SA/CRS is sometimes described by classification into “types”. There are four types of sacral agenesis that were determined through a research study done by T. S. Renshaw from 1959-1978. Twenty three (23) individuals with SA/CRS were studied. The classification system looks at what parts of the lower spine are missing and also at how the spine connects to the pelvis. Based on this information, 4 categories were found. Keep in mind that this study was conducted a long time ago and with a small group of individuals. There may be “types” of SA/CRS that do not fit these specific classifications.
Usually the sacrum (a group of fused bones at the lowest part of the spine) sits between two sides of the pelvis. The sacral bones and the pelvis look a little bit like a butterfly. The “wings” of the pelvis are called the ilia, on each side, and the sacrum is between the ilia, connected on each side.
Here is an artist’s rendering of the front and back view of the sacrum as it sits between pelvic bones with the spine above.
Renshaw (1978) classification: 4 categories of variations (“types”) described, as compared to the typical structure, shown here:
- Type I: total or partial unilateral sacral agenesis
- Type II: partial sacral agenesis with a partial but bilaterally symmetrical defect and a stable articulation between the ilia and a normal or hypoplastic first sacral vertebra (most common)
- Type III: variable lumbar and total sacral agenesis with the ilia articulating with the sides of the lowest vertebra present.
- Type IV: variable lumbar and a total sacral agenesis, the caudal end-plate of the lowest vertebra resting above either fused ilia or an iliac amphiarthrosis
