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PubMed This is a summary of 18 peer-reviewed journal articles Updated
Rheumatology

The Biology of SAPHO: Why Your Body Reacts This Way

At a Glance

SAPHO syndrome is mainly an autoinflammatory disorder, not a straightforward bone infection: immune activity can cause bone inflammation, thinning, and overgrowth alongside skin disease. C. acnes may trigger the process in some people, but its role remains uncertain.

Understanding SAPHO/Adult CNO requires a shift in how we think about “infection” and “inflammation.” For a long time, doctors weren’t sure if this was a slow-growing infection or an autoimmune problem. Modern research has clarified that it is primarily an autoinflammatory disorder—a condition where the body’s innate immune system (the “first responders”) becomes overactive and attacks its own bone tissue without a clear external threat [1][2].

The Biological “Glitch”

Adult SAPHO/CNO biology is heterogeneous and remains incompletely understood, but several mechanisms are proposed to play a role.

  • IL-1 Signaling: A specific protein called Interleukin-1 (IL-1) acts as a powerful “on” switch for inflammation. In SAPHO, researchers believe pathways involving this protein may be dysregulated [3]. This can lead to the activation of neutrophils, white blood cells that usually fight bacteria but instead cause swelling and damage in your bones and skin [4][1].
  • Bone Remodeling: Your bones are constantly being broken down and rebuilt. In this condition, the inflammation causes a “mixed” reaction: some areas of bone are eaten away (osteolysis), while other areas grow back too thick and dense (hyperostosis or sclerosis) [5][6].
  • Genetic Clues: While genetic findings are not routine diagnostic tests, researchers have proposed associations with specific genes like LPIN2, IL1RN, and FBLIM1 in some studies [2]. These genes are all involved in controlling how the body turns off inflammation, though no single gene explains all cases.

The Role of C. acnes: Trigger, Contaminant, or Pathogen?

One of the biggest controversies in SAPHO is the role of Cutibacterium acnes, the bacterium usually associated with common acne.

Studies show that C. acnes is found in many bone biopsies from SAPHO patients [7]. However, its significance remains contested:

  1. Sampling: This bacterium is very common on human skin, making it hard to tell if it was truly in the bone or just a “contaminant” from the biopsy procedure [8][9].
  2. Antibiotics: Long-term antibiotics rarely “cure” the condition the way they would a standard bone infection [1][10].
  3. The Trigger Theory: Many experts propose that C. acnes may act as a trigger. In people who are genetically susceptible, the presence of the bacteria might “set off” the autoinflammatory process, which then continues even after the bacteria are gone [10][7].

Differentiating SAPHO from “Look-Alikes”

Because SAPHO is rare, it is frequently confused with more common conditions. Distinguishing them requires careful clinical context, serial imaging, and specialist assessment.

Condition How it differs from or relates to SAPHO/Adult CNO
Infectious Osteomyelitis A true bacterial infection. While acute infections may involve high fever, chronic infectious osteomyelitis can be indolent and afebrile. It requires microbiology testing and targeted antibiotics [7][11].
Bone Metastasis Cancer that has spread to the bone. Lesions can be purely destructive (osteolytic), bone-forming (osteoblastic), or mixed. Biopsy is often needed to rule cancer out [12][13].
Tietze Syndrome A benign, self-limiting inflammation of the chest cartilage. It causes swelling but does not show the complex bone remodeling (sclerosis/thickening) seen on a SAPHO MRI [14][15].
Psoriatic Arthritis (PsA) & Axial Spondyloarthritis (axSpA) There is significant clinical overlap; some patients have both SAPHO and PsA or axSpA. PsA or axSpA can also involve the anterior chest wall and produce osteitis-like findings. Diagnosing an overlap requires careful specialist judgment [16][17][18].

While SAPHO overlaps with other arthritic conditions, recent consensus frameworks emphasize viewing its distinct clinical pattern as an independent entity to tailor monitoring and treatment [16].

Common questions in this guide

Is SAPHO syndrome an infection or an autoimmune disease?
SAPHO syndrome is generally considered an autoinflammatory disorder, meaning the body's first-line immune system becomes overactive and causes inflammation without a clear outside threat. It is not usually treated as a straightforward bacterial bone infection, although infection must be considered and ruled out when appropriate.
Does finding C. acnes mean that SAPHO is caused by an infection?
Not necessarily. C. acnes, a bacterium commonly found on skin and associated with acne, can appear in bone biopsies because of contamination or may act as a trigger in genetically susceptible people. Its role remains uncertain, and long-term antibiotics rarely cure SAPHO as they would a typical bone infection.
How does SAPHO change the bones?
Inflammation can cause two opposite changes: some bone is broken down, while other areas become unusually thick and dense. These changes are called osteolysis and hyperostosis or sclerosis and may be seen on imaging.
How do doctors tell SAPHO apart from infection or bone cancer?
Doctors use the full clinical pattern, serial imaging, and specialist assessment rather than one finding alone. Microbiology testing and, when needed, biopsy help evaluate infection or bone metastasis, which is cancer that has spread to bone.
Can SAPHO occur with psoriatic arthritis or axial spondyloarthritis?
Yes. SAPHO can overlap clinically with psoriatic arthritis and axial spondyloarthritis, and some people may have more than one of these conditions or similar chest-wall findings. A specialist must interpret the pattern to decide whether the conditions overlap.
Do skin flares and SAPHO bone pain happen at the same time?
Not always. Skin symptoms and bone pain can follow different patterns, so doctors may need to track each separately when assessing disease activity.
Can genetic testing confirm SAPHO syndrome?
Not usually. Studies have proposed links with LPIN2, IL1RN, and FBLIM1, but no single gene explains all cases, and genetic findings are not routine diagnostic tests. Care teams interpret any genetic information alongside symptoms and imaging.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Do my symptoms and imaging suggest a 'pure' case of adult CNO, or do I have overlapping features of psoriatic arthritis or axial spondyloarthritis?
  2. 2.Since my bone pain and skin flares don't always happen together, how should we monitor my disease activity?
  3. 3.If we found C. acnes in my biopsy, does that change my treatment plan, or is it considered a 'trigger' rather than a direct infection?
  4. 4.Are my genetic markers, like HLA-B27, helpful in predicting how my disease will progress?
  5. 5.What specific findings on my imaging help distinguish my bone changes from more dangerous conditions like bone metastasis or a standard infection?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (18)
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    New Insights into Adult and Paediatric Chronic Non-bacterial Osteomyelitis CNO.

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    Deen Hayatu M, Hüffmeier U

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    Increased neutrophil infiltration, IL-1 production and a SAPHO syndrome-like phenotype in PSTPIP2-deficient mice.

    Liao HJ, Chyuan IT, Wu CS, et al.

    Rheumatology (Oxford, England) 2015; (54(7)):1317-26 doi:10.1093/rheumatology/keu481.

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    New discoveries in CRMO: IL-1β, the neutrophil, and the microbiome implicated in disease pathogenesis in Pstpip2-deficient mice.

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    Serum levels of proinflammatory, anti-inflammatory cytokines, and RANKL/OPG in synovitis, acne, pustulosis, hyperostosis, and osteitis (SAPHO) syndrome.

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    Modern rheumatology 2019; (29(3)):523-530 doi:10.1080/14397595.2018.1469580.

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    A case of chronic recurrent multifocal osteomyelitis associated with crohn's disease.

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    The role of Cutibacterium acnes in auto-inflammatory bone disorders.

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    Growth of Cutibacterium acnes is common on osteosynthesis material of the shoulder in patients without signs of infection.

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    Acta orthopaedica 2018; (89(5)):580-584 doi:10.1080/17453674.2018.1489095.

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    SAPHO, autophagy, IL-1, FoxO1, and Propionibacterium (Cutibacterium) acnes.

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    Clinical features and radiological findings of 67 patients with SAPHO syndrome.

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    PMID: 28880693
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    Purely osteolytic bone lesions in the early osteoarticular phase of synovitis, acne, pustulosis, hyperostosis and osteitis syndrome.

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    F-18 FDG PET/CT in 26 patients with SAPHO syndrome: a new vision of clinical and bone scintigraphy correlation.

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    Journal of orthopaedic surgery and research 2018; (13(1)):120 doi:10.1186/s13018-018-0795-0.

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    Spondyloarthritis and Tietze's syndrome: A re-evaluation.

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    Modern rheumatology 2024; (35(1)):1-6 doi:10.1093/mr/roae086.

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    Chronic Nonbacterial Osteomyelitis of the Sternocostoclavicular Region in Adults: A Single-Center Dutch Cohort Study.

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    [Diagnosis and treatment of chronic nonbacterial osteitis (CNO) and SAPHO syndrome : Implications of the current consensus recommendations of an international commission of experts for German rheumatology].

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    Pro and contra: is synovitis, acne, pustulosis, hyperostosis, and osteitis (SAPHO) a spondyloarthritis variant?

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This page is for informational purposes only and is not medical advice. It describes proposed SAPHO syndrome mechanisms and look-alike conditions; your healthcare team must interpret your symptoms, imaging, and biopsy.

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