青少年脊柱侧凸患者胸椎椎弓根螺钉置入的准确性和安全性评价

目的：探讨青少年脊柱侧凸患者胸椎椎弓根螺钉置入的准确性和安全性，以减少相关手术并发症。方法：32例青少年脊柱侧凸患者术前均对畸形脊柱进行标准俯卧位CT加密扫描，测量进钉点至椎体前缘的深度、进针角度、椎弓根直径和椎体的旋转角度，根据测得数据确定椎弓根螺钉置入的深度和方向，置入螺钉后再行脊柱全长X线片及CT扫描评价置钉的准确性和安全性。结果：32例共置入226枚胸椎椎弓根螺钉，术后CT加密和X线片观察到205枚螺钉（90．7％）完全在椎弓根皮质骨内。10例21枚螺钉（9.3％）发生错置，7枚螺钉（3．1％）偏外，5枚螺钉（2．2％）偏前外侧（其中2枚螺钉靠近节段血管），4枚螺钉（1．8％）偏下，4枚螺钉（1．8％）直径过大导致椎弓根内壁膨胀内移，1枚螺钉（0．4％）误入椎管导致完全性脊髓损伤。T1～T4错置12枚（18．2％），T5～T12错置9枚（6．1％）；凸侧椎根螺钉置入的准确率为93．8％，凹侧为83．1％。结论：脊柱畸形患者术前应常规采用标准俯卧位CT加密扫描，根据扫描图像测得的相关数据可为术中准确置入椎弓根螺钉提供重要参考依据。在青少年脊柱侧凸患者胸椎椎弓根螺钉置入有一定的误置率，螺钉发生错置多见于上胸椎和凹侧．术中应高度重视。     

个体化选择脊柱侧凸患者胸椎椎弓根螺钉进钉点的研究

目的:探讨个体化选择脊柱侧凸患者胸椎椎弓根螺钉进钉点对置钉准确性的影响.方法:2006年3月至2008年6月手术治疗脊柱侧凸患者57例,其中青少年特发性脊柱侧凸44例,先天性脊柱侧凸12例,马凡综合征1例.根据患者术前CT设计拟固定胸椎的椎弓根螺钉进钉点并用于指导术中的进钉点选择,术后根据螺钉是否突破椎弓根的皮质壁来判断置钉准确性.结果:全部患者共置入椎弓根螺钉591枚,胸椎417枚,腰椎174枚,术后530枚螺钉的轴线完全位于椎弓根皮质内,准确率为89.7%, 其中胸椎置钉准确率为86.8%(362,417).61枚螺钉的轴线突破椎弓根皮质壁,胸椎55枚,腰椎6枚.55枚偏置的胸椎椎弓根螺钉中52枚螺钉的实际进钉点与术前设计一致,其中19枚钉尖位于椎体内;3枚螺钉为术中实际进钉点选择失误,螺钉轴线突破椎弓根皮质壁的距离均不超过4mm.无脊髓、大血管及脏器损伤等严重并发症发生.结论:个体化选择胸椎椎弓根螺钉进钉点可提高脊柱侧凸患者胸椎置钉的准确率,减少术中进钉点选择失误所致的并发症.     

特发性脊柱侧凸患者胸椎椎弓根的CT测量及其临床意义

目的：测量特发性脊柱侧凸患者胸椎椎弓根的有关数据，探讨其临床应用价值。方法：在30例特发性脊柱侧凸患者术前CT扫描片上测量胸椎椎弓根的宽度、深度、角度、椎体旋转角度等数据，根据所得数据选定置入螺钉的直径、长度．确定置入方向和深度。术后对置入螺钉的胸椎椎弓根节段行CT扫描，判断置钉位置。结果：CT测量的各项数据显示胸椎椎弓根适合椎弓根螺钉的置入。以此为依据术中置入胸椎弓根螺钉共245枚，228枚(93％)置入无误，6枚穿破椎弓根外壁，9枚穿破椎弓根下壁，2枚穿破椎弓根内壁，无神经系统并发症。结论：术前CT扫描测量特发性脊柱侧凸患者的胸椎椎弓根的有关数据可为选择适当长度和直径的螺钉并将其准确置入胸椎椎弓根内提供参考。从而保证螺钉安全置入。     

个体化导航模板在胸椎椎弓根螺钉置入中的初步临床应用

目的:通过临床应用评价个体化导航模板辅助胸椎椎弓根螺钉置入的准确性和安全性。方法:2008年7月～2009年9月,对11例需要行胸椎椎弓根螺钉置入手术的患者(青少年特发性脊柱侧凸7例,先天性脊柱侧凸2例,胸椎结核后凸畸形1例,多发性胸椎骨折1例)术前根据CT三维重建图像利用计算机辅助设计及快速成型技术设计制作46个胸椎个体化导航模板,术中应用个体化导航模板辅助在T2～T12置入椎弓根螺钉92枚,术后CT扫描评价螺钉位置,记录有无与螺钉置入相关的并发症。结果:通过个体化导航模板辅助置入的92枚胸椎椎弓根螺钉中,83枚完全在椎弓根内,9枚穿破椎弓根壁(其中椎弓根内侧壁穿破2枚、椎弓根外侧壁穿破7枚),其中5枚螺钉因椎弓根宽度小于4mm(3.0～3.8mm)而采用椎弓根旁固定方法(椎弓根螺钉轻度穿破椎弓根外侧壁经胸肋关节内侧进入椎体),椎弓根壁非故意穿破率为4.3%,置钉准确率为95.7%,所有穿破椎弓根壁的螺钉的穿出距离均小于2mm,螺钉位置可接受率为100%。无与螺钉置入有关的神经、血管、内脏损伤等并发症的发生。结论:个体化导航模板辅助胸椎椎弓根螺钉置入的置钉准确率高,安全、可行。     

胸椎椎弓根螺钉置入治疗胸椎骨折的准确性和安全性评价

目的探讨胸椎椎弓根螺钉置入技术治疗胸椎骨折的准确性和安全性。方法50例胸椎骨折患者术前均行脊柱标准俯卧位CT加密扫描,测量进针点、入钉点至椎体前缘的深度、进针角度和直径,根据测得数据确定椎弓根螺钉置入的深度和方向,术后再行脊柱X线片及CT加密扫描评价置钉的准确性和安全性。结果50例患者共置入240枚胸椎椎弓根螺钉,术后CT加密扫描和X线片观察到220枚（91．7％）螺钉完全在椎弓根皮质骨内;20枚（8．3％）螺钉发生错置,其中7枚（2．9％）螺钉偏外;5枚（2．1％）螺钉偏前外侧,有2枚（0．8％）螺钉靠近主动脉;3枚（1．3％）螺钉偏下;3枚（1．3％）螺钉直径过大导致椎弓根内壁膨胀内移;2枚（0．8％）螺钉误入椎管内。螺钉完全在椎弓根皮质内的百分比在不同的胸椎节段之间有显著性差异。结论术前CT扫描测量胸椎骨折患者椎弓根的有关数据可为术中准确置入螺钉提供重要参考依据。术中标准的X线透视指导和解剖标记定位是保证胸椎椎弓根螺钉准确置入的关键因素。术后CT加密扫描能准确地反映椎弓根螺钉位置偏差,并能反映椎弓根螺钉与相邻结构的位置和关系。螺钉发生错置多见于上胸椎。     

胸椎椎弓根螺钉在脊柱侧凸矫形术中的应用效果评价

[目的]评价胸椎椎弓根螺钉在脊柱侧凸矫形术中的应用效果。[方法]2008～2010年采用后路胸椎椎弓根螺钉技术治疗特发性和先天性脊柱侧凸患者26例。根据术后临床表现、矫正率和术后X线片判定胸椎椎弓根螺钉置钉情况,对胸椎椎弓根螺钉在脊柱侧凸矫形术中的应用效果进行评价。[结果]本组病例术后平均矫正率为63.81%,术后患者脊柱长度平均增加6.2 cm;术后X线片判定置入的308枚胸椎椎弓根螺钉中,置钉不良率为16.9%;所有患者术后均无胸部脏器及神经系统损伤表现。[结论]在脊柱侧凸临床治疗中,采用胸椎椎弓根螺钉进行侧凸矫形是有效、安全的方式。     

非影像监视下行脊柱侧凸胸椎椎弓根螺钉置入的临床应用

目的:探讨脊柱侧凸胸椎椎弓根螺钉非影像监视下徒手置入的方法及可行性。方法:57例脊柱侧凸患者行后路椎弓根螺钉系统矫形手术,徒手法置入胸椎椎弓根螺钉。术后常规拍摄脊柱全长X线片,随机选取10例患者行CT扫描观察,了解螺钉置入的准确性。结果:共置入胸椎椎弓根螺钉362枚。术后X线片观察到10枚螺钉偏外,4枚螺钉偏下,其中2枚螺钉引起轻微肋间神经痛,3周后完全缓解。CT观察47枚螺钉有2枚螺钉导致椎弓根内壁膨胀内移,没有相应神经症状。主弯Cobb角术前平均60.4°(32°～121°),术后平均18.3°(1°～70°),平均矫正率71.9%(38.1%～98.0%)。结论:徒手法置入脊柱侧凸胸椎椎弓根螺钉是可行的。     

徒手胸椎椎弓根螺钉置入技术治疗青少年特发性脊柱侧凸的安全性评价

目的 评价徒手胸椎椎弓根螺钉置入技术治疗青少年特发性脊柱侧凸的安全性。方法从2002年7月～2004年6月对38例青少年特发性脊柱侧凸患者，徒手应用胸椎椎弓根螺钉进行后路矫形内固定，所有患者术中进行神经电生理监测及X线透视确认，术后进行X线成像、CAT扫描评估螺钉位置，并对其中35例进行随访，从而评价本技术的安全性。结果共置入胸椎椎弓根螺钉326个，每一水平置入的螺钉数如下：T1,n=2；T2，n=10;T3，n=19；T4，n=27；L，n=28；T6，n=24；T7，n=23；TB，n=25；T9，n=29；T10,n=34；T11,n=48；T12，n=57。通过胸椎CT扫描评价326枚置入畸形胸椎的螺钉位置。共有19枚螺钉（5．8％）有中等程度的皮质穿破，即螺钉的中线在椎弓根壁皮质之外，其中6枚螺钉（1．8％）穿破椎弓根内侧壁。对35例患者进行术后跟踪随访，平均随访时间2年，未发现任何与置入的胸椎椎弓根螺钉（全部326枚螺钉）相关的神经、血管或内脏并发症。结论遵循严格步骤，逐步置入胸椎椎弓根螺钉的徒手技术在治疗青少年特发性脊柱侧凸中具有可靠的安全性。     

对徒手置入胸椎椎弓根螺钉的安全性评价

目的：评价徒手置入胸椎椎弓根螺钉的安全性并探讨其置钉方法.方法：372例患者采用徒手方法置入胸椎椎弓根螺钉,记录置入操作中和术后并发症,其中37例患者术后行CT断层扫描检查判断螺钉的位置,记录所有穿透骨皮质螺钉的数目和距离.结果：共徒手置入胸椎椎弓根螺钉2261枚,平均每例患者置入螺钉6.08枚,术中6例次置钉过程中出现脑脊液从钉道中流出,术中和术后未出现神经、血管和内脏损伤等并发症.37例患者术后行CT扫描判断螺钉位置,405枚螺钉中124枚（30.62%）穿透骨皮质,1枚（0.02%）穿透椎弓根内侧壁超过4mm.结论：徒手置入胸椎椎弓根螺钉穿透骨皮质的发生率较高,应该根据每个椎体旋转、倾斜等差异个体化确定置钉位置和方向,操作仔细认真,保证准确、安全、可靠地置入胸椎椎弓根螺钉.     

低龄儿童脊柱侧凸矫正术中椎弓根螺钉置入的精确性和安全性评估

目的:评估10岁及以下脊柱侧凸患儿侧凸矫正术中椎弓根螺钉置入的精确性及安全性,并分析其相关影响因素。方法:回顾性分析2008年2月~2008年7月我院收治的行后路椎弓根螺钉固定的10岁及以下脊柱侧凸41例患儿的临床资料,所有患者术前、术后均行CT检查,男26例,女15例,年龄2~10岁,平均5.4岁。先天性脊柱侧凸36例,特发性脊柱侧凸2例,神经肌肉源性脊柱侧凸2例,先天性软骨发育不全伴脊柱侧凸1例。术中根据解剖标志徒手置入椎弓根螺钉。在PACS系统上通过Pacs Client软件测量螺钉尖距椎弓根内壁、外壁、上壁、下壁以及椎体前缘的距离。若左侧椎弓根螺钉穿破椎弓根外壁或椎体前缘,测量钉尖与主动脉的距离。根据椎弓根螺钉所在位置(节段、凹凸侧、脊椎发育是否异常)分析其破壁率差别。不良置钉定义为椎弓根螺钉穿破椎弓根内、外壁或椎体前缘的距离超过2mm,和椎弓根螺钉进入椎间孔或穿破终板进入椎间盘。结果:本组病例共置入242枚椎弓根螺钉,胸椎128枚,腰椎114枚,平均每例患者置入5.8枚螺钉。螺钉完全在椎弓根内208枚,占86.0%。破壁34枚(占14.0%),其中不良置钉18枚(占7.4%),18枚中有5枚穿破外壁,8枚穿破内壁,5枚穿破椎体前缘。形态异常椎和凹侧的椎弓根螺钉的破壁率较高(分别为24.1%和17.9%)。术中一枚螺钉拔出,未出现其他螺钉置入相关并发症。穿破椎体前缘螺钉距离主动脉距离平均2.3mm。结论:10岁及以下儿童椎弓根螺钉的徒手置入有较高的精确性和安全性,但在发育不良椎体及凹侧置钉时应谨慎。     

Pedicle screw diameter selection for safe insertion in the thoracic spine

Objective

Many thoracic pedicles are too small for the safe acceptance of a transpedicular screw. However, few studies have so far reported on the methods to select a proper pedicle screw size and to confirm the morphologic changes for such a small thoracic spine pedicle. The objective of this work was to determine the potential limits of a pedicle screw diameter for transpedicular screw placement in the thoracic spine.

Methods

T2–T9 vertebrae from eleven patients that underwent posterior thoracic instrumentation with the use of fluoroscopically assisted insertion method were analyzed. The outcome measures were the pedicle widths, the gap between the outer pedicle width and the selected pedicle screw diameter, and the penetration length of the pedicle screws using computed tomography. The screws were distributed into two groups according to the pedicle width and screw diameter, and the screw perforation rate of the two groups was compared. The relationships of the gap and the distance of the screw penetration were compared and investigated in regard to the pedicle screw diameter selection.

Results

A total of 16 screws demonstrated a smaller diameter than the inner pedicle widths, while 22 screws had a larger diameter than the inner pedicle widths. One screw (6.3%) perforated the pedicle cortex in the smaller screw group, and twelve screws (54.5%) perforated the pedicle cortex in the larger screw group (P?=?0.006). A linear regression analysis in the larger screw group revealed that when the gap was less than 0.5?mm, a risk of a pedicle wall violation was observed.

Conclusions

When the screws with a larger diameter than the inner pedicle width are selected, the screw perforation rate increases. Therefore, the size of the screw diameter must be at least 0.5?mm less than the outer pedicle width to ensure safe transpedicular screw placement.     

Anatomic analysis of pedicle cortical and cancellous diameter as related to screw size

The effective thoracic and lumbar pedicle diameter as related to screw size for that pedicle was studied in six fresh-frozen human cadaver spines. Measurements of the pedicle were obtained before screw insertion using axial and coronal reformatted computed tomographic (CT) images, as well as graduated sounding of the pedicle. After sequentially loading each pedicle with increasingly larger screws, measurements were taken of the outer cortical diameters. Plastic deformation of the pedicle preceded pedicle fracture or cutout when the screw thread diameter became larger than the endosteal diameter or within 80% of the outer cortical diameter as measured from the CT scan. Pedicle screws did not obtain cortical purchase within the pedicle.     

Thoracic pedicle screw placement guided by computed tomographic measurements.

Cadaveric pedicle screw placement guided by the measurements from axial computed tomography (CT) scans in the thoracic spine was assessed in this study. Axial CT scans were performed on four cadaveric thoracic spines, and the measurements included the pedicle transverse angle, inner pedicle width, and distance between the midline of the vertebra and the pedicle axis on the dorsal aspect of the lamina. With utilization of the data from CT scans, screws were directly placed into the thoracic pedicle from T1 to T10. Screw penetration of the pedicle was determined by gross examination. The results showed that the largest pedicle transverse angle was found at the levels of T1-2, and the smallest occurred at the T3 through T8 levels. The value of the pedicle inner width was quite different between specimens with a minimum of 3.0 mm at T4 and a maximum of 9.2 mm at T10. Gross examination of the pedicle showed that 13 (16.3%) of 80 screws penetrated the pedicle wall, with a Grade I penetration in 11 pedicles and a Grade II penetration in 2 pedicles. Screw penetration of the medial wall was found in four pedicles and penetration of the lateral wall was noted in nine pedicles. No screw penetration of the superior and inferior walls of the pedicle was identified in any of the four specimens. Thoracic pedicle screw placement guided by the measurements from axial CT scans significantly reduced the incidence of pedicle penetration. Axial CT measurements of the pedicle inner diameter and transverse angle as well as the starting point for screw insertion are recommended if pedicle screw fixation is intended in the thoracic spine.     

Thoracic pedicle: surgical anatomic evaluation and relations

This anatomic study investigated the thoracic pedicle and its relations. The objective was to emphasize the importance of the thoracic pedicle for transpedicular screw fixation to avoid complications during surgery. Twenty cadavers were used to observe the cervical pedicle and its relations. The isthmus of the pedicle was exposed after removal of whole-posterior bony elements, including spinous processes, laminas, lateral masses, and the inferior and superior facets. The pedicle width and height, interpedicular distance, pedicle-inferior nerve root distance, pedicle-superior nerve root distance, pedicle-dural sac distance, root exit angle, and nerve root diameter were measured. There was no distance between the pedicle and dural sac in eight specimens. There was, however, a short distance in 12 remaining specimens in the upper and lower thoracic regions. The distances between the thoracic pedicle and the adjacent nerve roots ranged from 1.5 to 6.7 mm and 0.8 to 6.0 mm superiorly and inferiorly at all levels. The mean pedicle height and width at T1-T12 ranged from 2.9 to 11.4 mm and 6.2 to 21.3 mm, respectively. The interpedicular distance decreased gradually from T1 to T5 and then increased gradually to T12. The mean root exit angle decreased consistently from 104 degrees to 60 degrees. The nerve root diameter was between 2.3 and 2.5 mm at the T1-T5 level and then increased consistently from 2.5 to 3.7 mm. All significant differences were noted at p < 0.05 and p < 0.01. The following suggestions are made based on these results. 1) More care should be taken when a transpedicular screw is placed in the horizontal plane. 2) Improper medial placement of the pedicle screw, especially in the middle thoracic spine, should be avoided, and the anatomic variations between individuals should be considered. 3) Because of substantial variations in the size of thoracic pedicles, utmost attention should be given to the findings of a computed tomographic evaluation before thoracic transpedicular fixation is begun.     

3D打印技术辅助中上胸椎椎弓根置钉的准确性分析

目的评价3D打印技术在中上胸椎"分水岭"区域损伤(T3~7)椎弓根置钉时的应用价值。方法回顾性分析自2013-03—2016-02手术治疗的7例T3~7骨折或脱位。对患者进行薄层CT扫描,采用3D打印技术打印出中上胸椎实体模型,在模型上标定需固定节段的椎弓根钉进钉点(Magerl法),测量每一个节段螺钉的直径、长度、内倾角、头倾(尾倾)角,然后选择合适的椎弓根钉模拟置钉。结果本组共置入56枚椎弓根钉,按目前通行的评估椎弓根钉准确性的方法,在CT平扫的影像上对椎弓根穿透的程度进行分级:0级33枚,1级18枚,2级4枚(穿破外侧壁),3级1枚(穿破外侧壁,未造成不良后果),准确率91.1%。结论对于中上胸椎椎弓根置钉困难的患者,尤其是需要长节段置钉时,可以在3D打印的实体模型上模拟置钉,选择安全、有效的椎弓根钉,提高徒手置钉准确率,缩短了年轻医师的学习曲线。     

Pullout strength of misplaced pedicle screws in the thoracic and lumbar vertebrae - A cadaveric study

Background:

The objective of this cadaveric study was to analyze the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of lower thoracic and lumbar pedicle screws. We also investigated the effect of bone mineral density (BMD), diameter of pedicle screws, and the region of spine on the pullout strength of pedicle screws.

Materials and Methods:

Sixty fresh human cadaveric vertebrae (D10–L2) were harvested. Dual-energy X-ray absorptiometry (DEXA) scan of vertebrae was done for BMD. Titanium pedicle screws of different diameters (5.2 and 6.2 mm) were inserted in the thoracic and lumbar segments after dividing the specimens into three groups: a) standard pedicle screw (no cortical perforation); b) screw with medial cortical perforation; and c) screw with lateral cortical perforation. Finally, pullout load of pedicle screws was recorded using INSTRON Universal Testing Machine.

Results:

Compared with standard placement, medially misplaced screws had 9.4% greater mean pullout strength and laterally misplaced screws had 47.3% lesser mean pullout strength. The pullout strength of the 6.2 mm pedicle screws was 33% greater than that of the 5.2 mm pedicle screws. The pullout load of pedicle screws in lumbar vertebra was 13.9% greater than that in the thoracic vertebra (P = 0.105), but it was not statistically significant. There was no significant difference between pullout loads of vertebra with different BMD (P = 0.901).

Conclusion:

The mean pullout strength was less with lateral misplaced pedicle screws while medial misplaced pedicle screw had more pullout strength. The pullout load of 6.2 mm screws was greater than that of 5.2 mm pedicle screws. No significant correlation was found between bone mineral densities and the pullout strength of vertebra. Similarly, the pullout load of screw placed in thoracic and lumbar vertebrae was not significantly different.     

Computer-assisted thoracic pedicle screw placement: an in vitro feasibility study

STUDY DESIGN: In this cadaveric study, a computer-assisted image guidance system was tested for accuracy of thoracic pedicle screw placement. OBJECTIVES: Evaluate the system's accuracy for thoracic pedicle screw placement in vitro. SUMMARY OF BACKGROUND DATA: The effective use and reliability of pedicle screw instrumentation in providing short-segment stabilization and correction of deformity is well known in the lumbar spine. Pedicle screw placement in the thoracic spine is difficult because of the small dimensions of the thoracic pedicles and risk to the adjacent spinal cord and neurovascular structures. Investigators have shown the improved accuracy of computer-assisted lumbar pedicle screw placement; but the accuracy of computer-assisted thoracic pedicle screw placement, which is becoming more widely used, has not been shown. METHODS: In five human cadavers, 120 thoracic pedicle screws were placed with computer-assisted image guidance. The largest clinically feasible screw was used based on the cross-sectional dimensions of each pedicle. The accuracy was assessed by postoperative computed tomography and visual inspection. RESULTS: The overall pedicle cortex violation was 23 of 120 pedicles (19.2%). Nine violations (7.5%) were graded as major and 14 (11.7%) as minor. A marked and progressive learning curve was evident with the perforation rates that decreased from 37.5% in the first cadaver to 4.2% in the last two cadavers. CONCLUSIONS: Accurate thoracic pedicle screw placement is feasible with computer-assisted surgery. However, as with any other new surgical technology, the learning curve must be recognized and incorporated into the necessary fundamental knowledge and experience for these procedures.     

The USS pedicle hook system: a morphometric analysis of its safety in the thoracic spine. Universal Spine System.

The Universal Spine System (USS) pedicle hook design includes a fixation screw that passes obliquely in the anterocranial direction in the pedicle. The addition of the fixation screw was to address concerns with rotation of the hook and hook disengagement. This study was designed to evaluate the safety of the USS screw locked pedicle hook. Eleven cadaveric thoracic spines were instrumented posteriorly with USS pedicle hooks from T1 to T12. Spinal instrumentation was performed by a spinal surgeon experienced with the USS system. Spinal deformity was created prior to instrumentation, ranging from 0 to 55 degrees in the horizontal plane (rotation) and from 0 to 50 degrees in the frontal plane (scoliosis). Radiographs, computed tomography (CT), and segmental dissection were used for data acquisition. Morphometric CT analysis before instrumentation demonstrated that the transverse pedicular diameter was the smallest at T5 with a mean of 3.7 mm. The transverse pedicular angle (TPA) was found to always point toward the midline. The largest TPA was observed at T1 with a mean TPA of 28.4 degrees. The pedicle with the least angular deviation from the midline was T11 with a mean TPA of 7 degrees. Postinstrumentation CT analysis and segmental dissection revealed perforations of the pedicle cortex by the fixation screw in 15% of instrumented pedicles (26/172). There were 6 medial and 20 lateral perforations. Medial perforations occurred exclusively in the three most proximal spinal segments, whereas the lateral perforations occurred throughout the thoracic spine. The mean encroachment of the fixation screw was 1.67 mm medially and 1.95 mm laterally. This study demonstrates the variation in caliber and direction of the thoracic pedicles. Medial and lateral perforations of the pedicle can occur with the USS pedicle hook instrumented system.