Inversion and eversion are distinct movements of the foot, allowing dynamic interaction with varying terrains. Inversion turns the sole of the foot inward, towards the body’s midline. Conversely, eversion turns the sole outward, away from the midline. These motions contribute to the foot’s adaptability and balance during activities like walking or running.
The Ankle Joint
Inversion and eversion movements predominantly occur at the ankle, specifically within the subtalar and transverse tarsal joints. The subtalar joint, located beneath the main ankle joint, forms between the talus and calcaneus bones. This joint allows the foot to adapt to uneven surfaces. While the talocrural joint primarily permits up-and-down movements (dorsiflexion and plantarflexion), the subtalar joint handles side-to-side tilting of the sole. Inversion’s range of motion is around 25° to 30°, while eversion is more limited, ranging from 5° to 10°.
Anatomical Structures for Movement
Several bones contribute to inversion and eversion. The talus and calcaneus are the primary bones forming the subtalar joint, facilitating these rotations. The navicular and cuboid bones, part of the transverse tarsal joint, also coordinate these foot movements.
Ligaments provide stability and limit excessive ankle motion. The lateral collateral ligaments, including the anterior talofibular (ATFL), calcaneofibular (CFL), and posterior talofibular (PTFL) ligaments, resist inversion. On the medial side, the deltoid ligament complex limits eversion.
Muscles in the lower leg initiate and control these movements. For inversion, the primary muscles are the tibialis anterior and tibialis posterior. The fibularis (peroneus) longus and fibularis (peroneus) brevis muscles are the main contributors to eversion. These muscles, via their tendons, connect to foot bones to execute these actions.
Functional Role of Ankle Movement
Inversion and eversion allow the foot to adapt to uneven ground, providing stability and preventing falls. They also maintain balance during dynamic actions like walking, running, and jumping. The ability to invert and evert the foot contributes to shock absorption, reducing impact on joints and muscles. This dynamic stability aids efficient locomotion and protects the lower limbs from injury.
Common Injuries
Ankle sprains are common injuries from excessive inversion and eversion. An inversion ankle sprain occurs when the foot rolls inward excessively, stretching or tearing outer ankle ligaments. These lateral ankle sprains are the most common type, accounting for 70-85% of all ankle sprains, often involving the anterior talofibular ligament (ATFL). The mechanism often involves plantar flexion and inversion.
Eversion ankle sprains, though less common, happen when the foot rolls outward excessively, affecting the deltoid ligament on the inner ankle. These injuries are often more severe due to the deltoid ligament’s strength, and can sometimes lead to inner ankle bone fractures. Both sprains result in pain, swelling, and reduced mobility, with severity depending on ligament damage. Prompt evaluation and treatment are necessary for healing and to prevent chronic instability.