Normal Distal Excursion of the Peroneus Brevis Myotendinous Junction

A low-lying peroneus brevis muscle belly has been described as a risk factor for the development of peroneal tendon pathology, but this finding has primarily been described based on cohorts with pre-existing clinical findings. Therefore, the objective of this investigation was to evaluate the frequency of apparently abnormal low-lying muscle bellies from a series of subjects without clinical or imaging findings of peroneal tendon pathology. One hundred consecutive MRIs were reviewed with measurement of the distance from the distal peroneal myotendinous junction to the tip of the fibula. This distance was observed to be 23.9 ± 8.8 mm (10.8-55.4 mm; 95% confidence interval 22.2-26.7 mm). If one assumed that a myotendinous junction within 2 cm of the distal tip of fibula represented an abnormal low-lying muscle, then we observed 37% of extremities without clinical or radiographic evidence of peroneal tendon pathology that would be considered anatomically "abnormal." When a low-lying muscle belly was defined as occurring within 2 cm of the distal tip of the fibula, then a probability analysis of our data distribution found a 32.6% probability for individuals to have an "abnormally" low-lying muscle belly. These results indicate that what has traditionally been defined intraoperatively as an abnormally low-lying peroneus brevis muscle belly might simply represent intraoperative confirmation bias of relatively normal structural anatomy.