How is linear motion of the Tibia and Fibula converted to rotary motion at the feet?

[sraj]

The linear motion of the tibia and fibula is converted to rotary motion at the feet primarily through the arthrokinematics of the talocrural (ankle) joint, specifically by the interaction between the talar dome (a wedge-shaped, cone-shaped, or pulley-like structure) and the tibiofibular mortise (the socket formed by the ends of the tibia and fibula). As the tibia/fibula move linearly forward (dorsiflexion) or backward (plantarflexion), the unique geometry of the talus forces the ankle into rotational motion, resulting in inversion/eversion and internal/external rotation of the foot. 

Mechanism of Conversion (Linear to Rotary)

• The Talar Wedge: The top part of the talus (trochlea) is wider anteriorly and narrower posteriorly.
• Dorsiflexion (Upward Rotation): As the tibia and fibula move forward (linear), the wider anterior part of the talus moves into the mortise, causing the mortise to widen, the fibula to move proximally, and the foot to experience relative external rotation and eversion.
• Plantarflexion (Downward Rotation): As the tibia and fibula move backward, the narrower posterior part of the talus enters the mortise, allowing the fibula to move distally, causing relative internal rotation and inversion.
• Syndesmotic Accommodation: The distal tibiofibular joint is a fibrous joint (syndesmosis) that allows small amounts of motion, allowing the fibula to glide superiorly and slightly posteriorly (lateral rotation) during dorsiflexion, and inferiorly during plantarflexion. 

Key Anatomical Components:

• Talus: The "intermediate" bone connecting the lower leg to the foot.
• Tibiofibular Mortise: Formed by the medial malleolus of the tibia and the lateral malleolus of the fibula.
• Ligaments: The anterior/posterior tibiofibular ligaments and the interosseous membrane provide stability while allowing these necessary gliding movements of the fibula. 

During a normal gait cycle, from heel contact to mid-stance, the fibula moves medially/laterally (1.79–1.95 mm), anteriorly/posteriorly (2.65–2.84 mm), and superiorly/inferiorly (1.89–1.91 mm), creating a 3D conversion of motion.

AI responses may include mistakes.

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https://www.youtube.com/watch?v=2q0pG88bp0c

Ankle joint anatomy