Interesting case Steph,
I don’t think there is an ancones epitrochlearis present as I can see the medial head of triceps muscle and there is nothing superficial to the ulnar nerve.
Anatomically the anterior band of the ulnar collateral ligament sits really close to the ulnar nerve as you head slightly distal to the medial epicondyle of the humerus.
It is tricky without scanning myself, but anatomically it looks like a medial joint capsule injury involving the proximal origin of the UCL from the medial epicondyle.
This is something I never understood until a couple of years ago, that the UCL anterior band is so intimate to the ulnar nerve.
I have seen many cases in Taiwan with baseball pitchers who develop ulnar neuropathy due to this type of medial elbow joint capsule injury.
I think you can see some enthesis damage related to the capsule origin as well on your images.
So I think it less likely to be an Osborn ligament and more likely to be a medial joint capsule injury at the UCL proximal origin just posterior to where we take out usual picture. Remember it is part of the joint capsule so where we take out picture is not the whole story and the capsule extends back to where your injury is,
Nice one,
Steve