Sacroiliitis

Etiology

Various conditions result in the inflammation of the SI joint, leading to significant pain. Osteoarthritis can cause degeneration of the joint resulting in pathologic articulation and motion leading to this condition. Spondyloarthropathies themselves can cause significant inflammation of the joint itself. Pregnancy is another cause of the inflammation due to the hormone relaxin leading to the relaxation, stretching, and possible widening of the SI joint(s). The increased weight of pregnancy also causes extra mechanical stress on the joint, leading to further wear and tear. Trauma can cause direct or indirect stress and damage to the SI joint. Pyogenic sacroiliitis is the most frequently reported cause of acute sacroiliitis. Pain can originate from the synovial joint but can also originate from the posterior sacral ligaments.[4][5]

Epidemiology

Reports on the prevalence of sacroiliac pain vary widely. Some studies report the prevalence as 10% to 25% of those with lower back pain. In those with a confirmed diagnosis, the presentation of pain was ipsilateral buttock (94% cases) and midline lower lumbar area (74%). As mentioned previously the presentation can vary. Up to 50% of cases have radiation to the lower extremity: 6% to the upper lumbar area, 4% percent to the groin, and 2% percent to the lower abdomen.[6][7]

Pathophysiology

The sacrum articulates with the ilium, which helps to distribute body weight to the pelvis. The SI joint capsule is relatively thin and often develops defects that enable fluid, such as joint effusion or pus, to leak out onto the surrounding structures. As surrounding muscles and structures are affected different presentations of pain can present because different nerve roots innervate these structures. Common distributions of pain are L4-L5 dermatomes, but distributions can certainly present in dermatomes as high as L2 and as low as S3. The asymmetric motion of the pelvis can cause mechanical dysfunction, leading to degeneration and significant pain. Differential diagnosis includes leg-length discrepancies, unilateral weaker limb or gluteal muscles, tight or strained surrounding muscles structures, or hip osteoarthritis.[8]

Histopathology

Common degenerative findings show destruction and fibrosis of the cartilage and disruption, erosion, and ossification of sub-chondral bone. Synovitis also is commonly found which reveals hyperplasia of linings cells with infiltration of inflammatory cells. Degenerative enthesitis similarly reveals inflammatory cells. Infectious etiologies of sacroiliitis reveal lymphocytes, plasma cells, and fibrosis. Notably, some patients will show no histopathologic changes.

History and Physical

Sacroiliitis commonly presents as lower back pain. The pain presentation varies greatly, and patients could describe pain in one or both buttocks, hip pain, thigh pain, or even pain more distal. Patients may report that their pain is worse after sitting for prolonged periods or with rotational movements. Characterization of pain also varies widely and is commonly described as sharp and stabbing but can also be described as dull and achy. It is very important to ascertain more than just the timing and descriptions of the pain. Ask about a history of inflammatory disorders as discussed previously. Also, obtain a thorough review of systems to evaluate for systemic symptoms such as fevers, chills, night sweats, and weight loss. These symptoms are indicative of a more serious process indicating likely systemic illness.

Although not always obvious, inspection can reveal pelvic asymmetry. Measurement of the limbs can rule out a leg-length discrepancy. Inspect the spine for any abnormal curvatures or rotational abnormalities. Typically the range of motion, neurologic, and strength testing are unremarkable although the patient may experience pain during some of these tests.

Special provocative tests can be very helpful in reproducing the patient’s pain:

• “Fortin finger sign”- reproduction of pain after applying a deep palpation with the four-hand fingers posteriorly at the patient's SI joint(s).
• FABER test- reproduction of pain after flexing the hip while also abducting and externally rotating the hip.
• Sacral distraction test- reproduction of pain after applying pressure to the anterior superior iliac spine.
• Iliac compression test- reproduction of pain after applying pressure downward on the superior aspect of the iliac crest.
• Gaenslen test- reproduction of pain after having the patient flex the hip on the unaffected side and then dangle the affected leg off the examining table. Pressure is then directed downward on the leg to extend further the hip, which causes stress on the SI joint.
• Thigh thrust test- reproduction of pain after flexing the hip and applying a posterior shearing force to the SI joint.
• Sacral thrust test- reproduction of pain with the patient prone and then applying an anterior pressure through the sacrum.

The likelihood of SI joint mediated pain increases as the number of positive-provocative tests increase.

Evaluation

As discussed previously, various inflammatory conditions may cause or contribute to SI joint pain. If an inflammatory condition is suspected, consider ordering complete blood cell count, erythrocyte sedimentation rate, C-reactive protein, antinuclear antibody, human leukocyte antigen (HLA-B27), and rheumatoid factor. Although cancer is a far less common cause of sacroiliitis, if a cancerous process is suspected, consider ordering labs to assess for malignancy. [9][10]

In most clinical evaluations, common laboratory findings in the aseptic setting include:

• WBC count - usually normal (elevated in infection/septic presentation)
• ESR - elevated
• CRP - elevated
• HLA-B27 - About 50% to 92%% of patients with ankylosing spondylitis will be HLA-B27 positive
• Rheumatoid Factor (RF) - Negative in the setting of true ankylosing spondylitis

If trauma or inflammatory causes are not suspected, then radiographs are typically not needed, as there is usually no correlation between pain and radiographic images. If radiographs are performed an anteroposterior view of the pelvis/lumbar spine should be ordered. These images can show sclerosis or other degenerative changes of the SI joint. Specific sacroiliac views also can be taken at a 25 to 30-degree angle to help delineate widening or sclerotic changes at joint margins. CT scan also can show sclerotic changes and reveal reactive spurring or subluxation. The previously noted imaging findings are usually later changes. MRI may show subchondral edema, which is the earliest sign of sacroiliitis. PET scans can be ordered to evaluate for bony metastatic disease.

The most informative test for the diagnosis of sacroiliitis is SI joint injection with local anesthetics and steroids. If this procedure relieves the pain, it is likely that the inflammation at this site was the cause of the pain. Potentially, a single injection or series of injections can completely treat the pain. If the injection provided a very significant amount of pain relief, even if for a short period, then the chronic pain can likely be attributed to the SI joint. In such cases, a surgical intervention should be considered as the injections were able to localize the pain to the SI joint(s). No more than three injections should occur over a year.

Treatment / Management

Physical therapy can be very helpful if the pain is due to hypermobility. Therapy can help to stabilize and strengthen lumbopelvic musculature. If the pain is due to immobility, then physical therapy can help increase mobilization of the SI joint.  NSAIDs and muscle relaxants can be prescribed during the acute phase of presentations. These are less effective as cases become more chronic. Real-time image-guided intra-articular anesthetic/steroid injections can be performed for diagnostic and therapeutic effect. If the previous treatments do not provide adequate relief, then some providers will consider radiofrequency ablation. Usually, surgery is reserved as a last resort for patients with chronic pain. In such cases, one can consider SI joint fusion with SI screws.[11][12][13]

Differential Diagnosis

• Ankylosing spondylitis
• Hip tendonitis/fracture
• Piriformis syndrome
• Sacroiliac joint infection
• Trochanteric bursitis

Complications

• Chronic back and hip pain
• Muscle wasting