Ankle Joint Anatomy and Biomechanics

Ankle Joint as a Mortise and Tenon Joint

• The ankle can be referred to as a mortise and tenon joint, similar to woodworking joints.
• A mortise and tenon joint consists of a notch (mortise) and a protrusion (tenon) that fits into the notch, creating a strong joint.
• Analogy to IKEA furniture construction.
• In the ankle:
  • The tenon is the talus bone of the foot.
  • The mortise is formed by the fibula (lateral side) and tibia (medial side).

Talocrural Joint: A Specialized Hinge Joint

• The ankle joint is a specialized hinge joint due to its many structural components.
• Anatomical name: Talocrural joint.
  • Talo refers to the talus.
  • Crural refers to the leg (tibia and fibula).
• Articular surface:
  • Internal aspect of the fibula
  • Inferior and lateral aspect of the tibia.
• These articulate with the trochlear surface of the talus.
• The joint is a synovial joint, containing synovial fluid within a joint capsule.
• Ankle's open mortise and tenon design allows sagittal plane movements like flexion and extension.
• The open structure (notch in the mortise) is anterior and posterior, facilitating movement.
• The bony structure prevents movements in the frontal plane (inversion/eversion) which maintains stability.

Syndesmosis Joint

• The tibia and fibula are held together by a syndesmosis joint.
• This joint is crucial for stability; separation would compromise the joint's integrity.
• Components:
  • Anterior and posterior inferior tibiofibular ligaments (AITFL and PITFL).
  • Interosseous membrane: a tough fibrous band connecting the tibia and fibula along their length.
    • Contains hiatuses for blood vessels and nerves to pass through.
    • Serves as an attachment site for leg muscles.

Ankle Movement and Terminology

• The ankle hinge joint operates on the sagittal plane with a medial-lateral axis of rotation.
• Movements: Flexion and extension.
• Specific Foot Movements:
  • Dorsiflexion (true extension): Toes are brought up and out (similar to extending fingers).
  • Plantar flexion (true flexion): Pointing the toes downwards (similar to making a fist).
  • Dorsiflexion \approx Extension
  • Plantarflexion \approx Flexion

Muscle Function and Location

• The calf muscles on the posterior aspect of the leg are large and powerful because they lift the entire body weight during plantar flexion.
• During dorsiflexion, weight is primarily distributed through the calcaneus and talus (hindfoot).
• Posterior muscles facilitate lift-off during plantar flexion.

Tibial Torsion and Foot Positioning

• The axis of rotation in the ankle is oblique, not straight, causing the feet to point outwards.
• This is due to tibial torsion, where the tibia rotates externally during fetal development.
• The feet form a slight "V" shape, increasing the area of balance.
• This alignment increases the mechanical advantage of the calf muscles.

Mechanical Advantage and Retinaculum

• Calcaneal (Achilles) tendon insertion point is far from the axis of rotation, increasing leverage for plantar flexion.
• Anterior muscles have tendons closer to the axis, resulting in less mechanical power.
• Retinaculum: a band of connective tissue that keeps tendons in place.
• Extensor retinaculum: on the anterior side, connects the fibula to the tibia, containing the tendons of the anterior compartment muscles (dorsiflexors).
• Flexor retinaculum: on the posterior side, wraps around and keeps flexor tendons (plantar flexors) in place.

Bowstringing of Tendons

• Retinaculum prevents bowstringing, where tendons take the shortest line when a muscle contracts if the reticulum is severed.
• Bowstringing reduces the specificity and efficiency of muscle action.
• Without the retinaculum, the foot moves into full dorsiflexion instead of controlled toe movements.
• Analogy to the carpal tunnel in the hand (flexor retinaculum).
• Severed retinaculum leads to bowstringing, decreasing tendon efficiency.

Retinacula of the Lower Limb

• Extensor retinaculum:
  • Superior and inferior aspects on the anterior side of the talocrural joint.
  • Contain and protect tendons of the anterior compartment muscles.
• Fibular (Peroneal) retinaculum:
  • Superior and inferior aspects on the lateral side of the foot.
  • Contain and protect fibularis longus and brevis muscles (lateral compartment).
• Flexor retinaculum:
  • On the posterior side of the foot and ankle.
  • Covers the flexor muscles and creates the tarsal tunnel on the medial side.

Synovial Sheaths

• Synovial sheaths reduce friction between tendons and retinacula.
• These sheaths are made of synovial membrane and contain synovial fluid for frictionless movement.

Ligaments of the Ankle

• Ligaments are essential for joint stability by preventing excessive movements.
• Collateral ligaments on the medial and lateral sides prevent inversion and eversion at the talocrural joint.
• Lateral Collateral Ligament (LCL):
  • Calcaneofibular ligament.
  • Anterior talofibular ligament.
  • Posterior talofibular ligament.
• Medial Collateral Ligament (Deltoid Ligament):
  • Anterior tibiotalar ligament.
  • Posterior tibiotalar ligament.
  • Tibiocalcaneal ligament.
  • Tibionavicular ligament.

Inversion and Eversion Injuries

• Inversion injuries more commonly affect the lateral collateral ligaments.
• Eversion injuries are less common due to the strength of the deltoid ligament.

Summary of Talocrural Joint

• The talocrural joint is structurally and functionally a hinge joint.
• Collateral ligaments prevent coronal plane movements.
• Inversion and eversion occur at other joints in the foot, which will be covered in the next video.