寰椎椎弓根螺钉置入技术的研究进展

目前寰枢椎后路固定常用的技术有Gallie钢丝、Brooks钢丝、Halifix椎板夹、Apofix椎板钩、Magerl经侧块关节螺钉等方法固定。钢丝联合经关节螺钉（Brook+Magerl术）方法固定能提供坚强的固定和较高的融合率,也有助于通过维持脊柱的力线而使脊髓得到间接减压,但经关节螺钉固定有并发椎动脉损伤的可能,而且不管采用哪种置钉技术,均需处理寰枢椎侧块关节后方的椎静脉丛方可显露进钉点。自从谭明生创造性应用寰椎椎弓根螺钉技术以来,该项技术得到了广泛的应用[1]。Tan等[2]在对50具亚洲裔人寰椎标本进行形态学研究后提出,螺钉的进钉点应位于后弓的背侧,而不是从后弓的下方进入寰椎侧块,螺钉经寰椎后弓、峡部至侧块内,Resnick等[3]称之为寰椎的椎弓根螺钉。寰椎椎弓根螺钉固定技术,即通常所说的经寰椎后弓侧块螺钉固定技术,与寰椎侧块螺钉固定技术不是等同的概念。近年来在寰枢椎后路内固定术中寰枢椎椎弓根螺钉固定技术发展越来越快,显示出其他固定方法无可比拟的优越性。国内外关于寰枢椎椎弓根螺钉固定技术的应用解剖学研究、生物力学研究及临床应用研究的报道屡见不鲜,现综述如下。     

寰椎测量及其经后弓侧块螺钉固定通道的研究

目的：提供国人寰椎的数据，探讨经寰椎后弓侧块（类似椎弓根）螺钉固定的可行性。方法：用游标卡尺对50例干燥寰椎标本进行测量。在实验研究的基础上，对5例寰枢椎不稳患者采用经寰椎后弓侧块螺钉和枢椎椎弓根螺钉加钢板内固定，术中用卡尺和量角器测量寰椎后弓进钉点及进钉方向，术后在X光片和CT片上测量经寰椎后弓侧块螺钉通道。结果：测量得出寰椎后弓、侧块、椎动脉沟、后弓进钉点、后弓侧块螺钉通道的轴线长度及进钉方向等参数。临床5例10枚经寰椎后弓侧块螺钉均准确置入寰椎内，无穿破骨皮质或损伤神经血管病例。结论：寰椎具备行经后弓侧块螺钉内固定的条件。     

寰椎后弓侧块螺钉进钉角度的影像学研究及临床应用

目的探讨寰椎后弓侧块螺钉置钉安全角度及临床应用价值。方法对30名寰枢椎形态正常者行颈椎侧位X线摄片和寰枢椎轴位CT扫描,在影像学图像上对寰椎后弓最小高度、矢状面最大进钉倾斜角、最大进钉内倾角等参数进行测量。应用影像学观测的进钉角度对21例上颈椎不稳定患者行后路寰椎后弓侧块螺钉固定,术中观察后弓侧块螺钉置入后对周围重要组织结构的影响,术后观察有无神经症状加重。其中13例行手术前后CT检查,并测量螺钉置入前后的相关参数,评估置钉的安全性。结果(1)寰椎后弓侧块矢状面进钉倾斜角最大头倾10°、最大尾倾6°,最大倾斜角随寰椎后弓最小高度的增加有增大的趋势;(2)当进钉点由内侧改为外侧时,置钉内倾角需作0°～30°的相应调整;(3)手术实际进钉角度与术前预测有一定的差异,但均在预测范围内;(4)临床病例均置钉成功,影像学显示在常用的进钉区域内置钉无螺钉伤及椎动脉、脊髓等周围组织的现象。结论寰椎后弓侧块置钉在矢状面和轴平面上均有一定的安全角度范围,依据此范围置钉可增加寰椎后弓侧块螺钉固定的安全性和可操作性。     

寰枢椎后路内固定技术的研究进展

寰枢椎不稳多采用后路手术治疗,但由于其解剖及生物力学上的特殊性,内固定方式也与下颈椎明显不同,目的是提供即刻稳定性,解除脊髓及神经压迫,促进融合。目前固定方式主要包括钢丝固定技术、椎板夹或椎板钩技术、Magerl技术、寰椎侧块螺钉技术及椎弓根钉技术、枢椎椎弓根钉、枢椎椎板螺钉、峡部螺钉等,其后又出现寰椎后弓螺钉、后弓锁定钛板、寰枢侧块关节间融合器等新技术,但各固定方式在技术及恢复上颈椎稳定性上存在优缺点,故熟悉各内固定技术并对该领域研究进展有所了解有极其重要的意义。     

经后路寰椎椎弓根螺钉固定的置钉研究

目的探讨经后路寰椎椎弓根螺钉固定的可行性. 方法利用20具颈椎尸体标本,模拟经后路寰椎椎弓根螺钉固定.在寰椎后弓后缘表面,经枢椎下关节突中心点纵垂线与寰椎后弓上缘下方3 mm水平线的交点作为进钉点,按内斜10度、上斜5度钻孔,经寰椎椎弓根置入直径3.5 mm的皮质骨螺钉.测量进钉点与寰椎椎弓根中线平面的距离、螺钉最大进钉深度、螺钉内斜角度和螺钉上斜角度等解剖指标,观察螺钉是否突破椎弓根和侧块骨皮质,以及椎动脉、硬膜、脊髓是否损伤等. 结果共放置40枚寰椎椎弓根螺钉,测得进钉点与寰椎椎弓根中线的平均距离为(2.20±0.42) mm,螺钉最大进钉深度平均(30.51±1.59) mm,螺钉内斜角度平均(9.70±0.67)度,上斜角(4.60±0.59)度.其中1枚螺钉因上斜角度过大穿破椎弓根上缘,8枚因后弓高度过小而突破椎弓根下缘,5枚进钉过深突破寰椎侧块前缘皮质,但均未对脊髓和椎动脉造成损伤. 结论经后路行寰椎椎弓根螺钉固定是安全可行的,但应注意进钉角度和深度.     

寰椎后弓侧块螺钉固定的解剖学测量

目的:为寰椎后弓侧块螺钉固定提供解剖学依据。方法:用电子游标卡尺和量角器对40例干燥寰椎标本测量与进钉技术相关的解剖学数据。结果:测量得出寰椎后弓平均高度为4.58mm,寰椎“椎弓根”平均宽度为7.89mm,侧块中部平均高度和宽度分别为12.94mm和12.76mm,最大进钉深度为28.50mm,螺钉内斜角度为9.1°,上斜角度为4.7°。结论:进行寰椎后弓侧块螺钉固定是安全可行的。该术式可望成为寰枢段后路短节段固定的优选术式。     

后路寰椎后弓螺钉内固定研究进展

寰椎后弓螺钉内固定技术的提出,为治疗枕颈部不稳提供了新的补救方法。许多文献报道了寰椎后弓螺钉固定的可行性,初步显示了其置钉点易暴露、出血少、手术时间短等优势,尤其在侧块被肿瘤、炎症、创伤等破坏无法使用寰枢关节螺钉、寰椎侧块螺钉、椎弓根螺钉时补救固定。但如果外科医生术中操作不当,则可引起医源性骨折、椎动脉及颈脊髓损伤等严重并发症。因此,寰椎后弓螺钉内固定技术的安全性和有效性成为当下研究的热点。     

不同内倾角置入寰椎后路侧块螺钉的安全性比较

目的:将寰椎后路侧块螺钉上倾角固定在15°的情况下,比较不同内倾角度置钉相对应的寰椎侧块有效宽度和进钉深度,以确定寰椎后路侧块螺钉在横断面上的最佳内倾角度。方法:募集颈椎结构正常的健康成年国人64名,男32名,女32名;年龄32.7±7.8岁(18~60岁),进行颈椎CT平扫,通过Mimics软件对志愿者的颈椎CT数据进行测量,在寰椎后路侧块螺钉上倾角为15°的情况下,分别测量内倾角度为0°、5°、10°、15°、20°、25°、30°和理想内倾角(β角)时的寰椎侧块有效宽度和寰椎后路侧块螺钉的进钉深度;测量正中矢状面与寰椎后弓最后缘的交点与进钉点之间的距离和进钉点处的后弓下缘侧块高度;并进行统计学数据分析。结果:寰椎后路侧块螺钉的β角男性为20.01°±2.97°,女性为21.89°±3.14°。寰椎后路侧块螺钉内倾角度为0°、5°、10°、15°、20°、25°、30°和β角时的寰椎侧块有效宽度男性分别为8.95±1.18、9.51±1.14、10.00±1.17、10.42±1.20、10.39±1.29、9.41±1.25、8.04±1.21、11.02±0.96mm,女性分别为7.64±1.01、8.31±1.11、8.88±1.10、9.32±1.11、9.51±1.09、8.90±1.02、7.62±1.07、9.97±0.98mm;进钉深度男性分别为19.03±1.34、19.60±1.39、20.28±1.47、21.20±1.48、22.11±1.54、23.06±1.63、24.19±1.97、22.37±1.52mm,女性分别为17.47±1.68、17.90±1.64、18.50±1.79、19.31±1.93、20.47±2.03、21.67±2.18、22.89±2.38、21.26±1.99mm。相较于其他内倾角,β角对应的寰椎侧块有效宽度最大且差异有统计学意义(P0.01)。寰椎后路侧块螺钉内倾角为β时,寰椎后路侧块螺钉进钉点距离正中矢状面与寰椎后弓最后缘的交点距离男性为28.35±1.75mm,女性为26.70±1.60mm;进钉点处的后弓下缘侧块高度男性为4.38±0.85mm,女性为4.13±0.80mm。结论:寰椎后路侧块螺钉上倾15°时,以所对应的寰椎侧块有效宽度最大的内倾角β角进钉,理论上损伤椎动脉、脊髓等结构的几率最低。     

寰椎经后弓侧块螺钉固定通道的影像学测量

目的 :提供国人寰椎CT测量的数据 ,评价寰椎经后弓侧块螺钉置钉的可行性。方法 :针对寰椎经后弓侧块螺钉固定通道 ,对 5 0具寰椎骨标本进行多个切面的CT断层扫描并测量。结果 :将寰椎后结节中点旁开 18～ 2 0mm的矢状面与后弓下缘的上方 2mm处的冠状面的交线在后弓后方的投影点确定为进钉点 ;于冠状面上保持垂直进钉 ,矢状面上钉头向头侧倾斜约 5° ,是寰椎经后弓侧块螺钉固定较理想的置钉通道。结论 :寰椎具备行经后弓侧块螺钉内固定的条件。     

后路经关节螺钉固定颗粒状植骨融合治疗寰枢关节不稳定

目的:探讨后路经C1、C2侧块关节螺钉固定、颗粒状松质骨植骨行寰枢关节融合治疗寰枢关节不稳的效果。方法:自1999年12月~2003年4月对58例因齿状突不连、寰椎横韧带断裂或松弛导致寰枢关节不稳定的病例施行了后路经C1、C2侧块关节的螺钉固定术,然后在C1、C2后弓间植入颗粒状松质骨。术中不用钛缆固定寰椎后弓与枢椎棘突。术后不需任何外固定。结果:无手术中损伤脊髓和椎动脉的病例。49例获得随访,时间6个月～3年10个月,平均20个月,全部获得了骨性融合。结论:当寰枢关节不稳定时用两枚螺钉由后路经C1、C2侧块关节固定即可起到足够的稳定作用;在C1、C2后弓间植入颗粒状松质骨可获得很高的融合率。     

Insertion of lateral mass screw of the atlas via the posterior arch: anatomical study of screw insertion using dry bone samples of the atlas from Japanese cadavers

BACKGROUND: A new technique involving screw fixation of the atlas via the posterior arch and lateral mass has recently been reported for atlantoaxial instability. Because the posterior arch is thin, lateral mass screws risk penetrating the upper part of the posterior arch and damaging the vertebral artery running along the upper part of the posterior arch. METHODS: A total of 50 dry bone samples of the atlas from Japanese cadavers were used. We manually measured the shortest distance from the vertebral canal to the transverse foramen and the thickness at the thinnest part of the groove using calipers and investigated the frequency of dorsal ponticuli at the posterior arch. RESULTS: The area from the vertebral canal to the transverse foramen was thick enough to allow screw insertion, but the thickness of the posterior arch at the thinnest part of the groove was less than the screw diameter (3.5 mm) in 22% of vertebrae and <4 mm in 39%. A dorsal ponticuli was present in 10% of these samples. CONCLUSIONS: The size and shape of the posterior arch must be evaluated using radiography and computed tomography before inserting a lateral mass screw of the atlas via the posterior arch.     

Biomechanical comparison of the pullout strengths of C1 lateral mass screws and C1 posterior arch screws

Background contextConditions of the atlantoaxial complex requiring internal stabilization can result from trauma, malignancy, inflammatory diseases, and congenital malformation. Several techniques have been used for stabilization and fusion. Posterior wiring is biomechanically inferior to screw fixation. C1 lateral mass screws and C1 posterior arch screws are used for instrumentation of the atlas. Previous studies have shown that unicortical C1 lateral mass screws are biomechanically stable for fixation. No study has evaluated the biomechanical stability of C1 posterior arch screws or compared the two techniques.PurposeThe purpose of the study was to assess the differences in the pullout strength between C1 lateral mass screws and C1 posterior arch screws.Study designBiomechanical testing of pullout strengths of the two atlantal screw fixation techniques.MethodsThirteen fresh human cadaveric C1 vertebrae were harvested, stripped of soft tissues, evaluated with computed tomography for anomalies, and instrumented with unicortical C1 lateral mass screws on one side and unicortical C1 posterior arch screws on the other. Screw placement was confirmed with postinstrumentation fluoroscopy. Specimens were divided in the sagittal plane and potted in polymethylmethacrylate. Axial load to failure was applied with a material testing device. Load displacement curves were obtained, and the results were compared with Student t test. DePuy Spine, Inc. (Raynham, MA, USA) provided the hardware used in this study.ResultsMean pullout strength of the C1 lateral mass screws was 821 N (range 387?1,645 N±standard deviation [SD] 364). Mean pullout strength of the posterior arch screws was 1,403 N (range 483?2,200 N±SD 609 N). The difference was significant (p=.009). Five samples (38%) in the posterior arch group experienced bone failure before screw pullout.ConclusionsBoth unicortical lateral mass screws and unicortical posterior arch screws are viable options for fixation in the atlas. Unicortical posterior arch screws have superior resistance to pullout via axial load compared with unicortical lateral mass screws in the atlas.     

Routine insertion of the lateral mass screw via the posterior arch for C1 fixation: feasibility and related complications

Background contextTo our knowledge, there is no clinical study analyzing the feasibility and complications of the routine insertion of the lateral mass screw via the posterior arch for C1 fixation in a live surgical setting.PurposeTo evaluate the feasibility of routine insertion of the lateral mass screw via the posterior arch and related complications.Study designProspective clinical-radiological analysis.Patient sampleFifty-two consecutive patients with 102 C1 lateral mass screws inserted via the posterior arch.Outcome measuresCortical perforation, vertebral artery injuries, and visual analog scale score of occipital neuralgia recorded on a prospective database.MethodsAll consecutive patients in whom lateral mass screw placement via the posterior arch was attempted as the first choice whenever C1 posterior fixation was necessary were enrolled. Prospective database, clinical records, questionnaires regarding occipital neuralgia, pre- and postoperative computed tomography (CT) angiograms, and follow-up radiographs and CT scans were analyzed. This study was supported by a $9,000 academic research grant by the first author's hospital. The last author receives royalties for a posterior cervical fixation system, which is not the topic of this study and is not used or mentioned in this article.ResultsOne hundred two screws were attempted in 52 consecutive patients by a single surgeon. The height of 43 posterior arches (42%) was smaller than 4 mm on preoperative CT angiography. Lateral mass screws could be inserted via the posterior arch in all cases including eight with nine ponticuli posticus and seven with seven persistent first intersegmental arteries, but the posterior arch was perforated cranially by 7, caudally by 30, and craniocaudally (partially) by 3 screws and vertically split by 14 screws. Among the last 28 screws for which the authors' overdrilling technique was used, only one vertical split occurred, whereas among the first 74 screws without overdrilling, 13 vertical splits occurred. None of them led to screw loosening or nonunion. There were no vertebral artery injuries. Among the 19 patients with preoperative occipital neuralgia, 12 had complete resolution and seven had alleviation at the last follow-up. Among the 33 patients without preoperative neuralgia, seven developed new neuralgia postoperatively. Three of them underwent C2 root transection and the other four underwent C2 root dissection for intraarticular fusion of the facet joints. Of the seven, five had complete resolution and two had mild discomfort at the last follow-up.ConclusionsRoutine insertion of the lateral mass screw via the C1 posterior arch was feasible in even those with a small posterior arch, ponticulus posticus, or persistent first intersegmental artery. Although cortical perforation or vertical splitting of the posterior arch was often inevitable, it did not lead to significant weakening of the fixation or nonunion. Vertical split could be minimized by overdrilling the posterior arch. Vertebral artery injury was preventable by mobilization before screw insertion. Occipital neuralgia was not uncommon but thought to be unrelated to screw placement in most cases.     

经寰椎后弓上颈椎稳定性重建的解剖学研究

目的 通过解剖学测量,探讨经寰椎后弓螺钉固定重建上颈椎稳定性的解剖学基础.方法 测量39具干燥寰椎标本椎管内径宽度、螺钉进钉点后弓高度和宽度、寰椎椎动脉沟底后弓高度和宽度、经后弓螺钉固定钉道长度和侧块钉道长度、后弓固定与侧块固定进钉角度等解剖学数据.统计学分析所测量数据并对经寰椎后弓螺钉固定钉道长度与经侧块螺钉固定钉道长度两组数据配对student t检验,检验水准为α=0.05.结果 寰椎椎管内径宽度为(26.80±2.58)mm;进钉点寰椎后弓高度和宽度分别为(6.83±1.97)mm和(6.40±1.36)mm;椎动脉沟底后弓高度和宽度分别为(4.37±1.11)mm和(8.05±1.33)mm;经后弓螺钉固定钉道长度为(30.54±1.70)mm.进钉点置钉时钉道外倾角度为15°～20°、吻侧前倾角度为0°～5°.椎动脉沟底骨质高度小于颈椎常用螺钉直径3.5 mm的占25.6%(10具).经侧块螺钉固定钉道长度为(16.91±1.13)mm,进钉点置钉时钉道内倾角度为32.05°±6.03°、头侧前倾角度为5°.经后弓螺钉固定钉道长度与经侧块钉道长度两组数据配对student t检验统计分析结果显示差异有统计学意义(t=59.528,P＜0.001).结论 约有74.4%的人群可顺利经寰椎后弓螺钉固定完成上颈椎稳定性的重建,该技术不仅可使寰椎得到即刻、牢靠的稳定性,同时较寰椎经侧块螺钉固定技术具有较小的手术风险和操作难度,是一项具有推广价值的寰椎稳定技术.

Abstract：

Objective To investigate the anatomy of upper cervical vertebrae's stable reconstruction by poster arch of atlas screw by anatomic study. Methods To collect the anatomical data of 39 dry atlas with the average inner diameter of vessel of atlas, the average high and wide of the screw point of poster arch of atlas, the average high and wide of poster arch of atlas beneath the groove for vertebral artery, the tract length of poster arch screw, the tract length of lateral mass screw, the direction and angle of poster arch screw, and the direction and angle of lateral mass. Statistical comparison were performed with Student test between the tract length of poster arch screw and the tract length of lateral mass screw. Results The average inner diameter of vessel of atlas was (26.8±2.58) mm(21.7-31.0 mm). The screw point high and wide of poster arch of atlas were (6.83±1.97) mm and (6.40±1.36) mm respectively. The average high and wide of poster arch of atlas beneath the groove for vertebral artery were (4.37±1.11) mm and (8.05±1.33) mm respectively. The tract length of poster arch screw were (30.54±1.705) mm. The direction and angle of poster arch screw was 15°-20° incline laterally and 0°-5°incline cephalo. 25.6%(10 cases) patients with the diameter less than 3.5mm beneath the groove for vertebral artery. The direction and angle of lateral mass screw was 32.05°±6.03° incline medially and 5° incline cephalo. There were significant differences between the tract length of poster arch screw and the tract length of lateral mass screw(t=59.528, P＜0.001 ). Conclusion About 74.4% patients can reconstruct the upper cervical stable by poster arch of atlas screw. The technique can gain instant upper cervical stable, and reduce the risk of injury of blood vessel by over explore the atlas arch compare with the technique of lateral mass screw fixation of atlas. The technique is worth to promoting with its character of safe, easy perform and rigid fixation.     

后路寰椎侧块螺钉结合枢椎椎弓根螺钉固定治疗上颈椎不稳

目的：总结应用寰椎侧块螺钉与枢椎椎弓根螺钉技术固定融合治疗寰枢椎不稳的效果，探讨寰枢椎不稳的治疗方法。方法：采用寰椎侧块螺钉与枢椎椎弓根螺钉技术对15例寰枢椎不稳的患者进行了固定术，同时行自体髂骨融合。分别应用Vertex 7例，Axis 3例，和cervifix 5例，齿状突陈旧性骨折5例，新鲜Ⅱc型齿状突骨折6例，先天性游离齿状突4例。寰椎侧块螺钉进钉点选择在寰椎后结节中点旁开18mm～20mm，与后弓下缘以上2mm的交点，钉道方向在冠状面垂直，矢状面上螺钉头端向头侧倾斜约5°.枢椎进钉点为枢椎下关节突根部中点，钉道与矢状面夹角约15°，横断面夹角约30°。螺钉直径3．5mm，寰椎侧块螺钉长度28mm～32mm，枢椎椎弓根螺钉长度为22mm～26mm。结果：所有患者均未发生脊髓损伤和椎动脉损伤。随访10～25个月，平均14个月。术前JOA评分5．1～10．9分，平均7．6分。术后JOA评分13．2～16．8分，平均14．8分，改善率87．5％。植骨块全部融合，无内固定断裂、松动。结论：后路寰椎侧块螺钉与枢椎椎弓根螺钉技术稳定性良好，具有三维固定的优点，值得推广。     

颈椎侧块钢板螺钉固定术治疗颈椎失稳

为治疗颈椎失稳症,采用颈椎侧块钢板螺钉固定术治疗颈椎失稳症9例,通过6～26个月随访,均固定牢靠,融合良好。结果见颈椎侧块钢板螺钉固定对颈椎侧块具有牢固固定作用,同时对滑脱有良好的复位作用。认为后路颈椎侧块钢板螺丝钉固定加植骨是治疗颈椎失稳症的可靠方法。     

采用椎弓根螺钉内固定治疗寰枢椎骨折脱位

[目的]探讨寰枢椎椎弓根螺钉技术治疗创伤性寰枢椎不稳的临床效果。[方法]2009年9月~2017年3月,对6例创伤性寰枢椎不稳患者采用颈椎后路寰枢椎椎弓根螺钉固定技术治疗。术前均常规行颅骨牵引,椎弓根进钉点位于寰椎椎弓两侧距后结节20 mm、后弓下缘2 mm,内倾5°~10°、头倾10°~15°,置入直径3.5 mm、长22~26 mm螺钉。枢椎螺钉的进钉点位于侧块内上象限,内倾约25°,头倾约25°置入直径3.5 mm、长24~28 mm螺钉。[结果]术中无血管及神经损伤。术后随访6~36个月,患者临床症状得到明显改善。术后检查显示螺钉位置良好,无内置物松动、断裂。术后12个月JOA评分为11.00~17.00分。[结论]寰枢椎椎弓根螺钉固定治疗创伤性寰枢椎不稳疗效满意。     

Ideal screw entry point and projection angles for posterior lateral mass fixation of the atlas: an anatomical study

Although various posterior insertion angles for screw insertion have been proposed for C1 lateral mass, substantial conclusions have not been reached regarding ideal angles and average length of the screw yet. We aimed to re-consider the morphometry and the ideal trajections of the C1 screw. Morphometric analysis was performed on 40 Turkish dried atlas vertebrae obtained from the Department of Anatomy at the Medical School of Ankara University. The quantitative anatomy of the screw entry zone, trajectories, and the ideal lengths of the screws were calculated to evaluate the feasibility of posterior screw fixation of the lateral mass of the atlas. The entry point into the lateral mass of the atlas is the intersection of the posterior arch and the C1 lateral mass. The optimum medial angle is 13.5 ± 1.9° and maximal angle of medialization is 29.4 ± 3.0°. The ideal cephalic angle is 15.2 ± 2.6°, and the maximum cephalic angle is 29.6 ± 2.6°. The optimum screw length was found to be 19.59 ± 2.20 mm. With more than 30° of medial trajections and cephalic trajections the screw penetrates into the spinal canal and atlantooccipital joint, respectively. Strikingly, in 52% of our specimens, the height of the inferior articular process was under 3.5 mm, and in 70% was under 4 mm, which increases the importance of the preparation of the screw entry site. For accommodation of screws of 3.5-mm in diameter, the starting point should be taken as the insertion of the posterior arch at the superior end of the inferior articular process with a cephalic trajection. This study may aid many surgeons in their attempts to place C1 lateral mass screws.