What causes sesamoiditis?

The two sesamoid bones move around the curved underside of the first metatarsal head within two corresponding grooves. Embedded in the medial and lateral tendons of the flexor hallucis brevis, with the flexor hallucis longus tendon passing between them, they work with the ligaments, capsule, plantar fascia and muscles as an integrated movement and load-management system.

The sesamoids act like paired cams within the windlass mechanism. With the flexor tendons relaxed and the great toe lowered, they can remain behind the metatarsal head. As the toe rises, they rotate forward, changing tendon direction and leverage, tensioning the plantar fascia and contributing to arch stability.

Sesamoiditis develops when repeated pressure, compression, shear or tensile demand exceeds the complex’s capacity to manage and recover. The tissue becomes painful and irritated; a sudden impact can cause the same demand–capacity failure acutely.

Movement and timing matter

Ideally, the great toe rises before ground contact and moves the sesamoids forward of the first metatarsal head. They maintain that position as the forefoot becomes weight-bearing and through toe-off. Once loaded, the forward-positioned sesamoids cannot simply rotate backward. This creates a semi-locking cam relationship that works with the windlass mechanism to help maintain arch stability.

If the great toe does not rise before ground contact, the sesamoids remain behind the metatarsal head as the forefoot is loaded. In that position, loading resists their forward rotation, limiting formation of the windlass mechanism and the corresponding arch stability. If the great toe rises only slightly, the sesamoids can be positioned directly beneath the metatarsal head when forefoot loading reaches its maximum. They must then bear a concentrated share of those forces—the repeated loading pattern that can lead to sesamoiditis.

This is not the work of one isolated muscle. The extensor and flexor muscles of the great toe, the intrinsic foot muscles, the arches, the ankle and the rest of the lower-limb kinetic chain function synergistically. Weakness, delayed activation or imbalance anywhere in that system changes the position, timing and magnitude of the forces reaching the sesamoids.

How footwear contributes

Several footwear characteristics can restrict movement or concentrate load beneath the first metatarsal head:

• Shallow or stiff toe boxes restrict the dynamic rise of the great toe and the corresponding movement of the sesamoid complex.

• Narrow or tapered toe boxes displace the great toe toward the smaller toes and can alter first-ray and sesamoid alignment.

• Elevated heels shift additional pressure toward the forefoot, first metatarsal and great toe.

• Restrictive uppers and tight lacing inhibit the coordinated lowering and rising of the arches and limit the foot’s ability to prepare for changing loads.

• Stiff midsoles and outsoles replace natural forefoot bending with an external footwear pivot. If that pivot does not align with the first metatarsophalangeal joint, pressure can be focused beneath the sesamoids.

• A concave, raised or rigid supporting surface beneath the first metatarsal head can create a localized pressure point directly under the sesamoid complex.

• Thick, wide or highly tractioned soles change the external lever and the speed at which momentum is arrested or redirected during running, jumping and directional change.

• Cushioning, passive support and restriction change the sensory and movement environment within which the nervous system organizes muscular participation and timing.

Excessive pronation can rotate and destabilize the first ray, altering how the sesamoids track and load beneath the metatarsal head. Oversupination can leave the foot comparatively rigid and concentrate demand along the first metatarsal or one sesamoid. The exact pressure pattern depends on the person, the footwear and the activity.

The mechanical sequence: Restricted pre-contact great-toe elevation → the sesamoids remain behind or directly beneath the first metatarsal head → the forefoot loads before the semi-locking windlass relationship is established → forces concentrate at the sesamoids → repeated demand exceeds tissue capacity → pain and tissue irritation develop.

Why symptoms differ between people

The same footwear does not produce the same outcome in every person. Genetic predisposition, sesamoid shape, a naturally bipartite sesamoid, first-ray alignment, previous injury, tissue condition, muscular coordination, activity intensity, fatigue and recovery all influence available capacity.

Pain begins when the total demand exceeds that capacity. The activity that finally produces symptoms can be recent, but the movement pattern and loss of capacity that made the area vulnerable can develop over many years.

Consult with your healthcare practitioner to ask about employing soft tissue mobilization therapies to address the fibrotic scar tissue that may have formed prior to using Biopods and Barefoot Science.