Station 5: The pelvic girdle and the leg

Os Coxa (innominate)

The os coxae are the two large bones of the pelvis, which join with the sacrum to form the pelvic girdle. Although the os coxa is one unit in the adult, it consists of three separate bones (the ilium, pubis, and ischium), which all come together in the acetabulum, the large cup-shaped cavity that articulates with the head of the femur. Another significant feature is the greater sciatic notch, which transmits several important nerves and blood vessels and is useful for determining an individual’s sex.

Ilium: the flat, somewhat curved upper portion. The iliac crest is the most superior edge of the ilium, and can be felt through the skin.

Ischium: the most posterior portion of the os coxa. The ischial tuberosity supports the body in the sitting position.

Pubis: the most anterior portion. It articulates with the pubic bone from the opposite side at the pubic symphysis to form the anterior wall of the pelvis.

Sacrum

Sacral vertebrae are fused in the adult to form a single bone, the sacrum. The weight of the upper body in humans is passed to the os coxae (innominate bones) and femora (plural for femur- thigh bone) through the upper part of the sacrum, so this bone must be strong and stable.  

The sides of the upper sacral vertebrae are flared into wings, or alae to form a large surface that articulates with the ilium.

The coccyx, also known as the tailbone, is a small, triangular bone resembling a shortened tail located at the bottom of the spine. It is composed of three to five coccygeal vertebrae or spinal bones, and is at the very end of the sacrum.

Femur

The femur is the thigh bone, the longest bone in the body. The lower end joins the tibia (shin) to form the knee joint. The upper end is rounded into a ball, known as the head that fits into a socket in the pelvis to form the hip joint. The neck of the femur gives the hip joint a wide range of movement, but it is a point of weakness and a common site of fracture. At the lower end of the femur, there are two bony projections (the lateral and medial condyles) where the femur meets the tibia.

Tibia

The tibia is the inner and thicker of the two long bones in the lower leg. It is also called the shin bone. Its upper end is expanded into medial and lateral condyles, which have concave surfaces and unite with the condyles of the femur. On the inside of the ankle, the tibia widens and sticks out to form a large bony prominence called the medial malleolus. On the outside of the ankle is a protrusion called the lateral malleolus, which is sometimes called the ankle bone.

Fibula

The fibula is the outer and thinner of the two long bones of the lower leg. It is much narrower than the other bone (the tibia), to which it runs parallel and to which it is attached at both ends by ligaments. The upper end of the fibula does not reach the knee, but the lower end descends below the shin and forms part of the ankle at the lateral malleolus. Its main function is to provide attachment for muscles. It doesn’t give much support or strength to the leg, which explains why the bone can safely be used for grafting onto other bones in the body.

Patella

The patella, also known as the kneecap, is a flat, circular-triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint.

Foot

The bones of the foot provide mechanical support for the soft tissues; helping the foot withstand the weight of the body whilst standing and in motion. They can be divided into three groups:

• Tarsals – a set of seven irregularly shaped bones. They are situated proximally in the foot in the ankle area.
• Metatarsals – connect the phalanges to the tarsals. There are five in number – one for each digit.
• Phalanges – the bones of the toes. Each toe has three phalanges – proximal, intermediate, and distal (except the big toe, which only has two phalanges).