1. What do you think has happened?

Rupture of EPL (Extendor Pollicis Longus)

2. How has it happened?

Swelling around the undisplaced fracture site has resulted in a very local "compartment syndrome" resulting in tendon ischaemia and tendon rupture. The patient confirmed that there had been local swelling and that she had only noticed the failure of thumb movements later. She described it as being like a snapped elastic band.

3. How would you manage this?

Refer to Hand Surgeon for repair, possibly transposition of the Extensor Indicis tendon. She decided to continue wearing a future splint in the meantime.

Further reading:
http://www.wheelessonline.com/ortho/extensor_pollicis_longus_rupture