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Use of the Thoracolumbar Facet Transition as a Method of Identifying the T12 Segment

Abstract

Scott E Forseen, Bruce C, Gilbert, Sumir Patel, Juan Ramirez and Neil M Borden

Purpose: Evaluate the reliability of the change from the flat, posterolaterally oriented facets of the thoracic segments to the curved, posteromedially oriented facets of the lumbar spine, the “facet transition”, as a marker for the T12 segment and determine if variations in rib number are associated with lumbosacral transitional segments.

Materials and methods: 244 patients underwent whole spine CT examinations and the positions of the thoracolumbar facet transition, type of thoracolumbar facet transition (gradual or abrupt), position of the lowest thoracic ribs, and presence or absence of lumbosacral transitional anatomy were recorded. A Fisher Exact Test was used to determine if there was an association between a variant number of ribs and transitional anatomy at the lumbosacral junction.

Results: The thoracolumbar facet transition was located at the eighteenth segment in 50/244 (20%), nineteenth segment in 184/244 (75%), and twentieth segment in 10/244 (4%) of cases. The thoracolumbar facet transition was abrupt in 227/244 (93%). The lowest set of ribs was observed at the nineteenth segment in 225/244 (92%), twentieth segment in 11/244 (5%), and eighteenth segment in 8/244 (3%). The lowest fully-formed intervertebral disc was located between the twenty-third and twenty-fourth segments in 9/244 (4%), twenty-fourth and twenty-fifth segments in 216/244 (88%), and twenty-fifth and twenty-sixth segments in 19/244 (8%). Coexistent lumbosacral transitional anatomy was seen in 5/7 (71%) with eleven, 8/11 (73%) with thirteen, and 24/234 (10%) with twelve ribs. There was an association between variant numbers of ribs and coexistent lumbosacral transitional anatomy (p<0.5).

Conclusion: The thoracolumbar facet transition is not a reliable method of identifying the T12 segment. There are no known landmarks that reliably identify the lumbar segments. Accurate numbering of the lumbar spine requires counting caudally from C2.