Undeveloped semiconic posterosuperior ligament and susceptibility to avulsion of the C-7 spinal nerve in Erb palsy.

OBJECT

In Erb palsy, the C-7 spinal nerve has been found to be more subject to avulsion than the C-5 and C-6 spinal nerves. This study investigated the morphological and biomechanical characteristics of the semiconic posterosuperior ligaments (SPLs) at the C-5, C-6, and C-7 spinal nerves in neonates.

METHODS

Twenty-four brachial plexuses from 12 fresh neonate cadavers were used in this study. In 12 brachial plexuses from 6 cadavers, the following studies were performed with respect to the SPLs at the C-5, C-6, and C-7 spinal nerves: gross observation of morphological and histological characteristics; measurement of length, thickness, and width; and a semiquantitative analysis of collagen. In the other 6 cadavers, biomechanical tension testing was performed bilaterally on the C5-7 SPLs to assess the tensile strength of the ligaments.

RESULTS

The C5-7 spinal nerves are fixed to the transverse process through the SPL, a structure not observed at the C-8 and T-1 spinal nerves. Except for the width of the SPL insertion on the spinal nerve, which was found to increase gradually from C-5 to C-7, there was no statistically significant difference in the dimensions of the C-5, C-6, and C-7 SPLs. The sectional area percentage of collagen was 51% ± 10% in SPLs for C-5, 51% ± 11% for C-6, and 41% ± 10% for C-7; and this percentage was significantly lower in SPLs for C-7 than for C-5 or C-6 (1-way ANOVA, F = 4.3, p = 0.02; Tukey honestly significant difference test, p = 0.04 and 0.04, respectively). Sharpey fibers were observed at the transverse process origin of the SPL at C-5 and C-6 but not at C-7. Biomechanical tension testing showed that the mean failure load was 6.6 ± 0.9 N for the C-5 SPL, 6.4 ± 1.0 N for the C-6 SPL, and 5.4 ± 0.9 N for the C-7 SPL, and the failure load was significantly lower in SPLs at C-7 than in those at C-5 or C-6 (1-way ANOVA, F = 5.1, p = 0.01; Tukey honestly significant difference, p = 0.01 and 0.048, respectively). Nine of 12 C-7 SPLs failed at their origin from the transverse process, while only 4 of 12 C-5 SPLs and 3 of 12 C-6 SPLs failed at the origin site.

CONCLUSIONS

These findings suggest that the lower density of collagen and absence of Sharpey fibers decrease the biomechanical properties of the C-7 SPL, and this may account for the higher frequency of avulsion of the C-7 spinal nerve (in comparison with the C-5 or C-6 nerve) in Erb palsy.