Hi Dave,
It is really common for these 2 joints to be involved in arthritis and synovitis together and hence we often inject the 1st CMC and STT joints at the same time.
I have attached some images from my teaching material.
Scan the FCR tendon in the long axis and you can see it passing between the scaphoid and the trapezium. At this level inject between the bones and you have done an STT joint injection.
Some may flow into the FCR tendon sheath and I think this is OK as synovitis of this sheath goes along with STT joint arthritis and the sheath often communicates with the degenerative joint.
The photo I have attached is from a journal article, however I am concerned about the location of the radial artery if you use such a flat approach so we use a more vertical approach and check the location of the radial artery.
Steve.