骶2椎弓根前外侧置钉法的应用解剖学研究

目的:观察S2椎弓根的解剖特点及与周围血管神经的毗邻关系,探讨以第一骶后孔与骶外侧嵴为解剖标志的S2椎弓根前外侧置钉技术的可行性。方法:取20具成人干燥骶骨标本,其中15具分别从S1-S2和S2-S3椎体融合遗迹(骶横线)处平行骶骨底平面截断骶骨,观察S2椎弓根的形态;在第一骶后孔下缘最低切迹处平行骶骨底平面截断骶骨。在S2椎弓根前部(即盆面),截面与骶前孔外侧缘连线的交点为A点,与耳状面前缘(即骶髂关节处)的交点为B点。在截面上测量骶外侧嵴(X点)与A、B两点连线即XA线、XB线长度及其与正中矢状面夹角α角、β角;在截面上测量骶中间嵴(Y点)与A、B两点连线即YA线、YB线长度及其与正中矢状面夹角γ角、θ角;在5具完整的干燥骶骨标本上于X点向前外侧置入椎弓根螺钉,置钉方法为矢状面平行骶骨底,角度在α角与β角之间,并行CT扫描观察螺钉的位置。在10具经福尔马林浸泡固定的成人尸体标本上观察骶骨前血管神经的毗邻关系,观察骶前孔、骶髂关节与骶前交感干、髂内动静脉的关系。并按照上述测量结果分别在X、Y点向前外侧置入S2椎弓根螺钉,使螺钉穿出骶骨前方,落在骶前孔外侧缘与耳状面前缘之间。观察螺钉的出钉点与骶前血管神经的关系。结果:S2前方有大量的骨质可供螺钉置入。XA为26.0±1.7mm,XB为33.1±1.1mm,α角为22.1°±2.0°,β角为36.0°±3.7°;YA为30.1±0.8mm,YB为35.5±1.0mm,γ角为36.7°±1.5°,θ角43.8°±1.3°。在骶髂关节前方从骶前孔外侧缘开始向外侧依次有髂内静脉、髂内动脉、髂外静脉、髂外动脉。第1骶前孔与第2骶前孔之间的前外侧有S1神经通过。骶前交感干走形在骶前孔内侧缘,出钉点在骶前孔外侧,髂内动静脉后方。结论:S2椎弓根前外侧置钉安全可行,向前外侧固定时螺钉不能突破前方骨皮质,以免伤及骶前血管及神经。以骶外侧嵴与第一骶后孔下缘最低切迹水平面交点(X点)为进钉点,前外侧置钉更适用。     

骶骨椎弓根及侧块的应用解剖研究

【摘要】 目的：研究骶骨椎弓根及侧块的解剖学结构特点,为骶骨椎弓根和侧块螺钉内固定技术应用提供理论依据。方法：随机选择60例成人患者骶尾骨螺旋CT图像资料,应用三维重建技术确定骶骨椎弓根和侧块的进钉点,测量骶骨椎弓根和侧块的螺钉置入钉道长度和角度。选择15例成人尸体标本进行大体解剖,确定骶骨椎弓根和侧块的进钉点,并测量螺钉置入的钉道长度和角度。结果：S1～S5的椎弓根和侧块变化较大但左右对称,椎弓根进钉点位于横突中线与骶后孔中点连线交点,侧块进钉点位于横突中线与骶外侧嵴交点内侧,椎弓根钉的钉道角度外倾约2     

经S2椎弓根骶髂关节螺钉固定CT和解剖学研究

目的 :为经S2 椎弓根水平骶髂关节螺钉固定提供解剖学依据。方法 :对 2 2个骨盆标本进行解剖学研究 ,测量经S2 椎弓根骶髂关节螺钉固定的进针点、进针方向、钉道直径、进针点至S2 椎体中心和椎弓根中心距离、S2 椎弓根横断面形状。结果 :螺钉进针点在髂后下棘前方 2 8 7mm (15 0～34 5mm) ,坐骨切迹上方 15 3mm(10 0～ 2 2 0mm) ;进针方向垂直于正中矢状面且与髂骨翼外侧面夹角为 6 2 4°(5 5°～ 72°) ;钉道直径为 11 5mm (8 5～ 14 0mm) ;进针点至S2 椎体中心距离 6 4 7mm(5 5 3～ 77 6mm) ;进针点至S2 椎弓根中心距离为 5 3 1mm(45 3～ 6 4 8mm) ;S2 椎弓根横截面呈三角形。结论 :经S2 椎弓根水平骶髂关节螺钉固定的进针点应位于髂后下棘前方 30mm ,坐骨切迹上方15mm ;进针方向垂直于正中矢状面且与髂骨翼外侧面夹角为 6 0°;S2 椎弓根钉道直径较小 ,经S2 椎弓根水平骶髂关节螺钉固定应选择直径较小的短螺钉。     

第三、四骶骨螺钉应用的解剖学研究

目的通过对第三、四骶骨螺钉应用的解剖学测量,探讨第三、四骶骨螺钉应用的安全性。方法 32具成人骶骨标本。设定A点、B点和C点为螺钉进钉点,分别位于相邻骶后孔内侧缘连线的中点,相邻骶后孔连线的中点,经相邻骶后孔连线中点的水平线与骶外侧嵴的交点。D1、D2、D3、α和β、分别为水平面上骶骨内侧置钉,垂直置钉和外侧置钉的钉道长度,内侧置钉和外侧置钉最大偏移角度。d1、d3、θ和φ分别为矢状面上骶骨头侧置钉和尾侧侧置钉钉道长度,头侧置钉和尾侧置钉最大偏移角度。结果 S3螺钉在A和B点进钉,水平面上偏向外侧小于45.21°±10.66°,矢状面上偏向头侧小于8.50°±3.53°;C点进钉,水平面上偏向内侧小于52.00°±7.55°,矢状面上偏向头侧小于19.05°±6.36°。S4螺钉在A和B点进钉,水平面上偏向外侧小于49.50°±9.53°,矢状面上偏向头侧小于12.00°±4.24°,偏向尾侧小于14.00°±2.83°;C点进钉,水平面上偏向内侧小于66.19°±8.14°,矢状面上偏向头侧小于13.50°±3.54°,偏向尾侧小于28.50°±2.12°。结论了解第三、四骶骨的解剖结构和精确的螺钉放置可以保证骶骨螺钉固定成功。     

腰骶移行椎S_1椎弓根解剖测量及临床应用

目的探讨腰骶移行椎S1椎弓根进钉方法及临床初步应用效果。方法测量17例腰骶移行椎标本及30例正常骶骨标本的S1椎椎弓根横径、上下径、椎弓根中心轴线长度及角度,采用特定进钉点治疗15例腰骶移行椎患者。结果根据测量结果确定椎弓根螺钉置钉点。S1椎弓根横径:腰骶移行椎(22.47±4.67)mm,正常椎(26.72±3.76)mm(P0.05)。上下径:腰骶移行椎(20.71±2.97)mm,正常椎(25.38±4.62)mm(P0.05)。椎弓根中心轴线长度:腰骶移行椎(41.27±5.65)mm,正常椎(49.83±6.72)mm(P0.05)。移行椎椎弓根A角:腰骶移行椎6.54°±1.98°,正常椎6.31°±1.72°(P0.05);B角:腰骶移行椎43.55°±11.21°,正常椎35.71°±10.63°(P0.05)。腰骶移行椎的上关节突形态与正常者相近,但双侧不对称者多见。腰骶移行椎17例中11例出现不对称,正常椎30例中2例不对称,两者比较差异有统计学意义(P0.05)。V形槽形态在正常骶骨中不易见到,在腰骶移行椎中出现率为100%。结论腰骶移行椎后部横突变异,上关节突不对称率高,而V形槽出现恒定。以V形槽外缘3 mm处为纵线,以上关节突关节面下缘水平线为横线,其交点作为椎弓根螺钉进钉点。角度选择A角在6°左右、B角在43°左右,长度选择35~40 mm之间,临床应用收到理想疗效。     

腰骶移行椎椎弓根螺钉进钉方法的解剖学研究

目的 探讨腰骶移行椎椎弓根螺钉的进钉方法,为临床应用提供形态学依据.方法 共91具腰骶移行椎标本.生前资料完整者59具,男54具,女5具;平均年龄(41.2±16.7)岁,身高(162.4±7.3)cm.对椎弓根、上关节突、人字嵴、横突后部突起及"V"形槽进行观测,确定进钉点、进钉角度及深度.结果 按照Castellvi分型法,移行椎ⅡA型13具(左6具,右7具),ⅡB型10具.ⅢA型7具(左2具,右5具).ⅢB型60具,Ⅳ型1具.移行椎人字嵴均不明显,横突后结节突起与上关节突形成的"V"形槽均较明显.以移行椎上关节突关节面下缘水平线与"V"形槽最凹陷处交点作为移行椎椎弓根螺钉的进钉点,该点至椎弓根上缘距离,左(8.6±1.2)mm,右(8.3±1.2)mm;与矢状面呈20°时钉道至侧隐窝外侧壁距离,左(7.5±3.1)mm,右(7.2±2.7)mm;进钉角度,左21.3°±4.1°,右21.0°±4.6°;进钉深度,左(39.1±2.8)mm,右(39.1+2.7)mm.结论 腰骶移行椎后部横突变异,人字嵴结构不明显,以此作为进钉定位标志不可靠,而"V"形槽恒定存在,可以将上关节突关节面下缘的水平线与"V"形槽的最凹陷处的交点作为椎弓根螺钉的进钉点.     

漏斗技术结合徒手椎弓根探针技术在椎弓根置钉术中的应用

目的探讨利用漏斗技术结合徒手椎弓根探针技术置入椎弓根钉的安全性及准确性。方法术前测量目标椎的e角、f角及椎弓根直径,选择准确的进针点:骶椎以下关节突左7点右5点为进针点,腰椎以人字嵴顶点作为进针点,胸椎按Kim推荐的入点进针。在椎弓根入口去皮质后据e、f角徒手推进椎弓根探针至椎体前缘,测量所需螺钉长度,再沿钉道扩孔、攻丝及置入相应直径与长度的螺钉;术后CT片按照椎弓根内侧壁穿透<2 mm,外侧壁穿透<4 mm作为安全评价标准。回顾性分析自2009年6月~2012年1月利用该技术在184例中置入736枚椎弓根钉的情况。结果所有螺钉椎弓根内侧壁穿透<2 mm,外侧壁穿透<4 mm,总合格率100%;无一例出现神经根或脊髓损伤加重的表现,无一例出现脑脊液漏。结论利用合理的进针点,在个体化置钉的基础上,利用漏斗技术结合徒手椎弓根探针技术能成功置入胸腰椎椎弓根钉。     

骶骨后路钉板固定的解剖学研究

目的 探讨骶骨横行或斜行骨折时钉板固定的安全性.方法 30具新鲜成人尸体标本,男15具,女15具;年龄38～82岁,平均65岁.在骶骨上做标记:O点为骶骨螺钉进钉点(O_1点位于关节突关节的外下缘,O_2和O_3点位于经相邻骶后孔连线中点的水平线与骶外侧嵴的交点,O_4点位于S_3、S_4骶后孔连线的中点至骶外侧嵴水平线的中点),D_1为骶骨内侧置钉钉道长度,D_2为骶骨垂直置钉钉道长度,D_3为骶骨外侧置钉钉道长度,α为骶骨横截面上内侧置钉最大偏移角度,β为骶骨横截面上外侧置钉最大偏移角度.结果 S_1的D_1、D_2、D_3、α,β值分别为(38.73±5.29)mm、(31.15±4.68)mm、(41.47±6.18)mm、31.88°±4.32°、34.31°±5.12°;S_2的D_1、D_2、D_3、α,β值分别为(28.53±2.19)mm、(23.95±2.59)min、(34.69±5.13)mm、14.70°±3.54°、20.48°±5.35°;S_3的D_1、D_2、D_3、α,β值分别为(30.62±3.15)mm、(17.18±3.51)mm、(24.66±5.13)mm、52.00°±7.55°、39.00°±5.50°;S_4的D_1、D_2、D_3、α,β值分别为(23.21±4.14)mm、(9.07±2.20)mm、(14.52±3.51)mm、18.47°±4.42°、10.23°±2.69°.结论 S_1螺钉在O_1点进钉,平行于L_5棘突是理想的置钉路径.S_2-S_4理想的置钉路径:S_2螺钉在O_2点进钉,横截面上偏向外侧小于20.48°±5.35°;S_3螺钉在O_3点进钉,横截面上偏向内侧小于52.00°±7.55°;S_4螺钉在O_4点进钉,横截面上偏向内侧小于18.47°±4.42°.     

枢椎椎弓根螺钉进钉点的解剖定位研究

目的研究枢椎下关节突与枢椎椎弓根的位置关系,建立以枢椎下关节突为解剖标志的枢椎椎弓根螺钉进钉定位技术。方法取50套成人干燥枢椎标本,测量枢椎椎弓根的内、外缘和枢椎下关节突的内缘、中点、外缘分别与正中矢状线的垂直距离,以及枢椎椎弓根的宽度与高度。通过分析测量值间的关系,建立枢椎椎弓根螺钉的进钉定位技术。结果枢椎下关节突内缘在枢椎椎弓根内缘的外侧(3.67±0.41)mm处,枢椎下关节突中点在枢椎椎弓根外缘的外侧(1.15±0.44)mm处。建立了两种以枢椎下关节突为标志的进钉点定位方法,进钉点A位于枢椎下关节突内上象限,即中心点内、上各2mm处;进钉点B位于经枢椎下关节突内缘的纵垂线与枢椎下关节突中上1/4水平线的交点。结论枢椎下关节突与枢椎椎弓根间存在较恒定的解剖位置关系,枢椎下关节突可作为术中判断枢椎椎弓根位置和确定枢椎椎弓根螺钉进钉点的简易解剖学标志。     

中上颈椎侧块与寰椎椎弓根位置关系的解剖研究

目的研究中上颈椎侧块与寰椎椎弓根的位置关系,建立以中上颈椎侧块为解剖标志的寰椎椎弓根螺钉进钉点定位技术。方法20具尸体标本,分别测量寰椎椎弓根和中上颈椎侧块的内缘、中点、外缘与正中矢状面的垂直距离,通过分析测量值间的关系,建立寰椎椎弓根螺钉进钉定位技术。结果C2-4侧块的内缘分别在寰椎椎弓根内缘外侧0．37mm、0．27mm、0．24mm处;C2-4侧块的中点分别在寰椎椎弓根中点外侧1．18mm、1．41mm、1．74mm处;C2-4侧块的外缘分别在寰椎椎弓根外缘外侧1．96mm、2．54mm、3．24mm处。结论中上颈椎侧块与寰椎椎弓根间存在较恒定的解剖位置关系,C3和C4侧块与枢椎侧块一样,可作为术中确定寰椎后弓显露范围和判断寰椎椎弓根螺钉进钉点的解剖学标志。     

An anatomic study on the placement of the second sacral screw and its clinical applications

Background

The fixation of lumbosacral and sacral pelvis can be performed on the ilium and the Second Sacrum Vertebrae (S2). Although several studies on the anatomical and biomechanical features of S2 screw fixation have been published, little clinical application has been reported, especially combination of anatomical investigation and clinical study. This study was performed to design and optimize the method of pedicle screw placement for S2.

Materials and methods

Fifteen adult dry sacrum specimens were prepared and truncated from the S1–S2 and S2–S3 vertebral fusion remnants, and the morphology of the S2 vertebral body was observed from this section. The intersection of the horizontal line through the lowest point of the inferior edge of the first posterior sacral foramen and the lateral sacral crest was the entry point (Point X). The screws were inserted anterolaterally or anteromedially at Point X in 10 cadavers, with all of the screws penetrating the sacrum. Finally, the S2 sacral screw fixation technique was applied to a total of 13 patients with lumbosacral lesions, and the clinical outcome was evaluated at a minimum follow-up of 1 year.

Results

Two S2 sacral screw placement methods were developed, i.e., the anterolateral and anteromedial insertions. Seven patients had complete preoperative, postoperative, and follow-up data. In all cases, the bilateral S2 screws were placed in good position and the fixation was firm. There was no surgical wound infection or internal fixation loosening. All the patients achieved partial bone graft healing, which was verified by computed tomography.

Conclusions

The intersection of the horizontal line through the lowest point of the inferior edge of the first posterior sacral foramen and the lateral sacral crest can be used as the entry point for S2 sacral screw fixation. The S2 pedicle screw fixation shows good clinical effectiveness and safety for stable reconstruction of lumbosacral lesions.     

经骶髂关节置入骶1椎弓根螺钉的解剖学和影像学测量

目的:通过S1椎弓根的解剖学和影像学测量,探讨经骶髂关节置入S1椎弓根螺钉的可行性。方法:测量16具尸体骨盆标本双侧S1椎弓根前后缘的高度、深度(S1椎弓根最狭窄处的宽度)、骶翼深度、骶翼高度。测量骨盆出口位X线片上S1椎弓根的高度,并与解剖学测量结果比较。在轴位CT上测量髂骨后缘到骶翼、S1椎弓根、S1椎弓根纵轴的距离、髂骨外板与骶椎前缘皮质的距离。观察S1椎弓根矢状切面,评估置入2枚经S1椎弓根骶髂螺钉的安全区。结果:S1椎弓根前、后缘的高度平均为30.2mm和26.1mm,椎弓根深度和骶翼深度平均为27.8mm和45.8mm,骶翼后部平均高度为28.7mm。骨盆出口位X线片上S1椎弓根的平均高度是20mm,小于解剖学测量结果(P<0.0001)。轴位CT上,S1椎弓根纵轴在髂骨外板投影点到髂骨后缘的距离平均为32.5mm,到坐骨大切迹最高点的距离平均为38.6mm,髂骨外板到S1椎体前缘皮质的距离平均为105.2mm。结论:置入1枚S1椎弓根螺钉是安全的,常规置入2枚椎弓根螺钉可能较困难。     

Anatomical and biomechanical analysis of sacral pedicle and lateral mass

Objective: To study the anatomical and biomechanical features of sacral pedicle and lateral mass so as to provide reference for clinical screw fixation technology of sacral pedicle and lateral mass.Methods: A total of 60 adult patients' spiral CT images of the sacrum and coccyx were selected randomly. The entry points of sacral pedicle and lateral mass screws were determined, and the screw trajectory was measured using the three dimensional reconstruction method. Meanwhile,the gross anatomy was scrutinized in 15 adult cadaver specimens to determine the sacral pedicle and lateral mass screw entry points. The length, width and angle of sacral pedicle and lateral mass screw trajectory were measured. Eight of 15cadaver specimens were selected to test the maximal extraction force of sacral pedicle and lateral mass screws. The clinical data of 15 cases treated by pedicle and lateral mass screw technology were collected and analyzed.Results: The diameter and length of S1-S5 sacral pedicle and lateral mass screw trajectory were regular, with about 20° inclination angle. The S1 pedicle screw entry point was located at the intersection point of the basal lateral part of articular process and median line of transverse process, and no significant difference was found for the maximal extraction force between pedicle and lateral mass screws (P＞0.05). The entry points of S2-S5 pedicle screws were located at the intersection point of the line connecting adjacent posterior sacral foramina and median line of the transverse process.The lateral mass screw entry point of S2-S5 was on the median side of intersection point between median line of the transverse process and lateral sacral crest. The maximal extraction force of pedicle screws was significantly greater than that of lateral mass screws (P＜0.05).Conclusion: Both the sacral pedicle and the lateral mass screw fixation techniques can offer effective fixation and reconstruction for fracture of the sacrum and coccyx,but pedicle screw fixation may be more convenient, safe and reliable than lateral mass screw fixation.     

Neurovascular risks of sacral screws with bicortical purchase: an anatomical study

The aim of this cadaver study is to define the anatomic structures on anterior sacrum, which are under the risk of injury during bicortical screw application to the S1 and S2 pedicles. Thirty formaldehyde-preserved human male cadavers were studied. Posterior midline incision was performed, and soft tissues and muscles were dissected from the posterior part of the lumbosacral region. A 6 mm pedicle screw was inserted between the superior facet of S1 and the S1 foramen. The entry point of the S2 pedicle screw was located between S1 and S2 foramina. S1 and S2 screws were placed on both right and the left sides of all cadavers. Then, all cadavers were turned into supine position. All abdominal and pelvic organs were moved away and carefully observed for any injury. The tips of the sacral screws were marked and the relations with the anatomic structures were defined. The position of the sacral screws relative to the middle and lateral sacral arteries and veins, and the sacral sympathetic trunk were measured. There was no injury to the visceral organs. In four cases, S1 screw tip was in direct contact with middle sacral artery. In two cases, S1 screw tip was in direct contact with middle sacral vein. It was observed that the S1 screw tips were in close proximity to sacral sympathetic trunk on both right and the left sides. The tip of the S2 screw was in contact with middle sacral artery on the left side only in one case. It is found that the tip of the S2 screw was closely located with the middle sacral vein in two cases. The tip of the S2 pedicle screw was in contact with the sacral sympathetic trunk in eight cases on the right side and seven cases on the left side. Lateral sacral vein was also observed to be disturbed by the S1 and S2 screws. As a conclusion, anterior cortical penetration during sacral screw insertion carries a risk of neurovascular injury. The risk of sacral sympathetic trunk and minor vascular structures together with the major neurovascular structures and viscera should be kept in mind.     

Location of the first and second sacral nerve roots in relation to pedicle screw placement

Dissection and measurements of the first 2 sacral nerve roots with regard to the commonly used entrance points for S1 and S2 pedicle screw placement were performed to determine the location of the first 2 sacral nerve roots in relation to the pedicle screw entrance points in the upper 2 sacral vertebrae. The sacral nerve roots, dural sac, and pedicles were exposed after laminectomy. The mean distance from the reference point to the adjacent nerve roots superiorly and inferiorly at the S2 pedicle level was smaller than those at the S1 pedicle level. The medial angle of the sacral nerve roots progressively decreased from L5 to S3. The nerve root passing through the next foramen formed an immediate medial relation to the sacral pedicle rather than the dural sac. Pedicle screw placement in the first 2 sacral vertebral pedicles has been recommended for lumbosacral fusion and internal fixation of sacral fractures. No anatomic study is available regarding the location of the sacral nerve roots relative to the entrance points of sacral pedicle screw placement. Violation of the sacral canal and foramina by a sacral pedicle screw may injure the sacral nerve roots, especially at the level of the S2 pedicle.     

Evaluation of the upper sacrum by three-dimensional computed tomography.

Axial and sagittal computed tomographic (CT) scans of 40 sacrum specimens were obtained. The measurements of the upper sacral canal and S1-2 and S2-3 anterior sacral foramina were performed on axial scans, and the evaluation of the upper sacral pedicle was based on sagittal scans. The results showed that there were statistically significant differences between male and female specimens in 3 of 29 measurements. In general, the measurements of male specimens were slightly larger than those of the female specimens, and the linear dimensions of the sacral canal, anterior foramina, and pedicle decreased from the S1 to S3. This study indicated that the critical area of the sacral pedicle for screw insertion lies in the junction between the pedicle and vertebral body. CT scans provide more accurate information about the essential sacral anatomy.     

Variation in bone mineral density of the sacrum in young adults and its significance for sacral fixation

STUDY DESIGN: Bone mineral density variations throughout the sacrum were measured and correlated with sacral screw insertion torque. OBJECTIVE: To quantify bone mineral density variations within the S1 body and ala of young human specimens, especially along the pathways of sacral screws, and to examine the relation between sacral screw fixation and bone mineral density. SUMMARY OF BACKGROUND DATA: Vertebral bone quality is an essential factor in anterior or posterior screw fixation of the spine. Several studies have been conducted regarding bone mineral density variations in the cervical and thoracolumbar spine. However, such variations in bone mineral density in the sacrum have not been well documented. METHODS: The bone mineral density of 13 sacral specimens from young male cadavers (mean age, 31 years) was measured using highly accurate quantitative computed tomography. Variations in bone mineral density were measured in five transverse layers and seven vertical columns within the S1 body, and in four transverse layers and six vertical columns within the ala. The sacral screw insertion torque was measured (unicortical and bicortical), and the correlation with bone mineral density was calculated. RESULTS: The mean bone mineral density of the S1 body was 381.9 +/- 59 mg/cm3, which was 31.9% higher than that of the sacral ala (mean, 296.9 +/- 86 mg/cm3) (P < 0.05). Bone mineral density of the superior sacral endplate was higher than that of any other transverse layer. Columns near the lateral posterior and lateral anterior of the S1 body had the highest bone mineral density. In the ala, bone mineral density values of the internal columns (pedicle) were the highest. Screw insertion torque for bicortical purchase along the S1 pedicle correlated well with the bone mineral density of the S1 body (r = 0.67, P < 0.05). CONCLUSION: This study quantified the volumetric bone mineral density variations within the S1 body and ala, and a significant linear correlation between the screw insertion torque and bone mineral density was found. Optimal sacral screw insertion pathways were also outlined based on bone mineral density values.     

Pedicle screw placement at the sacrum: anatomical characterization and limitations at S1.

Anatomical and biomechanical data have suggested that pedicle screw fixation at the sacrum is optimum in the anteromedial direction into the S1 vertebral body, yet the possibility of posterior iliac crest interference with this screw pathway has been considered but not defined. This study aimed to determine if the anteromedial direction of screw placement into the vertebral body is possible in all cases at S1 and to assess the limiting effect of the posterior iliac crest. Computed tomography scans of the upper sacrum at the S1 pedicle parallel to the sacral endplate were examined in 100 patients. Analysis using a digitizer allowed characterization of an ideal screw pathway with variable screw and screw head diameters in an anteromedial direction into the S vertebral body. The effects of the posterior iliac crest upon these pathways were studied. The study demonstrated that anteromedial placement with bicortical fixation at the vertebral body was theoretically possible in almost all (98.5%) cases. Because the sacral body is often wider than the sacral spinal canal, a straight-ahead screw direction will often achieve placement into the S1 vertebral body, if the starting point for the screw allows screw placement adjacent to the medial border of the S1 pedicle with only 1.5 mm of cortical bone separating the canal and the screw. The space between the posterior iliac crest and the lateral aspect of the screw corridor ranges from a maximum of 52.4 mm to a minimum of 12.8, 6.2, and 0 mm for the 7-, 10-, and 12.5-mm screw corridors. On only three occasions (1.5%) was the ideal screw corridor not possible because of posterior iliac crest overlap. In each case, this occurred only unilaterally and when the widest of the screw corridors (12.5 mm) was used. Both the distance between the posterior iliac crests and the space available for optimum screw placement are greater in females than males.