The mechanical properties of human alar and transverse ligaments at slow and fast extension rates

Objective. To study biomechanics and failure characteristics of the alar and transverse ligaments of the upper cervical spine at two different extension rates. Design. In vitro biomechanical study using human and alar and transverse ligament specimens. Background. Previous studies of these ligaments have been at relatively slow extension rates, even though the real life trauma occurs at significantly higher extension rates. Methods. Fresh human cadaveric alar and transverse ligaments were subjected to slow (0.1 mm/s) and then fast (920 mm/s) extension rates. Force and elongation curves were recorded during the testing, and several biomechanical parameters were computed and compared. First the specimen was stretched at the slow extension rate up to 70 N for the alar ligament and 140 N for the transverse ligament. Then the specimens were stretched until failure at the fast extension rate. Rasults. Avarage initial lengths of the alar and transverse ligaments were 11.2 and 18.0 mm, respectively. At 70 N, the average alar ligament strain decreased from 16.4 to 0.3%, the stiffness increased from 80 to 2316 N/mm and the energy absorbed decreased from 47.4 to 1.3 Nrom, as the extension rate increased from the slow to the fast. At failure, the average strain, force and energy absorbed at the fast extension rate were respectively 3.1%, 367 N and 123 Nmm. For the transverse ligament at 140 N, the strain decreased from 12.5 to 0.6%, the stiffness increased from 141 to 1472 N/mm, and the energy absorbed decreased from 96.1 to 7.6 Nmm as the extension rate went from slow to the fast. At failure, strain, force and energy absorbed for the transverse ligament at the fast extension rate were, respectively, 2.3% and 436 N and 101 Nmm respectively. Conclusion. Within the physiological limits, the strain and energy absorbed decreased to less than one tenth, while the stiffness increased to greater than ten times as the extension rate increased, for both the alar and transverse ligaments. When failed at the faster rate, the alar ligament, although weaker of the two, absorbed greater energy to failure because of its higher failure strain. Relevance Injuries to the upper cervical spine have extremely serious clinical consequences because of the anatomic location. Alar and transverse ligaments provide most of the stabilit in this region. This is the first study to determine the mechanical properties of these ligaments at an extension rate that is closer to real life trauma.