semilunar

Semilunar is an adjective used in anatomy and related fields to describe crescent-shaped, or half-moon-shaped, structures. The term comes from Latin semi (“half”) and luna (“moon”).

There are two semilunar valves of the heart: the aortic valve and the pulmonary valve. Each valve consists of three crescent-shaped leaflets (cusps) attached to a fibrous annulus. They open during ventricular systole to allow ejection into the aorta or pulmonary artery and close during diastole to prevent backflow. Unlike the atrioventricular valves, semilunar valves have no chordae tendineae or papillary muscles; their closure relies on cusp coaptation and pressure gradients. The aortic valve has sinuses of Valsalva behind each cusp, from which the coronary arteries arise. The pulmonary valve also has sinuses, but they do not give rise to coronary arteries.

The term semilunar also appears in reference to the knee joint’s menisci. The medial and lateral menisci are crescent-shaped fibrocartilaginous structures between the femur and tibia. They help stabilize the joint, distribute loads, and facilitate lubrication and smooth articulation. Tears or degeneration of the menisci can impair knee function and lead to pain or instability.

In neuroanatomy, the semilunar ganglion, also called the trigeminal or Meckel’s ganglion, is the sensory ganglion of the trigeminal nerve (CN V). It houses the cell bodies of sensory neurons for the three divisions of CN V, whose afferent fibers project from the face to the brainstem.

The term semilunar is also used more broadly to describe other crescent-shaped structures in anatomy and historical literature.