Biomechanical Fundamentals of Sacroiliac Joint Pain

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Description[edit | edit source]

Sacroiliac joint pain is a condition that is defined as a pain experienced in the sacroiliac joint or its surrounding structural tissues. There are many biomechanical determinants that contribute to this pain. Injuries to the sacroiliac joint, whether acute or chronic, are not the only causes for pain to arise [1]. This article will cover different schools of thoughts that are related to the biomechanics behind sacroiliac joint pain. There are different types of forces that can act on a tissue and cause stress and strain upon it. The type of force is defined by the directions of the force vectors that are acting on the tissue. The following Figure 1 has been provided in order to help imagine the different forces can look like: A. Compression forces, B. Tensile Forces, C. Shear Forces

Compressive tensile shear loading.svg.png

What are the Different Mechanisms?[edit | edit source]

The sacroiliac joints transmit the weight of the trunk, arms, and head to the lower extremities. This can sometimes lead to tension or extraneous compression and shear forces to act upon the ligaments that surround the joints. The mechanism of Sacroiliac Joint injuries was shown to be due to sudden compressive loading and rapid rotation, which as mentioned previously, increase the levels of strain and stress [2].

Figure 2 of the pelvic bone has been provided below to be used as a reference.

Sacroiliac Joint

Difference Between Sexes[edit | edit source]

It is important to signify that the mechanics of the Sacroiliac Joint is different across the sexes. A study in 2018 shows that the different joint structure between the female and male provides a higher range of motion in females. Thus, this causes for the mechanics of the female’s Sacroiliac Joint to be different in terms of force distribution (tension, compression, shear, torsion, etc.). Therefore, the higher loads and range of motion in the female joints indicated a higher strain on the ligaments and stress on the sacroiliac joint. This indicates that females are more susceptible to pain and injury to start in that area compared to men due to the difference in structure of these two joints [3].

Sacroiliac Pain and Other Conditions[edit | edit source]

Since pain can arise from many different causes, there is an association between sacroiliac joint pain and many other different conditions. Sacroiliac joint dysfunction is another source of pain developing in that area. The line of gravity for body mass relative to the location of the SI joints is affected by posture. For example, in a lordotic posture, the line of gravity for body mass shifts anteriorly compared to a neutral posture. Moving the gravitational line of action anteriorly increases a moment encouraging an anterior tilt of the pelvis. This positioning increases the load placed on the SI joint. Sacroiliac joint dysfunction has been shown to have a correlation to what is known as Lower Limb Length Discrepancy. This condition is recognized when an individual’s lower limbs have a noticeable difference in length. This different in lower limb length showed to cause an increase pain in the sacroiliac joint region. In the literature article written by Kiapour et al. in 2012, the study conducted showed the mechanism by which lower limb length discrepancy impacted forces acing upon the sacroiliac joint. This study used finite element analysis, which is a computerized method to model the element we are studying and predict its reaction to applied forces. Using the computerized model, the right limb was lengthened to three different lengths. At the same time, a load and a moment were applied on lumbar spine in order to mimic many different movements. The results were explained using two parameters, the contact load and the load distribution (or the stress). In the case with the highest leg discrepancy (most lengthening), the long limb indicated higher levels of stress compared to no long limb discrepancy. In addition, it is important to note that a higher contact load does not indicate a higher stress. The reason to that is because stress equals to the distribution of load upon the contact area in the sacroiliac joint. If the lengthened leg was to be shortened this would reduce the stress on the SIJ due to the lower applied force and thus decrease the risk of pain and maybe even injury [4]. Furthermore, it is has been indicated that Sacroiliac Joint Dysfunction, can ultimately lead to Low Back Pain. Due to the reason that the trunk weight travels through both sacroiliac joint, when these is pain in these joint due to a degeneration, then it impacts the laxity of the joints. The laxity of the joint impacts the impacts the loading upon those two joints and can increase the shear forces acting upon the sacroiliac joint and thus the lumbar back [5].


  1. Cohen S. Sacroiliac Joint Pain: A Comprehensive Review of Anatomy, Diagnosis, and Treatment. Anesthesia & Analgesia. 2005;101(5):1440-1453.
  2. Kiapour A, Joukar A, Elgafy H, Erbulut D, Agarwal A, Goel V. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain. International Journal of Spine Surgery. 2019;14(s1):S3-S13.
  3. Joukar A, Shah A, Kiapour A, Vosoughi A, Duhon B, Agarwal A et al. Sex Specific Sacroiliac Joint Biomechanics During Standing Upright. Spine. 2018;43(18):E1053-E1060.
  4. Kiapour A, Abdelgawad A, Goel V, Souccar A, Terai T, Ebraheim N. Relationship between limb length discrepancy and load distribution across the sacroiliac joint-a finite element study. Journal of Orthopaedic Research. 2012;30(10):1577-1580.
  5. Richardson C, Snijders C, Hides J, Damen L, Pas M, Storm J. The Relation Between the Transversus Abdominis Muscles, Sacroiliac Joint Mechanics, and Low Back Pain. Spine. 2002;27(4):399-405.