Hi Ben,
Indeed,
With this type of compressive tendinosis at the apex of the cup shaped tendon it gets thinner and thinner until it eventually splits and becomes 2 peroneus brevis tendons.
This is an easy diagnosis of a longitudinal split, but I will suggest it when I start to see cystic myxoid degeneration forming in this area. Also I do compression testing with transducer pressure and you can sometimes see the split more obviously by the way the tendon moves. When it is split you can move the 2 halves of the tendon separate from each other with transducer pressure.
In the longitudinal plane with tendinosis the tendon will appear like “particle board” rather than linear lines of collagen and when they split you can sometimes see a longitudinal defect like a syrinx running through the tendon.
Basically any time I see some disorganization in the architecture of the tendon and not just simple particle board transformation I will suggest the possibility of a longitudinal split tear.
I apply the same logic to tibialis posterior which loves to split as well.
Cheers,
Steve