The SI joint is a real yet underappreciated pain generator in an estimated 15% to 25% of patients with axial LBP. Whereas historical and physical examination findings have been previously advocated as useful tools in identifying patients with SI joint pain, more recent studies have demonstrated they have limited diagnostic value. Presently, small-volume diagnostic blocks remain the most commonly used method for diagnosing this disorder, although their validity remains unproven. Intra articular and peri articular corticosteroid injections have been shown in most, but not all, studies to provide good to excellent pain relief lasting up to 10 months in patients with and without spondylarthropathy. One promising area in the treatment of SI joint pain is RF denervation, although the conclusions that can be drawn are limited by the heterogeneous methods used and the lack of controlled studies.
The sacroiliac (SI) joint is the largest axial joint in the body. There is wide variability in the adult SI joint, encompassing size, shape, and surface contour. The SI joint is most often characterized as a large, auricular-shaped, diarthrodial synovial joint. In reality, only the anterior third of the interface between the sacrum and ilium is a true synovial joint; the rest of the junction is comprised of an intricate set of ligamentous connections. Because of an absent or rudimentary posterior capsule, the SI ligamentous structure is more extensive dorsally, functioning as a connecting band between the sacrum and ilia. The main function of this ligamentous system is to limit motion in all planes of movement. In women the ligaments are weaker, allowing the mobility necessary for parturition.
The SI joint is also supported by a network of muscles that transmit regional muscular forces to the pelvic bones. Some of these muscles, such as the gluteus maximus, piriformis and biceps femoris, are functionally connected to SI joint ligaments, so their actions can affect joint mobility.
Age-related changes in the SI joint begin in puberty and continue throughout life. During adolescence, the iliac surface becomes rougher, duller, and coated in some areas with fibrous plaques. These senescent changes accelerate during the third and fourth decades of life and are manifested by surface irregularities, crevice formation, fibrillation and the clumping of chondrocytes. In the sixth decade, motion at the joint may become markedly restricted as the capsule becomes increasingly collagenous and fibrous ankylosis occurs. By the eighth decade of life, erosions and plaque formation are inevitable and ubiquitous.
The innervation of the SI joint remains a subject of much debate. The lateral branches of the L4-S3 dorsal rami are cited by some experts as composing the major innervation to the posterior SI joint. The innervation of the anterior joint is similarly ambiguous.
The SI joints are designed primarily for stability. Their functions include the transmission and dissipation of truncal loads to the lower extremities, limiting x-axis rotation, and facilitating parturition. Compared to the lumbar spine, the SI joints can withstand a medially directed force 6 times greater but only half the torsion and 1/20th of the axial compression load. These last 2 motions may preferentially strain and injure the weaker anterior joint capsule.
Risk factors that operate by increasing the stress borne by the SI joints include true and apparent leg length discrepancy, gait abnormalities, prolonged vigorous exercise, scoliosis and spinal fusion to the sacrum. Causes of SI joint pain can be divided into intra articular and extra articular sources. Intra articular causes include infection and arthritis. Extra articular sources are more common of the two and include enthesopathy, fractures, ligamentous injury and myofascial pain.
Pregnancy can predispose women to SI joint pain by combination of increased weight gain, exaggerated lordotic posture, the mechanical trauma of parturition, and hormone-induced ligamental laxity. The laxity associated with pregnancy is attributable to increased levels of estrogen and relaxin, and it predisposes parturients to sprains of the SI joint ligaments. SI subluxation has also been reported to cause back pain in pregnancy.
Inflammation of one or both SI joints is considered to be an early and prominent symptom in all seronegative and HLA-B27 associated spondylarthropathies. Although the precise etiology of spondylarthropathy remains unknown in most patients, the strong association with HLA-B27 supports the view that these conditions are attributable to a genetically determined immune response to environmental factors in susceptible individuals. In a subset of patients with Reiter’s syndrome/reactive arthritis, the disease is clearly induced by infection.
Patients complain of pain in the lower back with radiation into the buttock, lower lumbar region, lower extremity, groin area, upper lumbar region and abdomen. Pain can rarely be radiated below the knee and in to the foot. Based on the existing data, the most consistent factor for identifying patients with SI joint pain is unilateral pain (unless both joints are affected) localized predominantly below the L5 spinous process.
Pain is frequently worse with standing and sitting down for prolonged periods. Pain is also exacerbated by turning in bed or lying on the affected side.
One of the most challenging aspects of treating SI joint pain is the complexity of diagnosis. Dozens of physical examination tests have been advocated as diagnostic aids in patients with presumed SI joint pain . Many involve distraction of the SI joints, with two of the most common ones being Patrick’s test and Gaenslen’s test. Neither medical history nor physical examination findings are consistently capable of identifying dysfunctional SI joints as pain generators.
It is often assumed that an analgesic response to a properly performed diagnostic block is the most reliable method to diagnose SI joint pain. Although this may seem to be self-evident, the validity of intraarticular SI joint blocks remains unproven.
SI joint block can be one of the most challenging spinal injection procedures. Extravasation of LA to surrounding pain-generating structures such as muscles, ligaments, and lumbosacral nerve roots can lead to false-positive blocks. Conversely, failure to obtain adequate LA spread to the anterior and cephalad portions of the SI joint can result in false-negative blocks.
Regardless of the imaging modality used to confirm intra articular injection, SI joint injections should never be performed blindly because of a high failure rate. It is also accepted that diagnostic injections can frequently be therapeutic as well. In effect there is no infallible, universally accepted method for diagnosing pain originating in the SI joint.
Treatment of SI joint pain can be challenging.
As with any pain condition psychosocial factors can influence both the development of chronic pain conditions and the response to treatment. To optimize outcomes, the identification and treatment of concomitant psychosocial issues is of paramount importance. This is best accomplished via a multidisciplinary approach.
This includes controlling the pain with non-pharmacological option (TENS, acupuncture), oral analgesics and exercise regimes to strengthen the core muscles in the back.
Intra articular injections with local anaesthetic and steroid often serve the dual function of being diagnostic and therapeutic. Most investigators have found radiologically guided SI joint injections to provide good to excellent pain relief lasting from 6 months to 1 year. Various studies have also shown a beneficial effect for peri articular corticosteroid treatment as well.
Botulinum toxin injections have been used in management of SI joint pain. Both peri articular and intra articular botox injections have been reported.
Cooled radiofrequency is a relatively new concept developed for conditions that are difficult to treat effectively using conventional radiofrequency treatment. Cooled radiofrequency utilises a special radiofrequency probe designed to produce a larger lesion size and therefore treat a larger area than is possible with conventional radiofrequency treatment.
There is some evidence that suggests that cooled RF treatment may be more efficacious in management of SI joint pain, compared to standard RF and intra articular injections, though more robust studies are needed to confirm this.