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Tendons and ligaments are notoriously poorly vascularized and do not heal together again on their own. The fact that one has come loose could be due to age (osteoporosis therefore not good bone to anchor in), activity (highly active in ways that stress those tendons), or some sort of accidental injury among other things.
After 6 months a healthy bone should have been able to retain the anchor. I suspect that you may have some degree of osteoporosis although it may not be enough to have effected you in other ways. This is a common problem among post menopausal women. It would be a good idea to ask the doctor to do a bone density test to see how strong the bone is. Of course, there is always the possibility of a manufacturer's defect or something wrong with placement not detected on X Rays taken during surgery. These are the least likely of the whole list of things to have happened. The manufacturer will most likely need to be notified by your doctor to see if there has been some sort of recall, etc of this type of anchor. Not knowing which he used, I will mention that most are a matter of heavy non dissolving suture is placed through the tendon and that suture is permanently attached to a bone anchor at the manufacturer's. This bone anchor was placed by your doctor with the suture already attached to the anchor and then the anchor was placed either before or after the tendon was sutured to it. It is also possible that the suture pulled out of the tendon.
As you can see there are a number of reasons this could have happened. If it was the actual bone anchor itself that pulled out, it would be a good idea to have that bone density scan done.
I hope this helps you understand the processes involved and what may have happened. I would be greatly appreciative of any feedback you could leave after ACCEPTING my answer.