**Core Concept**
Gissane's angle is a radiological measurement used in the assessment of intra-articular fractures of the calcaneum. It is defined as the angle formed by the intersection of the posterior facet and the posterior tuberosity of the calcaneum on a lateral X-ray. This angle is crucial in determining the displacement of the posterior facet and the potential for subchondral bone damage in the subtalar joint.

**Why the Correct Answer is Right**
Gissane's angle is an important prognostic indicator for the severity of intra-articular fractures of the calcaneum. A decrease in Gissane's angle indicates a more significant displacement of the posterior facet, which can lead to subchondral bone damage and compromise the outcome of the fracture. The angle is measured by drawing a line along the posterior facet and another line along the posterior tuberosity on a lateral X-ray. A normal Gissane's angle is around 20-30 degrees, and any decrease below 20 degrees is considered abnormal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct definition of Gissane's angle.
**Option B:** This option is incorrect because it refers to a different radiological measurement, which is not relevant to intra-articular fractures of the calcaneum.
**Option C:** This option is incorrect because it mentions a different anatomical landmark, which is not involved in the measurement of Gissane's angle.

**Clinical Pearl / High-Yield Fact**
When assessing intra-articular fractures of the calcaneum, it is essential to measure Gissane's angle to determine the severity of the fracture and plan appropriate treatment. A decrease in Gissane's angle indicates a more significant displacement of the posterior facet, which may require surgical intervention to restore the subtalar joint.

**Correct Answer:** C. Gissane's angle is formed by the intersection of the posterior facet and the posterior tuberosity of the calcaneum on a lateral X-ray.