Video:

Introduction:

Subtalar joint (ST) instability is often misdiagnosed as simple lateral ankle instability or even totally missed. It is defined as a varus tilt with anterior and medial subluxation of the calcaneus in relation to the talus.
Clinical assessment, range of motion testing and stability of the ST are often not exact due to soft tissue around the hindfoot. The clinical examination of the ST stability must be done with the tibiotalar joint in dorsiflexion ‘‘blocking’’ the talus within the mortise limiting tibiotalar motion. A specific test using internal rotation while doing a varus stress test on the calcaneus, patients with subtalar instability demonstrate a medial shift of the calcaneus and an increased talocalcaneal angle. Congruency of the posterior compartment of the ST joint, determines mobility and guidance of motion. The intrinsic ligaments corresponding to the interosseous talocalcaneal ligament (ITCL), the oblique talocalcaneal ligament (TCO), the anterior cervical ligament (ACL), and the lateral talocalcaneal ligament (LTCL) are derived from the inferior extensor retinaculum. The extrinsic ligaments; fibulocalcaneal ligament (FCL) and the tibiocalcaneal part of the deltoid ligament control sliding, rolling and torsional ST motion. Still there is no single axis of motion identified, but several axes due to the curvatures of the facettes. Shallower angle of the posterior facet with subtalar instability thought to be a predisposing factor to developing subtalar instability in the anteroposterior plane.

Material and method:

One the first cadaver, we selectively cut the deltoid ligament then the intrinsic ligaments. On the second cadaver, we cut the FCL then the intrinsic ligaments. On the third cadaver we cut only the intrinsic ligaments.
The clinical assessments of the increase in motion and instability of the subtalar joint was performed by varus tilting and by anterior and medial subluxation.

Results:

Each group of ligaments (medial, lateral and instrinsic) has a different contribution for the axes of rotation. Complete tearing of the ACL, CFL and the ATFL were implicated in inducing ST instability.
Conclusion: Subtalar instability is a complex pathology. More accurate testing with objective assessment of the contribution of each group of ligaments should be performed in future studies.