The canthus is the corner where the upper and lower eyelids meet. This anatomical junction forms a pair of angles on each eye, defining the boundaries of the palpebral fissure, which is the opening between the eyelids. The term “canthi” is the plural form, referring to both the inner and outer corners of the eye. These corners are integral to the eye’s function, particularly in tear distribution and protection.
Defining the Canthus and General Structure
The canthus is a point of convergence for multiple tissues, including the orbicularis oculi muscle and the conjunctiva. This area secures the eyelids to the bony orbit through fibrous connective tissue structures known as the canthal tendons. The upper and lower eyelids meet here to form a continuous margin that helps efficiently spread the tear film across the ocular surface. The coordinated movement of the eyelids, guided by the canthi, allows for the self-cleaning mechanism of the eye and protection from foreign particles.
The canthal tendons play a significant role in maintaining the structural integrity and proper tension of the eyelids. Without this stabilization, the eyelids would lose their shape and ability to protect the eye effectively.
Medial Versus Lateral Canthi
Each eye has two distinct corners, the medial canthus and the lateral canthus, which differ significantly in structure and function. The medial canthus is the inner corner, located closer to the nose, and is characterized by a softer, more rounded angle. This corner is functionally complex because it houses the lacrimal drainage system, which is where tears exit the eye. Within the medial canthus, one can find the lacrimal caruncle, a small, pink mound of modified skin, and the plica semilunaris, a crescent-shaped fold of conjunctiva.
The medial canthus also contains the puncta, which are small openings on the upper and lower eyelids that lead into the tear-collecting tubes called canaliculi. The lateral canthus, in contrast, is the outer corner, situated closer to the temple, and forms a sharper, more acute angle. This outer corner lacks the drainage structures found medially and is primarily a point of stabilization for the eyelids, anchored firmly to the bony orbital rim by the lateral canthal tendon. The lateral canthus is typically positioned slightly higher than the medial canthus.
Medical Significance of the Canthal Region
The canthal region holds considerable importance in eye health and clinical practice due to its anatomical complexity. Issues with the medial canthus often involve the tear drainage system, leading to conditions like epiphora, or excessive tearing, if the puncta or canaliculi become blocked. The measurement of the distance between the two medial canthi, known as the intercanthal distance, is used to assess facial development and diagnose conditions like telecanthus, where the inner corners are widely spaced.
Surgical procedures frequently involve this area, such as canthoplasty, which is a reconstructive or cosmetic alteration of the canthal angle to address eyelid malposition or change the eye’s shape. The canthal tendons are also subject to trauma or laxity, which can cause the eyelids to droop or turn inward or outward, requiring surgical repair. Furthermore, the thin skin of the canthal region is a common site for skin cancers, such as basal cell carcinoma, making its integrity a focus of dermatologic and reconstructive surgery.