Extensor Digitorum Brevis (EDB) Interposition: A Cause of Blocked Ankle Fracture Reduction
Key Takeaway
The Extensor Digitorum Brevis (EDB) muscle or its tendons can block ankle reduction by becoming entrapped within the fracture site, especially in specific lateral malleolar fractures or posteromedial dislocations. This soft tissue interposition mechanically prevents accurate anatomical reduction of the lateral malleolus or talus, posing a significant challenge to successful open reduction and internal fixation.
You are in the operating theater performing an ORIF on a 35-year-old patient with an unstable, closed, bimalleolar ankle fracture-dislocation. After standard manipulation and provisional fixation of the lateral malleolus, your fluoroscopic imaging reveals a persistent 4mm gap at the fracture site and the lateral malleolus appears slightly malrotated. What are your immediate differentials and how will you systematically evaluate this?
Candidate: I would immediately suspect a soft tissue interposition. My differentials would include an interposed deltoid ligament, osteochondral fragment, or entrapment of a tendon such as the tibialis posterior or flexor hallucis longus. I would extend the exposure of the fracture site to perform a formal visual inspection, irrigate, and remove any physical block to ensure anatomical reduction.
Failing to mention the Extensor Digitorum Brevis (EDB) in the differential for a lateral-sided block. Candidates often fixate on medial-sided blocks (like the deltoid or PT tendon) while forgetting that the EDB muscle belly or tendinous slips can frequently interpose in lateral malleolar fractures with talar shift.
I would perform a systematic intraoperative check: 1. Verification: Confirm the malreduction under multi-planar fluoroscopy. 2. Inspection: Perform an open formal exploration of the fracture site. 3. Differential Diagnosis for Block: I will specifically inspect for the Extensor Digitorum Brevis (EDB) muscle belly or tendinous slips, which is a known cause of blocked reduction in lateral malleolar fractures, alongside osteochondral fragments, deltoid ligament entrapment, or the Tibialis Posterior/FHL tendons if the displacement is significant. 4. Action: Use a blunt retractor to clear the soft tissue, ensuring the lateral malleolus is reduced anatomically to the fibular notch before definitive fixation.
Regarding your mention of the Extensor Digitorum Brevis (EDB), what is its anatomical relationship to the ankle joint, and how does this proximity lead to it becoming an intra-articular block?
Candidate: The EDB originates from the lateral surface of the calcaneus and the sinus tarsi. During high-energy rotational ankle injuries, the fibula may fracture and the talus may shift significantly. Due to the EDB's position directly over the tarsal bones and lateral ankle structures, the muscle belly can be pulled into the fracture gap of the fibula or wedged into the space between the talus and the distal fibula, physically preventing reduction.
Confusing the EDB with the Extensor Digitorum Longus (EDL). While both can be involved, the EDB's specific origin on the calcaneus makes it uniquely susceptible to being "trapped" during talar subluxation and fibular displacement.
The EDB arises from the superolateral calcaneus and the sinus tarsi. In cases of fracture-dislocations with significant lateral talar shift and fibular displacement, the anatomical continuity of the lateral structures is disrupted. The EDB muscle belly, being mobile and located immediately deep to the lateral ankle tissues, can be sucked into the space vacated by the displaced talus or the widened fibular fracture gap. Its entrapment prevents the anatomical repositioning of the distal fibula back into the incisura tibiae, necessitating formal retraction of the muscle belly to achieve reduction.
