MON-510 Progression of Atypical Femur Stress Fracture after Discontinuation of Bisphosphonate Therapy

Background: The risk of bisphosphonate (BP)-induced atypical femur fracture (AFF) is believed to be markedly reduced following BP discontinuation. In this case, we report development and worsening of incomplete AFF occurring years after BP was discontinued. Clinical Case: A 70-year old woman with osteopenia presented with atraumatic, unilateral, right mid-thigh pain (dull, achy, worse with weight-bearing). Prior and current prescription medications included alendronate (over 8 years duration), estrogen for menopausal symptoms, levothyroxine and antihypertensive medications. Initial X-ray of the right femur revealed focal thickening of the lateral mid-shaft femoral cortex. Radionuclide bone scan demonstrated focal scintigraphic uptake consistent with stress fracture at that site and also contralateral uptake in the left subtrochanteric region. X-ray of the left femur showed thickening of the lateral cortex extending to the subtrochanteric region, but more extensive and slightly irregular thickening of the mid-femur lateral cortex. Urine N-telopeptide was 17 nmol BCE/mmol creatinine and osteocalcin was 8.4 ng/mL. Alendronate was discontinued but estrogen was continued due to ongoing menopausal symptoms. One year later, the irregular thickening of the left lateral cortex was noted to be more prominent. Urine N-telopeptide was 15 nmol BCE/mmol creatinine and osteocalcin was 12 ng/mL. Four years after initial presentation, a right horizontal linear translucent defect (dreaded black line) became evident within the area of cortical hypertrophy, thus, meeting radiographic criteria for AFF. Because of these findings, the patient underwent prophylactic intramedullary rodding of the right femur and estrogen was discontinued. Over the ensuing 3 years (total of 7 years after initial presentation), x-ray showed progression of the irregular cortical thickening in the left femur, with development of 3 focal areas of lateral cortical protuberance, each showing a horizontal linear translucent line. The patient was started on teriparatide for 1 year, with subsequent imaging showing resolution of the translucent defects, but persistence of pronounced abnormal cortical thickening. Conclusion: Our case illustrates the importance of long-term follow-up in patients who have complete or partial AFF. Monitoring is particularly important in cases with extensive and irregular cortical thickening, since partial AFF can progress and worsen even years after discontinuation of BP and other antiresorptive medications. In such cases, teriparatide may be a treatment option to enhance bone turnover, thus allowing improvement in bone repair mechanisms.