The meniscus is crescentic, with two horns (like a new moon) - and the posterior horn is the one at the back.

The horn at the front is called the anterior horn.
Issues of the posterior horn
Because the rounded ends of the femur obscure much of the posterior horn, it is difficult to see during routine arthroscopy. By putting the knee into a 'figure-4' position, the back of the joint can be opened up and the posterior horn can be examined with a probe.
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FORUM DISCUSSIONS
Grade III tear in posterior horn of medial meniscus.
Horizontal tear in posterior horn of medial meniscus. What do you think?
Posterior Horn of medial meniscus tear grade 3a How to treat?
CLINICAL PUBLICATIONS
Clinical Evaluation of the Root Tear of the Posterior Horn of the Medial Meniscus in Total Knee Arthroplasty for Osteoarthritis. Choi E-S and Park S-J. Knee Surg Relat Res. 2015 Jun; 27(2): 90–94.