By Alan Williams, PT, OTR/L, ATC, CSCS

It turns out that not all joints are created equal. Allow me to take you back to high school anatomy class for a moment… Nope, forget about the cutie sitting behind you. Now stop laughing at the teacher’s haircut. C’mon, focus. There we are! You may remember that the human body contains different types of joints – ball and socket joints (such as the shoulder and hip) and hinge joints (like the knee and elbow). But here’s a pop quiz – raise your hand if you remember the third type of joint. No one? The answer is synovial joint, the subtly-moving, sliding and gliding type of joint (like is found between the bones in your feet).

The sacroiliac (SI) joint is the synovial joint between the sacrum (the bottom five immobile vertebrae bones) and the iliac bones (the two large kidney-shaped halves of your pelvis; what most people refer to as their “hip bones” are actually the outer edges of their iliac bones). Between these three bones lie two types of cartilage, and the whole structure is supported by extremely strong ligaments – so strong, in fact, that dislocations of the joint are rare, and when put under enough stress, the pelvis will usually fracture before the ligaments will tear. Clearly this is not your average joint.

The SI joint has two huge jobs to perform – 1) as the base of the spine, it provides support for the weight of the entire upper body; 2) as the anchor for the pelvis and legs, it allows for the twisting and shock absorption needed for walking. Therefore the SI joint must be a strong foundation without sacrificing flexibility. It accomplishes this through its ability to lock and unlock.

When we are born, the surface of our SI joint is smooth, but as we learn to walk, the friction and stress of the suddenly increased physical demand creates angles, bumps, and ridges on the joint’s surface. These act like the teeth of a key, allowing the bones of the SI joint to lock together and hold strong or unlock and relax, depending on what is needed. When you walk, half of your SI joint will lock to provide stability during the transfer of weight from one leg to the other. Then that half will release and the other half will lock for the weight transfer of the next step, and so on. Another feature of the SI joint is its ability to expand during special circumstances – that is, pregnancy. The pregnancy hormone relaxin affects the SI ligaments, causing them to loosen and allowing for the widening of the pelvis necessary for birth.

Despite the incredible strength, stability, and coordination of the SI joint, problems do arise. SI joint dysfunction is commonly caused by trauma or injury (such as in a car accident or by using poor weight lifting techniques), pregnancy, certain illnesses (like gout or psoriasis), arthritis, and normal degenerative changes associated with aging. The result is pain in the lower back, hips, groin, or thighs. Sacroiliitis (general inflammation of the SI joint) is often a symptom of a movement problem, such as overuse in athletes or those who make compensations in their gait due to pain or mechanical difficulty (such as happens when one leg is longer than the other).

SI joint problems are notoriously difficult to diagnose. Because the symptom presentation is extremely similar to sciatica, herniated disks, and other low back issues, often the SI joint is overlooked. For this reason, it is important to see your physical therapist if you suspect your back, hip, and/or leg pain may have a SI joint origin. They will be able to design a treatment program to reduce inflammation and increase strength and stability.

The SI joint is the ironman of joints, but it’s not invincible. Maintaining a healthy weight, a nutritious diet, and a regular exercise program will go a long way in keeping all three of your joint types happy.

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