Working Diagnosis:
Chronic Myeloid Leukemia, medial collateral ligament tear

Treatment:
Patient was started on dasatinib, a targeted tyrosine kinase inhibitor.

Outcome:
After discussion of his results with a hematologist the patient was advised to present to the emergency department where he was admitted to the inpatient hematology service for further diagnostic testing and empiric treatment for tumor lysis syndrome.
He was discharged and later underwent outpatient treatment with a targeted tyrosine kinase inhibitor which he tolerated well. On his last visit to sports medicine clinic he had undergone 4 weeks of physical therapy and experienced significant improvements in his pain and functional status.
He was able to resume his preferred physical activity and exercise routine after using a hinged knee brace for a short period of time.
Patient followed up with his hematologist every 3 months and had an excellent response. He continued on dasatinib and tolerated this medication without significant side effects or complications. He underwent a formal course of physical therapy and continued to experience improvements in his pain and functional status. He was able to return to his regular level of physical activity prior to his injury.

Author's Comments:
Chronic myeloid leukemia is a malignant hematopoietic stem cell disorder which is characterized by cells carrying a Philadelphia chromosome which is a fusion of the BCR-ABL oncogene. This BCR-ABL gene results in a deregulated tyrosine kinase enzymatic activity that promotes growth and replication through downstream pathways which promote the bone marrow to produce stem cells which later go on to become granulocytes (neutrophils, basophils, eosinophils).
The median age of diagnosis is 56 years old. Since the introduction of imatinib in 2000, the annual mortality in CML has decreased from 10–20% to 1–2%.2 About 50% of patients diagnosed with CML in the US are asymptomatic thus the diagnosis of CML often occurs during a routine physical examination or blood tests.

Editor's Comments:
This case highlights the importance of ordering appropriate advanced imaging for an unstable joint, as well as further evaluation of abnormal musculature appearance on point of care ultrasound. Oftentimes, there are additional benign incidental findings related to bone or marrow, however in this case, there was a pathologic diagnosis that required additional lab evaluation and referral to a hematologic specialist. Differentials for musculoskeletal pain should remain broad, even in the acute setting with seemingly clear etiology.

References:
Knee pain: ACL, MCL, or CML?. Blood 2010; 115 (26): 5287. doi: https://doi.org/10.1182/blood-2009-04-190819

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