The Implant-Abutment Interface: Success Depends On It

Dating back only to the 1950’s when the first dental implant was placed into a human volunteer’s mouth by Swedish orthopedic surgeon Per-Ingvar Brånemark, dental implants are a relatively new innovation in their own right. Despite the industry’s youth, exhaustive, well-documented research has been performed over the course of these six and half decades that have allowed us to gain a deep understanding of the implant system and how to implement it for long-lasting success. A rapid evolution in materials, techniques and treatments used in the implant process has brought us to a point where we are able to achieve high-quality mechanical and esthetic results, as well as successful osseointegration. Countless studies have found that one of the fundamental components that contributes to the overall success of the implant system is the implant-abutment interface. Let’s explore why.

Understanding the implant-abutment interface

The implant-abutment interface is defined as the surface at the connection point of the prosthetic abutment and dental implant. The implant system’s joint stability relies heavily upon the implant-abutment interface and it’s quality of fit, which directly depends on the implant connection type. The three most commonly used implant-abutment connections are the external hex, internal hex, and conical connection, each with varying effects on the interface, which will be examined later on in this article.

The impact of the implant-abutment interface on successful implant therapy

What defines successful implant therapy? Fully- and well-healed soft tissue and bone around the implant define the first aspect of success, which is a result of proper osseointegration. Choosing high quality materials, optimal interface design, and proper connection geometry that inhibits abutment rotation all impact the success of osseointegration. The second and long-term measure of success is the occurrence of continuous beneficial effects on the implant’s surrounding tissue and bone. This includes the formation and preservation of a peri-implant soft tissue barrier, leading to a ‘cold-weld’ phenomenon that hermetically seals the implant site from any potential microleakage.

The risks of poor alignment of the implant-abutment interface

In any implant system, the use of components that are not precisely aligned and fitted can result in damage of the implant-abutment interface. An improper match between the implant and abutment poses a dual-sided problem in the patient’s mouth. Microgaps may form and expose the implant site to microbial leakage, putting the patient at risk of bacterial infection, which can result in short-term or persistent acute inflammation, and/or bone loss. The second issue is that of uncontrolled peak forces that are accompanied by high risk of implant fracture. Once the implant is fractured and needs to be replaced, the trauma that comes with invasive surgery puts the patient at risk for additional health issues, which are exacerbated by undergoing a second healing process (the first being from the original surgery). Surgical trauma of implant placement results in the formation of fibrotic tissue at the implant-abutment complex, which reduces potential vascularization to soft tissue adjacent to the implant, hindering the defense of microbial infiltration by the peri-implant soft tissue barrier.

Implant-abutment connections and their efficacy

External hex connection

The external hex is considered the original implant-abutment interface, invented by Brånemark and used by him in his early trials of dental implantology. It is characterized by a hexagonal connection that protrudes from the base of the implant. The anti-rotational capacity of the external hex made its popularity grow during Brånemark’s time, but its mechanical instability and considerable list of disadvantages drove researchers to seek out alternative designs. The external hex has incidence of screw loosening, rotational misfit, and micromovement — all issues that can arise due to lateral loading as a result of masticatory forces. Micromovements that occur as a result of lateral loading are particularly dangerous, as they can lead to seal failure against the mouth’s microbial environment. The subsequent microgaps formed from excessive micromovements leave the patient susceptible to bacterial infection and jeopardized oral health. The strong advantage that the external hex does have over its successor, the internal hex, is that it is compatible with a wider range of implant systems.

Internal hex connection

The internal hex, the second design in the evolution of implant-abutment connections, differs from the external hex in that the connection sitting inside the implant allows the entire system to be placed at or below bone level. This difference comes with several advantages over the external hex, most notably a significant increase in stability with strengthened joints, lower occurrence of screw loosening, improved microbial seal, and better esthetic outcome. The disadvantage of this system, however, is that the bone, rather than the retaining screw, bears the weight of the lateral forces in this system, which over the course of years of micromovements can cause bone loss and subsequent implant loosening. Furthermore, while the initial stability of the implant system with an internal hex is superior to its predecessor, microgaps can also occur over time in this system, exposing the implant area to microbial leakage and subsequent infection.

Conical connection

The conical connection is the most recent advance in implant-abutment connections, and was in fact stumbled upon by accident due to lack of materials. Rather than using an implant and abutment that are the same diameter, a new line of wide-diameter implants released in 1991 were paired, due to lack of another option, with smaller-diameter abutments. Over time, the radiographic follow-ups observed that these ‘platform-switched’ implants resulted in preservation of crestal bone, due to horizontal inward repositioning of the edge of the implant-abutment interface. The preservation of crestal bone results in a more stable implant and further improved esthetic results, due to the fact that the soft tissue around the implant also benefits from this process, creating a peri-implant soft tissue barrier. This barrier occurs naturally in healthy teeth and serves as a protective barrier against bacterial infiltration, and in prosthetic dentistry, its formation is found to be most successful with conical connections.

Implant-abutment interfaces: Closing thoughts

Screw loosening, implant fracture and poorly healed bone and soft tissue around the implant site are all critical considerations with concern to dental implant success. Ensuring that the implant-abutment interface chosen for each patient is both mechanically and biologically appropriate to minimize the risk of these common causes of failure will improve chances for long-term success.