So, is "minimal sclerosis" always Grade II sacroiliitis, or could it be something completely different?
No. "Minimal sclerosis" does not necessarily mean Grade II sacroiliitis. Sclerosis is a fairly vague term that simply means "a thickening or hardening of a body part" in this case, bone or tissue. As a general term, it is also used to describe osteoarthritic changes.
Sclerosis is a part of the clinical definition of Grade II Sacroiliitis, but it is not the only component to the diagnosis. Evidence of erosions or loss of definition of the joint margins (sometimes referred to as fuzzy joint space margins) is also required. One of the best definitions I have found of sacroiliitis classification can be found here: http://www.sma.org.sg/smj/4302/4302me1.pdf
"Sacroiliitis is usually graded according to the New York criteria.
* Grade 0 indicates normal SI joints within thin sharp cortical margins and normal cartilage space.
* Grade I refers to non-specific “suspicious” findings.
* Grade II changes are due to minimal sacroiliitis and consist of loss of definition of the SI joint margins. There may be minimal sclerosis and erosions. There may or may not be joint space narrowing.
* Grade III or moderate sacroiliitis manifests as definite sclerosis on both sides of the joint, erosions, and loss of joint space.
* Grade IV changes consist of complete bony ankylosis of the SI joints.
Changes are seen initially in the lower and middle thirds of the SI joint, with periarticular osteoporosis. Erosions and subchondral sclerosis progress to bony ankylosis. There is an eventual return to normal bone density."
This document lists other causes of sacroiliitis as well:
"Although the radiographical appearances of sacroiliitis may appear similar, differences in symmetry and distribution, as well as other clinical manifestations, provide clues to the correct diagnosis(2). The causes of sacroiliitis and mimicking conditions are summarised in the Table" below. http://www.sma.org.sg/smj/4302/4302me1.pdf
Table I. Patterns of disorders causing or mimicking sacroiliitis.
A. Bilateral and symmetrical
• Ankylosing spondylitis • Enteropathic arthropathies e.g. ulcerative colitis, Crohn’s disease, Whipple’s disease • Psoriatic arthropathy • Osteitis condensans ilii* • Hyperparathyroidism*
B. Bilateral and asymmetrical
• Psoriatic arthropathy • Reiter’s disease • Rheumatoid arthritis • Depositional arthropathies e.g. gout, CPPD, ochronosis • Osteoarthritis
Key: * indicates conditions which may mimic sacroiliitis.
Sacroiliitis and Sacroiliac Joint Dysfunction are not interchangeable terms.
Sacroiliac Joint Dysfunction is a condition that causes pain in the SI joints from a variety of causes. SI Joint Dysfunction includes many different sacroiliac joint problems, including SI joint dysfunction, SI joint syndrome, SI joint strain, and SI joint inflammation.
Sacroiliitis refers to inflammation, and the damage caused by inflammation, of the sacroiliac joint. Sacroiliitis could be called a form of SI Joint Dysfunction.
***edited to add***
Can your doctor review the films directly or ask the Radiologist to re-review them??