Inside Dental Technology
CAM-Milled Implant Bars
The keys to success involve careful case consideration combined with collaborative prosthetic planning
The success of an implant prosthesis is determined by the collaborative efforts of the dental implant team, which typically includes the surgeon, restorative dentist, and dental technologist. All members of this prosthodontic clinical and technical team must understand the responsibilities, needs, and desires of other team members if they are to provide optimal prosthetic care for a compromised edentulous patient.
The literature is replete with discussions about the independent roles and responsibilities of clinicians and technicians. However, one constant remains. With implant prosthetic dentistry, a symbiotic relationship must exist among these implant team members in order to understand all the variables involved for completing a complex milled-bar restoration.
An implant company and its local sales representative will play a critical role for this team as well as the facility that fabricates the milled bars. The implant sales representative can assist with product handling information, tools, parts, and education. The milling facility or its representative will help with design protocols for various milled bar options. It is critical to success that your laboratory should collaborate with the milling center and take ownership of the virtual design.
Due to the increasing demand for esthetic and functional complete prosthetics, implant-retained and -supported overdentures have rapidly become an integral part of most removable prosthetic dental laboratories’ product lines. As a result, laboratories are confronted with an increasing demand for implant prosthetics amidst a decreasing supply of dental technicians. With these challenges, laboratories must learn how to effectively and efficiently design implant prosthetics for compromised edentulous situations. Dental technicians who have knowledge and experience in removable prosthetics have adapted the best in understanding the complex variables of implant overdentures using CAD-milled bars. Utilizing CAD/CAM digital technology in implant prosthetic dentistry enables dental laboratories to work more efficiently with predictable outcomes in treatment. Digital communication and case planning have become keys to collaborative success for the dental implant team.
Understanding prosthetic needs, desires, and expectations is critical to achieving a successful level of satisfaction for an edentulous patient seeking implant treatment. Through the use of digital photos and mounted casts, we can visualize the patient’s oral condition and evaluate existing restorative space required for an implant overdenture milled bar. Utilizing implant-planning software with cone-beam computed tomography (CBCT) scans helps the prosthetic implant team evaluate potential placement of the implants and need for bone reduction to achieve the desired restorative space.
CBCT data can be evaluated with surgical planning software to place the dental implant in the ideal prosthetic and anatomic position.
The required restorative space is dependent on prosthetic needs and desires. To determine the restorative space needed, we must understand which milled bar design will be our objective during case planning. If an edentulous patient desires an implant prosthesis that is screw retained and can be removed only by the surgeon or restorative dentist, then a hybrid or profile-type milled bar would be indicated. If the patient wants a removable implant prosthesis that is stable, retentive, and removable from the milled bar for hygiene purposes, then a fixed-removable or detachable overdenture would be indicated. This type of overdenture prosthesis uses a milled bar for support, stability, and attachments for retention.
Restorative space must be evaluated using CBCT scans, radiographs, and mounted study models. The dental implant team should communicate and collaborate on defining the existing inter-residual or occlusal spaces compared with the required space for implant restoration. The options are to increase vertical dimension of occlusion/inter-residual arch relationship or to decrease bone with alveoplasty or a combination of both options. I have found, for example, that a 2-mm increase of vertical dimension of occlusion combined with a 4-mm to 6-mm bone reduction is common in case planning. Once the dental implant team has agreed on the type of restoration, number of implants, placement, required restorative space, and esthetic expectations, a diagnostic tooth arrangement can be set up on mounted study casts after model surgery of tooth extraction and/or bone reduction. This diagnostic setup will be the guide for making a surgical implant and bone-reduction guide as well as the provisional prosthesis that may be converted for immediate loading after surgery.
Milled bars have various names and are available in several designs depending on the manufacture or milling facility. Dental laboratory technicians specializing in removable and implant prosthetics should understand the basic design requirements for each type of overdenture milled bar. Every software design tool has specific names for bar types, so knowing the design names and restrictions according to the milling facility is important. For example, Biomet 3i, Dentsply’s ISUS, Nobel Biocare, Panthera, Preat, Straumann, and Zimmer use different terminologies with their respective design software products. These companies also have various milling capacities and capabilities. The parameters for vertical and horizontal dimensions of milled-bar design as well as space requirements superior to the bar in relation to the plane of occlusion and inferior in relation to the residual ridge must be understood. The advantages and disadvantages of a metal intaglio compared with a wraparound-style milled bar must be taken into consideration as a design option.
Understanding proper placement of external finish lines in milled bars is critical for preventing leakage of oral fluids between the metal and acrylic interface. Newer software incorporates vertical struts that help to retain denture teeth on the bar and prevent tooth breakage during mastication or parafunctional movements.
Today we have materials options for milled bars such as titanium, chromium-cobalt, and zirconia. Chrome-cobalt nonprecious bars are becoming more popular due to recent studies and their ability to be easily soldered or welded.
It has been said that a patient’s satisfaction is based on the appearance and function of the prosthesis. A patient truly understands a pleasing or more youthful smile, the self-confidence to laugh or sing, and the ability to enjoy a good meal.
Implant restorations in edentulous patients require excellent collaboration and communication between the patient, restorative dentist, surgeon, and dental technologist. The definitive goal and sequential clinical and technical steps needed to achieve the best possible outcome should be visualized and then planned before treatment is even begun to ensure optimal biomechanics, esthetics, and function.
Once a prosthetic plan is developed and accepted implants are placed, a master impression is taken after integration. The implant master casts are created from impression, a verification jig and a screw-retained baseplate are made to record maxillomandibular relationships. An option at this time is to take a second master impression picking up the verification jig in an open-tray technique. This secondary impression technique ensures accuracy and prevents movement of impression copings in vinyl polysiloxane material. If a pickup impression with verification jig is performed, then a screw-retained baseplate for records with either wax occlusal rims or a central bearing device would be next. After the records are transferred and master casts are mounted to a dental articulator, a tooth arrangement is made for an esthetic and functional wax try-in. Once the try-in is approved, the master casts are scanned using an optical scanner, and the design of the milled bar is performed virtually on the computer with 3D design software, thereby reducing working time and effort.
In prosthetic situations in which mucosal-supported complete dentures are to be replaced by an implant-supported milled bar prosthesis, numerous variables regarding the smile design, such as pink and white esthetics, phonetics, shape, transition line, color of teeth, and functional occlusion, are of extreme importance to successfully meeting a patient’s desires and expectations. Patients are paying premium prices for these complex implant restorations that replace entire arches of teeth. As dental prosthetic professionals, it is our moral obligation to use the best possible materials and techniques that will enable us to achieve excellent esthetic and functional results for these compromised edentulous patients.
Many implant brands are on the market today. Some manufacturing facilities will mill bars for only their implants, while others can mill bars for competitors, as well. Third-party milling centers can usually service many implant brands while having different mechanisms for a laboratory to interact with provider and milling facility. These may range from selling scanners to laboratories, which gives the laboratory total control of all aspects of the design process, to basic review of a provider’s generated 3D PDF.
A true advantage of utilizing CAD-milled implant bars is the significant reduction in laboratory time and the degree of complexity when compared with using more traditional approaches that involve the fabrication of the bar patterns with acrylic resin, investment, and casting of dental alloys. More dental laboratories are becoming equipped with CAD/CAM technology and are performing an increasing number of digital dental procedures every year. This type of prosthetic knowledge and experience will become part of every dental technician’s daily routine. Careful case consideration combined with the dental implant team’s collaborative prosthetic planning are the keys to success with the use of CAD-milled implant bars.
About the author
Robert Kreyer, CDT, is a partner of Custom Dental Prosthetics Inc. in Cupertino, CA.