The Pelvis & Hips - Bones

What is the Pelvis?

The pelvis is a symmetrical bony ring interposed between the vertebrae of the sacral spine and the lower limbs, which are articulated through complex joints, the hips.

It supports the spinal column and connects the upper body to the lower extremities.

Importantly, the pelvis functions as the reservoir for the abdominal organs including the bladder, intestine, and uterus in women or prostate in men.

• What are the 3 parts?

  The pelvis consists of three strong bones fused together:

  the ilium, ischium and pubis.
  
  These bones merge to form bilateral concave sockets named acetabulum, which accommodate the femur heads to form the hip joints.
  

  The ilium is the largest flat bone located on either side of the upper portion of the pelvis, including the iliac crest, which is the protruding tip bone of the pelvis that is easily felt with palpation.
  
  The pubis is the smallest bone at the anterior side of the pelvis. Both ends of the pelvis are fused by the symphysis pubis consisting of a cartilaginous flexible tissue.

• Male vs Female

  Males and females differ significantly in the anatomy of the pelvis:
  
  Females’ pelvis is wider and the pubis shorter than males’.
  
  This is an adaptation to child bearing and birth. In pregnant women the symphysis is extended to allow for the passage of the child through the birth canal.

The Sacrum

The sacrum consists of five fused vertebrae (S1 to S5) adopting a triangular shape located in the posterior side of the pelvis.

• The upper, wider region of the sacrum articulates with the ilium (sacroiliac joint) on each side.

• In the lower portion it connects to the tail bone or coccyx formed by five small fused bones.

• What does it do?

  The sacrum is particularly important in forming and stabilising the pelvic ring via the presence of numerous ligaments between bones and allows the connection of muscles of the pelvis and muscles of the hip joint
  
  At the lower tract of the spinal cord numerous nerves of the cauda equina extend from the foramen of the lumbar spine into the sacral canal. These nerves exit the sacrum through four pairs of cavities (sacral foramina) on each side of the canal or through the sacral hiatus located at the lower extremity of the canal.

What connects the pelvis?

• SI Joints

  The sacroiliac joint is a synovial joint that forms between the lateral articulating surfaces of the sacrum, and the articulating surfaces of the ilium.

  The articulating surfaces of the sacrum are L-shaped while those of the ilium are C-shaped. They allow for a small amount of movement.

  The sacral surface of the joint is covered by hyaline cartilage, and the ilium is covered by fibrocartilage.

  The joint is supported by several strong ligaments.

  The joint allows for some mobility during walking and other movements, and gives the pelvis some flexibility, but maintains its strength.

• Pubic Symphysis

  The pubic symphysis is a secondary cartilaginous joint (a joint made of hyaline cartilage and fibrocartilage) located between the left and right pubic bones near the midline of the body.

  The pubic symphysis can be found above the penis in males and above the vulva in females. 

  - In males, the joint connects to the ligament of the penis. 

  - In females, the joint is located near the clitoris.

  The pubic symphysis is covered by hyaline cartilage and united by a disc of fibrocartilage. The fibrocartilage disc is then reinforced by ligaments, which are bound to the disc itself, and tendons from the obliquus externus, rectus abdominis, and gracilis.

  Normally, the joint can be moved up to two millimeters and with a one degree of rotation. This flexibility changes during childbirth.

What is the Hip?

The hip is a major ball-and-socket joint connecting the long bones of the lower limbs (femur) to the pelvis.

• This joint allows a wide range of movements of the lower limbs and is used when walking, running, climbing, lunging and bending. 

• Because it bears the body weight, the hip joint is supported by large muscles, strong tendons and ligaments.

• What is the hip joint?

  The hip joint consists of the round head of the thighbone named femur, which inserts into the pelvis ring in a socked called acetabulum.
  
  The hip joint is protected by articular cartilage, which is a layer of elastic yet tough connective tissue surrounding both the femoral head and the acetabulum.
  
  The cartilage facilitates the movement of the joint by reducing the friction of the humerus head rotating inside the socket and importantly it absorbs shock impacts.

• The Femur

  The femur or thighbone is the longest and strongest bone in the human skeleton. 

  • It extends from the hip to the knee joint. 

  • The femoral head is the distal (upper) end of the femur that inserts into the acetabulum of the hip joint. 

  • As it descends towards the knee, it is separated from the longest tubular bone, or shaft, by the femoral neck.

  • The greater trochanter is the protruding extremity of the upper femur that can be felt laterally at the hip.

  • Below and on the opposite side of the great trochanter is the lesser trochanter, which connects to the iliopsoas muscle. 

  • The femoral shaft contains the bone marrow. 

What is the Coccyx?

The coccyx (also known as the tailbone) is the terminal part of the vertebral column. It is comprised of four vertebrae, which fuse to produce a triangular shape.

• Initially, the four coccygeal vertebrae are separate, but throughout life they fuse together to form one continuous bone
  

• How do they differ?

  There is considerable variation in structure between individuals.

  One common variant is failure of the first coccygeal vertebra (Co1) to fuse, remaining separate throughout adult life.

  In some individuals, there can be one more or one less coccygeal vertebra, giving the individual a coccyx with 5 or 3 vertebrae respectively

• What is it made up of?

  The coccyx consists of an apex, base, anterior surface, posterior surface and two lateral surfaces. 

  • The base is located most superiorly, and contains a facet for articulation with the sacrum. 

  • The apex is situated inferiorly, at the terminus of the vertebral column.

  • The lateral surfaces of the coccyx are marked by a small transverse process, which projects from Co1.

  • The coccygeal cornua of Co1 are the largest of the small articular processes of the coccygeal vertebrae. They project upwards to articulate with the sacral cornua.

What is Os Acetabuili?

Os acetabuli is an unfused secondary ossification centres of the acetabulum.

Ossa acetabuli are located at the anterosuperior margin. 

• They are rounded in shape with a concave lateral border and convex medial border. 

• They may be bilateral and partially fused to the acetabulum.

• Ossa acetabuli are normally seen in children but typically fuse in mid-to-late teens, being uncommonly seen in adults.

• Ossicles in the acetabular region may also be found in: 

• femoroacetabular impingement

• hip dysplasia

• osteochondrosis dissecans

• post-traumatic or degenerative bone fragments

• Some consider those present in displastic hips to represent fatigue fractures of the acetabular rim due to overload