| Background:Basilar invagination(BI)is a complex neurospinal compression syndrome,defined as an abnormal prolapse of the odontoid into the foramen magnum.Individuals may suffer from neural symptoms,syringomyelia,and cerebellar tonsillar herniation,even the paralysis and death.Basilar invagination is often associated with atlantooccipital assimilation,hypoplasia of clivus or occipital condyles,incomplete C1 ring,and C2-3 assimilation.The pathogenesis of BI remains unclear.Several studies were conducted to explore the features and therapies of BI.The posterior fixation is an important part of the cervical treatments,including the occipitocervical fixation and subaxial cervical fixation.Surgeons should have a deep understanding of the anatomy and a good master of the techniques to perform the operation successfully.Objections:1.To present a morphological map of the occipital bone in BI patients and help to guide the screw placement of the occipitocervical fusion.2.To evaluate the morphologic features of the subaixal cervical spine in BI patients and provide information for the accurate screw placement in this region.Materials and methods:1.A cohort of 50 BI patients(the BI group)and 50 cases with no head or cervical disease(the control group)was conducted in this part.Radiological measurements of the occipital bone were performed on computed tomography(CT)images based on a matrix of 99 points centered around the external occipital protuberance(EOP).2.A total of 42 BI patients were retrospectively included.Besides,forty-two patients without head or cervical disease were matched for sexes and ages.Information on radiographic features of the subaxial cervical spine was collected and compared systematically.Results:1.All thicknesses measured from points of the matrix in the BI group were thinner than those in the control group(p<0.05).The maximum thicknesses in both groups were located at the center of EOP,which were 15.11±2.84 mm in the BI group and 17.56 ± 3.03 mm in the control group,respectively.Additionally,thickness decreased with the distance away from the center of EOP.2.The BI group manifested a wider pedicle,laminar,and lateral mass than the control group in the subaxial cervical spine.The pedicle widths in the BI group and control group were 5.6±1.4mm vs.4.9±1.0 mm at C3,5.7±1.0 mmvs.4.8±0.8 mm at C4,5.6±1.3 mm vs.5.1±0.8 mm at C5.The laminar widths in the BI and control group were 4.9±1.7 mm vs.4.0±0.9 mm at C4,4.9±1.7 mm vs.4.1±0.8 mm at C5.The width of lateral mass:C3 12.7±2.7 mm vs.11.5±1.5 mm,C4 12.9±2.2 mm vs.11.5±1.5 mm,C5 12.5±2.3 mm vs.11.7±1.6 mm,C7 13.0±2.2 mm vs.12.3±3.4 mm.There were no differences in most measurements of the length of pedicle,laminar,and lateral mass.Conclusions:1.The occipital bone in BI patients is thinner than that in the general population.The entry point of occpital screws is advised to be in the area 1.5 cm lateral from the center of the external occipital protuberance,1cm lateral from median at the level of 0.5 cm inferior to the external occipital protuberance and 0.5 cm lateral from median at the level of 1 cm inferior to the external occipital protuberance.2.BI patients show a greater value in the C3-C5 pedicle width and C4-5 laminar width than the general population,but a smaller laminar width in C7.Particularly,the anatomic differences of the subaxial cervical spine should be taken into considerations to choose the proper screw placements for BI patients preoperatively. |
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