Soft tissues around long-term platform-switching implant restorations: histological evaluation in humans

Background: Although the Literature emphasized appealing short-term radiographic outcomes of implants restored according to Platform Switching (PS), a lack of evidence on histological soft tissue response in humans was noted. Aim: This randomized controlled trial aimed to histologically evaluate the long-term effects of PS restored implants on soft tissue surrounding the implant/abutment interface. Materials and Methods: 40 implants were inserted in posterior maxilla of 15 patients. Implant sites were randomly treated as following: standard diameter implant restored using matching-diameter abutment (control), wide diameter implant restored according to PS (test). Thirty-six months after prosthetic rehabilitation, subjects underwent a peri-implant soft tissue biopsy. Additionally, peri-implant radiographic bone levels and clinical parameters on implants and adjacent teeth (BoP, PPD, mPlI, Keratinized tissue) were measured. Procedures were approved by an ethical committee. Sections were stained with hematoxylin/eosin to evaluate the tissue morphology in general and with Sirius Red to measure the collagen content in the connective tissue compartment. Immunostaining for CD31 was performed on four sections to highlight blood vessels. All sections underwent histomorphometrical analysis. Results: 4 patients dropped out at the end of the study. A total of 28 samples were analyzed (Control:7 Test:21) Peri-implant radiographic bone levels confirmed statistically significant differences between groups [Control:1.56mm (SD: 0.35), Test:0.80mm (SD=0.47mm)]. In most samples of all groups, a small concentrated population of lymphocytes (ICT) was mainly localized in the connective tissue close to the junctional epithelium [Control:0.26mm2 (SD:0.04), Test:0.15mm2 (SD:0.1)]. The remaining peri-implant connective tissue presented few scattered lymphocytes and macrophages. All samples showed microvessels mainly distributed underneath the oral epithelium and the vascular density decreased in the deep connective tissue [Control:10.66% (SD:4.5), Test:10.7% (SD:3.8)]. With polarized light, the collagen fibers under the oral epithelium were thick and closely packed and appeared well-oriented in a perpendicular structure of bundles. [Control:60.7 (SD:14.2), Test:62.3 (SD:11.5)]. No difference in ICT, microvascular density and collagen content was evident between groups. For ICT, microvascular density and collagen content, the correlation coefficients to the clinical variables (BoP, PPD, mPlI and keratinized tissue) resulted not significant. Conclusions and clinical implications: this study, the first histological one on a relevant implant sample on human being, showed that 36 months after restoration the peri-implant soft tissue around test and control sites had similar histological characteristics, confirming platform switching as a safe prosthetic concept leading to better maintenance of peri-implant bone levels. However, further histological studies are required to longitudinally confirm the present data.