| Background and objectivesAtlantoaxial joint is the most agility and weakest as well as the most dangerous movement segment as it has complex structures and special functions in occipito-cervical migration department. Because of spinal cord, vertebral artery and cervical nerve and other important structures in vicinity, diseases in that region often result in serious clinical consequence. The diagnosis and treatment of diseases in atlanto-axial joint are relatively difficult for they locate deep position and have unique biomecha-nical properties and have become one of the unsettled problem in spinal area. At present, there are many researches for atlantoaxial joint from anatomy to biomech anics of the atlantoaxial joint such as bony structures, ligaments and vertebral artery surrounding adjacent structures. Atlantoaxial subluxation is the common disease in atlantoaxial joint. Its definition and diagnosis have been controversy since first repor-ted by the Corner in 1907 and named'atlantoaxial rotatory subluxation'in 1968 by Wortzman. And manipunation as a treatment for this disease is widely used, the indication is that unequal spaces between odontoid process and lateral mass of two sides in open-mouth X ray examination. In the process of cervical rotatory manipunation, literature and monograph often describe the diagnosis of atlantoaxial subluxation by the findings of palpation that the transverse process of one side of atlas being prominence and the other side being concavity. This simple means of diagnosis must cause many problems because they do not take into account the fact that the anatomic variation and various pathologies of bone, tissues in atlantoaxial joint could cause the skew of odontoid and blindly use a variety of manipunation techniques, including the dangerous cervical rotatory manipunation. Researches have revealed that the unequal space between odontoid process and lateral mass of two sides exists in the normal people, the reason for this is the anatomic variation of odontoid. And the asymmetry of transverse processes of two sides of atlas also exists. Misdiagnosis would happen if not considering these anatomic variations.Although many researches have done on the function and mechanism of cervical rotatory manipunation, there has still a controversy on the technique that the diagnosis of subluxation by palpating the skew of cervical spinal process or transverse process. The reports on iatrogenic damage caused by clinical use of cervical spine rotation manipulation are not rare as the result of misdiagnosis and misuse of that manipulation. Previous studies of atlantoaxial anatomy tend to focus on the anatomic measurement of bones rather than the variation of atlas and axis which have more clinical significance. Because atlas and axis play an important role in diagnosis and treatment of cervical rotatory manipunation, it is vital to carry out an anatomic variation study for such bones. This study aimed at providing anatomical bases for the diagnosis of atlantoaxial diseases, by measuring dry speci-mens of atlas and axis the various tapes of anatomic variation and incidence rate of them were identified. Another study was carried out to understand atlantoaxial rotatory motion regularity by reconstructing 3D model of atlantoaxial joint in Mimics software using the data from cervical spine CT scan under different rotation position of a normal person.Materials and methods1.Anatomical variation of the atlas:The 100 complete drying specimens of atlas were obtained from Department of Anatomy, Southern Medical University with the birth region, gender and age unknown. Measured by vernier caliper (error is 0.02mm) and protractor (accurate degree is 1°).①Measurement of transverse process asymmetry of bilateral atlas:first fixed the atlas on the measuring board with vertical and horizontal coordinate system using soft sticky mud, then drew a straight line through the anterior and posterior tubercles and made it coincidence with ordinate line on the measuring board, last drew a line through the two tips of transverse processes. If the line coincidence with the abscissa line on the measuring board, there was no transve-rse process asymmetry; If not coincidence with the abscissa line on the measuring board, then the atlas exsisted transverse process asymmetry.②Measurement of the asymmetry of posterior arch of atlas:Measure the distance between posterior tubercle and the posterior lateral mass of two sides. If the distances were equal, there no asymmetry; If the distance were unequal, there existed asymmetry.③Measurement of the asymmetry of anterior and posterior tubercles and vertebrae foramen. Fixed the atlas as the way mentioned above, the decentration development of anterior and posterior tubercles was observed. If the decentration development exist, the asymm-etry of anterior and posterior tubercles exist. Drew a straight line through the anterior and posterior tubercles and made it coincidence with ordinate line on the measuring board, vertical dimension between this line and intra-posterior lateral mass of two sides was measured. According to the equal or unequal dimension, the asymmetry of vertebrae foramen was identified.④Measurement of the asymmetry of groove for vertebral artery: According to the morphology of groove for vertebral artery, the vertebral artery groove was categorized as supersulcus, major grove, semi-ring and full-ring. The asymmetry of the vertebral artery groove was observed too.⑤Measurement of superior articular surface of atlas.2.Anatomical variation of the axis:The 100 complete drying specimens of atlas were obtained from Department of Anatomy, Southern Medical University with the birth region, gender and age unknown. Measured by vernier caliper (error is 0.02mm) and protractor(accurate degree is 1°).①Measurement of the skew dens:First drew a vertical line through the axial ray of the body of axis, then drew a straight line through the axial ray of dens, if those lines were perpendicularity and coincidence with horizontal line, the skew dens not existed; If the axial ray of dens was not perpendicularity with horizontal line and existed a included angle with the perpen-dicular of body of the axis, there existed skew dens, according to the direction of included angle, the direction of skew dens was identified.②Measurement of the skew spinous process:The relation between axial ray of spinous process and bisection line of central canal was measured, if those lines were coincidence, there was no skew spinous process; If not, there existed skew spinous process.③Observation of the bifid spinous process, measurement of length of the bifid spinous process of two sides and ratio of their generation were performed.④Measurement of the distance between the tip of transverse and the inferior articular facet.3.Kinematics of the atlantoaxial joint:①Object of CT scan:A healthy male volunte-ers,38 years old, height 166cm, weight 65.3kg, requests of volunteers:A, developed normally, excluding neck deformity, especially the atlantoaxial vertebral deformity and the severe skew dens and so on; B, without supreme cervical spine surgery and symptom of pain and acroanesthesia in the upper limb or hand, no limitation of motion in cervical part and diseases related to head and neck; C, without a history of severe infection in upper respiratory tract and cervical part; D, excluding associated systemic diseases:such as rheumatic arthritis, rheumatoid arthritis, ankylosing spond-ylitis;E, physical examination:No tenderness in the cervical spinous process and the region of greater occipital nerve;F, normal in radiograph including the open-mouth X ray, lateral view, hyper-extention and hyper-flexation position.②Data acquisition: The volunteer had the axial CT scan by 16-row multi-slice spiral CT (CT Room of Wu Jing Zong Dui Hospital of Guangdong Provence (Biograph 16 HR type, Siemens, Germany), scan range from the upper edge of occipital tuberosity to the lower edge of C2 vertebra. There were five position of cervical spine including neutral position, rotation to 10°, rotation to 30°, rotation to 60°and maximum rotation position. These CT scan were performed per week. Neutral position CT scan:the subject was prostra-tion with sagittal plane of head in perpendicularity with bed and the line of lateral angle of two eyes paralle with bed,and the line of CT scan in perpendicularity with dens. Rotation position CT scan:The included angle-between sagittal plane of head and median sagittal plane of bed was measured by a large size conimeter, then according to different included angle in the process of cervical rotation, the CT scans of rotatory position were conducted.③Reconstruction of three-dimensional geome-tric models of atlantoaxial joint under different rotatory angle:CT scan image data were imported into Mimics software, using threshold tools,the initial separation of soft tissue and bone structure was conducted. Then using the editing tool edited images of each layer and using region growing tool extended the area edited, erased the straggling points which developed from the process of editing images. Thus the segmentation of images of atlas and axis as well as partial occipital bone was conducted. At last, using 3D calculate and Smoothing tools constructed the Cl, C23D geometric models.④Measurement of image and model:A, atlas rotatory angle (C1°): Included angle between sagittal axis of atlas and midline was measured by measure tools in Mimics software; B,axis rotatory angle(C2°):Included angle between sagittal axis of axis and midline was measured in axial view; C, rotatory angle of occipital bone(Oc°):Included angle between sagittal axis of occipital bone and midline was measured; D, relative rotatory angle of atlas and axis (C1C2°):On the 3D model of atlantoaxial joint, drew a line named a through the anterior and posterior tubercles, another line named b through dens and spinous process, the included angle between a line and b line constructed the relative rotatory angle of atlas and axis; E, relative rotatory angle of occipital bone and atlas(OcCl1°):atlas rotatory angle minus rotatory angle of occipital bone. F, measurement of ADI:The vertical dimension between anterior edge of dens and posterior edge of anterior arch of atlas on sagittal view. G, measurement of LADS:The vertical dimension between anterior lumbar part of dens and inner margin of two lateral masses on s coronal view.Result1.For all of the 100 atlases, there were 15 samples(15%) of the skew transverse process in abscissa axis; 10 samples(10%) of the asymmetry of the left and right posterior arch of atlas;5 samples(5%) of the asymmetry of the anterior and posterior tubercle in longitudinal axis; llsamples(11%) of the asymmetry of the vertebral foramen,9 samples(9%) of the asymmetry of groove for vertebral artery; According to the shape of superior articular surface of atlas, they could be divided into four types including nephric shape superior articular facet, oval superior articular facet, dumb bell shape superior articular facet and irregularity shape superior articular facet. Among four types, there are 32% nephric shape superior articular facet,27% oval superior articular facet,23% irregularity shape superior articular facet,18% dumb bell shape superior articular facet.2.For all of the 100 axises, there were 14 samples(14%) of the skew dens,3 samples(3%) of the skew spinous process,96 samples(96%) of the bifid spinous process, in which 56 samples(56%) had the same length in both bifid sides,21 samples(21%) were longer in left bifid spinous process than the right and 19 samples(19%) were longer in right than the left. The distance between the tip of transverse process and the inferior articular facet in the left side was 17.61±2.48mm (11.2～23.7), the right side was 17.90±2.52mm (12.7～23.60). 3.Reconstruction of three-dimensional geometric models of atlantoaxial joint under different rotatory angle was completed. The rotatory angles of occipital bone(Oc°) under different positions were:1.2°,9.2°,34.9°,61.2°,90.5°;the atlas rotatory angles(C1°)were:2.2°,10.3°,36.7°,63.7°,92.7°;the axis rotatory angles(C2°) were:2.2°,2.1°,2.7°,18.7°,47.0°;the relative rotatory angles of atlas and axis (C1C2°)were:0°,8.2°,34.0°,45.0°,45.7°;the relative rotatory angles of occipital bone and atlas(OcC1°) were:1.0°,0.9°,1.8°,2.5°,2.2°;the ADI were:2.09mm, 1.97mm,2.97mm,2.28mm,1.60mm; the LADS(the left were:3.18mm,3.81mm, 4.78mm,4.20mm,3.08mm;the right were:2.58mm,2.62mm,2.06mm,2.50mm, 4.03mm)Conclusion1.The skew transverse process in abscissa axis of atlas is more common among the anatomic variation of atlas, so it should not be seen abnormal as well as signs of atlantoaxial subluxation. The asymmetries of anterior and posterior and vertebrae may cause stress changes in muscles and fascial of atlantoaxial joint, thus further influence the stability of atlantoaxial joint or cause clinical symptoms. For the asymmetry of groove for vertebral artery, there is no positive evidence that it can influence the blood of the vertebral artery and need further studies.2.The skew dens is more common among the anatomic variations of axis, so it should be taken into account in the X-ray examination. And atlantoaxial subluxation should not be diagnosed at the sign of unequal space between the dens and lateral mass. Because of existence of the skew spinous process, it should be taken into account while applying the C2 rotatory manipunation.3.Rotatory motion of Atlantoaxial joint initiated from atlas, in the early stage (0°-30°),the axis remains immobile; In the middle stage(30°-60°),the axis begins move, but at a slower speed compared with atlas; In the last stage(60°to maximum rotation),rotation of atlas and axis have reached to maximum, the rotation of the neck occurs exclusively at segments below the axis.In the process of rotatory motion, there is little relative motion in atlantooccipital joint, the angle is less than 2.5°. During normal head rotation, changes of ADI are relative small. But changes of LADS of two sides are manifest, from the neutral position to rotation to 60°, the left LADS is bigger than the right; When the head rotates to the limit, the right LADS is bigger than the left.4. Shortcomings of this study and future prospects:The measurement of the anatomic varation of atlas and axis was based on dry bone samples, thus the reason for the variation and the possible clinical consequences were lack of vivo studies.Further research should explore the reason for variation and clinical consequences related to the variation in order to provide more exact guidance for the diagnosis and treatment of atlantoaxial joint. On the other hand, the study of rotatory motion of atlantoaxial joint was based on the single sample, so in order to explore the regularity of kinematics of the atlantoaxial joint, further research should observe samples as large as possible and explore the ligaments structure which influence the motion of atlantoaxial joint.
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