Introduction

Figure 1. The Transition Part in Implant Design. The transition part refers to the distance from the connection between the fixture and abutment to the final cervical area of the crown. Aesthetically, if the transition part is visible supramucosally, it tends to appear unnatural. Conversely, when it lies submucosally, the appearance is more natural. This section includes both vertical and horizontal components, which together determine the angle of the emergent profile of the final restoration.

Figure 1. The Transition Part in Implant Design. The transition part refers to the distance from the connection between the fixture and abutment to the final cervical area of the crown. Aesthetically, if the transition part is visible supramucosally, it tends to appear unnatural. Conversely, when it lies submucosally, the appearance is more natural. This section includes both vertical and horizontal components, which together determine the angle of the emergent profile of the final restoration.

Emerging Phenomenon and the Transitional Zone

The traditional concept of biologic width, derived from studies on natural teeth, is often applied to interpreting the soft tissue around dental implants. Anatomically, this biologic width comprises epithelium and connective tissue, and while generally consistent, it does exhibit some variability. Functionally, it serves as the primary defense line for both natural teeth and implants against foreign invasions through its mechanical sealing ability and immunological functions. This transition area, known as the 'biologic width' in natural teeth, primarily has a vertical component. Some tissue-level implants with wide diameters also display only vertical components because the diameter difference between crowns and connection parts is minimal, simplifying the application of natural teeth's biologic width concepts to dental implants. However, implants with Internal Platform Switching (IPS) design feature horizontal distance differences, which constitute the horizontal component while the prostheses pass through the transitional zone, either submucosally or supramucosally.

When planning a crown that mimics the diameter of natural teeth, several factors must be considered to determine the depth of implant placement. One crucial consideration is respecting the biologic width, which entails allocating adequate space for the soft tissue around the transmucosal area of implants. When considering only a vertical component, the guideline for implant depth follows the traditional biologic width, typically 3-4 mm. This minimum soft tissue thickness or space is the primary concern to adhere to biological principles. However, when both vertical and horizontal components are considered, the emergent angle affects the space for soft tissue, potentially creating a favorable or unfavorable environment depending on the angle. Thus, both the intended emergent angle and the horizontal component's distance difference must be considered as guidelines for determining the depth of implant fixture placement during the first surgery.

The average posterior molars have a breadth and width (or mesiodistal or buccolingual length) of about 10 mm. However, the starting points of implant crowns, which are the fixture-abutment connection joints, average 3.0 mm in diameter. The diameter of crowns should gradually increase to 10 mm from 3 mm. The calculated horizontal difference is (10−3)/2=3.5(10−3)/2=3.5 mm. This 3.5 mm horizontal increasing distance on each side from the center should increase within its accompanying vertical components if the emerging angle is 45 degrees. It was reported that when the emerging angle is less than 30 degrees, crestal bone changes were minimal due to more stable biologic width.^[13,14] For instance, when the horizontal difference distance is 3.5 mm and the intended emergent angle is 30 degrees, the required vertical distance is 3.5×1.7=6.13.5×1.7=6.1 mm. Alternatively, when the horizontal difference distance is 2 mm (in the premolar area) and the intended emergent angle is 30 degrees, the required vertical distance is 2×1.7=3.42×1.7=3.4 mm.

Figure 2. Table of Vertical Distance Calculations. This table presents examples of how to calculate the required vertical distance based on different emergent angles and horizontal distances of difference for bone level implants designed with Internal Platform Switching (IPS). Each entry illustrates the necessary adjustments in vertical distance to accommodate specified emergent angles, providing a precise guide for implant placement to achieve optimal aesthetic and functional outcomes.

Figure 2. Table of Vertical Distance Calculations. This table presents examples of how to calculate the required vertical distance based on different emergent angles and horizontal distances of difference for bone level implants designed with Internal Platform Switching (IPS). Each entry illustrates the necessary adjustments in vertical distance to accommodate specified emergent angles, providing a precise guide for implant placement to achieve optimal aesthetic and functional outcomes.

Figure 3 and Figure 4: Impact of Implant Placement Depth on Clinical Crown Shape

These figures demonstrate the effect of the depth of implant placement on the shape of the clinical crown. Figure 3 shows an equicrestal placement of a second molar implant, where insufficient vertical space necessitates a smaller diameter or an abrupt emergent profile for the clinical crown. In contrast, Figure 4 illustrates a subcrestal placement for a first molar implant, which provides sufficient vertical space allowing for the creation of a more natural-looking clinical crown with a gradual emerging profile.

Figure 3. Case Study of Epi-Crestal Placement of a Second Molar Implant. This figure illustrates a case where a second molar implant was placed epi-crestally, resulting in a smaller clinical crown size and an unnatural appearance due to an improper emergent profile compared to the adjacent natural molar. This outcome is attributed to the shallow depth of placement, which causes the transition part to be exposed supramucosally.

Figure 3. Case Study of Epi-Crestal Placement of a Second Molar Implant. This figure illustrates a case where a second molar implant was placed epi-crestally, resulting in a smaller clinical crown size and an unnatural appearance due to an improper emergent profile compared to the adjacent natural molar. This outcome is attributed to the shallow depth of placement, which causes the transition part to be exposed supramucosally.

Figure 4. Comparative Case of Equicrestal vs. Subcrestal Implant Placement. This figure demonstrates the differences between equicrestal and subcrestal implant placements. For the implant in the position of the second molar, equicrestal placement results in a molar diameter reaching a short vertical distance, leading to an abrupt emergent profile with the transition part located supramucosally. In contrast, for the implant in the position of the first molar, subcrestal placement yields a more natural appearance, as it allows for a gradual emergence with the vertical distance matching the horizontal distance needed for diameter increase, and the transition part located submucosally.

Figure 4. Comparative Case of Equicrestal vs. Subcrestal Implant Placement. This figure demonstrates the differences between equicrestal and subcrestal implant placements. For the implant in the position of the second molar, equicrestal placement results in a molar diameter reaching a short vertical distance, leading to an abrupt emergent profile with the transition part located supramucosally. In contrast, for the implant in the position of the first molar, subcrestal placement yields a more natural appearance, as it allows for a gradual emergence with the vertical distance matching the horizontal distance needed for diameter increase, and the transition part located submucosally.

Situations When Implant Fixtures are Placed Subcrestally

• Esthetic Reasons:
  • Due to the emerging phenomenon associated with Internal Platform Switching (IPS) design implants.
  • To compensate for the transition zone required by IPS design implants.
  • Particularly important in molar regions where esthetic considerations are critical.
• Accommodating Implants in Narrow Crestal Ridges:
  • Enhances the strength of soft tissue around the implant fixtures by increasing blood supply.
  • Provides sufficient bone thickness (more than 2mm) to securely enclose fixtures in areas where the width of the crestal bone is insufficient for epicrestal placement.
• Proximity to Healthy Neighboring Natural Teeth:
  • When implants are placed next to healthy natural teeth, especially in the upper premolar and anterior regions, the distomesial depth of the implants is positioned subcrestally to optimize alignment and integration.

Additional Reasons for Subcrestal Implant Placement

Subcrestal placement of implant fixtures is often necessary beyond aesthetic or anatomical considerations. Typically, the standard reference point for determining fixture depth is the most apical part of the crestal area—the most apical point. Outside of this area, placing the fixture subcrestally becomes inevitable unless the bone is trimmed. However, bone trimming can be detrimental, especially when the adjacent teeth share the crestal (septal) bone. Furthermore, preserving the crestal bone above a subcrestally placed fixture can enhance mucosal support by maintaining essential blood supply, which is crucial for achieving stable and aesthetically pleasing results.

Figure 5. This figure illustrates a case where a plain X-ray image suggests that the implant was placed subcrestally, but a CBCT (Cone Beam Computed Tomography) image reveals that the fixture was actually placed epi-crestally in the buccolingual aspect.

Figure 5. This figure illustrates a case where a plain X-ray image suggests that the implant was placed subcrestally, but a CBCT (Cone Beam Computed Tomography) image reveals that the fixture was actually placed epi-crestally in the buccolingual aspect.

3. -Dimensional Classification of the Soft Tissue around Implants for 3DSTA

The traditional model for biologic width faces some limitations in explaining the relationship between the soft tissue and bone with subcrestally placed implants in a three-dimensional manner. For the purpose of this study, the soft tissue around implants has been categorized into three distinct zones: 1) Marginal Zone, 2) Transitional Zone, and 3) Subcrestal Zone. The dimensions of the Transitional Zone and the Subcrestal Zone were measured for their width and length using CBCT.

Ramon Gomez-Meda et al. proposed the "esthetic biologic contour" concept for implant restoration emergency profile design, in which the subgingival contour is divided into three zones: Zone E for esthetic conditioning composed of sulcus epithelium, Zone B for biologic boundary area composed of junctional epithelium, and Zone C for crestal bone stability composed of connective tissue.^[4] However, there are notable differences between his division and the classifications in this article regarding the boundaries of each zone and the approach to managing the connective tissue area in relation to the position of implants and the design of the prostheses for SPIs.

Figure 6. Schematic Drawing of Submucosal Soft Tissue Zones. This figure presents a schematic drawing that differentiates the submucosal soft tissue around implants into three zones: 1) Marginal Zone, 2) Transitional Zone, and 3) Subcrestal Zone.

Figure 6. Schematic Drawing of Submucosal Soft Tissue Zones. This figure presents a schematic drawing that differentiates the submucosal soft tissue around implants into three zones: 1) Marginal Zone, 2) Transitional Zone, and 3) Subcrestal Zone.

Evaluating Soft Tissue around Implants: Limitations and Techniques

There are limited clinical methods available for examining soft tissue around implants beyond probing the depth of the sulcus. Probing, especially around curved sulci with a metal probe, is challenging, and no universally accepted standard values exist for the sulcus depth of subcrestally placed implants. Despite these challenges, Subcrestally Placed Implants (SPIs) are highly valued for achieving the ultimate aesthetic goals in implantology. Numerous clinical reports affirm the positive outcomes of SPIs from a biological perspective. However, the traditional concept of biologic width, typically viewed in a two-dimensional context, falls short in accurately depicting the phenomena occurring around SPIs. This limitation may cause the deep positions after SPIs to be viewed negatively, despite positive clinical outcomes, thus necessitating a reevaluation to address the complexities of soft tissue dynamics around implants. This is particularly relevant given the long-term clinical data indicating that subcrestally placed implants can remain stable without clinical signs and symptoms of peri-implantitis or peri-implant mucositis.

The purpose of this report is to address these shortcomings and to provide a more comprehensive explanation of the phenomena around SPIs.

Figure 7. Long-Term Stability of Lower First Implants. This figure displays a panoramic view X-ray taken in 2023, showing the stable results of both lower first implants that were placed in 2012. These implants, which utilize Subcrestally Placed Implants (SPI) with Internal Platform Switching (IPS) design (specifically Bicon implants), demonstrate long-term stability and successful integration.

Figure 7. Long-Term Stability of Lower First Implants. This figure displays a panoramic view X-ray taken in 2023, showing the stable results of both lower first implants that were placed in 2012. These implants, which utilize Subcrestally Placed Implants (SPI) with Internal Platform Switching (IPS) design (specifically Bicon implants), demonstrate long-term stability and successful integration.

Materials and Methods

Procedure Description

• Implant Placement: All patients had one missing posterior molar replaced. Implants were placed subcrestally, ensuring complete embedding in both buccolingual and mesiodistal aspects.
• Implant Types: The majority (18 cases) utilized 5 x 8 mm Well Lock implants from Withwell company, South Korea. These implants are modified Bicon types designed for screwing healing abutments. In two cases, standard Bicon implants were used.
• Healing Abutment Installation: At the first surgery, healing abutments with a lower height but matching the diameter and profile of the final ready-made abutments were connected to the fixtures. All flaps were primarily closed without wound dehiscence.
• Transmucosal Connection: A second procedure to attach transmucosal connections was performed three months post-first surgery, where longer healing abutments matching the profile of the final ready-made abutments at the neck collar area were installed.
• Final Prosthesis Fabrication: Single crowns were crafted to match the size of adjacent natural teeth using the extraoral cementation (EOC) technique.

Surgical Procedure for SPI with IPS in Eight Well Lock Cases:

• Initial Setup: Healing abutments, matching the size and profile of the final ready-made abutments in stock, were connected to the subcrestally placed fixtures. This was done to protect the subcrestal bone during the procedure.
• Surgical Preparation: After reflecting the periosteal flap, the initial drilling was performed using a 2.0 mm diameter drill at a speed of 750 rpm with saline irrigation to cool the area and minimize heat generation.
• Sequential Drilling: Subsequent drillings were carried out with increasing drill sizes (2.5, 3.0, 3.5, 4.0, 4.5, 5.0 mm) at a reduced speed of 50 rpm, without saline irrigation, to prepare the bone hole for the implant.
• Implant Placement: Once the final drill size of 5.0 mm was reached, a fixture connected with a 5 mm diameter / 2.5 mm height healing abutment was passively installed into the prepared bone hole.
• Closure: Primary closure of the periosteal flap was achieved using 4-0 monocryl sutures (Ethicon com.).

Procedural Details for Bicon Implant Cases In the two cases involving Bicon implants, the plastic plug supplied by Bicon was utilized to cover the connection hole of the fixture.

This approach facilitates the easy replacement of the healing abutment with the predetermined ready-made final abutment, eliminating the need for additional bone reduction to fit the final abutment.

Figure 8. Subcrestal Placement of Fixtures and Healing Abutments. This image illustrates the subcrestal placement of fixtures, with healing abutments measuring 5 mm in diameter and 2.5 mm in height connected. The flap was then sutured to achieve primary closure, ensuring secure and proper healing conditions.

Figure 8. Subcrestal Placement of Fixtures and Healing Abutments. This image illustrates the subcrestal placement of fixtures, with healing abutments measuring 5 mm in diameter and 2.5 mm in height connected. The flap was then sutured to achieve primary closure, ensuring secure and proper healing conditions.

Secondary Operations for Prosthetic Procedures (after 3 months):

• Uncovering and Abutment Connection: The second surgical operations involved uncovering the sites and connecting longer healing abutments that matched the profile of the initially embedded healing abutments.
• Impression and Prosthesis Fabrication: Using the impression coping technique, final implant crowns were fabricated. These crowns were designed with predetermined ready-made abutments from stock, maintaining the same size as the healing abutments (5 mm diameter and 2.5 mm gingival cuff). This sizing helps to avoid bony interference during the fitting of implant abutment prostheses.
• Crown Design: The clinical crowns were shaped with a gradual emerging angle from the abutment margin toward the cervical part of the neighboring natural teeth, effectively emulating a natural tooth shape. This design is enabled by the ample vertical distance afforded by subcrestal placement, which facilitates a smooth and gradual emergent profile.
• Cementation Technique: The Complexes of Abutment and Crown (CACs) were set using the Extraoral Cementation (EOC) technique. This method involves cementing the components extraorally, thoroughly removing any excess cement, and ensuring complete cleaning of the CAC before seating it into the fixture.

Figure 9. X-Ray Sequence of Healing Abutment Changes. This series of X-rays illustrates the procedural sequences for implant placement and subsequent adjustments. Initially, healing abutments measuring 5 mm in diameter and 2.5 mm in height were placed and covered with primary flap closure. After three months, a second operation was undertaken for the prosthetic procedure. During this stage, the sites were uncovered and longer healing abutments, maintaining the same profile as the initially embedded abutments, were connected to prepare for final prosthesis installation.

Figure 9. X-Ray Sequence of Healing Abutment Changes. This series of X-rays illustrates the procedural sequences for implant placement and subsequent adjustments. Initially, healing abutments measuring 5 mm in diameter and 2.5 mm in height were placed and covered with primary flap closure. After three months, a second operation was undertaken for the prosthetic procedure. During this stage, the sites were uncovered and longer healing abutments, maintaining the same profile as the initially embedded abutments, were connected to prepare for final prosthesis installation.

Data Collection

Figure 10. Measurement Items Recorded from CBCT. This figure outlines the specific items used for recording measurements from the CBCT scans of each patient, enhancing understanding of the data collection process used for analysis.

Figure 10. Measurement Items Recorded from CBCT. This figure outlines the specific items used for recording measurements from the CBCT scans of each patient, enhancing understanding of the data collection process used for analysis.

Procedures for Measurement

Measurements from the panoramic views and CBCT were conducted using Sirona Dental Systems (Device: XIOX Plus ORTHOPHOS_XG_3D; Programs: SIDEXIS, GALILEOS Implant Viewer; Version 1.9.5605.25519 ID7).

Calibration of CBCT: To ensure accuracy, the presentation plane of the CBCT was centered around the long axis of the fixture. This allowed for the confirmation of the fixture's size—both diameter and length—against its known original dimensions.

Figure 11. Calibration of CBCT Images. This figure demonstrates the calibration process where images are coordinated along the long axis of the fixtures. This alignment is crucial for accurate measurement and analysis, ensuring that the fixture dimensions are accurately reflected in the CBCT images.

Figure 11. Calibration of CBCT Images. This figure demonstrates the calibration process where images are coordinated along the long axis of the fixtures. This alignment is crucial for accurate measurement and analysis, ensuring that the fixture dimensions are accurately reflected in the CBCT images.

1. Measurement of Peripheral Placement Depth (pPD) from Panoramic View

The Peripheral Placement Depth (pPD) was measured using panoramic views rather than 3-dimensional soft tissue analysis (3DSTA). This involved determining the perpendicular distances from the top of the crestal bone to the top of the implant fixture at both the mesial and distal sides. Measurements were taken after a calibration process utilizing the known fixture length as a reference.

2. Measurement of Placement Depth (PD) from CBCT - 3DSTA

Placement depth was assessed using CBCT in the 3DSTA context, focusing on both the central and peripheral areas in mesiodistal and buccolingual aspects:

a) Central Placement Depth (cPD): - The depth at the central area was measured as the perpendicular distance from the top point of the crestal bone to the platform of the implant fixture at its most outer point (e.g., at the 5 mm point of a 5 mm diameter fixture).

b) Peripheral Placement Depth (pPD) - Mesiodistal and Buccolingual Aspects: - In the mesiodistal aspect, the depth was measured as the perpendicular distance from the top point of the crestal bone to the platform of the implant fixture at the most outer point of the crown. - In the buccolingual aspect, the depth was measured from the most coronal top point of the crestal bone to the platform of the implant fixture.

3. Soft Tissue Thickness (STT) from CBCT - 3DSTA

Soft Tissue Thickness was measured using CBCT, focusing on both the central and peripheral areas, and assessing the dimensions mesiodistally and buccolingually. This measurement is crucial for understanding the vertical component of the soft tissue surrounding the implants, analogous to the traditional concept of biologic width.

a) Central Soft Tissue Thickness (cSTT) or Subcrestal STT (STTsub): - Measured as the perpendicular distance from the outer part of the abutment to the adjacent crestal bone at the subcrestal area. It reflects the thickness of soft tissue directly covering the abutment's embedded part, serving as a functional demarcation between the living bone tissue and the external environment, similar to the Schneiderian membrane in the sinus.

b) Peripheral Soft Tissue Thickness (pSTT) or Supracrestal STT (STTsupra): - Measured as the perpendicular distance from the outer part of the crown to the adjacent crestal bone at the supracrestal area. The reference plane used for measuring pSTT is identical to that used when measuring the Peripheral Placement Depth (pPD), ensuring consistency in measurement criteria across different dimensions.

4. Crestal Bone Thickness (CBT) from CBCT - 3DSTA

Crestal Bone Thickness is measured buccolingually, focusing solely on the dimension from the most inner point of the crestal bone contacting the fixture at the level of the fixture top, to the most outer point of the surrounding crestal bone. This specific measurement provides critical information regarding the thickness and health of the bone encompassing the implant, which is vital for assessing the stability and long-term viability of the implant.

5. Crestal Bone Length (CBL) = Transitional Zone Length from CBCT - 3DSTA

Crestal Bone Length, corresponding to the Transitional Zone Length, is measured as long as the peripheral soft tissue thickness between the crown-abutment complex and the surface of the crestal bone is maintained within 1.0 mm. The measurement is taken from the most inner contacting point of crestal bone with the fixture to the most outer point of crestal bone, conducted both buccolingually and mesiodistally. It can be regarded as the horizontal component of the soft tissue surrounding the implants.

Results

1. 

The crestal bone changes by the measurements of pPD (from panoramic view X ray)

		Time of Initial X ray	Time of F/U X ray	change	F/U duration
		pPD	pPD	pPD
Average	M	3.8	3.9	0.05	3.3 years
	D	2.8	2.8	0.08

2. 

The depth of placement (from the 3DSTA)

	The depth of placement at the central area						The depth of placement at the peripheral area
	cPD/M	cPD/D	cPD/B	cPD/L	average	deepest cPD	pPD/M	pPD/D	pPD/B	pPD/L	average	Deepest pPD
mean	1.9	1.6	1.8	1.7	1.7	2.3	3.7	2.9	2.0	2.3	2.7	4.0

3. 

The soft tissue thickness (from the 3DSTA)

	Subcrestal Soft Tissue Thickness
	STTsub					STTsup
	M	D	B	L	mean	M	D	B	L	mean
average	0.3	0.4	0.2	0.2	0.2	0.7	0.7	0.4	0.5	0.6

4. 

The crestal bone thickness (from the 3DSTA)

	CBT (Crestal Bone Thickness)
	B	L
average	3.0	3.7

5. 

The TZ (transitional zone) length (from the 3DSTA)

	CBL = TZL (Transitional Zone Length)
	CBL m	CBL d	CBL b	CBL l	mean
average	5.4	4.2	2.9	2.9	3.8

6. 

The emergency angle

Average of emergency angle

32

Discussion

Having Natural Crown Shape and Size

Proper implant prosthetic design, including appropriate size, allows implant crowns to function like natural teeth by effectively transferring masticatory forces and offering an aesthetically pleasing appearance. The implant prosthesis can be divided into supramucosal and submucosal parts. The supramucosal part, which is the visible crown, plays a crucial role in aesthetics. If this part harmoniously matches the neighboring natural teeth in terms of size, color, and morphology, it can seamlessly blend in, appearing entirely natural. For the crown to be perceived as natural, its diameter must correctly match that of natural teeth from the cervical area—the starting point of the supramucosal part.

Emerging Phenomenon in Implant Design

The emerging phenomenon observed in dental implants is primarily due to the horizontal distance between the crown diameter and the abutment-fixture connection diameter at the submucosal level. This spatial difference allows the crown to transition smoothly from the narrower abutment to the broader coronal structure, mimicking the natural contour of teeth where the crown gradually flares out from the gum line.

In contrast, certain types of tissue-level implants that exhibit no significant diameter difference between the implant fixture (or abutment/fixture junction) and the crown do not demonstrate this emerging phenomenon. This design more closely resembles natural teeth, which typically show minimal diameter differences between roots and crowns, thus eliminating concerns associated with the emerging phenomenon in implants.

Figure 12. mm of facial soft tissue thickness around the final restorations^[18]. However, these guidelines often overlook the specific requirements for subcrestal soft tissue thickness and do not fully address challenges associated with the emerging phenomenon.

Figure 12. mm of facial soft tissue thickness around the final restorations^[18]. However, these guidelines often overlook the specific requirements for subcrestal soft tissue thickness and do not fully address challenges associated with the emerging phenomenon.

Ideally, the bone table of an edentulous crestal ridge should be broad enough to comfortably accommodate an implant of regular diameter (3.5 – 5.0 mm) with adequate marginal crestal bone, allowing uniform placement depth across the implant. However, such ideal conditions are rare in clinical practice^[19]. More commonly, clinicians face narrower crests that necessitate placing implants subcrestally based on the most apical part of the crest, unless bone grafting is performed. This approach is more reflective of real-world clinical scenarios and underscores the need for careful planning and adaptation to varying anatomical landscapes.

Despite the prevalence of these conditions, there remains a lack of extensive research on the topographic relationship between the implant components (abutment and prosthesis complex) and the biological environment. Such studies are crucial for understanding the long-term biologic integrity and stability of implant structures.

Figure 13. Unlike a natural gingival sulcus, which is a normal anatomical structure when the periodontium is healthy, a periodontal pocket indicates pathological deepening and is characterized by inflammation, bone destruction, and detachment of the junctional epithelium^[21]. This difference is crucial in dental practice, as the mere measurement of depth can predispose clinicians to judge the periodontal state as compromised. However, this perspective may be a preconception when analyzing implants, where a longer soft tissue transitional area (sometimes misinterpreted as a pathologic pocket) is often considered undesirable despite clinical evidence showing stable results in deeply placed implants.

Figure 13. Unlike a natural gingival sulcus, which is a normal anatomical structure when the periodontium is healthy, a periodontal pocket indicates pathological deepening and is characterized by inflammation, bone destruction, and detachment of the junctional epithelium^[21]. This difference is crucial in dental practice, as the mere measurement of depth can predispose clinicians to judge the periodontal state as compromised. However, this perspective may be a preconception when analyzing implants, where a longer soft tissue transitional area (sometimes misinterpreted as a pathologic pocket) is often considered undesirable despite clinical evidence showing stable results in deeply placed implants.

Three-Dimensional Analysis and Subcrestal Implant Placement: The advent of Cone Beam Computed Tomography (CBCT) and its widespread use in implantology have shown that many implants are placed subcrestally, at least partially, without causing distinguishable pathological issues compared to epi-crestally placed implants^[12]. A lack of systematic methods for analyzing the submucosal soft tissue around subcrestally placed implants in a three-dimensional way likely contributes to unfounded pessimistic views. Three-dimensional soft tissue analysis (3DSTA) is particularly vital for subcrestal placements known to help reduce bone loss incidence^[1,22,23,24,25]. This technique also supports the creation of more natural-looking prostheses by providing sufficient space to offset the diameter difference between crowns and abutment-fixture connections, ensuring an appropriate emergence profile. Some authors have pointed out the limitations of two-dimensional analysis in studying implants and have suggested using micro-computed tomography for more detailed and accurate assessments^[26].

The Role of Epithelial Health and Integration in Dental Implant Success

The successful healing and maintenance of epithelial and connective tissues around dental implants are critical to the overall success of the implant^[30]. Epithelial downgrowth, a natural immune response to protect against pathogens, occurs to compensate for the disruption of tissue integrity, typically following accidents or surgical interventions. This process, known as epithelialization, is a primary response for repairing damaged tissues and is not harmful in itself.

However, unlike natural tissues, the structures around dental implants are artificially created and require integration into the body's systems to be considered healthy. This integration involves establishing a form of structural integrity that includes adequate blood supply, normal physiological function, and an immune defense mechanism. For implants, this integrity must be maintained without disruption to prevent unhealthy epithelial downgrowth, despite the inherently longer and broader soft tissue barriers around implants compared to natural teeth.

Figure 14. This focused source means that a larger soft tissue area receiving blood from the alveolar bone results in healthier tissues, reducing susceptibility to inflammation and resistance to bone loss. This is particularly beneficial since dental implants inherently lack connective tissue attachment and have a poorer blood supply due to the absence of periodontal ligaments. The enlarged TZ in subcrestal placements can compensate for these disadvantages by strengthening the biologic width.

Figure 14. This focused source means that a larger soft tissue area receiving blood from the alveolar bone results in healthier tissues, reducing susceptibility to inflammation and resistance to bone loss. This is particularly beneficial since dental implants inherently lack connective tissue attachment and have a poorer blood supply due to the absence of periodontal ligaments. The enlarged TZ in subcrestal placements can compensate for these disadvantages by strengthening the biologic width.

Figure 15. Similarity in Biologic Width Between Implants and Natural Teeth. When healing abutments or similar structures are placed without horizontal widening, the soft tissue around implants parallels the biologic width of natural teeth. In such cases, the biologic width closely matches the thickness of the adjacent surface mucosa. This alignment is particularly noted with the use of external hex connection type implants.

Figure 15. Similarity in Biologic Width Between Implants and Natural Teeth. When healing abutments or similar structures are placed without horizontal widening, the soft tissue around implants parallels the biologic width of natural teeth. In such cases, the biologic width closely matches the thickness of the adjacent surface mucosa. This alignment is particularly noted with the use of external hex connection type implants.

Enhanced Biologic Sealing in SPIs: A CBCT Analysis of Transmucosal Soft Tissue Dimensions

The cases examined in this study demonstrate that the area of transmucosal soft tissue, which corresponds to the junctional epithelium and connective tissue (the biologic width), around dental implants is enlarged due to deep subcrestal placement. This enlargement was quantitatively analyzed using Cone Beam Computed Tomography (CBCT). The results indicated that the average length of the Transitional Zone (TZ) was 3.8 mm, with an average thickness of 0.6 mm. These measurements suggest that the majority of the TZ is composed primarily of connective tissue. As the TZ extends toward the peripheral area and its thickness increases to more than 1.0-1.5 mm, it typically becomes covered with junctional epithelium. The predominant structure of the submucosal area is a connective tissue zone with junctional epithelium primarily present at the entrance, measuring approximately 0.5-1 mm, as detailed by Ikiru Atsuta et al.^[30].

Figure 16. Transmucosal Soft Tissue Structures in Subcrestally Placed Implants. This figure presents clinical images showcasing the transmucosal area related to the junctional epithelium and connective tissue (the biologic width) in subcrestally placed implants (SPIs), captured either after the removal or before the placement of the prosthetic parts. Compared to epi- or equi-crestal implant placements, these SPIs exhibit a larger area of transmucosal soft tissue. The right image distinctly identifies three layers: the subcrestal zone, the Transitional Zone (TZ), and the marginal zone, where the pale pink color closely matches the adjacent oral epithelium.

Figure 16. Transmucosal Soft Tissue Structures in Subcrestally Placed Implants. This figure presents clinical images showcasing the transmucosal area related to the junctional epithelium and connective tissue (the biologic width) in subcrestally placed implants (SPIs), captured either after the removal or before the placement of the prosthetic parts. Compared to epi- or equi-crestal implant placements, these SPIs exhibit a larger area of transmucosal soft tissue. The right image distinctly identifies three layers: the subcrestal zone, the Transitional Zone (TZ), and the marginal zone, where the pale pink color closely matches the adjacent oral epithelium.

Histologic Review of Connective Tissue in Dental Implants

Histologic evaluations of connective tissue in implant cases are challenging to obtain but essential for understanding tissue interfaces. This study utilized CBCT to analyze the dimensions of the Transitional Zone (TZ) in Subcrestally Placed Implants (SPIs), finding that minimal changes in this dimension indicate stable structural integrity and absence of bone loss. It suggests that maintaining a close proximity between the submucosal components of implants and the subcrestal bone is crucial for preserving tissue health, as confirmed by extensive research on connective tissue properties.

Figure 17. Quoted from 'Clinical Periodontology and Implant Dentistry, Sixth Edition'. This image illustrates key concepts discussed in the text, providing visual support for the theoretical knowledge presented.

Figure 17. Quoted from 'Clinical Periodontology and Implant Dentistry, Sixth Edition'. This image illustrates key concepts discussed in the text, providing visual support for the theoretical knowledge presented.

The connective tissue around dental implants plays crucial roles, contributing significantly to implant health and stability through the following functions:

• Structural Support and Nutrition: It provides a robust foundation and essential blood supply for the junctional epithelium, ensuring its proper function and health.
• Immune Response: Connective tissue facilitates immune reactions independently and supports the immune functions of the junctional epithelium. It also acts as a medium for the movement and transportation of cells and biochemicals, aiding systemic immune responses.
• Mechanical Sealing and Resilience: The connective tissue maintains a hermetic mechanical seal around the implant through its inherent positive pressure and resilience, attributes derived from its protein-carbohydrate macromolecular structure.
• Tissue Repair: It serves as a reservoir for stem cells and fibroblasts, which are crucial for ongoing repair and regeneration processes.
• Bone Protection and Homeostasis: Similar to the periosteum, connective tissue protects the underlying bone and aids in maintaining bone homeostasis, ensuring the longevity and durability of the implant.

These characteristics of connective tissue around implants are essential for successful integration and long-term success, providing mechanical stability and biological compatibility.

Probing Implants’ Sulcus: Evaluating Peri-Implant Soft Tissue Health

Probing the sulcus of dental implants, similar to natural teeth, involves using a periodontal probe with controlled force to measure the depth from the gingival margin to the deepest penetration of the junctional epithelium. Abrahamsson and Sodini found that a light probing force of 0.2 N reached similar depths in healthy tooth and implant sites, but in inflamed sites, the probe penetrates deeper into peri-implant tissues^[35]. The 2017 World Workshop advises that probing of dental implants should use approximately 0.25 N, aiming for pocket depths generally under 5 mm^[36].

Although automated periodontal probes were suggested, using metal probes on implants remains challenging due to the curvature in prosthetic designs^[37,38]. To assess the sealing and tightness of connective tissue, the author employed a probing technique using paper probes with a force range of 0.4-0.6 N. This method effectively demonstrated the health and resilience of peri-implant soft tissue, especially in SPIs, suggesting a substantial contribution to long-term implant stability and minimal bone change.

The roughness of implant surfaces affects bone to implant contact and initial survival rates but may increase bacterial adhesion and inflammation^[39]. Deep implant placement can protect against oral biofilm exposure, further enhanced by a robust soft tissue barrier.

Figure 18. Probing Peri-Implant Sulcus with Paper Points. This figure illustrates the technique of probing the sulcus depth around dental implants using paper points with a strength exceeding 0.6 N. The paper points are shown to penetrate easily into the sulcus of implants affected by peri-implantitis, with blood on the points upon withdrawal, signaling inflammation. In contrast, for healthy implants, the paper points do not penetrate the sulcus despite a longer soft tissue area, effectively demonstrating the integrity of the tissue and confirming the absence of inflammation. This visual comparison underscores the diagnostic value of paper points in assessing peri-implant health.

Figure 18. Probing Peri-Implant Sulcus with Paper Points. This figure illustrates the technique of probing the sulcus depth around dental implants using paper points with a strength exceeding 0.6 N. The paper points are shown to penetrate easily into the sulcus of implants affected by peri-implantitis, with blood on the points upon withdrawal, signaling inflammation. In contrast, for healthy implants, the paper points do not penetrate the sulcus despite a longer soft tissue area, effectively demonstrating the integrity of the tissue and confirming the absence of inflammation. This visual comparison underscores the diagnostic value of paper points in assessing peri-implant health.

Surgical Techniques Using Ready-Made Abutments for Subcrestally Placed Implants

To streamline the surgical process and enhance the biologic environment, using healing abutments that match the profile of the final ready-made abutments can eliminate the need for bone modification to accommodate abutment placement. Alejandro et al. noted the significance of the distance between the implant platform and the alveolar bone crest, which can dictate the choice of gingival collar height on abutments, influencing both mucosal barrier and bone stability^[45].

Maintaining optimal connective tissue space between the submucosal prosthetic components and the bone is crucial for leveraging the connective tissue's role effectively. While subcrestal implant placement often employs bone profiler drills to remove bony interferences and facilitate soft tissue accommodation, this approach may expose the mature bone to potential surgical trauma post-osseointegration, consuming additional time and energy. This drawback can be mitigated by employing a technique where matching healing abutments are placed during the initial surgery. Subsequently, for the final prosthesis, stock abutments with matching gingival cuff sizes are utilized to ensure seamless crown attachment, thereby reducing procedural complexity and enhancing tissue compatibility.

Figure 19. Connective Tissue Layers in Subcrestally Placed Implants. This figure illustrates the two distinct layers of connective tissue observed in subcrestally placed implants. The deeper layer, adjacent to the subcrestal bone, is a very thin, periosteum-like layer with a pale white color. The superficial layer, situated above the crestal bone, is thicker and appears red. This visual distinction highlights the varied structural composition of connective tissue in different zones around dental implants.

Figure 19. Connective Tissue Layers in Subcrestally Placed Implants. This figure illustrates the two distinct layers of connective tissue observed in subcrestally placed implants. The deeper layer, adjacent to the subcrestal bone, is a very thin, periosteum-like layer with a pale white color. The superficial layer, situated above the crestal bone, is thicker and appears red. This visual distinction highlights the varied structural composition of connective tissue in different zones around dental implants.

Figure 20. Comparison of Transitional Zones with and without Horizontal Extensions. This figure presents a comparison of two distinct scenarios in dental implant placement concerning the transitional zone (TZ). The first scenario showcases an implant without horizontal extensions of the TZ on either side, while the second includes horizontal extensions. The traditional interpretation of biologic width, typically considered vertical, is demonstrated at the peripheral end of the crestal bone where the junctional epithelium begins. This is similarly observed at the peripheral ends of the horizontal extensions of the TZ when present.

Figure 20. Comparison of Transitional Zones with and without Horizontal Extensions. This figure presents a comparison of two distinct scenarios in dental implant placement concerning the transitional zone (TZ). The first scenario showcases an implant without horizontal extensions of the TZ on either side, while the second includes horizontal extensions. The traditional interpretation of biologic width, typically considered vertical, is demonstrated at the peripheral end of the crestal bone where the junctional epithelium begins. This is similarly observed at the peripheral ends of the horizontal extensions of the TZ when present.

The Distribution and Angulation of the Transitional Zone

The slope of the transitional area in the connective tissue layer is typically oblique. When implant fixtures are placed at the epi-crestal level with sufficient horizontal crestal width (more than 2 mm), and the crown maintains close contact with its underlying soft tissue, it can function as a transitional zone as long as this intimate contact is maintained within a 1.0 mm range, for example. If this tight intimacy is not maintained, a gap may form where foreign material can intrude, causing the epithelial lining to extend until it reaches a healthy layer of connective tissue. If the crestal bone width is insufficient, maintaining intimate contact becomes challenging, requiring the crown at the cervical area to be thinner^[41]. This often results in a less natural emergence profile.

Figure 21. Visualizing Submucosal Areas during Abutment Repair. This figure shows the submucosal area exposed during the repair of a broken abutment neck after extensive use. It clearly depicts the Transitional Zone (TZ) and marginal zone, which extend obliquely or even horizontally along the mesiodistal sides. Notably, the subcrestal zone is absent on the buccolingual sides, indicating that the implant was initially placed at the epi-crestal level. This image provides a detailed view of the soft tissue layers affected by the placement level of the implant.

Figure 21. Visualizing Submucosal Areas during Abutment Repair. This figure shows the submucosal area exposed during the repair of a broken abutment neck after extensive use. It clearly depicts the Transitional Zone (TZ) and marginal zone, which extend obliquely or even horizontally along the mesiodistal sides. Notably, the subcrestal zone is absent on the buccolingual sides, indicating that the implant was initially placed at the epi-crestal level. This image provides a detailed view of the soft tissue layers affected by the placement level of the implant.

Need for Further Research on Subcrestal Connective Tissue Width

Further research is necessary to determine the optimal width of the subcrestal connective tissue space. If this width is excessively large, it may compromise its functional integrity, allowing for the invasion of foreign materials, including bacteria, into this critical area. Establishing the appropriate dimensions is essential to maintain the protective and structural roles of this tissue in dental implantology.

Figure 22. Natural-Looking Results Achieved by SPI Technique. This figure illustrates the effectiveness of the Subcrestally Placed Implants (SPI) technique in achieving results that closely mimic natural teeth. It demonstrates how the biologic width—comprising the transitional zone and subcrestal zone connective tissue—differs from that of natural teeth.

Figure 22. Natural-Looking Results Achieved by SPI Technique. This figure illustrates the effectiveness of the Subcrestally Placed Implants (SPI) technique in achieving results that closely mimic natural teeth. It demonstrates how the biologic width—comprising the transitional zone and subcrestal zone connective tissue—differs from that of natural teeth.

Paradigm Shifts in Implant Dentistry

The evolution of implant therapy has seen significant paradigm shifts, with the focus transitioning from primarily bone-centric to an emphasis on the stability of soft tissue, recognizing their interdependence^[43]. The optimal placement of implants is crucial as it influences the design of prosthetic components and ensures adequate space for soft tissue, impacting the overall therapeutic outcome.

Some experts argue that dental implants should ideally be placed 3 to 4 mm apical to the prospective gingival zenith on the restoration, cautioning that placing implants deeper can increase the risk of biological complications such as mucositis and peri-implantitis^[44]. However, this viewpoint often overlooks critical variables such as variations in soft tissue thickness, implant connection designs, and techniques for managing excess cement. Moreover, traditional analyses lacking a three-dimensional perspective may not accurately depict the true implant depth, potentially misrepresenting it as excessively deep on plain X-rays.

Figure 23. Illustrating the Scalloping Effect. This figure demonstrates the scalloping effect, emphasizing the importance of using the midfacial point, rather than the interproximal point, as the reference for implant placement. It shows that to accurately create a prosthesis with a natural emergence profile, the scalloping effect distance (approximately 2 mm) must be added to the vertical distance from the fixture-abutment joint to the crestal bone level as seen in plain X-rays. This adjustment ensures that the final prosthesis aligns correctly with the natural contour of the gingiva, enhancing both aesthetic and functional outcomes.

Figure 23. Illustrating the Scalloping Effect. This figure demonstrates the scalloping effect, emphasizing the importance of using the midfacial point, rather than the interproximal point, as the reference for implant placement. It shows that to accurately create a prosthesis with a natural emergence profile, the scalloping effect distance (approximately 2 mm) must be added to the vertical distance from the fixture-abutment joint to the crestal bone level as seen in plain X-rays. This adjustment ensures that the final prosthesis aligns correctly with the natural contour of the gingiva, enhancing both aesthetic and functional outcomes.

A three-dimensional analysis is essential, particularly due to the scalloping effect, where the interproximal gingival zenith appears more coronal than the midfacial zenith. This necessitates a more nuanced approach to implant depth evaluation to avoid biologic complications while achieving aesthetically and functionally sound results.

Gathering Soft Tissue Information for Prosthesis Fabrication

Traditionally, the primary information for fabricating dental prostheses comes from stone models, which replicate the oral mucosa using impression materials, or from oral scanning data. The design and dimensions of the prosthetic components that contact the tissue are influenced by the size and shape of the impression coping used. The choice of impression coping aids in determining the appropriate size and shape of the crown at the mucosal contact side. It's important to consider that during oral scanning, the soft tissue size may appear swollen due to the hydraulic positive pressure in the Transitional Zone (TZ) while the healing cap is removed for scanning. Conversely, the volume of the subcrestal connective tissue tends to remain relatively constant, providing a more stable basis for measurement and design.

Figure 24. Submucosal Swelling During Prosthesis Removal. This figure illustrates the temporary swelling of the submucosal area, which reflects the resilience and hydraulic positive pressure exerted by the connective tissue. This phenomenon typically occurs when prostheses or healing abutments are removed for temporary repairs or impression procedures, highlighting the dynamic nature of the tissue's response to external manipulations.

Figure 24. Submucosal Swelling During Prosthesis Removal. This figure illustrates the temporary swelling of the submucosal area, which reflects the resilience and hydraulic positive pressure exerted by the connective tissue. This phenomenon typically occurs when prostheses or healing abutments are removed for temporary repairs or impression procedures, highlighting the dynamic nature of the tissue's response to external manipulations.

Alejandro et al. suggested that the selection of abutment height, which influences marginal bone loss, should be guided by radiographic parameters—specifically, the distance between the implant platform and the alveolar bone crest at the time of implant placement. This approach supports the "one abutment, one-time" concept^[45]. Chiyun Won adjusted the selection of the definitive abutment based on the biologic environment, which includes subcrestal bone, connective tissue, and junctional epithelium, as demonstrated in this study. The design of the final restoration was incorporated into the surgical planning for implant placement, allowing the SPI technique to influence both the subcrestal bone morphology and the subgingival shape of the prostheses, actively molding the soft tissue in the Transitional Zone (TZ).

The TZ is present when the crestal bone, thicker than 2 mm, supports the overlying soft tissue and maintains intimate contact with the prostheses. Conversely, when the crestal bone is thinner than 2 mm, it cannot support a TZ, and only free-standing marginal gingiva is present, corresponding to the conventional biologic width which primarily consists of a vertical component.

Figure 25. Comparison of Marginal Zones with and without Transitional Zones in Dental Implants This image illustrates the differences in soft tissue around an implant. On the buccal side, unsupported soft tissue extending about 3 mm adheres to the conventional biologic width seen in natural teeth. In contrast, on the palatal side, the supported soft tissue includes a Transitional Zone (TZ) that maintains extended and intimate contact with overlying prostheses when healthy. Captured after a screw loosening incident ten years post-placement of an Astra Tech implant, the image shows no significant complications other than a metal tattoo in the soft tissue, resulting from friction caused by the abutment moving as the screw was unscrewed.

Figure 25. Comparison of Marginal Zones with and without Transitional Zones in Dental Implants This image illustrates the differences in soft tissue around an implant. On the buccal side, unsupported soft tissue extending about 3 mm adheres to the conventional biologic width seen in natural teeth. In contrast, on the palatal side, the supported soft tissue includes a Transitional Zone (TZ) that maintains extended and intimate contact with overlying prostheses when healthy. Captured after a screw loosening incident ten years post-placement of an Astra Tech implant, the image shows no significant complications other than a metal tattoo in the soft tissue, resulting from friction caused by the abutment moving as the screw was unscrewed.

This article introduces the concept of the Transitional Zone (TZ) of connective tissue to elucidate the phenomena occurring around Subcrestally Placed Implants (SCIs) from functional, structural, and biological perspectives.

Conclusions

Abbreviations

References

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