BACKGROUND Although C1 screw fixation is becoming popular, only a few studies have discussed about the risk factors and the patterns of C1 screw complications. OBJECTIVE To investigate the incidence of C1 screw complications and analyze the risk factors of the C1 screw complications. METHODS A total of 358 C1 screws in 180 consecutive patients were analyzed for C1 screw complications. Screw malposition, occipital neuralgia, major complications, and total C1 screw complications were analyzed. RESULTS The distribution of C1 screw entry point is as follows: inferior lateral mass, 317 screws (88.5 %); posterior arch (PA), 38 screws (10.7 %); and superior lateral mass, 3 screws (0.8 %). We sacrificed the C2 root for 127 screws (35.5 %). C1 instrumentation induced 3.1 % screw malposition, 6.4 % occipital neuralgia, 0.6 % vascular injury, and 3.4 % major complications. In multivariate analysis, deformity (odds ratio [OR]: 2.10, P = .003), traumatic pathology (OR: 4.97, P = .001), and PA entry point (OR: 3.38, P = .001) are independent factors of C1 screw malposition. C2 root resection can decrease the incidence of C1 screw malposition (OR: 0.38, P = .012), but it is a risk factor of occipital neuralgia (OR: 2.62, P = .034). Advanced surgical experience (OR: 0.09, P = .020) correlated with less major complication. CONCLUSION The incidence of C1 screw complications might not be uncommon, and deformity or traumatic pathology and PA entry point could be the risk factors to total C1 screw complications. The PA screw induces more malposition, but less occipital neuralgia. C2 root resection can reduce screw malposition, but increases occipital neuralgia.
An observational study. To evaluate the safeties of placing three different alternative C2 screws using the freehand technique under high riding vertebral artery (HRVA) and to analyze the C2 morphometry in patients with HRVA. A retrospective analysis of radiologic data was performed on patients that underwent C2 instrumentation from September 2004 to December 2017. Two hundred fifty-one patients were included, and 90 of these patients (35.9%) had a unilateral or bilateral HRVA. We placed three alternative C2 screws including superior pars, inferior pars, and translaminar screws. Computed tomography was used to assess cortical breeches of screw placement and obtain morphometric measurements of C2 pars and lamina, that is, superior pars height/length, inferior pars length, and laminar thickness/length. We used the modification of the all India Institute of Medical Sciences outcome to define cortical breach. In total, 117 alternative C2 screws were inserted in 90 patients; 7 superior pars screws (6%), 69 inferior pars screws (59.0%), and 41 translaminar (35%) screws. Although cortical breaches occurred during 31 screw placements (26.5%), these were unacceptable in only two cases (1.7%). No symptomatic neurovascular complication was observed after screw placement in any case. Mean height of C2 superior pars was 3.8 ± 1.8 mm and mean thickness of C2 lamina was 5.2 ± 1.1 mm. Mean lengths of superior pars, inferior pars, and lamina were 17.8 ± 3.0 mm, 13.6 ± 2.2 mm, and 26.7 ± 3.3 mm, respectively. Superior pars height and lamina thickness < 3.5 mm that was a minimal diameter of cervical screw were 49.6% and 6.8%, alternative C2 screw was not available in these cases. Placements of alternative C2 screws using the freehand technique were achieved accurately and safely in patients with HRVA. However, preoperative morphometric evaluation is essential to determine the best option for C2 instrumentation and C2 screw length to avoid neurovascular complications.
High-speed-ratio differential speed rolling (HRDSR) is a newly developed procedure in which a material, in either plate or sheet form, is subjected to severe plastic deformation by inducing a large shear deformation during rolling for a large thickness reduction of 60$70% in a single pass. [1][2][3] Early investigations of the microstructures of the AZ31, [1] AZ61 [2] and AZ91 alloys [3] produced by HRDSR led to the recognition that this processing method is capable of reducing the grain size of Mg alloys to the sub-micrometer level achieved using equal-channel angular pressing (ECAP) with multiple passes. Like many Mg alloys processed by ECAP, [4,5] the HRDSRprocessed AZ61 and AZ91 alloys [2,3] exhibited excellent superplasticity at relatively low temperatures (473-523 K). The superplasticity of the HRDSR-processed AZ31 alloy was, however, not so impressive at similar temperatures, despite it being processed under identical experimental conditions as for the AZ61 and AZ91 alloys. The result was attributed to the presence of a relatively small volume fraction of a second phase (b-Mg 17 Al 12 ) in AZ31, which affects grain refining and growth during deformation. [6] It is well known that addition of Ca to magnesium is effective in the grain refining of magnesium castings. [7] However, there have been few reports on the superplastic behavior of Mg-Al-Ca alloys, since the effective break-up of coarse (Al, Mg) 2 Ca phase, in the form of a continuous network in grain boundaries of matrix, is not easy through conventional thermo-mechanical working routes. [8] In a previous work, [9] the effect of high-frequency electromagnetic forces on the surface quality and microstructure of a 1 wt.-% Ca-AZ31 billet was examined. Under optimum processing conditions, high-surface-quality billets with a homogeneous microstructure consisting of fine equiaxed grains (L ¼ 40-50 mm, where L is the linear intercept grain size) and a thin layer of (Al, Mg) 2 Ca phase distributed along the grain boundaries could be obtained. During the subsequent hot extrusion process, significant grain refinement (L ¼ 2.1 mm) and effective break-up of the (Al, Mg) 2 Ca phase took place. The extruded 1 wt.-% Ca-AZ31 exhibited high-strain-rate superplasticity at 573 and 673 K, at which temperatures tensile elongations larger than 550% were achieved at 10 À2 s À1 . [10] In this work, we applied the HRDSR technique to extruded 1 wt.-% Ca-AZ31 plates in order to produce sheets capable of exhibiting superplasticity at warm temperatures (473-523 K), in the range where most forming operations on magnesium alloy sheets are conducted in practice. The use of superplastic magnesium alloy sheets is exciting since it provides accommodation for a higher degree of design freedom in making magnesium components. ExperimentalA cylindrical AZ31 (Mg, 3 wt.-%Al, 1 wt.-% Zn) billet with 1 wt.-% Ca added (denoted 1CaAZ31) was produced by electromagnetic casting in the presence of electromagnetic stirring; details of this casting technique are available elsewhere. [9] Billets with ...
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