"Repeat Bone Mineral Density Screening and Prediction of Hip and Major Osteoporotic Fracture" 
JAMA Sep 25
http://jama.jamanetwork.com/article.aspx?articleid=1741821 
"Conclusions and Relevance  In untreated men and women of mean age 75 years, a second BMD measure after 4 years did not meaningfully improve the prediction of hip or major osteoporotic fracture. Repeating a BMD measure within 4 years to improve fracture risk stratification may not be necessary in adults this age untreated for osteoporosis. . . . "