Great case Sara,
And amazing images!
I was going to suggest the plate was normal until I saw the video.
I think on the video you can clearly see blood of the haemarthrosis passing between the volar plate and where it should be attached to the pharyngeal base. So on the basis of this I would have to assume that the plate has been at least partially torn from the phalangeal base without any significant avulsion fracture being present. I think this also explains the slightly sublimed posture of the joint.
The images of the collateral ligaments on both the radial and ulnar aspect are also abnormal with loss of the expected fibrillar architecture and a tiny avulsed fragment on the radial side.
So in summary it is a joint capsule injury with volar plate avulsion and strain injuries to the radial and ulnar collateral ligaments. The haemarthrosis suggests an underlying bony injury, however the volar plate avulsion may be enough to cause the intra-articular bleeding.
Do you mind if I use the video as it is an amazing example of dynamic assessment leading to the correct answer.
Great work!
Steve.