Instrumentation And Fusion Of The Spine 301 To The Sacrum And Pelvis

Vincent J. Devlin, MD, Joseph Y. Margulies, MD, PhD, and William O. Shaffer, MD

1. When is fusion across the L5-S1 motion segment indicated?

• L5-S1 spondylolisthesis

• Symptomatic degenerative disorders involving the L5-S1 level

• Tumor, infection, or fractures involving the lumbosacral junction

• Spinal deformities (e.g. neuromuscular scoliosis with pelvic obliquity, adult idiopathic or de novo scoliosis with associated L5-S1 degenerative changes)

• Revision/salvage situations (e.g. distal extension of a prior scoliosis fusion due to degenerative changes below previously fused levels)

2. What complications are associated with fusion across the lumbosacral junction?

• Pseudarthrosis

• Loss of lumbar lordosis resulting in sagittal imbalance (flatback syndrome)

• Recurrent or progressive spinal deformity

• Implant loosening or failure

• Sacroiliac pain or arthrosis

• Pelvic stress fracture

3. Why is the L5-S1 level considered the most difficult level of the spine to fuse?

• Unfavorable biomechanical conditions. The lumbosacral junction is a transition zone between the highly mobile L5-S1 disc and the relatively immobile sacropelvis. Tremendous loads are transferred across the lumbosacral junction (up to 11 times body weight) as axial weight-bearing forces are transmitted from the vertebral column to the pelvis. In addition, the oblique orientation of the L5-S1 disc results in increased shear forces across this level

• Unique anatomy of the sacrum and pelvis. The sacrum is composed of cancellous bone and possesses limited sites for screw fixation. The large-diameter S1 pedicle provides less secure screw purchase compared with proximal vertebral levels

4. What is the 80-20 rule of Harms? How is it relevant to L5-S1 fusion procedures?

Biomechanical studies of the lumbosacral region have demonstrated that approximately 80% of axial load is transmitted through the anterior spinal column, and the remaining 20% is transmitted through the posterior column. Spinal fusion and instrumentation procedures that do not restore anterior column load sharing across the lumbosacral junction are destined for failure.

5. Explain why it is more difficult to achieve successful fusion between T10 and the sacrum compared with fusion between L4 and the sacrum.

Long-segment fusion constructs (e.g. T10-S1) have a higher rate of failure than short-segment constructs (e.g. L4-S1) because of the following factors:

• Increased risk of pseudarthrosis. The pseudarthrosis rate increases as the number of levels undergoing fusion increases

• Increased forces placed on distal sacral fixation. Increasing the number of instrumented levels proximal to the sacrum increases the lever arm exerted by the proximal spine on the distal sacral implants. The degree of strain on S1 screws increases as the number of segments immobilized above the sacrum increases. These unfavorable biomechanical factors increase the risk of distal fixation failure

6. What types of anterior spinal implants are used when arthrodesis is performed across the lumbosacral junction?

The most common anterior implants utilized when arthrodesis is performed across the lumbosacral junction are intracolumnar implants. These include structural bone graft (autograft or allograft) and fusion cages (e.g. titanium mesh, carbon fiber, PEEK) used in combination with autograft, allograft, or biologics (e.g. bone morphogenic protein). Placement of extracolumnar implants is challenging due to proximity of vascular structures and the osseous anatomy of the lumbosacral junction. Low-profile plates may be placed anteriorly distal to the bifurcation of the great vessels. Large-diameter cancellous screws may be used to secure bone grafts/cages between the L5 and S1 vertebral bodies.

7. What are the options for posterior implant fixation in the sacrum and pelvis when arthrodesis is performed across the lumbosacral motion segment?

Posterior implant fixation in the sacrum and pelvis can be categorized based on the three anatomic zones of the sacropelvic unit:

• Zone 1: Composed of the S1 vertebral body and upper margin of the sacral ala. Zone 1 fixation most commonly consists of S1 pedicle screws directed medially into the S1 pedicle and body. Alternate zone 1 fixation options for special situations include bilateral L5-S1 transfacet screws and "S" rods (as described by Dunn-McCarthy or Warner-Fackler).

• Zone 2: Composed of the sacral ala and the middle and lower sacrum. Fixation options in zone 2 include S1 alar screws (directed laterally into the sacral ala at the level of S1), S2 alar screws (directed laterally into the sacral ala at the level of S2), and intrasacral (Jackson) rods.

• Zone 3: Composed of the ilium bilaterally. Fixation is most commonly achieved with large-diameter (7-10 mm) screws placed in the lower iliac column. An alternative is placement of a solid rod (Galveston technique). Occasionally, the upper iliac column is utilized as a supplementary fixation site. Alternate zone 3 fixation techniques include the S2-alar-iliac screw fixation technique, transiliac (sacral) bar fixation, and sacroiliac screw fixation. Figure 30-1.

8. Describe the technique of S1 pedicle screw placement.

The dorsal bony cortex at the base of the superior S1 articular process is removed with a rongeur or burr. A pedicle probe is directed perpendicular to the sacrum and directed medially and angled toward the S1 endplate. Careful penetration of the anterior cortex of the sacrum permits bicortical fixation and increases screw purchase. Screw purchase can be further enhanced by directing the S1 pedicle screw superiorly to engage the anterior margin of the endplate of Sland is termed tricortical fixation (posterior sacral cortex, anterior sacral cortex, and superior endplate cortex). Due to the cancellous nature of the S1 pedicle, unlcortlcal screws (short screws that do not engage the anterior sacral cortex) provide less secure fixation and are prone to loosening and failure.

9. Describe the technique of laterally directed sacral screw placement.

Laterally directed sacral screws (alar screws) may be placed at the level of S1 and/or S2. The S1 alar screw entrance point is located just distal to the L5-S1 facet joint in line with the dorsal S1 neural foramen. The S2 alar screw entrance point is located between the dorsal S1 and S2 neural foramina. A starting point is created with a burr or drill in this area. A probe is placed through the sacrum until it contacts the anterior sacral cortex. The desired trajectory is 35° laterally and parallel with the S1 endplate. Length of this pilot hole is determined with a depth gauge. The anterior sacral cortex is then perforated in a controlled fashion to achieve bicortical fixation. Laterally directed sacral screws may be used in combination with medially directed S1 screws. Figure 30-2.

Figure 30-1. The fixation zones of the sacropelvic unit. (From O'Brien MF, Kuklo TR, Lenke LG. Sacropelvic instrumentation: anatomic and biomechanical zones of fixation. Semin Spine Surg 2004;16:76-90.)
A

Figure 30-2. Sacral screw options. S1 pedicle screw (A) and S1 alar screw (B). (From Kim DH, Henn JS, Vaccaro AR, et al. Surgical Anatomy and Techniques of the Spine. Saunders; 2006. p. 241.

10. What structures are at risk when a screw is placed through the anterior cortex of the sacrum?

Medially directed S1 screws, which are directed parallel to the upper S1 endplate or toward the sacral promontory, do not endanger any neurovascular structures with the exception of the middle sacral artery and vein. If a screw is inserted in a straightforward direction without medial angulation, the L5 nerve root is at risk of injury where it crosses the anterior sacrum. Screws placed laterally at the S1 level have a greater potential to injure critical structures including the lumbosacral trunk, internal iliac vein, and sacroiliac joint. Laterally directed screws at the S2 level have potential to injure the colon, which may be adjacent to the lateral sacrum in this region (Fig. 30-3).

11. When is it reasonable to perform a fusion across the sacrum with only bilateral S1 screw fixation?

Bilateral S1 screw fixation is effective for short-segment instrumentation and fusion across the lumbosacral junction (i.e. L4 or L5 to sacrum). A typical instrumentation construct consists of bilateral lumbar pedicle screws at each proximal level undergoing fusion. Supplemental anterior structural grafts and/or cages are utilized to provide anterior column load sharing as needed.

12. List situations where it is desirable to supplement S1 screw fixation with additional fixation strategies.

Indications for use of S1 screw fixation combined with additional sacropelvic fixation include:

• Long-segment scoliosis fusions that extend to the sacrum

• When correction of pelvic obliquity is required

• Stabilization and/or reduction and fusion of high-grade spondylolisthesis

• Lumbar revision surgery (e.g. osteotomy for sagittal imbalance syndrome; decompression and fusion for distal degeneration and stenosis below a long-segment fusion ending at L5)

• Multilevel fusion procedures in patients with osteopenia/osteoporosis

13. Describe the technique for placement of iliac fixation.

A rongeur is used to remove bone from the region of the posterior superior iliac spine (PSIS) at the level of S2-S3. A blunt probe or drill is used to develop a channel for insertion of a screw or rod between the cortices of the iliac bone along a trajectory extending from the posterior superior iliac spine and passing above the greater sciatic notch toward the anterior inferior iliac spine. Typically an anchor of at least 80 mm in length can be safely placed in adult patients. A rod-connector is often required to link the iliac screw with the longitudinal rod as the iliac screw is not colinear with the proximal screw anchors (Fig. 30-4).

0 = Pivot point

Q = Common iliac vessels

= L5 nerve root ■ = S1 pedicle screw □ = Sacral alar screws

= L5 nerve root ■ = S1 pedicle screw □ = Sacral alar screws

Figure 30-3. Sacropelvic fixation options in relation to the lumbosacral pivot point and adjacent critical structures. (From Polly DW, Latta LL. Spinopelvic fixation biomechanics. Semin Spine Surg 2004;16:101-6.)

14. What risks may be associated with iliac fixation?

• Need for extensive surgical exposure that can be associated with increased bleeding and prolonged operative time

• Injury to surrounding neurovascular structures including the superior gluteal artery, sciatic nerve, and cluneal nerves

• Potential for damage to the acetabulum or hip joint by misdirected anchor placement

• Implant prominence leading to the need to remove implants after fusion has occurred

• Sacroiliac pain

15. What is S2-alar-iliac fixation?

This technique for spinopelvic fixation is a modification of the traditional technique for iliac fixation (starting point in the posterior superior iliac spine). S2-alar-iliac fixation utilizes a modified starting point located 1 mm lateral and 1 mm distal to the S1 dorsal sacral foramen. The screw is directed laterally through the sacral ala, across the lower portion of the sacroiliac joint, and between the cortical tables of the ilium toward the anterior inferior iliac spine. As the starting point is colinear with an S1 pedicle screw, an offset connector is not required to link the screw to a longitudinal rod. As the starting point is more medial compared with traditional iliac fixation, there is decreased implant prominence. A potential disadvantage of this technique is the unknown long-term consequence of placing a screw across the sacroiliac joint compared with traditional iliac fixation, which spans the posterior sacroiliac region without violation of the sacroiliac joint.

16. Describe the technique for placement of an intrasacral rod.

A rod is inserted into the lateral sacral mass through the canal of a previously placed S1 pedicle screw. The rod and screw interlock within the sacrum providing secure fixation. The implants are buttressed by the posterior ilium. Fixation provided by this technique is superior to fixation provided by S1 screws alone. Figure. 30-5.

Figure 30-5. Anteroposterior (A) and lateral (B) views of the lumbosacral junction depicting an intrasacral rod construct. (From Margulies JY, Armour EF, Kohler-Ekstrand C. Revision of fusion from the spine to the sacropelvis: Considerations. In: Margulies JY, Aebi M, Farcy JP, editors. Revision Spine Surgery. St. Louis: Mosby; 1999. p. 623-30.)

Figure 30-5. Anteroposterior (A) and lateral (B) views of the lumbosacral junction depicting an intrasacral rod construct. (From Margulies JY, Armour EF, Kohler-Ekstrand C. Revision of fusion from the spine to the sacropelvis: Considerations. In: Margulies JY, Aebi M, Farcy JP, editors. Revision Spine Surgery. St. Louis: Mosby; 1999. p. 623-30.)

17. What is transsacral fixation?

In cases of high-grade isthmic spondylolisthesis (grades 3 and 4) screws and/or bone grafts/cages may be placed obliquely across the L5-S1 disc space and obtain purchase in both the L5 and S1 vertebrae. Traditionally, a fibular graft is placed from either a posterior approach (from S1 into L5) or anterior approach (from L5 into S1). The addition of screw fixation from the upper sacrum across the L5-S1 disc space and into the L5 vertebral body increases distal screw fixation, increases the fusion rate, and decreases the risk of graft fracture.

18. What is a transiliac (sacral) bar?

This type of fixation uses rod(s) that spans the sacrum passing horizontally from ilium to ilium. Initially, sacral bars were used for fixation of sacral fractures. They have been modified to serve as an anchor within the pelvis to form the basis for complex reconstruction procedures involving fusion across the lumbosacral region. Specialized connectors have been developed to link the trans-iliac bar with longitudinal rods anchored to proximal spinal levels.

19. What is meant by the lumbosacral pivot point in relation to the biomechanics of lumbosacral fixation?

The stability provided by sacropelvic fixation devices has been conceptualized in relation to a pivot point located at the posterior aspect of the L5-S1 disc space (see Fig. 30-3). Iliac fixation provides the most stable method of sacropelvic fixation because it extends fixation for a greater distance anterior to the lumbosacral pivot point than any other technique. S1 screw fixation provides the least resistance to counteract flexion moments around the pivot point compared with other sacropelvic fixation techniques. The addition of a second point of sacral fixation provides improved fixation compared with use of S1 fixation alone.

20. What type of brace should be prescribed if the surgeon wishes to restrict motion across the lumbosacral junction?

The surgeon should prescribe a thoracolumbosacral orthosis (TLSO) with a thigh cuff. Lumbar orthoses that do not immobilize the thigh will increase rather than decrease motion across the L5-S1 level.

21. A 75-year-old woman with severe osteoporosis and adult degenerative scoliosis underwent treatment with posterior spinal instrumentation and fusion from T10 to sacrum 2 weeks ago. Distal fixation in the sacrum consisted of bilateral S1 screw fixation. Anterior column support at L5-S1 was provided with a structural femoral allograft. The patient complains of new-onset severe sacral pain, which has developed over the past several days. Radiographs show no obvious signs of sacral screw pull-out. What is the most likely explanation for the patient's severe sacral pain?

A fracture of the proximal sacrum at the level of the S1 screws should be suspected. If the fracture is nondisplaced, it may be overlooked on radiographs. A computed tomography (CT) scan of the sacrum with sagittal and coronal reconstructions should be obtained to confirm this diagnosis. Prompt treatment should be initiated before displacement of the fracture occurs and before traumatic spondyloptosis develops. Fixation distal to the fracture can be achieved with iliac fixation or S2-alar-iliac fixation.

22. Summarize the techniques that a surgeon can utilize to increase the rate of successful fusion across the L5-S1 segment.

• Perform an L5-S1 interbody fusion with a structural spacer (e.g. allograft, autograft, fusion cage) to restore anterior column load sharing and increase likelihood of successful arthrodesis

• Use multiple fixation points within the sacropelvic unit (iliac fixation is the most stable type of fixation)

• Cross-link the longitudinal members in the region of the sacrum

• Use an appropriate orthosis to restrict lumbosacral motion

Key Points

1. S1 pedicle fixation is the most common sacral fixation technique utilized when performing fusion to the sacrum.

2. For high-risk fusions to the sacrum, bilateral S1 pedicle screws supplemented with iliac fixation is the most reliable instrumentation technique.

3. Structural interbody support at the L4-L5 and L5-S1 levels increases the rate of fusion and decreases the risk of construct failure when performing fusion to the sacrum.

Websites

S2-alar-iliac pelvic fixation: http://thejns.org/doi/pdf/10.3171/2010.1.F0CUS09268

Comparison of pelvic fixation techniques in neuromuscular deformity correction: Galveston rod versus iliac and lumbosacral screws:

http://www.medscape.com/viewarticle/545049 Spinopelvic fixation: http://www.bioline.org.br/pdf?ni05154

BiBLiOGRAPHY

1. Devlin VJ, Asher MA. Biomechanics and surgical principles of long fusions to the sacrum. Spine State Art Rev 1996;10:515-29.

2. Jackson R, McManus A. The iliac buttress: a computed tomographic study of sacral anatomy. Spine 1993;18:1318-28.

3. Lebwohl NH, Cunningham BW, Dmitriev A, et al. Biomechanical comparison of lumbosacral fixation techniques in a calf spine model. Spine 2002;27:2312-2320.

4. Lehman RA Jr, Kuklo TR, Belmont PJ Jr, et al. Advantage of pedicle screw fixation directed into the apex of the sacral promontory over bicortical fixation: a biomechanical analysis. Spine 2002;27:806-11.

5. Margulies JY, Floman Y, Farcy JP, et al, editors. Lumbosacral and Spinopelvic Fixation. Philadelphia: Lippincott-Raven; 1996.

6. Moshirfar A, Kebaish KM, Riley LH. Lumbosacral and spinopelvic fixation in spine surgery. Semin Spine Surg 2009;21:55-61.

7. McCord DH, Cunningham BW, Shono Y, et al. Biomechanical analysis of lumbosacral fixation. Spine 1992;17:S235-S243.

8. O'Brien MF, Kuklo TR, Lenke LG. Sacropelvic instrumentation: anatomic and biomechanical zones of fixation. Semin Spine Surg 2004;16:76-90.

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