A novel technique for sacropelvic fi xation using image-guided sacroiliac screws:a case series and biomechanical study

Kee D.Kim,Huy Duong,Aditya Muzumdar,Mir Hussain,Mark Moldavsky,Bandon Bucklen

1 Department of Neurological Surgery,University of California,Davis,Sacramneto,CA 95817,USA;

2 Department of Neurosurgery,Kaiser Permanente Medical Group,Sacramento,CA 95825,USA;

3 Musculoskeletal Education and Research Center(MERC),Globus Medical Inc,Audubon,PA 19403,USA.

AbstractIn this study,we sought to assess the safety and accuracy of sacropelvic fi xation performed with image-guided sacroiliac screw placementusing postoperativecomputed tomography and X-rays.Thesacroiliac screwswereplaced with navigation in fi ve patients.Intact specimens were mounted onto a six-degrees-of-freedoMspine motion simulator.Long lumbosacral constructsusing bilateral sacroiliac screwsand bilateral S1 pedicleand iliac screwswere tested in seven cadaveric spines.Ninesacroiliac screwswerewell-placed under an imageguidancesystem(IGS);one was placed poorly without IGSwith no symptoms.Both fi xation techniques signi fi cantly reduced range of motion(P＜0.05)at L5-S1.The research concluded that rigid lumbosacral fi xation can be achieved with sacroiliac screws,and imageguidance improves itssafety and accuracy.Thisnew techniqueofimage-guided sacroiliac screw insertion should prove useful in many types of fusion to the sacrum,particularly for patients with poor bone quality,complicated anatomy,infection,previous failed fusion and iliac harvesting.

Keywords:lumbosacral fi xation,sacropelvic fi xation,sacroiliac screw,computer-assisted surgery,image-guided surgery

Introduction

Posterior lumbosacral instrumented fusion is widely used to treat spinal instability caused by a variety of conditions including degenerative disease,infection,tumor,trauma and deforMity[1-3].When the instrumented constructis long and extendspast the lumbar region,a largemoment arMisplaced on the caudal end,putting the construct at risk for fusion failure,instrumentation pullout,sacral fracture and loss of lordosis[4-5].The construct w ill be in additional jeopardy when the spine is osteopenic or osteoporotic,and post-radiation.

One commonly used approach to strengthen the caudal end of lumbosacral constructs is to extend fi xation to the pelvis with the use of bilateral iliac screws.They are placed with bicortical purchase in an anterolateral direction froMthe posterior superior iliac spine into thehard cortical boneabove thesciatic notch,thereby resisting the cantilever moment of the long construct at the cranial end[2,6].The iliac screw heads are generally positioned lateral to the axis of lumbar/sacral screws,and therefore require additional bending of the rods,or the use of an offset connector.In this article,we present an alternative technique for strengthening the lumbosacral construct using bilateral sacroiliac screws.Thesacroiliac screwsarealso inserted in the anterolateral direction into the hard cortical bone above the sciatic notch to optiMize resistance against the cantilever moment of a long construct superiorly.The headsof the sacroiliac screwsare in-linewith the rest of the lumbosacral screws in a long construct,which obviates the need for additional bending of the rod or the use of offset connector.

In this study,we evaluated the biomechanical parameters of sacroiliac I screws in cadavers with nondestructive range of motion(ROM)and flexibility testing.We present the results of our study,and w ill discuss the advantages of sacroiliac screws over traditional iliac screws.We w ill also describe the technique of sacroiliac screw placement with use ofimage guidance/fluoroscopy to make sacropelvic fi xation safer and more accurate..

Materials and methods

Sacroiliac screws were placed in fi ve patients under image guidance froM2005 through 2007 with permission from,and in accordancewith theguidelinesof,the Institute of Research and Ethics(Table 1).All patients were referred to the senior author(KDK)through the UC Davis Health System.The decision to perforMsacropelvic fi xation was mainly based on the need for additional points of fi xation in the pelvis to attain successful lumbosacral fusion.A ll patientshad received preoperative lumbosacral CTand MRIscans—and,in some cases,plain radiographs.

Image guidance and surgical procedure

Fig.1 Artist's rendering of the pelvis showing sacroiliac screw position.

The Stealth Station Image Guidance System(IGS)(Medtronic Navigation,Louisville,CO,USA)wasused for preoperative trajectory planning and intraoperative screw placement.Preoperative CT data were imported into the IGS computer workstation,and anatomic registration points were selected.The points selected on bony landmarks were those that were most likely to be identi fi ed once standard posterior lumbosacral exposure for posterolateral fusion was attained.Registration landmarks usually consisted of one or two spinous processes,and two to four facet joints in the lumbosacral region.In patients undergoing reoperation,any easily identi fi able bony landmarks were chosen.The ideal screw trajectory traversed the cortices of the sacroiliac joint,with the tip of thescrew within the iliuMin the cortical bone above the sciatic notch(Fig.1).Sacroiliac screw trajectories were planned using twodimensional navigational and three-dimensional(3D)reconstructed views(Fig.2).MaximuMideal screw length and diameter were determined froMthe preoperative virtual plan.

A midline skin incision and subperiosteal dissection were performed to expose the relevant spinal levels outto the transverse processes and the sacral ala.A passive reference arc was clamped onto the spinous process of the most cephalad vertebra that was exposed.The reference arc was cranially angled away froMthe lumbosacral region to minimize the possibility that it could be inadvertently moved after registration.Point registration and surface merging were completed to optimize accurate registration.Good registration accuracy was consistently veri fi ed by touching observable bony landmarks with the IGS probe tip and correlating probe location,with the virtual image displayed on the monitor.Pedicle screws were typically inserted at lumbar levels using standard techniques.

Table 1 Characteristics of fi ve consecutive cases ofimage-guided sacropelvic fi xation performed by senior author froM2005 through 2007

Fig.2 Stealth IGS monitor snapshot.A:Upper left:parasagittal view show ing the trajectory froMthe sacruMto the pelvis.Upper right:probe’s eye view of the ilium.This view is particularly helpful for planning a screw trajectory that avoids cortical breech and has an ideal location,where a screw may be surrounded by stronger cortical bone(arrow).Lower left:theaxial view of sacropelvic screw trajectory.Lower right:3D view of thesacropelvic region.Note theextensive iliac defect(arrow)on the right froMan iliac crest harvestperformed during previous surgery.Image guidance is helpful in planning the trajectory to avoid the area of defect/weakness.B:Postoperative axial CT show ing good placement of bilateral sacroiliac screws.

Fig.3 DiagraMs,photographs and radiographs with the two surgical constructs(Sacroiliac fi xation vs.bilateral S1 pedicle screw and iliac screw[S+I]fi xation).SI:sacroiliac.

Next,in keeping with the preoperative plan,IGSwas used to select sacroiliac screw entry points on the sacrum.A new entry point not included in the preoperative plan may have been selected to facilitate rod alignment,contouring and attachment to screw heads.Typically,the point of entry was noted at S1 or S2.Thecortical wall of thesacruMwaspenetrated by an IGS aw l.Next,an IGS sounding probe was used to createapilothole for thescrew.Thepilothole trajectory may have been modi fi ed froMthe virtual plan for placement of the screw in the ideal portion of the ilium(the cortical bone above the sciatic notch).The sounding probe was advanced with IGS tracking.The virtual navigation view was used with tactile feedback to discern when the sacroiliac joint wasbeing traversed.Pilot hole integrity was checked with a ball-tip probe;then,the IGS tap was used to tap the same pilot hole.The tapped holewas typically probed again to assess for cortical breach.Sacroiliac screws were then inserted,with care taken to avoid deviating froMthe tapped trajectory.Sacroiliac screws were connected to the remaining pedicle screws with contoured rods to forMa physiologic lumbosacral curve.The fusion surface was routinely decorticated,and bone graft was placed after the rod had been secured with setscrews.A postoperative CT scan was performed to check the screw position.

Biomechanical testing

Seven cadaveric spines(ilio-lumbosacral)were harvested and stripped of all musculature while all of their ligamentous structures,vertebral bodies and intervertebral discs were kept intact.X-rays were obtained to assess the specimens’bone quality.Spine specimens were secured into test fi xtures at L1 and the sacrum.Intact specimens were mounted onto a sixdegrees-of-freedoMspine motion simulator,and nondestructive ROMflexibility testing was conducted by applying pure moments(±6 Nm)in three randoMly selected physiologic planes:flexion-extension,lateral bending and axial rotation[7-8].Segmental L5-S1 motions were collected via an optoelectronic motion measurementsysteMwith infrared light-eMitting diodes placed at L5 and S1.After intact testing,6.5-mmdiameter bilateral pedicle screws(REVERE®Stabilization System,Globus Medical,Inc.,Audubon,PA,USA)were placed at L1-L5,without violation of facet joints or disc spaces.Iliac screws that were 7.5 mMin diameter were used,and a 5.5-mm-diameter rod was instrumented for all biomechanical testing to be performed.Sacroiliac fi xation was performed by using(1)bilateral sacroiliac screws and(2)bilateral S1 pedicle screws and iliac screws(S+I)(Fig.3).Lumbosacral rods were sized and contoured appropriately for the seven specimens.Additional offset connectors were used to connect iliac screws to rods(S+Iconstruct).ROMdatawerenormalized relative to the value of the intact spine.Statistical analysis was performed on the raw data using repeated measures analysis of variance and Tukey’s post hoc test(P＜0.05).The Data Analysis Tool Pack available in Microsoft Excel(Microsoft Corp.,Redmond,WA,United States)was used to run the analysisof variance,and the post-hoc test was calculated based on the equation for Tukey’s honest signi fi cant difference test.

Fig.4 Biomechanial study constructs.A:Stealth IGS snapshot show ing preoperative trajectory planning.Note the lack ofinclusion of the rightiliuMin the CT fi eld of view.B:Postoperative CTshow ingaxial imagesof malpositioned right SIscrew placed under fluoroscopic guidance(indicated by arrows)compared with a properly positioned left sacroiliac screw placed via IGS.IGS:image guidance system.

Fig.5 Normalized ROM(%)at L5-S1 for all loading modes.ROM:range of motion.*P＜0.05 vs.intact.

Fig.6 Stealth IGS snapshot taken during preoperative p lanning.Red arrows indicate iliac crest defect froMprior graft harvest site.The planned trajectory allows for avoidance of this defect,enhancing fi xation strength.IGS:image guidance system.

Results

In vivo sacroiliac screw p lacement

Five patients(four males,and one female)with a mean age of 58 years(range:29-84 years)underwent screw placement under an IGS to allow visualization of the screw trajectory(Table 1).In all cases,the surgical indication for placement of sacroiliac screws was the need to supplement a planned lumbosacral fusion with sacropelvic fi xation to add strength to the construct.In two cases(Table 1,Cases2 and 5),anterior instrumented fusion was combined with posterior fusion.Diagnoses of treated patients included severe degenerativedisease(n=2),infection(n=2)and trauma(n=1).A total of nine sacroiliac screws were placed under IGS,and one without.Eight sacroiliac screws were placed with entry points in S2,and two were placed with entry points at the caudal aspect of S1(Table 1,Case 5).All image-guided screws were placed in good anatomic positions.The right-sided screw in Case5 was placed without image guidance,as the fi eld of view on the preoperative CT scan did not include the iliuMon the right side.Postoperative CT showed that this screw was malpositioned with ventral perforation into the retroperitoneal space adjacent to the iliopsoas(Fig.4),but the contralateral image-guided screw position was ideal.In the present series,no vascular or neurologic injuries were detected postoperatively.Follow-up at longer than 2 years revealed no construct failures.During the fi rst postoperative month,one patient(Table 1,Case 5)died froMcomplications of aspiration and sepsis not related to the surgery.

In vivo comparisons with intact

During flexion-extension,ROMfor sacroiliac and S+Iconstructs was(31.1±16.7)%and(35.9±14.9)%respectively,compared with the intact.In lateral bending,SI and S+I constructs reduced motion to(37.3±17.5)%and(38.5±17.4)%,respectively,compared with the intact.Finally,during axial rotation,sacroiliac and S+I constructs reduced ROMto(77.6±21.3)%and(72.6±23.0)%,respectively,compared with the intact.

In vivo range of motion

Both fi xation techniques signi fi cantly reduced ROM(P＜0.05)at L5-S1 compared with the intact in all loading modes(flexion-extension,lateral bending and axial rotation;Fig.5).We observed no statistically signi fi cant differences(P＜0.05)in the stability offered by both techniques in any of the loading modes.ROMwith sacroiliac fi xation showed a slight decrease in flexion-extension and lateral bending,aswell asaslight increase in axial rotation in comparison to S+I fi xation.

Discussion

Various sacral screws,and iliac bolts or screws,have been utilized for lumbosacral constructs[9].Orthopaedic surgeonshave described the percutaneousplacement ofiliosacral screws inserted froMthe iliuMto the sacruMfor pelvic ring reconstruction,including the use ofimaged-guided fluoroscopy[10-15].To our know ledge,this is the fi rst report in English literature that describes theuseofimageguidance for a long screw with an entry point in the sacrum,traversing the SI joint and terminating within the ilium.

Rationale

Pseudoarthrosis rates as high as 45%,with in situ fusion of the lumbosacral junction,have been reported for high-grade spondylolistheses[16].Achieving a solid arthrodesis is made more dif fi cult by additional challengesposed by prior surgery,infection,osteoporosisor severe deforMity.Consequently,variousaugmentation techniques for lumbosacral arthrodesishavebeen reported[6,16].Oneoption for increasing the rateof solid fusion involves using an interbody graft at the L5-S1 disc space[17-18].

Another option for augmenting lumbosacral fusion involves using the iliuMto share the stress and load placed on the sacrum.Iliac screw fi xation has been shown to improve biomechanical strength at the lumbosacral junction,with reasonable clinical outcomes[6,19].In comparison to other methods of sacral fi xation such as intrasacral rods,iliac screws are more effective in preventing S1 pedicle screw failure[6,9,18].The additional points of fi xation attained with iliac screws provide greater protection than what is provided by interbody arthrodesis with posterior instrumentation at L5-S1 alone[17].

Researchers have asserted that the best available construct for MiniMizing lumbosacral instrumentation failure and enhancing fusion consists of long,bilateral iliac screws and bicortical sacral screws with anterior column support.Iliac screws frequently require removal due to the discoMfort associated with their prominence over the posterior iliac w ing at the PSIS[6,20].Sacroiliac screws,on the other hand,are no more prominent than sacral pedicle screws.

Iliac screws(in thebilateral S1 pediclescrew and iliac screw[S+I]construct)introduce the disadvantage of awkward attachments to the lumbosacral rods via short offset connectors,contributing to the potential for mechanical failure at this additional interface.Alternatively,sacroiliac screws can be aligned readily with lumbosacral screw heads,enabling easeof anatomic rod contouring.

Often,iliac screws are placed by creating a breach in the iliac cortex(at the PSIS)to allow seating of the screw head,which diMinishes screw purchase to only one cortex(cortical bone above the sciatic notch).Akesen et al.[20]noted that iliac screws have a much weaker bone and screw interface than lumbar or sacral pedicle screws as a result of their positioning in cancellous bone.These authors theorized that a better sacropelvic fi xation technique would diMinish screwtoggling stress,which,in turn,could reduce screw loosening.This iliac-toggling stress can be reduced by the sacroiliac screw via its insertion through four cortices(sacral dorsum,both sacroiliac joint cortices,and the thick cortical bone above the sciatic notch).

Indications

Indications for sacropelvic fi xation are broad.Previously reported indications include long fusion to the sacruMfor spinal deformity,medium-and high-grade isthmic spondylolisthesis,revision surgery,and surgery involving patients at high risk for fusion failure at the lumbosacral junction[2,4,20-24].

Advantages

The method proposed for sacropelvic fi xation with bilateral sacroiliac screws provides several advantages over iliac screw fi xation.Sacroiliac screws do not requiremodi fi cation of midlineexposure for placement.In contrast,more lateral exposure is needed to allow visualization of the iliac entry point for iliac screws.Thisapproach lengthens the surgical procedure,leading to increased blood loss.Sacroiliac screws are less prominent than iliac screws because of their deeper,more medial screw head position.In contrast,iliac screw s frequently are proMinent and can cause discoMfort,prompting their removal.Iliac screws and their offset connectors have been associated with a substantial incidence of breakage in some series,although pseudoarthrosis or pain is not always reported[2,6,20,25].

Additional advantages offered by sacroiliac screws include easier rod contouring and a simpler fi nal construct,both of which reduce the possibility ofinstrumentation failure.Image guidance obviates the need for typical anatoMic landmarks when screws are placed.Rather,an entry point on the dorsal sacruMaligned with ipsilateral lumbosacral screwsmay beused to allow easy positioning of the rod.Theentry point for iliac screws,on the other hand,is typically not aligned with the lumbosacral pedicle screws.This poses an additional challenge in bending the rod,or results in an additional point of construct failure when offset connectors are used.

A lthough not tested,sacropelvic fi xation with sacroiliac screws should have greater pullout strength than sacropelvic fi xation with iliac screws because additional cortical walls are traversed.Four cortical walls are traversed by sacroiliac screws as opposed to oneor two by iliac screws,depending on whether or not the cortical iliac entry point is removed in an effort to bury the iliac screw,thus avoiding its proMinence.Additional pullout strength of the sacroiliac screw sacropelvic fi xation construct may enhance successful bony fusion.The biomechanical study demonstrated L5-S1 stiffness in the sacroiliac construct comparable with that in the currently used S+Iconstruct.

Enhanced safety afforded by image guidance

Attempting sacroiliac screw placement without the use of an IGS is not recommended.Sacroiliac screw trajectory often requires that the surgeon’s hand be in contact with the skin and paraspinal muscle while a sounding probe isadvanced past the sacroiliac joint and into the ilium.Tension froMtheparaspinal musclemass reduces the bony tactile feedback upon which the surgeon normally depends to avoid cortical perforation.The use ofimage guidance dramatically decreases the risk to nearby pelvic structures,and effectively obviates the need for intraoperative radiographic veri fi cation.The iliac screws used are typically 7.5 mMin diameter and 60 to 80 mMlong[6,22,25].However,at least one radiologic and anatomic study has suggested that the maximal length ofiliac screws can be up to 141 mMin men and 129 mMin women,and a diameter of 6 to 8 mMallowsgood purchase[26].Imageguidanceallows for the placement of larger-diameter,longer screwswith improved safety.Furthermore,using the IGS during preoperative planning w ill allow the practitioner to determine screw sizes in advance.This facilitates appropriate selection of the instrumentation set,as some systems do not include screws of longer length or greater diameter.

Two published reportshave described sacroiliac screw fi xation with the sacral insertion point starting at S2.In a study by Chang et al.[27],a screw pathway froMsacrum(S2)to ilium was proposed on the basis of 3D radiographic analysis of skeletally mature adolescents with normal pelves.In another study,O’Brien et al.[28]described percutaneous placement of sacrum(S2)-iliac screws in cadaveric specimens on the basis of measurements derived froM3D CT images.However,we have comeacrossno studiesof sacroiliac screw placementthat used image guidance.The proposed technique,which is not limited to S2 of the sacrum,may begin at S1.

Multiple studiesusing cadaveric specimensor plastic models have veri fi ed the safety and effectiveness ofimage-guided percutaneous placement ofiliosacral screws[10-11,14-15].Gautier et al.[10]found that image guidance was safe for open or percutaneous screw fi xation of the SI joint.Hinsche et al.[11]reported that radiation exposure was considerably lessened,and that the major advantage was surgical guidance in four planessimultaneously.SMith et al.[14]found thatscrews were placed more accurately under image guidance,although these investigators did encounter errors with all methods of screw placement.

Tonetti etal.[15]used four human pelves,placed all 12 screws correctly,and found the method to be safe.Inserting sacroiliac screws under IGS should not signi fi cantly lengthen operative time compared with that required for iliac screw fi xation.The time consumed by registration may be balanced out by the time saved avoiding lengthy fluoroscopic imaging,which would be necessary without IGS.Furthermore,patients undergoing reoperation frequently have had a prior iliac crest graft harvest on one or both sides.With the use ofiGS,the placement of sacroiliac screws in patients with prior iliac harvesting was simple.With an appropriate preoperative plan,screw insertion was effectively guided,and the previous iliac harvest site was avoided(Fig.6).

Potential complications

Potential complications can be inferred froMthose seen in the context of sacral pedicle screws,iliac screws and iliosacral screws,including percutaneous screws[12-13,20].Practitioners may encounter iliac or sacral fracture,vascular or nerve root injury,spinal fluid leak,screw malpositioning,haloing,loosening,fatigue or failure.Technical vigilance and image guidance should minimize theoccurrence of these complications.The use of sacropelvic fi xation does not totally eliMinate the incidence of pseudoarthrosis at L5-S1,even when anterior column support is provided[6,29].Available literature provides no evidence that iliac screws are deleterious to the SI joint at 5 to 10 years of follow-up[6].

LiMitations

Current IGS systems have limitations.The length of the sounding probe and tap is liMited,as these instruments are designed for pedicle screws no longer than 50 mm.In many cases,shorter screws have had to be used,because screws longer than 70 mMwere not available.Tracking the screw itselfis not possible with many imageguidancesystems.Thus,greatcaremustbe taken to vigilantly adhere to the trajectory already established by the image-guided sounding probe and tap.Sacroiliac screw placementwithoutimageguidance may be too risky.with image guidance,most spine surgeons w ill be able to place sacroiliac screws successfully despite their level of expertise.However,the IGS accuracy must always be veri fi ed before navigation to avoid complications.As noted in the case of malpositioning,it is important to ensure that the preoperative CTobtained with IGShasan adequate fi eld of view that includes both iliac crests.

In conclusions,lumbosacral fusion continues to evolve through technological advancements in spinal instrumentation,and in the development of tools that facilitate the safe and accurate placement ofimplants.As the U.S.demographic shifts toward a larger geriatric population,more problems with the osteoporotic spine w ill be encountered.Consequently,more patients w ill develop complex lumbosacral pathologies that w ill require stronger and more durable spinal fi xation techniques.This new technique ofimage-guided sacroiliac screw insertion should prove useful in many types of fusion to the sacrum,particularly for patients with poor bonequality,complicated anatomy,infection,previous failed fusion and iliac harvesting.