Originally Posted by kerstin
Thanks, I am sure that the tibialis posterior is a main factor in the developing proces but why it gives a solid region around navicular/talus/calcaneal? there is no reconstruction possible and there is lots of force in that region too.
Yes, a research on that topic would be nice, so I know a very good clinical researcher who lives in sacramento and listens to the name Kevin, so I challenging you ;-)).
It would not take much change in the elastic modulus
of the plantar ligaments in a woman with a medially deviated subtalar joint (STJ) axis to start the cascade of events that cause posterior tibial dysfunction after menopause.
First of all, a reduction in elastic modulus in all the ligaments of the foot would tend to cause increased STJ pronation moment since the medial longitudinal arch has 50% more plantar ligaments than does the lateral longitudinal arch. This increase in number of ligaments means that any increase in elongation of the plantar ligaments for a given force being applied will cause more flattening in the height of the medial arch than it will cause a flattening in the height of the lateral arch which, in effect, produces an increase in forefoot varus (or a decrease in forefoot valgus) deformity.
Secondly, with the dorsiflexion of the medial metatarsal rays relative to the lateral metatarsal rays, there will be increased ground reaction force (GRF) on the lateral metatarsal heads which will increase the STJ pronation moment which will, in turn, cause increased STJ pronation motion and increased internal rotation, plantarflexion and medial translation of the STJ axis relative to the plantar foot (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.) This medial deviation of the STJ axis will not only shorten the supination moment arm for the posterior tibial tendon but will also lengthen the pronation moment arm for GRF, both of which will cause a demand for the posterior tibial tendon to have increased tensile force within it in order to produce the same supination effect on the foot when compared the tensile force required within the tendon when the STJ axis is less medially deviated (Kirby KA: Conservative treatment of posterior tibial dysfunction. Podiatry Management, 19:73-82, 2000.)
The more internally rotated STJ axis will have its greatest effects on increasing the magnitudes of STJ pronation moment during the late midstance phase of gait when the GRF has moved towards the forefoot and the moment arms for GRF to cause STJ pronation moments become larger. Late midstance is also the time of gait when the dorsiflexion moments on the medial metatarsal rays are the greatest which will also tend to progressively cause more medial metatarsal ray dorsiflexion during late midstance over time, especially if the STJ axis deviates further medially and the posterior tibial muscle/tendon complex and spring ligament complex becomes weaker and/or becomes plastically elongated.
Therefore, theoretically, in a woman that already possesses a medially deviated STJ axis in her pre-menopausal years, the following cascade of events may occur, simply due to the initiating event of a reduction in elastic modulus of all her plantar ligaments after menopause:
1. Increased elongation of plantar ligaments of medial longitudinal arch.
2. Increased dorsiflexion of medial metatarsal rays, increased STJ pronation moment and increased STJ pronation motion.
3. Increased medial deviation of STJ axis.
4. Decreased supination moment arm for posterior tibial tendon and increased pronation moment arm for GRF in late midstance phase of gait.
5. Increase tensile stress on posterior tibial tendon and spring ligament complex.
6. Plastic elongation/tearing of posterior tibial tendon and/or spring ligament complex.
7. Processes #1-7 continue until some source for STJ supination moment prevents further STJ pronation motion and stops the deformation of the foot architecture.
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
Voice: (916) 925-8111 Fax: (916) 925-8136