Osteitis condensans Ilii (OCI) is a self-limiting condition marked by sclerosis of the iliac bone, found either incidentally on imaging in asymptomatic patients or those presenting with lower back pain. Imaging and clinical findings are localized to the sacroiliac joint(s) and thus, must be differentiated from other conditions that may present in a similar fashion (spondyloarthritis, degenerative arthritis, septic arthritis). Although some of the presenting features at times may overlap, OCI is distinct in that it spares the joint space, is not progressive, and most commonly presents in the absence of lab value abnormalities.
Although there have been several proposed theories, the pathophysiology involved in the development of OCI essentially remains unclear. Because this condition is most prevalent in perigestational/postpartum women , it was initially postulated that it was related to physiologic changes brought on by pregnancy. These changes are marked by both mechanical and vascular components. Mechanical considerations include joint space widening and ligamentous laxity of the sacroiliac joints  as well as an increased stress load placed on the pelvis in the gravid state.
From a vascular standpoint, an enlarged uterus may exert a mild mass effect upon the adjacent abdominal aorta/iliac arteries. This may cause mild hemodynamic flow alteration and transient ischemia to the downstream distal ileum . This does not account for the fact that OCI also occurs in males and nulliparous females. But the findings do remain commonly associated with weight gain , and thus, a mechanical etiology is still felt most likely. Of note, a genetic predisposition has also been questioned, as there have been cases in which human leukocyte antigen-B27 (HLA-B27) was found; however, a consistent link has not been established. Most patients are HLA-B27 negative . While there have been case reports of patients with an elevated erythrocyte sedimentation rate (ESR) suggesting an inflammatory component to the process, the vast majority of patients have normal ESR levels .
OCI was named in 1926 by Sicard et al., as an exclusive entity used to describe a condition with characteristic sclerotic radiographic imaging findings localized to the periarticular (auricular) portion of the ileum at the sacroiliac joint without other joint space abnormality such as erosions, effusion, or periarticular bone marrow or soft tissue edema . The prevalence of OCI in the general population has been reported between 0.9% and 2.5% . The prevalence of OCI in those specifically being imaged in evaluation for inflammatory arthropathy, however, has been shown in target populations to be as high as 8.9% . As previously noted, the condition is more common in females , especially in those with recent weight gain or in pregnancy.
Histolopathologic findings are limited in OSI; however, it has been shown that in affected regions of sclerosis, there is focal bone marrow fibrosis. In addition, there is increased density in trabeculation, as well as an increase in osteoblastic activity. There has also been shown to be a quantitative increase in lamellar bone in the affected region .
OCI is often asymptomatic, but can also present as nonspecific lower back pain. If present, pain symptoms are usually reported as worsened with exertion and improve with rest. In contrast, the lower back pain symptoms in spondyloarthritis commonly improve with activity. As previously noted, OCI symptoms are most commonly seen in association with weight gain or pregnancy . Systemic symptoms such as fever, fatigue, weakness, and weight loss are absent, and for this reason, a detailed history and complete review of systems should be obtained to exclude other processes.
Pain is usually bilateral and may extend to the buttocks and posterior thighs in a non-radicular fashion . Symptoms may or may not correlate with point tenderness at the sacroiliac joint. Fortin finger sign reproduces pain when deep pressure is applied posteriorly over the joint. Likewise, the FABER/Patrick test (pain elicited by hip flexion in conjunction with abduction and external rotation) is variably positive and thus equally nonspecific. Additional provocative maneuvers to include sacral distraction test, thigh/sacral thrust test, and iliac compression tests are also unreliable indicators.
It is important to perform a full musculoskeletal exam to assess for other symptomatic joints, as OCI only localizes to the lower back. Additionally, post-traumatic and congenital causes for the pain, including leg-length discrepancy and spinal malalignment, should also be excluded on physical exam. In addition, there should be no neurologic findings of sensory or motor loss .
As previously noted, OSI most commonly presents without lab value abnormality. Therefore laboratory studies are most helpful in ruling out other causes of sacroiliac disease. Inflammatory markers such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are negative and should be obtained to rule out an inflammatory arthropathy such as rheumatoid or psoriatic arthritis, or infectious sacroiliitis. Rheumatoid factor is also negative and may, therefore, offer some diagnostic discriminatory value as well. HLA-B27 is rarely positive , but in these instances, nonspecific, for a positive value, is also seen in as many as 85-95% of ankylosing spondylitis cases . As previously described, a direct correlation between HLA-B27 and OSI has not been well established.
Classic radiographic findings of OSI include sclerosis arising along the subcortical articular surface of the iliac bone extending into the adjacent medullary space, a pattern that classically appears as a triangular area of increased density/sclerosis  with apex cephalad. The findings are most commonly symmetric. The important discriminator in the radiographic findings is no associated joint space narrowing or widening, ankylosis, bony destruction, erosion, effusion, bone marrow edema, or surrounding soft tissue edema . Although isolated iliac involvement is a hallmark, it has also been shown that in some cases, mild sacral sclerotic involvement is seen as well .
CT scan demonstrates the same findings as radiographs but to greater advantage and detail. Additionally, the axial nature of the exam may show an anterior predilection for sclerosis . On MRI, regions of sclerosis appear as low signal intensity on both T1 and T2 weighted imaging. Bone marrow edema has not typically been described in association with this process; however, it has recently been reported in a minority of cases . Despite these case reports surrounding edema should increase suspicion for alternative inflammatory etiologies. Pertinent negatives on MRI include no periosteal reaction and no localized soft tissue abnormality. Lastly, a nuclear medicine bone scan will generally demonstrate localized increased uptake in the sacroiliac region due to increased sclerosis (bone formation) . Advanced imaging is seldom required to diagnosis OSI as the triangular, symmetric, iliac sided sclerosis in the absence of elevated inflammatory markers is an "Aunt Minnie" or pathognomic presentation.
Given the non-progressive nature of OSI, the prognosis is highly favorable, and conservative therapy is preferred. This includes NSAIDs, physical therapy, and rest. Although not an inflammatory condition, corticosteroid/anesthetic therapeutic injections have been employed , and in one refractory case, a surgical core decompression was reported .
Sacroiliitis refers to inflammation of the sacroiliac joint. This a nonspecific term that includes a multitude of etiologies with varying clinical presentations ranging from unilateral, bilaterally symmetric, or bilateral asymmetric .
The unilateral processes can be distinguished from OSI primary based on the distribution as OSI is most commonly bilateral symmetric. An infection will most commonly present with elevated laboratory values and fever in addition to surrounding soft tissue and bone marrow edema on advanced imaging. All of those features are absent in OSI. A neoplastic process will be easily distinguished based on history in the setting of a known primary malignancy and/or advanced imaging as neoplasm will commonly be a destructive process, and not have the symmetric triangular sclerotic appearance.
Other differential considerations would include spondylosis of the lumbar spine, osteoarthritis of the femoroacetabular joints, piriformis syndrome, trochanteric or iliopsoas bursitis, muscle strain or tendinosis. Low back and hip pain can have similar clinical presentations and physical exam findings; however, this processes can be easily distinguished from OSI on advanced MRI imaging of the lumbar spine and pelvis.
OCI is a self-limiting and nonprogressive disease. Most patients with OCI have an excellent outcome.
There are no known complications associated with OCI.
Not applicable as this condition is self-limiting with no proven cause. Weight gain may be a risk factor, and thus patient education should focus on promoting healthy lifestyle choices and proper nutrition.
An interprofessional approach to this condition is generally not needed as conservative management is favored.
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