The Basic And Clinical Research Of Malalignment At Subaxial Cervical Spine After Atlantoaxial Fusion

Objective1. To evaluate the influences of fixed C1-C2 and different C1-C2 angles on the range of motion(ROM) and the intradiscal pressure(IDP) of subaxial cervical spine.2. To analyze the kinematic changes of the subaxial cervical spine through the establishment of 3-dimensional finite element model of C1-C2 fixation in different angles.3. To evaluate the relationship between the C1-C2 angel and the C2-C7 sagittal alignment after C1-C2 was fixed using Harms technique or Magerl technique through clinical observation in a long-term follow-up.Methods1. We simulated 3-dimension cervical motions on 8 human specimens with C1-C2 fixed in 3 different angles(neutral position, neutral position-10°, neutral position +10°)following intact analysis in the material test system. The ROM changes of each motion segment and the IDP changes of 4 subaxial motion segments(C2-C3, C3-C4, C4-C5 and C5-C6) were monitored.2. The 3-d finite element model of the cervical spine loaded with C1 lateral mass screws and C2 pedical screws was simulated. Three cervical 3-d finite element models with different C1-C2 angles were created and the subaxial cervical biomechanical characteristics of each model was analyzed.3. 30 patients who underwent the Harms procedure or Magerl with wiring procedure by a single surgeon for atlantoaxial instability from 2010 to 2012 were retrospectively analyzed. Before, after the operation and at the final follow-up, ADI, SAC and angles at Oc-C1, C1-C2, C2-C3 and C2-C7 were measured.Results1. ROM change patterns at all the subaxial segments were similar. Fixed C1-C2 led to a significant ROM increase relative to the intact condition during flexion/extension testing and a larger C1-C2 angle(neutral position +10°) caused an additional ROM increaseduring flexion while a smaller C1-C2 angle(neutral position-10°) induced a further ROM increase during extension. Axial rotation testing revealed the most striking and similar ROM increases in the instrumented groups relative to the intact group. Lateral bending testing didn’t reveal significant ROM change between the instrumented groups and the intact group. For IDP analysis, C1-C2 fixed in a larger angle(neutral position +10°) caused significant IDP increases at the C2-C3, C3-C4 and C4-C5 levels during flexion.2. When C1-C2 angle was fixed in neutral position +10°, there was more flexion at the subaxial cervical spine and neutral position-10°induced more extension at the same segments. Excessive subaxial rotation was observed in all three models with fixed C1-C2.More lordotic C1-C2 induced greater subaxial intradiscal pressure(IDP) during flexion.Less lordotic C1-C2 induced greater tension in the anterior longitudinal ligaments and greater stress in the lateral mass facets with increased subaxial extension. Excessive subaxial rotation made greater stress in the opposite lateral mass facets.3. Significant corrections in postoperative ADI and SAC were observed in both groups. However, the average postoperative C1-C2 sagittal angles were more lordotic in the Magerl with wiring group than in the Harms group. There were statistically significant negative correlations between the C1-C2 angle and C2-C7 angle changes in both groups.Conclutions1. The results from this study indicate that fixed C1-C2 will lead to compensatory motions at the subaxial segments to make up the motion loss at C1-C2 under the hybrid testing protocol. When C1-C2 is fixed in a more lordotic position, there will be more compensatory flexion at the subaxial segments. On the contrary, C1-C2 fixed in a relatively kyphotic position will induce compensatory extension at the subaxial segments.What’s more, C1-C2 fixed in a lordotic position will also significantly increase the IDPs at the subaxial segments during flexion. Previous studies support a theory that increased ROM and IDP result in impaired nutrition of the intervertebral disc followed by progressive degeneration which lead to malalignment of subaxial cervical spine. Therefore,to maintain a physiologic sagittal alignment of subaxial cervical spine, C1-C2 should befixed in the neutral position or a relatively smaller angle instead of a more lordotic position.2. C1-C2 fixation in neutral position +10 ° will induce excessive compensatory flexion and make IDP higher during flexion. C1-C2 fixation in neutral position-10°will induce excessive compensatory extension which will make greater tension in the anterior longitudinal ligaments and greater stress in the lateral mass facets. Excessive subaxial rotation will make greater stress in the opposite lateral mass facets. In reality, all these changes contribute to accelerated degeneration of subaxial anterior longitudinal ligament,intervertebral disc and lateral mass facet which result in decreased height of intervertebral space and subsequent subaxial cervical malalignment. The optimal C1-C2 fixation angle is neutral position.3. No matter Harms technique or Magerl technique, postoperative lordotic change at C1-C2 is associated with kyphotic change at C2-C7. Harms technique is more effective to control C1-C2 angle. To lower the risk of subaxial kyphotic change, surgeons should be extremely cautious about the choice of C1-C2 fixation angle. Optimum choice should be made based on a comprehensive consideration before operation.