Lower back pain is one of the most prevalent sports maladies, affecting athletes in nearly every sport. Diagnosing the cause of a back injury is quite difficult and challenging because multiple structures in the lower back region can cause pain. However, an accurate diagnosis is paramount to providing successful treatment of the spine injury.
Although still somewhat controversial, the sacroiliac joint (SIJ) is generally accepted as an anatomic structure within the lumbar complex that if injured can be a cause of lower back pain. Mechanical dysfunction, inflammation, infection, trauma, and degeneration all have been attributed to the SIJ. Once the diagnosis of SIJ injury is established, specifically directed treatment can lead to satisfying results. This article discusses the diagnosis, management, and rehabilitation of sacroiliac injuries and pain.
The incidence of lower back pain in humans parallels the incidence of the common cold, with a lifetime rate approaching 95%. Goldwaith and Osgood first discussed the possibility that SIJ injury could cause low back pain as early as 1905.  In the decades since then, several attempts have been made to establish the prevalence of SIJ syndrome in persons with back pain, and the results of these reports vary widely.
Schwarzer et al remarked that "the prevalence of sacroiliac pain would appear to be at least 13% and perhaps as high a 30%" in patients with low back and buttock pain.  Bernard and Kirkaldy-Willis reported the prevalence rate to be 22.5% in 1293 patients with back pain. 
The SIJ is a true diarthrodial joint that joins the sacrum to the pelvis. [4, 5, 6] In this joint, hyaline cartilage on the sacral side moves against fibrocartilage on the iliac side. The joint is generally C shaped with 2 lever arms that interlock at the second sacral level. The joint contains numerous ridges and depressions, indicating its function for stability more than motion. However, studies have documented that motion does occur at the joint; therefore, slightly subluxed and even locked positions can occur. [2, 7]
Stability is provided by the ridges present in the joint and by the presence of generously sized ligaments. The ligamentous structures offer resistance to shear and loading. The deep anterior, posterior, and interosseous ligaments resist the load of the sacrum relative to the ilium. More superficial ligaments (eg, sacrotuberous ligament) react to dynamic motions (eg, straight-leg raising during physical motion). The long dorsal sacroiliac ligament can become stretched in periods of reduced lumbar lordosis (eg, pregnancy).
Many large and small muscles have relationships with these ligaments and the SIJ, including the piriformis, biceps femoris, gluteus maximus and minimus, erector spinae, latissimus dorsi, thoracolumbar fascia, and iliacus. Any of these muscles can be involved with a painful SIJ. As a true joint, the SIJ is a pain-sensitive structure richly innervated by a combination of unmyelinated free nerve endings and the posterior primary rami of L2-S3. The wide possibility of innervation may explain why pain emanation from the joint can manifest in so many various ways, with different and unique referral patterns for individual patients.
The function of the SIJ is to dissipate loads of the torso through the pelvis to the lower extremities and vice versa. The pelvis acts as a central base through which large forces are accepted and dissipated. Although the main role of the joint is to provide stability, the SIJ has limited motion that allows it to dissipate and transfer significant loads and stresses. Studies by Weisel indicate that most movement occurs when rising from the sitting to the standing position. However, the amount of motion is small, making assessment of sacroiliac motion during physical examination quite difficult. Selvik suggested that hyperextension produces the greatest degree of motion (2° on average, with only minimal translation of 0.5-1.6 mm).
If the motion in the pelvis is asymmetric, then dysfunction can occur. Some conditions that cause asymmetric motion include leg-length inequalities, a unilaterally weak lower limb (eg, polio), tight myofascial structures (eg, iliopsoas), and scoliosis. Hip osteoarthritis can lead to leg-length shortening and SIJ pain.
Women may be at increased risk for SIJ problems because their broader pelvises, greater femoral neck anteversion, and shorter limb lengths lead to different, possibly predisposing, biomechanics. In addition, pregnancy often leads to stretching of the pelvis, specifically targeting the sacroiliac ligaments and possibly leading to dysfunction, hypermobility syndromes, and chronic pain.
The nerve supply of the SIJ originates from multiple lumbosacral root levels with partial innervation from L2 (anterior joint) to S3 (posterior joint). Because the root innervation can vary so widely, the pain referral patterns from primary sacroiliac pain can also vary. Fortin et al interviewed multiple patients documented to have sacroiliac pain by anesthetizing the joint with lidocaine injections under fluoroscopic guidance. [8, 9] He found referral patterns ranging from localized buttocks pain to frank radicular leg pain and many other descriptions in between.
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