14/10/2022
NASCE PRIMA L'UOVO O LA GALLINA?
SINDROME PRONATORIA E LIMITAZIONE FUNZIONALE DELLA MOBILITÀ DELLA 1^ MF (FHL = functional hallux limitus).
LE CONSIDERAZIONI DI KEVIN KIRBY
https://www.facebook.com/100064687777625/posts/477761901056732/
Does Functional Hallux Limitus Cause a Low-Arched Foot, or Does a Low-Arched Foot Cause Functional Hallux Limitus?
Functional hallux limitus (FnHL) occurs when a foot has normal hallux dorsiflexion in the non-weightbearing examination but has limited hallux dorsiflexion during standing and weightbearing activities. Functional hallux limitus was first described in 1972 by the late Patrick Laird, DPM, as being due to excessive rearfoot pronation which increased first ray dorsiflexion (Laird PO: Functional hallux limitus. The Illinois Podiatrist. 9:4, 1972).
Then, 14 years later, in 1986, Dananberg described how FnHL could lead to flattening of the medial longitudinal arch (MLA), "Lowering of medial longitudinal arch is related to the inability of the hallux to extend." Dananberg believed that FnHL causes MLA flattening whereas Laird believed that MLA flattening precedes and causes FnHL. There is no research evidence to date to support the idea that FnHL caused MLA flattening. However, mechanical modelling of the foot does strongly support that idea that flattening of the MLA can cause FnHL.
In the illustration below, a model of the foot with the plantar fascia attaching to the plantar heel and hallux is presented. In the upper illustration, the MLA of the foot is higher than in the lower illustration. Static analysis of this mechanical system shows that the plantar fascia will have reduced tension within it in feet with a higher MLA and the plantar fascia will have increased tension force in lower MLA height feet. A similar mechanical model was first described by John Hicks in his classic paper on foot biomechanics from 1961(Hicks JH: The Three Weight Bearing Mechanisms of the Foot. In F.G. Evans (ed): Biomechanical Studies of the Musculoskeletal System. C.C. Thomas Co., Springfield, Ill., pp. 161-191, 1961).
Since tension within the plantar fascia produces an internal hallux plantarflexion moment, then increased plantar fascia tension will produce increased internal hallux plantarflexion moment, which will lead to increased tendency to cause FnHL. Lower MLA height will increase plantar fascia tension, increase hallux plantarflexion moment and will decrease the likelihood that ground reaction force plantar to the hallux during propulsion will result in hallux dorsiflexion.
As a result, feet with lower MLA height have a greatly increased risk of developing FnHL. It is highly unlikely that FnHL is the cause of feet developing a reduced MLA height.
References:
Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 139-152.
