That is a really perfect ultrasound image and it is technically difficult to obtain,
You have worked out the secret of heel toeing the transducer firmly into the palm of the hand to show that distal FCR insertion to the base of the 2nd M/C
Yes indeed I agree with your bony annotations and if you see bony change in the trapezoid on this image it helps to make the diagnosis of STT arthritis.
In clinical practice for me the scaphoid and the trapezium are far easier to assess than the trapezoid and hence I tend to make the diagnosis of STT arthritis on these 2 bones.
But you will certainly see it on the trapezoid as well if you get a great leek like you have achieved.
Remember always to review the plain radiographs as well before starting the ultrasound as the answer is often on these beautiful old fashioned images.
I do love a plain radiograph!
Steve.