Ankle biomechanics

Functional Goals

• The foot must be both flexible and rigid, depending on the phase of gait and activity.
• Foot and ankle joints transmit significant loads:
  • 2x body weight at rest.
  • 3x body weight while walking.
  • 13x body weight when running.
• The foot needs to be a rigid lever during push-off and heel strike and supple during stance, especially on uneven ground.

Foot and Ankle Movements

• Dorsiflexion/Plantarflexion: Occur in the sagittal plane at the ankle joint.
• Varus/Valgus: Movement in the coronal plane at the hindfoot.
• Abduction/Adduction: Transverse movement of the midfoot from the midline.
• Pronation/Supination:
  • Supination: Combines ankle plantarflexion, subtalar varus, and midfoot adduction.
• Inversion/Eversion:
  • Inversion: Combines subtalar varus and forefoot supination.
  • Eversion: Combines subtalar valgus and forefoot pronation.

Ankle and Talus

• The talus is wider anteriorly and moves within a 10-degree oblique axis between the malleoli.
• During dorsiflexion, the talus rotates externally, while during plantarflexion, it rotates internally.
• As the foot moves, it transitions from down and in to up and out.

Distal Tibiofibular Joint

• Well-supported by interosseous membrane (IOM) and syndesmotic ligaments, with minimal motion (2mm).
• Large contact area typically prevents arthrosis, but if stabilizers are disturbed, rapid arthritis can develop.

Ankle Movements During Gait

• Ankle provides:
  • 20 degrees of dorsiflexion (DF) and 30 degrees of plantarflexion (PF).
  • 11 degrees of tibial rotation (supplemented by subtalar motion for forward propulsion).
• Gait cycle:
  • At heel strike, the ankle is neutral or slightly PF, followed by further PF in early stance.
  • During mid-stance, DF occurs as the body moves over the foot, followed by PF again during push-off.
  • Swing phase: Progressive DF occurs to clear the floor, followed by PF in preparation for heel strike.

Kinetics of the Ankle

• Large contact area lowers stress compared to the hip or knee, hence less arthritis.
• Sensitivity to disruptions: A 1mm taller shift can increase contact pressure by 42%, leading to early arthritis.
• Gastrosoleus complex (via Achilles tendon) works to maintain upright stance, causing compressive forces on the ankle joint.
• Ankle joint reaction force is proportional to gastrosoleus activity, which explains pain during tiptoeing in osteoarthritis (OA) patients.

Achilles Anatomy

• The Achilles tendon spirals as it moves distally, with medial fibers inserting posteriorly and lateral fibers inserting anteriorly.
• Achilles tendon lengthening cuts should align with the fiber arrangement: Distal Anterior, Medial Proximal (DAMP procedure).

Subtalar and Foot Biomechanics

• Subtalar joint (STJ) acts as a torque converter, translating tibial rotation into foot pronation and supination.
• STJ provides 20 degrees inversion and 5 degrees eversion.
• Axis of rotation: In normal feet, 1 degree of tibial rotation yields 1 degree of inversion/eversion.
• Variations:
  • Flat feet: More foot rotation due to a more horizontal subtalar axis.
  • Cavus feet: Less rotation due to a more vertical axis.
• In the absence of subtalar motion (e.g., tarsal coalition), inversion/eversion occurs at the ankle joint.

Midtarsal/Transverse Tarsal Motion (Chopart's Joint)

• The talonavicular (TN) joint is a ball-and-socket joint, and the calcaneocuboid (CC) joint is saddle-shaped.
• In pronation, the joints unlock, making the foot supple.
• In supination (e.g., during push-off), the joints lock, providing a rigid lever arm for efficient push-off.

Tarsometatarsal Joint Motion

• Movement is restricted to less than 5 degrees dorsiflexion and 15 degrees plantarflexion by strong ligaments and bony architecture.
• The joints form the transverse arch of the midfoot, centered around the 2nd tarsometatarsal joint (Lisfranc's joint).

Metatarsal Break

• Refers to the oblique axis at which the metatarsophalangeal joints (MTPJs) extend, varying between 50-70 degrees in individuals.

Plantar Fascia

• Originates at the calcaneus and inserts into the tarsal joints and proximal phalanges.
• Functions as a truss that supports the arches and works via the windlass mechanism.
• During toe-off, the plantar fascia tightens, locking the tarsal joints and raising the longitudinal arch, making the foot a rigid lever.

Ankle Replacement Considerations

First-Generation Implants (1970s)

• Cemented, constrained, or unconstrained designs, prone to failure due to:
  • Cementing requiring large bone resection.
  • Constrained implants loosening at the bone-implant interface.
  • Unconstrained implants leading to instability due to soft tissue balancing issues.

Second-Generation Implants

• Semi-constrained designs with three components.
  • Allow sliding in AP, mediolateral directions, and dorsi-plantar flexion.
  • Uncemented, reducing subsidence and loosening.
  • Improved balance with thicker polyethylene inserts (fixed or mobile).
  • Examples: LCS and STAR (mobile poly), Agility (fixed poly with tibiofibular fusion).
• Indications: OA or rheumatoid arthritis in low-demand elderly patients.
• Contraindications: Infection, lack of bone stock, deformity, high demand, neuropathy, or vascular insufficiency.
• Complications: Malpositioning leading to impingement, instability, malleolar fracture, or premature wear.

Third-Generation Designs

• Introduce conical-shaped talar components, designed to be more anatomical. Long-term results are still pending.