Newly proposed classification of plantaris tendon insertion based on branching patterns: a morphological study.

Background: Although the plantaris muscle (PM) is considered vestigial in humans, it plays a significant clinical role in procedures such as grafting. Recent reports have suggested its potential involvement in mid-portion Achilles tendon tendinopathy. Therefore, the aim of this study was to evaluate the morphological variations of the PM, focusing on its branching patterns at the level of insertion.

Materials and methods: Dissections were performed on 58 lower limbs obtained from 29 adult cadavers preserved in a 10% formalin solution. The morphology of the PM was assessed concerning its branching patterns at the insertion level and its relationship with the calcaneal tendon.

Results: The PM was present in 52 out of 58 cases (89.65%). In most of these cases (37 limbs, 63.79%), there was no splitting of the plantaris tendon (PT) at the level of the extensor expansion (ExP). Bifurcation occurred in 10 cases (19.23%), trifurcation in four (7.69%), and in one instance, the tendon divided into four slips. In 11 cases (22.4%), the insertion blended with the Achilles tendon (Type VII), which may increase the risk of Achilles tendinopathy.

Conclusions: Our study indicates the existence of new types of PM tendon insertions that could potentially influence the development of Achilles tendinopathy. Variations in the anatomical morphology of the PM tendon may lead to potential conflicts with the Achilles tendon and the tibialis posterior tendon, thereby increasing the risk of tendinopathy.