The main sources of ALP in GCF are neutrophils, bacteria within dental plaque, fibroblasts, and osteoblasts [28]. Plagnat et al. [29] suggested that longitudinal monitoring of ALP in PICF might confirm its possible use as a marker of implant failure. Considering the change in the median values of the ALP level over time, in the test group, the ALP level decreased at 1–4 weeks and then increased at 6, 8, 10, and 12 weeks. These results are similar to those of a previously reported animal study of gene expression of ALP during the osseointegration period [30]. However, no significant differences in the ALP level were noted over time. This may be due to the variation of ALP sources in PICF and/or the small sample size at each measurement in this study.

Osteocalcin or bone gamma-carboxyglutamate protein is a noncollagenous protein in the bone matrix. It is a small (6000 Da) polypeptide. Monjo et al. [30] studied 372 implants in 93 rabbits, and suggested that osteocalcin is the best biological marker for implant osseointegration after 4-week healing periods. In our study, there were statistically significant increases in the OC level at 6, 8, 10, and 12 weeks when compared with 1 week. These results are similar to those of previous clinical studies. Slotte et al. [31] studied gene expression of bone healing in PICF using a quantitative real-time PCR technique [31]. Their pilot study demonstrated that gene expression for OC was high 7 weeks after implant placement. Prati et al. [32] studied the release of bone markers during the osseointegration period using the Luminex assay. They revealed that OC level was high 8 weeks after implant placement in their nonloaded group.

Regarding the relationship between the ISQ values and ALP or OC levels, although the ISQ values were weakly correlated with the bone markers (r = 0.226 for ALP level and r = 0.245 for OC level, P = 0.05), there were significant and positive correlations between the ISQ values and ALP or OC levels at all measurements from week 1 to week 12. These results are in harmony with the findings of a previous clinical study that investigated gene expression of bone markers in PICF [31]. That pilot study showed that ALP gene expression was correlated with RFA at week 4 (r = 0.44, P = 0.03), and OC gene expression was correlated with RFA at week 2 (r = 0.5, P = 0.04). To our knowledge, correlations of the ISQ value with the bone markers which were detected at protein level in PICF are reported for the first time.