The subtalar joint (talocalcaneal joint) is a complex articulation located in the rearfoot, just beneath the main ankle joint. It is fundamental for stability and movement, allowing the foot to adjust to the ground during walking and running. Its primary function is to enable side-to-side movement, which is necessary for navigating uneven surfaces. The joint works with other foot joints to translate the rotational forces of the leg into adaptable foot movements, ensuring shock absorption and efficient propulsion.
Anatomy: Bones and Joint Structure
The subtalar joint is formed by the meeting of two large tarsal bones: the talus, which sits superiorly, and the calcaneus, the heel bone, located inferiorly. This articulation is classified as a synovial joint, meaning it is encased within a fibrous capsule and contains lubricating fluid to facilitate smooth movement. The capsule is lined with a synovial membrane, which produces fluid that nourishes the cartilage and reduces friction.
The connection between the talus and calcaneus involves three distinct articular facets: anterior, middle, and posterior. The posterior facet is the largest and forms the main anatomical subtalar joint. These facets are separated by the tarsal canal, which contains the interosseous talocalcaneal ligament.
This ligament is a thick band of tissue that anchors the talus and calcaneus together, providing significant stability. Additional ligaments, such as the medial and lateral talocalcaneal ligaments, further reinforce the joint capsule externally. The complex arrangement of these three gliding facets and the intricate ligament network allows for the specialized, multi-directional motion that defines the joint’s function. In clinical practice, the joint is often considered a functional unit that includes the posterior articulation and the talocalcaneonavicular joint anteriorly.
Biomechanics: The Role in Foot Movement
The primary movements occurring at the subtalar joint are inversion and eversion, which are side-to-side motions of the heel. Inversion turns the sole of the foot inward, while eversion turns the sole outward. These actions are coupled with other motions in three planes of movement, occurring around an oblique axis.
This coupled movement is referred to as pronation and supination, which are components of the walking cycle. Pronation is a triplanar motion consisting of eversion, abduction (moving away from the midline), and dorsiflexion. Conversely, supination involves inversion, adduction (moving toward the midline), and plantarflexion.
During the initial contact phase of walking, the subtalar joint pronates, allowing the midfoot to “unlock” and become a flexible structure. This flexibility enables the foot to absorb the shock of impact and conform to the ground surface irregularities. Pronation also helps absorb the internal rotation of the leg as the foot strikes the ground.
As the body moves over the foot, the subtalar joint begins to supinate during the latter half of the stance phase. Supination causes the midfoot to “lock up,” making the foot a rigid lever. This rigidity provides a stable base for the calf muscles to push off and propel the body forward efficiently. The ability of the joint to transition smoothly between a flexible adaptor and a rigid lever is fundamental to efficient human gait.
Dysfunction: Common Issues Affecting the Subtalar Joint
When the subtalar joint is compromised, it can lead to pain and instability, often felt around the heel or on the outside of the ankle. One common issue involves misalignment, where excessive pronation contributes to a flat foot, or excessive supination is associated with a high arch. These misalignments alter the kinetic chain, potentially causing compensatory strain in the knees, hips, and lower back.
The joint can also be affected by trauma, such as a severe sprain, which can stretch or tear the supporting ligaments, particularly the interosseous talocalcaneal ligament. Injuries to the subtalar joint are sometimes misdiagnosed as simple ankle sprains due to their proximity, but they typically involve pain and difficulty walking on uneven ground. Such injuries often cause instability and persistent pain, especially when bearing weight.
Arthritis is another frequent problem, often developing years after a fracture of the calcaneus or talus (post-traumatic arthritis). The degeneration of the articular cartilage leads to stiffness, swelling, and chronic pain that worsens with prolonged standing or activity. Progressive loss of motion in the subtalar joint, making it difficult to turn the foot side-to-side, is a characteristic sign of advanced arthritic changes.