Characterization of a first thoracic rib ligament: anatomy and possible clinical relevance

Cadaver dissection and measurement.

To describe a previously undocumented intracostal ligament that limits the potential space through which the T1 ventral ramus passes before joining the C8 ventral ramus.

Preclavicular entrapment of the T1 ventral ramus can lead to radiculopathy, neurogenic thoracic outlet syndrome, or both, the so called "double crush" phenomenon. The usual sites of entrapment include the neural foramen, the interscalene interval, an aberrant cervical rib, the first rib itself, or an apical thoracic mass.

A total of 42 shoulders from 21 embalmed cadavers (13 male, 8 female) were dissected. The presence of the ligament was noted and its anatomic characteristics were measured with digital calipers by 3 independent investigators. Means, ranges, and standard deviations were calculated.

The average ligament length was 31.0 mm (SD, 4.3). The ligament was trapezoidal in shape, and wider anteriorly. The mean anterior width was 7.1 mm (SD, 3.8), midsubstance width 3.6 mm (SD, 1.5), and posterior width 3.5 mm (SD, 1.3). The mean thickness was 0.5 mm (SD, 0.3), and the maximal opening through which the T1 nerve passed between the first rib and the ligament was 6.3 mm (SD, 1.6). The ligament was present on at least one side in 81% of individuals (67% of shoulders): 52% bilateral and 29% unilateral.

This previously undescribed ligament is a robust structure, present on at least one side in over 80% of the individuals studied. When present, the ligament creates a narrow interval between the ligament and the first rib that the T1 ventral ramus traverses before crossing the first rib superiorly and contributing to the inferior trunk of the brachial plexus. Although the actual clinical significance has not been demonstrated, this ligament may represent another entrapment site for the T1 ventral ramus.