A 64-year-old woman had undergone bilateral total knee arthroplasty in 1998. In 2010, she presented with a large, painless, diaphyseal soft-tissue mass of the lower leg. She indicated that she had no history of knee pain, trauma, or infection. Ultimately, the mass was found to be a synovial fluid-filled cyst that communicated with the knee joint, which was a result of severe osteolysis.Conclusion:
Large diaphyseal tibial masses in the presence of total knee arthroplasty should raise a high index of suspicion not only for tumors and infections, but also for severe osteolysis. Knowledge of the various ways that osteolysis can present as well as an appropriate workup will help to guide diagnosis and management.