Summary

The Chopart Joint (Midtarsal Joint) consists of the Talonavicular and Calcaneocuboid joints. It is the "S-shaped" joint that locks the foot rigid for push-off. Injuries range from occult sprains ("Midfoot Sprain") to devastating fracture-dislocations. The classic "Nutcracker Fracture" occurs when the forefoot is abducted, crushing the cuboid between the calcaneus and the metatarsals. These injuries are frequently missed (up to 40%) on initial X-rays. If untreated, they lead to a stiff, painful, arthritic foot that cannot push off. [1,2,3]

Key Facts

• The Locking Mechanism: When the subtalar joint inverts, the axes of the Talonavicular (TN) and Calcaneocuboid (CC) joints diverge, locking the Chopart joint rigid (Lever Arm). When everted, they are parallel, unlocking the foot (Shock Absorber). Disruption destroys this mechanism.
• The Nutcracker: An abduction force compresses the lateral column (Cuboid) like a nut in a cracker.
• The "Piano Key" Sign: Instability of the TN or CC joint allows the metatarsal heads to be pushed up and down independently.

Clinical Pearls

"Fleck Sign": A small avulsion bone fleck dorsal to the Talonavicular or Calcaneocuboid joint on lateral X-ray represents a major ligamentous avulsion (Bifurcate Ligament). It is the tip of the iceberg.

"Lateral Column Length": The goal of surgery in a Nutcracker fracture is to push the crushed cuboid back out to length. If the lateral column is short, the foot drifts into Abduction (Flatfoot).

"Bridge Plating": You cannot put screws into a crushed cuboid (mush). You must put a plate that spans from the Calcaneus to the Metatarsal, "bridging" the cuboid to hold it out to length while it heals.

                 MIDFOOT INJURY
                        ↓
            X-RAY VISIBLE FRACTURE?
           ┌────────────┴─────────────┐
          YES                        NO
           ↓                          ↓
        CT SCAN             CAN WEIGHT BEAR?
    (Classify Pattern)      ┌─────────┴─────────┐
           ↓               YES                 NO
   DISPLACED / SHORT?      ↓                   ↓
      ┌────┴────┐      SPRAIN              CT SCAN / MRI
     NO        YES        ↓                   ↓
      ↓         ↓       BOOT            OCCULT FRACTURE?
    CAST     SURGERY    (4w)           ┌──────┴──────┐
   (6-8w)  (ORIF/Bridge)              NO            YES
                                      ↓              ↓
                                   SPRAIN        TREAT AS
                                                FRACTURE

Indications

• Non-displaced fractures.
• Ligamentous sprains with NO instability on stress view.
• Lateral column length maintained.

Protocol

• Boot / Cast: 6 weeks.
• Weight Bearing: Non-weight bearing for 4 weeks usually required to prevent displacement.

Goal

• Restore Column Length (Medial and Lateral).
• Restore Joint Congruity.

1. Cuboid Nutcracker Fracture

• External Fixation: Distractor placed between Calcaneus and Metatarsal to pull lateral column out to length.
• ORIF:
  • Bridge Plate: A plate screwed to Calcaneus and 4th/5th Metatarsal. Spans the cuboid. Removed at 3 months.
  • Bone Graft: To fill the void in the crushed cuboid.

2. Navicular Fracture

• ORIF: Screws or Plates (Medial column plate).
• Fusion: Primary fusion of Talonavicular joint is sometimes needed for comminuted fractures because TN arthritis is poorly tolerated.

3. Ligament Repair

• Direct repair of Bifurcate ligament (rarely done alone). Usually reduction of bones restores stability.

Primary Fusion?

• Displaced fracture-dislocations of the Talonavicular joint have a very high rate of post-traumatic arthritis (arthritis occurs within 1-2 years). Some surgeons advocate Primary Arthrodesis (Fusion) of the TN joint immediately to save the patient 2 years of pain.

Missed Diagnosis Stats

• Richter et al: Reported that Chopart injuries are missed in up to 41% of polytrauma patients.

The Injury

You have damaged the "S-joint" in the middle of your foot (Chopart Joint). This is the locking mechanism that turns your foot from a flexible shock absorber into a rigid lever for walking.

The "Nutcracker"

Specifically, you have crushed the cube-shaped bone on the outside of your foot (Cuboid). It has been squashed like a nut in a cracker.

The Fix

We need to pull the foot back out to its proper length. We will put a metal plate across the crushed bone to "bridge" it, holding it open while it heals. We will remove the plate in 3 months.

The Future

Your midfoot will likely be stiff. You might find it hard to walk on uneven ground or stand on your tiptoes.

Q1: What is the "Bifurcate Ligament"? A: A Y-shaped ligament originating on the anterior process of the calcaneus and inserting into the cuboid and navicular. It is the primary stabilizer of the Chopart joint.

Q2: What is the consequence of a short lateral column (crushed cuboid)? A: Abduction of the forefoot -> Pes Planovalgus (Flatfoot) deformity.

Q3: Why is the Talonavicular joint called the "Acetabulum Pedis"? A: Because acts like a ball-and-socket joint (Talar head = Ball, Navicular = Socket). It is essential for complex hindfoot motion. Fusing it eliminates 90% of subtalar motion.

Q4: Differentiate a Lisfranc from a Chopart injury. A:

• Lisfranc: Tarsometatarsal joint (Distal to Chopart).
• Chopart: Midtarsal joint (Talonavicular/Calcaneocuboid).

(End of Topic)