Difference between Sacrum and Coccyx

Sacrum

Sacrum is a triangular bone present just below the lumbar vertebrae. The sacrum is composed of five segments (S1 to S5); its upper part fuses with the last component of the lumbar vertebrae (L5) to form a sacral vertebrae between the ages of 18 and 30.

This bone fits between the two halves of the pelvis and connects the spine to the lower half of the body. The two dimples at the lower back are the point where the sacrum joins the hip bones forming a sacroiliac joint. The nerves emerging from the sacral region control bowel movements and bladder control, and also provide sensation in the crotch area.

The sacrum acts as a shock absorber for the spine and also helps in weight bearing and stabilising of the pelvis. Spina bifida is a congenital disorder that arises from incomplete closure of the vertebral arch, mainly at the lumbar and sacral bones. Fractures of the sacral region are uncommon, however cancerous tumours in the sacral bone are very common.

Refer: Bones of spine: Anatomy of Vertebral Column

Coccyx

Coccyx is a vestigial bone (also known as tailbone) that forms the final component of the vertebral column. It is the remnant of the tail evolved from animals with bony tails. It comprises maybe three, four or five rudimentary vertebrae that join superiorly with sacral bones forming a fibrocartilaginous joint.

The first coccyx bone is the largest and slightly resembles the last sacral bone; the rest of the bones diminish as we go lower. The coccyx bone is attached anteriorly to the levator ani and coccygeus muscles. It is attached to the gluteus maximus posteriorly.

Just because it is a vestigial bone does not mean that the coccyx are totally useless. They are attached to several tendons and ligaments anteriorly and posteriorly, and also support the vertebral column.

Coccydynia is a condition that is developed as a result of damage to the coccyx bone by either falling or sitting too long. Tumours in the coccyx bone are also common. Remedy for both the conditions is to remove the coccyx bone.

Sacrum vs. Coccyx

Sacrum

Coccyx

Description

The sacrum is a triangular bone, concave anteriorly and convex posteriorly, present just below the lumbar vertebrae.

The coccyx is a triangular bone present just below the sacral vertebrae.

Number of Bones

It is composed of five bones that fuse at adult stages of life.

It is composed of three to five fused rudimentary bones.

Location

It is located just below the lumbar vertebrae.

It is located below the sacral vertebrae.

Vestigial/Non-vestigial

It is not vestigial.

It is a vestigial bone.

Articulations/Attachments

It has these articulations:

  • With the last segment (L5) of the lumbar vertebrae
  • With the first segment of the coccyx vertebrae
  • With the hip bones on either side

It has the following attachments:

  • With the last segment (S5) of the sacral vertebrae
  • With the levator ani coccygeus muscles anteriorly
  • With the gluteus maximus posteriorly

Function

The sacral vertebrae acts as a shock absorber for the spine, and also stabilises the pelvis.

It helps in stabilising the vertebral column.

Clinical Significance

  • Spina bifida is a continental disorder that arises due to malfunctioning in the lumbar and sacral area.
  • Fractures are uncommon.
  • Tumours are common.
  • Fractures in the coccygeal bones are common and can arise due to prolonged sitting or falling.
  • Tumours are common.

Visit BYJU’S Biology for more detailed information.

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Frequently Asked Questions

Can you live without a tailbone?

Yes, the tailbone or coccygeal bones are remnants of the bony tails that helped our ancestors in movement and balance. Today, the tailbone serves us no need. Having said that, just because it is a vestigial bone does not mean that the coccyx are totally useless. They are attached to several tendons and ligaments anteriorly and posteriorly, and also support the vertebral column.

What causes the sacrum to be out of alignment?

The sacrum can go out of alignment in both males and females. The main reason is external taruma, injury, overextension, wrong posture and sometimes pregnancy.

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