Background: A 5-year longitudinal cohort study was carried out to evaluate the influence of anatomical crown to implant ratio (CIR) on peri-implant marginal bone level (MBL) in single implants. Materials and Methods: The longest possible implants, according to the availability of pristine bone, were inserted, one per patient, among periodontally healthy teeth in consecutively recruited subjects. CIR and MBL changes were measured on standardized radiographs. The relationship between MBL and multiple predictors was investigated. A statistical analysis suitable for mixed type distributions was conducted: for the discrete component a logistic regression model was used and for the continuous component the impact of the variables on MBL was examined by using robust nonparametric comparison tests. Results: Seventy-eight dental implants were inserted in 34 mandibles and 44 maxillae, with one stage procedure in 40 cases and two stage in 38 cases. Thirty-five implants were <10 mm, while 43 were ≥ 10 mm long; 28 implants had a CIR ≤1 and 50 had a CIR >1. No drop-outs or implant loss were observed. Bone loss occurred only in a few cases, measuring less than 0.5 mm and being significantly more pronounced for implant length ≥10 mm, for lower CIR values and for the two stage procedure. Conclusion: Higher CIR values were not related to increased peri-implant bone loss; a <10 mm long implant insertion may be safely considered for reduced bone heights.
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