Cadaveric study of anatomical and histological characteristics of flexor hallucis longus tendon at possible sites of tendon rupture
Authors: Goral Joanna , Inouye Sandra E , Ferrel Benjamin , Cheung Andrew , Belford Zachary , Leland Eric , Meyer Alice , Fanter Nathan 
Published: November 2023

RationaleThe purpose of this study was to examine the frequency of mechanical wear, chronic inflammation and tendinosis of the flexor hallucis longus (FHL) tendon at areas of structural vulnerability. FHL tendonitis is strongly associated with ballet dancers and athletes, thus clinicians typically do not expect to find such a condition in the non‐athletic patient with foot pain. This commonly leads to misdiagnoses or delayed diagnosis, incurring expensive testing and raising the risk of complications. We hypothesized that signs of chronic mechanical wear and inflammation of the FHL tendon are commonly present in the general population, thus providing evidence that clinicians should entertain a higher level of suspicion for this condition in the non‐athletic population.Materials and methodsThe subjects in this study were 20 donor cadavers (11 females and nine males) fixated in formalin, provided by the Department of Anatomy at Midwestern University. The mean age was 81 (±12). Twenty left, and 19 right legs were dissected. First, tendon samples were assessed based on their gross anatomical characteristics. Next, samples of the FHL tendon were harvested from four sites of suspected mechanical wear (medial talar tubercle, sustentaculum tali, knot of Henry, sesamoid bones) and from a control site, where FHL tendon was not in contact with any bony structures. The collected specimens were fixed, processed for paraffin embedding, sectioned, and stained with hematoxylin and eosin for histological analysis. Tissues were examined for the presence of inflammatory cells and for the evidence of the local compressive force‐induced conversion of the tendons’ dense connective tissue into fibrocartilage.ResultsHistologically, no evidence of inflammatory cells was found at any of the tendon sites. However, cartilage‐specific transformation of tendons with a significant presence of chondrocytes was observed in the FHL tendons harvested from the areas associated with sesamoid bones (94.7% of specimens), the medial talar tubercle (79.5% of specimens), and the sustentaculum tali (20% of specimens). Only a few chondrocytes were present in the area of the knot of Henry and in the control tendons. There appeared to be no correlation between the gross anatomical characteristics of individual tendons and the presence of chondrocytes.ConclusionFHL tendons examined in this study did not show any signs of inflammation. However, the significant evidence of chondrocytes at specific sites may indicate the presence of consistent mechanical wear resulting in stress‐related changes in tendons. Fibrocartilage may be a sign of pathology that could lead to degeneration and rupture of the tendon. Thus, the areas of FHL tendon associated with sesamoid bones and the talar tubercule appear most vulnerable to injury.Support or Funding InformationBenjamin Ferrel and Zachary Belford were supported by the Midwestern University Research Fellowship Program. Andrew Cheung and Eric Leland were supported by the Midwestern University work‐study program.