From anatomy to imaging: Towards a reliable MRI classification of the plantaris tendon

The plantaris tendon is common but anatomically variable, and some distal insertions may contribute to Achilles tendon disorders. No validated, reproducible MRI-based classification currently exists.

We retrospectively reviewed 281 ankle MRI examinations performed between 2020 and 2023 using a standardized 3 T protocol. Tendon morphology was classified according to Olewnik's five-type cadaveric system. Inter- and intra-observer reproducibility were assessed with Cohen's kappa and positive agreement. Because reproducibility was limited, a simplified four-type MRI-based classification was developed and tested.

The original five-type classification showed only moderate reproducibility (inter-observer κ = 0.56; intra-observer κ = 0.60), mainly because type 5 was unreliable. The simplified four-type classification improved reproducibility substantially (inter-observer κ = 0.75; intra-observer κ = 0.78).

Olewnik's classification is only moderately applicable to MRI. A simplified four-type MRI-based system appears more reproducible and may provide a reliable framework for clinical research.

III - Retrospective diagnostic study.