Analyzing the Transitional Zone of Peri-Implant Soft Tissue in Subcrestally Placed Implants: A 3D CBCT-Based Model for Esthetics and Biological Stability in Molar Cases
Authors: Won Chiyun 
Published: January 2025

Objectives
This study aimed to evaluate the outcomes of the Subcrestally Placed Implants (SPI) with Internal Platform Switching (IPS) design in single molar replacements utilizing the matching abutment technique. The investigation focused on achieving biological stability and esthetics while proposing a schematic model to seek the relationship between the biological stability of peri-implant soft tissue and crestal bone.

Materials and Methods
Twenty cases of SPI with IPS design were evaluated. Panoramic radiographs were employed to assess changes in crestal bone levels at the mesial and distal sides over an average follow-up period of 3.4 years, providing a practical method for routine monitoring. CBCT scans, acquired at least one year after surgery, were comprehensively analyzed to determine the dimensions of peri-implant soft tissue in a three-dimensional context. Parameters assessed included:

Placement depth (PD), both central (cPD) and peripheral (pPD).
Soft tissue thickness (STT), both central (cSTT) and peripheral (pSTT).
Transitional Zone (TZ) length at the mesial, distal, buccal, and lingual aspects.

To evaluate the health of peri-implant soft tissue, the following clinical assessments were performed:

IPPP (Implant Paper Point Probe)
Bleeding on Probing (BOP)
Visual Assessment for redness and swelling

Results
1. Panoramic Analysis
Panoramic radiographs revealed stable crestal bone levels with minimal changes, with mean pPD change observed to be less than 0.1 mm over an average follow-up period of 3.3 years.
2. CBCT Analysis
o The average central placement depth (cPD) was 1.4 mm, and the peripheral placement depth (pPD) was 2.7 mm.
o The average central soft tissue thickness (cSTT) was 0.3 mm, and the peripheral soft tissue thickness (pSTT) was 0.6 mm.
o The average Transitional Zone (TZ) length was 4.3 mm.
3. Clinical Observations
o Depth of Probing: Probing depths were consistently less than 1 mm in all cases, demonstrating minimal pocket formation and stable peri-implant conditions.
o Bleeding on Probing (BOP): No bleeding was observed in any case, suggesting an absence of inflammation around the implants.
o Visual Signs of Inflammation: No swelling, redness, or other visual indicators of inflammation were detected, reflecting healthy peri-implant soft tissue in all cases.

Conclusions
The findings of this study demonstrated that subcrestally placed implants (SPI) with an internal platform switching (IPS) design, utilizing the matching abutment technique, provide stable outcomes, particularly in terms of crestal bone levels and peri-implant soft tissue dimensions. This stability is likely attributed to the biological and structural integrity at the soft tissue-implant restoration interface (Transitional Zone). The study investigated the three-dimensional topography of peri-implant soft tissue, emphasizing its relationship with implant restoration, crestal bone changes, and clinical outcomes. Further histological research is necessary to validate the unique characteristics and mechanical properties of these zones, offering deeper insights into their contribution to long-term implant success and enhanced esthetics.