T1 is the first thoracic vertebra, transitional between the cervical and thoracic spine. Its body is larger than C7 and has complete (not hemi-) costal facets for the first rib articulation. The T1 transverse process is large and projects posterolaterally to articulate with the first rib. T1 contributes to the brachial plexus via its ventral ramus (C8-T1 lower trunk).
T1 compression or injury (Klumpke pattern) affects the lower brachial plexus, producing intrinsic hand weakness and Horner syndrome (ptosis, miosis, anhidrosis) from sympathetic chain involvement. T1 is the reference level for thoracic spinal surgery — the prominent C7 spinous process (vertebra prominens) identifies the C7-T1 junction. T1 corpectomy for pathological fracture or tumour at this level requires posterior stabilisation due to the transition zone mechanical demands.
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