# Variations in the Extensor Pollicis Brevis-Extensor Pollicis Longus Tendon Complex

## TL;DR

This study examines the diverse tendon patterns of the extensor pollicis brevis in cadaver hands, suggesting these variations may reflect an ongoing evolutionary process.

## Contribution

The study provides new insights into the developmental and evolutionary aspects of EPB tendon variation.

## Key findings

- EPB tendons show diverse running patterns and insertion variations.
- Tendon diversity is likely due to a distinct developmental process.
- These variations may reflect an ongoing evolutionary process.

## Abstract

Despite several reports on the running of the extensor pollicis brevis (EPB) tendons, the classification of tendon insertions remains ununified due to differences in reports. This diversity in tendon patterning is attributed to the process of tendon development. In this study, we assessed the running of the EPB tendons of 44 cadaver hands fixed in ethanol/formalin in detail and examined the existing classification method. The specimens were obtained from 15 women and seven men, with an average age of 86 years. Consistent with previous reports, we observed a wide diversity in the running of the EPB tendons. Further, we found that EPB tendon insertions showed diverse variations in the proportion and running of fibers, making it difficult to classify them into independent patterns. It is speculated that the EPB tendon develops through a different process than that of the muscle body of the EPB and that the entire muscle-tendon module of the EPB is evolving. The diversity of the EPB tendons observed in this study may reflect the ongoing process of evolution. In clinical practice, a wide variation in the running of the EPB tendons should be considered.