CAD/CAM Technology in Implant Abutment Design
Achieving repeatable success and longevity in implant restorations
Daniel Alter, MSc, MDT, CDT
With today’s rapidly evolving digital dental technology, creating a consistent and repeatable custom abutment has become easier to achieve. Prior to the onset of computer-aided design and computer-aided manufacturing (CAD/CAM), manufacturing custom abutments required a keen understanding of both physics and the biomechanical functions of implant systems and their restorative protocols. Furthermore, the practice by which the custom abutments were fabricated—using pre-manufactured components in conjunction with manual dexterous processes and a sequence of procedures utilizing the lost wax technique—presented potential inconsistencies and jeopardized the viability of the implant and, ultimately, the dental restoration.
Implant dental restorations have quickly become the leading restorative option, and this trend appears to be growing.1 For years, the implant technician assessed the treatment protocol contingent on specific variables in conjunction with the practicing dentist’s instructions. Once a dental implant treatment had been established and the patient acceptance had been granted, a sequence of events leading to the surgery took place. Today, both implant assessment and oral surgery procedures have become significantly more successful and easier to execute through the utilization of digital technologies and digitally verified guided surgery protocols. Subsequently, the restorative process has become more systematic and unadulterated with the use of digital dentistry.
Traditionally, after placement, a clinical impression was required to capture the implant fixture(s) intraorally, either with a closed- or open-tray technique. In the dental laboratory, these impressions were translated into a master implant model encompassing the implant fixture(s), soft tissue, and surrounding intraoral characteristics. A knowledgeable and skilled implant technician would then use the model to place corresponding pre-manufactured titanium internal components with a plastic burnout sleeve. Reliant on angulation and parallelisms, the technician would then wax onto the sleeve in order to create appropriate patient-specific custom abutment(s). Careful attention was taken not to place wax in the burnout plastic sleeve or on the precision pre-manufactured internal titanium component during waxing. Furthermore, attention had to be given to the physics and biomechanical processes of the implant fixtures to ensure that failure did not occur due to torque or physical contra forces. Once waxing was completed, the abutment was subjected to the lost wax technique, meaning the wax patterns with precision components were invested and placed into a burnout oven. There, the wax—along with the plastic sleeve burnout and the investment ring—was prepared to accept the molten gold that would be forced into the void left by the burnt out wax/plastic sleeve. Unfortunately, this process is prone to issues that could potentially lead to casting failure. These issues include miscasts caused by cold rings, split/broken rings, incomplete castings, bubbles, etc. Assuming that the casting process was a success, the implant(s) were then subjected to a series of manual dexterous and strategic grinding and polishing to achieve the final product that would eventually be delivered to the dentist.
Digital Implant Abutment Design
Digital dentistry, through CAD/CAM, facilitates solutions to the aforementioned potentially negative issues. Furthermore, the digital process makes additional material options a reality for the custom implant patient.
The digital design stage begins with a scan of the intra-oral/master implant model with specific scan flags (Figure 1). These scan flags are manufactured specifically for individual implant manufacturers or systems and provide the fixture’s position in the oral environment (Figure 2). When scanning a model with pink tissue, careful attention must be given to the scan technology. Depending on the scanner’s data capture source, spraying the pink tissue might be necessary. Once the position and angle is established in the software, design can begin. The software will propose its best possible scenario, confirming it with the parameters previously established and taking into account the physics and biomechanical considerations. These are still at the discretion of the implant technician; however, the software will not allow the technician’s design to infringe on the absolute parameters that were previously set. These parameters are measures to ensure long-term success and viability of the implant(s).
Using the design software, the technician needs to annotate the tooth position desired for the abutment(s) and verify the path of insertion (Figure 3). The software progresses intuitively, prompting the technician to the next stage. It will eventually propose an abutment according to the physical characteristics that are currently in the data rendering and provide tools to manipulate for customization (Figure 4). Each and every step or phase is completely controlled by the user, and although the software has basic proposals generated, there are multiple tools by which to alter the shape of the abutment. For example, the 3Shape implant software ( www.3shape.com) offers a snap-to-tissue button (Figure 5). This feature takes the topography of the site and fills it from the fixture to the designer’s accepted line specification (subgingiva, gingiva, or supragingiva). It is important to note that the implant gingiva space should maintain a concavity to promote good blood flow.
The final shape of the custom abutment, including the angulation, height, path of insertion, and anti-rotational features, is ultimately produced by the implant CAD designer. In abutment design, there is a side tool bar in the 3Shape implant module called the “Sculpt” tool (Figure 6). In that tool bar, there are two anti-rotational tools—they are the plane cut and attachment. The plane cut provides a slice along an axis of the abutment, leaving the abutment non-cylindrical, and provides a singular positive seat. An added measure of anti-rotation could be used in the attachment tab. There, the technician would chose the hole (2.0 mm X 2.00 mm) attachment and place it on the abutment along the long axis (Figure 7), making sure not to affect the margin of the abutment. This groove will create a positive lock and anti-rotation mechanism for very short custom abutments (Figure 8).
Abutment Solutions
Through manipulation of an open-sourced CAD/CAM implant module, the material selections are broad for the manufacturing of custom abutments. Four of the most prominent restorative choices are: zirconia bonded to a titanium interphase; full-contour (screw-retained) zirconia bonded to a titanium interphase; complete zirconia abutment; and titanium abutment. The more popular choice is the zirconia abutment bonded to a titanium interphase. A custom abutment is manipulated and designed to the technician’s/laboratory’s specifications and artistic touch, and then is sent to the existing laboratory’s mill or production service and milled out of a zirconia block. The milled custom abutment core portion is then sintered according to the manufacturer’s specifications. Once sintered, the abutment is cemented to a prefabricated titanium interphase purchased from the implant manufacturer in the correct orientation. The reason this is an optimal choice is due to the uncompromised fit between the implant platform and the abutment, coupled with the esthetic value of the zirconia abutment core. An all-zirconia abutment can be milled utilizing the same digital technology without cementing a titanium interphase, but rather milling the interphase. However, this method possesses its problems with regards to inconsistency, potentially inadequately sealed implant junctions, and zirconia failures/breakage.2,3
In order to achieve other implant material selection solutions, the same aforementioned steps are taken with a final destination difference. Once the abutment design is fabricated and completed by the implant designer, an .stl file is generated and sent to the implant manufacturer or milling center. These enterprises have very sophisticated 5-axis, industrial CNC machines that can perform the detailed and exact execution of milling very sensitive surfaces, which is required for the implant interphases. Furthermore, these CNC milling machines are capable of milling out a block of pure titanium, a task that a smaller milling machine cannot perform.
All of the dental laboratory industry’s conventional knowledge and wisdom can now easily be channeled from the traditional manual dexterous fabrication process to the virtual and digital world. This movement yields a better treatment protocol that can be assessed, verified, and manufactured precisely, in conjunction with the manual dexterous skill set and knowledge previously required in order to reach such a sophisticated implant solution. Ultimately, the level of dentistry is elevated, providing the dental practitioner and laboratory with tools to achieve repeatable success and the dental patient with the best dental treatment.
References
1. iData Research Inc., 2009; Figure 4-4: Number of Dental Implant Prosthetic Devices, U.S. 2005-2015.
2. Nguyen HQ, Tan KB, Nicholls JI. Load fatigue performance of implant-ceramic abutment combinations. Int J Oral Maxillofac Implants. 2009;24(4):636-646.
3. Aboushelib MN, Salameh Z. Zirconia implant abutment fracture: clinical case reports and precautions for use. Int J Prosthodont. 2009;22(6):616-619.
About the Author
Daniel Alter, MSc, MDT, CDT
Professor of Restorative Dentistry
New York City College of Technology,
City University of New York
Brooklyn, NY
