Resumen de Upper cervical spine kinematics at the intersegmental level and the role of the alar ligaments
The upper cervical spine is often excluded from the kinematic studies of the cervical spine due to the specific morphology of its vertebrae: atlas (C1) and axis (C2). Widening the knowledge about the head-C2 biomechanics is of high clinical interest. Previous studies disagree with the maximum head-C2 mobility and how an alar ligament injury alters it. The alar ligament is a bilateral connection between the occipital bone and C2. Manual techniques in the upper cervical spine might imply a risk of neurovascular damage; to reduce this risk, the upper cervical spine stability is assessed with the mobility in pure lateral bending (side-bending stress test) and in axial rotation (rotation stress test). However, contradictions exist as to how an alar ligament injury alters the response during these manual tests. Furthermore, indirect mobilization approaches have shown good results in patients, e.g., improvement of C1-C2 mobility with C0-C1 mobilizations, but no biomechanical evidence supports these indirect approaches.
The objectives of this thesis were (1) to provide the intersegmental range of motion (ROM) of the upper cervical spine in lateral bending, axial rotation, and flexion-extension, (2) to analyze the effects of an alar ligament injury on the intersegmental mobility (with a unilateral ligament transection), (3) to assess the screening of this injury with in vitro manual tests (side-bending stress test and rotation stress test), and (4) to quantify how limiting the C0-C1 mobility influences the C1-C2 mobility. Ten head-C2 specimens were manually mobilized in lateral bending, axial rotation, and flexion-extension in three conditions: (1) with intact alar ligaments, (2) with a C0-C1 screw stabilization, and (3) with a unilateral alar ligament transection. The motion was tracked with reflective markers and an optoelectronic capture system. To quantify the kinematics, a local coordinate system was created in each bone by measuring the coordinates of the reflective markers and the coordinates of anatomical landmarks. A load cell quantified the applied load through the full ROM.
These in vitro tests have reinforced the intersegmental ROM that can be expected in the upper cervical spine in a healthy population, consistent with previous in vitro and in vivo studies. The transection of one side of the alar ligaments has provided a better insight into which alterations can be expected when a patient has an alar ligament injury, and how these alterations might be detected with two manual clinical tests (side-bending stress test and rotation stress test) considering the ROM and the degree of stiffness. A bilateral increase of the ROM was quantified after the unilateral cut of the ligament, which is consistent with previous studies, but other studies had unilateral effects only on the contralateral side to a unilateral alar ligament cut. Lastly, related to the C0-C1 stabilization, the influence of limiting C0-C1 mobility has been seen in the axial rotation and flexion of C1-C2.
This thesis has provided new insights related to two clinical tests (side-bending and axial stress tests) and has deepened the understanding of the biomechanics behind the effect clinically observed in C1-C2 mobility with C0-C1 mobilizations.
