Abstract
Fibrous dysplasia (FD) is an uncommon, benign bone disorder caused by a somatic mutation in the GNAS gene on chromosome 20, leading to impaired osteoblastic differentiation and the replacement of normal bone with structurally weak, fibro-osseous tissue. It is typically diagnosed during childhood, though some cases remain undetected until incidentally discovered in adulthood. We present a patient with symptomatic knee osteoarthritis and an incidental finding of monostotic femur FD. The presence of FD in this anatomical location complicates an otherwise straightforward total knee arthroplasty (TKA). A bone biopsy was performed to rule out malignant transformation of the FD. The resulting bone defect posed a risk of periprosthetic fracture due to the stress riser created by the biopsy. A long femoral stem was used to mitigate this risk, and the patient recovered fully. This case underscores the critical importance of meticulous planning in managing patients with incidental findings of potential bone tumors. Biopsy site selection must consider definitive surgical management while avoiding pitfalls, such as periprosthetic fractures that may arise from stress risers created by biopsy site fenestration.