The Femur

The femur is the thigh bone, and it's the longest bone in the body. Its lower end meets with the tibia to form the knee joint; the upper end is rounded and fits into a socket in the pelvis. The femur also provides attachment points for the leg and buttocks muscles.

The ball and socket joint where the femur meets the pelvis is the type of joint that gives the largest range of movement. Because the ball at the end of the femur fits deeply into the socket in the pelvis, this joint is very stable; a similar but looser-fitting joint at the shoulder has more mobility.

A rim of cartilage lining the socket helps grip the femur firmly; the ligament binding the two bones is among the strongest in the human body.

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