A 53-year-old male laborer presents to his primary care physician with complaints of acute onset of left knee pain. He has had mild episodes of knee pain in the past and is two years status post a left partial medial meniscectomy. He has had mild relief with the use of anti-inflammatories. His past medical history is significant only for hyperparathyroidism and mild hypertension. He denies any fevers or chills. His exam reveals a moderate knee effusion and diffuse pain and tenderness with palpation and range of motion. Weightbearing radiographs are shown below. The most likely etiology of the patient's knee pain is characterized by which finding?