 Many women with breast cancer are treated with a class of drugs called aromatase inhibitors mainly letrozole or anastrozole, which lowers the amount of estrogen being produced in the body. Women on aromatase inhibitors appear to experience joint pains and arthralgia. The aim of this study is to determine whether the joint pains experienced by some women on aromatase inhibitors is associated with more defects in their cartilage, compared to women not receiving this therapy. Using the magnetic resonance imaging MRI technique, the knee joint will be examined to assess changes in cartilage volume over time. 