The Sacroiliac Joint

The sacroiliac (SI) joint is perhaps the most difficult joint in the skeleton to image adequately to make an accurate diagnosis of a disorder affecting it. This is partially due to obscuration of the joint by multiple overlying soft tissue structures and variations in the obliquity of the joint within an individual and among individuals. A modified anteroposterior (AP) Ferguson view (see Chapter 1 ) is the most useful view to eliminate the confusing soft tissue shadows and to profile that part of the joint that is affected by all disease processes.

The SI joint consists of two parts: (1) the true joint and (2) the ligamentous attachment between the two adjacent bones ( Fig. 7-1 ). The anteroinferior one third of the SI joint is a true synovial joint. The posterosuperior one third is fibrous. The transition between the true synovial and fibrous components of the joint is a region of complex anatomic variation that includes developmental defects in the articular cartilage. The iliac side is covered by fibrous cartilage that is 1 mm thick; the sacral side is covered by hyaline cartilage that varies from 3 mm to 5 mm in thickness. Owing to the thinness of the cartilage on the iliac side compared to the sacral side, all disease processes involve the iliac side first and the sacral side second. The cartilage-covered area is surrounded by synovium. The posterosuperior portion of the SI joint is nothing more than a cleft between the sacrum and the ilium. There is no cartilage covering either bone in this area. Intraosseous ligaments extend between the sacrum and ilium, joining the two bones together. The AP modified Ferguson view images the anteroinferior-most aspect of the SI joint, which is the area where disease first begins.

The obliquity of the SI joint varies from person to person. Therefore, no two individuals have identical-appearing SI joints. There are two criteria for determining normality of an SI joint. First, although the width of the SI joint varies from person to person according to the thickness of the cartilage on the sacral side, the SI joint should be of uniform width within the individual. Second, the white cortical line along the iliac and sacral side should be intact ( Fig. 7-2 ). If these criteria are not met, then the SI joint must be considered abnormal. The diagnosis of disease involving the sacroiliac joint depends upon observing the following: (1) the width of the joint space, (2) the presence and type of erosions, (3) the presence and type of sclerosis, (4) the presence and type of bone bridging, and (5) the distribution of these changes.

Width of the joint space

Apparent widening of the SI joint is observed with infection and the inflammatory spondyloarthropathies. Uniform narrowing of the SI joint is observed in rheumatoid arthritis. Irregularity of the width of the SI joint, where some parts are too narrow and other parts are too wide, is observed in the crystalline arthropathies and in osteoarthritis.

Presence and types of erosions

Erosions are present in all of the inflammatory arthropathies. Small and succinct erosions tend to be present in ankylosing spondylitis and rheumatoid arthritis, whereas large and extensive erosions tend to be present in psoriatic, reactive, and septic arthritis. A large erosion may occur in gout, but it will have a sclerotic, well-defined border as opposed to the ill-defined border seen in the inflammatory arthropathies. Erosions are not seen in calcium pyrophosphate dihydrate (CPPD) crystal deposition disease or osteoarthritis.

Presence and type of sclerosis

Reparative bone is seen behind or adjacent to erosive changes. This sclerosis tends to be minimal in ankylosing spondylitis and much more extensive in reactive, psoriatic, and septic arthritis. Reparative bone is seen in CPPD arthropathy, gout, and osteoarthritis. This sclerosis abuts the articular surface, usually at the inferior and superior aspects of the true joint. Sclerosis is seen in a wedge-shaped configuration on the iliac side of the SI joint in osteitis condensans ilii. The widest part of the wedge in osteitis condensans ilii is along the inferior aspect of the ilium.

Presence and type of bone bridging

There are two types of bone bridging: (1) a true bone ankylosis of the joint itself and (2) anterior osteophyte formation bridging across the ilium to the sacrum anterior to the joint. Bone ankylosis is seen in the inflammatory arthropathies and in septic arthritis. Anterior osteophyte formation is seen in the crystalline arthropathies and osteoarthritis.

Distribution of changes

Disease entities are unilateral, bilateral and symmetrical, or bilateral and asymmetrical. Septic arthritis is almost always unilateral. Ankylosing spondylitis, spondylitis associated with bowel disease, CPPD arthropathy, and osteitis condensans ilii tend to be bilateral and symmetrical. Psoriatic arthritis, reactive arthritis, gout, and osteoarthritis tend to be bilateral and asymmetrical.

Radiographic changes in sacroiliac joint disorders

Generalized Inflammatory Disease

The most significant disorders of the SI joint are the inflammatory ones. Therefore, knowledge of radiographic changes occurring in these disorders is important. All of the inflammatory disease processes affect the SI joint in a specific sequence, varying only in degree of change within the sequence. Erosive changes begin on the iliac side of the SI joint. The earliest change observed is poor definition or loss of the white cortical line on the iliac side ( Fig. 7-3 ). As the erosive disease progresses, the SI joint becomes apparently widened ( Fig. 7-4 ). The body then responds by laying down reparative bone behind the erosive changes ( Fig. 7-5 ). The reparative process then becomes the dominant part of the radiographic picture. Following this, bone ankylosis occurs across the SI joint ( Fig. 7-6 ). Once the SI joint is ankylosed, the surrounding bone becomes osteoporotic secondary to loss of normal mechanical stress across the SI joint ( Fig. 7-7 ). Although each inflammatory disease process goes through this sequence of events, disease entities can be distinguished through their extent and distribution of involvement. Specific disorders of the SI joints and their distinguishing radiographic characteristics are now illustrated and discussed.