The ankle joint is a highly mobile structure that bears the entire weight of the body, making it one of the most frequently injured joints. An ankle sprain occurs when the stabilizing ligaments are stretched or torn during an unnatural twisting motion. When the foot rolls inward, it is called an inversion or lateral sprain, damaging the ligaments on the outside of the ankle. Conversely, an eversion or medial sprain happens when the foot rolls outward, stressing the ligaments on the inside. Inversion sprains account for approximately 85% of all ankle sprains, a disparity rooted in the joint’s fundamental anatomy and biomechanics.
How Bony Architecture Favors Inversion
The structure of the ankle joint itself, often called the ankle mortise, creates an inherent bias against the foot rolling outward. The lower leg bones, the tibia and the fibula, form a protective socket around the talus, which is the main ankle bone. The bony prominences on either side of the ankle are the malleoli, acting like mechanical barriers to motion.
The outer ankle bone, known as the lateral malleolus of the fibula, extends significantly lower toward the foot than the inner ankle bone, the medial malleolus of the tibia. This longer projection on the outside acts as a deep physical restraint, limiting how far the foot can naturally roll into eversion. Any forceful attempt to roll the foot outward is quickly blocked by this extended bony structure. The shorter inner malleolus, however, offers less resistance to the foot rolling inward toward the midline of the body. This structural difference means that the joint is mechanically less stable against inversion, allowing the foot to move further in that direction before any bone-on-bone contact provides a physical stop.
The Strength Differential in Ankle Ligaments
Ligaments are the passive restraints that connect bones and prevent excessive movement, and their arrangement in the ankle demonstrates a clear difference in strength between the medial and lateral sides. The medial side is secured by the Deltoid ligament complex, a broad, fan-shaped structure composed of four distinct, fused bands. This complex collectively forms an exceptionally strong and resilient stabilizing unit.
Because of its immense strength, a force large enough to tear the Deltoid ligament during an eversion event is rare. Such a force often results in a fracture of the outer ankle bone before the ligament fails. Only about 15% of ankle sprains involve this medial complex, and they frequently occur alongside a bone break.
The lateral side, which resists inversion, is stabilized by three separate ligaments: the Anterior Talofibular Ligament (ATFL), the Calcaneofibular Ligament (CFL), and the Posterior Talofibular Ligament (PTFL). The ATFL is the weakest and thinnest of these lateral ligaments and is the one most frequently injured during an inversion sprain. This ligament is often the first to tear when the foot rolls inward, sometimes in conjunction with the CFL.
The comparative fragility of this three-part lateral complex to the robust, fused Deltoid complex creates a significant mechanical vulnerability. The weaker lateral ligaments are simply more prone to tearing than the Deltoid ligament is to stretching or rupturing.
Why Functional Range of Motion is Greater for Inversion
The movement of the foot, particularly the side-to-side tilting, is primarily controlled by the subtalar joint located just below the ankle joint. This joint is naturally designed to permit a greater degree of inversion than eversion.
The functional range of motion for inversion is greater than for eversion. Studies show that inversion can range from 30 to 40 degrees, while eversion is restricted to 15 to 20 degrees. This larger arc of motion means the foot has a wider window to accidentally roll inward during activities like stepping on uneven ground or landing awkwardly.
The inherent flexibility and greater movement allowance into inversion increases the opportunity for the lateral ligaments to be overstretched. The limited range of eversion acts as an additional biomechanical safeguard, reducing the likelihood of the foot rolling far enough outward to injure the stronger Deltoid ligament.