Foot and Ankle Biomechanics

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Medial longitudinal arch

• Two models, both have validity
  • Beam model:
    • arch is curved beam made up of interconnecting joints. The stability is dependent on the joints and ligamentous interconnections.
    • Tensile forces on inferior surface of the beam, compressive forces on the superior surface
  • Truss model:
    • arch has a triangular structure with two struts connected at the base by a tie rod.
    • The struts are under compression and the tie rod is under tension.
• Windlass effect
  • Plantar fascia is the tie rod in truss model
  • Dorsiflexion of MTPJ puts traction on the plantar fascia and causes elevation of the arch through the windlass effect.
  • During toe-off, the toes are dorsiflexed passively while the body passes over the foot. The plantar fascia tightens and shortens the distance between the MTP heads and heel, and thereby elevates the arch. This traction also helps to invert the calcaneus through the attachment on the medial plantar aspect of the calcaneus.
• Arch stability, in order of importance
  • Plantar fascia
  • Long and short plantar ligaments
  • Spring ligament

Soft Tissues

• Dorsal skin loosely attached
• Plantar skin firmly attached by extensions of the plantar fascia
• Heel pad absorbs shock, consists of comma-shaped or U-shaped fat-filled columns arrayed vertically. The septae are reinforced internally with elastic transverse and diagonal fibres to produce a spiiral honeycomb effect. SEptal degeneration and fat atrophy occur with age.

Movement Terminology

• Abduction and adduction
  • occur around a vertical axis rather than anterior-posterior axis. This motion is called internal and external rotation elsewhere in the body.
  • Primarily occurs at the subtalar and midtarsal joints and is limited
• Inversion and eversion
  • Occur in the coronal plane about an anterior/posterior axis. This motion is called abduction/adduction elsewhere in the body.
  • Primarily occurs at the subtalar and midtarsal joints.
• Dorsiflexion and plantarflexion
  • Occurs around a medial/lateral axis in the sagittal plane
  • Primarily occurs at the talocrural joint.
• Supination and pronation
  • Occurs around the oblique axes of the foot, around a single axis, as a component of motion in three planes. Often called triplanar, as the same three components of motion always occur.
  • Supination: components of plantarflexion, adduction, and inversion
  • Pronation: components of dorsiflexion, abduction, and eversion.
  • Occurs at the talocrural, subtalar, and midtarsal joints.

Load Distribution

• The ankle has a large load-bearing surface area of 11 to 13 cm2
• Lower stresses than in the knee or hip
• Most load is transmitted through he tibial plafond to the talar dome
• Remainder transmitted to the medial and lateral talar facets
• Inversion -> increases medial talar facet load
• Eversion -> increases medial talar facet load
• Ankle dorsiflexion -> greatest talar contact and lowest pressure

Kinetics of the Foot

• Peak vertical forces 120% body weight during walking, and 275% during running
• Medial column (talus, navicular, cuneiforms, 1-3 metatarsal) -> bears most of load
• Lateral column (calcaneocuboid joint, 4-5 metatarsals) -> transmits lesser load
• Standing: greater pressures on the heel. Forefoot peak pressure is under the second metatarsal head.
• Barefoot walking: centre of pressure initially central heel, then moves rapidly across the midfoot to the forefoot where the velocity decreases.
• Shoe wearing: reduced peak heel pressure. Forefoot load shifts medially, maximum pressure is under the 1st and 2nd metatarsal heads. Pressure under the toes increase.
• Running: two types of runners, rearfoot strikers and midfoot strikers. Rearfoot strikers: initial ground contact with posterior third of shoe. Midfoot strikers: middle third of shoe. Both groups, first contact is along the lateral border of the foot. Centre of pressure is in the distal most 20 to 40% of the shoe, indicating that most time is spent on the forefoot

Effects of Shoes on Biomechanics

• Narrow toebox compresses the forefoot contributing to hallux valgus, hammer toes, and bunionettes.
• Elevated heels increase forefoot pressure, cause pain under the metatarsal heads, contribute to intermetatarsal neuroma formation, and lead to Achilles contracture with limited ankle dorsiflexion and altered gait
• Athletic shoes: questionable whether it can control foot motion
• Barefoot running: a big topic in itself. Increase in stress fractures in certain situations.
• Rocker bottom shoes: decrease plantar pressure especially over the forefoot, but unproven effects on the proximal joints.

Reading

• open access article general overview
• old but open access article on the first ray