The sacroiliac joints, which connect our sacrum to our pelvis, are curious and particularly unique joints in our body. The sacrum forms the connection between our spine and our pelvis, physically connecting our upper and lower body. The joints where that connection happens are essential for shock absorption and load transfer between our torso and the legs. They make it possible for us to stand, walk, and run. Let’s take a look at what we know and what we don’t know about the anatomy and function of the sacroiliac (SI) joints.

What structures make up the sacroiliac or SI joints?

Bones of the SI joint

We actually have two sacroiliac joints, one on each side of our pelvis. The SI joints are found where the sacrum, the triangular bone at the base of our spine, meets our two ilium bones. Remember the ilium is the large, sort of elephant-ear-shaped bone of our pelvis. Where the sacrum articulates with the ilium on each side of our pelvis are our right and left SI joints. 

The SI joint is tiny. It’s considered a synovial joint, however, some sections of this joint are more fibrous than typical synovial joints (Ashby et al., 2021). The distal section is cartilaginous and functions like a synovial joint, but the proximal section is more fibrous (Ashby et al., 2021; Vleeming et al., 2012).

Muscles and the SI joint

Let’s take a look at the muscles that affect the sacroiliac joint. Interestingly, there aren’t any muscles that attach directly and have a line of pull from the sacrum to the ilium. The closest direct muscular connection we have is the piriformis muscle which attaches at one end to the anterior (front/inside) side of the sacrum and attaches at the other end to the greater trochanter of the femur. We also have the gluteus maximus which attaches at its proximal end to both the ilium and the sacrum (along with other sites) and then attaches at its distal end to the gluteal tuberosity on the femur and into the iliotibial band.

Unlike other joints in the body, muscular actions on the SI joints are primarily indirect. However, muscular tension and position do affect how force moves through those joints. The cascade of relationships between the muscular and ligamentous interactions is extremely complex though, making it very difficult to pinpoint sources of pain or dysfunction when they occur at the sacroiliac joints. 

You could imagine that any muscle or fascial sheet which attaches to the sacrum or the ilium could potentially affect tension at the SI joint. Those could include (but not be limited to): piriformis, gluteus maximus, erector spinae, multifidus, and the thoracolumbar fascia. The same could be said about muscles that attach to the ilium. Additionally, muscles that affect the pull and position of the other pelvic bones, or the spine, could indirectly affect the direction and amount of tension at the SI joints. It’s easy to see how the relationship of tension between muscles and the sacroiliac joint quickly gets complicated.

Ligaments

When it comes to the sacroiliac joints, the structures that connect the bones are just one more way that these joints are unique among joints in the body. Arguably, the most important structures that cross the SI joints are not the muscles. The primary connections around the sacrum are actually ligaments, which makes the sacroiliac joints very different than typical synovial joints. 

The ligaments that cross the SI joints or extend from ligaments which do, include:

• Anterior sacroiliac ligament
• Long posterior sacroiliac ligament
• Short posterior sacroiliac ligament
• Interosseous sacroiliac ligament
• Sacrotuberous ligament
• Sacrospinous ligament

While movement is both possible and necessary at the sacroiliac joints, stability is at least as important as movement at these joints. In the move from a four-legged horizontal being to a two-legged vertically oriented being, the work of the sacrum changed in some ways. Compared to a horse, dog, or cat, for example, more of the shock absorption from walking and running is handled by our pelvic girdle, so our SI joints are less mobile. In four-legged animals, some of this shock absorption is managed by the shoulder girdle (Myers, 1998). 

The many strong ligaments that connect the sacrum to the ilium are critical to creating the necessary pelvic stability, which is one of the main functions of the sacroiliac joints (Ashby et al., 2021; Casaroli et al., 2020). In fact, one reference even refers to the ligamentous connections around the sacrum as a “continuous ligamentous stocking” (Ashby et al., 2021). 

The ligaments that directly cross the sacroiliac joint, as well as ligaments that connect the sacrum to other sites, change in tension as the sacrum moves. For example, Vleeming, 1996 reported that the long dorsal sacroiliac ligament is slack when the sacrum is in nutation, but is tense when the sacrum is in counternutation. Working in balance with that, the sacrotuberous ligament is slack when the sacrum is in counternutation, but tense when the sacrum is in nutation (Vleeming, 1996).

Movements at the sacroiliac joint

Movement at the SI joint is something researchers are still trying to understand. The movement at the sacroiliac joints is multi-dimensional and complex. It’s also quite complicated to measure and scientists have yet to create a really good way of doing that. For that reason, there is still considerable debate about exactly how and how much movement happens there (Klerx et al., 2020). 

Nutation and counternutation

The simplest movements that we’re aware of at the sacroiliac joints are referred to as nutation and counternutation. Nutation describes the movement of the sacrum when it nods forward in relation to the ilium bones. Counternutation is the opposite. It describes the movement of the sacrum when it nods backward in relation to the ilium bones.

However, there has been some documented movement at the SI joints in all three directional planes (Casaroli et al., 2020; Vleeming et al., 2012). Movement at these joints varies considerably between individuals. More or less movement is possible depending on bone shape, ligamentous tension or laxity, and muscle tension (Casaroli et al., 2020).

However, what’s as important as the idea of the sacrum tilting forward and back, is the idea of the main function of the sacroiliac joints as part of the shock absorption and load transfer system between the torso and the legs (Ashby et al., 2021; Casaroli et al., 2020; Klerx et al., 2020). Sacral movement is not the same thing as the movement of the pelvis at the hip joints, but they are related.

How we move each side of our pelvis at the hip joints influences how force is transferred from the legs through the pelvis to the spine above via the sacrum (Ashby et al, 2021). The movement at the sacroiliac joints and their function in force transfer is complicated by the fact that we can move the two sides of the pelvis independently. That’s necessary, of course, for whole-body movement like walking. 

Conclusion

The sacroiliac joint is a fascinating, complex part of our human anatomy. Its structure and function are unique among the joints of our body. It’s a critical structure for standing, walking, and running. Due to its location, it’s difficult to study, so much about our understanding of its specific functions remains unclear.

References

Ashby, K., E. Yilmaz, M. Mathkour, L. Olewnik, D. Hage, J. Iwanaga, M. Loukas, and R.S. Tubbs. 2021. Ligaments stabilizing the sacrum and sacroiliac joint: A comprehensive review. Neurosurgical Review. 8pgs. https://doi.org/10.1007/s10143-021-01625-y

Casaroli, G., T. Bassani, M. Brayda-Bruno, A. Luca, and F. Galbusera. 2020. What do we know about the biomechanics of the sacroiliac joint and sacropelvic fixation? A literature review. Medical Engineering and Physics. 76:1-12.

Enix, D.E. and J.M. Mayer. 2019. Sacroiliac joint hypermobility biomechanics and what it means for health care providers and patients. Physical Medicine and Rehabilitation. 11:S32-S39.

Klerx, S.P., J.J.M. Pool, M.W. Coppieters, E.J. Mollema, A.L. Pool-Goudzwaard. 2020. Clinimetric properties of sacroiliac joint mobility tests: A systematic review. Musculoskeletal Science and Practice. 48: 9pgs. https://doi.org/10.1016/j.msksp.2019.102090

Myers, T. W. 1998. Poise: Psoas-piriformis balance. Massage Magazine, Issue 72, March/April, p.31-39. (Reprinted in Myers, T. W. (March 8, 2022) Body3: A therapist’s anatomy reader. Published by Tom Myers.

Richardson, C.A., C.J. Snijders, J.A. Hides, L. Damen, M.S. Pas, and J. Storm. 2002. The relation between the transverse abdominis muscles, sacroiliac joint mechanics, and low back pain. Spine. 27(4):399-405.

Vleeming, A., M.D. Schuenke, A.T. Masi, J.E. Carreiro, L. Danneels, and F.H. Willard. 2012. The sacroiliac joint: An overview of its anatomy, function and potential clinical implications. Journal of Anatomy. 221:537-567.

Vleeming, A. et al 1996. The function of the long dorsal sacroiliac ligament. Spine. 21(5):556-562.

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