Transitional lumbosacral segment with unilateral transverse process anomaly (Castellvi type 2A) resulting in extraforaminal impingement of the spinal nerve: a pathoanatomical study of four specimens and report of two clinical cases.

The spinal nerve can be pinched between the transverse process of the fifth lumbar vertebra and the sacral ala. The patients are divided into two types: elderly persons with degenerative scoliosis and somewhat younger adults with isthmic spondylolisthesis. For the first time, we describe extraforaminal impingement of the spinal nerve in transitional lumbosacral segment with unilateral transverse process anomaly. Selective nerve root blocks were performed in two clinical cases. One patient underwent nerve root decompression via a posterior approach. One year after operation, this patient reported no radicular or lumbar pain. The pathoanatomical study demonstrated pseudoarthrosis between the transverse process and the ala of the sacrum and showed dysplastic facet joints at the level below the transitional vertebra in all specimens. Furthermore, we present the oldest illustration of this pathological condition, published in a book by Carl Wenzel in 1824. Extraforaminal entrapment of the spinal nerve in transitional lumbosacral segment with unilateral transverse process anomaly can cause radiculopathy, and osteophytes are the cause of the entrapment. Dysplastic facet joints on the level below the transitional vertebra could be one reason for "micromotion" resulting in pseudoarthrosis with osteophytes. Sciatica relief was obtained by means of selective nerve root blocks or posterior decompression via a dorsomedial approach.