The purpose of this study is to describe normal dynamic computed tomography anatomy of hind foot in inversion/eversion movement, and to define diagnostic criteria for subtalar instability.

Dynamic CT (Aquilion One Toshiba) of the ankle during inversion and eversion movement will be performed in patients from three groups: healthy volunteers, patients with subtalar arthritis, and patients with suspicion of subtalar instability.

Inclusion Criteria:

• Group Instability: suspicion of subtalar instability
• Group Arhtritis: subtalar stiffness
• Group Healthy volunteers: healthy volunteers
• conscious and cooperating
• written consent obtained after oral and written clear and intelligible information
• no pregnancy

Exclusion Criteria:

• pregnancy
• unable to understand information about this study
• unable to understand the movement of inversion / eversion of ankle
• absence of affiliation to social security