger (and usually less expensive). However, the material is harder than the implant body connection; it is more difficult to control the variance of size; and any misfit will wear the antirotation feature, which will increase the risk of screw loosening. The machined coping is more accurate to engage the antirotational component of the implant body and is made of a similar material hardness, which decreases the risk of abutment screw loosening. However, the cemented interface of the metal coping to ceramic abutment may be an intermediate to long-term complication not yet appreciated.
The pre-manufactured angled abutment may be made of titanium or its alloy or of ceramics. The weaknesses of prefabricated angled abutment designs have been discussed previously, and their use should be limited to situations in which a facial or angled implant position precludes restoration without correction of the angulations. When the implant body is not favorably positioned, the angled abutment may be the only rational solution to restore the patient. The angled abutment is most efficient when the flat of the antirotational design within the implant body is positioned to the midfacial position. Otherwise, the angle of the abutment will be rotated from the center, making it more difficult to ideally prepare and restore.
A trend toward laboratory-customized anatomical or esthetic abutments has emerged. In the past, a custom-made abutment usually was fabricated from a plastic castable pattern or machined cylinder and plastic castable coping. This abutment type was developed originally by the University of California, Los Angeles (hence UCLA abutment). The laboratory waxes and casts metal for the customized abutment design to the plastic or metal sleeve.
The primary advantage of the custom abutment is that the abutment is fabricated for each specific patient condition. The subgingival crown margin position and contours can be extended only where necessary. Another major advantage of the customized abutment is that the abutment may be fabricated with crown and bridge precious metal, so the facial region may be covered with porcelain (the color of the tooth, the root, or the soft tissue) and may be extended close to the implant abutment connection for improved facial esthetics. The facial crown margin on this abutment is usually a porcelain butt joint 1 mm below the tissue that is fabricated and developed in the laboratory. In this way, if tissue shrinkage occurs in the long term, a metal margin of the abutment is not seen. This is most beneficial when placing a single-tooth crown in a young patient to account for a potential remodeling of the gingival profile over time.
The disadvantages of the custom-made abutment are related primarily to the laboratory phase. With the castable plastic pattern, the abutment can be fabricated out of any precious metal (to decrease corrosion risk between the coping and cast metal) without a two-metal interface. The plastic pattern abutment also is much less expensive. However, the abutment to implant connection is less precise. A diamond lapping tool is suggested to improve the flat-to-flat finish of the abutment to implant connection, but overzealous finishing may create a gap. In addition, the internal platform for the abutment screw cannot be cast accurately. Hence, the shoulder of the abutment screw does not seat precisely, which causes an increased risk of abutment screw loosening and associated complications. Therefore, the plastic coping is not suggested whenever a customized situation is used.
UCLA abutments with a machined metal coping within a castable custom abutment have a high degree of fit at the implant–abutment connection with the added advantage of decreased screw loosening. Therefore, although the cost may be greater, the machined abutment metal coping is suggested.
The CAD-CAM abutment may be customized and fabricated from almost any material and to almost any angulations. An indirect laboratory technique is necessary. The dentist makes an implant body impression (using a closed- or open-tray technique) that uses an impression transfer that engages the antirotational feature of the implant body. After the abutment is designed in the laboratory with the computer, it is milled or fabricated by digital technology. As digital technology increases in the fabrication of implant prostheses, this restorative approach will become more popular.
In conclusion, the restoring dentist has many abutment options to restore an anterior single tooth. Most often when the implant body was positioned correctly, a direct approach is used with a prefabricated abutment, slightly wider than the implant body. It may be easily modified when too flared to the facial or too close to an adjacent tooth. When the implant is positioned correctly, the 2- to 3-mm vertical depth of tissue on the facial most often hides the color of the metal abutment below the crown. When the prefabricated titanium alloy abutment has a gold-color titanium nitride coating, it is an esthetic subgingival benefit.
When the implant was positioned too coronal and the depth of tissue is minimal or thin tissue biotype exists, a ceramic abutment or customized abutment with cervical ceramics is of benefit. These abutments permit the subgingival areas below the crown margin to be more favorable for the cervical esthetics of the restorations. When the interdental papilla is flat and a prosthetic method is desired to elevate the papilla height, a custom abutment also has many benefits.