Versatility in Dental Implants and Prostheses

The purpose of this article is to reflect on the influences of the oral biomedical process that affect decisions in treatment protocol selection when choosing between a fixed or removable implant-supported prosthesis. It also highlights custom abutments and substructures and the differences between screw-retained, cementable, and screw-mentable restorations in simple to complex dental implant prosthetics, as well as some of the innovations that may improve outcomes.

Currently, the most common dental implants are endosteal, which closely resemble tooth roots and are osseo-integrated with the bone. Osseo-integration is the human body’s ability for the bone to heal and connect around the titanium implant. Implants are vital in restoring oral function and esthetics for the patient, in addition to preserving the adjacent healthy natural teeth, bone, and soft tissue. However, unlike natural teeth, dental implants have no movement into the bone because the periodontal ligament is lost along with the missing teeth. Therefore, the design of the substructure for the final restoration should compensate for the even distribution of forces exerted during mastication explained by the biomechanical principles. According to Dr. Carl Misch, there are several forces that affect integration of the implants.

“Three clinical moment arms exist in implant dentistry: (1) occlusal height, (2) cantilever length, and (3) occlusal width. Minimization of each of these moment arms is necessary to prevent unretained restorations, fracture of components, crestal bone loss, and…complete implant system failure” (Figure 1 and Figure 2).¹

The application of the principles of dental biomechanics provides long-term integration of the dental implants and the success of the restorative process, which includes fixed or removable prostheses. The selection of the final restoration, fixed, or removable prosthesis depends on the clinical aspects of the patient’s oral function and his or her medical well-being (Figure 3).

For fixed prostheses, the options are cement-retained and screw-retained restorations. In both cases, the design of the abutment is essential in protecting the implant from failure, maintaining the soft tissue surrounding the implant, and ensuring that the restoration meets oral function and esthetic expectations. For cement-retained restorations, there are certain requirements, such as 5-mm abutment height for better retention of final restoration, including 1-mm sub-gingival margin for soft-tissue protection and better esthetics (Figure 4).

There are requirements and advantages in addition to those indicated by Dr. Misch above. “Considerable advantages can be gained with cement-retained restorations, including retrievability, ease of splinting implants, reduced incidence of unretained prostheses, more passive castings, improved correction of non-passive castings, progressive loading, improved direction of loads, improved hygiene of the implant sulcus, enhanced esthetics, improved access, reduced fracture of components, reduced crestal bone loss, reduced porcelain fracture, reduced cost, and less chairtime. As a result, the vast majority of the fixed prostheses are cement-retained in the United States.”¹

Screw-retained restorations—in which the abutment and restoration are integrated into one piece—are used in cases where the inter-occlusal space and the mesial/distal dimension are compromised. Also, these types of prostheses are most often used in full-mouth dental rehabilitation because “screw retention provides the most definitive and rigid splinting when multiple implants are used and therefore enhances implant primary stability.”²

Certain innovations in dental materials and technologies allow for a variety of manufacturing methodologies that lead to better dental rehabilitation. Some of these innovations include:

• Prefabricated titanium bases, which can be cemented to the milled zirconia abutment (Figure 5 and Figure 6). This method allows the dental technician to customize the abutments to precise details using CAD software, such as for emergence profile and contours to promote healthy blood flow, which is important in preserving the soft-tissue.
• Angulated screw access holes, which can be angulated (or an angulated screw channel used) for easier access during the insertion phase and to correct the access hole in an esthetically compromised area. The angulated screw channel allows the insertion of a screw-retained restoration in cases where a labial/facing screw access hole would usually require a cement-retained solution, and in posterior cases where there is limited opening/access.
• Platform switch abutments, though they may present multiple advantages and purposes, including in cases where a larger implant is necessary but the edentulous space is limited. They are also useful in anterior areas where conservation of the crestal bone can help the process of osseo-integration and improve esthetics (Figure 7). Platform switching will also prevent the atrophy of bone from moving away from the implant platform rim.
• Engaging or non-engaging platforms (Figure 8 and Figure 9) that are used for either a strong or a passive fit of the abutments into the implants. “An engaging abutment in a screw-retained fixed cantilevered [prosthesis] provides a mechanical advantage, and engaging the implant furthest from the cantilever when designing a screw-retained cantilever [prosthesis] increased resistance to fracture of the distal abutment screw. Non-engaging abutments are mostly used for multi-unit cases where there is a need for them to have a passive fit and not put too much force on the implants.”³

For removable prostheses, there are two options for retention: bar substructure and attachments (locators). The design of the bar substructure depends on the span of the cantilever and the number and placement of the implants. Here, the principles of the anterior-posterior (AP) spread must be followed (Figure 10). The AP spread is the measurement of the distance between the anterior and posterior implants as they relate to each other. The cantilever portion of the bars must not exceed one-and-a-half times (1.5x) that spread.

Bar substructure design requires (Figure 11) that:

• The bar should have 1-mm to 2-mm clearance from the soft tissue allowing for proper patient hygiene.
• The height of the bar must be considered in conjunction with the overdenture to help restore the patient’s vertical dimension.
• The connections between the bar abutments must be strong and have no sharp edges, which may increase denture fraction.
• The cantilever span should not be too long.

Ignoring the bar design requirements leads to “overload of an implant [which] may result in marginal bone resorption, periodontal bone loss, pressure necrosis, and, finally, failure of osseointegration. Crestal bone loss and early implant failure after loading results most often from excess stress at the implant-bone interface. Also, the design of implant-supported overlay dentures should ensure proper stress distribution to the bone around implants.”⁴

Attachments or locators are used as an alternative to bar substructure for retention (Figure 12). In this case, it is important to select the right abutment attachment, which can allow for even distribution of forces. “However, attachments transmit vertical and/or horizontal load to the supporting implants and consequently cause stress in the bone surrounding the implants.”⁴ A recent study observed that, “the stress in peri-implant bone was lower with the application of ball attachments than that of the bar-supported overlay dentures.”⁴

Attachments are best used in cases where limited inter-arch space is present and they can compensate for inter-implant angulation up to 40 degrees, which can contribute to the restoration’s stability and implant integration. As presented above, the critical importance of an adequate treatment plan is based on applying the principles of oral biomechanics for dental rehabilitation.

In conclusion, advanced dental implants technologies and materials help dental practitioners and dental technicians to successfully plan treatments and fabricate implant-supported prosthetics that effectively satisfy patients’ needs and desires, while maintaining the proper integrity of the oral environment’s function, esthetics, and, ultimately, increasing standards and levels of successful dental care for the patient.

Laura Andreescu, CDT, BS, MBA, is a lecturer at New York City College of Technology and a technician at a private dental practice.