Diagnosis:
Tenosynovitis

Findings: (Images 1 - 6)
The dominant finding is the presence of a moderate amount of fluid within the extensor pollicis longus tendon sheath. In light of clinical history, this accounts for the "lump" on the dorsoradial aspect of the wrist. No evidence of high-grade extensor pollicis longus tendinopathy or tendon macrotear. A small amount of sheath fluid is identified in the extensor carpi radialis longus and brevis tendon sheaths.

A residual, transversely oriented fracture line is seen in the distal radial metaphysis parallel the growth plate. This fracture demonstrates near complete healing, with a mild degree of residual marrow edema about the fracture site.

Discussion:
The findings in this case are consistent with tenosynovitis. This may be related to recent cast or splint placement for fracture healing, versus altered mechanics post-trauma. Other etiologies include infectious, overuse, rheumatoid arthritis, and autoimmune disease.