Two dimensional view of SIJ surfaces. The articular part appears to be "L" shaped. Weight bearing occurs at the posterior syndesmosis, which is 1.57 times as large as the synovial part. Roughenings show the interosseous and other posterior ligament attachment points. The axial joint is marked by an oval.
Two dimensional view of SIJ surfaces. These landmarks are more meaningful when seen in three dimensions. We can better visualize that the sacrum is suspended by ligaments.