S1椎弓根螺钉结合髂骨板间螺钉治疗骶髂关节骨折脱位

目的 探索S1椎弓根螺钉结合髂骨板问螺钉治疗骶髂关节骨折脱位的临床疗效，评价两者结合对骶髂关节骨折脱位的治疗价值。方法 对11例骶髂关节骨折脱位患者用脊柱内固定系统(TSRH)之S1椎弓根螺钉结合髂骨板间螺钉进行固定，该组患者涉及骶髂关节的垂直移位及旋转的骨盆环变形，归于Tile分型的B类或C类骨盆损伤。11例患者均伴有前环损伤，其中9例予以加压钢板(smith nephew)内固定，余2例患者单纯采用后路手术内固定。结果 7例患者垂直移位完全复位，9例旋转畸形纠正，未发现感染及神经损伤等并发症。结论 S1椎弓根螺钉结合髂骨板问螺钉固定技术治疗骶髂关节骨折脱位，可获得即刻稳定性并良好地维持了复位的效果．这一混合技术对于涉及垂直及旋转损伤的骨盆环损伤有稳定的作用。     

Unilaterale trianguläre lumbopelvine Stabilisierung

Operative fixation has become treatment of choice for unstable sacral fractures. Osteosynthesis for these fractures results in loss of reduction in up to 15%. Vertical sacral fractures involving the S1 facet joint (Isler 2 and 3) may lead to multidirectional instability. Multidirectional instability of the posterior pelvic ring and lumbopelvic junction may be stabilized and forces balanced by a so-called lumbopelvic triangular fixation. Lumbopelvic triangular fixation combines vertical fixation between the lumbar vertebral pedicle and the ilium, with horizontal fixation, as an iliosacral screw or a transiliacal plate osteosynthesis. The iliac screw is directed from the posterior superior iliac spine (PSIS) to the anterior inferior iliac spine (AIIS). Thereby, lumbopelvic fixation decreases the load to the sacrum and SI joint and transfers axial loads from the lumbar spine directly onto the ilium. Triangular lumbopelvic fixation allows early full weight bearing and therefore reduces prolonged immobilization. The placement of iliac screws may be a complex surgical procedure. Thus, the technique requires thorough surgical preparation and operative logistics. Wound-related complications may occur. Preexisting Morell–Lavalée lesions increase the risk for infection. Prominent implants cause local irritation and pain. Hardware prominence and pain are markedly reduced with screw head recession into the PSIS.     

Reduction and fixation of sacroiliac joint dislocation by the combined use of S1 pedicle screws and the galveston technique

STUDY DESIGN: This retrospective study was designed to analyze the results of the treatment with S1 pedicle screws and the Galveston technique of seven patients with sacroiliac dislocation. OBJECTIVES: To evaluate the effectiveness of the combined use of S1 pedicle screws and the Galveston technique for the treatment of sacroiliac dislocation. SUMMARY OF BACKGROUND DATA: Although several procedures for internal fixation of sacroiliac dislocation have been reported, there have been no reports discussing surgical treatment of sacroiliac dislocation by the combined use of S1 pedicle screws and the Galveston technique. METHODS: Seven patients with sacroiliac dislocation were treated with pedicle screws of S1 and iliac rod according to the Galveston technique. In the seven patients, the dislocation was associated with vertical displacement of the sacroiliac joint and rotational deformity of the pelvic ring. They were classified into Type-C pelvic disruption according to the Tile's classification. Three patients with disruption of the symphysis pubis underwent additional fixation of the symphysis using a dynamic compression plate. The remaining four patients were treated by the posterior procedure alone. RESULTS: The vertical displacement was completely reduced in five patients, and the rotational deformity was completely corrected in four patients. The reduction was maintained at the time of the final follow-up evaluation. There were no perioperative complications with the exception of late infection in one patient. CONCLUSIONS: The combined use of S1 pedicle screws and the Galveston technique provided immediate stability and sufficient reduction for sacroiliac dislocation in seven patients in this study. This hybrid internal fixation procedure is useful for reduction and fixation of sacroiliac dislocation associated with the vertical and rotational instability of the pelvic ring.     

Anatomically correct reduction and fixation of a Tile C-1 type unilateral sacroiliac disruption using a rod and pedicle screw system between the S1 vertebra and the ilium: experimental and clinical case report

 We have developed a new surgical technique for the treatment of Tile C-1 type sacroiliac disruption. We tried this procedure first in a cadaveric specimen and then applied it to a clinical case. We used the Texas Scottish Rite Hospital (TSRH) rod and pedicle screw system to insert one screw into the S1 vertebra without using an image intensifier and the other screw into the bone marrow of the ilium from the posterosuperior iliac spine. A straight rod was connected between the two screws by using a manipulator to attempt to reduce and fix the sacroiliac disruption. The combined pubic symphysis diastasis could be simultaneously reduced and fixed by using a plate through another incision, resulting in anatomically correct reconstruction of the pelvic ring. In this procedure, the alignment of the sacroiliac joint can be reversibly and directly changed during reduction and fixation. The sacroiliac joint can be strongly fixed because the screws can be freely inserted into the intact portion of the pelvis and the adjacent lumbar spine, if necessary. Good reduction is obtained because direct compression force is applied to the fracture site. The posterior and anterior procedures can be simultaneously performed under the same lateral position. Received: December 25, 2001 / Accepted: May 2, 2002     

Pedicle screw fixation in the cervical spine

Pedicle screw fixation of the lower cervical spine is a new technique that provides an alternative to posterior lateral mass plating. Although biomechanical studies support the use of pedicle screws to reconstruct the cervical spine, placing screws into the small cervical pedicle poses a technical challenge. Penetration of the pedicle is the primary complication associated with screw insertion in the lower cervical spine. Pedicle screw fixation at the C2 and C7 pedicles in conjunction with use of plates for occipitocervical or cervicothoracic plating is becoming an accepted technique; however, pedicle screw fixation should not be routinely used at the C3-C6 levels. It may be indicated in patients who have osteoporotic bone or when rigid internal fixation cannot be achieved by conventional techniques.     

骶髂关节经皮拉力螺钉固定钉道参数的解剖学测量

目的 为经S_1椎弓根水平骶髂关节拉力螺钉固定术提供应用解剖学依据.方法 2008年6月至2009年7月收集50个成人骨盆的螺旋CT扫描数据,重建骨盆三维模型,模拟经皮托力螺钉固定.测量S_1椎弓根的宽和高、骶髂关节拉力螺钉的进针点和进针方向、进针点至S_1椎体对侧前皮质和髂后上棘的距离.结果 S_1椎弓根的宽和高分别为(20.43±1.63)mm和(20.26±0.99)mm;2枚螺钉的进针点均在髂前上棘和髂后上棘的连线上方,至髂后上棘的距离分别为(49.87±6.80)mm和(51.11±7.15)mm.螺钉平行进入S_1椎弓根,与髂骨翼后外侧面垂直,与冠状面和欠状面的夹角分别为18.35°±5.20°和77.62°±3.98°.进针点到S1椎体对侧前皮质的距离分别为(76.08±4.32)mm和(77.62±3.98)mm.骶髂关节拉力螺钉的长度、S_1椎弓根的高度和宽度、进针点到髂后上棘的距离、进针点与冠状面的夹角在男女之间差异有统计学意义(P<0.05).结论 正常成人在S_1椎弓根水平均町置入2枚直径为6.5 mm的拉力螺钉,钉道参数的解剖学测量为骶髂关节拉力螺钉固定手术导航模板的设计提供了理论基础.     

Sacroiliac fixation: a new technique after pelvic trauma

We propose a new technique for sacroiliac fixation for the treatment of pelvic fracture with vertical and horizontal instability (Tile class C). This fixation technique allows control of vertical displacement while allowing, if needed, a certain degree of movement in the horizontal plane to facilitate reduction of anterior lesions. The technique involves insertion of two sacral screws, one in S1 and one in S2, and two iliac screws. The iliac screws are inserted in the posterior iliac crest through two sacroiliac connectors placed on a rod linking the two sacral screws. Vertical displacement is controlled by blocking the screw heads on the connecting rod. If needed, a certain degree of horizontal mobility of the half pelvis can be allowed by loosening the connectors on the rods. This technique was used for 4 patients. Anatomic reduction was achieved and no secondary movement of the osteosynthesis material nor secondary displacement were observed. The quality of the fixation allowed rapid weight bearing in the standing position and early walking without crutches. This type of fixation can only be used for type C12 fractures in the Tile classification.     

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.     

Pedicle and transverse process screws of the upper thoracic spine. Biomechanical comparison of loads to failure

STUDY DESIGN: An In vitro biomechanical load-to-failure test. OBJECTIVES: To determine the comparative axial pullout strengths of pedicle screw versus transverse process screws in the upper thoracic spine (T1-T4), and to compare their failure loads with bone density as seen on computed tomography. SUMMARY OF THE BACKGROUND DATA: The morphology of the upper thoracic spine presents technical challenges for rigid segmental fixation. Though data are available for failure characteristics of cervical-lateral mass screws, analogous data are wanting in regard to screw fixation of the upper thoracic spine. METHODS: Ten fresh-frozen human spines (T1-T4) were quantitatively scanned using computed tomography to determine trabecular bone density at each level. The vertebrae were drilled and tapped for the insertion of a 3.5-mill meter-diameter cortical bone screw in either the pedicle or the transverse process position. A uniaxial load to failure was applied. RESULTS: The mean ultimate load to failure for the pedicle screws (658 N) was statistically greater than that of the transverse process screws (361 N; P < 0.001). The T1 pedicle screw sustained the highest load to failure (775 N). No significant difference was found between load to failure for the pedicle and transverse process screws at T1. A trend toward decreasing load to failure was seen for both screw positions with descending thoracic level. Neither pedicle dimensions nor screw working length correlated with load to failure. CONCLUSIONS: Upper thoracic pedicle screws have superior axial loading characteristics compared with bicortical transverse process screws, except at T1. Load behavior of either of these screws was not predictable based on anatomic parameters.     

Low-profile pelvic fixation with sacral alar-iliac screws

Background

In the last years, a new technique for pelvic fixation using ‘sacral alar-iliac’ screws has been proposed as an alternative to the classic ‘iliac wing’ screws and the old Galveston technique.

Methods

The authors provide a ‘step-by-step’ review of the technical details regarding this technique, as well as some important anatomical and radiological landmarks for its successful performance.

Conclusions

‘Sacral alar-iliac’ screws provide several advantages in relation to pelvic fixation with iliac bolts such as: better alignment with the rods, use of a ‘low-profile’ screw, and fixation of the sacroiliac (SI) joint, which avoids postoperative SI joint-related pain.     

钉道强化提高椎弓根螺钉固定强度的生物力学研究

目的 评价椎弓根钉道局部强化技术及其结合膨胀式椎弓根螺钉提高椎弓根螺钉固定强度的效果.方法 通过自行设计及加工的钉道局部强化装置,向钉道周壁点状注入CaSO4骨水泥以强化椎弓根钉道.5具新鲜冻存人体脊柱标本,每具随机选取4个腰椎共20个腰椎标本,采用随机区组设计方法分为10个区组.设计四种固定方法:A组(普通椎弓根螺钉)、B组(普通椎弓根螺钉+钉道局部强化)、C组(膨胀式椎弓根螺钉+钉道局部强化)、D组(普通椎弓根螺钉+钉道内灌注CaSO4骨水泥),随机应用在每个区组的2个椎体共4个椎弓根钉道.分别测试每个椎弓根螺钉的最大轴向拔出力及能量吸收值.从剩余腰椎标本中任意取两个腰椎,应用钉道局部强化技术后利用Micro-CT观察钉道周壁微观结构变化.结果 C、D两组的最大轴向拔出力均值及能量吸收值均值高于A组(P＜0.01)及B组(P＜0.05),B组最大轴向拔出力均值及能量吸收值均值高于A组(P＜0.01),C、D两组之间的差异无统计学意义(P＞0.05).结论 钉道局部强化技术可显著提高椎弓根螺钉的固定强度,结合使用膨胀式椎弓根螺钉可进一步提高螺钉的固定强度.     

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.     

Biomechanics of sacropelvic fixation: a comprehensive finite element comparison of three techniques

Sacropelvic fixation is frequently used in combination with thoracolumbar instrumentation for complex deformity correction and is commonly associated with pseudoarthrosis, implant failure and loosening. This study compared pedicle screw fixation (PED) with three different sacropelvic fixation techniques, namely iliac screws (IL), S2 alar-iliac screws (S2AI) and laterally placed triangular titanium implants (SI), all in combination with lumbosacral instrumentation, accounting for implant micromotion. Existing finite element models of pelvis-L5 of three patients including lumbopelvic instrumentation were utilized. Moments of 7.5 Nm in the three directions combined with a 500 N compressive load were simulated. Measured metrics included flexibility, instrumentation stresses and bone–implant interface loads. Fixation effectively reduced the sacroiliac flexibility. Compared to PED, IL and S2AI induced a reduction in peak stresses in the S1 pedicle screws. Rod stresses were mostly unaffected by S2AI and SI, but IL demonstrated a stress increase. In comparison with a previous work depicting full osteointegration, SI was found to have similar instrumentation stresses as those due to PED. Fixation with triangular implants did not result in stress increase on the lumbosacral instrumentation, likely due to the lack of connection with the posterior rods. IL and S2AI had a mild protective effect on S1 pedicle screws in terms of stresses and bone–implant loads. IL resulted in an increase in the rod stresses. A comparison between this study and previous work incorporating full osteointegration demonstrates how these results may be applied clinically to better understand the effects of different treatments on patient outcomes. These slides can be retrieved under Electronic Supplementary Material.     

骶髂关节空心钉内固定导向器的研制及临床应用

目的 自行设计并研制一种骶骼关节空心钉内固定导向器,以期提高S1椎弓根空心钉置入的成功率及置入质量。方法 自行设计并研制的骶髂关节空心钉内固定导向器由带有测量长度的定位器、带有角度刻度表的水平面S1椎弓根空心钉置入角（TSA）、矢状面置入角(SSA)调节器及固定螺丝等组成。2006年6月至2009年10月共收治15例不稳定型骨盆骨折累及骶髂关节者,男6例,女9例;年龄19 ～ 60岁,平均45.6岁。不稳定型骨盆骨折经骶髂关节韧带3例,经骶骨Ⅰ区5例,累及骶骨Ⅱ区5例,累及骶骨Ⅲ区2例。15例患者均采用骶髂关节空心钉内固定导向器引导术中空心钉的操作,共置入18枚空心钉。结果 术后CT及X线片示18枚空心钉置入位置准确,与术前设计相比,具有相同或相近的TSA及SSA。结论 骶骼关节空心钉内固定导向器设计合理,使用方便,能提高S1椎弓根空心钉置入的准确性及置入质量,且可显著减少C型臂X线机的透视次数。     

Open versus endoscopic lumbar pedicle screw fixation and posterolateral fusion in a sheep model: a feasibility study.

BACKGROUND CONTEXT: Conventional open procedures for lumbar pedicle screw instrumentation are often associated with significant morbidity resulting in longer postoperative recovery and hence have led to the advent of less invasive techniques of spinal instrumentation and fusion. PURPOSE: This study compares lumbar pedicle screw fixation and posterolateral fusion using endoscopic and open techniques in a sheep model. STUDY DESIGN: Endoscopic and open techniques of lumbar pedicle screw instrumentation and posterolateral fusion are compared in a sheep model (12 sheep in each group). The safety and fusion outcome are assessed by postoperative computed tomography (CT) scans performed at 6 months. METHODS: Twelve sheep underwent bilateral L4-L5 pedicle screw fixation and posterolateral fusion by means of endoscopic instrumentation and another 12 sheep by means of conventional open technique. For posterolateral fusions, an autologous iliac crest bone or bone graft substitute (Healos/MP52; Orquest, Mountain View, CA) was used on either the animal's left or right side. The animals were euthanized 6 months after surgery and the lumbar spines were harvested. Pedicle dimensions and screw insertion angles were measured and the two techniques were compared using CT and image analysis software. Pedicle screw misplacement was detected by means of direct inspection and CT scan, and fusions were evaluated by CT scans. RESULTS: The endoscopic group's screw insertion angle was more convergent, corresponded more closely to the human pedicle angle and was less variable than in the open technique. Three of the endoscopic group's 48 screws were misplaced medially, whereas in the open technique, 5 screws were misplaced laterally. Posterolateral bone fusion masses were palpable 6 months after surgery for both the autologous bone grafts and graft substitutes. There were no complications. The ratio of screw to pedicle diameter was approximately 0.6, greater than in human studies. CONCLUSIONS: Despite differences between pedicle dimensions and screw angles, most screws could be inserted into the sheep pedicle using endoscopic or open techniques. Bone fusion was successful in both groups. Thus, minimally invasive, endoscopic transpedicular screw-plate fixation and posterolateral fusion is feasible, and as effective as the conventional open technique.     

Vertically unstable pelvic fractures fixed with percutaneous iliosacral screws: does posterior injury pattern predict fixation failure?

OBJECTIVE: To measure the failure rate of percutaneous iliosacral screw fixation of vertically unstable pelvic fractures and particularly to test the hypothesis that fixations in which the posterior injury is a vertical fracture of the sacrum are more likely to fail than fixations with dislocations or fracture-dislocations of the sacroiliac joint. DESIGN: Retrospective review. SETTING: Level 1 trauma center. METHODS: All patients with pelvic fractures admitted between January 1, 1993, and December 31, 1998, were identified from the trauma registry. Hospital records were used to identify patients treated with iliosacral screws. Radiologic studies were examined to identify patients who had unequivocally vertically unstable pelvic fractures. Immediate postoperative and follow-up anteroposterior, inlet, and outlet radiographs from a minimum of 12 months postinjury were examined. Position, length, and numbers of iliosacral screws and any evidence of screw failure (eg, bending or breakage) were recorded. Residual postoperative displacement and late displacement of the posterior pelvis were measured. The main outcome measure was failure, defined as at least 1cm of combined vertical displacement of the posterior pelvis compared with immediate postoperative position. The main analysis was for association between fracture pattern and failure. Patient demographic data, iliosacral screw position, and anterior pelvic fixation method also were studied. RESULTS: The study group comprised 62 patients with unequivocally vertically unstable pelvic fractures in whom the posterior injury was treated with closed reduction and percutaneous iliosacral screw fixation. Of patients, 32 had dislocations or fracture-dislocations of the sacroiliac joint, and 30 had vertical fractures of the sacrum. Fixation failed in four patients, all with vertical sacral fractures and all within the first 3 weeks after surgery. These four patients required revision fixation. In two further cases with vertical sacral fractures, there was evidence that the fracture had only barely been held by the fixation, but these fractures healed, and follow-up radiographs did not meet the displacement criteria for failure. A vertical sacral fracture pattern was associated significantly with failure (Fisher exact test, P = 0.04); the excess risk of failure compared with sacroiliac joint injury was 13% (95% confidence interval 1% to 25%). There was no significant association between failure and anterior fixation method, iliosacral screw arrangement or length, or any demographic or injury variable. CONCLUSIONS: Percutaneous iliosacral screw fixation is a useful technique in the management of vertically unstable pelvic fractures, but a vertical sacral fracture should make the surgeon more wary of fixation failure and loss of reduction.     

Postoperative complications of S2AI versus iliac screw in spinopelvic fixation: a meta-analysis and recent trends review

BACKGROUND CONTEXTIliac screw constructs have been a major advancement in spinopelvic fixation demonstrating superior biomechanics as compared with earlier pelvic spanning constructs. However, drawbacks such as screw site prominence and wound complication have led to the development of a lower profile S2AI iliac screw.PURPOSEIn this study, we aimed to study the differences in complication rates between the traditional iliac and S2AI fixations via a pooled analysis of the available head-to-head comparisons between S2AI and iliac screws. We also aimed to study the iliac screw complications trend over the years particularly with reference to recent modifications in its screw insertion techniques.STUDY DESIGNA meta-analysis with attention to the comparison of patients who underwent iliac screws and S2AI screws was conducted.METHODSThe following databases were utilized: PubMed, Scopus, Web of Science, Embase, and Cochrane Central Register of Controlled Trials database. Using the search terms: iliac, iliac bolts, S2AI, sacral 2 alar iliac, sacral two alar iliac, reviewers independently selected eligible studies, analyzed data and evaluated the risk of bias. Data analysis was conducted using RevMan 5.3 software.RESULTA total of 215 articles were identified, with 6 clinical studies directly comparing outcomes of S2AI pelvic fixation versus iliac screw fixation. A total of 477 patients were included, of which 255 patients (53.5%) underwent S2AI screw and 222 (46.5%) underwent iliac screw fixation. Our pooled analysis favored S2AI screws with regards to postoperative complications of screw prominence (odds ratio [OR]=5.99, 95% confidence interval [CI]=2.168–16.523, p<.001), screw loosening (OR=3.36, 95% CI=1.415–7.998, p=.006), implant breakage (OR=2.30, 95% CI=1.189–4.443, p=.013), and revision surgery (OR=7.84, 95% CI=3.224–19.080, p<.001). Although there was a trend toward more wound complications in conventional iliac screw techniques when compared with S2AI, it was not statistically significant.CONCLUSIONSpinopelvic fixation is an evolving technique. The results from this study showed that S2AI screws with a lower profile have made a significant impact in reducing complications associated with conventional iliac screws. With recent entry point modification and further advancement in the conventional iliac screw technique, such as the “subcrestal iliac screw technique” which reduces the iliac screw complication rate but avoids S2AI-associated SIJ violation. Further studies may be needed to investigate whether these newer iliac screw techniques can narrow the difference in complication rates between iliac screws and S2AI screw techniques.     

寰枢椎后路椎弓根螺钉固定的生物力学评价

目的：评价寰枢椎后路椎弓根螺钉固定的生物力学稳定性。方法：6具新鲜颈椎标本，按随机顺序，对每一标本先后行C1-C2椎弓根螺钉、Magerl螺钉、Brooks钢丝以及螺钉联合钢丝固定，在脊柱三维运动实验机上测量其三维运动范围。结果：Magerl螺钉或C1-C2椎弓根螺钉联合Brooks钢丝组成的固定系统的三维运动范围最小。C1-C2椎弓根螺钉固定的前后屈伸运动范围与Brooks钢丝固定无差异，但大于Magerl螺钉；其左右侧屈运动范围小于Brooks钢丝固定，大于Magerl螺钉；其轴向旋转角度明显小于Brooks钢丝固定，但与Magerl螺钉无统计学差异。结论：C1-C2椎弓根螺钉的三维稳定性与Magerl螺钉相当，联合Brooks钢丝固定可进一步提高其稳定性。     

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out

BACKGROUND CONTEXT: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better "fit and fill" of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion. PURPOSE: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. STUDY DESIGN/SETTING: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. METHODS: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student's t test. RESULTS: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. CONCLUSIONS: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.     

Biomechanical advantages of dual over single iliac screws in lumbo-iliac fixation construct

The development of iliac screws has provided a markedly easier way for spino-pelvic instrumentation than the classical Galveston technique. However, high rates of iliac screw loosening and breakage are usually reported, especially in cases where bilateral single iliac screws are used. Therefore, there is a need for exploring more stable iliac fixation techniques. Thus, the objective of this study was to compare the biomechanical effects of bilateral single and dual iliac screws on the stability of L3-iliac fixation construct under total sacrectomy condition. In this study, L2-pelvic specimens were harvested from seven fresh human cadavers. After biomechanically testing the intact state simulated by L3-L5 pedicle screw fixation, destabilization was introduced by total sacrectomy. Upon destabilization, L3-iliac screw-rod reconstructions were performed by four different techniques as follows: (1) bilateral single short iliac screws (Single-Short); (2) bilateral single long iliac screws (Single-Long); (3) bilateral dual short iliac screws, placed in the upper and lower iliac columns (Dual-UL); and (4) bilateral dual short iliac screws, all placed in the lower iliac column (Dual-Lower). These four iliac screw fixation techniques were sequentially preformed in the same specimen, and the lengths of the short and long iliac screws were 70 and 130 mm, respectively. Biomechanical testing was performed on a material testing machine under 800 N compression and 7 Nm torsion loading modes to evaluate the construct stiffness. In compression, the stiffness of the L3-iliac fixation constructs of Single-Short, Single-Long, Dual-UL, and Dual-Lower techniques were 73, 76, 98, and 108% of the intact state, respectively. No significant differences were detected between Single-Short and Single-Long (P = 0.589) techniques. However, the compressive stiffness of these two techniques was significantly lower than the intact state, and the Dual-UL and Dual-Lower techniques (P < 0.05). There was no statistical difference between the intact condition and the Dual-Lower technique (P = 0.109). Interestingly, Dual-Lower exhibited notably higher compressive stiffness than Dual-UL (+10.3%, P = 0.049). In torsion, the stiffness of Single-Short, Single-Long, Dual-UL, and Dual-Lower techniques were 72, 79, 105, and 109% of the intact condition, respectively. No significant differences were detected between Single-Short and Single-Long techniques (P = 0.338), and also among Dual-UL, Dual-lower techniques, and the intact state (P > 0.05). However, Single-Short and Single-Long techniques provided markedly lower construct torsional stiffness than the other three groups (P < 0.05). For lumbo-illiac reconstruction after total sacrectomy, even the use of bilateral single, long iliac screws do not help in restoring the local stability to the intact condition. However, dual iliac screws provide much higher construct stability than single iliac screw techniques. Therefore, dual iliac screw technique should be preferred for treating the unstable situation caused by total sacrectomy.