This is a case of a 17-year-old male patient with a chief complaint of nagging pain in his left knee. They sought a consult to a physician, and the initial radiograph was read as unremarkable. One month after, despite the intake of the prescribed pain medications, his condition persisted and worsened. Hence, they sought a second opinion from a specialist. He was then immediately scheduled for a needle biopsy and revealed high-grade osteosarcoma. The patient underwent three cycles of neoadjuvant chemotherapy and was subsequently scheduled for limb salvage surgery using a tumor prosthesis. The tumor necrosis for the specimen has less than 90%; hence, a new set of chemotherapy regimens was used for the subsequent three cycles. The latest follow-up revealed an excellent integration and bony ingrowth around the prosthesis.