Redefining Congenital Atlantoaxial Dislocation: Objective Assessment in Each Plane Before and After Operation

The C1-C2 joint has multiple degrees of freedom of movement and C1-C2 dislocation (AAD) is often multiplanar. The existing methodology to assess the dislocation is limited to few planes. The object of this study is to redefine and objectively assess congenital AAD in each possible plane, before and after the operation. This study consisted of 95 patients of irreducible congenital AAD operated on with the posterior approach alone. Preoperative and postoperative computed tomography imaging was studied in the axial, coronal, and sagittal planes. The relationship of C1-C2 along with the C1-C2 joint inclination was studied in each plane. The extent and type of dislocation was objectively assessed in each plane (newer indices) and compared with follow-up imaging for correction. The preoperative and postoperative Japenese orthopaedics association scores were compared. The commonest variety (61 patients) was a combination of anteroposterior (AP) and vertical C1-C2 dislocation. Five patients had predominant APnteroposterior, 6 vertical, 4 axial rotational, 9 lateral angular tilt, and 3 had lateral transalational. Seven patients had a combination of dislocation in AP, vertical, and rotational planes. AP dislocation was seen with sagittal inclination of C1-C2 joints and vertical dislocation with coronal inclination. Asymmetry in the joint's sagittal inclination added to a rotational component, whereas asymmetry in the coronal angulation caused lateral angular tilt. Pure rotational or lateral translation dislocation had near-normal C1-C2 orientation. Preoperative Japenese orthopaedics association score was worst in the lateral tilt and the lateral translation. Correction in all planes was achieved in all patients. The objective assessment of C1-C2 dislocation and joints in each plane was to determine its management and help in achieving multiplanar correction.