Imaging description

Transitional vertebrae are fairly frequently identified in the lumbosacral region, where they are referred to as lumbosacral transitional vertebrae (LSTV). Lumbosacral transitional vertebrae have a reported prevalence of 4–30%, and can reflect sacralization or lumbarization [1]. Sacralization is more common [2].

In sacralization, there is assimilation (fusion) of the lowest lumbar segment into the sacrum. In lumbarization, the S1 segment is not fused with the remainder of the sacrum, and appears as a separate “lumbar type” vertebral body. Partial sacralization or lumbarization often occurs in which there is partial fusion or separation of the transitional vertebra. (Figure 27.1 and 27.2).

While transitional vertebrae may be identified on radiographs, CT with multiplanar reformations (MPRs) provides more optimal evaluation of the vertebrae, pseudarthroses, and fusion.

Castellvi [3] described a classification system for LSTV, ranging from type I to type IV anomalies, with A and B subtypes for unilateral or bilateral involvement respectively.

Importance

The lucency between the transitional vertebra and the sacrum may be mistaken for infection or fracture.

Lumbosacral transitional vertebrae may result in numbering errors, when the radiologist counts up from the last lumbar appearing vertebra to identify and enumerate the vertebra on a lumbar spine study. A sacralized L5 may cause them to mistakenly number T12–L4 as L1–L5, or a lumbarized S1 may cause them to mistakenly number L2–S1 as L1–L5. Errors in enumeration of the vertebrae can result in surgery or procedures being performed at the wrong levels. Spine imaging of all other levels should be reviewed to ensure a correct enumeration. If this is not possible (for example if the thoracic spine was not imaged), a clear description of your method for numbering the spine should be provided, to avoid ambiguities leading to errors.