Kevin, Bruce, Others,

I have enjoyed reading the recent posts...very educational and entertaining.

Kevin, given this statement :The amount of 1st and 2nd ray dorsiflexion motion will depend on the dorsiflexion stiffness of each metatarsal ray (dorsiflexion stiffness of metatarsal ray = the force on the metatarsal head that is required to produce a certain number of degrees of metatarsal ray dorsiflexion). Dorsiflexion stiffness of the individual metatarsal ray is, in turn, dependent on elastic modulus of the plantar ligaments, medial longitudinal arch height and relative contractile activity of the plantar intrinsic muscles and extrinsic muscles of the plantar arch of the foot (i.e. FDL, PT, PL & FHL) at any instant in time.

Would you further expound on this statement:the erroneous conceptual idea of independent movement of the longitudinal axis of the MTJ (LMTJ) has been used by many podiatrists to explain the ability of the forefoot to remain plantigrade while the calcaneus everts in closed kinetic chain (CKC).

Thank you kindly,

Kevin M

Kevin Kirby <[log in to unmask]> wrote:

Bruce and Colleagues:

<<9. Movement of plane of metatarsal heads relative to the calcaneus is not necessarily caused by MTJ motion but is more likely caused by different degrees of sagittal plane rotations of each of the metatarsal rays relative to each other.>>

Bruce (Mr. Greenjeans) replied:

<<Please see my above statement re: this. If the cuboid and navicular rotate away from each other in the frontal plane and "unlock" the MTJ, so to speak, or cause it to become unstable, then the metatarsals 2-4 can become exttemely plantarflexed in relation to the 1st and 5th metatarsals, and in relation to the calcaneus. You need to be more specific here Kevin. Relative to the calcaneus in what plane, frontal?, Transverse?, Sagittal? I believe a MTJ collapse causing a plantarflexion of the calcaneus, and plantarflexion of the 3rd metatarsal and dorsiflexion of the 1st and 5th metatarsal is a very important relationship to visualize.>>

Sorry, Bruce, but I did say I took these 12 MTJ items off the top of my head (probably about 10 minutes of writing). The frontal plane relationship of the metatarsal heads to the calcaneus has been suggested by many podiatrists to be primarily a MTJ "deformity". However, I believe that the frontal plane relationship of the metatarsal heads to the calcaneal bisection (i.e. forefoot varus, forefoot supinatus, forefoot valgus "deformities") are better thought of as being caused by different sagittal plane positions of each individual metatarsal ray relative not only to each other and but also relative to the rearfoot, rather than being caused by some "MTJ mechanism".

For example, the erroneous conceptual idea of independent movement of the longitudinal axis of the MTJ (LMTJ) has been used by many podiatrists to explain the ability of the forefoot to remain plantigrade while the calcaneus everts in closed kinetic chain (CKC). However, this ability of the plantar metatarsal heads to remain plantigrade during CKC pronation of the rearfoot is more likely related to increased dorsiflexion of the more medial metatarsal rays at the metatarsal-cuneiform joints, cuneiform-navicular joint and the talo-navicular joint rather than just by a movement at the talo-navicular joint and calcaneo-cuboid joints themselves.

The way I would explain it, if I had to teach some podiatry students who understood the concepts of moments, forces, moment arms, etc, would be as follows: Increased STJ pronation moment during weightbearing increases the pronation motion of the STJ in CKC. In response, there is increased ground reaction force (GRF) on the medial metatarsal heads (1st and 2nd especially) which, in turn, increases the dorsiflexion moment on these metatarsal rays. The increase in 1st and 2nd ray dorsiflexion moment increases the dorsiflexion motion of the 1st and 2nd metatarsal rays relative to the other metatarsal rays and relative to the rearfoot, therefore allowing all the metatarsal rays to stay in a plantigrade position during CKC STJ pronation. The amount of 1st and 2nd ray dorsiflexion motion will depend on the dorsiflexion stiffness of each metatarsal ray (dorsiflexion stiffness of metatarsal ray = the force on the metatarsal head that is required to produce a certain number of degrees of metatarsal ray dorsiflexion). Dorsiflexion stiffness of the individual metatarsal ray is, in turn, dependent on elastic modulus of the plantar ligaments, medial longitudinal arch height and relative contractile activity of the plantar intrinsic muscles and extrinsic muscles of the plantar arch of the foot (i.e. FDL, PT, PL & FHL) at any instant in time.

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Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA

Voice: (916) 925-8111 Fax: (916) 925-8136
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