Accessory Sacroiliac Articulations with Arthritic Changes

An interesting condition of some clinical importance has been demonstrated on the posterior surface of the sacrum. Accessory articulations between the ilium and sacrum may occur, and in certain cases show well advanced arthritic changes. One or two such joints may appear on either or both sides. Anatomy of the Sacroiliac Joint The sacrum is normally formed by the fused elements of five primary vertebral segments. The alae, in front, originate from the costal elements. Posteriorly, the transverse processes join to form the crests which are lateral to the posterior sacral foramina. The ear-shaped, cartilage-covered, articular surfaces arise from the anterior portions of the first two segments. These correspond to the alae of the sacrum. There is a sacral tuberosity above and behind the articular surface on either side. That portion of the innominate bone posterior to the iliac fossa faces toward the sacrum. About one-third of this medially directed surface—the so-called auricular surface, occupying the anterior inferior portion—articulates with the sacrum. The posterior superior two-thirds, the tuberosity of the ilium, does not normally articulate with the sacrum. The bone here is roughened and connected to the sacral tuberosity by the posterior sacroiliac ligaments. The sacroiliac joint is stated to be a diarthrosis capable of some minimal gliding and rotary movements. It may or may not possess a joint cavity with a synovial lining. Its nerve supply originates from the superior gluteal, the sacral plexus, and the posterior divisions of the first and second sacral nerves. The Accessory Sacroiliac Joints Various anatomists have described accessory sacroiliac joints. Most commonly these occur between the medial surface of the posterior superior iliac spine and the rudimentary transverse tuberosity just lateral to the second posterior sacral foramen. Normally the posterior superior iliac spine is 3 to 6 mm. from the posterior surface of the sacrum. When an accessory articulation is formed, the plane of the joint forms an angle with the plane of the normal sacroiliac articulation. By such an imperfect mechanical structure two bones articulate at two separate surfaces, the planes of which are angulated. Less frequently an accessory articulation may develop between the tuberosity of the ilium and the sacral tuberosity, opposite the first posterior sacral foramen. At these points articular facets form. These are covered by hyalin or fibrocartilage and appear as true diarthroidial joints 1 or 2 cm. in diameter. A small foramen between this joint and the true sacroiliac articulation furnishes passage for nutrient blood vessels. Trotter quotes the incidence of accessory sacroiliac joints as reported by different authors. The figure varied from 36 per cent in her own series of 958 pelves, down to that of Derry, 10.4 per cent in 192 skeletons.