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A 21-year-old male ultimate Frisbee player presented with one year of anterolateral right knee pain that was worse with activity. There was a remote history of trauma. Physical exam was unremarkable except for tenderness at the superolateral patella. X-ray revealed a bipartite patella. MRI revealed mild cystic change, sclerosis and bone marrow edema around the synchondrosis of the bipartite patella. Bipartite patella is an uncommon anatomic variant that is often asymptomatic and discovered incidentally; however, it rarely can be a primary cause of knee pain, can be diagnosed with MRI, and is amenable to conservative and surgical interventions. Treatment options are varied and include conservative management, injections, and various surgical approaches.

Case Description: Patient presented for medial knee pain and swelling she noticed 3 weeks ago after mountain skiing. She did not recall any injures at that time, nor prior. On exam there was no instability with cruciate or collateral ligament testing. Full strength and sensation. Mild fullness of knee was appreciated. She had tenderness to palpation proximal to the medial joint line. A diagnostic ultrasound exam was performed of medial knee which revealed a well circumscribed hyperechoic mass within vastus medialis. An MRI was ordered and the patient was referred to an oncologic surgeon for biopsy.
Setting(s): Outpatient clinic.
Patient(s): 55F without significant PMH. Assessment/Results: MRI showed 2.7x1.7x1.5cm ovoid circumscribed indeterminate mass within the vastus medialis muscle. A CT scan was additionally ordered at the request of the musculoskeletal radiologist to help rule out internal calcification from traumatic/inflammatory conditions prior to performing biopsy. The CTscan was negative for internal calcium. Subsequent core needle biopsy result proved mass to be lowgrade spindle cell lesion, further characterized by fluorescence in situ hybridization testing to be nodular fasciitis. The patient elected to monitor mass at 6-month intervals for progression, with option for excision in future. MRI at 6 months showed interval decreased volume of mass.
Discussion(s):Without sonographic evidence of a mass, the working diagnosis for medial knee pain in a middle-aged woman with swelling after a skiing trip is traumatic arthropathy. If this patient had not been scanned, she could have gone through a course of conservative treatment including therapy with an incorrect diagnosis and possible exacerbation of symptoms.
Conclusion(s): This case illustrates the utility of diagnostic ultrasound in clinic to provide good patient care through accurate diagnosis