Electronical supplement
The analysis of the instantaneous center of rotation (ICR) of C1-C2 following C2-fractures.
Methods:
For the purpose of assessing changes of the instantenous center of rotation (ICR) of the C1-2 joints as part of the current study the authors applied a technique similar to White and Panjabi [2] on each subject. In general the location of the ICR is determined by drawing perpendicular bisectors of intervals connecting like points on two positions of the moving vertebra. The point of intersection of the perpendicular bisectors marks the loction of the ICR. In the current study with the digital axial CT-slices in both maximum left and right rotation, distinct anatomical landmarks such as the anterior atlas tubercle and the upmost posterior border of the C1 transverse foramina were marked. An ellipse was drawn fitting the cortical boundaries of the odontoid in axial plane at the level of the previous markers set. The drawings made in both axial CT-slices were captured and processed digitally using a commercial desktop publishing software (Adobe photoshop v7.0, Adobe Systems Incorp. San Jose /USA). Captured axial CT-scans were superimposed on each other with 70% transparency and identical accentuated landmarks of both axial CT-scans were matched, the identical landmarks were connected and bisector lines drawn. The intersection points of both lines resembled the ICR for the individual configuration of atlantoaxial joints after the C2-fracture. A sketch of the C1-2 segment illustrates how the location of the ICRC1-2 was determined by geometry (fig. 1).
Results:
The position of the ICRC1-2 in axial plane for each patient was plotted on an axial scetch of the odontoid taking physiological dimensions into account. The ICRs were plotted against those of normals[1](fig.2). The graph illustrates that following C2-fracture healing, there was an obvious anterior shift of the ICR in several individuals compared to distribution of normalcy [1].
Currently there exists no gradingof the deviation of the ICR from normalcy, but the detailed investigation showed that assessment of the ICR and graduation of changes might a valuable new parameter to be assessed in future research on C2-fracture outcome and C1-2 motion analysis. From the anterior shifting of the ICR in several individuals, we conduct that the deformity of the C2-vertebra after the fracture has healed healed can result in a deviated ICR, thus, beside local restraints like osseus and cartilagineous joint deformities, distorted ICR mightcause deviation of motional axis and reduced ROTC1-2 .
Discussion:
In addition to articular surface deformities, distortion of the rotational axis of C1 and C2 in terms of deviated ICRC1-2 , e.g. as a sequelae of a widened C2-vertebra or tilted odontoid is thought to contribute to reduced ROTC1-2: The median atlantoaxial joint confers movement of the anterior arch of C1 pivoting on the odontoidand sliding around its ipsilateral aspect [3] with the rotational axis situated inside the odontoid and merely posterior in normals [1]. Hence, from an anatomical point of view any malunion with a tilted odontoid might deviate the ICRC1-2and affect ROTC1-2. We investigated the ICRC1-2 and observed a trend revealing an anterior shift. Currently there is no grading of the severity of any deviation of the ICRC1-2 but these observations warrant further research.
1. Roche CJ, King SJ, Dangerfield PH, Carty HM. The atlanto-axial joint: Physiological range mof rotation on MRI and CT. Clin Radiol 2002;57:103-108.
2.White AA, Panjabi MM. Kinematics of the spine. In White AA Panjabi MM ed. Clinical biomechanics of the spine. Philadelphia, U.S.A: J B Lippincott, 1978:87-125.
3. Bogduk N, Mercer S. Biomechanics of the cervical spine. I: Normal kinematics. Clin Biomech 2000;15:633-648.