Peri-implant pocket depth was measured at four sites per implant (mesial, buccal, distal, and lingual) using a pressure sensitive probe (KerrHawe Click Probe®, Bioggo, Switzerland) (probe force of 0.25 N). Bleeding and suppuration were scored up to 30s after pocket probing. Microbiological peri-implant sulcus samples were collected from each implant with peri-implantitis using four sterile paperpoints per implant. Paperpoints were collected in a vial containing RTF and were analyzed in the same manner as the intra-operative samples. Outcome variables were total anaerobic bacterial load and the presence and numbers of the periodontal pathogens A. actinomycetemcomitans, P. gingivalis, P. intermedia, T. forsythia, F. nucleatum, P. micra, and C. rectus.

Randomization

Fourteen notes with the word “phosphoric acid” and 14 notes with the word “saline” were put into 28 identical, sequentially numbered, non-transparent envelopes according to a randomization list generated by a computer program. The envelopes were irreversibly sealed. During the surgical procedure, after flap deflection and mechanical cleansing, the surgeon temporarily left the operating room. The surgical assistant opened an envelope and prepared the materials as needed according to the information on the note. A third person (YDW) performed the decontamination procedure according to group allocation. The materials were removed, and the surgeon continued the surgical procedure. The researcher (performing the clinical measurements, DH) was blind to treatment allocation and did not have access to the randomization code until the end of the research period.

Statistical methods

Sample size

Sample size was based on the microbiological data from a previous study evaluating the effect of implant surface decontamination with a chlorhexidine solution versus a placebo solution [10]. The decontaminating effect of phosphoric acid was expected to be similar to the decontaminating effect of chlorhexidine (reduction in log-transformed mean anaerobic bacterial load = 4.21 (chlorhexidine group) versus 2.77 (placebo group), SD = 2.12).