Ligamentous Laxity Across C0-C1-C2 Complex: Axial Torque-Rotation Characteristics Until Failure

The axial torque until failure of the ligamentous occipitoatlanto-axial complex (C0-C1-C2) subjected to axial angular rotation (8) was characterized using a biaxial MTS system. A special fixture and gearbox that permitted right axial rotation of the specimen until failure without imposing any additional constraints were designed to obtain the data. The average values for the axial rotation and torque at the point of maximum resistance were, respectively, 68.1° and 13.6 N-m. The specimens offered minimal resistance (∼ 0.5 N-m), up to an average axial rotation of 21° across the complex. The torque-angular rotation (T-0) curve can be divided into four regionsregions of least and steadily increasing resistances, a transition zone that connects these two regions, and the increasing resistance region to the point of maximum resistance. The regions of least and steadily increasing resistances may be represented by two straight lines with average slopes of 0.028 and 0.383 N-m/degree, respectively. Post-test dissection of the specimens disclosed the following. The point of maximum resistance corresponded roughly to the value of axial rotation at which complete bilateral rotary dislocation of the C1-C2 facets occurred. The types of injuries observed were related to the magnitude of axial rotation imposed on a specimen during testing. Soft-tissue injuries alone (like stretch/rupture of the capsular ligaments, subluxation of the C1-C2 facets, etc.) were confined to specimens rotated up to or close to the point of maximum resistance. The specimens that were subjected to rotations up to the point of maximum resistance of the curve spontaneously reduced completely on removal from the testing apparatus. Spontaneous reduction was not possible for specimens tested slightly beyond their points of maximum resistance. Specimens that were rotated well beyond the point of maximum resistance showed, in addition to the soft tissue damage and subluxation, avulsion fractures of the bone at the points of attachment of the alar ligament to either the occipital condyles or odontoid process (dens), or fractures of the odontoid process inferior to the level of alar ligament attachment. The alar ligaments did not rupture in any of the specimens.