Anatomic Study And Biomechanical Evaluation About The Posterior Atlantoaxial Transarticular Screw Fixation

Atlantoaxial instability was common in clinical disease, its Causes include trauma, congenital malformation, rheumatoid arthritis, atlantoaxial transarticular screw fixation has been used usually to treat these cases. The current authors modified the location mark of Magerl screw fixation in operation and got good clinical effect. To provide the flexability of the new location mark, we took direct measurements from 40 dry axis and atlas vertebrae. We pointed out the position of the new location mark and definited the anatomic parameters correlating X-ray fluoroscopy, then discussed two screw trajectory. To assess whether our new location mark associated with C1-C2 transarticular screw fixation was superior to other entry point location mark, we compare the biomechanical stability in a cadaver model. To learn surgical unsuitability which was the first problem in inserting screw, we took direct measurements and got some data associated with 2.7mm screw, then pointed out solve thought as it was surgical unsuitability. (After that, we introduced some clinical experience about new entry point.)40 dry axis and atlas vertebrae of Chinese adult skeletons were selected for measurement, Specimens having gross evidence of congenital or acquired vertebral pathologic features were excluded from the current study. All liner parameters were measured using a electronic digital caliper accurate 0.02mm, angle using a conimeter accurate 1°. First definited our new location mark, then made the anatomic measurement correlating X-ray fluoroscopy. Measured items: (1) pedicle height (gd); (2) pedicle width (kd); (3) pedicle superior angle; (4) pedicle median angle; (5) screw trajectory superior angle; (6) screw trajectory median angle; (7) trajectory general length; (8) trajectory axis length; (9) trajectory atlas length (10) distance of pediclebisector (11) distance to afterwards edge (AH); (12) distance to outside edge (AW); (13) distance to inside edge(AN); (14) distance to tuberculum anterius (CF); (15) distance to arcus anterio(SF)The point of 1/3 postmedian areas on the superior articular surface of C2 intersected by the trajectories, are clarified as a important guide to the ideal and safe trajectories which should master in operation. But how to grasp it and quantify it had not more record. We work out a solution by took direct measurements from three direction, that was from the screw exit point to outside edge, to afterwards edge, and to inside edge. Obtaining the distance to three direction, we could definite X-ray fluoroscopy position where the screw went through the superior articular surface of C2. Getting the point of 1/3 postmedian areas on the inferior articular surface of d, remaining screw trajectory superior angle and median angle, we found the parameters of atlas anterior border;Most domestic scholar admit entry point which was in the area where the center of lower edge on axis processus articularis inferior join the axis vertebral plate inferior margin. AO throerial entry point was join point of the axis pedicle bisector and the axis vertebral plate, but definited with 2cm distance to inside edge, 3cm distance to lower edge. This and other entry point was indirect localization. To find ideal direct location mark of entry point and pursuit longer screw trajectory, we find out the new location mark.The new location mark is the center of lower edge on axis processus articularis inferior ,it was the outside of pedicle bisector or just on it ,The radiologic parameters of the screw exit point on processus articularis superior: Distance to outside edge were left (7.1±0.9)mm, right (6.8±0.9)mm; Distance to afterwards edge were left (6.3±0.7)mm, right (6.5±0.7)mm; Distance to inside edge were left (15.2±1.2)mm, right (15.5±l.l)mm; The general length of screw trajectory in bone were left (36.7±2.8)mm, right (36.2±2.9)mm.The bony landmark and radiologic parameters can be easily grasped in dry skeletons and biomechanical model specimens, it was direct, so the judgement in experimentation or operation of posterior atlantoaxial transarticular screw fixation became simply and certain. The new location mark was the outside or gastro of pedicle bisector, or just on it, that could do it best to remain the screw lengh. AO screw trajectory which was theoretically pedicle bisector was obviously shorter then our screw trajectory. To articular surface of Cl-2, more big screw trajectory superiorangle was, more firm the fixation was. Screw trajectory superior angle was bigger then pedicle median angle, so the fixation was firmer then AO screw trajectory.Pursuiting longer screw trajectory, we also think highly of inserting screw safe. The determine of screw exit point in superior articular surface of C2 ensure screw was included in pedicle near groove, avoiding vertebral artery injury or spinal cord injury.The same dry skeletons Specimen was measured, the new location mark was the gastro of pedicle bisector, position relation let us contract stereo view. Thus, we can know the spatial position of pedicle bisector, then judge the screw position by screw angle, entry and exit point. The accuracy and safety of atlantoaxial transarticular screw insertion were improved.By anatomic study we made a Conclusions: the new location mark is the center of lower edge on axis processus articularis inferior; the bony landmark and radiologic parameters can be easily grasped in operation of posterior atlantoaxial transarticular screw fixation, the length of screw trajectory were long, so the fixation were firm0Based on these Conclusions, The three-dimensional motions of Q relative to Ci were measured in 8 human cadaveric specimens, which was under four models: nomal, odontoid fractures, two posterior atlantoaxial transarticular screw fixation.The biomechanical comparison of different screw trajectory hadn't more report. We compared the biomechanical stability imparted to the atlantoaxial complex by either new screw trajectory or AO screw trajectory technique in a cadaver model and provide evidence for clinical operation.Egiht fresh ligamentous human cervical spine specimens (C0-C3) were harvested from donated cadavers shortly after death. The specimens were double-bagged and kept frozen at -20 [degrees] C. The spines were radiographed to exclude highly degenerated specimens. Before testing, the specimens were thawed to room temperature and stripped of muscles, leaving ligamentous tissue and discs intact. Co and C3 was potted to a plastic material base..During mounting, the specimen was oriented in the neutral position with the C2-C3 disc space in the horizontal plane. The prepared specimens were attached to the rigid base with special clamp. Experimental loads were applied to the spine in the form of pure moments through a system of weights, nylon strings, and pulleys. Six polyhedral body marker were inserted onto Ci, C2 and C3 vertebral bodies, which couldn't attach each other when vertebral bodies moving.All moving experiment were made on spinal three diamensions moving machine.The specimens were loaded under increments of 1.5 Nm, thus the specimen shouldn't be injuryed. Each load step was applied for 3 times before data collection to minimize the viscoelastic effects. The specimens were frequently sprayed with normal saline to prevent tissue drying during testing. The video camera obtain image on zero and biggest loading, then save it to computer image processing system; This system reconstructed spinal three diamensions moving and calculate motion range.All screw trajectory were parpered on specimens by the posterior atlantoaxial transarticular screw fixation, The entry point was the center of lower edge on axis processus articularis inferior in method A and was pedicle bisector in method B. Co and C3 was potted to a plastic material base after screw trajectory were parpered. The specimens were tested sequentially as follows: Intact spine(having four screw trajectory), TT model odontoid fracture, the posterior Atlantoaxial Transarticular screw fixation method A, method B.Method A: The new location mark is the center of lower edge on axis processus articularis inferior, using 0.8mm rod as guide pin. Reduce the Atlantoaxial complex, guide pin span the C1-C2 segment and exit from atlas lateral mass, screw trajectory were parpered with 3.2mm drill. The screw had different specification.Method B: AO entry point is inserting screw by pedicle bisector,the rest procedure were the same as above. TO judge whether our screw inserting method was superior to AO method in biomechanics, we inserted screw by method B first then by method A on the same specimen.lt could eliminate error.Results:Posterior atlantoaxial transarticular screw fixation significantly decreased motion in all directions, There was generally no significantly difference on the amount of motion among the two screw trajectory. Conclusions: Posterior atlantoaxial transarticular screw fixation provide satisfactory stability for clinical use. Biomechanical stability of two screw trajectory was close to each other.To learn surgical unsuitability which was the first problem in inserting screw, we took direct measurements and got some data associated with 2.7mm screw, then pointed out solve thought as it was surgical unsuitability.Measurement content: 1.pedicle height: the height of isthmus where was 2.5cm to pedicle superior border; 2. pedicle width:the width of middle pedicle.we measured 80 lateral pedicle data of 40 cases .we regarded height or width less then 4.5mm as being unsuitability to 3.5 mm screw and regarded height or width less then 3.8mm as being unsuitability to 2.7 mm screw.we made conclusions:part patients wereunsuitabile to unilateral or bilateral Transarticular screw. 22% cases were defined as "unacceptable" when using 3.5mm screw, 11% cases were defined as "unacceptable" when using 2.7mm screw,that was say we could increase 11% cases to accept atlantoaxial transarticular screw fixation by reduce screw diameter.The solve thought of screw unsuitability.The dislocation fractionate 3 varietas: reducible atlantoaxial dislocation, irreducible atlantoaxial dislocation,and non- reducible atlantoaxial dislocation.Only reducible atlantoaxial dislocation, and irreducible atlantoaxial dislocation with surgical lysis through transoral approach could use Magerl screw fixation.We inserted one lateral screw first if the other lateral was unsuitable,combined with other posterior fixation such as: Gallies method, Brooks methods Halifix method ^ Apofix method.This could assure atlantoaxial stability. If the bilateral was unsuitable for Magerl screw fixation, we used other posterior fixation alone or anterior atlantoaxial transarticular screw fixation.