Working Diagnosis:
Right sacroilitis and acetabular labral tear

Treatment:
The patient was treated with three weeks of non-weight-bearing and sacroiliac corticosteroid injections were performed. The patient then resumed weight lifting, aerobic exercise, and was recommended to follow up in 3-6 months.

Outcome:
Upon treatment completion, the patient was allowed to resume normal weight lifting activity and aerobic exercise. He subsequently returned to running and experienced pain reduction.

Author's Comments:
This case illustrates an complicated presentation with co-existence between sacroiliitis and acetabular labral tear. Although sacroiliitis typically presents with low back, buttock, hip, or thigh pain that worsens with prolonged sitting; the case demonstrates that sacroiliitis being the main pain generator is less likely due to the pain worsening with activity. The radiograph and MRI were also more suggestive of acetabular labral tear as the main pain generator for this patient, as confirmed by the treatment regimen of non-weight-bearing, crutches and sacroiliac corticosteroid injections whereas the typical treatment for true sacroilitis is physical therapy, structured exercise, and NSAIDs. Recognizing this presentation can reduce morbidity and improve functionality.

Editor's Comments:
It should be noted that there are no universally accepted diagnostic criteria for seronegative spondyloarthropathy including ankylosing spondylitis (AS). This is due to the wide heterogeneity of the disease and the lack of serological markers. Rather, several classification criteria have been standardized to create more uniformity in disease categorization (namely for inclusion in clinical trials, rather than for diagnosis in everyday practice). That said, classification criteria can still be useful tool to help guide the clinical diagnosis. Of the various criteria, the Assessment of SpondyloArthritis international Society (ASAS) criteria largely holds higher sensitivity and specificity compared to other designed criteria and can capture milder (and earlier) disease forms, in part by incorporating advanced imaging and extra-articular signs. It is important to note that there are different criteria used for axial vs. peripheral spondyloarthritis.

This patient fulfills the ASAS classification criteria for peripheral spondyloarthropathy (i.e. did not present with back pain). This includes one or more of the following: Psoriasis, Inflammatory Bowel Disease, HLAB27 positivity, Uveitis, or Sacroiliitis on Imaging; PLUS two or more of the following: Arthritis, Enthesitis, Dactylitis, Inflamatory back pain in the past, or Family history of a spondyloarthropathy.

The positive Human Leukocyte Antigen B27, also known as HLAB27, is included in both the ASAS axial and peripheral spondyloarthritis classification criteria and has an important role in the early diagnosis of spondyloarthritis. It is still unclear how HLAB27 (a specific protein found on cell surfaces) participates in the inflammatory process of autoimmune disease. Only 6 to 7% of the general population are HLAB27 positive, of which only 5 to 10% will have a spondyloarthritis or ankylosing spondylitis. Conversely, upwards of 70% for those with a spondyloarthropathy (up to 90% for those with ankylosing spondylitis, specifically) will have a positive HLAB27. Hence, despite its high specificity for AS and other spondyloarthropathies, HLAB27 has a low sensitivity, making it a poor choice as a screening marker, so it should not be ordered in the absence of inflammatory joint changes or suspicion of a spondyloarthropathy.

References:
1. Banga K, Racano A, Ayeni OR, Deheshi B. Atypical hip pain: coexistence of femoroacetabular impingement (FAI) and osteoid osteoma. Knee Surg Sports Traumatol Arthrosc. 2015;23(5):1571-1574.

2. Swenson KM, Erickson J, Peters C, Aoki SK. Hip pain in young adults: diagnosing femoroacetabular impingement. JAAPA. 2015;28(9):39-45.

3. Chamberlain R. Hip Pain in Adults: Evaluation and Differential Diagnosis. Am Fam Physician. 2021;103(2):81-89.

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