Are knee joints different from elbow joints?
The elbow is a very complex joint. In quadrupeds, it permits (basically) bending and straightening (flexion and extension), just like a knee.
But in primates (which includes humans) a whole new movement is available: pronationand supination.
This is the movement which allows you to turn your hand over from palm down to palm up. It’s necessary for freedom of movement of the hand (to permit climbing and swinging in the first instance). It involves a very complex movement at both the wrist and the elbow at the same time. If your knee could do it, you could turn your feet around and point them backward. In order to permit this movement, the radius rotates like a key in a lock, twisting the whole forearm.
Imagine a human on all fours. At the elbow, most of the weight is transmitted from thehumerus (upper arm bone) to the ulna. But at the wrist, most of the weight is transmitted from the radius to the carpus (wrist). The ulna hardly carries any weight at the wrist. That means there needs to be some way of transferring the load between the radius and ulna in the forearm, and there is: the tough but flexible interosseous ligament which transmits force but still permits pronation and supination.
So the elbow is really cool, but it can’t work alone and requires a complex wrist and a complex forearm to permit its wide range of flexibility.
The knees, however, have a very different role, in that they need to support most of our weight and they need to enable us to run, to sit, to get up, etc.
The knee is one of the largest and most complex joints in the body. The knee joins the thigh bone (femur) to the shin bone (tibia). The smaller bone that runs alongside the tibia (fibula) and the kneecap (patella) are the other bones that make the knee joint. Tendons connect the knee bones to the leg muscles that move the knee joint. Ligaments join the knee bones and provide stability to the knee:
- The anterior cruciate ligament prevents the femur from sliding backward on the tibia (or the tibia sliding forward on the femur).
- The posterior cruciate ligament prevents the femur from sliding forward on the tibia (or the tibia from sliding backward on the femur).
- The medial and lateral collateral ligaments prevent the femur from sliding side to side.
Two C-shaped pieces of cartilage called the medial and lateral menisci act as shock absorbers between the femur and tibia. Numerous bursae, or fluid-filled sacs, help the knee move smoothly.
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