| Objective1.To reveal the relation between alignments of upper and subaxial cervical spine by a radiological study2. To establish a three—dimensional finite element model of the cervical spine and evaluate its feasibility in clinical practice3. To study the biomechanical changes of subaxial cervical spine after atlantoaxial fusion by three—dimensional finite element analysis4. To evaluate the subaxial sagittal alignment after atlantoaxial fusion in patients without rheumatoid arthritis and to find contributing factorsMethod1. Six hundred asymptomatic volunteers were recruited. Including criteria:①no history of cervical trauma or disease,②no nuchal pain or symptoms of arms and shoulders,③the inferior aspect of C7 can be revealed on plain lateral cervical radiographs in neutral position. All the volunteers underwent plain lateral cervical radiographs with neck in neutral position. They were instructed to look straight ahead in a relaxed standing position while filmed. The film–tube distance was set at 1.5 m. The following angles were measured on the radiographs: occipitocervical angle (Oc-C2), atlantoaxial angle (C1-C2) and subaxial angle (C2-C7). Lordosis was recorded as a positive value while kyphosis as a negative value. The measurements were repeated three times and the averaged values were recorded. Relations among these three angles and relations between angles and age were analyzed.2. A finite element model of the whole cervical spine with the skull base and their soft tissues was established based on the data of the CT scan in healthy volunteers. The parameters of the structure were cited from the literatures. The model was adjusted and verified in flexion, extension, rotation and lateral bending.3. Insert the C1-C2 fixation instrument at different C1-C2 angle and analyze the stresses of the discs and joint capsules and the range of motion (ROM) of different levels. The instrument was consisted of C1-C2 transarticular screw and C1 laminar claw.4. The C1-C2 angle and C2-C7 angle of the patients were measured on lateral radiographs. C1-C2 angle was formed by inferior aspect of atlas and axis; and C2-C7 angle by inferior aspect of axis and the superior aspect of C7. Lordosis was recorded as a positive value while kyphosis as a negative value. The measurements were repeated three times by the same author and the averaged values were recorded. The age, gender and the pathology of the patients were recorded and the contributing factors on C2-C7 angle was analyzed.Results1. The mean Oc-C2 angle was 16.3±7.02°in females, significantly larger than 14.9±6.50°in males. The mean C1-C2 angles were 28.2±4.04°in females and 26.4±4.60°in males, and C2-C7 angles were 12.7±6.61°and 16.3±7.32°correspondingly. The mean C1-C2 angle in females was significantly larger than that in males while C2-C7 angle smaller than that in males. The C2-C7 angle correlated significantly not only with C1-C2 angle but also with Oc-C2 angle. And correlation between C1-C2 angle and C2-C7 angle was stronger than that between Oc-C2 angle and C2-C7 angle. There were also significant positive correlations between C1-C2 and Oc-C2 angles. Oc-C2 angle, C1-C2 angle, and C2-C7 angle correlated significantly with age in both sexes.2. The final intact cervical spinal model consisted of 664026 elements and 228557 nodes. The model had the same similarity and profile as the clinical case and can be used in biomechanical analysis.3. In flexion and extension operating condition, the ROM of the model with instruments was less than the model without instrument(P<0.01). But there were no differences in ROM among models with different C1-C2 fixation angle(P>0.05). The internal stresses of discs at C3-C7 was larger in model with instruments than in model without instruments(P<0.05), but the stresses at C2-C3 was smaller in model with instruments. The internal stresses of discs at C3-C7 was larger in model with larger C1-C2 fixation angle than in normal C1-C2 fixation angle(P<0.05). The stresses of JC in extension became larger after C1-C2 fixation. The stresses of JC in the model with larger and smaller C1-C2 fixation angle were all larger than that with normal C1-C2 fixation angle, and significant differences was observed between the model with larger C1-C2 fixation angle and normal angle(P<0.05).4. All the patients obtained bone fusion. Decreased C2-C7 angle was observed in 17 patients. Cervical kyphosis was observed in 2 patients before surgery, one of which deteriorated during follow-up. 4 patients with lordotic curve developed cervical kyphosis. 2 patients with postoperative kyphosis developed nuchal pain and one of them complicated with numbness of the hands. All of them did not need surgical intervention. The C1-C2 angle before and after operation were 18.2±13.3°(-18.7°～42.1°)and 24.0±7.3°(7.8°～37.1°),respectively, and the C2-C7 angle were 13.7±9.6°(-3.4°～28.3°)and 9.5±10.9°(-11.9°～31.8°)correspondingly. There was a statistically significant difference in C1–C2 angle before and after surgery, so did in C2-C7 angle. There was a significant negative correlation between preoperative C1–C2 angle and C2–C7 angle. But the negative correlation between postoperative C1–C2 angle and C2–C7 angle was not significant. There are negative linear correlation between preoperative and postoperative C1–C2 angle changes and C2–C7 angle changes. This result indicates that the patients who have lordotic fixation at C1–C2 decrease lordosis at C2–C7 after surgery. As the degree of C1–C2 lordosis increases, the chances of postoperative kyphosis at the subaxial cervical spine become higher. there was no correlation between postoperative C2–C7 angle and nonradiographic parameters, including age, sex, pathology.ConclusionC1-C2 fixation angle was the key to regulate postoperative subaxial alignment in atlantoaxial arthrodesis. There were negative correlations between C1-C2 angle and C2-C7 angle. If C1-C2 was fixed at excessive lordosis, causing an increase of C1 inclination, the lordosis of subaxial cervical spine would be decreased to offset the C1 inclination. The compensated decrease in lordosis of subaxial cervical spine was correlated with the...
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