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[Instability in injury of the alar ligament. A biomechanical model].

Fresh human cadaveric specimens of occiput (C0) to C3 were subjected to 1.5 nm of flexion, extension and bilateral bending and axial torque. The resulting physiological motions were studied in an unconstrained three-dimensional manner. The effects of sequential transections of the left and right alar ligaments on the relative motion of C0-1 and C1-2 were studied. After transection of the left alar ligament, the percentage increases in neutral zones (NZ) and ranges of motion (ROM) were documented at both the C0-1 and C1-2 joints. In the sagittal plane, the most increase was at C1-2 due to the flexion moment, e.g., 47.4% in NZ and 27.6% in ROM. In lateral bending, the left alar transsection resulted in mostly right lateral bending increases and at the C0-1 joint, 37.1% in NZ and 19.6% in ROM. In axial rotation, changes in the total motion of the C0-2 joint complex were more reliable indicators. For left alar transsection, most increases were in right axial rotation, e.g., 25.6% for right rotation versus 11.2% for left rotation in the NZ parameter. Functional loss of the alar ligaments indicates a potential for instability which, however, must be determined in conjunction with other clinical findings, such as neurological dysfunction, pain and deformity.