Quantifying the Emergence Profile Contour for Immediate Provisionalization: A Proposed Mathematical Model

In implant dentistry, identifying the ideal position of the final restoration before the surgical phase is essential in achieving optimal esthetics. Consequently, the placement of an implant based on fixed reference points allows for the ideal location of the implant. Ideal placement provides for the continual stability of proximal hard and soft tissues and ultimately the final esthetic outcome.

In a healed ridge, regardless of implant design, definitive guidelines for implant placement have been established over the years (Figure 1). This placement has been promoted as a 3-dimensional concept.¹  In the vertical dimension, the implant position should be 2 to 4 mm apical to the future gingival margin,²  2 mm of buccal bone thickness should be observed after placement,³  and implants should be placed 1.5 mm from adjacent teeth⁴  and 3 mm from adjacent implants.⁵  Establishing similar guidelines for immediate implants has been debated for the past 20 years. Without a definitive consensus, many practitioners have established their own protocols.

In immediate implant placement, to avoid buccal recession, the implant is often placed toward the palatal or lingual aspect of the socket, leaving a gap of 2 mm between the implant and the buccal plate.⁶  In 2010, the animal study by Covani et al⁷  provided histological support for undersizing the implant diameter and lingual placement, which led to minimal resorption of the buccal wall. The periodontal phenotype should also be evaluated before extraction, as studies have shown greater peri-implant mucosal dimensions in the presence of a thick peri-implant phenotype as compared with a thin phenotype.^8,9 

The recommended vertical distance of 3–4 mm from the free gingival margin (FGM) to the implant platform has been consistently used by surgeons to gauge the depth to which they should place the implant.¹⁰  In addition to this value, the recession of the FGM after atraumatic extraction should also be considered, as well as the gap distance, which is the buccolingual location of the implant in the extraction socket.

Another variable to consider is the degree of convexity of the prosthetic restoration, as it affects the final position of the soft-tissue margin. How does the consideration of the dimension from the FGM to the crest of bone as a curved line rather than a linear value change our expected esthetic outcome? The purpose of this study is to review the effect of vertical implant depth, distance of buccal gap on the emergence profile contour by introducing a measurement tool that can help design the buccal contour, improving the esthetic result of the temporization of immediate implants.

Although each case has unique variables to consider when evaluating risks for gingival recession, there are certain surgical guidelines that can direct implant placement based on a fixed reference point (Figure 2).

Buser et al¹  recommend placement of the implant at a position 1–2 mm lingual to the emergence of adjacent teeth to ensure an adequate width of buccal bone after healing and stable soft tissue. Evans and Chen¹¹  brought this concept to immediate implant placement and showed that implants with a shoulder position placed at or buccal to a line drawn between the cervical margins of adjacent teeth had 3 times greater recession than implants placed with a shoulder position lingual to this line.

The nature of this recommendation leaves a gap between the lingual aspect of the buccal bone and the implant surface. Chen et al¹²  found greater mucosal recession when this gap measured less than 1.1 mm as compared with implants placed with a gap of 2.3 mm. This finding suggests that to maximize the final esthetic result, a larger safety margin should be adopted with the implant shoulder positioned at least 2 mm from the internal buccal socket wall.

Ferrus et al¹³  studied the influence of this gap size on the three-dimensional changes in hard tissue after 4 months of healing. The thickness of the buccal bone crest significantly influenced not only the amount of horizontal gap fill but also the amount of vertical crest resorption. Even without implant placement, the buccal bone of an extraction socket demonstrates greater resorption in thin bony morphotypes versus those with thicker dimensions.¹⁴  Therefore, determining the height and width of the buccal plate before extraction is essential to accurately predict the amount of bone surrounding the implant after immediate placement and the nature of the healing process.

Management of the circumferential space between the immediate implant and the facial wall has been discussed thoroughly, as it affects both soft- and hard-tissue healing. Paolantonio et al¹⁵  stated that when immediate implants are placed with a gap of 2 mm or less, healing without regenerative materials is similar to implants placed in mature bone. However, it was also found that if the space is greater, the decreased bone-to-implant contact may require use of a membrane to prevent ingrowth of fibrous tissue. In 2004, Botticelli et al¹⁶  provided histological evidence that grafting this circumferential space with particulate bone and resorbable membrane can result in osseointegration within the gap area. Chen et al¹²  also found that with the use of anorganic bone grafts and/or barrier membranes, the extent of the horizontal resorption is limited to approximately 25% of the original buccal dimension. Thus, although there may be varying opinions on how the gap should be managed, we can aim for placement of the implant shoulder at a distance of 2 mm from the facial aspect of the extraction socket (an ideal orofacial position) to minimize the risk for not only buccal bone resorption but marginal gingival recession as well.

In 2001, Kois¹⁷  described the importance of the osseous crest position as a critical foundation for stable gingival levels. Likewise, the position of the interproximal crest is important after surgical intervention, such as immediate implant placement. Kois discussed previous clinical data from 100 healthy patients and developed quantitative data for 3 different biologic variations. These variations (low, normal, and high) are based on the vertical distance of the osseous crest to the FGM. The greater the distance of the osseous crest to the FGM, the greater the risk of tissue loss after an invasive procedure. If the vertical distance of the total dentogingival complex on the midfacial aspect is 3 mm, a slight apical loss of tissue (up to 1 mm) is anticipated after extraction and immediate fixture placement.

Based on Kois, greater or less than a 3-mm vertical distance indicates that the change will range from potentially greater than 1 mm to a negligible amount of migration of the gingival margin in an apical direction. Measuring the distance from the FGM to the osseous crest before extraction is therefore an important diagnostic procedure. If the facial gingival levels are harmonious and the distance to the osseous crest is 3 mm or greater, orthodontic extrusion is recommended to facilitate movement of the osseous crest coronally to account for subsequent osseous resorption and potential greater soft-tissue loss.¹⁸ 

Another component that must be considered is primary implant stability. This is achieved during sequential osteotomy by engaging the palatal wall and the bone 3–5 mm beyond the apex of the extraction socket.^19,20 

A major concern for patients with regard to immediate implants is leaving the office the day of extraction of an anterior tooth without a fixed restoration. The concept of immediate provisionalization has numerous benefits for patients, such as reduced number of surgical procedures, improved esthetics, elimination of the need for removable provisional restorations, and cost.^21–23  However, the biological benefits of this technique are controversial throughout the literature.

DeRouck et al²⁴  performed a systematic review and reported that immediate loading or provisional procedures allowed for improved management of interproximal tissue levels; however, the benefits of immediate provisionalization on the mid-facial marginal tissue were less apparent.

In 2014, Tarnow et al²⁵  found that placement of an immediate provisional restoration alone did little to prevent contour change compared with a control group. However, the authors still advocated for the placement of immediate provisionals, since the number of procedures can be reduced, thereby streamlining the overall treatment time and increasing patient comfort. They concluded that the smallest change in facial-palatal contour was achieved when placing a bone graft into the residual labial gap around a postextraction socket anterior implant in addition to a contoured healing abutment or custom-contoured provisional restoration.

Another study by Amato et al²⁶  evaluated the effect of grafting and provisionalization of immediate implants on the soft tissue. Implants were placed with either a healing abutment or a provisional restoration with a convex emergence profile at time of placement. They found a significant horizontal reduction of approximately 1 mm in the most coronal aspect of the gingival margin in the healing abutment group due to the lack of mechanical support to the soft tissue.

The use of a provisional restoration as a shell to protect the graft has been proposed to enhance the integrity of the supporting tissue around the implant.^27,28  It has been demonstrated that in tooth-supported restorations, overcontouring will cause apical migration of the gingival margin. On the contrary, undercontouring will encourage coronal positioning of the gingival margin and enhance the thickness of the soft tissue.

This concept has further been evaluated by Su et al²⁹  using the critical and subcritical concept whereby the emergence area is divided into 2 zones. The first is the critical contour, which is the area of the profile located immediately apical to the gingival margin and describes a 1-mm-wide band extending circumferentially around the restoration. It is theorized that the location of the gingival zenith may be modified through manipulation of this critical contour location. Apical to the critical contour is the subcritical contour, which may be designed as either convex, straight, or concave. In a recent study by the same group of authors, clinical guidelines for contour management of immediate and delayed provisional restoration in terms of the critical and subcritical contour were further demonstrated.³⁰ 

Although the restoration contour is related to the esthetic outcome and has been proposed to be a risk factor for peri-implantitis,³¹  direct study of the manipulation of the design of emergence contour of implant restorations has been limited. The objective is to compile the factors that may affect the final esthetic outcome and determine whether these factors can be integrated into an easy-to-use model that would allow for control in clinical research and eventually be applied to surgical guidelines.

The operator-controlled variables that contribute to the prosthetic contour include the depth of the implant platform and horizontal placement of the implant platform, each of which make up the 3-dimensional location of the implant within space, and the convexity of the restoration. Estimations that consider the emergence profile based on only a linear analysis, such as the location of the critical contour on the crown surface, without taking into account the entire curvature of the restoration, underestimate the distance between the implant-abutment junction and its eventual effect on the location of the soft tissue.

The goal of establishing a mathematical model to predict prosthetic implant components was previously assessed by Goldstein et al³²  in 2014, in which the curve was considered for a provisional restoration in a healed implant site. We introduce a proposed mathematical model that can quantify the soft tissue–to-prosthetic interface distance in immediately placed and immediately provisionalized implants. This model takes into account the surgical location of the platform within the extraction socket as well as the curve associated with the transmucosal profile of the restoration.

The mathematical model begins by quantifying the location of the implant platform in relation to the FGM. The vertical depth is measured from a line corresponding to the height of the FGM directly above the implant platform to the most buccal point on the implant platform (Figure 3, line a to b). The horizontal distance is measured from the buccal wall of the socket to the implant platform (Figure 3, line a to d). These lines make up the legs of a right triangle.

From these, we can use the Pythagorean theorem to solve for the linear distance from the implant platform to the FGM (Figure 4). We will call this distance the “emergence line.” However, when reproducing the anatomy of a natural tooth, we see that the emergence profile of the prosthesis is not a straight line but rather a curve. The model must define a curved line that passes through these 2 points: the implant platform and the FGM.

To better represent the actual shape of the emergence profile, we can think of this geometrically as part of a circle (Figure 5). We can change the degree of convexity over this distance by using circles of different sizes. A smaller circle represents a more contoured emergence profile with greater convexity. A larger circle represents a more gradual emergence profile with less convexity (Figure 6). The size of a circle can be measured using the circle's radius or R-value. Applying this clinically, changing the R-value of our circle represents the clinical process of over- or undercontouring a restoration.

So far, we have established 2 components (independent variables) of the geometric model representing the clinical situation for the emergence line: (1) the linear distance created by the placement of our implant platform in relation to the FGM and (2) a circle representing the degree of convexity of our emergence profile. Putting these 2 concepts together (line value and circle value) gives us an arc along the circle that represents the length of the prosthetic–to–soft tissue interface, which we will refer to as the emergence curve. The next step in our process is to establish a model that will allow us to relate our variables to the length of this emergence curve.

In doing so, our hypothesis confers that any changes in the placement of our implant as represented by the emergence line will have a direct, quantitative impact on the length of the emergence curve. Furthermore, the degree of convexity of the prosthesis represented by a circle with radius R will have a direct, quantifiable impact on the length of the emergence curve (Figure 7).

Putting all our variables together and using trigonometry and geometry, we come up with an arc length that is mathematically dependent on both the implant placement and the convexity of the prosthesis. As we found no current literature with a numerical norm for implant convexity, we used a fixed reference point of the height of a sample prosthetic abutment to demonstrate the prosthesis as shown (Figure 8).

The model uses a spreadsheet (Table 1) into which we can input our variables and solve for arc length. An important test for the model was whether it correlated to what the clinician could expect to find in a given clinical situation. A comparative table was created showing all distances between 1 and 5 mm, and the arc length calculated was greater than the chord length at each possible measurement.

As an example, using a defined vertical dimension of 3 mm from the implant platform to the future soft-tissue margin and a horizontal distance of 2 mm, a linear estimation would yield a contour of 3.61 mm, while a convex profile would instead measure 3.74 mm.

In addition, manipulation of the included variables yielded the expected change in arc length. For example, if the surgeon places an implant platform at a more apical position, the arc length is expected to increase. Accordingly, increasing the platform depth from 3 mm to 4 mm with the same parameters as described above would increase the arc length from 3.74 mm to 4.76 mm. Similarly, modifying the prosthetic radius variable, by decreasing it or increasing it, will lead to a change in the prosthetic contour (either a more convex emergence profile or more flatter) because it will change the arc length (Figures 6 and 7).

A digital workflow has now eliminated the guesswork from the surgical phase. Immediate implant cases begin with a cone-beam computerized tomography (CBCT) scan of a failing tooth to obtain 3-dimensional volume data, which are captured as a DICOM (Digital Imaging and Communications in Medicine) file. An intraoral surface scanner is then completed and captured as an STL (stereolithography) file, including soft-tissue positioning and tooth morphology. These files are then stitched together, allowing for orientation of hard-tissue landmarks with soft-tissue data. This allows us to better execute a restoratively driven treatment plan based on the position of the implant planned in the CBCT in relation to the position of the teeth and soft tissue captured on the surface scan.

From here, practitioners can make precise decisions for surgery based on known 3-dimensional immediate implant concepts from both hard- and soft-tissue anatomical landmarks. The concept described in this article becomes valuable in this particular step. If we establish anatomical norms relating the hard and soft tissue, we can use the mathematical model described above to produce the ideal emergence profile without guessing. The implementation will become valuable when a planning software can use a mathematical model to print a specific contour that relates the position of the implant to the position of the provisional restoration, as opposed to an arbitrary contour as in contemporary practices. In the clinical setting today, we are able to mimic the extracted tooth only at the gingival margin, but the area between the gingival margin and the implant platform is arbitrarily designed by the dentist in a chairside procedure or by the lab technician if the restoration is fabricated before the extraction is done. Use of this proposed mathematical model may lead to more consistent and predictable clinical outcomes and warrants further study and development.

To illustrate the concept, we have included an example of what we can do today and what we are hoping to achieve with this mathematical model. A 62-year-old man presented with a resorbed root on the buccal of the maxillary right central incisor #8. Tooth extraction was decided as the best course of treatment. To prepare and execute the case with a fully digital workflow, as described before, an intraoral scan and CBCT were taken, providing data in an STL and DICOM format, which can then be entered in an implant-planning software, such as coDiagnostiX. The STL data will be imported in Meshmixer Software, which allows us to digitally extract tooth #8. (Figure 9). That file will be imported and superimposed on the DICOM file in coDiagnostiX. The implant planning will begin by digitally placing the proper implant to the set criteria discussed previously: 3 mm from the gingival margin and with a minimum 2-mm buccal gap (Figure 10).

This planning guide will be synergized with the prosthetic software Straumann CARES Visual Chairside, which allows for chairside provisionalization, among other features. By using these steps, the clinician will end up with a design that allows the clinician or technician to further shape the submarginal contour (Figures 11 and 12), especially from the mesiobuccal line angle to the distobuccal line angle. Once the design is established, it can be routed to make a digital model with a soft-tissue design (Figure 13). When the project is finalized, all 3 models: designed temporary, soft tissue, and the model with the analog location, can be exported into STL format for 3-dimensional printing. The designed temporary can then be printed with the proper shade and adapted to a prosthetic abutment appropriate to the chosen implant system using the printed model with the printed soft tissue. Finally, the fit of the surgical guide over the model and its orientation to the lab analog is verified (Figures 14 through 17).

It is hoped that this mathematical model, when integrated with prosthetic and implant-planning software, will enable the automated design of the most crucial buccal subgingival marginal contour (running from the buccal mesial line angle to the distal line angle). This innovation will remove the need for estimating the arbitrary submarginal contour design. The integration will facilitate fabrication of an immediate prosthesis, which will result in the ideal buccal or labial gingival tissue contour.

Abbreviations

CBCT:

cone-beam computerized tomography

DICOM:

Digital Imaging and Communications in Medicine

FGM:

free gingival margins

STL:

stereolithography