Hi Jean:  Many people worry about osteonecrosis of the jaw, so you and your dental hygenist are not alone.  However, the risk of contracting this from oral bisphosphonates (Fosamax) are very small.

Here's a link from the ADA (American Dental Association) that explains the small risk associated with oral bisphosphonate use, and the recommendations for those preparing for dental surgery while taking this drug.

Intravenous use of bisphosphonates put the patient at a slightly higher risk of contraction, but even those numbers are low, when you consider how many people worldwide take these medications to prevent fractures.

I would talk to your Dentist and ask about this to see what he or she has to say about taking Fosamax, and if your hygenist has some specific information about your current dental health that would lead her to believe that if you had oral surgery you wouldn't heal.  Since this condition is rare, I would be interested to know why the hygenist has come to this particular conclusion.

Good luck...