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173 KISEP Experimental Research J Korean Neurosurg Soc 26173-177, 1997 Acute Sciatica from Sacral Screw Impingement on the Lumbosacral PlexusEmphasis on the Safe Zones for Sacral Screw Placement - Case Report - Jae Won Doh, M.D., Bark Jang Byun, M.D., Edward C. Benzel, M.D.* Department of Neurosurgery, Soonchunhyang University Chonan Hospital, Chonan, Korea Division of Neurosurgery, University of New Mexico School of Medicine, Albuquerque, USA* = Abstract = he authors present the case of sciatica due to bicortical sacral screw impingement on the lumbosacral plexus across the anterior sacrum. The placement of sacral screw across the anterior sacral cortex carries significant inherent risks to neurovascular and visceral structures. However, the clinical reports of lumbosacral plexus invol- vement by the misplaced screw are not well documented in the literature. This is an unique case of sciatica due to sacral screw impingement on the lumbosacral plexus after motor vehicle accident(MVA), and confirmed by CT scan and intraoperative electrical stimulation. We reviewed sacral anatomy and preventive measures for avoiding complication of this type are discussed. KEY WORDS :Sacrum·Screw penetration·Lumbosacral plexus·Sciatica·Safe zone. Introduction Although transpedicular screw fixation of the spine is frequently employed in the treatment of many spinal defor- mity, optimal screw fixation of sacrum remains a clinically challenging problem 12)15)23) . Complex neurovascular and visceral structures lie anterior to the sacrum. Since anterior sacral anatomy is not familiar to many spine surgeons, injury to any of these due to screw penetration can lead to possible life-threatening consequences. Some reports using cadavers showed a variable risk of sacral screw penetra- tion 9)12)15) , but surprisingly clinical reports regarding compli- cations of sacral screw penetration are very rare, and most of them have been suggested only by the plain X-ray findings 4)13)14) . Reported herein is an unusual case of acute sciatica caused by misplaced bicortical sacral screw which was confirmed by computed tomography(CT) scan and intraoperative electrical stimulation. A review of sacral anatomy, sacral biomechanics, and preventive measures follows. Case Report On November 2, 1995, a 42-year-old male was admitted because of right sciatica and difficult walking with cane. These symptoms began immediately after MVA in April 14, 1995 and have continued to worsen prior to admission. At the time of accident, he was suddenly jolted from side to side and immediately experienced a severe right sciatica. The pain is somewhat relieved by lying down, but becomes severe whenever in upright position. He had a past history of discectomy, decompressive foraminotomy, posterolateral bone fusion and L4-S1 pedicle screw fixation with Danek titanium system at same time for the L5-S1 retrolisthesis and herniated disc in March 9, 1993 at another hospital. ExaminationThe physical examination revealed mild tenderness and muscle spasm over the lumbosacral junction. The neurologic examniation showed a positive straight leg raise to 30°on the right and hypesthesia along the right lateral leg and dorso-lateral aspect of the right foot. He also has a motor deficit of 4/5 gastrocnemius muscle and an absent ankle jerk on the right. The electromyographic study T

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Page 1: Acute Sciatica from Sacral Screw Impingement on …Acute Sciatica from Sacral Screw Impingement on the Lumbosacral Plexus:Emphasis on the Safe Zones for Sacral Screw Placement 174

173

KISEP Experimental Research J Korean Neurosurg Soc 26::::173-177, 1997

Acute Sciatica from Sacral Screw Impingement on the Lumbosacral Plexus::::Emphasis on the Safe Zones

for Sacral Screw Placement ---- Case Report ----

Jae Won Doh, M.D., Bark Jang Byun, M.D., Edward C. Benzel, M.D.* Department of Neurosurgery, Soonchunhyang University Chonan Hospital, Chonan, Korea

Division of Neurosurgery, University of New Mexico School of Medicine, Albuquerque, USA* = Abstract = he authors present the case of sciatica due to bicortical sacral screw impingement on the lumbosacral plexus

across the anterior sacrum. The placement of sacral screw across the anterior sacral cortex carries significant

inherent risks to neurovascular and visceral structures. However, the clinical reports of lumbosacral plexus invol-

vement by the misplaced screw are not well documented in the literature. This is an unique case of sciatica due to

sacral screw impingement on the lumbosacral plexus after motor vehicle accident(MVA), and confirmed by CT scan and

intraoperative electrical stimulation. We reviewed sacral anatomy and preventive measures for avoiding complication

of this type are discussed. KEY WORDS:Sacrum·Screw penetration·Lumbosacral plexus·Sciatica·Safe zone.

Introduction

Although transpedicular screw fixation of the spine is

frequently employed in the treatment of many spinal defor-mity, optimal screw fixation of sacrum remains a clinically challenging problem12)15)23). Complex neurovascular and visceral structures lie anterior to the sacrum. Since anterior sacral anatomy is not familiar to many spine surgeons, injury to any of these due to screw penetration can lead to possible life-threatening consequences. Some reports using cadavers showed a variable risk of sacral screw penetra-tion9)12)15), but surprisingly clinical reports regarding compli-cations of sacral screw penetration are very rare, and most of them have been suggested only by the plain X-ray findings4)13)14). Reported herein is an unusual case of acute sciatica caused by misplaced bicortical sacral screw which was confirmed by computed tomography(CT) scan and intraoperative electrical stimulation. A review of sacral anatomy, sacral biomechanics, and preventive measures follows.

Case Report

On November 2, 1995, a 42-year-old male was admitted because of right sciatica and difficult walking with cane. These symptoms began immediately after MVA in April 14, 1995 and have continued to worsen prior to admission. At the time of accident, he was suddenly jolted from side to side and immediately experienced a severe right sciatica. The pain is somewhat relieved by lying down, but becomes severe whenever in upright position. He had a past history of discectomy, decompressive foraminotomy, posterolateral bone fusion and L4-S1 pedicle screw fixation with Danek titanium system at same time for the L5-S1 retrolisthesis and herniated disc in March 9, 1993 at another hospital.

Examination:The physical examination revealed mild tenderness and muscle spasm over the lumbosacral junction. The neurologic examniation showed a positive straight leg raise to 30° on the right and hypesthesia along the right lateral leg and dorso-lateral aspect of the right foot. He also has a motor deficit of 4/5 gastrocnemius muscle and an absent ankle jerk on the right. The electromyographic study

TTTT

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Acute Sciatica from Sacral Screw Impingement on the Lumbosacral Plexus:Emphasis on the Safe Zones for Sacral Screw Placement

대한신경외과학회지 제 26 권 제 2 호 1997 174

(EMG) is unremarkable. The plain X-ray films showed penetration of sacral screws through the anterior sacrum, and a small gap on the plate-nut interface due to loosening on the right S1 screw(Fig. 1). However, posterolateral fusion mass appear to be solid and there was no movement with flexion/extension. On the spine CT scan, the screws were penetrating across the anterior sacral cortex and then compressed the lumbosacral plexus more severe on the

right(Fig. 2). Operation:Revision was carried out. There was obvious

wide laminotomy at L4-5, L5-S1 were noted and L5, S1 nerve roots had been completely decompressed within the neural foramen without impingement from screws. The previous spinal implants at L4-S1 were removed without difficulty;the nuts, longitudinal plates, and pedicle screws were removed according to the order. To confirm the cause

Fig. 2. Left:Spine CT scan showing the screws directed to lumbosacral trunk(arrows). Right:The screws are penetrating across the anterior sacral cortex, and then compressing the lumbosacral trunk more severe on the right.

Fig. 1. Left:Plain x-ray AP view showing the sacral screws in a near-straight line with slight medial angulation. Right:The S1 screws are penetrating through the anterior sacrum, and a small gap(arrow) on the plate-nut interface due to loosening.

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Jae Won Doh · Bark Jang Byun · Edward C. Benzel

대한신경외과학회지 제 26 권 제 2 호 1997 175

of right L5 nerve root irritation, the right S1 screw path hole through the sacral ala was gently sounded with a Penfield dissector, and electric current was then placed on the Penfield with monopolar cautery, which caused contraction of the right leg.

Postoperative course:The sciatic pain was completely disappeared immediately after surgery and the patient could walk without any support. He resumed his normal daily activity and returned to previous work.

Discussion

Transpedicular screw fixation in the thoracolumbar spine has become popular method in worldwide. However, sacral screw placement for optimal fixation remains a challenging clinical problem12)15)23). Sacrum has an inherent weakness both anatomically and biomechanically. Many cadaveric stu-dies have described the anatomical structures at risk along the anterior sacrum with sacral screw penetration9)10)12)15). In general, lumbosacral trunk, S1 nerve root, and internal iliac vein are at high risk because these structures are firmly attached to the anterior bony sacral surface. Biomechanically, sacrum is weaker than thoracolumbar spine;thin cortical bone and more fatty tissue, in which the sacral screws have a poor purchase6)7). In addition, relatively vertical orientation of the lumbosacral joint exposes the sacral spine to an increased risk of translational deformation. Therefore, bone screws in the sacral pedicle are subjected to posteriorly directed force during flexion that results in tendency for screw pullout3)4)20).

Many spine surgeons now increasingly use screws but some anatomical studies have showed the neurovascular and visceral structures at risk with bicortical sacral screw purchase through the anterior sacral cortex9)10)12)15). However, it is surprising that clinical reports on complications regarding the sacral screw penetration are very rare. Camp, et al.4), rep-orted one case of left sciatica due to anterior screw penetration which was demonstrated by the plain X-ray. Matsuzaki, et al.14), reported one case of S1 root impingement as the screw was inserted too caudally, and demonstrated by the S1 radiculogram. Louis13) reported six cases of radiculopathy without evidence of radiological findings, which subsided after offending screws were removed. In this case, L5 nerve impingement from the penetrating screw was demonstrated by CT scan and confirmed intraoperatively with electrical stimulation test.

Although many techniques have been proposed for op-timal sacral screw fixation1)5)7)8)9)13)16)17), two screw orien-tations are commonly used for posterior fixation to the sacrum:(1) anteromedially into the sacral promontory, and (2) anterolaterally into the sacral ala. Mirkovic, et al.15), showed that with screws aimed 45° laterally into the sacral ala, 55% abutted the lumbosacral trunk and 8% contacted the internal iliac vein. He determined two“safe zones”for S1 screw placement;one laterally and one much larger zone medially(Fig. 3). The medial safe zone lies between the sacral promontory medially and internal iliac vein laterally. The lateral safe zone is bordered laterally by the sacroiliac joint and medially by the lumbosacral trunk. He pointed out that screws placed in the S1 pedicle(medial safe zone) were least likely injure to neurovascular bundle. Licht, et al.12), demonstrated midline“safe zone”, which is small area approximately 2cm wide. Xu, et al.22), demonstrated that safer area for S1 pedicle insertion was in pedicle zone 2, which was located between the lower lateral portion of the L5-S1 facet and the lateral sacral crest. Esses, et al.9), stressed the need to evaluate the position of the sacral foramen and recommended placing screws above the S1 sacral foramen level directed medially toward the sacral promontory. dePertti, et al.7), recommended S1 screw placement with an inward direction as the prominence of the iliac crest allows. There

Fig. 3. Schematic drawing showing the anatomical two safezones for S1 sacral screw placement. The larger medialzone lies between the sacral promontory and internaliliac vein. The lateral zone is bordered between thesacroiliac joint and lumbosacral trunk. The screwsdirected to medial safe zone are more safe and pre-vent neurovascular injury. S/I=sacroiliac(Reprinted bypermission from Van Rightuskirks CS, Baldwin NG:Sur-gical techniques:lumbosacral and sacropelvic fixation.In Benzel EC, Tator CH(eds):Conte-mporary manag-ement of spinal cord injury. Park Ridge, AANS, 1995pp169)

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Acute Sciatica from Sacral Screw Impingement on the Lumbosacral Plexus:Emphasis on the Safe Zones for Sacral Screw Placement

대한신경외과학회지 제 26 권 제 2 호 1997 176

is general agreement that medially-directed screws are sig-nificantly stronger and carry minimal risk of injury than laterally-directed ones5)7)9).

The question of how deep to insert screw is clinically important, yet remains incompletely answered. Although bicortical purchase across the anterior sacral cortex is bio-mecanically stronger than unicortical purchase, Zindrick, et al.23), found no statiscal difference in pullout strength between screws placed to a 50% depth and screws placed just up to but not penetrating the anterior cortex. Smith, et al.18), de-monstrated unicortical and bicortical screw fixations of sacrum sustained similar strength in older population. Licht, et al.12), recommended that anterior cortical penetration not be used during pedicle screw fixation of the sacrum due to the lack of documented benifit of enhanced stability and the well defined risks to the sacral structures. Georgis, et al.10), recommened that depth of penetration should be more than 2 to 3mm through the anterior sacral cortex, even though engagement of the anterior cortex for greater fixation strength may be necessary. Therefore, risks and benifits must be established whether the additional biome-chanical advantages of bicortical sacral purchase outweigh the risks associated with potential neurovascular penetra-tion16)20).

Some reports11)14)19)21)23) have showed that lateral roent-genogram or fluoroscopic technique commonly used intra-operatively was inaccurate for determining actual penetration of anterior cortex by the screw because there are“blind spot”on cylindrical vertebral body. Considering the varia-bility regarding the anatomy of sacrum and inaccuracy of intraoperative lateral x-ray to determine the screw depth, the use of individualized preoperative CT evaluation is needed to minimize risks when sacral screw placement is considered16).

Although pedicle screw fixation has become popular in worldwide, the sacral screw placement may not be a benign procedure and may require steep learning curve. There have been many mistakes by the neurosurgeons who have little experiences2). To prevent this kind of complications, we emphasize;(1) awareness of the detailed anterior sacral anatomy and anatomical safe zones for sacral screw place-ment, (2) medial screw orientation into the sacral promontory, (3) avoidance of bicortical screw purchase across the ante-rior sacral cortex, and finally (4) the use of individualized preoperative CT scan for determining safe trajectory angle and screw depth.

Conclusion

In summary, we reported an unusual case of sacral screw

complication with screw penetration across the anterior sacrum associated with acute sciatica which developed immediately after MVA. When placing the sacral screw, it is important to aware the possibility of screw penetration;therefore we should bear in mind exact anatomical safe zones, correct screw direction and appropriate screw depth to prevent this kind of unusual complication. • 논문접수일:1997년 1월 24일 • 심사완료일:1997년 2월 13일

References 1) Asher MC, Strippen WE:Anthropometric studies of the human

sacrum relating to dorsal transsacral implant designs. CLIN Orthop 203:58-62, 1986

2) Ausman JI:Who is going to find out? Surg Neurol 44:403-404, 1996

3) Benzel EC:Biomechanics of spine stabilization:principles and clinical practice, ed 1. New York:McGraw-Hill, 1995, pp11-15

4) Camp JFc, Caudle R, Ashmum RD, et al:Immediate compli-cations of Cotrel Dubousset instrumentation to the sacropelvis:A clinical and biomechanical study. Spine 15:932-942, 1990

5) Carson GD, Abitbol JJ, Anderson DR, et al:Screw fixation in the human sacrum:An in vitro study of the biomechanics of fixation. Spine 17(Suppl):S 196-S 203, 1992

6) dePeretti F, Argenson C:Anatomy of the thoracolumbar and sacral spine with special consideration for implant insertion, in Floman Y, Farcy JPC, Argenson C(eds):Thoracolumbar spine fractures, ed 1. New York:Raven press, 1993, Vol 1, pp11-33

7) dePeretti F, Argenson C, Bourgeon A, et al:Anatomic and experimental basis for the insertion of a screw at the first sacral vertebra. Surg Radiol Anat 13:133-137, 1991

8) Dubousset J:Cotrel-Dubousset instrumentation for paralytic neuromuscular spinal deformities with emphasis on pelvic obli-quity, in Bridwell KH, DeWald RL(eds):The textbook of spinal suegery, ed 1. Philadelphia:Lippincott, 1991 pp347-365

9) Esses SI, Botsford DJ, Huler RJ, et al:Surgical anatomy of the sacrum:A guide for rational screw fixation. Spine 16(Suppl):S 283-S 288, 1991

10) Georgis T, Rydevik B, Weinstein JN, et al:Complications of pedicle screw fixation, in Garfin SR(ed):Complications of spinal surgery, ed 1. Baltimore:Williams and Wilkins, 1989, pp200-210

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대한신경외과학회지 제 26 권 제 2 호 1997 177

11) Krag MH, Van Hal ME, Beynnon BD:Placement of transpe-dicular vertebral screws close to anterior vertebral cortex:Description of methods. Spine 14:879-883, 1989

12) Licht NJ, Rowe DE, Ross LM:Pitfalls of pedicle screw fixation in the sacrum:A cadaver model. Spine 17:892-896, 1992

13) Louis R:Fusion of the lumbar and sacral spine by internal fixation with screw plates. Clin Orthop 203:18-33, 1986

14) Matsuzaki H, Tokuhashi Y, Matsumoto F, et al:Problems and solutions of pedicle screw plate fixation of lumbar spine. Spine 15:1159-1165, 1990

15) Mirkovic S, Abitbol JJ, Edwards CC, et al:Anatomic consid-eration for sacral screw placement. Spine(Suppl):S 289-S 294, 1991

16) Morse BJ, Ebraheim NA, Jackson WT:Preoperative CT de-termination of angles for sacral screw placement. Spine 19:604-607, 1994

17) Roy-Camille R, Saillant G, Mazel C:Internal fixation of the lumbar spine with pedicle screw plating. Clin Orthop 203:7-17, 1986

18) Smith SA, Abitbol JJ, Carlson GD, et al:The effects of depth

of penetration, screw orientation, and bone density on sacral screw fixation. Spine 18:1006-1010, 1993

19) Steinmann JC, Mirkovic S, Abitbol JJ, et al:Radiographic assesment of sacral screw placement. J Spinal Disord 3:232-237, 1990

20) Van Buskirk CS, Baldwin NG:Surgical techniques:lumbo-sacral and sacropelvic fixation, in Benzel EC, Tator CH(eds):Contemporary management of spinal cord injury:Neurosur-gical topics. Park Ridge:AANS, 1995, pp167-176

21) Whitecloud TS, Skalley TC, Cook SD, et al:Roentgeno-graphic measurement of pedicle screw penetration. Clin Orthop 245:57-68, 1989

22) Xu R, Ebraheim NA, Yeasting RA, et al:Morphometric evaluation of the first sacral vertebra and the projection of its pedicle on the posterior aspect of the sacrum. Spine 20:936-940, 1995

23) Zindrick MR, Wiltse LL, Widdel EH, et al:A biomechanical study of intrapeduncular screw fixation in the lumbosacral spine. Clin Orthop 203:99-112, 1986

요천골 신경총에 나사못 충돌로 인한 급성 좌골신경통 - 증 례 보 고 -

순천향대학 천안병원 신경외과학교실 뉴-맥시코주립대학 신경외과학교실*

도재원·변박장·Edward C. Benzel*

= 국 문 초 록 = 천추골 앞쪽을 관통하는 나사못이 제5요추신경을 눌러서

발생한 좌골신경통 1례를 보고한다. 강한 고정력을 얻기 위

하여 천추체 앞쪽을 관통하는 나사못고정수술을 할 때에는

천추골 앞쪽에 놓여 있는 혈관과 신경을 손상시킬 위험이 크

다. 사체를 이용한 여러 실험에서는 천추골의 나사못관통으

로 인한 다양한 위험성에 대한 보고가 있으나, 임상보고례

는 매우 드물다. 본 증례는 잘못된 각도로 관통한 천추나사

못이 원인이었으며, 요추 CT와 수술중에 전기자극을 가함

으로써 이를 확인할수 있었다. 천추주위의 해부학적 구조와

이러한 합병증을 예방하기 위한 방법에 대해 고찰하였다.