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 [6]. 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 [3]. 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 [3].

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 [1][3][4], but in these instances, nonspecific, for a positive value, is also seen in as many as 85-95% of ankylosing spondylitis cases [8][9]. 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 [1] 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 [3]. Although isolated iliac involvement is a hallmark, it has also been shown that in some cases, mild sacral sclerotic involvement is seen as well [10]. 

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 [3]. 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 [1].  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) [11][12]. 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 [3], and in one refractory case, a surgical core decompression was reported [12].

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 [10].

• Unilateral 
  • Infection: In the setting of unilateral sacroiliitis, infection has to be excluded as delay in diagnosis is associated with severe morbidity. The patients generally present with severe pain, decreased range of motion, fever, elevated inflammatory markers, and purulent fluid on joint aspiration. Risk factors include recent therapeutic injection, bacteremia, IV drug users, or immunocompromised states. Delayed diagnosis can lead to severe joint destruction leading to long term disability, sepsis, or death in extreme cases. However, most patients have a positive outcome.[13]
  • Destructive neoplastic process: Although uncommon, a primary osseous lesion or metastatic disease can involve the iliac wing or sacrum and mimic sacroiliac disease. In young patients, the primary considerations would include osteosarcoma, Ewing sarcoma, or lymphoma. In adults, metastatic disease is more common.
  • SAPHO: This is a rare condition that presents with the constellation of findings to include synovitis, acne, pustulosis, hyperostosis, and osteitis. [14]

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.

• Bilateral asymmetric
  • Gout: This is a crystalline arthropathy most commonly affecting middle-aged men. The most common joints involved are the hands and feet, specifically the first metatarsophalangeal (feet) and radiocarpal (hands). Patients may have elevated serum uric acid levels on laboratory analysis. On imaging, gout is an erosive process, so there should be periarticular erosions, soft tissue swelling, and in some cases, a soft tissue mass (tophus). The demographics, involvement of the hands/feet, and inflammatory changes are key distinguishing features.[15]
  • Psoriatic arthritis (PA): PA is a seronegative inflammatory arthropathy that most commonly affects the hands and feet. Most patients will have a precedent diagnosis of psoriasis with skin manifestations. Unlike gout or rheumatoid arthritis, there is no gender predilection. The involvement of the hands and feet is distal more than proximal. In other words, the interphalangeal joints are more commonly involved than the joints of the midfoot or wrist. There is commonly soft tissue swelling, erosive changes, periostitis, joint subluxations, or ankylosis. The characteristic imaging features and multifocal involvement distinguish it from OSI. [10][16][17]
  • Osteoarthritis: The hallmarks of osteoarthritis are sclerosis, osteophyte formation, and subchondral cystic change. Osteoarthritis involvement of the sacroiliac joints will manifest as changes of both sides of the joint with small osteophytes. In addition, it will not have the classic triangular sclerosis seen in OSI. 

• Bilateral symmetric
  • Ankylosing spondylitis (AS): AS is seronegative spondyloarthropathy most commonly in young adult males. Unlike other arthropathies discussed previously, this is a process that predominately involves the large joints, specifically the SI joints and spine, but can also involve the hips and shoulders. Patients will commonly be HLA-B27 positive, unlike OSI. Like OSI, the process is bilaterally symmetric. However, the gender predilection is male, and the dominate imaging feature is ankylosis starting in the sacroiliac joints progressing to the spine. OSI does not involve the joint space, will not result in ankylosis, and does not involve the spine.[18]
  •  Rheumatoid arthritis (RA): RA is a chronic inflammatory arthropathy more commonly involving women than men with peak presentation in middle-age. The hands and feet are most commonly involved; however, other joints such as the knee, shoulder, hip, sacroiliac, and spine can be involved. It is rare for a patient to have involvement of a large joint without involvement of the hands or feet. The patients will commonly present with periarticular osteopenia and erosions, unlike OSI.
  • Enteropathic arthritis: The association with inflammatory bowel disease is the distinguishing feature from OSI in these patients.

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.