Contour angle and peri‐implant tissue height: Two interrelated features of the implant supracrestal complex

Abstract Objectives Recent research has suggested the contour of the prosthesis and the vertical height of the peri‐implant mucosa as important parameters that can influence the long term health and stability of the peri‐implant tissue. In particular, overcontouring of the prosthesis has been correlated with an increased risk for peri‐implantitis, while reduced soft tissue height has been associated with marginal bone loss, recession, and other soft tissue complications. Although these two parameters have been investigated as independent in the current literature, clinical experience points toward a close interrelation between transmucosal tissue height and prosthesis contour angle. It is often found that a reduced vertical height of the implant supracrestal complex is the main reason for overcontouring of the prosthesis. At the same time, achieving a favorable contour of 30o or less is not possible unless the clinician has ensured an adequate vertical height of the soft tissue. The purpose of this short communication is to establish the relation between tissue vertical height and prosthesis contour by utilizing a theoretical geometry equation based on the Pythagorean theorem. In doing so, one can use the dimensions of the implant as well as those of the prosthesis at the mucosal margin to calculate the essential vertical height for achieving a favorable prosthesis contour. Conclusions As the treatment plan of the implant supracrestal complex is “top‐down,” in case of deficient vertical height, subcrestal placement of the implant should be considered to achieve a proper prosthesis contour.

independent in the current literature, clinical experience points toward a close interrelation between transmucosal tissue height and prosthesis contour angle. It is often found that a reduced vertical height of the implant supracrestal complex is the main reason for overcontouring of the prosthesis. At the same time, achieving a favorable contour of 30o or less is not possible unless the clinician has ensured an adequate vertical height of the soft tissue. The purpose of this short communication is to establish the relation between tissue vertical height and prosthesis contour by utilizing a theoretical geometry equation based on the Pythagorean theorem. In doing so, one can use the dimensions of the implant as well as those of the prosthesis at the mucosal margin to calculate the essential vertical height for achieving a favorable prosthesis contour.
Conclusions: As the treatment plan of the implant supracrestal complex is "topdown," in case of deficient vertical height, subcrestal placement of the implant should be considered to achieve a proper prosthesis contour. between the design of the implant prosthetic elements (Mattheos, Vergoullis, et al., 2021) and the health of the peri-implant tissue. The contour of the prosthesis and the dimensions of the peri-implant mucosa are the features most frequently implicated in the long term health and stability of the peri-implant tissue .
Three cross-sectional studies based on peri-apical radiographs (Katafuchi et al., 2018;Majzoub et al., 2021;Yi et al., 2020) have suggested that overcontouring of the prosthesis of more than 30°is correlated with an increased risk for peri-implantitis in bone-level implants. Although two cross-sectional studies that have followed a similar methodology have not confirmed the correlation of overcontouring with peri-implant inflammation (Inoue et al., 2020;Lops et al., 2022), these results overall certainly warrant further research exploration. Aside from peri implantitis, animal (Souza et al., 2018) and clinical (Spinato et al., 2019) studies have shown that a wide angle of the profile of the abutment as it is ascending from the bone-level can lead to early marginal bone loss. Furthermore, the convexity of the prosthesis profile has been correlated with increased recession (Siegenthaler et al., 2022), marginal bone loss (Valente et al., 2020), and when combined with overcontouring, Peri-implantitis (Yi et al., 2020).
The peri-implant soft tissue dimensions and, in particular, the vertical "height" of the mucosa have also been shown to be important parameters for health and stability. The initial concepts of the periimplant mucosa height were influenced by concepts first described in the periodontium, from the "Biologic Width" (Gargiulo et al., 1961) to the recently introduced "Supracrestal Tissue Attachment" (Caton, et al., 2018). Terms such as "Peri-Implant Soft Tissue Barrier" (Glauser et al., 2005), "Peri-implant Mucosa" (Araujo & Lindhe, 2018), and "Peri-implant Phenotype" (Avila-Ortiz et al., 2020) have been utilized to describe the peri-implant tissues. These studies have advocated the importance of an essential minimum height of the peri-implant mucosa to ensure the stability of the soft tissue and marginal bone in the long term. This essential height, which has previously corresponded to the concept of the "Biologic width," has been approximated to be between 2.5 and 4 mm in human studies (Glauser et al., 2005;Romanos et al., 2010;Tomasi et al., 2014). Failure to secure this soft tissue height has been associated with marginal bone loss (Linkevicius, Puisys, Linkevieiene, et al., 2015;Linkevicius, Puisys, Steigmann, et al., 2015), recession, and other soft tissue complications.
The recently introduced concept of the Implant Supracrestal Complex (Mattheos, Vergoullis, et al., 2021)  CT in implants appears with no vascular supply close to the abutment and very few fibroblasts, resembling more scar tissue; this is likely attributed to a lack of the PDL vascular complex (Glauser et al., 2005). Blood vessels originating from the supra-periosteal complex are located in the lateral borders of the CT and JE zone.
These blood vessels are the origin of the immune response to bacteria in the sulcus (Berglundh et al., 1994;Kawahara et al., 1998).
Similar to teeth, peri-implant crevicular fluid is produced, which flows into the sulcus through the junctional epithelium. Analysis of periimplant crevicular fluid (PICF) for protein biomarkers such as proinflammatory cytokines, chemokines, and bone turnover markers can reveal clinical and subclinical inflammation (Güncü et al., 2012).
Apart from structural observations, animal studies were used to identify the dimensions of the peri-implant mucosa, especially in comparison to natural teeth ( Table 1). The very diverse anatomic conditions and sizes, however, on the different animal models, combined with the different conditions of implant placement (e.g., absence of mucosal scalloping, diverse sulcus depth, no esthetic needs), prevent extrapolation of any conclusions on peri-implant tissue dimensions relevant to humans.
The few human studies available have pointed to significant diversity of dimensions not only between different studies but also among individuals in the same study sample, as well as between different sites in the same individual (Table 2). This should not be T A B L E 1 Calculation of the peri-implant tissue vertical dimensions in animal histomorphometric studies. studies, the range of 2.5-4 mm appears as a "comfort zone," representing the essential height for the establishment and maintenance of the peri-implant tissue seal in healthy conditions. This resembles the concept of the "Biologic width," which was first introduced around natural teeth and represented the essential supracrestal height of the periodontal tissue. If violated by restorations or trauma, marginal bone loss would follow. Today, there is significant evidence pointing to a similar concept around implants (Linkevicius, Puisys, Linkevieiene, et al., 2015;Linkevicius, Puisys, Steigmann, et al., 2015); thus, it becomes important to establish a minimum peri-implant supracrestal vertical tissue height of about 3 to 4 mm, which will accommodate adequately the biologic demands of sustainable health.
In cases where the vertical height of the peri-implant tissue is less than 3 mm, marginal bone resorption has often been reported around the implant platform . This might be physiological remodeling, resulting in re-establishing the vertical dimensions required to accommodate the soft tissues at the expense of the crestal peri-implant bone.

| OVERCONTOURING OF THE PROSTHESIS, EMERGENCE ANGLE, AND CLINICAL IMPLICATIONS
"Overcontouring"-an excessively wide profile of the abutment or prosthesis-has been associated with two main unfavorable clinical outcomes: early "aseptic" marginal bone loss or "remodeling" and peri-implantitis. The pathogenetic mechanisms involved in the two can be very different. Histomorphometry on animals showed that a healing abutment with a 45°angle led to significantly more marginal bone loss than the equivalent with 15°after only 4 months of healing (Souza et al., 2018). This angle corresponds to a steep widening of the abutment diameter right coronal of the implant platform and in close proximity to the marginal bone. At the same time, cross-sectional studies (Katafuchi et al., 2018;Majzoub et al., 2021;Yi et al., 2020) have suggested that overcontouring of the prosthesis more than 30°as it appears in peri-apical radiographs is correlated with an increased risk for peri-implantitis in bone-level implants. Finally, the convexity of the prosthesis profile has been correlated with increased recession (Siegenthaler et al., 2022), marginal bone loss (Valente et al., 2020), and when combined with overcontouring, peri-implantitis (Yi et al., 2020).
Collectively seen, these findings appear to relate to the previously discussed concepts of vertical tissue height and biologic width, as an excessively wide angle of ascendance from the bone margin can also be seen as limiting the essential vertical space for the establishment of the soft tissue seal.

| DESIGNING FAVORABLE DIMENSIONS FOR THE IMPLANT SUPRACRESTAL COMPLEX
In clinical practice, overcontouring is not uncommon, and in most cases encountered is a compensation for deficiencies in the implant position. In particular, possibly the most frequent reason for overcontouring is the inability to secure an adequate vertical height between the implant platform and the margin of the prosthesis. The Computer Assisted Design can now empower the designer to visualize and design the Implant Supracrestal Complex from every angle, converting the 2-dimensional design principles into a proper fully 3-dimensional structure.

F I G U R E 2
Application of the Pythagorean theorem in the trapezoid approximation of the Implant Supracrestal Complex for a posterior tooth (a) calculation of essential height for a 30°contour angle (b) increase of the angle for height reduction to 2 mm (red line).

CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
Data are available from the corresponding author upon reasonable request.