Tricuspid annulus

The tricuspid annulus (TA) is the fibromuscular ring-like structure that serves as the attachment point for the three leaflets of the tricuspid valve, forming the junction between the right atrium and the right ventricle. Rather than a well-defined fibrous ring (as the term "annulus" might imply), it is more accurately described as a functional unit defined by the atrioventricular junction and the basal attachment of the tricuspid valve leaflets into the right ventricular endocardium.

Shape and Structure

The TA is nonplanar, saddle-shaped (or D-shaped), with an elliptical configuration. It has two higher points oriented superiorly toward the right atrium (anteroseptal and posterolateral portions) and two lower points oriented toward the right ventricular apex (anterolateral and posteroseptal portions). The annular plane sits nearly vertically in the chest and is rotated approximately 45° from the sagittal plane. Histologically, the TA contains very little fibrous tissue or collagen, particularly along the right ventricular free wall segment; instead, it is composed primarily of epicardium and endocardium with adipose tissue filling the atrioventricular groove. Multiple muscular bars (approximately 24 mm in diameter) are found within the annulus in a crisscross pattern, which may provide structural support.

Dimensions

Normal tricuspid annular circumference is approximately 12 ± 1 cm, with an area of 9.811 ± 2 cm² as measured by 3D echocardiography. The TA is approximately 20% larger than the mitral annulus, making the tricuspid valve the largest of the four cardiac valves.

Dynamic Properties

The TA is a highly dynamic structure, with its area changing by up to ~30% during the cardiac cycle. It is largest during end-systole/early diastole and during atrial systole, and smallest during mid-systole. These dynamic changes are also influenced by loading conditions.