The sacroiliac (SI) joint's primary function is to transfer the weight of the upper body to the lower extremities. In the upper portion of the joint, the sacrum and the ilium are not in contact but rather connected with powerful posterior, inter-osseous, and anterior ligaments. The anterior and the lower half of the joint is a typical synovial joint with hyaline cartilage on the joint surfaces. The SI Joint is an axial joint with an approximate surface area of 17.5 square cm. The joint surface is smooth in juveniles and becomes irregular over time. Motion (primarily rotation) decreases with aging, and increased motion may be associated with pregnancy.
The SI joint is stabilized by a network of ligaments and muscles, which also limit motion in all planes of movement. The normal SI joint has a small amount of normal motion, approximately 2-4 mm in any direction.
SI Joint Anatomy, Biomechanics and Prevalence
Sacroiliac (SI) Joint Pathology
Mechanical strain and injury to the sacroiliac (SI) joint are produced by either a combination of vertical compression and rapid rotation (i.e. carrying a heavy object and twisting), or by falls to the backside. SI joint pain can also be caused by SI joint disruption or degenerative sacroiliitis.
Patients can feel problems in their leg(s), buttocks and sometimes even higher on the spine. This is especially true with sitting, lifting, running, walking, or even sleeping on the involved side. In all of these cases, the symptoms can be felt anywhere from the lower leg to the lower spine. The most significant indication is the specific location of the pain below the L5 joint.
In addition to pregnancy, women may be at increased risk for sacroiliac joint problems because of their broader pelvises, the greater curve of their lumbar spines.
Sacroiliac (SI) Joint Disorders are a Significant Cause of Low Back Pain
The SI joint is a real yet underappreciated pain generator in an estimated 15% to 25% of patients with axial low back pain. Bernard & Kirkaldy-Willis found that 22.5% of patients with reported low back pain had SI Joint pain.2 Sembrano showed that up to 25% of low back pain patients have significant pain coming from their hip and/or SI joints.3
The Significance of the SI Joint in Low Back Pain
The SI joint is often overlooked as a low back pain generator because of its unique diagnostic challenges:
- It's common to link low back pain with protruding disc even when neurological signs are absent (Reference: Weksler, et al)4
- SI joint problems can occur in conjunction with other spinal problems, such as herniated disc, degenerative disc disease, osteoarthritis, or sciatica
- It's common for pain from SI joint disorders to feel like discogenic or radicular low back pain. Many patients go on to receive lumbar fusion instead of SI joint fusion – so SI joint disease should be strongly considered in the differential diagnosis of low back pain. (Reference: Weskler, et al)4
The Importance of Diagnosing SI Joint Disorders in Surgical Practice
According to a study by Ha, et al, lumbar fusion patients have been shown to have a higher incidence of SI joint degeneration than non-fusion patients.5
- The incidence of SI joint degeneration in patients was 75% at 5 years post-fusion, which was significantly higher than in the non-fusion group, 38.2%.
- Among patients with one-segment fusion, 91% developed SI joint degeneration.
- Regardless of whether the fusion includes the sacrum, the SI joint is influenced by increased mechanical stress arising from lumbar/lumbosacral fusion.
- Posterolateral lumbar/lumbosacral fusion can be a cause of SI joint degeneration.
Studies by DePalma and Liliang, et al., demonstrate that 40-61% of symptomatic post-lumbar fusion patients were found to have SI joint disorders based on diagnostic blocks. 6,7
These data suggest that:
- Patients who are still symptomatic after lumbar spine surgery may have actually had SI joint problems instead of or in addition to another spinal problem
- Patients who have had prior lumbar spine surgery may develop issues with their SI joint
- The increasing incidence of SI joint problems supports the inclusion of the SI joint in a differential diagnostic work-up of patients with low back pain.
- Cohen, Steven P. Sacroiliac Joint Pain: A Comprehensive Review of Anatomy, Diagnosis, and Treatment. Anesth Analg 2005; 101:1440-1453.
- Kirkaldy-Willis WH, Bernard TN Jr. Making a specific diagnosis. In: Managing Low Back Pain. 4th ed. Philadelphia (PA): Churchill Livingstone; 1999. p. 206-26.
- Sembrano, Jonathan N.*, et al. How Often Is Low Back Pain Not Coming From the Back?. Spine 2009; 34(1): E27-E32. *Conducts clinical research for SI-BONE Inc.
- Weksler, Velan, et al. The role of SI joint dysfunction in the genesis of low back pain: the obvious is not always right. Archives of ortho and trauma surgery. 2007 Dec; 10(127) 858-8.
- Ha, et al. Degeneration of Sacroiliac Joint After Instrumented Lumbar or Lumbosacral Fusion: A Prospective Cohort Study Over Five-Years Follow-Up. Spine (Phila Pa 1976). 2008 May 15;33(11):1192-8.
- Michael J DePalma*, Jessica M Ketchum, and Thomas R Saullo, “Etiology of Chronic Low Back Pain in Patients Having Undergone Lumbar Fusion,” Pain Medicine 2011; 12: 224–233.
- Po-Chou Liliang et al., “Sacroiliac joint pain after lumbar and lumbosacral fusion: findings using dual sacroiliac joint blocks,” Pain Medicine (Malden, Mass.) 12, no. 4 (April 2011): 565-570.
The iFuse Implant System® is intended for sacroiliac fusion for conditions including sacroiliac joint dysfunction that is a direct result of sacroiliac joint disruption and degenerative sacroiliitis. This includes conditions whose symptoms began during pregnancy or in the peripartum period and have persisted postpartum for more than 6 months. There are potential risks associated with the iFuse Implant System. It may not be appropriate for all patients and all patients may not benefit. For information about the risks, click here.