Sacroiliac arthritis can cause buttock or low back pain. Inflammation of the synovial compartment resulting from erosion and laxity of the ligaments can decrease the fluidity of the pelvic girdle and cause a faulty posture or gait.
Degeneration of the sacroiliac joints has primary or secondary causes. The primary causes commonly affect many joints and include rheumatoid arthritis, ankylosing spondylitis, Reiter's disease, and osteoarthritis. Secondary causes of sacroiliac dysfunction include trauma, obesity, contact sports, and septic arthritis. The pattern of involvement of sacroiliac disease can be characterized by its presentation, such as bilateral, unilateral, symmetrical, or asymmetrical. Ankylosing spondylitis and rheumatoid arthritis are commonly bilateral and symmetrical. Psoriatic arthritis, Reiter's disease, and osteo-arthritis may be bilateral but are asymmetrical in presentation. The unilateral presentation can be associated with septic arthritis or osteoarthritis.
History The pain from sacroilitis is caused by peri-osteal irritation at the myofascial insertions. The joint line is innervated from several levels including L3-S1. A deep dull ache or hypersensitivity to the ipsilateral joint line is often found. In males the discomfort may radiate to the groin or testicles. Pain emanating from the sacroiliac joint can cause buttock discomfort that can be referred to the hip or anterior thigh. The pain of the sacroiliac joint may become worse with sitting and relieved with walking. Patients have reported to be worse in the morning and relieved as mobility progresses throughout the day.
Another etiology of low back pain is the anomalous lumboiliac transitional articulation. The articulation between the L5 transverse process and the sacrum or ilium is present in 5-7% of the general population-(i9). A diverse array of pain symptoms will arise during evaluation of this subset of patients, but a majority of patients report discomfort in the buttocks and pain radiating to the lower limb. In addition, most patients are found to be symptomatic on the side of the anomalous articulation.
Insufficiency fractures of the sacrum and of the sacro-iliac joint can often occur in patients with primary or secondary osteoporosis such as transplant patients (20). Transplant recipients with osteoporosis who develop low back pain with or without trauma should be screened for insufficiency fractures.
Physical Examination Examination of the hips should be part of the complete back examination. The
patient may present with an obvious antalgic gait. In the sitting position there may be a pelvic tilt. Point tenderness is instructive where the patient is asked to point to the area of pain with one finger. Positive identification of the sacroiliac joint as the area of pain is significant.
The pelvic pressure test is also informative. On the prone patient, a vertical posterior to anterior compression of the central aspect of the sacrum or adjacent to a sacro-iliac joint can elicit pain or pressure symptoms that are concordant with the patient's usual symptoms (Fig. 6).
The pelvis can be manipulated to re-create the zone of discomfort. The Gaenslen test is also a good provocative test. The patient is placed supine, and the hip and knee are maximally flexed toward the trunk. The opposite knee is raised. A positive result is pain across the sacroiliac joint.
Diagnostic Imaging Plain film imaging of the sac-roiliac joint is commonly negative in the initial phase of sacroilitis. Later in the disease process, erosion and sub-chondral cyst formation create pseudowidening. However, this widening represents an early radiographic change. As the process becomes advanced, narrowing of the joint space occurs. The joint cartilage is roughened and wears away. Spur formation and lipping occur at the edge of the joint surface. Sharpened articular margins, osteophyte formation, and lipping of the marginal bone formation take place and create thickened, dense subchondral bone, ankylosis, and osteophytes. In ankylosing spondylitis, ankylosis or erosions may be seen depending on the stage of the process.
CT scan findings of sacroiliac spondylitis may show normal bone density with fusion and syndesmophytes across the joint. The higher resolution of bone detail pro-
vided by CT imaging can show the degenerative and erosive changes of the joint earlier than plain films.
The radionuclide bone scan is commonly helpful in characterizing the pattern of inflammatory reaction. Increased activity in the sacroiliac joints may be unilateral or bilateral. Bilateral and symmetrical appearance of activity may lead to characterization of an inflammatory process, rheumatoid arthritis, or other detectable cause (such as fracture). Involvement of other joints may demonstrate the pattern and distribution of disease. Sacral insufficiency fractures may go undiagnosed on plain film radiographs. A characteristic "butterfly" or "half butterfly" appearance of the fractures on radionuclide bone scans can establish the diagnosis. Other diagnostic clues include blood tests for rheumatoid factor, sedimentation rate, and the HLA-B27 haplotype.
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