Aneurysmal bone cyst, unspecified forearm

This code signifies a benign, expansile, and often hemorrhagic bone lesion characterized by blood-filled spaces separated by fibrous septa. It typically affects the metaphysis of long bones, but in this instance, it specifically refers to an unspecified forearm location. While benign, it can cause pain, swelling, and pathological fractures due to bone destruction.

Apply this code when documentation confirms a diagnosis of an aneurysmal bone cyst located in the forearm, and the laterality (right or left) is not specified. This diagnosis is often made based on imaging studies (X-ray, MRI) and confirmed by biopsy. Use this code when the provider has not indicated a more specific anatomical site within the forearm.

• Ensure documentation clearly states "aneurysmal bone cyst" and specifies the forearm as the affected site.
• Avoid using this code if laterality (right or left forearm) is documented; instead, select the more specific M85.531 or M85.532.
• Differentiate from other bone cysts like unicameral bone cysts (M85.6-) or fibrous dysplasia (M85.0-), which have distinct pathological features.