A horizontal cleavage tear is a degenerative tear of a meniscus, extending into the meniscus substance, and cleaving it horizontally into two parts, like a clam.
The meniscus is the wedge-shaped shock absorber between the femur and tibia (two menisci in each knee). After developing a horizontal cleavage tear the meniscus becomes incompetent. Also, with walking there may be a valve-like effect, with joint fluid being trapped inside the meniscus under pressure - this may cause the tear to extend until the joint fluid is squeezed out of the edge of the meniscus, forming a meniscal cyst.
The tear may not be immediately apparent during arthroscopy, and is frequently missed by the inexperienced surgeon.
Current treatment is usually to resect (cut away) the damaged surface, leaving an intact rim.