A Guide to Your Sacroiliac (SI) Joint and Why It Hurts

By Jonathan S. Chu, M.D.

Interventional Pain Management

Physical Medicine and Rehabilitation

The sacroiliac joint, or SI joint, is one of the most common sources of low back and buttock pain.


Yet, most people have never heard of if this joint, or even know what it is.


Many patients who come to my office are surprised when I diagnose them with sacroiliac (SI) joint dysfunction as the source of their pain. They assumed that their symptoms were from either “sciatica” or herniated discs, since these are the most commonly discussed diagnoses in pop culture and the mainstream media.


However—little do most people know—SI joint pain is actually one of the most common diagnoses that pain management doctors see. In fact, it’s been estimated that 26% of low back pain originates from the SI joints.


The SI joint is a very crucial structure that allows you to do most of your day to day activities, supporting your entire upper body weight while standing and walking.


Follow along with me and I’ll explain everything you need to know about the joint.

First, let’s talk about where it’s located.


The SI joint is a key joint in the pelvis.

It connects the sacrum (the tailbone), to a bone called the ilium, which is the outer flared portion of the pelvis.


There is one SI joint on each side, located just below the lumbar spine.


The SI joint has a very important job, as it bears the load from the entire upper body when walking, standing, and most other normal daily activities.


Given the massive mechanical burden that the SI joints need to handle on a day to day basis, it’s not surprising that these joints can become very inflamed and painful when subjected to excess stresses.

Next, let’s take a closer look at the structure of the joint.


The SI joint is considered a synovial joint, similar to the knee joint and hip joint.


This means that the joint is surrounded by a tough fibrous capsule on the outside, and the joint is lined by cartilage on the inside, which helps to cushion the joint.


Synovial fluid fills the inside of the capsule, which helps to lubricate the surfaces inside the joint.


Unlike other synovial joints, which usually slide and function like hinges—the SI joint is not designed to move much—and often excess motion can actually be the cause of SI joint pain.


The joint is supported by tough fibrous ligaments in the back which also form part of the joint capsule.


The SI joint is also bolstered by strong muscles in the area as well, which are often targeted by strengthening exercises in a dedicated physical therapy program.

Why Does the SI Joint Hurt?


As we discussed earlier, the SI joint has a very important job and bears tremendous loads with even simple day-to-day activities.


This includes shear stress, torsional stress, and simple repetitive stress.


Sometimes the joints are subjected to an even greater intensity of strain than usual, such as during bouts of intense physical training, heavy lifting, trauma, or excess bending and twisting.


This can cause micro-injury of the cartilage within the joint, as well as the surrounding capsule, ligaments, and musculature.


Ultimately, this can lead to a flare of painful inflammation of the joint and its capsule.

Specific factors that Predispose you to Developing SI Joint Pain


1) If you have pre-existing arthritis and degenerative changes within the SI joint, then it is definitely easier to experience a flare-up of SI joint pain after a strenuous or traumatic event.


2) Another major factor is obesity. The reason is simple: excess body weight subjects the SI joint to constant stress greater than it was designed to handle.
Here’s the good news: it’s a factor largely within your control. So many of my patients have felt tremendous relief of their SI joint pain simply by losing some weight.


3) In that vein, anything that changes the way you move and walk can subject the SI joint to extra stresses and cause it to flare up.
This is why patients born with a leg length discrepancy are more prone to developing SI joint pain.
More frequently, pain in different area of the body can often throw off your gait and lead to SI pain.
Here’s a common scenario I see in the office:

Let’s say you develop a knee or ankle injury and are placed into a brace or boot. This will result in an awkward and abnormal gait for a length of time—placing a lot of extra strain on the SI joint. Then, unfortunately, you may find yourself dealing with two areas of pain!


4) It’s not surprising then that SI joint pain is very common in pregnancy. The weight gain during pregnancy obviously places a lot of additional stress on the joint.
In addition to this, many hormones are released during the pregnancy cause the SI joint to become more lax, increasing movement at the joint.
Also, carrying the baby changes how the mother moves in various ways, causing excess swayback posture and altered gait.


5) Lastly, SI joint pain is also exceedingly common in people who have had prior spinal fusion surgeries.
To understand why, you have to look at all of the biomechanical changes that occur after a spinal fusion.
The placement of hardware restricts the ability of the lower levels of your lumbar spine to bend and twist. As a result, the SI joints are subjected to greater loads during day to day activities.
Also, the surgery in that region can also weaken or damage some of the ligaments that support the SI joint, allowing more motion at the joint, and potentially, increased pain.

How Do You Diagnose SI Joint Pain?


It can be tricky to accurately identify pain coming from the sacroiliac joint, since its symptoms overlap with a lot of other painful conditions in the low back.


In my office, I always look for a pattern of several symptoms and exam findings to help me make a confident diagnosis.


First, SI joint pain almost always presents with pain the in the buttock.


The pain also very commonly extends up into the low back and into the thigh as well.

The pain can even radiate into the groin and as far down as the calf, mimicking conditions like sciatica, though this is less common.


In addition to this, the SI joint will nearly always be tender to applied pressure.

In my office I also like to do a battery of provocative tests specifically designed to place extra stress on the SI joint. If pain is reproduced with these tests, then it helps to confirm my diagnosis.

Many of my patients ask if imaging studies are needed for diagnosing SI joint pain.

And the answer to that, surprisingly, is actually no.

Studies have compared various types of imaging studies to diagnostic injections, which are considered the gold standard.


They found that X-rays, CT scans, and even radionucleotide bone scans of the SI joint are just not very sensitive when compared to diagnostic injections.


So although imaging studies can be crucial part of investigating other types of pain, they aren’t very useful for diagnosing SI joint pain. In this case, your history and examination are far more important.

Treatment of SI Joint Pain


So let’s say you have low back and buttock pain, and your doctor thinks it’s coming from the SI joint.


What next?


In my opinion, its best to approach SI joint pain on multiple fronts.


First, if the pain is very intense, your physician can start off by performing an SI joint injection.


This is done by placing a small needle into the SI joint space using X-ray guidance (fluoroscopy), and injecting steroid and numbing medication directly into the joint. This acts as a potent anti-inflammatory to help calm everything down.


Read an in-depth explanation of SI joint injections here:

If you don’t have any pre-existing stomach or kidney issues, your doctor may also prescribe you an anti-inflammatory medication for a short course (about 2 weeks) to help quell the pain.

These include medications like meloxicam (Mobic), celecoxib (Celebrex), naproxen (Aleve), and ibuprofen (Advil, Motrin).

Once the pain has been controlled to a reasonable level, it’s very critical to begin an exercise program designed specifically to rehabilitate the SI joint.


As I mentioned above, the SI joint is supported by an intricate network of muscles. These mainly include the core muscles, as well as the hip muscles.


It is very important to strengthen these muscles so that the SI joints have adequate scaffolding to protect them through all of the stresses of your day-to-day life.


Building flexibility by stretching the muscle groups surrounding the SI joint is also critical.

By stretching out key muscles like the hamstrings (back of the thigh), the hip adductors (inside of the thigh), and hip flexors (front of the hip), you make your body more limber and improve your biomechanics.


After you’ve developed better flexibility, you’ll find that your everyday movements will be far more natural and place less strain on your SI joints.


Along these lines, if you are overweight, it’s also imperative to work on weight loss to reduce the day-to-day stresses on the SI joint.

Thank you so much for reading. I greatly commend you for taking the time and effort to educate yourself about your pain.

As a physician, I believe it is of the utmost importance to take a proactive approach to your health.

I know that your dedication and positive efforts towards improving your pain will pay off over time.

Until Next Time,

Jonathan S. Chu, M.D.


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