A demifacet is one of two partial articular surfaces (facets) found on a vertebra that contributes to the articulation with the head of a rib.
Demifacets are bilaterally paired and located on the superior and inferior posterolateral aspects of certain vertebrae. They are designed to articulate with the head of a rib. Specifically:
- The superior demifacet of the inferior vertebra articulates with the upper half of the rib head.
- The inferior demifacet of the superior vertebra articulates with the lower half of the same rib head.
This arrangement ensures that a typical rib head articulates with two adjacent vertebrae and the intervening intervertebral disc, contributing to the structural integrity and movement of the thoracic spine. The vertebrae that typically have demifacets are the thoracic vertebrae (T2-T8). T1 typically has a full superior facet and an inferior demifacet. T9 may have a full superior facet or superior demifacet, and typically no inferior demifacet. T10-T12 typically have full facets.
