 knee, anatomy, cruciates, the all-important ACL or anterior cruciate ligament. The ACL is a two-bundled ligament. It's got a smaller antiromedial and a larger postural lateral ligament that's tight in extension. Here's our ligament right here and this spot right here where it enters the femur is a critical position known as the over-the-top position. The ACL comes down, it slopes down obliquely from postural superior to antero inferior. We'll see in the coronal projection it looks a little bit like a water slide and as it gets down more distally it starts to fan out and that fanning is normal. The bundles of the ACL are arranged kind of like a celery stalk. They're mostly parallel and you know that the end of a piece of a celery does fan out. On an MRI the upper portion of the ligament looks a little pinched but that's rather artificial for when you look at it in the axial projection it's not quite as narrow. Now the functional ACL fiber recruitment is more complicated than just a one or two fiber bundle arrangement. The ACL is the primary restraint to anterior tibial displacement. So if you have an ACL that is deficient, sprained, stretched, partially torn, high grade or full thickness tear you will see when you lie on your back the anterior cruciate ligament supporting structures you will see when you're lying on your back the tibia drift anteriorly. We'll talk more about this as this is the sign of passive anterior tibial translation and is invaluable in the diagnosis of ACL deficiency whether the ACL is torn or not. Now the posterior lateral bundle provides principal resistance to hyper extension force. The cruciate ligament is surrounded by this aquamarine blue structure I'm going to color it in just for fun I'll make it kind of orangey right there and that sheet is common to the ACL and the PCL and together they have a little blowhole in the back right there. I should make my blowhole a little blacker or darker I'll pick black in fact. So there's a little blowhole coming out the back of this common sheet. I'm going to make it black and here it is it communicates with the sheet of the ACL and PCL and it'll just stand if there is fluid or injury in this joint and sometimes it'll even poke through this ligament right here sort of blue ligament which is known as the oblique popliteal ligament and it'll go right through it and can compress the popliteal fossa. So in summary we've got a two bundled structure, an antrometeal and a posterior lateral one restraint to the tibia, the other supporting the knee in hyperextension. With ACL transaction or deficiency you have anterior tibial translation they're surrounded by a sheath that has a little blowhole or connection hole in the back and when this sheath fills with fluid this area may distend and push backward on important key structures and there are key areas of penetration and attachment of the ACL one called the over the top position back here and the other one in the tibia to be discussed a little bit later on. Let's separate that as a vignette for now and you can move on to the next accompaniment vignette.