Anatomical Constraints to Flexor Hallucis Longus Passage Through the Digital Flexor Sheath: A Cadaveric Study
This cadaveric study demonstrates that while an intact flexor hallucis longus tendon can generally pass through the A2 pulleys of the index, middle, and ring fingers, its passage is frequently restricted or impossible at the A4 pulley and in the little finger and thumb, highlighting significant anatomical constraints for its use as a graft.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
When a tendon inside the hand is severely damaged, surgeons sometimes face a difficult choice: if the original tissue cannot be repaired, they must replace it with a new piece of tendon taken from elsewhere in the body. This is a standard procedure for restoring the ability to bend the fingers, but finding a suitable replacement is not always simple. The ideal donor tendon must be long enough to reach the injury, strong enough to handle the work of a finger, and available without causing significant disability to the part of the body where it is taken. While doctors have a few standard options for these replacements, they are not always present or large enough for every patient. In such cases, surgeons have looked to the foot, specifically a large, powerful tendon that controls the big toe, as a potential backup source. However, before this foot tendon could be considered a viable option for the hand, a fundamental physical question remained unanswered: could a tendon designed for the foot actually fit through the narrow, tunnel-like passages of the hand?
To answer this, a team of researchers in France conducted a careful examination using nine human cadavers. They harvested the intact tendon from the big toe of each foot and then attempted to thread it through the digital flexor sheaths of the corresponding hand. These sheaths are the protective tunnels that guide the tendons as they move the fingers. The researchers did not cut the tunnels open; instead, they tested whether the foot tendon could pass through the existing, unmodified structures of the hand. They focused on specific narrow points within these tunnels, known as pulleys, which act like rings holding the tendon close to the bone. The team tested the tendon in the index, middle, ring, and little fingers, as well as the thumb, observing how easily the tendon slid through or if it got stuck.
The results revealed that the success of this procedure depends entirely on which finger is involved and exactly where in the tunnel the tendon is trying to pass. In the index, middle, and ring fingers, the tendon moved through the first narrow point with very little trouble in most cases. However, as the tendon moved deeper into the hand toward the second narrow point, the difficulty increased significantly. In the little finger, the situation was even more restrictive; the tendon failed to pass through the first narrow point in more than half of the specimens and was completely blocked at the second point in nearly every single case. The thumb presented its own unique challenges, with the tendon unable to pass through the entire tunnel in a large portion of the tests.
These findings do not suggest that the foot tendon is a perfect solution, nor do they prove it is impossible to use. Instead, the study identifies specific anatomical roadblocks. The researchers found that while the tendon might slide through the beginning of the tunnel in some fingers, it frequently encounters a physical barrier further down that prevents it from moving freely. The study explicitly states that these observations describe the physical fit of the tendon within the preserved tunnels, but they do not measure the force required to push it through or how well it would glide once in place. Consequently, the authors conclude that while the foot tendon is a substantial piece of tissue, its ability to serve as a graft is limited by the narrow architecture of the hand, particularly in the little finger and at deeper levels of the other fingers. The work clarifies the physical constraints of the anatomy, showing that simply having a strong tendon is not enough; it must also be able to navigate the tight spaces of the hand without getting stuck.
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