Here is the most recent analysis I could find of the various studies done in the past few years: "Melatonin effects on bone: Implications for use as a therapy for managing bone loss." The full article is here:
https://onlinelibrary.wiley.com/doi/full/10.1111/jpi.12749
I waded through it, and the findings look promising, but need further research. Here was the author's conclusion:
"3 CONCLUSION
Osteoporosis is a significant economic burden comparable or greater than that of a range of chronic noncommunicable diseases (ie, rheumatoid arthritis, asthma, and hypertension).79 The mortality risk related to hip fracture for a 50-year-old woman is 2.8%, which is equivalent to her risk of death from breast cancer and four times higher from endometrial cancer.79 As in women, the mortality rate in men after hip fracture increases with age (even more than women) and is highest in the year after a fracture.79, 84 The mortality rate in men approximately doubled over the first 6 months compared with that in similarly aged women.79, 84

The uniqueness of melatonin as a molecule, which ranges from effects on melatonin (MT2) receptor-mediated osteoblastogenesis through MAPK/Wnt-beta catenin pathways,21 to osteoclast-mediated inhibitory effects through direct151, 173 or indirect (osteoblast-mediated RANKL) melatonin actions,119 to antioxidant properties through mitochondrial melatonin receptors22, 23 or through melatonin's direct actions on mitochondrial membranes,23 induction of antioxidant enzymes, and/or free radical scavenging,18, 26-28, 40, 113, 219-222, 235, 236 makes it an ideal if not essential molecule to promote bone health. The recent discovery of mitochondrial melatonin receptors22, 23 provides fertile ground for future research in understanding their role in melatonin-mediated processes in the body “in general” but especially in bone. These properties of melatonin coupled with its actions on the immune system, circadian entrainment, quality of life, safety profile, and cost284 argues strongly for its use clinically to slow the progression of bone loss in “at risk” populations (eg, aging population, menopausal women, RA, shift workers) or augment bone mass in those with osteopenia, osteoporosis or who have suffered a fragility fracture."

So...what does this mean for us? Well, for most people, it would appear that a small nightly dose of melatonin (3mg) could be helpful, if they have or are trying to prevent osteopenia. Not as clear if one already has osteoporosis, and there is no analysis or study I can find about adding melatonin to supplement other osteoporosis treatment. My bone scores are pretty good for my age (over 70), size and family history. But several family members developed severe osteoporosis in their late 80's. So I will probably add melatonin to my regimen after I run it past my PCP.

Do you already have a diagnosis of osteoporosis? Are you taking medication already?
Sue