Demystifying emergence profile: looking at implant success from the right angle!

Feb 12 / Pro. Nikos Mattheos,Dr. Martin Janda
Designing the optimal emergence profile is among the “hottest” areas in Implant Dentistry, with much ongoing research and debate. Although some studies have connected the contour of the abutment / prosthesis with inflammation and peri-implantitis, little is understood about where to look and what to avoid when designing an implant prosthesis. A recent clinical study from the team of Drs Mattheos and Janda utilising 3D imaging technology comes to shed light to this topic, showing how specific angles of the prosthesis can increase x30 the risk for inflammation!

Dental Diaries talked to the authors and brought today some important messages for your Implant practice and guidelines for the next time you design an implant crown!
Can the prosthetic design of an implant prosthesis influence the long-term health of peri-implant tissue? Yes, it can, and this is some fundamental clinical knowledge we have long had from prosthodontics on teeth. Implant Dentistry however has been initially so focused on osseointegration, that it took several decades to awaken to the importance of the prosthesis-tissue interface and how the design of the transmucosal complex can influence both aesthetics and long-term tissue health. This was our main drive to introduce the concept of the Implant Supracrustal Complex some years ago, aiming to study the close interrelation of tissue and components in what we often call as “emergence profile” (1).

The first evidence pointing to this direction came with 3 cross-sectional studies (2-4) which suggested that prosthesis contour of more than 30o as it appears in peri-apical radiographs, is correlated with increased risk for peri-implantitis in bone level implants. Furthermore, convexity of the prosthesis profile has been correlated with increased recession (5), marginal bone loss (6) and peri-implantitis when combined with over contouring of the prosthesis (3). This was the first time that there was an attempt to “quantify” elements of the prosthetic design at implants relate them with pathology. Although these results pointed to a certain direction, it was difficult to extract some concrete suggestions as to what the optimal design should be.

Why was it difficult to come up with specific design guidelines?

Although a tiny anatomical area, the transmucosal part of the implant-abutment-prosthesis is a highly complex structure. Trying to approach this complex 3-dimensional structure solely by means of the 2-dimensional interproximal projection on periapical radiographs is insufficient. Utilising a periapical radiograph to describe the 360o profile of an implant crown, is like taking a screenshot to describe a movie: it only shows one out of the many very different aspects. Furthermore, a major limitation was that the peri-implant soft tissue is not visible in periapical radiographs, thus there was no possibility to relate the crown contour to the actual position and shape of the healthy or inflamed mucosa. The mesial and distal contour was referred to as “Emergence Angle”, but in reality, there can be no “emergence” without the soft tissue which the crown is emerging from. Emergence Profile (EP) and Angle (EA) based on the Glossary of Prosthodontic Terms 9th Edition, can only be defined in relation to the “circumscribed soft tissues”, which cannot be accounted for in periapical radiographs. Finally, one of the difficulties with previous studies was that the angles were calculated using “tangent” lines. Geometry defines “tangent” as the line which can be drawn perpendicular to the diameter of a perfect circle, but in the complex and irregular curves of the implant crown a precise tangent is next to impossible to define precisely, seriously limiting the reproducibility of measurements (Fig 1). So, although these early studies pointed towards a serious interrelation between prosthetic contour and tissue health, the exact features that affect this relation could not be defined. Also, the threshold of 30o is more of a statistical cut-off point that describes the specific sample, rather than a biological fact. We should note that 2
lesser-known cross sectional studies which followed similar methodology did not confirm the correlation of >30o contour and peri-implantitis (7,8).

What new does your recent study bring?

We have thought a lot on the above limitations, and we have discussed extensively how we could come up with a methodology that can overcome much of these boundaries.

The first thing we decided was to move away from the periapical radiographs. Thus, we utilised CBCT matched with intraoral scans, where we could see in detail the full contour of the components in relation to the position, shape, and size of the soft tissue. We then selected six representative sites for each crown 45o apart for our measurements, thus we can see the contour from different angles (Fig 2). We excluded the directly mesiodistal view, because that is the site under the contact point of the crown, the only site where probing is not possible. It is a paradox that in research we often measure mesial-distal marginal bone loss in peri-apical radiographs and then correlate it with mesial-distal probing depth, when in 3D reality the site of probing and the site of bone loss are far apart.

The second important decision was to move away from the “tangent” lines. Instead, we used a methodology from Cephalometry, by defining specific and reproducible points and planes, such as the most coronal point of contact between the soft tissue and the crown (true emergence), or the most apical point of contact between the abutment surface and the implant (Fig 3). The methodology was rather complex, we would not get into all details to prevent your readers from getting bored. However, by following a pedantic process we increased precision and reproducibility to the level that two independent observers would come to the same result more than 95% of the times and when not the deviation was minimal.
Finally, we moved away from one single line to define “contour”. Instead, we defined two different angles: The Mucosal Emergence Angle (MEA), the point where the true emergence through the soft tissue takes place and the Deep Angle (DA), at the apical part of the implant supracrustal complex, where the abutment ascents from the implant platform and the bone level. We also measured the total contour angle (TA) as previously done in periapical radiographs as a control.


Furthermore, we used a patient sample as homogenous as possible: we only included posterior single crowns at bone level implants, and we excluded cases where the prosthesis axis was more than 10o divergent from implant axis. We also excluded all probing depth measurements in sites where the crown contour preventing proper insertion of the probe. This is a very important point, as we all know in clinical practice that it is very difficult to probe over-contoured implants without underestimating the probing depth (Fig 4). We ended up excluding about 4% of the measurements, but we have increased this way the validity of the results, by securing that the analysis did not include mistaken or underestimated probing depths. We also used some sophisticated statistics, accounting for three analysis levels: site, implant and patient.

What was the main result of the study?

The first important finding was - maybe not surprisingly - a very clear and strong correlation of the Mucosal Emergence Angle with inflammation. The risk for inflammation increased in proportion to the widening of the MEA, somewhat slower up to angles of 40o but rising steeper with angles of 50o, 60o, and 70o. As an indication, sites with mucosal angle of ≥ 70o had 33 times higher risk for bleeding. When all 6 sites of an implant prostheses had MEA ≥ 40o, the risk of having bleeding at all 6 sites was
9.5 times higher. At the same time, no such association was found for the Deep Angle. There was a strong message there: when it comes to inflammation, it’s the emergence that matters, not the entire contour. Thinking of how inflammation starts as a reaction to the biofilm, it makes absolute sense. The deep angle on the other hand is at the bottom of the Implant supracrustal complex and was not associated with inflamma- tion. This is not surprising either, this area is far from the reach of biofilm at least under healthy conditions. It could be an angle that relates with the early marginal bone loss, or “remodelling” in the short period after the implant placement. But this is something for another discussion.
So, what was the main clinical message from your study?

The message is simple, as much as it is powerful: Inflammation starts from the top of the soft tissue. The relation of the implant crown with the soft tissue margin is of strategic importance, as this is the very point of initial plaque accumulation. A narrow angle of the crown at this point will facilitate oral hygiene, 360o removal of the biofilm and will reduce the risk for inflammation. The wider this angle the higher the risk. Is there a
cut-off threshold? I would not think so. Such thresholds are more statistics than actual biological findings. Statisticians typically would choose the number that gives the highest statistical significance and use it as a threshold, e.g., ≥30o, but this does not necessarily reflect a biological phenom- enon. So complex and multifactorial phenomena as inflammation would be more correctly approached by a range, rather than one single cut-off value or an average. Looking at the risk curve we can say that the risk rises mildly from a MEA of 20o to 40o , then steeper towards 50o to 70o.
There is no absolute safety zone, but maintaining MEA no wider than 30o - 40o is advisable, while the aim should be to keep this angle as narrow as clinically feasible.

And now what next?

This was our first set of clinical data pointing out to the importance of design for clinical outcomes. The data we have compiled from patients included many more parameters, analysis of concavity or convexity, marginal bone levels, detailed tissue measurements. We even got some more ideas while analysing the data. This is the beauty of 3D imaging, once the reconstruction of the implant supracrustal complex is done, the data is saved and can be accessed and measured again and again with different approaches or methods. We are grateful to have the support of a great multidisciplinary team at Chulalongkorn University as well as a multicontinental team of experts, so we are on the way to bring up much more soon! For now, you can read the first paper for more details which is available open access thanks to Karolinska Institute and Chulalongkorn University. We are looking forward to your thoughts, suggestions, and comments!