Background and objective:Atlantoaxial dislocation(AAD),often caused by trauma,tumors or congenital malformations,is a challenging disorder of the craniocervical junction.It is always a difficult region for spine surgery because of its deep location and intricate anatomic structure.Congenital AAD is usually associated with various bone abnormalities,joint malformations.When occipitalization of atlas exists,it is more difficult to treat AAD due to the vertebral artery(VA)varations.In managing these types of complex diseases,most spine surgeons are still greatly challenged with issues regarding how to detect the complex anatomy and how to intraoperatively address the VA.Meanwhile,some types of VA varations is usually associated with narrow C2 pedicle which limit the C2 pedicle screw techniques and requires some salvage fixation techniques.In addition,the stability of C1LC2PS is still controversial.Therefore,how to choose C2 salvage fixation techniques and improve the stability of C1LC2PS have become an urgent clinical problem.This research intends to study and discuss the problems regarding:(1)To describe and further classify different types of VA variations at the CVJ with SFOF-VR technology and quantitative measurement of anatomical parameters related to the VA variations.(2)To analyze the effectiveness of cross-link assisted C1LC2PS fixation system during reduction of AAD.(3)The biomechanical comparison of different C2 salvaged technologies was discussed finally.Methods:120 patients with AAD and atlas occipitalization who had undergone 3D-CTA were retrospectively studied.Imaging data were processed via the SFOF-VR technique.Anatomic measurements related to VA abnormalities at the CVJ were measured by using MIMICS 15.0 and 3-matic software.Cadaveric specimens were tested using a 6-degree-of-freedom robotic arm under the following conditions:(1)intact,atlantoaxial rotational sub luxation(AARS),C1 lateral mass and C2 pedicle screw fixation(C1LC2PS),C1 lateral mass and C2 pedicle screw fixation with one cross-link(C1LC2PS+1CL),C1 lateral mass and C2 pedicle screw fixation with two cross-links(C1LC2PS+2CL).(2)intact,AARS,C1LC2PS,C1 lateral mass and C2 trans-laminar screw(C1LC2TLS),Cl lateral mass and trans-C2 inferior articular process(C1LC2IAPLS).Three-dimensional intervertebral motion was tracked by stereo-photogram metrically while flexion-extension(FE),lateral bending(LB),and axial rotation(AR)were applied(1.5 Nm)to specimens.ThenResults:(1)Seven different types VA varations were classified.Distance from the canal of Type Ⅱ VA to the posterior facet surface of atlas lateral mass(5.51±2.17mm)means a 3.5 mm screw usually can be safely inserted.Shorter distance from the midline(13.50±4.35)illustrates potential Type Ⅲ VA injury during exposure.Decreased height and width of axis isthmus in Type V indicate increased VA injury risks.(2)Compared with the intact and AARS,C1-C2 ROM of the three intervention groups decreased significantly in each direction of loading motion(p<0.05);comparisons between the three subgroups showed that the ROM of single and double cross-link structures decreased significantly under rotating load motion compared with the intact and non-cross-link(p<0.001);however,there was no significant difference in ROM between single and double cross-link structures in all load motion.(3)Compared with the C2IAPS and C2TLS,C1-C2 ROM decreased significantly in rotating and flexion-extension load motion.Compared with the C2PS and C2TLS,C1-C2 ROM decreased significantly in rotating load motion.Compared with the C2PS and C2IAPS,C1-C2 ROM decreased significantly in flexion-extension load motion.Conclusion:(1)The courses of the VA variations in patients with AAD and atlas occipitalization were described and classified into seven different types utilizing the SFOF-VR technique,which is highly recommended as a convenient and concise method for the preoperative evaluation.This new classification system and related anatomical parameters allow for improved understanding of the 3D relationship between the course of VA variations and CVJ anatomy,which is important for minimizing the risk of intraoperative VA injury in patients with congenital anomalies at the CVJ.(2)The cross-link can effectively reduce the rotational ROM of AAD after C1 lateral mass-C2 pedicle screw fixation and the number of cross-link device has no difference in assisting fixation system on the ROM of each loading movement.(3)C2TLS and C2IAPS can be used as a salvage technique in narrow C2 pedicle cases.From the biomechanical point of view,the ROM of axial rotation and flexion-extension in C2TLS is significantly lower compared with C2IAPS. |