Thoracic And Lumbar Spine Instrumentation

O Edward A. Smirnov, MD, D. Greg Anderson, MD, and Vincent J. Devlin, MD

GENERAL CONSIDERATIONS

1. Summarize the functions of spinal instrumentation in thoracic and lumbar fusion procedures.

• Enhance fusion. Spinal implants immobilize spinal segments during the fusion process and increase the rate of successful arthrodesis

• Restore spinal stability. When pathologic processes (e.g. tumor, infection, fracture) compromise spinal stability, spinal implants can restore stability

• Correct spinal deformities. Spinal instrumentation can provide correction of spinal deformities (e.g. scoliosis, kyphosis, spondylolisthesis)

• Permit extensive decompression of the neural elements. Complex spinal stenosis problems requiring extensive decompression create spinal instability. Spinal instrumentation and fusion prevent development of postsurgical spinal deformities and recurrent spinal stenosis

2. Why is surgical stabilization of the spine considered a two-stage process?

In the short term, stabilization of the spine is provided by spinal implants. However, long-term stabilization of the spine occurs only if fusion is successful. If the fusion does not heal, spinal implant failure will ultimately occur. The surgeon influences this process through meticulous fusion technique, selection of the appropriate location for fusion (anterior, posterior, or combined anterior and posterior column fusion), and use of appropriate spinal implants to adequately support the spine during this process.

3. What is meant by the terms tension band principle and load-sharing concept?

• A tension band is a portion of a construct that is subjected to tensile stresses during loading. In the normal spine, the posterior spinal musculature maintains normal sagittal spinal alignment through application of dorsal tension forces against the intact anterior spinal column. This is termed the tension band principle. The posterior spinal musculature can function as a tension band only if the anterior spinal column is structurally intact

• Biomechanical studies have shown that in the normal lumbar spine approximately 80% of axial load is carried by the anterior spinal column and the remaining 20% is transmitted through the posterior spinal column. This relationship is termed the load-sharing concept (Fig. 29-1)

4. What is the relevance of the load-sharing concept to the selection of appropriate spinal implants?

Load sharing between an instrumentation construct and the vertebral column is a function of the ratio of the axial stiffness of the spinal instrumentation and the axial stiffness of the vertebral column. If the anterior spinal column is incompetent, the entire axial load must pass through the posterior spinal implant. In the absence of adequate anterior column support, normal physiologic loads exceed the strength of posterior spinal implant systems. In this situation, posterior spinal implants will fail by fatigue, permanent deformation, or implant migration through bone. Thus, it is critical to reconstruct an incompetent anterior spinal column when using posterior spinal implant systems.

POSTERIOR SPINAL INSTRUMENTATION

5. Name three posterior spinal instrumentation systems that are considered to be the precursors of contemporary posterior spinal instrumentation systems.

Harrington instrumentation, Luque instrumentation, and Cotrel-Dubousset instrumentation.

I Tension X Compression

I Tension X Compression

I Tension | Compression

Figure 29-1. Anterior column load-sharing and posterior tension band principle.

I Tension | Compression

Figure 29-1. Anterior column load-sharing and posterior tension band principle.

6. What is Harrington instrumentation?

The initial instrumentation developed by Paul Harrington consisted of a single rod with ratchets on one end in combination with a single hook at each end of the rod. Distraction forces were applied to obtain and maintain correction of spinal deformities. This system was introduced 1960 in Texas and was utilized to treat various spinal problems, especially scoliosis, for more than 25 years. Shortcomings of this system included the need for postoperative immobilization to prevent hook dislodgement and the inability to correct and maintain sagittal plane alignment. Various modifications were introduced to address these problems, including square-ended hooks, use of compression hooks along a convex rod, and use of supplemental wire fixation (Fig. 29-2).

7. What is Luque instrumentation?

In the 1980s, Edwardo Luque from Mexico introduced a system that provided segmental fixation consisting of wires placed beneath the lamina at multiple spinal levels. Wires were tightened around rods placed along both sides of the lamina. Corrective forces were distributed over multiple levels, thereby decreasing the risk of fixation failure. The increased stability provided by this construct eliminated the need for postoperative braces or casts. The ability to translate the spine to a precontoured rod provided better control of sagittal plane alignment than Harrington instrumentation (Fig. 29-3).

Figure 29-2. A, B, Harrington instrumentation. (A from Winter RB, Lonstein JE, Denis F, et al. Atlas of Spine Surgery. Philadelphia: Saunders; 1995. B from Errico TJ, Lonner BS, Moulton AW. Surgical Management of Spinal Deformities. Philadelphia: Saunders; 2009.)
Figure 29-3. A, B, C, Luque instrumentation. (From Winter RB, Lonstein JE, Denis F, et al. Atlas of Spine Surgery. Philadelphia: Saunders; 1995.)

8. What is Cotrel-Dubousset instrumentation?

In 1984, Cotrel and Dubousset from France introduced their segmental fixation system, which became known as the CD system. It consisted of multiple hooks and screws placed along a knurled rod. The use of multiple fixation points permitted selective application of compression and distraction forces along the same rod by altering hook direction. A rod rotation maneuver was introduced in an attempt to provide improved three-dimensional correction of scoliosis. Rod contouring permitted improved correction of the sagittal contour of the spine. The stable segmental fixation provided by this system obviated the need for postoperative immobilization (Fig. 29-4).

Figure 29-4. A, B, C, Cotrel-Dubousset instrumentation. (A from Winter RB, Lonstein JE, Denis F, et al. Atlas of Spine Surgery. Philadelphia: Saunders; 1995. B, C, from Lonstein JE, Bradford DS, Winter RB, et al. Moe's Textbook of Scoliosis and Other Spinal Deformities. 3rd ed. Philadelphia: Saunders; 1995.)

Figure 29-4. A, B, C, Cotrel-Dubousset instrumentation. (A from Winter RB, Lonstein JE, Denis F, et al. Atlas of Spine Surgery. Philadelphia: Saunders; 1995. B, C, from Lonstein JE, Bradford DS, Winter RB, et al. Moe's Textbook of Scoliosis and Other Spinal Deformities. 3rd ed. Philadelphia: Saunders; 1995.)

9. What is meant by the term posterior segmental spinal fixation?

Posterior segmental spinal fixation is a general term used to describe a variety of contemporary posterior spinal instrumentation systems that attach to the spine at multiple points throughout the instrumented spinal segments. A complete implant assembly is termed a spinal construct. Typically, spinal instrumentation constructs consist of a longitudinal member (rod or plate) on each side of the spine connected by transverse connectors (cross-linking devices) to increase construct stability. Segmental fixation is defined as the connection of the longitudinal member to multiple vertebrae within the construct. Options for achieving segmental fixation include the use of hook, wire, and pedicle screw anchors. Various corrective forces can be applied to the spine by means of segmental anchors including compression, distraction, rotation, cantilever bending, and translation. The Isola system, developed by Marc Asher and colleagues, popularized the integration of hook, wire, and screw fixation within a single implant construct. Such implant constructs are referred to as hybrid constructs (Fig. 29-5).

Figure 29-5. A, B, C, Contemporary hybrid posterior segmental spinal instrumentation.

10. Describe the use of hook anchors in posterior segmental spinal constructs.

Hook anchors may be placed above or below the T1 to T10 transverse processes, under the thoracic facet joints, and above or below the thoracic and lumbar lamina. When blades of adjacent hooks face each other, this is termed a claw configuration. Compression forces can be applied to adjacent opposing hooks, thereby securing the hooks to the posterior elements. A claw may be composed of hooks at a single spinal level (intrasegmental claw) or hooks at adjacent levels (intersegmental claw). Hooks placed in a claw configuration provide more secure fixation than a single hook anchor. For this reason, claw fixation is typically used at the proximal and distal ends of spinal constructs.

11. Describe the use of wire anchors in posterior segmental spinal contructs.

Wire anchors (and more recently cables) can be placed at every level of the spine. Possible attachment points for wire anchors include the base of the spinous process, underneath the lamina (sublaminar position), or underneath the transverse process. Spinous process wires are placed through a hole in the base of the spinous process and remain outside the spinal canal. Sublaminar wires require careful preparation of the cephalad and caudad interlaminar spaces to minimize the risk of neurologic injury as wires are passed beneath the lamina and dorsal to the neural elements.

12. Describe the use of pedicle screw anchors in posterior spinal contructs.

Pedicle screw anchors can be used throughout the thoracic and lumbar spinal regions and have become the most popular type of spinal anchor currently. Advantages of pedicle screws include secure fixation, the ability to apply forces to both the anterior and posterior columns of the spine from a posterior approach, and the capability to achieve fixation when lamina are deficient. The disadvantages of pedicle screws include technical challenges related to screw placement and the potential for neurologic, vascular, and visceral injury due to misplaced screws. Pedicle screws may be broadly classified as fixed head screws (monoaxial), mobile head screws (polyaxial), or bolts (require a separate connector for attachment to the longitudinal member) (Fig. 29-6).

13. What are the anatomic landmarks for placement of pedicle screws in the thoracic and lumbar spine?

• In the thoracic region, screw placement is initiated at the lateral aspect of the pedicle. The pedicle entry site is determined by referencing the transverse process, the superior articular process, and the pars interarticularis. Exact position of the entry site is adjusted depending on the specific level of the thoracic spine and whether the screw trajectory is straight-ahead or anatomic

Figure 29-6. Pedicle screw-based instrumentation construct. (From Buchowksi JM, Kuhns CA, Bridwell KH, et al. Surgical management of posttraumatic thoracolumbar kyphosis. Spine J 2008;8:666-77.)

• In the lumbar region, the entry site for screw placement is located at the upslope where the transverse process joins the superior articular process just lateral to the pars interarticularis. This site can be approximated by making a line along the midpoint of the transverse process and a second line along the lateral border of the superior articular process. The crossing point of these two lines defines the entry site to the pedicle (Fig. 29-7)

Figure 29-7. Landmarks for pedicle screw placement. A, Thoracic spine. B, Lumbar spine. (Courtesy of DePuy Spine, Inc.)

14. What is dynamic stabilization of the spine?

Dynamic stabilization is a concept of placing anchors (generally pedicle screws) into the spine and connecting these anchors with a flexible longitudinal member (e.g. rod, cable, spring). The goal of this type of implant is to constrain but not eliminate motion. Proponents of this concept believe this type of implant will produce less stress on the adjacent spinal segments and may prevent some of the complications observed following spinal fusion (e.g. adjacent-level degenerative changes). Opponents worry that without concurrent spinal arthrodesis, these implants may loosen or fail prematurely and require revision surgery. Currently, there are limited data to prove or disprove the scientific utility of this concept (Fig. 29-8).

Figure 29-8. Dynamic spinal fixation system. Pedicle screws are linked by a flexible rod, allowing constrained motion between the screws.

15. What are interspinous implants?

Interspinous implants are designed and indicated 1) for treatment of symptomatic lumbar spinal stenosis when fusion is not intended and 2) as a method for achieving lumbar segmental fixation when fusion of a spinal segment is intended. Interspinous implants indicated for the treatment of lumbar spinal stenosis are inserted between adjacent spinous processes to slightly distract the spinous processes apart and induce segmental kyphosis. Spinous process distraction results in slight enlargement of the cross-sectional area of the spinal canal and may relieve position-dependent spinal stenosis symptoms. Various materials (titanium, silicone, polyethylene) have been proposed for this category of implant. Patients who experience positional relief of leg pain symptoms due to lumbar spinal stenosis while in a sitting position are considered surgical candidates. This type of device is a motion-preserving implant that avoids the need for spinal fusion. Interspinous implants have also been utilized as a means of achieving segmental fixation when fusion of a motion segment is intended (Fig. 29-9).

Nerve root Figure 29-9. Interspinous implant.

ANTERIOR SPINAL INSTRUMENTATION

16. What are the two main types of anterior spinal instrumentation?

• Anterior spinal implants may be broadly classified as extracolumnar or intracolumnar implants. Extracolumnar implants are located on the external aspect of the vertebral body and span one or more adjacent vertebral motion segments. Extracolumnar implants consist of vertebral body screws connected to a longitudinal member consisting of either a plate or a rod. Extracolumnar implants are placed on the lateral aspect of the thoracic and lumbar vertebral bodies with screws directed in a coronal plane trajectory. An exception to this principle occurs at the L5-S1 level where implants are placed in an anterior midline location due to anatomic constraints created by the vascular structures at this level

• Intracolumnar implants consist of implants that reside within the contour of the vertebral bodies. Implant options include bone, metal, or synthetic materials that are capable of bearing loads. Intracolumnar implants may or may not possess potential for biologic incorporation within the anterior spinal column

17. Contrast the utility of anterior plate and rod systems.

• Anterior plate systems (Fig. 29-10A) are useful for short-segment spinal disorders (one or two spinal levels). Tumors, burst fractures, and degenerative spinal disorders requiring anterior fusion over one or two levels are indications for use of an anterior plate system. The use of a plate system is problematic when significant coronal or sagittal plane deformity exists or when multiple anterior vertebral segments require fixation. Technical difficulties arise because restoration of spinal alignment is required prior to plate application in the presence of significant spinal deformity

• Anterior rod systems (Fig. 29-10B) offer advantages in comparison to plate systems. In short-segment spinal problems, anterior rod systems permit corrective forces to be applied directly to spinal segments, thereby restoring spinal alignment. For example, in the presence of a kyphotic deformity secondary to a burst fracture, initial distraction provides deformity correction and facilitates subsequent placement of an intracolumnar implant. Subsequent compression of the anterior graft or cage restores anterior load sharing and enhances arthrodesis. In long-segment spinal problems (e.g. scoliosis) single or double rod systems can be customized to the specific spinal deformity requiring correction

Figure 29-10. Anterior extracolumnar implants: plate system (A) and rod system (B). (From Devlin VJ, Pitt DD. The evolution of surgery of the anterior spinal column. Spine State Art Rev 1998;12:493-528.)

18. What are some guidelines for placement of vertebral body screws when using an anterior plate or rod system?

The screws should be parallel to the vertebral endplates. In the axial plane, the screws should be parallel with or angle away from the vertebral canal. The screw tips should purchase the far cortex of the vertebral body but should not protrude more than 5 mm beyond this point (Fig. 29-11).

Figure 29-11. Correct placement of anterior vertebral body screws. (From Zindrick MR, Selby D. Lumbar spine fusion: different types and indications. In: Wiesel SW, Weinstein JN, Herkowitz H, et al., editors. The Lumbar Spine. 2nd ed. Philadelphia: Saunders; 1996.)

19. Describe three possible functions of intracolumnar implants.

Intracolumnar implants may be differentiated based on their intended function:

• Promote fusion. Intracolumnar implants that have potential for biologic incorporation include autograft bone (e.g. ilium, fibula), structural allograft bone (tibia, femur, humerus), and synthetic cages (titanium mesh, carbon fiber, polyether ether ketone) filled with bone graft. Such implants are typically used after discectomy or corpectomy to reconstruct the anterior spinal column and promote spinal fusion

• Function as a spacer. Certain intracolumnar implants (e.g. polymethylmethacrylate [PMMA]) are intended to function as an anterior column spacer despite lack of potential for biologic incorporation

• Preserve motion. An emerging concept is the use of a disc spacer to maintain segmental mobility, stability, and disc space height without fusion

20. What factors should be considered in choosing among autograft, allograft, and cage devices when an intracolumnar implant is indicated?

• Autograft remains the gold standard from the standpoint of fusion success. However, significant donor site morbidity is associated with procurement of a structural autograft

• Allograft provides good early strength and avoids donor site morbidity. However, allograft possesses a lower and slower fusion rate compared with autograft. In addition, use of allograft exposes the patient to the infectious risk associated with donor tissue

• Cage devices possess excellent strength and provide the advantage of mechanical interdigitation with vertebral receptor sites, thereby decreasing risk of dislodgement (Fig. 29-12). Cage devices can be filled with cancellous autograft, allograft, or biologic agents (e.g. bone morphogenetic proteins) to promote fusion. However, cage devices may subside into the vertebral bodies, resulting in loss of anterior column height. In addition, radiographic assessment of anterior column fusion can be difficult in the presence of cage devices. Cage devices are grouped into two main categories:

• Static (cage dimensions determined prior to implantation)

• Expandable (possess capacity for expansion following implantation to optimize stability)

Figure 29-12. Intracolumnar implants. Commercially available vertebrectomy spacers: Titanium mesh (A), expandable titanium cage (B), and stackable modular polyetheretherketone (PEEK) (C). (From Kim DH, Henn JS, Vaccaro AR, et al. Surgical Anatomy and Techniques to the Spine. Philadelphia: Saunders; 2006.)

Figure 29-12. Intracolumnar implants. Commercially available vertebrectomy spacers: Titanium mesh (A), expandable titanium cage (B), and stackable modular polyetheretherketone (PEEK) (C). (From Kim DH, Henn JS, Vaccaro AR, et al. Surgical Anatomy and Techniques to the Spine. Philadelphia: Saunders; 2006.)

21. When is it reasonable to use polymethylmethacrylate (PMMA) as a spacer to reconstruct an anterior spinal column defect?

Currently, PMMA is used in two situations:

• Anterior spinal reconstruction of metastatic vertebral body lesions in patients with a finite lifespan. When used for this purpose, PMMA is subject to tensile failure and loosening secondary to development of a fibrous membrane at the cement-bone interface

• Reconstruction of osteoporotic compression fractures. Vertebroplasty and kyphoplasty procedures involve the injection of PMMA into the vertebral bodies to alleviate pain secondary to acute and subacute fracture

22. What are the approach options for placement of an intracolumnar implant?

Intracolumnar implants may be placed through anterior, posterior, or lateral surgical approaches. The best approach depends on the location and type of spinal pathology requiring treatment. Recently, minimally invasive approaches have been popularized for placement of intracolumnar implants.

Key Points

1. Spinal implants function to maintain or restore spinal alignment, stabilize spinal segments, and enhance spinal fusion.

2. Short-term stabilization of the spine is provided by spinal implants and long-term stabilization of the spine is traditionally provided by fusion.

3. A posterior spinal instrumentation construct consists of:

a. Vertebral anchors (hooks, wires, screws)

b. Longitudinal elements (rods) on each side of the spine c. Transverse connectors (cross-linking devices)

4. Reconstruction of the load-bearing capacity of the anterior spinal column is critical to successful application of spinal instrumentation.

5. The safety and efficacy of motion preserving spinal implants is an area of active investigation.

Websites

History of surgery for correction of spinal deformity: http://www.medscape.com/viewarticle/448306 Thoracic pedicle screw fixation for spinal deformity: http://www.medscape.com/viewarticle/448311 Classification of posterior dynamic stabilization devices: http://www.medscape.com/viewarticle/555030

BiBLiOGRAPHY

1. Asher MA, Strippgen WE, Heinig CF, et al. Isola implant system. Semin Spine Surg 1992;4:175-92.

2. Cotrel Y, Dubousset J, Guillaumat M. New universal instrumentation in spine surgery. Clin Orthop 1988;227:10-23.

3. DiPaola CP, Molinari RW. Posterior lumbar interbody fusion. J Am Acad Ortho Surg 2008;16:130-9.

4. Harms J, Tabasso G. Instrumented Spinal Surgery: Principles and Techniques. New York: Thieme; 1999.

5. Harrington PR. The history and development of Harrington instrumentation. Clin Orthop 1988;227:3-5.

6. Kim DH, Albert TJ. Interspinous process spacers. J Am Acad Orth Surg 2007;15:200-7.

7. Kim DH, Henn JS, Vaccaro AR, et al. Surgical Anatomy and Techniques to the Spine. Philadelphia: Saunders; 2006.

8. Kim YJ, Lenke LG, Kim J, et al. Comparative analysis of pedicle screw versus hybrid instrumentation in posterior spinal fusion of adolescent idiopathic scoliosis. Spine 2006;31:291-8.

9. Lenke lG, Betz RR, Harms J. Modern Anterior Scoliosis Surgery. St. Louis: Quality Medical Publishing; 2004.

FDA Disclosure: Pedicle screw system clearance by the FDA is limited to use as an adjunct to fusion in skeletally mature patients. The use of pedicle screws is not FDA approved in the pediatric population as of 8/1/2010.

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