The flexor hallicus brevis (FHB) is a diminutive, but giant of a muscle. It originates and terminates in the sole your foot and is just like the “little engine that could.” The FHB is responsible for propulsion of your foot and leg at toe-off. I can almost hear it saying, “I think I can, I think I can, ” as it powers runners up a hill. On the way down the other side, it’s saying, with a smug smile, “I knew I could, I knew I could.”
The FHB has it’s origins on the bottom of the mid foot on the cuboid bone, a large tarsal bone on the lateral side (outside) of the foot. Along with the third, fourth, and fifth metatarsals, it makes up what is referred to as the lateral column of the foot. The FHB then courses toward the great toe on a slight line across the foot. Finally, it splits and inserts into the medial and lateral sides at the bottom of the proximal phalanx, also known as the first bone in your great toe.
FHB Function
Repeatedly, with every step you take, this muscle flexes the great toe and the first metatarsal. This motion causes a powerful grasping of the great toe, resulting in propulsion of the foot and leg. Bottom line, this giant little muscle is a prime mover of your entire body. Just like the little engine, it keeps on working without signs of fatigue.
Possible problems
The FHB is called a bipinate muscle because it splits into two wings that form tendons. Both tendon wings surround and encompass a sesmoid bone, and then continue to travel toward the FHB’s insertion on the bottom of the proximal phalanx. The two sesmoid bones are called medial, or tibial, and lateral, or fibular, sesmoids. They are like,pulleys, creating an enormous mechanical advantage for the FHB. Injuries to the FHB are rare; rather, it is the sesmoid bones that are subject to injury.
Sesmoid injuries are usually related to one of three situations. First, during puberty, when the sesmoid is not completely formed into solid bone, it may suffer trauma and crack or split. This fracture usually does not heal because the sesmoid is under constant opposing stress from both sides. Often a tough fibrous band will form that acts like a hard bonding glue holding the sesmoid together and allowing it to continue to function without any problem. Strong, redundant forces like the repetitive motion of running, or hiking long distance, especially downhill, or a single incident that would hyperextend the toe as in a sudden slip or fall, can brake this fibrous binding. The result is that the sesmoid will act like a broken or fractured bone, with all the associated pain. This is called sesmoiditis and the sesmoid bone itself is then called a biparte sesmoid.
Second, in the adult foot the sesmoids are subject to fracture as a result of trauma or faulty biomechanics. A high-arched foot is at risk because it has decreased shock-absorbing capabilities. In this situation the sesmoids are punished with every step as they take on the extra stress from the overloading of the foot. Usually the medial (tibial) sesmoid is at greatest risk.
Third, the formation of an advanced bunion deformity can cause the lateral (fibular) sesmoid to displace from the normal position, creating complex stresses and possibly pain.
Dealing with Sesmoiditis
If you have sesmoiditis, you will feel pain with every step. As the sesmoid reaches and contacts the ground, it will send severe shock into the foot. Even just standing still will cause severe pain. The painful condition responds to rest, but will always return with activity. Often, using a shoe that offers cushion and shock absorption will be very helpful. Asics Nimbus, Mizuno Creation, Nike Volmero, and the new New Balance 1350 are my favorites. When the shoes offer no help, the standard of care is trigger point injection, usually with a fast-acting cortisone. I have been using non-steroid injectable medications with excellent results. I choose to use them as evidence continues to pile up that steroid injections can cause other problems to ligaments and tendons. Orthotics are usually very helpful. Treatment regimens that combine shoes, injections, and orthotics will solve most cases. Occasionally, conservative treatment fails when an ornery sesmoid is resistant to therapy. In these cases surgical excision of the sesmoid is the only option. The procedure is easy and quick. The recovery takes several weeks but in the end the athlete has no pain and can resume all normal activities.
Summary
The flexor hallicus brevis is a heroic little muscle that performs with the strength of a giant because of the mechanical advantages created by the pulley action of the sesmoids. When injury does occur, it’s usually to the sesmoids. Treatment options are varied but usually the problems can be solved. Early treatment is the key to success.
