SESAMOID BONES AND FLEXOR MUSCLES OF THE HALLUX

Sesamoid bones are important for a normal mechanics of the first Metatarsophalangeal joint (MTPJ) and of the Hallux (Great Toe)!

Let´s find out how!

Sesamoid from the Greek “sesamoeidés”, as its shape is like a sesame seed. It seems that “sesamum” was first used by Galen about 180 A.D because these little bones resembled the seeds of “Sesamum indicum” plant.
The Sesamoid bones (medial and lateral), beneath the base of the first Metatarsal head in the Metatarsophalangeal joint (MTPJ), are the two most important Sesamoid bones of the foot. The head of the first Meta tarsal is large and wide, forming the ball of the Hallux.

These little bones, which form the plantar pad of the MTPJ, contribute significantly to stabilizing the foot during propulsion. They serve as pulleys for the muscles, which stabilize the Hallux (Great Toe) against the ground during propulses.

With the heel lifted and the Hallux dorsiflexed the Flexor Hallucis Brevis utilizes the Sesamoids as a pulley below the anterior articular surface of Metatarsal head to angle more obliquely to its insertion on the base of proximal Phalanx. This results in a lever arm for a plantarflexion force of the Hallux against ground. Flexor Hallucis Longus provides a strong plantar force to the Hallux as it passes between the Sesamoids and stabilizes the Hallux on ground. (Fig. 1) (Valmassy, 1996)

By keeping the first Metatarsal weightbearing and stable against the Sesamoids, the first Metatarsal works as a solid basis for the Hallux stabilization in order to receive the body weight during propulsion.
If Flexor muscles pull without the pulley effect of the Sesamoids under the plantar flexed metatarsal head, it will stabilize the Hallux against the Metatarsal head and not against the ground. (Valmassy, 1996)
 
Sesamoids are then responsible for absorbing vertical pressures in push-off and for reducing friction on the Metatarsal head.
Plantar stability of Hallux against the ground during propulses is provided by Flexor Hallucis Longus and Flexor Hallucis Brevis muscles. The good function of these muscles is correlated with the normal function of Sesamoids.

Sesamoids determine the angle of insertion of the Flexor Hallucis Brevis tendons and maintain proper alignment of the Flexor Hallucis Longus tendon, protecting it also.

Let´s meet these muscles!

The Flexor Hallucis Longus tendon (in Layer 2 (of 4 layers) of plantar foot) towards the Hallux, lies in the groove between the Sesamoids and the two bellies (tendons) of the Flexor Hallucis Brevis muscle. It arises from the lower two-thirds of the posterior surface of the Fibula/Peroneus and inserts at the plantar surface of the base of the distal Phalanx of the Great Toe (Hallux). (Fig. 2)

Its function is plantarflexion of the Hallux at the distal Interphalangeal joint, plantarflexion in the Talocrural/Tibiotalar joint (ankle flexion) and foot inversion in Subtalar joint.

The Flexor Hallucis Brevis (in Layer 3 (of 4 layers) of plantar foot) is a short and deep muscle in the sole of the foot.  This muscle arises from the medial side of the plantar surface of the Cuboid, behind the groove for Fibularis (Peroneus) Longus and the adjacent part of the lateral Cuneiform and the Tibialis Posterior tendon.

Flexor Hallucis Brevis forwards and medially towards the Great Toe, separated in two fleshy bellies (medial and lateral tendons), passing above each Sesamoid. Each tendon inserts in one side of the base of the proximal Phalanx of the Great Toe. The medial one joins Abductor Hallucis muscle, while the lateral joins Adductor Hallucis muscle. Sesamoid bones act as a bearing for both Flexor Hallucis Brevis tendons.

The main function of Flexor Hallucis Brevis is flexion of the Hallux in the MTPJ. It also supports the medial longitudinal arch of the foot.
Imagine now if there are any injuries in Sesamoid bones and how they can cause pain and alter muscle functions by eventually change its alignment.
Donatelli (1990) refers that an altered position of the Sesamoid bones transforms the Flexor Hallucis Longus and Brevis muscles from flexors to adductors, contributing to plantar flexion loss.

Frowen, et al, (2010) say that if there are changes in the relation of the Sesamoids within the first MTPJ joint stability decrease and an efficient propulsion at toe-off is compromised; and the loss of normal orientation of the medial Sesamoid increases the conditions of the foot to pronate.
Studies show that there is a correlation between Sesamoid dislocations and Hallux Valgus. When there is a lateral dislocation the Hallux is abducted.
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How strategic is the position of Sesamoid bones!
 
REFERENCES:
Donatelli, Robert, 1990, “The Biomechanics of the Foot and Ankle”
Valmassy, Ronald L., 1996, “Clinical biomechanics of the lower extremities”
Netter, Frank H., 2013, “The Netter Collection of Medical Illustrations, Musculoskeletal System, Volume 6, Part II – Spine and Lower Limb”
Frowen, P. et al., 2010, “Neale´s Disorders of the Foot”

Raquel Dora Pinho, March 2025