Explanation
The deltoid ligament is divided into superficial and deep layers. The superficial layer (tibionavicular, tibiocalcaneal, and superficial tibiotalar ligaments) crosses two joints (ankle and subtalar) and primarily resists hindfoot eversion. The deep layer (anterior and posterior deep tibiotalar ligaments) is intra-articular and crosses only the ankle joint. Biomechanical sectioning studies have conclusively shown that the stout, deep posterior tibiotalar ligament is the primary tether preventing lateral translation and external rotation of the talus. Incompetence of the deep deltoid is the defining feature of a bimalleolar-equivalent ankle fracture.

**References:**
1. Harper MC. 'The deltoid ligament. An evaluation of need for surgical repair.' *Clinical Orthopaedics and Related Research*. 1988;(226):156-168.
2. *Rockwood and Green's Fractures in Adults*, 9th Edition. Chapter: Ankle Fractures.