Human clinical trials have documented individual bone sites reaching a substitution rate of 4.5% (and slightly higher) of strontium for calcium, though it is not a whole-skeleton average.
In clinical trials for the osteoporosis drug strontium ranelate, paired bone biopsies of postmenopausal women treated for 36 months showed that strontium was exclusively incorporated into newly formed bone. In these specific, newly mineralized bone packets, the mean substitution rate of calcium by strontium ions was 4.5%, with some individual tissue samples revealing that up to 10% of calcium atoms (1 out of every 10) were replaced by strontium in the apatite crystal lattice.
However, across the entire skeleton, the average retention is much lower—typically plateauing at around 1.5% to 2.0% of total bone tissue weight after 3 years of therapeutic dosing.
The DEXA Scan "Illusion": Strontium has a much higher atomic weight than calcium (87.6 vs. 40.1). Because it blocks X-rays far more effectively, every 1% of strontium substituted into the bone crystal causes a DEXA scan to overestimate bone mineral density (BMD) by roughly 10%. A person with a 5% substitution at a specific hip or spine site would see an artificially inflated BMD reading of up to 50%. Servier, the original manufacturer of strontium ranelate, stated that, after three years on strontium ranelate, BMD measured by DEXA scan could be inflated by 50%.
https://pubmed.ncbi.nlm.nih.gov/19874195/
https://pmc.ncbi.nlm.nih.gov/articles/PMC4708970/
