Biomechanical Fundamentals of Sacroiliac Joint Pain

Introduction[edit | edit source]

SIJ Human

The sacroiliac joints (SIJs), the largest axial joints in the body. They are located between the sacrum and pelvic bones on either side, and connect the spine to the pelvis, facilitating load transfer from the lumbar spine to the lower extremities.

The majority of low back pain (LBP) is perceived to arise from the lumbar spine, but another likely source of LBP that is largely overlooked is the SIJ.

Sacroiliac pain etiologies include: hypo- or hypermobility, extraneous compression or shearing forces, micro- or macro-fractures, soft tissue injury, inflammation, pregnancy, adjacent segment disease, leg length discrepancy, and prior lumbar fusion.[1] See Sacroiliac Joint Syndrome

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].

Biomechanical Factors in SIJ Pain[edit | edit source]

The following all contribute to SIJ pain syndromes

Compressive tensile and shear loading

Sheer Load. The stability against sheer comes from:

  • The wedging of the sacrum and the ligaments give the resistance to shear loads.
  • Grooves and ridges give added resistance to protect the joint against shearing.


  • The SIJ ROM is greatest in flexion-extension (about 3°), followed by axial rotation (about 1.5°), and lateral bendings (about 0.8°).
  • The male and female SIJ ROM values are also different with the maximum ROM of 1.2° degrees (men) and 2.8° (women).
  • The motion of the SIJ while standing on one leg is higher than when standing on both legs.

Sexual Dymorphism

  • Women are more prone to the incidences of SIJ and pelvic pain due to high mobility.
  • They also have greater stresses, greater loads, and more pelvis ligament strains compared to men.
  • The influence of hormones, for example relaxin in women, increases the mobility of the SIJ by providing ligament laxity for giving birth.

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 [3].

References[edit | edit source]

  1. Kiapour A, Joukar A, Elgafy H, Erbulut DU, Agarwal AK, Goel VK. Biomechanics of the sacroiliac joint: anatomy, function, biomechanics, sexual dimorphism, and causes of pain. International journal of spine surgery. 2020 Feb 1;14(s1):S3-13. Available: (accessed 22.10.2022)
  2. Kiapour A, Joukar A, Elgafy H, Erbulut D, Agarwal A, Goel VK. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain. International journal of spine surgery. 2020 Feb 1;14(s1):S3-13.
  3. Kiapour A, Abdelgawad AA, Goel VK, Souccar A, Terai T, Ebraheim NA. Relationship between limb length discrepancy and load distribution across the sacroiliac joint-a finite element study. Journal of Orthopaedic Research. 2012 Oct;30(10):1577-80.