Normal Gissane Critical Angle Size on Radiography
The Gissane critical angle (also called the crucial angle of Gissane) is formed by the downward and upward slopes of the superior calcaneal surface at the level of the posterior facet of the subtalar joint, visible on a lateral foot radiograph. It serves as a reliable indicator of the structural integrity of the calcaneus. Accurate measurement is essential in trauma settings, where alteration of this angle signals intra-articular calcaneal injury.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 120-145¡ |
Clinical Significance
Under normal conditions, the Gissane critical angle measures between 120° and 145° on a lateral radiograph. An increase beyond 145° is a well-recognized indicator of a depressed calcaneal fracture, particularly involving the posterior subtalar facet. Axial loading injuries — such as falls from height — commonly drive the lateral process of the talus downward into the calcaneus, widening this angle.
When the angle is elevated, careful assessment for associated injuries is warranted, including subtalar joint incongruity, peroneal tendon impingement, and lateral wall blow-out. The Gissane angle is typically evaluated alongside Böhler's angle; depression of Böhler's angle with concurrent increase of the Gissane angle strongly suggests significant intra-articular disruption. A normal or borderline Gissane angle does not exclude non-displaced fractures, and CT remains the gold standard for surgical planning.
- Intra-articular calcaneal fracture (most common cause of angle increase)
- Subtalar joint subluxation or dislocation
- Severe degenerative arthritis with posterior facet collapse
- Pathological fracture (e.g., metastatic disease, osteoporosis)
- Post-surgical or post-traumatic deformity
Reference: Iyer KM. Trauma Management in Orthopedics. Springer. (2012).
Imaging Notes
The Gissane critical angle is measured exclusively on a true lateral radiograph of the foot and ankle. Proper positioning requires the foot to be in a neutral position with the lateral aspect flat against the detector, avoiding rotation. The angle is drawn by placing one line along the downward slope of the anterior process of the calcaneus toward the posterior facet and a second line along the upward slope of the posterior facet; the angle is measured at their intersection at the highest point of the posterior facet.
Rotational malpositioning is a key technical pitfall — even slight obliquity can artificially alter the measured angle. When plain radiographs suggest an abnormal Gissane angle, CT with multiplanar reconstruction is recommended to fully characterize fracture pattern, posterior facet depression, and comminution prior to operative decision-making.