Atlantoaxial fixation using C1 posterior arch screws: feasibility study, morphometric data, and biomechanical analysis

Abstract

<jats:sec><jats:title>OBJECTIVE</jats:title><jats:p>C1–2 is a highly mobile complex that presents unique surgical challenges to achieving biomechanical rigidity and fusion. Posterior wiring methods have been largely replaced with segmental constructs using the C1 lateral mass, C1 pedicle, C2 pars, and C2 pedicle. Modifications to reduce surgical morbidity led to the development of C2 laminar screws. The C1 posterior arch has been utilized mostly as a salvage technique, but recent data indicate that this method provides significant rigidity in flexion-extension and axial rotation. The authors performed biomechanical testing of a C1 posterior arch screw (PAS)/C2 pars screw construct, collected morphometric data from a population of 150 CT scans, and performed a feasibility study of a freehand C1 PAS technique in 45 cadaveric specimens.</jats:p></jats:sec><jats:sec><jats:title>METHODS</jats:title><jats:p>Cervical spine CT scans from 150 patients were analyzed to determine the average C1 posterior tubercle thickness and size of C1 posterior arches. Eight cadavers were used to compare biomechanical stability of intact specimens, C1 lateral mass/C2 pars screw, and C1 PAS/C2 pars screw constructs. Paired comparisons were made using repeated-measures ANOVA and Holm-Sidak tests. Forty-five cadaveric specimens were used to demonstrate the feasibility and safety of the C1 PAS freehand technique.</jats:p></jats:sec><jats:sec><jats:title>RESULTS</jats:title><jats:p>Morphometric data showed the average craniocaudal thickness of the C1 posterior tubercle was 12.3 ± 1.94 mm. Eight percent (12/150) of cases showed thin posterior tubercles or midline defects. Average posterior arch thickness was 6.1 ± 1.1 mm and right and left average posterior arch length was 28.7 mm ± 2.53 mm and 28.9 ± 2.29 mm, respectively. Biomechanical testing demonstrated C1 lateral mass/C2 pars and C1 PAS/C2 pars constructs significantly reduced motion in flexion-extension and axial rotation compared with intact specimens (p &lt; 0.05). The C1 lateral mass/C2 pars screw construct provided significant rigidity in lateral bending (p &lt; 0.05). There was no statistically significant difference between the two constructs in flexion-extension, lateral bending, or axial rotation. Of the C1 posterior arches, 91.3% were successfully cannulated using a freehand technique with a low incidence of cortical breach (4.4%).</jats:p></jats:sec><jats:sec><jats:title>CONCLUSIONS</jats:title><jats:p>This biomechanical analysis indicates equivalent stability of the C1 PAS/C2 pars screw construct compared with a traditional C1 lateral mass/C2 pars screw construct. Both provide significant rigidity in flexion-extension and axial rotation. Feasibility testing in 45 cadaveric specimens indicates a high degree of accuracy with low incidence of cortical breach. These findings are supported by a separate radiographic morphometric analysis.</jats:p></jats:sec>

Computed Tomographic Morphometric Analysis of C1 and C2 for Lamina Cross Screw Placement in Malay Ethnicity

“…They also found that both the constructs had reduced motion in the flexion-extension and axial rotation compared with the intact specimen, and the C1 lateral mass/C2 pars screw rod construct provided rigidity in lateral bending. However, there was no notable difference between the two constructs in flexion-extension, rotation, and lateral bending [22]. According to all the biomechanical studies done and previous case reports on successful fusion, we believe that the C1 and C2 laminar screw has the potential to be used as an additional construct for added stability or to even replace the current method of fixation due to its similar stability with less of a risk of neurovascular injury.…”

“…The atlas posterior arch screw was later utilized as a salvage method of fixation when the usual method of fixation was not feasible because of the presence of a tumor, unfused C1 posterior arch, high-riding vertebral artery, or an aberrant vertebral artery course [7,8,20,21]. The C1 and C2 laminar screw offers less risk of vertebral artery injury as the procedure is done under direct visualization [3,10,22,23]. However, care should be taken when inserting the laminar screw in C1, especially to not breach the superior cortex where the vertebral artery passes at the superior aspect in the vertebral artery groove [20].…”

“…They found that all the constructs have similar stability in terms of flexion, extension, lateral bending, and axial rotation [23]. Cadena et al [22] performed a different study comparing C1 lateral mass/C2 pars screw rod construct with the C1 posterior arch/C2 pars screw rod construct. They also found that both the constructs had reduced motion in the flexion-extension and axial rotation compared with the intact specimen, and the C1 lateral mass/C2 pars screw rod construct provided rigidity in lateral bending.…”

“…This study also gives the surgeon a better understanding of the dimensions of C1 and C2, which will help in preoperative planning as well as in intraoperative selection of the method of fixation and screw sizes. A laminar screw would be a safer alternative for a patient with a highriding vertebral artery [22], anomalous vertebral arteries [7,8], a unilateral occluded vertebral artery [8], lateral mass destruction of C1 or C2 by a tumor [20,21], or any other condition that makes the current existing method of fixation not feasible.…”

Surgical nuances and construct patterns influence construct stiffness in C1-2 stabilizations: a biomechanical study of C1-2 gapping and advanced C1-2 fixation

“…Biomechanical and clinical studies have analyzed variations in construct patterns, including C1 arch and C1 hemi-arch screws, C1 hooks, C2 laminar screws and uniand bicortical screw fixation or extension into the subaxial spine [9,10,[13][14][15]. Most biomechanical studies have tested primary construct stability to compare the construct stiffness and the reduction in the range of motion (ROM) at C1-2 [15].…”