Comparison of stability of two kinds of sacro-iliac screws in the fixation of bilateral sacral fractures in a finite element model

ObjectiveTo compare the stability of lengthened sacro-iliac screw and sacro-iliac screw for the treatment of bilateral vertical sacral fractures to provide reference for clinical application.MethodsA finite element model of Tile C pelvic ring injury (bilateral type Denis II fracture of sacrum) was produced. (Tile and Denis are surgeons, who put forward the classifications of pelvic ring injury and sacral fracture respectively.) The bilateral sacral fractures were fixed with a lengthened sacro-iliac screw and a sacro-iliac screw in seven types of models, respectively. The translation and angular displacement of the superior surface of the sacrum in the case of standing on both feet were measured and compared.ResultsThe stability of one lengthened sacro-iliac screw fixation in the S1 or S2 segment is superior to that of two bidirectional sacro-iliac screws in the same sacral segment; the stability of one lengthened sacro-iliac screw fixation in S1 and S2 segments, respectively, is superior to that of two bidirectional sacro-iliac screw fixation in S1 and S2 segments, respectively; the stability of one lengthened sacro-iliac screw fixation in S1 and S2 segments, respectively, is superior to that of one lengthened sacro-iliac screw fixation in the S1 or S2 segment; the stability of two bidirectional sacro-iliac screw fixation in S1 and S2 segments, respectively, is markedly superior to that of two bidirectional sacro-iliac screw fixation in the S1 or S2 segment and is also markedly superior to that of one sacro-iliac screw fixation in the S1 segment and one sacro-iliac screw fixation in the S2 segment; the vertical stability of the lengthened sacro-iliac screw or the sacro-iliac screw fixation in S2 is superior to that of S1. The rotational stability of the lengthened sacro-iliac screw or sacro-iliac screw fixation in S1 is superior to that of S2.ConclusionS1 and S2 lengthened sacro-iliac screws should be used for the fixation in bilateral sacral fractures of Tile C pelvic ring injury as far as possible and the most stable fixation is the combination of the lengthened sacro-iliac screws of S1 and S2 segments. Even if lengthened sacro-iliac screws cannot be used due to limited conditions, two bidirectional sacro-iliac screw fixation in S1 and S2 segments, respectively, is recommended. No matter which kind of sacro-iliac screw is applied, the fixation combination of S1 and S2 segments is strongly recommended to maximise the stability of the pelvic posterior ring.     

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.     

骶髂拉力螺钉固定技术在骨盆后环损伤中的临床应用

目的 探讨应用骶髂拉力螺钉固定技术治疗骨盆后环损伤的适应证、手术方法及疗效。方法 1998年6月～2002年11月，对13例骨盆骶髂复合结构损伤患者采用后侧经骶髂关节的骶髂拉力螺钉固定方法治疗骨盆后环损伤，男10例，女3例；年龄21～49岁，平均37．5岁。通过影像学柱查明确13例患者有15侧骨盆骶髂复合结构损伤。按照AO骨盆环损伤方法分型：B型损伤6例．其中B2型4例，B3型2例；C型损伤7例．其中C1型2例，C2型3例，C3型2例。患者麻醉后俯卧位，行闭合或开放复位后用骶髂拉力螺钉固定。取髂骨翼后侧进钉点，根据S1椎体上缘和骶骨翼走行确定进钉方向。所有操作均在“C”型臂X线机透视下进行。结果 13例患者均获随访．随访时间6个月～4年6个月，平均19．7个月。手术平均时问为70min。13例共置入骶髂拉力螺钉21枚。患者在术后2-3周可持双拐下地行走，3～4个月双下肢可完全负重行走．并逐渐恢复体力劳动，未发生神经损伤及螺钉松动、断裂现象。腰骶及下肢活动接近正常。结论 骶髂拉力螺钉固定技术可使不稳定性骨盆后环损伤患者获得良好的即刻复位和固定，恢复骨盆的稳定性，适用于骶髂关节脱位及骶骨骨折的患者。     

Investigation of bone quality of the first and second sacral segments amongst trauma patients: concerns about iliosacral screw fixation

Background

Iliosacral screw fixation has become a common method for surgical stabilization of acute disruptions of the pelvic ring. Placement of iliosacral screws into the first sacral (S1) body is the preferred method of fixation, but size limitations and sacral dysmorphism may preclude S1 fixation. In these clinical situations, fixation into the second sacral (S2) body has been recommended. The objective of this study was to evaluate the bone quality of the S1 compared to S2 in the described “safe zone” of iliosacral screw fixation in trauma patients.

Materials and methods

The pelvic computed tomography scans of 25 consecutive trauma patients, ages 18–49, at a level 1 trauma center were prospectively analyzed. Hounsfield units, a standardized computed tomography attenuation coefficient, was utilized to measure regional cancellous bone mineral density of the S1 and S2. No change in the clinical protocol or treatment occurred as a consequence of inclusion in this study.

Results

A statically significant difference in bone quality was found when comparing the first and second sacral segment (p = 0.0001). Age, gender, or smoking status did not independently affect bone quality.

Conclusion

In relatively young, otherwise healthy trauma patients there is a statistically significant difference in the bone density of the first sacral segment compared to the second sacral segment. This study highlights the need for future biomechanical studies to investigate whether this difference is clinically relevant. Due to the relative osteopenia in the second sacral segment, which may impact the quality of fixation, we feel this technique should be used with caution.

Level of evidence

III     

Biomechanical comparison of sacroiliac screw techniques for unstable pelvic ring fractures

OBJECTIVE: To determine the stiffness and strength of various sacroiliac screw fixations to compare different sacroiliac screw techniques. DESIGN: Randomized comparative study on embalmed human pelvises. MATERIALS AND METHODS: In 12 specimens, we created a symphysiolysis and sacral fractures on both sides. Each of these 24 sacral fractures was fixed with 1 of the following methods: 1 sacroiliac screw in the vertebral body of S1, 2 screws convergingly in S1, or 1 screw in S1 and 1 in S2. On the left and right side of a pelvis, different techniques were used. The pubic symphysis was not stabilized. We measured the translation and rotation stiffness of the fixations and the load to failure using a 3-dimensional video system. RESULTS: The stiffness of the intact posterior pelvic ring was superior to any screw technique. Significant differences were found for the load to failure and rotation stiffness between the techniques with 2 screws and a single screw in S1. The techniques utilizing 2 screws showed no differences. CONCLUSIONS: Based on the results of this study, we can conclude that a second sacroiliac screw in completely unstable pelvic fractures increases rotation stiffness and improves the load to failure.     

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。     

Trans-iliosacral plating for vertically unstable fractures of sacral spine associated with spinopelvic dissociation: A cadaveric study

Background:

The treatment algorithm for sacral fracture associated with vertical shear pelvic fracture has not emerged. Our aim was to study a new approach of fixation for comminuted and vertically unstable fracture pattern with spinopelvic dissociation to overcome inconsistent outcome and avoid complications associated with fixations. We propose fixation with well-contoured thick reconstruction plate spreading across sacrum from one iliac bone to another with fixation points in iliac wing, sacral ala and sacral pedicle on either side. Present biomechanical study tests the four fixation pattern to compare their stiffness to vertical compressive forces.

Materials and Methods:

Dissection was performed on human cadavers through posterior midline paraspinal approach elevating erector spinae from insertion with two flaps. Feasibility of surgical exposure and placement of contoured plate for fixation was evaluated. Ten age and sex matched computed tomography scans of pelvis with both hips were obtained. Reconstructions were performed with advantage windows 4.2 (GE Light Speed QX/I, General Electric, Milwaukee, WI, USA). Using the annotation tools, direct digital CT measurement (0.6 mm increments) of three linear parameters was carried out. Readings were recorded at S2 sacral level. Pelvic CT scans were extensively studied for entry point, trajectory and estimated length for screw placement in S2 pedicle, sacral ala and iliac wing. Readings were recorded for desired angulation of screw in iliac wing ala of sacrum and sacral pedicle with respect to midline. The readings were analyzed by the values of mean and standard deviation. Biomechanical efficacy of fixation methods was studied separately on synthetic bone. Four fixation patterns given below were tested to compare their stiffness to vertical compressive forces: 1) Single S1 iliosacral screw (7.5 mm cancellous screw), 2) Two S1 and S2 iliosacral screws, 3) Isolated trans-iliosacral plate, 4) Trans-iliosacral plate + single S1 iliosacral screw.

Statistical Analysis:

Mean of desired angulation for inserting screws and percentage of displacement on biomechanical testing was evaluated.

Results:

Mean angulations for inserting sacral pedicel were 12.3° (SD 2.7°) convergent to midline and divergent of 14° (SD 2.3°) for sacral ala screw and 23° (SD 4.9°) for iliac wing screw. All screws needed to be inserted at an angle of 90° to sacral dorsum to avoid violation of root canals. Cross headed displacement across fracture site was measured and plotted against the applied vertical shear load of 300 N in five cycles each for all the four configurations. Also, the force required for cross headed displacement of 2.5 mm and 5 mm was recorded for all configurations. Transmitted load across both ischial tuberosities was measured to resolve unequal distribution of forces. Taking one screw construct (configuration 1) as standard base reference, trans-iliosacral plate construct (configuration 3) showed equal rigidity to standard reference. Two screw construct (configuration 2) was 12% stronger and trans-iliosacral plate (configuration 4) with screw was 9% stronger at 2.5 mm displacing on 300 N force, while it showed 30% and 6%, respectively, at 5 mm cross-headed displacement.

Conclusions:

Trans-iliosacral plating is feasible anatomically, biomechanically and radiologically for sacral fractures associated with vertical shear pelvic fractures. Low profile of plate reduces the risk of hardware prominence and decreases the need for implant removal. Also, the fixation pattern of plate allows to spare mobile lumbosacral junction which is an important segment for spinal mobility. Biomechanical studies revealed that rigidity offered by plate for cross headed displacement across fracture site is equal to sacroiliac screws and further rigidity of construct can be increased with addition of one more screw. There is need for precountered thicker plate in future.     

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.     

Biomechanical comparison of posterior pelvic ring fixation

OBJECTIVE: To determine relative stiffness of various methods of posterior pelvic ring internal fixation. DESIGN: Simulated single leg stance loading of OTA 61-Cl.2, a2 fracture model (unilateral sacroiliac joint disruption and pubic symphysis diastasis). SETTING: Orthopaedic biomechanic laboratory. OUTCOME VARIABLES: Pubic symphysis gapping, sacroiliac joint gapping, hemipelvis coronal plane rotation. METHODS: Nine different posterior pelvic ring fixation methods were tested on each of six hard plastic pelvic models. Pubic symphysis was plated. The pelvic ring was loaded to 1000N. RESULTS: All data were normalized to values obtained with posterior fixation with a single iliosacral screw. The types of fixation could be grouped into three categories based on relative stiffness of fixation: For sacroiliac joint gapping, group 1-fixation stiffness 0.8 and above (least stiff) includes a single iliosacral screw (conditions A and J), an isolated tension band plate (condition F), and two sacral bars (condition H); group 2-fixation stiffness 0.6 to 0.8 (intermediate stiffness) includes a tension band plate and an iliosacral screw (condition E), one or two sacral bars in combination with an iliosacral screw (conditions G and I); group 3-fixation stiffness 0.6 and below (greatest stiffness) includes two anterior sacroiliac plates (condition D), two iliosacral screws (condition B), and two anterior sacroiliac plates and an iliosacral screw (condition C). For sacroiliac joint rotation, group 1-fixation stiffness 0.8 and above includes a single iliosacral screw (conditions A and J), two anterior sacroiliac plates (condition D), a tension band plate in isolation or in combination with an iliosacral screw (conditions E and F), and two sacral bars (condition H); group 2-fixation stiffness 0.6 to 0.8 (intermediate level of instability) includes either one or two sacral bars in combination with an iliosacral screw (conditions G and I); group 3-fixation stiffness 0.6 and below (stiffest fixation) consists of two iliosacral screws (condition B) and two anterior sacroiliac plates and an iliosacral screw (condition C). DISCUSSION: Under conditions of maximal instability with similar material properties between specimens, differences in stiffness of posterior pelvic ring fixation can be demonstrated. The choice of which method to use is multifactorial.     

The influence of sacral morphology on the existence of secure S1 and S2 transverse bone corridors for iliosacroiliac screw fixation

Sacroiliac (SI) screw fixation for unstable pelvic fractures stands out as the only minimally invasive method among all other ORIF procedures. A strictly transverse screw trajectory is needed for central or bilateral fracture patterns up to a complete iliosacroiliac fixation. However, secure screw insertion is aggravated by a narrow sacroiliac bone stock. This study investigates the influence of a highly variable sacral morphology to the existence of S1 and S2 transverse corridors.     

S1 pediculoiliac screw fixation in instabilities of the sacroiliac complex: biomechanical study and report of two cases

OBJECTIVE: A new technique for posterior sacroiliac fixation is described and compared with conventional techniques. PATIENTS/MATERIAL AND METHODS: A patient with sacral alar fracture (zone 1) and another one with sacroiliac joint instability due to tuberculous infection underwent fixation using screws placed in the S1 pedicle and the iliac bone. Vertical stability of the new technique also was investigated using polyurethane pelvic bone analogs and compared with anterior double plating (group P) and iliosacral screw fixation (group ISS) techniques. RESULTS: Healing was obtained and reduction was maintained in both patients on the final follow-up examination at 2 years postoperatively. Vertical loading tests revealed that failure loads within the first 10 mm of displacement of the new pediculoiliac screw fixation technique (group PIS) was higher than plating (P = 0.03) and lower than ISS techniques (P = 0.002). Ultimate failure load of the PIS technique was slightly higher than plating (P = 0.277) and lower than ISS techniques (P = 0.003). With the addition of an iliosacral screw to the pediculoiliac screw construction (PIS+ISS), the PIS technique became more stable in early (P = 0.110) and ultimate failure loads (P = 0.003). CONCLUSIONS: Pediculoiliac screw fixation for sacroiliac joint disruptions and zone I sacrum fractures using iliac and S1 pedicle screws is a new and effective alternative for obtaining and maintaining anatomic reduction.     

加长骶髂螺钉固定相关参数的放射解剖学研究

[目的]通过测定S1、S2加长骶髂螺钉可置钉区域的相关指标,为加长骶髂螺钉固定的临床实践提供解剖学依据。[方法]对66名成人进行骨盆CT扫描,对S1、S2加长骶髂螺钉可置钉区域的长、宽、高进行测量,对进钉点作量化定位。对上述指标行统计学描述,并对左右两侧、S1、S2两节段、S1、S2各自不同层面的同种数值进行比较。[结果]模拟左右两侧置钉的相关测量所得同种数值无显著性差异;S1、S2加长骶髂螺钉安全置钉区域的髂骨长度均>16 mm;S1、S2可置钉区域的宽、高均>7.3 mm,且整体而言S1比S2具有更大的宽度值和高度值;加长骶髂螺钉最佳进钉点的量化定位参考范围,S1:髂后上棘前方42.21～63.69mm,坐骨大切迹最高点上方32.77～53.75mm。S2:髂后上棘前方22.68～54.28 mm,坐骨大切迹最高点上方14.06～33.70 mm。[结论](1)S1、S2加长骶髂螺钉的置入具有解剖学的可行性;(2)同一节段左右两侧置入加长骶髂螺钉的各项指标无差异;(3)螺纹长度为16mm的7.3 mm部分螺纹空心钉和6.5 mm部分螺纹松质骨螺钉均可作为加长骶髂拉力螺钉使用;(4)S1、S2均有置入至...     

S1 screw bending moment with posterior spinal instrumentation across the lumbosacral junction after unilateral iliac crest harvest

STUDY DESIGN: A biomechanical study comparing fixation across the lumbosacral junction. OBJECTIVES: To determine which long posterior construct across the lumbosacral junction produces the least bending moment on the S1 screw when only one ilium is available for fixation. SUMMARY OF BACKGROUND DATA: Recent in vitro studies have demonstrated the benefit of anterior support and fixation into the ilium when instrumenting a long posterior construct across the lumbosacral junction. METHODS: Four L2-sacrum constructs were tested on six synthetic models of the lumbar spine and pelvis simulating that the right ilium had been harvested. Construct 1: L2-S1 bilateral screws. Construct 2: L2-S1 + left iliac bolt. Construct 3: L2-S1 + left iliac bolt + right S2 screw. Construct 4: L2-S1 + bilateral S2 screws. The four constructs were then retested with an anterior L5-S1 strut. A flexion-extension moment was applied across each construct, and the moment at the left and right S1 pedicle screw was measured with internal strain gauges. RESULTS: Iliac bolt fixation was found to significantly decrease the flexion-extension moment on the ipsilateral S1 screw by 70% and the contralateral screw by 26%. An anterior L5-S1 strut significantly decreased the S1 screw flexion-extension moment by 33%. Anterior support at L5-S1 provided no statistical decrease in the flexion-extension moment when bilateral posterior fixation beyond S1 was present with either a unilateral iliac bolt and contralateral S2 screw, or bilateral S2 screws. CONCLUSIONS: There is a significant decrease in the flexion-extension moment on the S1 screw when extending long posterior constructs to either the ilium or S2 sacral screw. There is no biomechanical advantage of the iliac bolt over the S2 screw in decreasing the moment on the S1 screw in flexion and extension. Adding anterior support to long posterior constructs significantly decreases the moment on the S1 screw. Adding distal posterior fixation to either the ilium or S2 decreases the moment on S1 screws more than adding anterior support. Further, adding anterior support when bilateral distal fixation past S1 is already present does not significantly decrease the moment on the S1 screws in flexion and extension.     

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.     

拉力螺钉固定技术在骨盆后环损伤中的临床应用

目的探讨应用拉力螺钉固定技术治疗骨盆后环部损伤的适应证、手术方法及疗效。方法1998年8月至2005年12月，对47例骨盆骶髂结构复合损伤患者分别采用后侧经骶髂关节的拉力螺钉固定方法，后侧跨骶骨的髂-髂拉力螺钉固定方法及骶-髂拉力螺钉加髂：髂拉力螺钉固定方法治疗骨盆后环损伤，男31例，女16例；年龄18～53岁，平均32岁。通过影像学检查明确47例患者有60侧骨盆骶髂结构复合损伤。按照AO骨盆环损伤方法分型：B型损伤20例，其中B2型9例，B3型11例；C型损伤27例，其中C1型8例，C2型12例，C3型7例。患者麻醉后取俯卧位，行闭合或开放复位后用拉力螺钉固定。所有操作均在C型臂X线机透视下进行。结果47例患者均获随访，随访时间6个月至6年6个月，平均27．7个月。平均手术时间为80min。47例共置入骶髂拉力螺钉104枚。患者在术后2～3个月双下肢可完全负重行走，并逐渐恢复体力劳动。仅发生3枚螺钉松动，未发生神经损伤及螺钉断裂等并发症，腰骶及下肢活动接近正常。结论拉力螺钉固定技术可使不稳定性骨盆后环获得良好的即刻复位和固定，恢复骨盆环的稳定性，适用于骶髂关节脱位及骶骨骨折患者。     

Sacroiliac joint luxation after pedicle subtraction osteotomy: report of two cases and analysis of failure mechanism

Background

Sagittal decompensation after pedicle subtraction osteotomy (PSO) is considered as late onset complication. Several mechanisms have been suggested, but little attention has been paid to the caudal end of lumbar instrumented fusion, especially sacral iliac joint (SIJ) deterioration.

Methods

Clinical histories and radiographic sagittal parameters of two patients with SIJ luxation after PSO are presented. The biomechanical failure mechanism and risk factors are analysed.

Results

Two patients underwent correction of fixed anterior sagittal imbalance by PSO, followed by pseudarthrosis revision surgery. Both of them sustained persistent sacroiliac pain, progressive recurrence of anterior imbalance and progressive pelvic incidence (PI) increase around 10°. An acute bilateral SIJ luxation occurred in both patients leading to sharp increase or PI around 20°. One patient was treated by SIJ fusion and the other patient was placed on non-weight-bearing crutch ambulation for 1 year. Both patients had a high preoperative PI (95° and 78°). A theoretical match between lumbar lordosis (LL) and PI was not achieved by PSO. Osteopenia was present in both patients. Computed tomography evidenced L5–S1 pseudarthrosis and sacroiliac joint violation by pelvic or sacral ala screws.

Conclusion

Patients with high PI might seek for further compensation at their SIJ when lacking LL after PSO. Chronic anterior imbalance might lead to progressive weakening of sacroiliac ligaments. Initial circumferential lumbosacral fusion and accurate iliac screw fixation might reduce stress on implants, risk for pseudarthrosis, implant failure and finally SIJ deterioration. Bone mineral density should further be investigated preoperatively.

     

An anatomical update on the morphologic variations of S1 and S2

Although percutaneous fixation with iliosacral screws has been shown to be a safe and reproducible method for sacroiliac dislocation and sacral fractures, it is a technically demanding technique, and one of its contraindications is sacral anatomical variations and dysmorphism. The incidence and pattern of S1 and S2 anatomical variations were evaluated in 61 patients (35 women and 26 men) using magnetic resonance imaging of the sacrum in an attempt to explore the possible existence of groups of individuals in whom percutaneous sacroiliac fixation is difficult due to local anatomy. S1 and S2 dimensions in both the transverse and coronal planes were recorded and evaluated. In each individual, S1 and S2 dimensions both in the coronal and transverse planes were proportional, with S2 dimensions being 80% of those of S1 on average. Patients were separated into 4 groups based on the S1 and S2 body size and the asymmetry of dimensions in the transverse and coronal planes. In 48 patients (78.6%), dimensions in both planes were symmetrical despite the varying size of the S1 and S2 body. In 2 patients (3.3%) there was a combination of large transverse plane and small coronal plane dimensions, with large S1 and S2 body size. In 9 patients (14.8%), coronal plane dimensions were disproportionately smaller compared to those of the transverse plane, with a varying size of S1 and S2 body making effective sacroiliac screw insertion a difficult task. Thus, a preoperative imaging study, preferably computed tomography scan, of S1 and S2 body size and coronal plane dimensions and an intraoperative fluoroscopic control of S1 and S2 dimensions on the coronal plane are suggested for safe sacroiliac screw fixation.     

Incidence of and Risk Factors for Screw Loosening after Iliosacral Screw Fixation for Posterior Pelvic Ring Injury

Purposes

Screw loosening is a common complication of iliosacral screw fixation, with subsequent loss of stability and fracture re-displacement. This study aimed to investigate the incidence of and risk factors for screw loosening after iliosacral screw fixation for posterior pelvic ring injury.

Methods

A total of 135 patients with posterior pelvic ring injuries who were treated with iliosacral screw fixation in our department between July 2015 and April 2021 were selected for this retrospective analysis. The possible risk factors for screw loosening were investigated using univariate and multivariate logistic regression analyses of patient demographics and trauma-related and iatrogenic variables, including age, sex, body mass index, Osteoporosis Self-Assessment Tool for Asians (OSTA) index, mechanism of injury, Young–Burgess classification, site of injury, type of injury, type of screw, mode of fixation, numbers of guidewire adjustments, accuracy of screw position, and quality of fracture reduction.

Results

The incidence of screw loosening was 15.6% (n = 21). The mean duration for screw loosening was 3.2 ± 1.5 months after operation. Univariate analysis results showed that the Young–Burgess classification, type of injury, site of injury, type of screw, mode of fixation, and OSTA index might be related to screw loosening (p < 0.05). According to the multivariate logistic regression, vertical shear injuries (Odds ratios [OR] 9.80, 95% Confidence intervals [CI] [1.96–73.28], p = 0.008), type of injury (OR 0.25, 95% CI [0.13–0.79], p = 0.027), common screws (OR 6.94, 95% CI [1.53–31.40], p = 0.012), screws insertion only at the level of the first sacral segment (S1) (OR 8.79, 95% CI [1.18–65.46], p = 0.034), injury site located in the medial sacral foramina (OR 6.28, 95% CI [1.16–34.06], p = 0.033), and lower OSTA index [OR 0.41, 95% CI [0.24–0.71], p = 0.001] were significantly related to screw loosening.

Conclusions

Vertical shear injuries, sacral fractures, injury site located in the medial sacral foramina, and lower OSTA index are significantly associated with the postoperative occurrence of screw loosening. Transiliac–transsacral screw fixation and screws insertion both at the level of the S1 and second sacral segment can prevent screw loosening.     

Accuracy of computer-guided screw fixation of the sacroiliac joint.

Computer-assisted image guidance allows precise preoperative planning and intraoperative localization of surgical instruments. The technique recently was validated for the insertion of pedicle screws. In the laboratory, the precision of a surface-matching algorithm was evaluated for registration and accuracy and safety of screw placement into the vertebral bodies of S1 and S2 for fixation of the sacroiliac joint. Using six plastic pelves, 24 screw holes were made through the sacroiliac joint into the vertebral body of S1, and 12 holes were made through the sacroiliac joint into S2. The accuracy of the hole position was evaluated using a postoperative computed tomography examination. The safety factor was assessed by analysis of the remaining bone stock around the holes calculating a theoretical cylindrical volume being outside bone with increasing bore hole diameters. The registration was accurate with a mean error less than 1.4 mm in the posterior parts of the pelvis. The drilling followed precisely the preoperatively planned trajectories; perforation of the cortex of the sacrum was not observed. The safety factor of the S1 vertebral body is higher than that of S2 allowing larger diameter screw insertion into S1. This technique provides a safe and precise guide for transcutaneous or open insertion of iliosacral screws in cases of iliosacral dislocation or sacral fracture.