Sacroiliac Joint Anatomy Explained with Diagrams (Easy Guide for Students)

Introduction

The Sacroiliac (SI) Joint is one of the most important joints in the human body. It connects the spine to the pelvis and helps transfer body weight from the upper body to the legs. Although it has only a small amount of movement, it plays a major role in standing, walking, running, and maintaining balance.

This guide explains the anatomy of the Sacroiliac Joint in simple English, making it easy for radiology students, medical students, physiotherapy students, and beginners to understand.

What is the Sacroiliac Joint?

The Sacroiliac Joint (SI Joint) is the joint formed between:

Sacrum – the triangular bone at the bottom of the spine.
Ilium – the largest bone of the pelvis.

Every person has two Sacroiliac joints:

Right SI Joint
Left SI Joint

These joints connect the spine with the pelvic bones.

Simple Diagram of the Sacroiliac Joint

        Spine
          │
       Sacrum
      /       \
 SI Joint   SI Joint
    │           │
 Left Ilium  Right Ilium

Bones of the Sacroiliac Joint

1. Sacrum

The sacrum is a triangular-shaped bone located below the lumbar spine.

Functions:

Supports body weight
Connects the spine to the pelvis
Forms the back wall of the pelvis

2. Ilium

The ilium is the largest part of the hip bone.

Functions:

Supports the pelvis
Protects pelvic organs
Provides attachment for muscles

Type of Sacroiliac Joint

The SI Joint is a combination of two types of joints:

Synovial Joint (Front Part) – allows a small amount of movement.
Syndesmosis (Back Part) – connected by very strong ligaments.

This combination provides both stability and limited mobility.

Ligaments of the Sacroiliac Joint

Ligaments are strong bands that hold bones together.

1. Anterior Sacroiliac Ligament

Located in front of the SI joint.
Prevents excessive forward movement.

2. Posterior Sacroiliac Ligament

Located behind the SI joint.
One of the strongest ligaments.
Provides stability during standing and walking.

3. Interosseous Sacroiliac Ligament

Found deep between the sacrum and ilium.
The strongest SI joint ligament.
Holds the bones tightly together.

4. Sacrotuberous Ligament

Connects the sacrum to the ischial tuberosity.
Prevents excessive movement.

5. Sacrospinous Ligament

Connects the sacrum to the ischial spine.
Helps stabilize the pelvis.

Diagram of SI Joint Ligaments

      Sacrum
     /      \
Anterior   Posterior
 Ligament   Ligament
      │
Interosseous Ligament
      │
      Ilium

Blood Supply

The Sacroiliac Joint receives blood mainly from:

Superior Gluteal Artery
Iliolumbar Artery
Lateral Sacral Artery

Good blood supply helps maintain healthy joint tissues.

Nerve Supply

The SI Joint receives nerves from:

L4
L5
S1
S2
S3 spinal nerves

These nerves explain why SI joint problems can cause pain in the lower back, buttock, and legs.

Functions of the Sacroiliac Joint

The SI Joint performs several important functions:

Transfers body weight from the spine to the legs.
Stabilizes the pelvis.
Absorbs shock during walking and running.
Supports standing posture.
Helps maintain body balance.
Allows a small amount of pelvic movement during childbirth.

Movement of the SI Joint

The SI Joint has very limited movement.

Main movements include:

Nutation – the sacrum tilts forward.
Counternutation – the sacrum tilts backward.

These small movements help with walking and bending.

Cartilage in the SI Joint

The joint surfaces are covered by cartilage.

Sacral side – Hyaline cartilage
Iliac side – Fibrocartilage

Cartilage reduces friction and allows smooth movement.

Common Disorders of the Sacroiliac Joint

Some common conditions include:

Sacroiliitis (Inflammation)
Degenerative Arthritis
Ankylosing Spondylitis
SI Joint Dysfunction
Traumatic Injury
Infection (Rare)

Symptoms may include lower back pain, buttock pain, hip pain, stiffness, and difficulty walking.

MRI Appearance of the SI Joint

On MRI, radiologists evaluate:

Joint space
Bone marrow edema
Cartilage
Joint fluid
Erosions
Sclerosis
Ankylosis
Soft tissue inflammation

MRI is the best imaging method for detecting early sacroiliitis.

Clinical Importance

The Sacroiliac Joint is an important source of chronic lower back pain.

Doctors examine the SI joint in patients with:

Persistent low back pain
Buttock pain
Hip pain
Suspected inflammatory arthritis
Ankylosing spondylitis

Early diagnosis improves treatment outcomes.

Quick Revision Table

Feature	Description
Number of SI Joints	Two
Bones	Sacrum + Ilium
Joint Type	Synovial + Syndesmosis
Main Function	Weight transfer and stability
Main Movement	Nutation & Counternutation
Strongest Ligament	Interosseous SI Ligament
Blood Supply	Superior Gluteal, Iliolumbar, Lateral Sacral arteries
Nerve Supply	L4–S3
Best Imaging	MRI

Conclusion

The Sacroiliac Joint is a strong and stable joint that connects the spine to the pelvis. Although it moves only slightly, it is essential for posture, walking, and weight transfer. Understanding its anatomy helps students interpret X-rays, CT scans, and MRI images and recognize conditions such as sacroiliitis and SI joint dysfunction.

A strong foundation in SI joint anatomy is essential for radiology, anatomy, physiotherapy, orthopedics, and medical students.

Wednesday, August 5, 2026

Sacroiliac Joint Anatomy Explained: Easy Guide with Diagrams