Your knee is one of the most important joints in your body. It is also one of the more interesting ones, because humans are the only animal to stand with an upright posture and bear their body weight through the fully extended knee joint. Whilst most people mistakenly believe the knee is a simple hinge joint, this is not true. Knees provide stability and allow your legs to bend (flexion), straighten (extension) and swivel (rotation).
Doctors and scientists have been studying the knee for a long time and we are still learning new things about it. As recently as 2013, after several years of researching anterior cruciate ligament (ACL) injuries, Belgian scientists provided the first full anatomical description of the anterolateral ligament (ALL). However, back in 1879, a French surgeon called Paul Segond was the first to report the existence of a “pearly, resistant, fibrous band” connecting the outside front of the femur to the tibia. Yet it was not named and its existence was forgotten or ignored and often assumed to be part of iliotibial (IT) band.
The knee is made up of four structures: bones, cartilage, tendons and ligaments.
The bones involved in the knee joint are the femur (thighbone) and the tibia (shinbone). They meet at the patella (kneecap).
The cartilage provides shock absorption when you walk, run, climb or participate in any other impact activity. There are two types of cartilage—the menisci and the articular cartilage. Cartilage does not have blood vessels and it can't regenerate itself naturally, which is why damage to cartilage is very hard to repair.
The tendons connect the knee bones to muscles like the quadriceps and hamstrings, which then contract and relax to move the knee.
A ligament is a tough strip of connective tissue that joins one bone to another bone around a joint. The knee joint ligaments help to stabilize and support the knee when it is in motion. The four main ligaments of the knee are: two collateral ligaments— the medial collateral ligament (MCL) and the lateral collateral ligament (LCL), which resist side-to-side movement and help prevent rotation between the femur and the tibia; and two cruciate ligaments known as the anterior cruciate (ACL) and posterior cruciate (PCL) ligaments, which respectively stop the tibia moving forward or backward.
Another important function of the ACL is its role in the “screw-home mechanism.” This is an important element of knee stability when a person stands upright. It occurs towards the end of full extension of the knee (between 0° and 20°) when the tibia rotates, causing both cruciate ligaments to tighten, which locks the knee. This has the effect of putting most of the weight onto the cartilage, menisci and bones of the joint and giving the muscles of the thigh and calf a rest. Tearing of the ACL is very common in sports where there are a lot of abrupt movements, like in basketball and football.
So what does this all mean?
Researchers believe that this newly named anterolateral ligament (ALL), could be important in protecting the knee when we twist or change direction. Despite advancements in surgical techniques for ACL repair, between 10% and 20% of people who have the surgery are unable to recover full use of their knee joint. Interestingly, some of these patients say that their knees give way when they twist or turn. Researchers now think an injury to the ALL may be partly responsible for this. They believe some people may injure the ALL at same time as the ACL, leaving the knee less stable as the leg rotates. New questions are being raised about ACL injuries and this could signal a breakthrough in the treatment of affected patients.
The human knee just got a lot more interesting!
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Further Reading
Anatomy of the anterolateral ligament of the knee