The primary cause is a disruption or laxity of the superior peroneal retinaculum (SPR), a fibrous band that holds the peroneal tendons securely in the groove behind the lateral malleolus.

Causes

• Acute Trauma:
  • Forceful Ankle Dorsiflexion with Eversion/Inversion: This is the most common mechanism. A sudden, strong contraction of the peroneal muscles, often during activities like skiing, ice skating, or sports with rapid changes in direction (basketball, soccer, rugby), can cause the tendons to pull forcefully against the retinaculum, leading to a tear or avulsion (separation) of the SPR from the fibula.
  • Severe Ankle Sprain (especially inversion sprains): While typically associated with lateral ankle ligament injuries, a severe inversion sprain can sometimes injure the SPR, allowing the tendons to dislocate.
  • Direct Trauma: A direct blow to the outside of the ankle.
• Anatomical Predisposition
  • Shallow Fibular Groove: The groove behind the lateral malleolus where the tendons sit might naturally be too shallow or even convex instead of concave, making it easier for the tendons to slip out.
  • Absent or Insufficient Fibular Ridge: The bony ridge that helps deepen the groove may be small or absent.
  • Congenital Laxity of the SPR: Some individuals may be born with a naturally looser superior peroneal retinaculum.
  • Low-Lying Peroneus Brevis Muscle Belly: If the muscle belly extends too far distally, it can occupy more space in the groove, increasing the likelihood of subluxation.
  • Pes Cavus (High Arches): This foot type can put increased strain on the peroneal tendons and may be associated with hindfoot varus, predisposing to instability.
• Chronic Instability/Repetitive Stress:
  • Chronic Ankle Instability: Repeated ankle sprains or long-standing ankle instability can lead to chronic laxity of the SPR.
  • Repetitive Microtrauma: Activities involving frequent and forceful eversion/dorsiflexion (e.g., ballet, gymnastics) can lead to gradual stretching or weakening of the retinaculum.