A new approach to 3D imaging has shed new light on ankle kinematics.

Ankle injuries are among the most common reasons for emergency department visits. EDs treat more than 628,000 ankle injuries per year. Ankle injuries account for about 20% of all visits to the ED. While ankle injuries are not life threatening, they can cause disability that decreases quality of life, so accurate diagnosis is always essential.

Many ankle injuries involve the “body’s steering wheel,” the subtalar joint. Serving as the articulation between the facets on the inferior surface of the talus and the matching facets on the superior surface of the calcaneus, the subtalar joint allows for inversion and eversion of the foot. ED physicians often rely on imaging to evaluate ankle injuries, but the location of the subtalar joint has made in vitro imaging challenging.

Common problems affecting the subtalar joint include instability following ligament injury, arthritis, and pes planus in children and adults. Trauma to the subtalar joint, such as talar body or calcaneal fracture, is a common cause of subtalar joint arthritis.

These problems of the hindfoot can prevent participation in sports or exercise, disrupt normal daily activities, and have a negative effect on mobility and overall function. In fact, research shows end stage ankle and hindfoot arthritis can negatively affect quality of life as much as heart disease.

Revealing the Secrets of the Subtalar Joint

In the new study, researchers announced the development of a new approach to imaging that could give doctors their first glimpse of the subtalar joint in action. The research team created a novel technique for the non-invasive in vivo quantification of bone-to-bone motion of the subtalar joint under full weight bearing. This new technique could help improve the clinical evaluation of subtalar joint function, analysis of surgical procedure outcomes, and the design of replacement prosthesis.

The technique combines weight-bearing clinical computed tomography (CT) with digital volume correlation (DVC) to evaluate the center of rotation and helical axis of the subtalar joint during eversion and inversion motion.

Researchers enrolled eight healthy asymptomatic volunteers into the study. Each participant stepped onto a PedCAT standing CT scanner for an initial scan. Next, the researchers placed two custom wedges, which were 16 mm high and at 30 degrees inclination onto the scanner. Each subject stood on the wedges with one foot inverted and the other everted during the CT scan.

The subjects then turned around 180 degrees, to reverse the inversion and eversion for another scan. Following image acquisition, 3D reconstruction of the images allowed for multi-planar view of the subtalar joint in the three configurations. Image post-processing allowed for calculation of the subtalar joint axis and calculation of the joint’s center of rotation.

“This is the first time this technique has been used in humans,” said lead author of the study and Reader in Bioengineering at the University of Portsmouth, Dr. Gianluca Tozzi. “It is non-invasive and gives clinicians a perfect view of a patient's subtalar joint motion under full weight-bearing, making it possible for the first time to determine the joint's centre of rotation which, in turn, opens the possibility of much-improved design of joint replacements.”

The technique can someday help clinicians identify different joint kinematics in patients with ligamentous laxity and instability. It could also help clinicians develop personalized treatments for patients with stiffness and arthritis of the hindfoot.

"Being able to see the subtalar joint in action is made possible by a combination of 3D imaging (computed tomography) and digital volume correlation. The technology has a huge potential to be expanded, allowing doctors to see any strain in the bone, greatly improving clinical diagnosis.”

The researchers published their study in Nature’s Scientific Reports.