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When the big guy heads medially….Game Changer

Lately we have been seeing a lot of bunions (hallux valgus). While doing some research on intermetatarsal angles (that’s for another post) we came across the nifty diagram you see above. 

Regardless of the cause, as the 1st metatarsal moves medially, there are biomechanical consequences. Lets look at each in turn. 

  • the EHB (extensor hallucis brevis) axis shifts medially. this muscle, normally an extensor of the proximal phalanyx, now becomes more of an abductor of the hallux. It’s secondary action of assisting the descent of the head of the 1st metatarsal no longer happens and it actually moves the base of the proximal phalanyx posteriorly, altering the axis of centration of the joint, contributing to a lack of dorsiflexion of the joint and a hallux limitus
  • Abductor hallucis becomes more of a flexor, as it moves to the plantar surface of the foot. Remember, a large percentage of people already have this muscle inserting more on the plantar surface of the foot (along with the medial aspect of the flexor hallucis brevis), so in these folks, it moves even more laterally, distorting the proximal phalanx along its long axis (ie medially) see this post here for more info
  • Flexor hallucis brevis moves more laterally. Remember this muscle houses the sesamoid bones before inserting onto the base of the proximal phalannx; the medial blending with the abductor hallucis and the lateral with the adductor hallucis. Because the sesamoid bones have moved laterally, they no longer afford this muscle the mechanical advantage they did previously and the axis of motion of the 1st metatarsal phalangeal joint moves dorsally and posterior, contributing to limited dorsiflexion of that joint and a resultant hallux limitis. The lateral movement of the sesamoids also tips the long axis of the 1st metatarsal and proximal phalanyx into eversion. In addition, the metatarsal head is exposed and is subject to the ground reactive forces normally tranmittted through the sesamoids; often leading to metatarsalgia. 
  • Adductor hallucis: this muscle now has a greater mechanical advantage  and because the head of the 1st ray is not anchored, acts to abduct the hallux to a greater degree. The now everted position of the hallux contributes to this as well

As you can see, there is more to the whole than the sum of the parts. Bunions have many biomechanical consequences, and these are only a small part of the big picture. Take you time, learn your anatomy and examine everything that has a foot!

See you in the shoe isle…

Ivo and Shawn

pictures from: http://www.orthobullets.com/foot-and-ankle/7008/hallux-valgus and http://www.stepbystepfootcare.com/faqs/nakedfeet/