Hi Linh,
Interesting case.
In this case I suspect the thenar muscle atrophy is related to the underlying severe OA.
If the trapezoid has been removed there must have been severe STT joint OA changes.
These patients find it difficult to use their thumb in a normal way and I suspect the global atrophy of the thenar muscle group is disuse atrophy related rather than a primary neural problem.
The medial and ulnar nerves sound normal.
Steve