The Experimental Study Of Anterior Occiput-to-axis Fixation With Transarticular Screws

ObjectiveThis study is designed to find a novel internal fixation technique for clinicaloccipitocervical arthrodesis.By a series of studies,we attempt to achieve thefollowing goals: 1. Find anatomic clues for anterior occiput-to-axis fixation with transarticularscrews.Provide anatomic parameters of the occipitoatlantoaxial complex forclinical,scientific and Chinese constitution investigations . 2.Explicit the entry point,ideal angle of insertion,and allowable range and length ofthe trajectory in anterior occiput-to-axis fixation with transarticular screws;discussthe indication,complication and procedure of craniovertebral junction fixation withtransarticular screws.Materials and Methodsl.The occipitoatlantoaxial complex of 30 random normal adult Chinese were three-dimensionally reconstructed by CT,on which the hight of anterior and posterior edge of occipital condyle were measured;the width of occipital condyle articular facet were measured;the length of atlas superior articular facet were measured;the width and length of atlas inferior articular facet were measured;the maximum or minimus or optimal lateral angulation of the screw placement relative to sagittal plane were measured;the maximum or minimus or optimal posterior angulation of the screw placement relative to the coronal plane were measured ;the length of the medial or lateral or optimal screw path were measured;the relative location of hypoglossal canal and occipital condyle were observed.2.Calculate the mean and standard deviation of each side data.The data obtained were statistically analyzed.Owing to the major variation of some data during the course of observation in the experiment,we calculate the 80% confidence interval in order tomake the outcome have more reference value in clinic.3.Take 8 dry normal Chinese adult cadavers involving the integrity of occipitoatlantoaxial complex.The bone matrix were exposed.We can make clear the screw entry point,the adjacent anatomic structure and the extension of vertebral artery.4.According to the ideal entry point,the optimal angle and length of screw trajectories measured before, anterior occiput-to-axis screw fixation was performed on 8 dry craniovertebral junction specimens.The three dimensions were reconstructed by CT postoperatively,in order to conform the accuracy of data measured above and the feasibility of anterior occiput-to-axis screw fixation.Results1 .The occipital condyle and the superior articular facet of atlas and axis bit to form the axis-atlanto-occipital articulation,where the articular face of the occipital condyle was inside oblique and there was great variation in morphology of the hypoglossal canal. The hight of anterior edge of left occipital condyle is 6.06-9.97mm.The hight of anterior edge of right occipital condyle is 6.09-10.10mm. The hight of posterior edge of left occipital condyle is 4.30~10.62mm.The hight of posterior edge of right occipital condyle is 4.91-10.1 lmm. The width of left occipital condyle articular facet is 11.36-17.08mm. The width of right occipital condyle articular facet is 11.23-16.99mm. The length of atlas left superior articular facet is 11.71~19.98mm. The length of atlas right superior articular facet is 11.65-20.7lmm. The width of atlas left inferior articular facet is 12.74~17.00mm. The width of atlas right inferior articular facet is 12.80-16.96mm. The length of atlas left inferior articular facet is 14.54-18.84mm. The length of atlas right inferior articular facet is 12.80-16.96mm. The minimus left lateral angulation of the screw placement relative to sagittal plane is 6.29~20.07degree. The minimus right lateral angulation of the screw placement relative to sagittal plane is 5.24~20.04degree. The maximum left lateral angulation of the screw placement relative to sagittal planeis 17.18-29.41 degree. The maximum right lateral angulation of the screw placement relative to sagittal plane is 15.53-28.06 degree. The minimus left posterior angulation of the screw placement relative to the coronal plane is 8.14-22.18 degree. The minimus right posterior angulation of the screw placement relative to the coronal plane is 7.53-21.79 degree. The maximum left posterior angulation of the screw placement relative to the coronal plane is 21.44-36.61 degree. The maximum right posterior angulation of the screw placement relative to the coronal plane is 20.49-34.87 degree. The length of left medial screw path is 29.06-41.12mm. The length of right medial screw path is 29.46~42.62mm. The length of left lateral screw path is 28.49~40.21mm. The length of right lateral screw path is 28.69-41.15mm.2. Divide the above data into two groups.The histogram show there is no special partial data.So can adopt parameter estimation directly.In this study we adopt the two independent samples T test.Statistical analysis show there lie deference between the two group data.But there is no statistical significance.3.Six out of thirty normal adult hypoglossal nerves lie at posterior third of occipital condyle,which occupy 20% of the total .Twenty-four out of thirty normal adult hypoglossal nerves lie at anterior or middle third of the occipital condyle,which occupy 80% of the total.4. According to the optimal screw entry point, angle and length of screw trajectories measured before, Anterior occiput-to-axis screw fixation was performed on 8 dry craniovertebral junction specimens.Three dimension reconstructed CT show the screws are in bone matrix.There are no damage to hypoglossal nerve and vertebral artery.And there are no srews penetrate the occipital condyle.Conclusion1. The image analysis on the anatomy of normal adult occipitoatlantoaxial complex show as follws:(l) The anterior occiput-to-axis screw fixation is feasible in the occipitoatlantoaxial complex.(2) The location of hypoglossal nerve and vertebral artery is relatively constant. In some case hypoglossal nerve lie in theposterolateral part of the occipital condyle,and make the screw fixation dangerous.2. The ideal entry point is located caudal to the C2 superior facet joint in line with themedial third of the C2 superior facet. Take the middle length or angle of screw path between the maximum and minimus data as the optimal data.So the optimal length of screw path is 35.6mm. The optimal lateral angulation of the screw placement is 17 degree o The optimal posterior angulation of the screw placement is 22 degree o Owing to the great variation of the angle and length of screw path,the personality standard should be considered before the screw fixation.Every patient should be checked by three dimension reconstructed CT to determine the angle and length of the screw path.And the operation should be instructed by the data.3. The indication of anterior occiput-to-axis fixation with transarticular screwsoperat is as follows:(l) Congenital hypoplasia or absence of the bony elements of the upper cervical spine occurs rarely.(2)Failed attempts at posterior occipitocervical arthrodesis leave significant posterior scar tissue, disrupted osseous anatomy, and few landmarks for achieving safe and effective posterior stabilization. (3)Instability following procedures to remove tumors at the craniocervical junction may result from the removal of posterior elements of the upper cervical vertebrae in association with the removal of varying amounts of occipital bone, again diminishing posterior fixation alternatives. The operation is difficult and need high operative technique.We do not recommend this technique for routine stabilization of the occipitocervical junction; however, it may be useful in the rescue or salvage situation where more conventional stabilization alternatives are not technically possible, the availability of an anterior screw fixation technique may add stable fixation to further attempts at obtaining a posterior arthrodesis.4.The operation complications:the damage to the surrounding tissue such as blood vessel,nerve,spine,etc.Especially to hypoglossal nerve and vertebral artery when screw penetrate the bone matrix.5.We apply several means such as image measurement of normal adult occipitoatlantoaxial complex,cadaver dissection and operating on corpse,parameterestimation.And proved some feasibility of the anterior occiput-to-axis screw fixation in clinic.