Hi Johannah,
This is a great example, and lovely images.
It is indeed isolated wasting of teres minor. It most likely represents unilateral axillary nerve damage (quadrilateral space syndrome).
You sometimes see deltoid atrophy as well, however this is variable.
The common causes are a prior dislocation or surgery.
But there are other possibilities.
It might be a case of Parsonage Turner syndrome which is a brachial plexus neuritis which is usually self limiting and resolves. It loves the supra scapular nerve where you get wasting of supra and infra in a young person, but is can involve the axillary nerve.
I have also personally seen many cases of isolated teres minor wasting , both unilateral and bilateral in patients with no relevant symptoms with no history of trauma or surgery where I scratch my head and wonder why!
Speaking to my radiologists they also occasionally encounter it for no apparent reason.
I suspect the axillary nerve is sometimes a little unreliable with its work ethic !!!
Thanks so much for your lovely post and images,
Please enjoy the website,
Steve