Effects of alar ligament transection on upper cervical spine rotation.

Fresh human cadaveric specimens of occiput (C0) to C3 were subjected to axial torque. The resulting physiological motions were studied in an unconstrained three-dimensional manner. Effects of sequential transections of the left and right alar ligaments on the relative motions of C0-C1 and C1-C2 were studied. After transection of the left alar ligament, ranges of motion--due to 1.5 Nm torque--increased at both the C0-C1 and C1-C2 joints. Increases were small, on average 1.9 degrees to each side and at each level. Increases due to subsequent cutting of the right alar ligament were, on average, only 0.5 degrees and statistically not significant. In general, neutral zones showed greater increases, e.g., 3.9 degrees to each side at the C1-C2 joint. Comparing right and left axial rotations, after transection of the left alar ligament, showed greater percentage increases for the right, as compared to the left, axial rotation, at both C0-C1 and C1-C2 joints. Functional loss of the alar ligaments indicates a potential for rotatory instability, which, however, must be determined in conjunction with other clinical findings, such as neurological dysfunction, pain, and deformity.