AnatomyLowerSpine

SPINE Volume24,Number21,pp2183–2187©1999,Lippincott Williams&Wilkins,Inc.Peridiscal Vascular Anatomy of the Lower Lumbar SpineAn Endoscopic PerspectiveRobert T.Vraney,MD,Frank M.Phillips,MD,F.Todd Wetzel,MD, and Marshall Brustein,MDStudy Design.An analysis of the vascular anatomy relative to the underlying discs in the lower lumbar spine performed by using radiologic studies.Objective.To define better the vascular anatomy of this region from the perspective of the endoscopic spine surgeon,and to draw conclusions regarding surgical ac-cessibility of the lower lumbar intervertebral discs.Summary of Background Data.Cadaveric studies have highlighted the high degree of anatomic variability among the vascular structures overlying the anterior ele-ments of the lower lumbar spine.An endoscopic ap-proach to the intervertebral disc at these levels often is limited by arterial or venous structures.The growth of laparoscopic techniques to approach the intervertebral disc has renewed interest in the vascular anatomy of this region.Methods.A computer-generated series of abdominal vascular studies performed for unrelated indications were scrutinized to ensure clear demarcation of the overlying vascular structures and underlying bony anatomy of the lumbar spine in a true anteroposterior plane.For analysis, 21arterial(abdominal angiograms)and22venous (venograms)studies were selected.For the arterial data, level of aortic bifurcation and course of common iliac vessels were determined.The venous data were analyzed in a similar manner using the confluence of the common iliac veins as the major landmark.To draw conclusions about anterior surgical accessibility of the disc,the loca-tion of vessels was measured with respect to interverte-bral disc spaces.Results.The bifurcation level of the abdominal aorta was quite variable,occurring anteriorly to the L3vertebral body in3of21studies(14%),and anteriorly to L4and L5 in10(48%)and7(38%)of the studies,respectively.The cranial half of L5was the most common single area of bifurcation(in one third of the studies).Venous anatomy was more consistent,with86%of the patients displaying an iliac vein confluence at L5,and14%having a conflu-ence overlying L4.From an arterial perspective the L3–L4, L4–L5,and L5–S1discs were readily accessible in10%, 38%,and95%of the studies,respectively.From a venous perspective,these levels were accessible in100%,73%, and60%of the studies,respectively.Conclusions.The results of this study suggest that a laparoscopic approach to the L3–L4intervertebral disc will seldom be accomplished without significant retraction of the aorta.Access to the L4–L5space will be accomplished readily in approximately one third of the patients.In the others,it will require significant vascular dissection.TheL5–S1space,conversely,will be readily accessible by theauthors’definitions without significant vessel dissectionin the majority of patients.[Key words:laparoscopic spinesurgery,lumbar spine,vascular anatomy]Spine1999;24:2183–2187The use of endoscopic techniques in spinal surgery has grown rapidly since T.G.Obenchain described thefirst laparoscopic lumbar discectomy in1991.7Endoscopic procedures,which promise smaller incisions,less pain, shorter hospital stays,and quicker recovery times,have proven their worth in thefields of general,urologic,tho-racic,and gynecologic surgery.In many cases,endo-scopic techniques have supplanted their open counter-parts as the procedures of choice.Although early results in the lumbar spine have been encouraging,pioneers of these techniques have pointed out that these procedures are technically demanding and fraught with poten-tial complications.4,6,10,12Injuries to critical neurovascular structures with en-doscopic approaches to the lumbar spine make up a large proportion of the reported complications.4,6,12The inti-mate relation of the great vessels(inferior vena cava, aorta,common iliac arteries,and veins)to the anterior structures of the lumbar spine has,in many series,limited approaches.4,5,7,9,12Access to the anterior lumbar spine is limited by an assumed aortic bifurcation and iliac vein confluence at or near the L4body.Recent cadaveric stud-ies,however,have revealed marked variability in the vas-cular structures adjacent to the lumbar spine.3Because these structures represent the major factor limiting acces-sibility to the middle and upper lumbar spine,a clear delineation of the vascular anatomy and its relation to the disc space would provide useful information to the spinal surgeon wishing to use these techniques.This study represents an attempt to define the peridis-cal arterial and venous anatomy of the lower lumbar spine and its relation to the intervertebral discs.MethodsA series of radiologic abdominal vascular examinations per-formed for vascular conditions was selected from a computer-generated list.The studies were scrutinized carefully for a clear display of the pertinent vascular anatomy superimposed on the bony elements of the underlying lumbar spine in a true antero-posterior plane.Studies with significant rotation(assessed byFrom the University of Chicago Spine Center,Section of OthopaedicSurgery and Rehabilitation Medicine,Chicago,Illinois.Acknowledgment date:November19,1998.Acceptance date:March11,1999.Device status category:1.2183positioning of the pedicles and spinous processes)were dis-carded.Arterial data were obtained from 21abdomi-nal angiograms.For the analysis,13women and 8men ranging in age from 42to 80years (mean,66years)were selected.Measurements to the nearest millimeter were made by two independent observers.These measurements displayed a high degree of interobserver reliability and accuracy.The level of aortic bifurcation was determined.Measurements of the dis-tance between the right and left common iliac arteries were recorded with respect to underlying disc spaces at spinal levels caudal to the bifurcation.Measurements of the total width of the disc spaces at these levels also were recorded.These mea-surements allowed the percentage of disc space free of directly overlying artery to be calculated (Figure 1).Because the bifurcation of the aorta occurred roughly in the midline,the calculated “clear space”between the common iliac arteries was likewise located centrally.Because the central third of the disc space is the most frequently sought starting point for laparoscopic lumbar discectomy,discs with the central portion and adjacent 30%of the disc width uncovered by overlying artery was defined as readily accessible with respect to the ar-terial anatomy.Venous data were determined from 22venograms that had been performed for placement of inferior vena cava filters.Analysis was performed on 11women and 11men ranging in age from 27to 94years (mean,64years).Data were recorded in a fashion similar to that for the arterial studies.The conflu-ence level of the right and left common iliac veins was deter-mined and the width of the inferior vena cava recorded.Mea-surements of the portions of these vessels overlying the disc spaces from L3–L4to L5–S1were recorded.Measurements of the total width of the discs at these levels as well as the propor-tion of disc clear of overlying venous structures also were re-corded to define the proportion of disc space free of overlying veins.The accessibility of the disc space was defined as de-scribed for the arterial measures.ResultsArterial StudiesA total of 21arterial studies were analyzed (Table 1).The level of aortic bifurcation was determined with re-spect to the cranial or caudal half of the underlying ver-tebral body.Overall,the most common level of bifurca-tion was at L4.Five aortas divided over the cranial half of L4and five over the caudal half.Three of the 21patients displayed a bifurcation overlying the L3body (one cranial and two caudal).The level of aortic bifurca-tion was at the cranial half of L5in seven studies,whereas in a single study the aorta divided over the cau-dal half of L5.Overall accessibility with respect to the overlying ar-terial anatomy was determined for each of the poten-tially approachable levels studied (Figure 2).In three studies,the aorta bifurcated proximally to the L3–L4disc.However,L3–L4was readily accessible in only two instances (9.5%of the patients).In the remaining 18studies with the aortic bifurcation caudal to L3–L4,the aorta occupied the central portion of the disc space at L3–L4,rendering the disc space inaccessible.The L4–L5disc space was readily accessible from an arterial perspective in only 8of 21specimens (38%),whereas the L5–S1disc space was almost universally ac-cessible,available in 20of 21instances,or 95%of the time.Venous StudiesAt total of 22venous studies were available for analysis (Table 2).The confluence level of the common iliac veins was determined with respect to the underlying vertebral bodies.In general,there was substantially less anatomic variability in the venous studies.The single most com-mon level of iliac vein confluence was at the cranial level of L5,whereas the iliac veins converged in 14of the 22patients.In an additional five patients,this occurred at the caudal level of L5.The most cranial level of conflu-ence was the caudal level of L4,where the iliac veins converged in 3of the 22patients.Overall accessibility with respect to the venous anat-omy was determined for L3–L4to L5–S1(Figure 2).Because the inferior vena cava is oriented along the right side of the lumbar spine,the structures at greatest risk with exposure of the underlying discs are the left com-mon iliac vein and the inferior vena cava itself.Theright-Figure 1.Abdominal angiogram.The width of intervertebral disc clear of overlying vascular structures (black arrow)compared with the total width of the disc (white arrow)was used to calculate the percentage of disc free of overlying vessels.For the disc to be considered “accessible,”the central portion and adjacent 30%of the disc width should be uncovered by overlying vessel.2184Spine •Volume 24•Number 21•1999sided orientation of the inferior vena cava at L3–L4re-sulted in the central portion of the disc being universally accessible (in all 22specimens)by the definitions of the current study.At L4–L5,16of 22disc spaces (73%)were readily accessible from a venous perspective.Of the 22patients,13(60%)displayed ready accessibility of the L5–S1disc on the basis of the criteria used in the cur-rent study.DiscussionEndoscopic,and more specifically,laparoscopic lumbar spinal surgery continues to gain acceptance with sur-geons and patients.Proponents of these techniques cite decreased costs,decreased hospital stays,shorter recov-ery times,less pain,and elimination of spinal canal ex-posure as potential benefits of these “minimally inva-sive”procedures.10Laparoscopic cholecystectomy has indeed replaced its open counterpart as the procedure of choice when indicated.It remains to be seen if these pro-cedures will follow a similar evolutionary pattern in the field of spinal surgery.Perhaps the single most important factor related to the evolution of laparoscopic techniques in spine surgery is the long learning curve associated with these technically demanding procedures.Published results,although en-couraging,have come from a handful of select centersTable 1.Analysis of Arterial StudiesPatient No.Age (yr)Sex Bifurcation L3–L4L4–L5L5–S1%Disc Clear*Accessible†%Disc Clear*Accessible†%Disc Clear*Accessible†148F Caudal L4N/A No 17No 52Yes 270F Cranial L5N/A No N/A No 57Yes 369F Caudal L5N/A No N/A No 31Yes 476F Caudal L4N/A No 30Yes 66Yes 573M Cranial L4N/A No 55Yes 81Yes 664F Caudal L317No 73Yes 100Yes 770F Cranial L5N/A No N/A No 65Yes 870M Cranial L5N/A No N/A No 45Yes 958F Caudal L347Yes 96Yes 100Yes 1063F Cranial L4N/A No 43Yes 75Yes 1161F Cranial L5N/A No N/A No 88Yes 1261M Cranial L5N/A No N/A No 43Yes 1380F Cranial L4N/A No 60Yes 100Yes 1456M Cranial L5N/A No N/A No 44Yes 1553F Cranial L343Yes 41Yes 68Yes 1677M Cranial L4N/A No 16No 100Yes 1775M Caudal L4N/A No 20No 76Yes 1842F Caudal L4N/A No 21No 70Yes 1967F Cranial L4N/A No 44Yes 100Yes 2071M Caudal L4N/A No 15No 65Yes 2174MCranial L5N/ANoN/ANo29No*Percent of disc space that is clear of overlying vascular structures.†Readily accessible using defined criteria.N/A ϭnotapplicable.Figure 2.Disc Accessibil-ity Profile.2185Vascular Anatomy of the Lumbar Spine •Vraney et aland surgeons representing the vast majority of experi-ence with these techniques.4,9,10,12Results are prelimi-nary,and complications are not rare.McAfee et al6reported on complications in thefirst 100cases of endoscopic anterior thoracolumbar spinal surgery.These authors noted one laceration of a left common iliac vein in theirfirst22cases.Repair of the vessel required conversion to an open procedure. Mathews et al4reported a series offive patients who underwent a laparoscopic approach to the L5–S1disc space.A sixth patient required conversion to an open laparotomy as a result of an iliac vein tear.Zucherman et al12reported a series of17patients who underwent lapa-roscopically assisted anterior lumbar interbody fusions. Of the17patients,2required conversion to laparotomy to control bleeding.However,the authors did not de-scribe the nature of the vascular injuries.Reported complications highlight the importance of a thorough knowledge concerning the vascular anatomy of the prevertebral zone.Traditionally,textbooks have underestimated the variability in the anatomy of the great vessels and their relation to the anterior elements of the lumbar spine.1Anda et al1reported four cases of vascular perforations as complications of lumbar discec-tomies performed posteriorly.In their study of the pre-vertebral lumbar anatomy using computed tomography scans,they noted extensive variability in the vascular anatomy.This study additionally demonstrated the inti-mate association of vascular structures to the anterior portions of the lumbar spine,especially with regard to the venous system.Parke et al8confirmed this variability of the arterial system in a cadaver study.Recently,the development of aggressive approaches for total en bloc spondylectomy for malignant tumors of the spine has renewed interest in the vascular anatomy of this region.2,3,11Kawahara et al3dissected21cadavers, paying special attention to the anterior vascular struc-tures.The great vessels and their proximal branches were noted to be contiguous with the soft tissue of the anterior spinal elements,and to vary greatly in their location.The current study was undertaken as an attempt to better characterize the anterior peridiscal anatomy of the lumbar spine.The authors’own increasing experience with laparoscopic spinal surgery has revived their inter-est in the vascular anatomy of this region.Their study demonstrated an impressive amount of variability in the arterial anatomy of this region,with bifurcation of the abdominal aorta occurring anywhere from the cranial half of L3to the caudal half of L5.The confluence of the common iliac veins was,conversely,much more consis-tent in its location,occurring just anteriorly to the cranial portion of L5nearly two thirds of the time.According to the current authors’definition of acces-sibility,an anterior laparoscopic approach to the L3–L4 disc level will seldom be accomplished without signifi-cant arterial retraction.Although the right-sided orien-tation of the venous structures poses little obstacle at this level,the abdominal aorta or proximal portion of the common iliac arteries was found to be overlying the cen-tral portion of the disc in90%of the specimens studied. The intervertebral disc at L4–L5will be readily accessi-ble in approximately one third of patients.Significant dissection to approach this level will be required in the remainder of patients.At this level,again,arterial struc-Table2.Analysis of Venous StudiesPatient No.Age(yr)Sex ConfluenceL3–L4L4–L5L5–S1%Disc Clear*Accessible†%Disc Clear*Accessible†%Disc Clear*Accessible†159M Cranial L564Yes53Yes41No 283M Caudal L451Yes47Yes34No 394M Caudal L482Yes40No86Yes 469F Cranial L569Yes46Yes90Yes 550M Cranial L574Yes54Yes50Yes 658F Cranial L564Yes77Yes41No 764M Cranial L587Yes58Yes96Yes 856F Caudal L550Yes53Yes59Yes 958F Caudal L583Yes82Yes4No 1087M Cranial L584Yes51Yes32No 1173F Cranial L551Yes35No28No 1239F Cranial L560Yes38No46Yes 1372M Cranial L579Yes39No62Yes 1476F Cranial L574Yes68Yes59Yes 1556F Caudal L569Yes49Yes40No 1667F Caudal L568Yes75Yes57Yes 1781M Caudal L469Yes38No69Yes 1848F Cranial L575Yes68Yes47Yes 1940M Cranial L568Yes57Yes93Yes 2027M Cranial L573Yes38No44No 2172F Caudal L560Yes52Yes30No 2276M Cranial L579Yes77Yes69Yes *Percent of disc space that is clear of overlying vascular structures.†Readily accessible using defined criteria.2186Spine•Volume24•Number21•1999tures are more likely to pose a barrier to the anterior approach.In nearly three fourths of the current studies, the central portion of the L4–L5disc was free of overly-ing venous structures.The overlying anatomy at this level may be particularly important because this disc fre-quently is implicated in pathologic conditions.As expected,the current results showed the L5–S1 intervertebral disc to be most readily accessible of the levels studied,approachable without vessel dissection al-most two thirds of the time.It is likely that with vessel retraction and dissection,the L5–S1space will be acces-sible in a substantially greater number of patients.The current criteria for accessibility were,by design,conser-vative,and obviously cannot reliably predict the likeli-hood of a successful approach for a given surgeon.Ve-nous structures present the major limit to extensive exposure at this level,afinding consistent with reports of complications in published series.Previous studies on the anatomy of this region have used cadaveric material.The current study used radio-graphic studies of living patients.Several potential sources of bias existed in the current experimental tech-nique.The average age of the patients was66years for arterial studies and64years for venous studies.This age may be slightly higher than the average age of patients likely to require anterior lumbar surgery.It is possible that underlying spondylosis and loss of disc height in this age group may have contributed to the variability in vas-cular anatomy that was noted.In addition,it is difficult to ascertain the magnitude,if any,of measurement error inherent in the current tech-nique.In an attempt to minimize magnification errors, vascular data were reported in the form of percentages and ratios relative to measured disc dimensions.It is important that the relative measurements made on the radiographs(i.e.,width of vascular structures versus width of underlying disc)correlate closely with the true anatomy.If a significant distance exists between the disc and overlying structures measured,then magnification error could introduce a significant source of bias.Open cadaveric studies,however,do confirm the intimate re-lation of these vascular structures to the underlying ver-tebral bodies and intervertebral discs.It is likely,there-fore,that magnification error was not significant.In conclusion,the results of this study confirm the work of previous authors who have noted the marked variability of the vascular anatomy anterior to the lum-bar spine.Textbooks historically have underestimated this variability,especially with regard to the arterial sys-tem.The vascular structures of this region present a po-tential impediment to the laparoscopic approach to the lumbar spine.Access to the L3–L4and L4–L5interver-tebral discs via a laparoscopic transperitoneal approach will require significant arterial dissection.Conversely, the L5–S1intervertebral disc will be readily accessible without significant dissection of vascular structures in most patients.References1.Anda S,Aakhus S,Skaanes KO,Sande E,Schrader H.Anterior perforations in lumbar discectomies:A report of four cases of vascular complications and a CT study of the prevertebral lumbar anatomy.Spine1991;16:54–61.2.Boriani S,Biagini R,De lure F,et al.En bloc resections of bone tumors of the thoracolumbar spine.Spine1996;21:1927–31.3.Kawahara N,Tomita K,Baba H,et al.Cadaveric vascular anatomy for total en bloc spondylectomy in malignant vertebral tumors.Spine1996;21:1401–7.4.Mathews HH,Evans MT,Molligan HJ,Long paroscopic discectomy with anterior lumbar interbody fusion:A preliminary review.Spine1995;20: 1797–802.5.Mayer HM.A new microsurgical technique for minimally invasive anterior lumbar interbody 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Center4646N.Marine DriveChicago,IL60640E-mail:fphillip@2187 Vascular Anatomy of the Lumbar Spine•Vraney et al。