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Foot Tripod Concept Refuted

Since the nineteenth century the belief has become ensconced that the foot is a static structure, concerned primarily with allowing the human to balance whilst standing [i][ii]To re-inforce that the concept of the foot as a “tripod” evolved, with the sub-agenda that the “tripod” arrangement would allow greater stability and a more even distribution of weight across the plantar surface. However that concept fails to recognise that the human is primarily a mobile creature and is, in essence, a “balancing machine”.

The concept of the “tripod foot” acting to distribute weight evenly across the plantar surface was refuted in 1987[iii] That paper showed by load distribution analysis that the heel carried 60%, the midfoot 8%, and the forefoot 28% The toes were only minimally involved in the weight bearing process.

The concept of the tripod foot is bound in with the belief that the foot’s primary purpose is to accept weight when standing. That is incorrect. The foot has many functions, but its primary function is to act as a brake on forward movement. The human is designed to move forward and as a result significant design exists to counter forward falling (falling backwards is less frequent and far more injurious, at times catastrophically so). Thus numerous “fail safe” features are built into reducing the damage of falling forward. Perhaps the most important is the primary function of the foot which is to allow the moving human to decelerate safely. Said another way, the foot acts as a brake on forward movement.  To fulfill that role, and to adapt to other roles the marvelous capacity of the foot is to adept at adjusting to irregularities in the surface in all three axis. This is what the normal foot usually does with great capability.

To illustrate, it is relatively easy to walk on stilts or on a Syme’s amputation – where there is no forefoot.  But the stilt walker and bilateral Syme’s victims are unable to arrest forward movement. In other words, they cannot “brake”.

The “standing, weight bearing” function is relatively unimportant, and that normal flexible adaptability should never be sacrificed in order to create the fictional “weight bearing tripod”. Sadly the (obsolete) belief in the “Foot tripod” colours all thinking by “conventional” surgeons. This obsession with the “tripod” concept prevails in all the orthopaedic surgeons and surgical podiatrists with whom I discuss these matters, with frightening regularity as a “knee-jerk” rule-of-thumb.

Because (for other reasons) the foot is a relatively short lever arm it is necessary to have a powerful muscle to action the braking mechanism, and this muscle is the triceps suri. Being so powerful pathological shortening of the triceps can be significantly disabling if it is abnormal. The most common abnormality is shortening. This seems to be an “age related change” but is also associated with diabetes, peripheral vascular disease, rheumatoid arthritis, hypertonia of central origin, trauma, Guillaum Barre syndrome and (in the past) poliomyelitis

 

[i] [i] Dawid Burger, MD, Amiethab Aiyer, MD*, Mark S. Myerson, MD Evaluation and Surgical Management of the Overcorrected Clubfoot Deformity in the Adult Patient ) December 2015 Volume 20, Issue 4, Pages 587–599

[ii] Cavanagh PR, Rodgers MM, Iiboshi A Foot Ankle. Foot Ankle. Foot Ankle. 1987 Pressure distribution under symptom-free feet during barefoot standing.1987 Apr;7(5):262-76.

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