09/03/2021
Proximal and Distal Structures That Contribute to First Ray Dorsiflexion Stiffness
In order to increase the dorsiflexion stiffness of the first ray (i.e. prevent first ray hypermobility) during weightbearing activities, the body uses both active factors that are mediated directly by the central nervous system (CNS) and passive factors that are not mediated or altered by the CNS. Active factors, such as contractile activity of the peroneus longus (PL), posterior tibial (PT), flexor hallucis longus (FHL), abductor hallucis (ABH), flexor hallucis brevis (FHB), and adductor hallucis (ADH) muscles, may be voluntarily or involuntarily controlled by the CNS in order to either increase or decrease the first ray dorsiflexion stiffness depending on the functional needs of the individual’s foot.
Passive factors that increase first ray dorsiflexion stiffness are those factors that are not controlled by the CNS and include the tension forces generated within the plantar fascia and the plantar ligaments of the first ray when ground reaction force (GRF) is exerted on the plantar forefoot. Both the active and passive factors that mechanically affect first ray dorsiflexion stiffness are necessary to allow normal function of not only the first ray but also the rest of the foot and lower extremity during weightbearing activities (Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009, pp. 73-84).
For each of these passive and active factors to contribute toward increasing the dorsiflexion stiffness of the first ray, they must, by definition, also be able to generate first ray plantarflexion moments. Increases in the contractile activity of the PL, PT, FHL, ABH, FHB and ADH will generate a first ray plantarflexion moment which will, in turn, increase first ray dorsiflexion stiffness by actively resisting the first ray dorsiflexion moments from GRF. Passive increases in tension forces within the plantar aponeurosis and plantar ligaments of the first ray, which occur as a result of plantar loading of the forefoot, will also generate a first ray plantarflexion moment which will, in turn, increase first ray dorsiflexion stiffness and actively resist first ray dorsiflexion moments (see illustration below).
In my illustration below, the foot on the left with a lower medial longitudinal arch will have a decreased 1st ray plantarflexion moment arm available for the 1st MPJ compression force generated by the tension force within the plantar fascia, abductor hallucis and other tendons that attach to the sesamoids and hallux. As a result, this lower longitudinal arch will produce much less 1st ray plantarflexion moment when compared to the same 1st MPJ compression force in a foot with a higher medial longitudinal arch (shown on the right) due to the longer 1st ray plantarflexion moment arm that is present in the higher-arched foot.
The function of the first ray and first MPJ may be better understood by further dividing the tension load-bearing structures that increase first ray dorsiflexion stiffness into two categories that are dependent on where these muscles, tendons and ligaments insert onto their respective osseous structures in the first ray and hallux. Many of the muscles, tendons and ligaments that produce a first ray plantarflexion moment do not exert tension force directly on the osseous structures of the first ray, but rather exert increased tension force directly through their insertions onto osseous structures which are distal to the first metatarsophalangeal joint (MPJ). Included in these structures, which I will name distal first ray plantarflexion structures (DFRPS), are the FHL, which inserts on the base of the hallux distal phalanx, and the ABH, FHB, ADH and medial slips of the central component of the plantar aponeurosis, all of which insert onto the base of hallux proximal phalanx via the medial and lateral sesamoids.
All of the DFRPS will exert a posteriorly-directed force on either the proximal phalanx or distal phalanx of the hallux which will, in turn, result in an increase in posteriorly-directed force acting onto the anterior articular surface of the first metatarsal head from the base of the hallux proximal phalanx.
The other tension load-bearing structures that produce a first ray plantarflexion moment do so by exerting increased tension force directly through their insertions onto the osseous structures of the first ray, or, in other words, by inserting proximal to the first MPJ. Included in these structures, which I will name proximal first ray plantarflexion structures (PFRPS), are the PL, which inserts on the base of the first metatarsal and first cuneiform, the PT, which inserts onto the navicular and first cuneiform, and the plantar ligaments of the first ray including the plantar ligaments of the navicular-first cuneiform joint, first cuneiform-first metatarsal joint and, to a lesser extent, the ligaments between the first and second rays. Since all of these tension load-bearing structures directly attach to the osseous structures of the first ray, they can therefore directly act to either actively or passively generate a first ray plantarflexion moment so that first ray dorsiflexion stiffness may be increased as is necessary.
One of the interesting aspects about this division of tendons and ligaments that produce a first ray plantarflexion moment into either DFRPS or PFRPS is that it highlights the fact that many of the structures that increase first ray plantarflexion moment exert their stabilizing mechanical effect on the first ray as a result of their ability to cause increased compression forces at the first MPJ. For example, if the FHL, FHB, ABH, ADH tendons and medial slip of the central component of the plantar aponeurosis were all transected, these structures could then no longer contribute to increasing first ray plantarflexion moment and to increasing first ray dorsiflexion stiffness. As a result, the first ray would lose important load-support function with transaction of these DFRPS.
In addition, the amount of first ray plantarflexion moment caused by tension forces within the DFRPS are directly related to the degree of first ray declination angle which is also correlated to the height of the medial longitudinal arch (MLA) of the foot. For example, in a foot with a high MLA (e.g. pes cavus), the posteriorly-directed force on the first metatarsal head resulting from the increase in tension forces from the DFRPS will have a longer moment arm to produce first ray plantarflexion moment than in the foot with a low MLA (e.g. pes planus). Therefore, for a given amount of first MPJ compression force caused by the tension forces within the DFRPS, the foot with a higher MLA will have an increased first ray moment arm, an increased first ray plantarflexion moment and increased first ray dorsiflexion stiffness when compared to the foot with a lower MLA.
This article is from my May 2009 Precision Intricast Newsletter ", "Proximal and Distal Structures in First Ray Dorsiflexion Stiffness" in my book: Kirby KA: Foot and Lower Extremity Biomechanics IV: Precision Intricast Newsletters, 2009-2013. Precision Intricast, Inc., Payson, AZ, 2014, pp. 45-46.
My five books, both in English and Spanish language editions, may be purchased from Precision Intricast Orthosis Lab at www.precisionintricast.com/shop.
