Case: A seventy-three-year-old woman who had been given the tentative diagnosis of spontaneous osteonecrosis presented to our clinic for a second opinion. Additional imaging and evaluation revealed the lesion to be a subchondral metastasis consistent with breast cancer. She received proper oncologic treatment and subsequent total knee arthroplasty. Conclusion: Orthopaedic surgeons should be aware of the possibility of metastatic disease manifesting itself in the form of an isolated subchondral lesion about the knee. For patients with such lesions, treatment in the form of total knee arthroplasty is a viable option that can provide excellent pain relief and function.