Aneurysmal bone cyst, unspecified ankle and foot

This code signifies the presence of an aneurysmal bone cyst (ABC) affecting an unspecified ankle or foot. An ABC is a benign, expansile, osteolytic bone lesion characterized by blood-filled spaces separated by fibrous septa, often presenting with pain, swelling, or pathological fracture.

Use this code when documentation confirms an aneurysmal bone cyst in the ankle or foot, but the specific laterality (right or left) is not documented or cannot be determined. This applies to initial diagnoses, follow-up care, or surgical interventions for the condition.

• Avoid using this code if laterality is specified; instead, use the more specific codes for right (M85.571) or left (M85.572) ankle/foot.
• Ensure the documentation clearly identifies the lesion as an aneurysmal bone cyst, often confirmed by imaging (X-ray, MRI) and/or biopsy.
• Consider sequencing this code as a primary diagnosis if the ABC is the reason for the encounter, or as a secondary diagnosis if it's an incidental finding or comorbidity.