History: A 30-year-old woman presented with complaint of low-back pain following spontaneous vaginal delivery. She had no history of spinal anesthesia. The pain was dull and came on and off. She had no known precipitating or aggravating factors. There was no limitation of movement or interference with daily activities.

The patient underwent a lumbosacral spine CT scan. A selected sagittal image is shown below.

Coronal and axial CT images of the lumbosacral spine are shown below.

Lumbosacral spine MRI was performed and showed normal lumbar spine. There was no sign of inflammation in the visualized sacroiliac joints. Selected sagittal T2-weighted and T2-weighted fat-saturated images are shown, as well as an axial T2-weighted image. 

Findings

• CT: There is bilateral triangular-shaped sclerosis of the iliac side of the sacroiliac joints. No sclerosis of the sacral side of the sacroiliac joints is seen. The sacroiliac joint spaces are preserved. The lumbar spine is normal; no ankylosis of the spine is noted.




• MRI: MRI shows normal lumbar spine. There is no sign of inflammation in the sacroiliac joint.

Differential diagnosis

• Sacroiliitis

• Septic arthritis of the sacroiliac joints

• Osteitis condensans ilii

• Osteoarthritis of the sacroiliac joints

髂骨致密性骨炎

Discussion

Osteitis condensans ilii

Epidemiology

It is an uncommon condition usually detected incidentally. The incidence is less than 2.5%, and it is more common in females. It generally occurs more often in pregnancy and the puerperal period, similar to the findings in this case.

Etiology

The etiology of OCI is unknown. However, it has been suggested that the condition could result from imbalance of weight across the sacroiliac joints, which leads to a chronic stress response by the joints.

Clinical presentation

The condition is usually detected incidentally while investigating for other pelvic pathologies. However, in some cases, patients can present with low-back pain that is usually not localized to the sacroiliac joints. This is similar to the presentation in this patient.

Imaging features

Imaging features can be demonstrated on pelvic x-ray and CT scans. However, CT gives more detailed information than x-ray. 

The salient finding is bilaterally symmetrical triangular-shaped sclerosis of the iliac bones adjacent to the sacroiliac joints. The outer border of the triangular sclerosis is well demarcated and the base of the triangle points inferiorly. The sacroiliac joint surfaces are smooth and the joint spaces preserved. Also, the sacral aspects of the sacroiliac joints are normal.

Differentiating OCI from the early stage of sacroiliitis, such as in ankylosing spondylitis, can pose a challenge. The main distinguishing feature is that in sacroiliitis, the sclerosis usually first involves the sacral side of the sacroiliac joint (SIJ). This is because the sclerosis in sacroiliitis normally begins with the surface where the articular cartilage is thinnest; this is the sacral surface of the joint. Sclerosis of the iliac side of the SIJ occurs later in sacroiliitis after the sacral side has been involved. This is unlike OCI where the sacral side of the SIJ is normal and the iliac side is sclerosed. In other words, sclerosis of the iliac side of the SIJ with a normal sacral side of the joint makes sacroiliitis unlikely. In late-stage sacroiliitis, there is usually sclerosis of both the iliac and sacral sides of the SIJ with narrowing and subsequent ankylosis of the joint spaces. However, the sacroiliac joint spaces are usually preserved in OCI and the sacral side of the SIJ remains spared. Also, other associations such as ankylosis of the vertebrae may be seen in sacroiliitis but not in osteitis condensans ilii.

Septic arthritis of the SIJ is another possible differential. In early-stage septic arthritis, there could be periarticular osteopenia (due to hyperemia) and joint effusion. This is in contrast to OCI, which is associated with sclerosis and no joint effusion. Late-stage septic arthritis of the SIJ may lead to erosion and irregularity of the articular surfaces and narrowing of the joint spaces. However, the articular surfaces of the SIJ are smooth and the sacroiliac joint spaces are normal in OCI. Clinically, septic arthritis of the SIJ is associated with severe pain and limitation of movement. However, the pain of OCI is usually mild and does not interfere with movement.

Unlike sacroiliac joint osteoarthritis, where there is irregularity and erosion of the articular surfaces, subchondral cysts, osteophyte formation, and loss of the joint spaces, the articular surfaces in OCI are smooth and the joint spaces are preserved.

Treatment

No treatment is usually required; the condition generally resolves spontaneously. However, conservative management with the use of physical therapy and analgesics may be appropriate in some cases.

Key points

• Careful evaluation of the sacroiliac joint is important when imaging the spine.

• The side of sclerosis of the sacroiliac joint is important in distinguishing pathologies of the joint.

• Correct diagnosis of osteitis condensans ilii, when present, is necessary to prevent unnecessary workup and treatment.

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