Anatomy, Purpose and Movement
The SI joint seems to be a gigantic mystery to those that have pain in this area. In this blog post we are going to demystify the anatomy and purpose of this joint as well as take a look at how it moves. So much pain, but so little education surrounds this unique joint for people that are hurting. Let’s change that, shall we!
Often asked questions revolve around how much the joint moves, it’s stability, it’s ability to become hypo or hyper mobile and what muscles need to be strengthened to hold the joint in place. Many interventions for pain also hinge on understanding the innervation ( what nerves supply the SIJ) which is also part of the anatomy. So let’s dig in.
Functional anatomical and biomechanical models are required to analyze a puzzle as complex as low back pain (LBP) and pelvic girdle pain (PGP). Such an approach can help us understand that seemingly different structures are functionally related. In this respect, we quote Radin, who stated, “Functional analysis, be it biological, mechanical or both, of a single tissue, will fail as in all complex constructs, the interaction between the various components is a critical part of their behaviour” (Radin, 1990).
So basically, understand that while we can look at it’s bits and pieces separate from the body, we are taking it out of its context, which never works well. It’s a lot like the difference between studying a dead body for anatomy versus a live one, or looking at drawings of muscle and not seeing what that looks like inside a real person, they are very different. Remember that as you read on.
The sacroiliac joint is a synovial joint formed between the ilium and the sacrum. The left and right sacroiliac joints, together with the pubic symphysis and the sacrococcygeal joint, compose the articulations ( fancy word for joints) of the pelvic girdle. The sacroiliac joints connect the hip bones posterolaterally ( back and side), while the pubic symphysis connects them anteriorly (front).
The main function of the joint is to bear the weight of the axial skeleton ( the upright part) and transfer it to the hip bones. The weight can then be distributed to the two femurs in the standing position, or the ischial bones in the seated position. Notice the function of the joint isn’t movement. It’s weight transfer and distribution. Kinda an important point.
Still with me?
Good.
Yes there is a lot of medical terminology here, and yes it is dense, but trust me, it is a necessity. Gaining a better understanding beyond “this area hurts here” is key to getting the right help. The more we know about our bodies and the functions all the various parts perform, the easier it will be to identify what explanations of our pain are reasonable and which ones aren’t. It might be boring, but if you get through it, it will provide immense help.
And maybe just to give you a break I’ll throw in a fun story about monkeys, or Disneyland, or monkeys at Disneyland.
I might even do it mid sentence. Who knows? Now we return to the technical stuff!
The SIJs are highly specialized joints that permit stable (yet flexible) support to the upper body. In bipeds, such as humans (or monkeys…monkeys!) the pelvis serves as a basic platform with three large levers acting on it (the spine and two legs). Both the tightness of the well‐developed fibrous apparatus and the specific architecture of the SIJ result in limited mobility. Sacral movement involves the SIJ, and also directly influences the discs and most likely the higher lumbar joints as well. For example, forward and backward tilting of the sacrum between the iliac bones affects the joints between L5–S1, as well as most likely influencing joints at the higher spinal levels (Vleeming & Stoeckart, 2007).
It has been postulated that the SIJs act as important stress relievers in the ‘force–motion’ relationships between the trunk and lower limb. These joints ensure that the pelvic girdle is not a solid ring of bone that could easily fracture under the great forces to which it might be subject, either from trauma or its many bipedal functions (Lovejoy, 1988). Analysis of gait mechanics demonstrates that the SIJs provide sufficient flexibility for the intra‐pelvic forces to be transferred effectively to and from the lumbar spine and lower extremities (Lee & Vleeming, 2007).
In other words, think of the SIJs as your body’s version of the shocks on your car. When the shocks on your car are in fine working order, they absorb all the lumps and bumps on the road and you are left with a smooth ride to Disneyland (okay that felt a bit forced. Moving on!).
Now we’re going to move on to the parts of the SIJs, the capsules and ligaments, the innervation (where nerves get their power and sensation from), the function, and how other muscles act on them.
Joint capsule
The sacroiliac joint is completely encircled by a fibrous capsule that attaches on the articular margins of the sacrum and ilium. The internal surface of the capsule over the non-articular surfaces of the joint is lined by synovial membrane. Basically, the joint is in an enclosure of fluid that lubricates the joint, allowing for movement.
The ventral SIJ capsule is relatively thin and frequently has defects that allow fluid substances in the joint space to leak out onto surrounding structures. In 61% of 76 joints examined by injection and imaging, leakage of injected contrast was reported (Fortin et al. 1999a). Notably, contrast leaked into the ventral region in close juxtaposition with the lumbosacral plexus and into the dorsal sacral foramina, where it could be in contact with the dorsal sacral plexus (Fortin et al. 1999b). These findings may help explain the results of Indahl et al. (1999) who, using pigs, revealed that stimulation with bipolar wire electrodes in the ventral SIJ capsule initiated a muscular response of the gluteus maximus and the quadratus lumborum muscles. The close association of the dorsal sacral rami to the interosseous ligaments (Willard et al. 1998; McGrath & Zhang, 2005) may explain why stimulation directly dorsal of the SIJ capsule provoked a response in the deep medial multifidus fascicles lateral to the L5 spinous process.
So what does all this mean? Well, the capsule has a high percentage of defect which affects the nearby regions of the body. Cool huh. ALso means that nothing is wrong with the joint itself, but there is a lot of “noise” or input coming from the area. I bet you can hear future bog posts mentioning this face again when it comes to how effective those SIJ injections are at giving relevant information, can’t you.
Ligaments
Due to its position and weight-bearing nature, the sacroiliac joint has to be stabilized with several strong ligaments. The main ligaments of the joint are anterior and posterior sacroiliac ligaments, while additional stability is provided with accessory ligaments: sacrotuberous ligament and sacrospinous ligament.
The SIJ capsule closely follows its articular margins. In addition, the associated core ligaments are numerous and strong (Palastanga et al. 1998), including the ventral, dorsal and interosseous ligaments (Soames, 1995). Short and long dorsal sacroiliac ligaments complement the interosseous ligaments. The long dorsal ligament (LDL), which originates from the posterior superior iliac spine (PSIS), is the most superficially and dorsally located of the SIJ ligaments (Vleeming et al. 1996). The STLs, SSLs and ILs are strong accessory SIJ ligaments (Palastanga et al. 1998). Furthermore, the ILs are connected to both the dorsal and ventral sacroiliac transverse ligaments (Pool‐Goudzwaard et al. 2001). These ligaments play important roles in increasing joint stability via force closure (discussed later).
Kinda hard to imagine instability being anatomically possible with all of those ligaments, isn’t it.
Anterior sacroiliac ligament
The anterior sacroiliac ligament is composed of many thin strands that form a strong, broad and flat band. It lies on the pelvic surface of the joint, forming the anteroinferior component of the joint capsule. The anterior sacroiliac ligament extends from the ala of the ilium, just anterior to the auricular surface, to the pelvic surface of the sacrum.
The part of the pelvic surface that lies between the auricular surface and the upper rim of the greater sciatic notch often shows a rough preauricular sulcus that provides an attachment site (origin) for the lower fibers of the anterior sacroiliac ligament that then insert onto the third sacral segment. This part of the ligament is particularly well developed, whereas the rest of the ligament is rather thin.
Posterior sacroiliac ligament
The posterior sacroiliac ligament is a compound ligament composed of three distinct bands. It lies posterior and superior to the joint, filling the space between the corresponding tuberosities of the ilium and the sacrum. The posterior sacroiliac ligament is much thicker and stronger than its anterior counterpart. The bands that compose this ligament are as follows:
- Interosseous sacroiliac ligament: a short but very strong band that spans the gap between the sacrum and ilium on the posterior side of the joint by attaching on their tuberosities, just behind the auricular surfaces. Occasionally, one or two accessory joint cavities can be identified within the ligament.
- Short posterior sacroiliac ligament: a short ligament found superficial to the interosseous sacroiliac ligament. It fills the upper part of the gap between the ilium and sacrum. The short posterior sacroiliac ligament originates on the first and second transverse tubercles of the sacrum and extends horizontally to attach on the tuberosity of the ilium.
- Long posterior sacroiliac ligament: a long band that is the most superficial of the posterior sacroiliac ligaments. It extends from the posterior superior iliac spine to the third and fourth transverse tubercles of the sacrum.
Accessory ligaments
The accessory ligaments aid the main ligaments and contribute to the stability of the sacroiliac joint. The main function of these ligaments is to prevent forward tilting of the sacral promontory.
- Sacrotuberous ligament: a flat, triangular ligament that has several superior attachments; one band of the ligament arises on the gap between the posterior superior and posterior inferior iliac spines, where it is partially blended with the posterior sacroiliac ligaments. A second band arises on the lateral side of the sacrum below the auricular surface, and the third on the lateral side of the upper part of the coccyx. The three bands converge and extend inferiorly, forming a triangular shape before inserting on the lower margin of the ischial ramus. Right before the insertion occurs, the bands diverge again, thereby prolonging the inferior attachment point. This prolonged attachment is called the falciform process and is situated just below the pudendal canal, where it blends with the fascial sheath of the internal pudendal vessels and pudendal nerve. The posterior surface of the ligament is blended with the gluteus maximus, whereas the inferior fibers of the ligament continue into the tendon of biceps femoris.
- Sacrospinous ligament: a thin, triangular band found deep to the sacrotuberous ligament. It originates from the border of the lower sacral and upper coccygeal segments, anterior to the sacrotuberous ligament. The ligament is broad in its origin, but narrows as it extends laterally to insert onto the ischial spine. The sacrospinous ligament is thought to be a fibrous part of the coccygeus muscle, as it is largely blended with it.
Besides their main function, the sacrotuberous and sacrospinous ligaments also transform the greater and lesser sciatic notches into the greater and lesser sciatic foramina, respectively.
Okay, take a deep breath. We’re done with ligaments. That wasn’t so bad, was it? I think we all deserve a break.
- Greater sciatic foramen: an opening bounded by the posterior border of the iliac bone anterosuperiorly, the sacrotuberous ligament posteromedially, and the sacrospinous ligament and ischial spine inferiorly. The contents of this foramen are the piriformis muscle, gluteal vessels and nerves, internal pudendal vessels, pudendal nerve, sciatic and posterior femoral cutaneous nerves and the nerves to obturator internus and quadratus femoris.
- Lesser sciatic foramen: an opening bounded by the ischial body anteriorly, spine of the ischium and sacrospinous ligament superiorly, and the sacrotuberous ligament posteriorly. Its contents include the tendon of obturator internus, the nerve to obturator internus, and the internal pudendal vessels and pudendal nerve.
A short while ago I was at the San Diego Zoo on the Monkey Trails. There is an elevated pathway so you can watch the primates through the tree canopy. We came to a group of colobus monkeys, three of them. All three looked at me and went very still. Then the one in the middle, while looking directly at me, burped in my general direction. The other two started tittering while the burping one looked very proud. Moral of the story: I got burped on and then laughed at by monkeys. I promised you a monkey story and I monkey story you have received! That was fun, but we must now power through.
Innervation
The sacroiliac joint receives innervation primarily from branches of the anterior and posterior rami S1-S2 spinal nerves and the superior gluteal nerve. Additionally, the joint receives contributions from the obturator nerve and lumbosacral trunk. This is really important information when considering RFA.
Movements
Due to its arrangement and strong ligaments, the sacroiliac joint allows little to no movement. Like other plane-type synovial joints, the sacroiliac ligament only allows for some amount of gliding and rotatory movements. The rotatory movements of the sacroiliac joint are described as nutation and counternutation. Please note, we are talking millimeters of motion here, this is close to imperceptible. Especially by human hands because it is underneath all those ligaments and the muscle we have yet to talk about.
- Nutation: occurs as the superior segment of the sacrum tilts downwards and anteriorly, while the inferior portion of the sacrum and the coccyx tilt posteriorly in the opposite direction. This movement is also referred to as sacral flexion, and occurs only in conjunction with trunk flexion or hip extension. Think of nodding your head while trying to drink out of a glass.
- Counternutation: occurs as the superior segment of the sacrum tilts upwards and posteriorly, while the inferior portion of the sacrum and the coccyx tilt anteriorly in the opposite direction. This movement is also referred to as sacral extension and occurs only in conjunction with trunk extension or hip flexion. Thinking of throwing your head back while tilting your drinking glass in the opposite direction.
The main function of the sacroiliac joint lies in the fact that it comprises one of the pillars of the pelvic mechanism for transmitting the weight of the head, trunk and upper limbs to the lower limbs. The lack of free mobility in the sacroiliac joint favors strength and stability to maximize weight bearing capabilities. These characteristics are especially important in order to maintain stability in the erect position during standing or walking. Interestingly, during childbirth, this changes due to the hormone relaxin, which loosens the joint and its ligaments to allow for more mobility. This puts the sacroiliac joints in position to increase the pelvic inlet and outlet diameters to allow for the passage of the fetus.
Muscles acting on the sacroiliac joint
No muscles have a direct action on the sacroiliac joint. Instead, there are muscles that attach onto the joint and provide to its stabilization. Some of these muscles include the internal abdominal oblique, external abdominal oblique, erector spinae, hamstring muscles, piriformis, the superior fibers of gluteus maximus, psoas minor, coccygeus, levator ani and lots of others.
And that’s it, you made it! Again, I wouldn’t put you through all of that if I didn’t know it had a direct benefit to what we are attempting to do here. But thank you for sticking with me. We have taken a not-too-exciting yet crucially important step on the road to regaining your life. There will be many more to come. In the meantime, I’ll see if I can’t find some more monkey anecdotes.
References:
https://www.kenhub.com/en/library/anatomy/sacroiliac-joint
- Palastanga, N., & Soames, R. (2012). Anatomy and human movement: structure and function (6th ed.). Edinburgh: Churchill Livingstone.
- Lippert, L. S. (2011). Clinical Kinesiology and Anatomy (5th ed.). Philadelphia, PA: F. A. Davis Company.
- Standring, S. (2016). Gray’s Anatomy (41tst ed.). Edinburgh: Elsevier Churchill Livingstone.
- Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
- Hall, S. J. (2015). Basic biomechanics (7th ed.). New York, NY: McGraw-Hill Education.
https://onlinelibrary.wiley.com/doi/full/10.1111/j.1469-7580.2012.01564.x
