Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine

While the biomechanical properties of pedicle screws have proven to be superior in the lumbar spine, little is known concerning pullout strength of pedicle screws in comparison to hooks in the thoracic spine. In vitro biomechanical pullout testing was performed to evaluate the axial pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine with regard to surgical correction techniques in scoliosis. Nine human cadaveric thoracic spines were harvested and disarticulated. To simulate a typical posterior segmental scoliosis instrumentation, standard pedicle hooks were used between T4 and T8 and supralaminar hooks between T9 and T12 and tested against pedicle screws. The pedicle screws were loaded strictly longitudinal to their axis; the hooks were loaded perpendicular to the intended rod direction. In total, 90 pullout tests were performed. Average pullout strength of the pedicle screws was significantly higher than in the hook group (T4-T8: 531 N versus 321 N, T9-T12: 807 N versus 600 N, p < 0.05). Both screw diameter and the bone mineral density (BMD) had significant influence on the pullout strength in the screw group. For scoliosis correction, pedicle screws might be beneficial, especially for rigid thoracic curves, since they are significantly more resistant to axial pullout than both pedicle and laminar hooks.     

胸椎经关节突关节椎弓根螺钉固定的可行性研究

 目的明确胸椎后路经关节突关节椎弓根螺钉固定的解剖学可行性和技术参数.为临床应用提供参考。方法取 20具胸椎标本.仔细解剖胸椎的后侧和前侧方.以清楚地暴露胸椎椎板和椎弓根。以椎板下缘向上、外缘向内各 7 mm为进钉点.在 T_1.2、T_5.6、T_9.10直视下置入经关节突关节椎弓根螺钉.通过直接的置钉和 CT重建.观察胸椎后路经关节突关节椎弓根螺钉实际置钉的可行性.测量经关节突关节椎弓根螺钉内固定进钉角度和钉道长度。结果所有胸椎后路经关节突关节椎弓根螺钉均由上位胸椎下关节突经关节突关节.进入下位胸椎的椎弓根.成功置入下位胸椎的椎体内。重建 CT测量发现螺钉在横断面的外倾角度为 2.1°±0.7°.在矢状面的尾倾角度为 41.4°±3.2°.在各节段间略有不同.但差异无统计学意义。平均的螺钉钉道长度为(40.6±4.9) mm.钉道长度由上胸椎向中、下胸椎呈逐渐增加趋势.差异有统计学意义(F=74.09, P<0.01)。结论胸椎后路经关节突关节椎弓根螺钉具有解剖学可行性.可以作为胸椎椎弓根螺钉固定的一种补充内固定方法.但置钉时要求较高的准确性。     

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.     

Accuracy and safety of thoracic pedicle screw placement in spinal deformities

OBJECTIVES: To determine the safety of pedicle screw fixation in thoracic spine deformity correction. METHODS: One hundred twelve pedicle screws were surgically placed in 25 patients with degenerative, posttraumatic, and Scheuermann kyphosis and idiopathic and neuromuscular scoliosis. Screw position was evaluated using intraoperative and postoperative radiographs and thin-slice computed tomography. RESULTS: Of the total 112 thoracic pedicle screws that were inserted, 98 screws (87.5%) were fully contained within the cortical boundaries of the pedicle. When comparing proximal screws (T1-T8) with distal screws (T9-T12) and convex placed screws with concave ones, a statistically significant difference in screw placement was evident (P < 0.05). More misplaced screws were seen proximally and on the concave side. Of the 14 malpositioned screws, 2 (1.8%) demonstrated aortic abutment. There were no neurologic deficits, vascular injuries, or mechanical failures recorded. CONCLUSIONS: Placement of thoracic pedicle screws is both feasible and safe.     

上胸椎椎弓根螺钉固定并发症的探讨

目的分析上胸椎椎弓根螺钉固定的并发症,总结其手术技巧和经验。方法回顾性分析2009年4月至2012年4月采用T1~4椎弓根螺钉技术治疗的各类上胸椎损伤68例(共384枚螺钉),上胸椎骨折/脱位45例,均行Ⅰ期后路切开复位内固定术;结核合并后凸畸形23例,均行前路病灶清除植骨融合内固定术+后路矫形术。所有患者结合术中胸椎椎弓根四壁探查、术后手术节段椎体X线片、CT扫描,观察螺钉在椎弓根内的位置、角度及与椎弓根壁的关系和距离。结果椎弓根壁损伤54枚(14.06%),其中外侧壁损伤39枚(10.16%),包括Ⅰ级损伤27枚(7.03%),Ⅱ级损伤12枚(3.12%);内侧壁损伤15枚,均为Ⅰ级损伤。无1枚螺钉损伤上下壁,无1枚螺钉同时损伤超过2个壁,无术中置钉失败,螺钉松动位移2枚,创伤患者Frankel分级无加重,非创伤患者脊髓功能JOA评分由术前的5.9分提高至术后的11.5分,未发现植骨不融合、假关节形成或节段不稳表现。结论上胸椎椎弓根螺钉固定的并发症发生率低,是相对安全的操作方法。术前详细分析影像学资料、熟悉局部解剖特点、掌握合理的置钉技术,可有效避免并发症的发生。     

In vivo accuracy of thoracic pedicle screws.

STUDY DESIGN: A retrospective observational study of 279 transpedicular thoracic screws using postoperative computed tomography (CT). OBJECTIVE: To determine the accuracy of transpedicular thoracic screws. SUMMARY OF BACKGROUND DATA: Previous studies have reported the importance of properly placed transpedicular thoracic screws. To our knowledge, the in vivo accuracy of pedicle screw placement throughout the entire thoracic spine by CT is unknown. METHODS: The accuracy of thoracic screw placement within the pedicle and vertebral body and the resultant transverse screw angle (TSA) were assessed by postoperative CT. Cortical perforations of the pedicle were graded in 2-mm increments. Screws were regionally grouped for analysis. RESULTS: Forty consecutive patients underwent instrumented posterior spinal fusion using 279 titanium thoracic pedicle screws of various diameters (4.5-6.5 mm). The regional distribution of the screws was 39 screws at T1-T4, 77 screws at T5-T8, and 163 screws at T9-T12. Fifty-seven percent of screws were totally confined within the pedicle. Although medial perforation of the pedicle wall occurred in 14% of screws, in <1% there was >2 mm of canal intrusion. Lateral pedicular perforation occurred in 68% of perforating screws and was significantly more common than medial perforation (P < 0.0005). Seventeen screws penetrated the anterior vertebral cortex by an average of 1.7 mm. Screws inserted between T1 and T4 had a decreased incidence of full containment within the pedicle (P < 0.0005) and vertebral body (P = 0.039) compared with T9-T12. The mean TSA for screws localized within the pedicle was 14.6 degrees and was significantly different from screws with either medial (mean 18.0 degrees ) or lateral (mean 11.5 degrees ) pedicle perforation (P < 0.0005). Anterior vertebral penetration was associated with a smaller mean TSA of 10.1 degrees (P = 0.01) and with lateral pedicle perforation (P < 0.0005). There were no neurologic or vascular complications. CONCLUSIONS: Ninety-nine percent of screws were fully contained or were inserted with either < or =2 mm of medial cortical perforation or an acceptable lateral breech using the "in-out-in" technique. Anterior cortical penetration occurred significantly more often with lateral pedicle perforation and with a smaller mean TSA. The incidence of fully contained screws was directly correlated with the region of instrumented thoracic spine.     

Inserting pedicle screws in the upper thoracic spine without the use of fluoroscopy or image guidance. Is it safe?

Several studies have looked at accuracy of thoracic pedicle screw placement using fluoroscopy, image guidance, and anatomical landmarks. To our knowledge the upper thoracic spine (T1–T6) has not been specifically studied in the context of screw insertion and placement accuracy without the use of either image guidance or fluoroscopy. Our objective was to study the accuracy of upper thoracic screw placement without the use of fluoroscopy or image guidance, and report on implant related complications. A single surgeon inserted 60 screws in 13 consecutive non-scoliotic spine patients. These were the first 60 screws placed in the high thoracic spine in our institution. The most common diagnosis in our patient population was trauma. All screws were inserted using a modified Roy-Camille technique. Post-operative axial computed tomography (CT) images were obtained for each patient and analyzed by an independent senior radiologist for placement accuracy. Implant related complications were prospectively noted. No pedicle screw misplacement was found in 61.5% of the patients. In the remaining 38.5% of patients some misplacements were noted. Fifty-three screws out of the total 60 implanted were placed correctly within all the pedicle margins. The overall pedicle screw placement accuracy was 88.3% using our modified Roy-Camille technique. Five medial and two lateral violations were noted in the seven misplaced screws. One of the seven misplaced screws was considered to be questionable in terms of pedicle perforation. No implant related complications were noted. We found that inserting pedicle screws in the upper thoracic spine based solely on anatomical landmarks was safe with an accuracy comparable to that of published studies using image-guided navigation at the thoracic level.     

Biomechanical evaluation of pedicle screws versus pedicle and laminar hooks in the thoracic spine.

BACKGROUND CONTEXT: Pedicle screws have been shown to be superior to hooks in the lumbar spine, but few studies have addressed their use in the thoracic spine. PURPOSE: The objective of this study was to biomechanically evaluate the pullout strength of pedicle screws in the thoracic spine and compare them to laminar hooks. STUDY DESING/SETTING: Twelve vertebrae (T1-T12) were harvested from each of five embalmed human cadavers (n=60). The age of the donors averaged 83+8.5 years. After bone mineral density had been measured in the vertebrae (mean=0.47 g/cm(3)), spines were disarticulated. Some pedicles were damaged during disarticulation or preparation for testing, so that 100 out of a possible 120 pullout tests were performed. METHODS: Each vertebra was secured using a custom-made jig, and a posteriorly directed force was applied to either the screw or the claw. Constructs were ramped to failure at 3 mm/min using a Mini Bionix II materials testing machine (MTS, Eden Prairie, MN). RESULTS: Pedicle claws had an average pullout strength of 577 N, whereas the pullout strength of pedicle screws averaged 309 N. Hooks installed using the claw method in the thoracic spine had an overwhelming advantage in pullout strength versus pedicle screws. Even in extremely osteoporotic bone, the claw withstood 88% greater pullout load. CONCLUSION: The results of this study indicate that hooks should be considered when supplemental instrumentation is required in thoracic vertebrae, especially in osteoporotic bone.     

Computer navigation of parapedicular screw fixation in the thoracic spine: a cadaver study.

STUDY DESIGN: An in vitro study to investigate the advantages of computer assistance for the purpose of parapedicular screw fixation in the upper and middle thoracic spine. OBJECTIVES: To evaluate the feasibility and application accuracy of parapedicuar screw insertion with the assistance of an optoelectronic navigation system. SUMMARY OF BACKGROUND DATA: Because of anatomic limitations, thoracic pedicle screw insertion in the upper and middle thoracic spine remains a matter of controversy. The technique of parapedicular screw insertion has been described as an alternative, although the exact screw position is difficult to control. With the assistance of computer navigation for the screw placement, it might become possible to overcome these challenges. METHODS: Four human specimens were harvested for this study; 6-mm screws were inserted from T2 to T8 with the assistance of a CT-based optoelectronic navigation system. During surgery virtual images of the screw position were documented and compared with postoperative contact radiographs to determine the application accuracy. The following measurements were obtained: axial and sagittal screw angles as well as the screw distances to the anterior vertebral cortex and the medial pedicle wall. RESULTS: All 54 screws were inserted in a parapedicular technique without violation of the medial pedicle wall or the anterior or lateral vertebral cortex. The mean +/- standard deviation difference between the virtual images and the radiographs was 1.0 +/- 0.94 mm for the distance to the medial pedicle wall and 1.9 +/- 1.44 mm for the distance to the anterior cortex. The angular measurements showed a difference of 1.6 +/- 1.1 degrees for the transverse screw angle and 2.1 +/- 1.6 degrees for the sagittal screw orientation. CONCLUSION: With the assistance of computer navigation it is possible to achieve a safe and reliable parapedicular screw insertion in the upper and middle thoracic spine in vitro. The application accuracy varies for the linear and angular measurements and is higher in the axial than in the sagittal plane. It is important for the surgeon to understand these limitations when using computer navigation in spinal surgery.     

Biomechanical study on pullout strength of thoracic extrapedicular screw fixation

Objective:To identify the biomechanical feasibility of the thoracic extrapedicular approach to the placement of screws. Methods:Five fresh adult cadaveric thoracic spine from T1 to T8 were harvested. The screw was inserted either by pedicular approach or extrapedicular approach. The result was observed and the pullout strength by pedicular screw approach and extrapedicular screw approach via sagittal axis of the vertebrale was measured and compared statistically. Results:In thoracic pedicular approach, the pullout strength of pedicle screw was 1001.23 N±220 N (288.2-1561.7 N) and that of thoracic extrapedicular screw approach was 827.01 N±260 N when screw was inserted into the vertebrae through transverse process,and 954.25 N±254 N when screw was inserted into the vertebrae through the lateral cortex of the pedicle. Compared with pedicular group, the pullout strength in extrapedicular group was decreased by 4.7% inserted through transverse process (P>0.05) and by 17.3% inserted through the lateral cortex (P<0.05). The mean pullout strength by extrapedicular approach was decreased by 11.04% as compared with pedicular approach (P<0.05). Conclusions:It is feasible biomechanically to use extrapedicular screw technique to insert pedicular screws in the thoracic spine when it is hard to insert by pedicular approach.     

The biomechanical effect of pedicle screw hubbing on pullout resistance in the thoracic spine

Background contextThe biomechanical fixation strength afforded by pedicle screws has been strongly correlated with bone mineral density. It has been postulated that “hubbing” the head of the pedicle screw against the dorsal laminar cortex provides a load-sharing effect, thereby limiting cephalocaudad toggling and improving the pullout resistance of the pedicle screw.PurposeTo evaluate the pullout strength (POS) of monoaxial hubbed pedicle screws versus standard fixation in the thoracic spine.Study designBiomechanical investigation.MethodsTwenty-two human cadaveric thoracic vertebrae were acquired and dual-energy X-ray absorptiometry scanned. Osteoporotic (n=16) and normal (n=6) specimens were instrumented with a 5.0×35-mm pedicle screw on one side in a standard fashion. In the contralateral pedicle, 5.0×30-mm screw was inserted with hubbing of the screw into the dorsal lamina. A difference in screw length was used to achieve equivalent depth of insertion. After 2,000 cycles of cephalocaudad toggling, screws were pulled out with the tensile force oriented to the midline of the spine and peak POS measured in newtons (N). Four additional specimens were subjected to microcomputed tomography (micro-CT) analysis to evaluate internal pedicle architecture after screw insertion.ResultsHubbed screws resulted in significantly lower POS (290.5±142.4 N) compared with standard pedicle screws (511.5±242.8 N; p=.00). This finding was evident in both normal and osteoporotic vertebrae based on independent subgroup post hoc analyses (p<.05). As a result of hubbing, half of the specimens fractured through the lamina or superior articular facet (SAF). No fractures occurred on the control side. There was no difference in mean POS for hubbed screws with and without fracture; however, further micro-CT analysis revealed the presence of internal fracture propagation for those specimens that did not have any external signs of failure.ConclusionsHubbing pedicle screws results in significantly decreased POS compared with conventional pedicle screws. Hubbing predisposes toward iatrogenic fracture of the dorsal lamina, transverse process, or SAF during insertion.     

上胸椎椎板螺钉的生物力学研究

 目的比较上胸椎椎板螺钉(translaminar screw.TLS)、椎板关节突螺钉(translaminar facet screw.TLFS)和椎弓根螺钉(transpedicle screw.TPS)固定的拔出强度。方法取 9具新鲜尸体上胸段(T1~T3)完整脊柱标本.双能 X线骨密度仪测量后.游离成 3个独立完整节段(T1、T2、T3).在椎体两侧随机进行 TPS、标准 TLS和 TLFS置入.螺钉直径均为 4.0 mm。分别进行旋入扭距和拔出试验.比较三种固定方式的最大轴向扭矩和拔出力。结果上胸椎 TPS的平均最大扭矩为(0.40±0.01) N.m.TLS的平均最大扭矩为(0.35±0.01) N.m.TLFS的平均最大扭矩为(0.43±0.01) N.m;TPS与 TLS间差异无统计学意义(t=1.94,P >0.05).TPS与 TLFS间差异无统计学意义(t=-1.28, P>0.05).TLFS与 TLS间差异有统计学意义(t=-13.86, P0.05). TPS与 TLFS间差异无统计学意义(t=0.924, P >0.05).TLFS与 TLS差异有统计学意义(t=9.907, P约 0.05)。螺钉的旋入最大扭矩与螺钉的拔出力呈正相关性。结论上胸椎椎板螺钉、椎板关节突螺钉与椎弓根螺钉固定拔出强度差异并不明显。椎板关节突螺钉固定的拔出强度明显大于椎板螺钉。椎板螺钉和椎板关节突螺钉固定可以作为椎弓根螺钉固定的一种补充方法。     

Stereotactic navigation for placement of pedicle screws in the thoracic spine

OBJECTIVE: Pedicle screw fixation in the lumbar spine has become the standard of care for various causes of spinal instability. However, because of the smaller size and more complex morphology of the thoracic pedicle, screw placement in the thoracic spine can be extremely challenging. In several published series, cortical violations have been reported in up to 50% of screws placed with standard fluoroscopic techniques. The goal of this study is to evaluate the accuracy of thoracic pedicle screw placement by use of image-guided techniques. METHODS: During the past 4 years, 266 image-guided thoracic pedicle screws were placed in 65 patients at the University of Michigan Medical Center. Postoperative thin-cut computed tomographic scans were obtained in 52 of these patients who were available to enroll in the study. An impartial neuroradiologist evaluated 224 screws by use of a standardized grading scheme. All levels of the thoracic spine were included in the study. RESULTS: Chart review revealed no incidence of neurological, cardiovascular, or pulmonary injury. Of the 224 screws reviewed, there were 19 cortical violations (8.5%). Eleven (4.9%) were Grade II (< or =2 mm), and eight (3.6%) were Grade III (>2 mm) violations. Only five screws (2.2%), however, were thought to exhibit unintentional, structurally significant violations. Statistical analysis revealed a significantly higher rate of cortical perforation in the midthoracic spine (T4-T8, 16.7%; T1-T4, 8.8%; and T9-T12, 5.6%). CONCLUSION: The low rate of cortical perforations (8.5%) and structurally significant violations (2.2%) in this retrospective series compares favorably with previously published results that used anatomic landmarks and intraoperative fluoroscopy. This study provides further evidence that stereotactic placement of pedicle screws can be performed safely and effectively at all levels of the thoracic spine.     

New means in spinal pedicle hook fixation

Summary Pedicle hooks which are used as an anchorage for posterior spinal instrumentation may be subjected to considerable three-dimensional forces. In order to achieve stronger attachment to the implantation site, hooks using screws for additional fixation have been developed. The failure loads and mechanisms of three such devices have been experimentally determined on human thoracic vertebrae: the Universal Spine System (USS) pedicle hook with one screw, a prototype pedicle hook with two screws and the Cotrel-Dubousset (CD) pedicle hook with screw. The USS hooks use 3.2-mm self-tapping fixation screws which pass into the pedicle, whereas the CD hook is stabilised with a 3-mm set screw pressing against the superior part of the facet joint. A clinically established 5-mm pedicle screw was tested for comparison. A matched pair experimental design was implemented to evauluate these implants in constrained (series I) and rotationally unconstrained (series II) posterior pull-out tests. In the constrained tests the pedicle screw was the strongest implant, with an average pull-out force of 1650 N (SD 623 N). The prototype hook was comparable, with an average failure load of 1530 N (SD 414 N). The average pull-out force of the USS hook with one screw was 910 N (SD 243 N), not significantly different to the CD hook's average failure load of 740 N (SD 189 N). The result of the unconstrained tests were similar, with the prototype hook being the strongest device (average 1617 N, SD 652 N). However, in this series the difference in failure load between the USS hook with one screw and the CD hook was significant. Average failure loads of 792 N (SD 184 N) for the USS hook and 464 N (SD 279 N) for the CD hook were measured. A pedicular fracture in the plane of the fixation screw was the most common failure mode for USS hooks. The hooks usually did not move from their site of implantation, suggesting that they may be well-suited for the socalled segmental spinal correction technique as used in scoliosis surgery. In contrast, the CD hook disengaged by translating caudally from its site of implantation in all cases, suggesting a mechanical instability. The differences in observed hook failure modes may be a function of the type and number of additional fixation screws used. These results suggest that additional screw fixation allows stable attachment of pedicle hooks to their implantation site. Hooks using additional fixation screws passing obliquely into the pedicle apparently provide the most rigid attachment. The second fixation screw of the prototype hook almost doubles the fixation strength. Thus, the prototype hook might be considered as an alternative to the pepdicle screw, especially in the upper thoracic region.     

Accuracy of pedicle screw placement for upper and middle thoracic pathologies without coronal plane spinal deformity using conventional methods

There is a growing interest in the implantation of pedicle screws into the middle and upper thoracic spine. However, usage of the technique is still under debate for these levels because the pedicles in these regions are small and exhibit a high degree of inter- and intraspecimen variability. Twenty-four consecutive patients treated for upper and middle thoracic pathologies by pedicle screw instrumentation under biplanar or uniplanar fluoroscopy were evaluated retrospectively. The rate of screw misplacement on postoperative computerized tomography and complications caused by misplaced screws were determined. In 24 cases, a total of 113 upper-middle thoracic pedicle screws were inserted. Fifty-one of them were inserted between T2 and T5 with guidance of biplanar fluoroscopy, and 62 were inserted between T6 and T8 with uniplanar fluoroscopy. The rate of misplacement was 20.3% for 113 screws (27.4% for T2 to T5 screws, and 14.5% for 62 T6 to T8 screws). Four screws were incorrectly inserted which could have clinical significance (3.5%), and 1 of them required revision. The difference between the upper and middle thoracic screws was not statistically significant (P=0.089). Screw insertion with laminectomy did not cause significant difference in both upper and middle thoracic regions. Only 2 complications were seen because of screw insertion. Pedicle screw insertion with guidance of fluoroscopy may be a reliable and safe method for upper and middle thoracic pathologies.     

Torsional rigidity of scoliosis constructs

STUDY DESIGN: A biomechanical study of the rigidity of various scoliosis constructs instrumented with and without caudal pedicle screw anchors and with none, one, or two cross-link devices. OBJECTIVES: To determine whether the increased torsional rigidity provided by distal pedicle screw fixation might make cross-linking unnecessary. SUMMARY OF BACKGROUND DATA: Pedicle screws and cross-linking devices have been shown to increase the structural rigidity of spinal constructs. Their relative contributions to scoliosis construct rigidity has not been determined. METHODS: "Short" (T2-T11) and "long" (T2-L3) scoliosis constructs were mounted on an industrially fabricated spine model and tested in a hydraulic testing machine. Four different short and four different long constructs were tested: hooks only, hooks with concave side thoracic sublaminar wires, hooks with distal pedicle screw anchors, and hooks, distal pedicle screw anchors, and concave thoracic sublaminar wires. There were four iterations for each construct tested: no cross-links, one superior cross-link at T4-T5, one inferior cross-link at T9-T10, and two cross-links. Torsional rigidity was tested by applying a rotational torque at T2. Vertebral body motion was recorded with a three-dimensional video analysis system. RESULTS: Constructs with distal pedicle screws were statistically more rigid in torsion than those with hooks as distal anchors. The additional torsional rigidity from one or more cross-links was negligible compared with that provided by pedicle screws. CONCLUSIONS: With pedicle screws as distal anchors in scoliosis constructs, cross-linking with one or two devices adds very little additional rotational stiffness and may be unnecessary in many cases.     

The safety of fluoroscopically-assisted thoracic pedicle screw instrumentation for spine trauma

BACKGROUND: Pedicle screw fixation is considered biomechanically superior to other stabilization constructs. However, the potential for severe complications have discouraged its use in the thoracic spine. Our goal is to determine the incidence of major perioperative complications following the placement of thoracic pedicle screws using anatomic landmarks and intraoperative fluoroscopy in patients with spine fractures. METHODS: Retrospective review of 245 consecutive patients with spine fractures requiring pedicle screw fixation between T1 and T10 at a regional Level I trauma center between 1995 and 2001. Database and medical record review were used to identify the incidence of major perioperative complications. A major complication was defined as a potentially life-threatening vascular injury, neurologic deterioration, pneumothorax or hemothorax, and tracheoesophageal injury. Patients were monitored for these complications from the time of surgery until discharge. RESULTS: In all, 1,533 pedicle screws were placed between T1 and T10 in 245 patients. No patient sustained a major complication related to screw placement. Three patients (1.2%) required a secondary procedure for prophylactic revision of four (0.26%) malpositioned screws. CONCLUSIONS: This study supports the safety of pedicle screws in the thoracic spine using preoperative imaging evaluation, standard posterior element landmarks and intraoperative fluoroscopy.     

透视下分步引导上中胸椎椎弓根螺钉安全植入实验研究

目的：探索C形臂X线监测引导上中胸椎椎弓根螺钉植入的的方法,并评定其准确性与安全性。方法：①取6具正常成人T1-T8脊椎骨架标本,分解出单个椎体,导针沿椎弓根轴线进针,分别于进针点、针前端位于椎弓根中部、椎体后缘及椎体前缘皮质下,通过C形臂X线透视,记录、分析椎弓根轴线导针在进针点及不同进针深度时在正侧位透视图像上导针前端的位置,以及相关位置对应关系变化规律。②按上述椎弓根轴线导针C形臂X线透视监测对应位置变化规律,作为C形臂X线透视下分步引导上中胸椎椎弓根螺钉安全植入的方法。取6具T1-T8脊柱标本,C形臂X线机引导下分步植入椎弓根螺钉96枚,然后将脊椎标本作CT扫描,判定椎弓根螺钉位置。结果：根据CT扫描结果,优（椎弓根螺钉安全位于椎弓根内者）90枚,可（螺钉穿破椎弓根内或外侧骨皮质较少,突破在2mm以内者）6枚,差（螺钉穿破椎弓根内外骨皮质较多,突破在2mm以上）0枚。结论：C形臂X线透视下分步引导上中胸椎椎弓根螺钉植入,是一种能提高上中胸椎椎弓根螺钉植入的简单经济、确实可行的方法。     

Cervical spine pedicle screws: a biomechanical comparison of two insertion techniques

STUDY DESIGN: Biomechanical testing of the pullout strengths of pedicle screws placed by two different techniques in adult human cadaveric cervical spines. OBJECTIVES: To determine whether there is a significant difference in screw purchase of two commonly proposed methods of cervical pedicle screw insertion. SUMMARY OF BACKGROUND DATA: Wiring techniques remain the gold standard for posterior cervical fixation. However, absent or deficient posterior elements may dictate the use of alternative fixation techniques. Cervical pedicle screws have been shown to have significantly higher pullout strength than lateral mass screws. METHODS: Fifty fresh disarticulated human vertebrae (C3-C7) were evaluated with computed tomography for anatomic disease and pedicle morphometry. The right and left pedicles were randomly assigned to either a standard method or the Abumi insertion method. In the latter technique the cortex and cancellous bone of lateral mass are removed with a high-speed burr, which provides a direct view of the pedicle introitus. The pedicle is then probed and tapped and a 3.5-mm cortical screw inserted. Each screw was subjected to a uniaxial load to failure. RESULTS: There was no significant difference in the mean pullout resistance between the Abumi (696 N) and standard (636.5 N) insertion techniques (P = 0.41). There was no difference in pullout resistance between vertebral levels or within vertebral levels. Two (4%) minor pedicle wall violations were observed. CONCLUSION: In selected circumstances pedicle screw instrumentation of the cervical spine may be used to manage complex deformities and patterns of instability. Surgeons need not be concerned about reduced screw purchase when deciding between the Abumi method and its alternatives.     

Morphometric analysis of thoracic and lumbar vertebrae in idiopathic scoliosis

STUDY DESIGN: Prospective study on the morphometry of 337 pedicles in 29 patients with idiopathic scoliosis. OBJECTIVES: To analyze by means of computed tomographic scans the vertebral morphometry in idiopathic scoliosis treated by pedicle screw instrumentation. SUMMARY OF BACKGROUND DATA: Although several studies exist on the vertebrae's morphometry in normal spines, little is known concerning the morphometry of scoliotic vertebrae. METHODS: The pedicles' morphometry between T5 and L4 was analyzed by computed tomographic scans in 29 surgically treated patients with idiopathic right thoracic scoliosis. Measurements included chord length, endosteal transverse pedicle width, transverse pedicle angle, and pedicle length. RESULTS: The endosteal transverse pedicle width was significantly smaller (P < 0.05) on the concavity in the apical region of the thoracic spine and measured between 2.5 and 4.2 mm in the middle thoracic spine (T5-T9) and between 4.2 and 5.9 mm in the lower thoracic spine (T10-T12). In the lumbar spine, the width varied between 4.8 and 9.5 mm without significant differences between the concave and convex sides (P > 0.05). The chord length was shortest at T5, measuring 37 mm and increased gradually to 50 mm at L3 with significantly larger dimensions in male patients and on the concavity of the apical region in the thoracic spine (P < 0.05). The pedicle length varied minimally, with a range of between 20 and 22 mm, and was relatively consistent throughout the thoracic and lumbar spine. The transverse pedicle angle varied between 6 degrees in the lower thoracic spine and 12 degrees in the upper thoracic and lower lumbar spine. CONCLUSION: The morphometry in scoliotic vertebrae is substantially different from that of vertebrae in normal spines, with an asymmetrical intravertebral deformity shown in scoliotic vertebrae. Pedicle screw instrumentation on the concavity in the apical region of thoracic curves appears critical because of the small endosteal pedicle width.