Iliac crest

The iliac crest is the upper border of the ilium. It extends from the anterior superior iliac spine at the front to the posterior superior iliac spine at the back, following a gentle S-shaped curve. Viewed from the outside, it forms a gradual curve; viewed from above, it curves gently from front to back. Around its midpoint is a slightly outward-projecting tubercle, whose outline may be visible even through clothing in a thin person.

The crest has some width, so when viewed from above, it is divided into three margins: an outer lip, an intermediate line, and an inner lip. Different muscles attach to each.

• Outer lip: The external oblique, which forms part of the side of the abdomen, attaches here. Toward the back, the lower portion of the latissimus dorsi, which spans the back, also attaches here.
• Intermediate line: The internal oblique, deeper within the abdominal wall, originates here.
• Inner lip: The transversus abdominis, the deepest abdominal muscle, attaches here, and the quadratus lumborum at the side of the lower back also attaches near this inner margin.

The thick fascia covering the lower back, the erector spinae beside the spine, and the gluteus medius in the buttock also meet around this crest. In short, the muscles of the abdomen and back above connect with the muscles of the buttock and thigh below at the iliac crest.

Another attachment here is the iliolumbar ligament, which extends from the transverse process of the fifth and lowest lumbar vertebra to the inner side of the iliac crest, firmly linking the lumbar spine and pelvis.

The iliac crest is widely used as a landmark for locating structures in the body. This is because a horizontal line joining the highest points of the two iliac crests usually passes near the fourth lumbar vertebra. That is why healthcare professionals place their hands on your lower back to locate this line before an injection or examination in that area. In manual therapy and exercise instruction, the heights of the iliac crests are also compared to check whether the pelvis is tilted to one side.

You may notice it in situations like these:

• When your trousers or belt keep slipping down on just one side, or a bag strap slips over one side of your waist
• When you sleep on your side and pressure on the iliac crest against the surface beneath you causes pain
• When pressing along the upper edge of the iliac crest at the side of your lower back produces a dull tenderness. Standing while leaning to one side for a long time, or frequently bending your lower back sideways, can easily fatigue the muscles around this area.
• When the iliac crest becomes bruised and aches after a fall or collision. Because it lies just beneath the skin, it takes the impact directly. An injury to this area during vigorous contact sports is called a hip pointer.

• Stretch the side of your lower back: Raise one arm overhead and slowly lean your body to the opposite side to lengthen the muscles along your side that run from the iliac crest to the ribs. Try holding for about 10 seconds as you breathe out.
• Alternate sides: Switch shoulders when carrying a bag, and alternate the hip you sit your child on when carrying them, so the strain does not concentrate around just one iliac crest.
• Activate your buttock muscles: A gentle exercise in which you lie on your side and open your knees allows the gluteus medius to share the work, helping the area around the iliac crest feel more comfortable.
• Avoid positions that put pressure on it: If you lie on one side for a long time, try supporting your waist with a thin folded towel or placing a pillow between your knees.

The iliac crest can be felt just beneath the skin, and its interior is filled with bone marrow and spongy bone, so it has many medical uses. The back of the iliac crest, near the posterior superior iliac spine, is the most common site for collecting bone marrow for testing. It is also often chosen as a site for harvesting bone when a bone graft is needed.

The iliac crest is also used to estimate bone age in growing adolescents. A separate piece of bone, an epiphysis, develops along the top of the iliac crest and gradually fuses firmly from front to back during the mid-to-late teens. Using an X-ray to assess how much of this piece has fused and estimate how much more the spine will grow is called the Risser sign. Because repeated injuries to the iliac crest in a growing child can cause this piece to detach, pain in this area is not taken lightly when coaching young athletes.