The sesamoids (little pea shapes bones underneath the big toe joint) lay within the tendon of the flexor hallucis brevis tendon. They give the tendon a mechanical advantage by increasing the moment arm (like a pulley).
Sesamoid injuries can be tricky, the reason they are irritated or injured is typically related to their overall load. This load is compromised of both the compressive forces (ground reaction forces) acting on the sesamoids AND the shear forces that occur with the elongation of the flexor hallucis brevis tendon.
That function is great but it also means there are high shear forces on the sesamoids from the tendon working in this way. When the sesamoids are already irritated (or outright injured) these shear forces can really annoy the sesamoids and create significant pain.
How much this specific element is irritating the sesamoid is easy enough to test. I like to get my patients non-weight-bearing, push against the hallux and get them to push back with as much force as they can. If this aggravates the area or outright hurts when it's happens then this element is a big contributing factor. This test might mean that I add a Mortons extension (probably poron) to their orthotic or shoe innersole or actively get them to roll off the side of their foot at toe-off to try and reduce the shear forces associated with contracture of the FHL. Generally I wouldn't do this. I would be trying to encourage proper first mtpj dorsiflexion and encourage the flexion through the hallux whilst offloading by using a metatarsal dome or pad.
I find that the shear forces generally aren't the main causative factor leading to sesamoid pain/injury (the high ground reaction forces and compressive forces usually are) BUT this doesn't mean that extra shear force isn’t annoying it enough to stop it from getting better.
Nearly all patients with sesamoid pain will need their training or activity loads studied in more depth and modified to reduce their overall load. Some will benefit from strapping and taping whilst others might benefit from exercises to rehabilitate the flexor hallucis brevis tendon itself.
When patients don't respond to this, I would start to consider readdressing the exercise. Is a bipartite sesamoid and the cartilaginous connection causing grief? What is this? We will leave that for another day.
If you have any questions or think this is something affecting you, don't hesitate to reach out to wagga podiatry plus.
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