In the two previous posts on this subject a relatively straightforward case of a middle aged female, medium build and moderate activity level was presented, complaining of pain in the area of the sesamoids of the right foot. A biomechanical etiology was established without any complicating issues and recommendations for building an appropriate orthotic device was made, based on the P-14 model device available on the KevinRoot Medical website and order form. Why was this particular device recommended? Let’s talk!
First of all it is difficult to separate a discussion of the sesamoid apparatus from the 1st metatarsophalangeal joint as a whole. The foot sesamoids are two small bones embedded within the substance of the Flexor Hallucis Brevis tendons, which insert into the base of the proximal phalanx of the Hallux. The Flexor Hallucis Longus tendon courses between the sesamoid bones and inserts into the distal phalanx of the Hallux.The sesamoids provide a mechanical advantage to the flexor tendons and allow us to push down against the ground with our great toe while propulsing forward in upright stance. The correct anatomical alignment of the sesamoid is directly beneath the first metatarsal head and movement of the sesamoid bones should be directly forward and backward within the anatomical grooves on the plantar aspect of the first metatarsal head. In reality, the sesamoids are fixed against the ground and the 1st metatarsal rotates forward over them.
In order for this system to function at its optimum, the 1st metatarsal should be relatively locked in a more plantarflexed position, with adequate resistance to the ground reactive force that is pushing it in a superior direction. This is a primary function of the Peroneus Longus muscle during propulsion of the foot. When the foot is in an over pronated position at the moment of propulsion during gait, the mechanism is “unlocked”, the ground reactive force overwhelms the action of the Peroneus Longus Muscle, the first metatarsal elevates and the 1st Metatarsal planageal joint jams instead of bending properly, the first metatarsal is pushed forward into the base of the proximal phalanx. Generally this will result in the hallux abducting and everting, rolling over the medial aspect of the hallux instead of pushing forcefully through it, over time and repetition the 1st MTPJ joint adapts and Hallux Valgus, Hallux Limitus/Rigidus, and/or sesamoid displacement occurs.
In the treatment room, explaining this to a patient, I would often have them imagine a person lifting the handle of a wheelbarrow. The sesamoids are the wheels, the 1st metatarsal is the cart. Normally,the load of the cart is centered over the wheels and it is relatively easy to push a heavy load forward. Now imagine the wheels of the cart somewhat off to the side of the cart, not centered, and the wheelbarrow having to tilt the opposite side in response, and ultimately tipping over. That wheelbarrow has very little chance of being able to effectively move forward in this circumstance.
Classically trained foot surgeons have focused on the 1st-2nd intermetatarsal angle and Hallux Abductus angle present on the anterior-posterior x-ray in determining what type of corrective surgery to perform. Recently much more importance is being given to the eversion of the 1st metatarsal as the primary deformity of bunions. Surgical corrections aimed at axially rotating the metatarsal out of eversion have been developed, even to the point where removal of the apparently hypertrophic medial eminence of the 1st metatarsal head isn’t necessary in some cases. In other words, the surgery aims to un-tilt the cart and realign the wheels.This is the same objective of our orthotic device.
Orthotic devices work best in a functional way if they can create change of function, otherwise they can be used to accommodate whatever deformity has already occurred and help prevent further development of that deformity. In our case of displaced sesamoids, the anatomical malalignment present on the standard A-P x-ray view is reduced when the heel was raised during the sesamoid axial view. Therefore, reduction of the deformity is possible.
The purpose of the orthotic device in this case is to;
- Reduce pronatory force during transition from midstance to propulsion phase, (High arch, rigid frame)
- Reduce pronatory force during propulsion (balanced intrinsic forefoot post)
- Reduce reactive ground force underneath 1st MTPJ joint complex (varus extrinsic forefoot bar, 1st ray cutout, dancer’s pad or reverse Morton’s extension)
Once again, these are guidelines or suggestions Every case is different and unique unto itself, even on the same patient, left versus right foot. There are many variables to consider both objective and subjective.
Thank you to the reader for participating in this exercise, I welcome feedback. As always, the team at KevinRoot Medical is ready to help you!
