Chronic Ankle Instability: This condition often follows a significant ankle sprain, especially if the initial injury was not adequately rehabilitated. The presence of 1+ edema without pain suggests a possible chronic condition where the ankle joint's stability is compromised, leading to persistent swelling.

Residual or Undiagnosed Osteochondral Lesion: Although initial imaging was negative for fracture, an osteochondral lesion (a piece of cartilage and bone that can become damaged or detached) could have been missed, especially if the imaging was not of high quality or if the lesion was not visible at the time. These lesions can cause chronic symptoms, including swelling.
Synovial Impingement or Hypertrophy: Chronic inflammation of the synovial tissue in the ankle can lead to hypertrophy and impingement, causing swelling and potentially pain, although the latter is not present in this case.
Overuse or Repetitive Strain Injury: If the child has been engaging in activities that repeatedly stress the ankle, this could lead to chronic swelling without significant pain.

Osteomyelitis or Chronic Infection: Although less likely given the absence of pain and systemic symptoms, chronic infection of the bone or soft tissues could present with swelling alone. This diagnosis is critical to rule out due to its potential for serious complications.
Tumor (Benign or Malignant): A tumor in the ankle region could cause chronic swelling. While rare, this diagnosis is crucial not to miss due to the potential for malignancy.

Chronic Regional Pain Syndrome (CRPS) Type I: Previously known as reflex sympathetic dystrophy, this condition can follow trauma and is characterized by chronic pain, swelling, and changes in skin temperature and color. The absence of pain makes this less likely, but it's a consideration if other symptoms develop.
Hematologic Disorders (e.g., Hemophilia): Certain bleeding disorders could lead to chronic swelling due to recurrent hemarthrosis (bleeding into joints). This would be more likely if there were episodes of significant pain or if there's a family history of such conditions.
Neurogenic Causes: Rarely, chronic swelling can be associated with neurological conditions affecting the sympathetic nervous system, such as complex regional pain syndrome or other autonomic disorders.