C7 roots vulnerable to avulsion in newborns: histological and biomechanical basis for obstetric palsy of brachial plexus

Objective To investigate anatomical and biomechanical features of the semiconic pos-terosuperior ligaments at the brachial plexus roots in newborns in order to understand the specific mechanism of obstetric palsy of brachial plexus. Methods In 6 fresh newborn cadavers, the C5-7 nerve roots were histologically analyzed using hematoxylin and Mallory trichrome staining to observe the semiconic posterosu-perior ligaments. In 4 newborn cadavers, the complex of nerve roots-ligament-transverse process was bilaterally removed and a biomechanical test was performed to examine the failure load of the complex. Results The area of collagen fiber of the ligament around C7 nerve roots was significantly smaller than that of the ligaments around nerve roots of C5 and C6 (P<0.05). The semiconic posterosuperior ligament around the C5 and C6 nerve roots was reinforced by Sharpey fibers at its insertion to the transverse process. Sharpey fibers were hardly seen at the hgament around the C7 nerve roots. The failure loads of the attachments around the C5, C6 and C7 nerve roots were respectively (6.47±1.12), (6.23±1.21) and (5.03±1.05) N. The failure loads of the attachments around the C5 and C6 nerve roots were significantly larger than that around the C7 nerve roots (P=0.019 and 0. 046 respectively). Conclusion High incidence of C7 avulsion injury in ob-stetric palsy of brachial plexus may be attributed to the relatively weak semiconic posterosuperior ligament in newborns. Key words: Brachial plexus;  Birth injuries;  Ligament;  Biomechanics;  Histology