Inter-implant distances below 3 mm: interproximal bone stability up to 10 years with digital and radiographic comparative assessment of implant spacing

Purpose To evaluate interproximal bone stability in cases of inter-implant distance (IID) < 3 mm over a follow-up period of up to 10 years, and to assess the agreement between digital and radiographic methods for measuring implant spacing. Materials and methods This retrospective cohort study included 18 pairs of adjacent implants, featuring platform-switching and internal conical connection, placed with an IID < 3 mm and followed for a mean period of 7.32 ± 2.31 years. IID was determined using three approaches: periapical radiography with simple internal calibration, a composite radiographic calibration incorporating vertical and horizontal distortion factors of both implants, and a digital three-dimensional workflow based on intraoral scanning and STL superimposition. Peri-implant marginal bone levels and mid-interproximal bone height were evaluated relative to the implant–abutment junction (IAJ) and to a reference line connecting adjacent IAJs, respectively. Agreement between measurement methods was assessed using Bland–Altman analysis and intraclass correlation coefficients; equivalence testing was performed using two one-sided tests (TOST). Associations between IID and bone parameters were explored using Spearman correlation. Results The mean digitally measured IID was 1.81 ± 0.71 mm, with 55.6% of sites presenting an IID < 2 mm. Peri-implant marginal bone levels were coronal to the IAJ in 88.9% (32/36) of sites. Mid-interproximal bone was located 1.97 ± 0.88 mm coronal to the line connecting adjacent IAJs. No statistically significant association was found between IID and peri-implant or interproximal bone levels. IID measurement methods demonstrated a high agreement of ICC = 0.95; nonetheless, simple radiographic calibration tended to underestimate IID. At an equivalence margin of ± 0.1 mm, only composite calibration was interchangeable with the digital method. Conclusions Within the limitations of this retrospective cohort, inter-implant distances below 3 mm, with the majority below 2 mm, were not associated with interproximal bone loss after up to 10 years of function. The digital workflow enabled distortion-free assessment of implant spacing and provided a reference for evaluating radiographic calibration methods. The present findings warrant confirmation in prospective studies with larger cohorts.