postaxial

Postaxial is a term used in anatomy and embryology to describe the position of a structure on the limb or body that is toward the posterior aspect or away from the limb’s axis. In human anatomy, the postaxial side of the upper limb corresponds to the ulnar (little finger) side, while in the lower limb it corresponds to the fibular (little toe) side. By contrast, the preaxial side refers to the radial (thumb) side of the hand and the tibial (big toe) side of the foot.

In embryology, preaxial and postaxial designations describe the organization of the developing limb along the anterior-posterior axis. The postaxial region is typically involved in forming digits and structures toward the little finger or little toe, with signaling centers guiding patterning along this axis. A classic example of postaxial pathology is postaxial polydactyly, a congenital condition characterized by the presence of an extra digit on the postaxial side of the hand or foot. The condition can occur in isolation or as part of syndromic patterns and may vary in degree of duplication and functional impact.

The term postaxial is primarily used in clinical anatomy, developmental biology, and related medical fields. It provides a convenient reference for describing limb orientation, congenital anomalies, and the regional identity of limb structures during growth and development. The counterpart term, preaxial, is used similarly to denote the opposite (radial/tibial) side.