Gregori M. Kurtzman, DDS, MAGD, MIPS/ICOI, FPFA, DICOI and Allen L. Schneider, DDS, FAGD, DICOI
ABUTMENT FORCES
Many forces can be exerted on the abutment and crown that can affect long term success of the prosthetics. Rotational forces are well known to affect single units. The intimate mating of the abutment head to the implants platform greatly influences this. The less intimate the fit, the more play between the components and the greater the micro motion that will lead to either screw loosening or screw breakage. Thus, a component that has a milled platform interface will be a more accurate fit than a component that has been cast.
Lateral forces can be defined as any force that is exerted on the prosthetic components that is not along the long axis of the implant. In the posterior occlusal forces are easily placed along the long axis as the implant is placed in an almost vertical direction. Unfortunately, in the maxillary anterior, the implant can not be placed in a vertical orientation due to the anatomy of the premaxilla. If we observe natural maxillary anterior teeth we see that the incisal edge is inline with the apical tip. This directs the forces on the tooth along the long axis, which maximizes the load bearing ability of the tooth and surrounding bone. What we see in natural teeth is that bone overlaying the buccal aspect of the maxillary anterior teeth is very thin and dehisences and apical perforations through the bony plate are common. Natural teeth being supported by the ligamentary apparatus are able to distribute the forces to the bone and these naturally occurring boney defects do not seem to affect the stability of the tooth. Implants require bone to surround the entire body of the implant and an apical bony perforation may affect its long term stability.
Replacement of a single tooth in the anterior region with an implant supported restoration is a difficult treatment option due to numerous functional and biological requirements. As resorption occurs bone is lost on the buccal of the premaxilla and there is a shift toward the palate. The result is the placement of the implant with a rotation toward the horizontal plane. If the restoration were kept so that the incisal edge was on the long axis the result would be a flared appearance. To achieve a natural look, the incisal edge needs to be placed more vertically, increasing the angle from the long axis.3-7 This increases lateral forces on the components and can lead to screw loosening, screw breakage and abutment or implant fracture. Bone grafting can be utilized to replace missing bone but vertical placement of the anterior maxillary implants can not be achieved without severe compromise to the esthetics and general appearance. The solution is to maximize the strength of the implant restorative components to eliminate material fatigue and increase fit of the components to prevent loosening. This is especially important as the angulation increases from the long axis.
ABUTMENT TYPES
Single implants that will receive a cemented crown, require an abutment head to interface between the implant platform and the crown. Ideally, the abutment head should resemble a prepared tooth with good form, morphology and emergence profile. Proper implant positioning and appropriate preparation of hard and soft tissue are critical to creating optimal emergence profile, function, esthetics and periodontal health. Several types are available and have been in use and can be separated into three categories; stock, UCLA and computer milled.8-11
Stock (prefabricated) abutments heads may be milled either at the laboratory or in the office to customize them for the particular case. These are available either as straight or pre-angled. The limitations to stock abutments are; major modifications may not be possible due to weakening of the abutment structure, angles supplied may be either too great or not great enough, angle compared to hex orientation may not align ideally, and emergence profile may not be wide enough for a natural look.12,13
UCLA (laboratory wax and cast) abutments are available in two forms; all plastic and plastic sleeve with gold platform. Due to the high tempertures required to cast titanium, gold alloys have been used when casting UCLA abutments. The disadvantage of the all plastic UCLA type abutments is the platform mating portion is cast and will not provide an accurate fit as compared to a milled metal component fit. This is a critical feature in single units to prevent micro motion and screw loosening. With splinted multi-units this does not place a key factor. UCLA abutments also are provided with a machined gold platform component and a castable sleeve. The sleeve is wax and cast, locking to grooves in the machined gold portion locking the two pieces together. A metal of a lower melting point must be used to avoid changes in the milled component. Therefore, the bond between the cast portion and the milled portion is mechanical. As the angulation of the abutment head increases from the long axis of the implant these forces increase greatly. In high load situations fracture of the abutment at the interface of these two pieces has been reported.14
Computer milled solid abutments are the next evolution to abutment fabrication. A solid block of titanium is milled using a computerized milling machine to the operators specifications. The benefit is ideal shape can be achieved without compromise to strength within the abutment head. The finished abutment is a homogonous metal with no interfaces within that may lead to stress-induced fracture and failure. Unlike the all plastic UCLA, the platform mating surface is also machined giving a very accurate intimate fit with no significant “machinest slop.”
ATLANTIS MILLED ABUTMENTS
A transfer coping is attached to the implants and an index is fabricated orienting the transfer copings to the adjacent teeth. This is sent to the lab along with full arch impressions.
The laboratory incorporates the implant analogs into the master cast using the transfer coping and makes measurements directly on the cast. The measurements are made between the implant analog and adjacent anatomic landmarks. This determines what degree of emergence profile is needed, the length and shape of the abutments and the margins. The data is inputted into software that uses a Computer Aided Design and Computer Aided Machine (CAD/CAM) to customize the ideal abutment shape. The image generated can be modified according to instructions sent with the case.
The file is then transferred to a computer controlled precision milling machine and the abutment, including the platform interface between the analog and abutment is milled from a solid block of titanium. The milled abutments are then returned to the treatment casts to verify proper shape, contour and occlusal clearance. A duplicate matching abutment can be ordered at this time with or without a fitted temporary crown. The precision of the CAD/CAM milling unit ensures that If no modifications are made to the matching abutment heads, the permanent crown can be seated on the currently installed milled abutment. When modification of the abutment is necessary to place the margin further subgingival it will be necessary to utilize the milled abutment head that the permanent crown was fabricated on as the final abutment head.
The case is then returned to the restoring dentist for completion of the prosthetics following try-in and verification of fit.
CASE REPORT
A thirty six year old male presented following a sports related traumatic injury to the upper right cuspid. The cuspid had been knocked out the prior day with a portion of the buccal plate of bone. A 5 mm x 14mm RBM threaded implant (Lifecore Biomedical, Chaska, MN) was placed with an osseous graft to replace the lost bony plate.
Following healing, the implant was exposed at second surgery and a healing abutment placed. A week after the healing abutment was placed the patient returned to have impressions. A full arch impression was taken of both the maxillary and mandibular
arches. The healing abutment was removed and a transfer coping (Atlantis Components, Inc., Cambridge, MA) placed onto the implant. A polyvinyl bite registration material was injected around the transfer coping and an open tray impression was taken of the full arch. This was performed to capture the soft tissue and allow a soft tissue model to be fabricated. The open tray also increases the stability of the transfer coping and increases the accuracy of the model. Upon setting the transfer coping screw was removed, the impression tray was removed and sent to Atlantis for abutment fabrication. The resulting soft tissue model will also be used to fabricate a custom lab processed temp on the milled abutment head.15
The patient returned to the office and the healing abutment was removed. Two matching milled abutments were ordered. This would allow the patient to wear a fixed temporary crown while the permanent crown was being fabricated and allow the laboratory to fabricate the permanent crown on the milled abutment yielding a more precise fit. Both milled abutments were tried in to verify the fit and shape. The margin placement was checked and if necessary it can be modified intraorally. The prefabricated temporary crown was cemented over the second milled abutment intraorally. The first milled abutment was then sent to a laboratory on the treatment cast to have a porcelain fused to metal crown fabricated.
As the margins of the milled abutment heads were modified slightly, the two milled abutment heads did not match exactly any longer. When the permanent crown was returned, the milled abutment head was removed from the mouth and the other milled abutment head placed. Seating was verified with a radiograph and the crown tried in. This was followed by cementation of the permanent crown.
CONCLUSION
The Atlantis abutment offers both strength and precision to achieve a simplified natural prosthetic result. Solid milled abutments can offer stronger customized abutment heads than cast UCLA type. This can be critical as the angle increases from the long axis and more forces are brought to bear on the platform area of the implant assembly. With CAD/CAM technology, ideal abutment shapes can be achieved simplifying the prosthetics portion of treatment.
Indexing with the transfer coping can be accomplished at either the surgical placement of the implants (1st stage) or at second stage uncovering. When performed at second stage surgery less modification of the milled abutment heads may be necessary as the soft tissue has stabilized in position. OH
Dr. Kurtzman is in private practice in Silver Spring, MD and is an Assistant Clinical Professor at the University of Maryland School of Dentistry, Department of Restorative Dentistry.
Dr. Schneider is in private practice in Springfield, VA, and is a consultant in prosthetics at the Veterans Administration at Martinsburg, WV.
Oral Health welcomes this original article.
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ABSTRACT
Traditionally, implants restored with single tooth restorations have been complicated due to the difficulty in achieving ideal emergence profile and crown form. Utilizing CAD/CAM technology the Atlantis milled abutment overcomes these difficulties to develop a customized milled abutment with ideal emergence profile and shape to simplify crown fabrication.
