Correlation of the effect of restoration margin to alveolar crest distance on marginal bone loss in fixed implant treatments with keratinised mucosa width and phenotype

The aim of this cross-sectional study was to evaluate the effect of the restoration margin–alveolar crest (RM–AC) distance on peri-implant marginal bone loss (MBL) and probing depth (PD) in fixed implant treatments, and additionally to investigate the relationship of these parameters with keratinized mucosa width (KMW) and mucosal phenotype (MP) both clinically and radiographically. A total of 114 patients aged between 34 and 80 years and 531 implants were included. The RM–AC distance, PD, gingival index (GI), modified sulcus bleeding index (mSBI), plaque index (PI), MBL, KMW, and MP were recorded for all implants. Implants were categorized as RM–AC <1.5 mm and ≥1.5 mm; as inadequate KMW (<2 mm) and adequate KMW (≥2 mm); and as thin or thick MP. Clinical and radiographic data were analyzed statistically using Mann–Whitney U, Kruskal–Wallis, Chi-square, and Spearman correlation tests. Of the sample, 27.1% (n=144) were classified as RM–AC <1.5 mm and 72.9% (n=387) as ≥1.5 mm. In these implants, PD was greater (p<0.05) and MBL was more pronounced (p<0.05). Among the participants, 30.1% (n=160) had KMW <2 mm, while 69.9% (n=371) had KMW ≥2 mm. Individuals with inadequate keratinized mucosa exhibited a significantly higher risk of peri-implantitis and mucositis (p<0.05). No significant relationship was found between PD and KMW (p>0.05). However, implants with KMW <2 mm showed significantly higher GI, mSBI, and PI scores (p<0.05), and greater MBL (p<0.05). Regarding mucosal phenotype, 51.0% (n=271) of the implants were surrounded by a thick phenotype, and 49.0% (n=260) by a thin phenotype. A significantly higher incidence of peri-implantitis was observed in sites with a thin keratinized mucosa phenotype (p<0.05). No association was found between mucosal phenotype and PD (p>0.05). GI, mSBI, and PI scores were significantly higher in the thin mucosa phenotype group (p<0.05). In conclusion, this study found that an RM–AC distance <1.5 mm, inadequate keratinized mucosa, and a thin mucosal phenotype were significantly associated with peri-implantitis and mucosal inflammation. RM–AC distance <1.5 mm was also found to contribute to marginal bone loss. Furthermore, these unfavorable soft tissue conditions, combined with increased probing depth, plaque accumulation, and inflammatory indices, resulted in greater marginal bone loss.