High-Grade Undifferentiated Pleomorphic Sarcoma of the Distal Femur Masquerading as a Medial Collateral Ligament Sprain in a Recreational Runner: A Case Report

A 62-year-old recreational runner with osteopenia presented to a sports medicine clinic with left medial knee pain following a non-contact hyperextension injury sustained during a training run.Physical examination demonstrated medial joint line tenderness with positive valgus and varus stress testing, indicating laxity of both collateral ligaments that, in retrospect, was inconsistent with an isolated medial collateral ligament (MCL) injury and out of proportion to the low-energy mechanism.Initial radiographs revealed a moth-eaten pattern in the distal femoral meta-diaphysis.MRI demonstrated a 4.4 × 6.6 × 9 cm mass in the left distal femoral meta-diaphysis with cortical breakthrough and soft-tissue extension.Ultrasound-guided biopsy confirmed high-grade undifferentiated pleomorphic sarcoma of bone (UPS-B), staged T2NxM1a (Stage IVA).While awaiting pathology results, the patient sustained a displaced pathologic fracture through the lesion.She had an inadequate response to neoadjuvant chemotherapy and ultimately underwent an above-knee amputation.UPS-B is a rare primary malignant bone tumor that can present with symptoms mimicking common sports injuries.This case highlights the importance of recognizing an inconclusive examination and obtaining radiographs in older active adults with atypical musculoskeletal presentations.