I can almost believe the connection on the axial MRI, but the sagittal is an eye of faith for me.
When asked to tackle these I simply go through the volar forearm anatomy looking for any variations.
It is surprising how many variations you observe in the general population and the variation you have bought up for discussion is not that rare, but usually not symptomatic.
I agree the clinical test is easy. If you hold the fingers in extension and the patient can not fully flex the thumb they have it.
I think it is cool that it is all anterior interosseous nerve territory. That nerve only has 3 jobs and 2 of them pertain to the muscles that get connected (FPL and FDP index), the other muscle it supplies id pronator quadratis.
We all know how easy it is to follow FLB from the thenar eminence where any idiot can see it, through the carpal tunnel where it is always the most radially placed tendon, into the forearm where it becomes muscular and the muscle is always immediately anterior to the radius so easy to identify and follow.
FDP index is easy to pick up in the carpal tunnel by flexing the DIP jt of index only and you will see it move.
Follow it proximally watching its relationship with FPL and look for any collagen bridge between them or muscle variation. They sit next to each other anatomically.
You can also repeat the clinical test as I described and look for movement of FPD index as PFL tries to tug on it at the anastomosis site.
This is where we have a real advantage over MRI.
The connections are usually in the distal forearm / proximal carpal tunnel area as distal to this they diverge. If the connection is more proximal it is usually a whole extra muscle slip or extra tendon where the anatomy has a wholesale variation rather than just a small collagen bridge.
I owe my botulinum toxin work with spasticity management for my love of solar forearm and hand muscle anatomy.
Steve