Abutment Macro and Micro-design for the Improvement of the Soft Tissue Interface

How do abutment design and surface modification affect the behavior of soft and hard tissue over time? Dr. Mesquida opens this lecture by presenting the biology of tissue integration. Once an incision is opened and an implant and abutment are placed, there is rapid colonization of epithelial tissue, and within days hemidesmosomes appear, then the basal lamina generates an epithelium. Over the course of a few days, granulation tissue forms, and connective tissue appears, which provides protection of the underlying bone from bacterial down growth and infection. This soft tissue barrier is referred to as supracrestal fibers, and also known as biological width. Having an average length of about 3 to 4 mm, it can be separated into two components: the connective tissue area that is right above the bone or the implant platform (about 1.5 mm) and the junctional epithelium (about 2 mm). 

Dr. Mesquida discusses surgical techniques and other relevant factors, such as abutment materials and design, and their favorability for the restoration and the gingival environment, and tissue thickness. The type of abutment implant connection plays a crucial role in tissue integration, as does the relationship between the horizontal diameter of the abutment and the horizontal diameter of the implant, which establishes the platform-switching concept. It results in the stability of soft tissue, a reduced incidence of peri-implantitis, and a better aesthetic outcome. Most implant-abutment connections available today are external hex, internal hex, and flat-to-flat connections, also called tube-in-tube connections. The question is whether these different connections influence the supracrestal fibers that will form. The goal is to have the supracrestal fibers above the implant connection. 

Bone loss at the supracrestal complex is a common issue faced in implantology, caused by bone remodeling, epithelial downgrowth, and its migration to the apical area. Theories regarding the cause include bacterial load, gap size, and micro-movements at the connection. Looking at the evidence, it is clear that the size of the gap, unless clinically unacceptable, is irrelevant. What is relevant is the micromovement between the abutment and the implant. Dr. Mesquida presents clinical videos showing the differences in micromotion between an external hex connection and an internal connection. Both in-vivo and clinical studies show a 1.5 to 2 mm bone loss can be expected with external hex connections during the first year of function.

According to the Albrektsson criteria, an implant with 1 to 1.5 mm of bone loss in the first year of function is still considered a success. But, should this be considered sufficient today? Studies show that bone loss around implants with a conical connection is significantly less. Dr. Mesquida presents a case involving a patient with a failing mandibular incisor; immediate implant placement is not possible because 4-5 mm of the apical bone volume necessary to stabilize the implant is not available. In this case, he decided to extract the tooth, augment the site with a bone graft, and place an implant with an internal connection and platform-switching capability. At the two-year follow-up, despite the modern connection and platform switching, a small degree of bone loss is noted under critical observation. However, clinically, the outcome is satisfactory, and the patient is satisfied with the outcome.

Can we improve upon some degree of undesirable bone loss, however small? Dr. Mesquida recommends looking at new innovations in implant-abutment connections and presents one such innovation, a trioval implant system (N1™, Nobel Biocare). The tight seal of connection restricts any micromovement between parts of the implant system. In discussing this innovation, Dr. Mesquida dispels misconceptions about bone-to-implant contact and primary stability in relation to implant design. Circular implants, in general, tend to have a high initial bone-to-implant contact. When an implant is placed in a round osteotomy, the entire implant is surrounded by or in contact with bone. For the same reason, the peri-implant strains are evenly distributed. Clinical studies show that a high insertion torque will induce osteocytic cell death and apoptosis, thus compromising primary stability. During this time, the implant should not be loaded because it is surrounded by some degree of necrosis.

The new trioval implant system could promote tissue integration and widens eligibilty for immediate implants. At placement of the trioval implant, contact between the implant wall and the bone occurs mainly at the so-called minimal areas, the triangle vertex. At the same time, there is no contact at the so-called maximal areas. Consequently, less pressure is applied, and less osteoclastic activity is induced. As a result, Dr. Mesquida notes, osseointegration should be faster and, presenting an ideal scenario for immediate implant placement. An immediate implant is usually placed on a Type 1 socket, with a requisite minimum of 1 mm of palatal bone and a 2 mm space surrounding the implant at the buccal aspect in order to be able to graft that area. Dr. Mesquida demonstrates the advantage of a trioval implant for this clinical indication. Considering these prerequisites and the literature on buccal lingual dimensions of extraction sockets, he shows 58.1% of central incisors in the anterior premaxillary region cannot be treated with a cylindrical implant of 4 mm because the implant would be too wide. However, the remaining space buccally is larger when the trioval implant is used. This increases the proportion of patients eligible for immediate implant placement in an extraction socket, resulting in more patient treatments. Dr. Mesquida illustrates this with a clinical case, in which a patient with a failing central incisor and complete loss of the buccal palate was treated with an immediate implant using guided surgery and 3D planning to utilize maximum bone available and additional xenograft and tissue graft. 

Turning to the topic of platform switching, the concept was discovered by chance when restorative components in the matched dimension were not available, and abutments with a smaller dimension were used. Less marginal bone loss was observed with the smaller abutments, and so, the effect of platform switching was observed. There are two theories behind the platform switching concept: one relates to the aforementioned micro-movements between the abutment and implant, which result in a disruption to the microfibers that seal the implant. The second theory is the biological theory. This involves a small inflammatory infiltrate that is always observed around two-piece implants. When this inflammatory infiltrate has been displaced or moves inward towards the connection, the distance to the bone increases. A 2010 systematic review addressing platform switching shows that a greater degree of mismatch results in even less bone loss. This mismatch can only be limited as it would otherwise result in mechanical problems. The critical value that the investigators of the 2010 study defined as clinically relevant is that the difference should be at least 0.4 mm.

In terms of surgical and prosthetic techniques, Dr. Mesquida presents the various options, including submerged implants, healing abutment, and immediate provisionalization. Presenting several studies showing that there is no difference in terms of bone loss, he concludes that all surgical techniques work in the same manner. But what about the prosthetic techniques? Dr.Mesquida invites the audience to review the steps performed until a final restoration is placed. In general, an implant has been placed, and a healing abutment has been used to avoid the risk of a re-opening. In the presented case of a second molar, the abutment is removed, and a pink band of connective tissues is observed at the very base of the implant platform, and epithelial tissue or junctional epithelium is observed at the top. By removing the abutment, the seal of the tissues was broken, a new wound opened, and with it, a potential portal of entry for bacterial contamination. As early as 1997, a study by Abrahamsson demonstrated the bone loss induced by abutment removal. Dr. Mesquida illustrates the typical number of disconnections in a clinical case of a failing anterior central incisor. He extracted the tooth, placed an implant, placed a temporary crown, performed a connective tissue graft, and restored the case with a zirconia abutment. Counting the disconnections, he noted 13 disconnections. And the presented case may have been a favorable case, as the patient showed a thick biotype. The clinical implications led Dr. Mesquida to apply the “one abutment one time” concept, in which an abutment is placed at the time of surgery and never removed. Studies show that when the “one abutment one time” concept is applied, an average bone loss of around 0.2 to 0.5 mm is observed.

What impact does abutment material have?  When looking at abutments, four things are critical:

1. Biocompatibility
2. Aesthetics
3. Bacterial adhesion
4. Soft tissue cell affinity

When considering abutments, tissue thickness also comes into play. Modern studies show that at tissue thickness below 2 mm, bone loss of about 1.2 mm is observed, and at tissue thickness above 2 millimeters, bone loss begins to decrease significantly. A concept introduced some years ago is the On1 concept, for which Dr. Mesquida was one of the early researchers. In his ongoing prospective study, Dr. Mesquida and co-authors are following 72 patients with single tooth replacements: 36 implants with On1 base and 36 with implant level restorations. Despite challenges following the patients during COVID restrictions, results revealed a statistically significant difference in favor of the one abutment one time concept, the On1 base restored group.

The next frontier for innovation to improve tissue integration, identified by Dr. Mesquida, is the abutment surface. Even when all the previously discussed concepts of platform switching, one abutment one time, proper implant position, and biocompatibility of all materials are applied, some bone loss occurs. What might be the next opportunity for innovation? For Dr. Mesquida, it's the surface chemistry of the abutment. He proposes incorporating a surface that is favorable for fibroblast attachment, that is bacteriostatic and therefore does not support bacterial colonization. Until now, little literature is available on this topic, especially long-term studies. In his study, Dr. Mesquida found, when comparing machined and anodized abutments (Xeal™, Nobel Biocare), that anodized abutments tended to have less recession. He is now investigating two clinical questions: is there any difference in bone loss and is there a difference in periodontal parameters? In contrast to a study by Hall, Dr. Mesquida observed 15% of machined abutments had bleeding on probing, while 80 % of the anodized abutments had bleeding on probing. Furthermore, using a visual analog score (VAS), patients were asked to report pain during probing on a scale of 0 to 100. Patients with machined abutments answered an average of 14, while patients with anodized abutments consistently felt slightly more pain. Dr. Mesquida asks, what could be the reason for this? He concludes that more bleeding on probing and an increase in pain perception could indicate better mucointegration, i.e. better integration of the abutment into the soft tissue.

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