M u s c u l o s k e l e t a l I m a g i n g • R ev i ew Ha and Petscavage-Thomas Lumbar Spine Hardware
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Musculoskeletal Imaging Review
Alice S. Ha1 Jonelle M. Petscavage-Thomas 2 Ha AS, Petscavage-Thomas JM
Keywords: disk replacement, fusion, lumbar spine, spinous process distraction DOI:10.2214/AJR.13.12217 Received November, 10, 2013; accepted after revision February 18, 2014. 1 Department of Radiology, University of Washington, Seattle, WA. 2 Department of Radiology, Penn State Hershey Medical Center, 500 University Dr, Hershey, PA 17033. Address correspondence to J. M. Petscavage-Thomas ([email protected]).
This article is available for credit. AJR 2014; 203:573–581 0361–803X/14/2033–573 © American Roentgen Ray Society
Imaging of Current Spinal Hardware: Lumbar Spine OBJECTIVE. The purposes of this article are to review the indications for and the materials and designs of hardware more commonly used in the lumbar spine; to discuss alternatives for each of the types of hardware; to review normal postoperative imaging findings; to describe the appropriateness of different imaging modalities for postoperative evaluation; and to show examples of hardware complications. CONCLUSION. Stabilization and fusion of the lumbar spine with intervertebral disk replacement, artificial ligaments, spinous process distraction devices, plate-and-rod systems, dynamic posterior fusion devices, and newer types of material incorporation are increasingly more common in contemporary surgical practice. These spinal hardware devices will be seen more often in radiology practice. Successful postoperative radiologic evaluation of this spinal hardware necessitates an understanding of fundamental hardware design, physiologic objectives, normal postoperative imaging appearances, and unique complications. Radiologists may have little training and experience with the new and modified types of hardware used in the lumbar spine.
Lumbar Spinal Fusion and Instrumentation Lumbar spinal stenosis due to degenerative change is the fastest growing reason for spinal fusion surgery in adults older than 65 years [1]. The procedure involves bilateral partial laminectomies followed by diskectomy. As in the cervical spine, fusion consists of placement of graft material and an interbody spacer in the disk space by use of stabilization hardware. The location of the spacer can be assessed on radiographs with two to three radiopaque markers. The markers should be no less than 2 mm from the posterior vertebral body margin to prevent protrusion into the spinal canal [2] (Fig. 1). Posterior instrumentation provides support until bone fusion occurs. The posterior pedicle screws should not extend beyond the anterior margin of the vertebral body or lateral or medial to the pedicle [2]. Metal or radiolucent rods or plates may connect the pedicle screws. The Aspen device (Lanx) is a less invasive alternative to pedicle screws that requires little or no bone removal and has rates of fusion as high as 94% [3, 4] (Fig. 2). It is indicated for posterior stabilization with interlaminar fusion; anterior, transforaminal, and posterolateral interbody
fusion; and revision procedures. The device consists of two locking metal plates placed on the sides of the spinous process and connected by spikes into the spinous process [3, 4]. Bone graft material is placed in the radiolucent hole of the plate. Despite high rates of fusion and clinical success, complications related to dural exposure and canal disruption continue to be a problem with posterior fusion. Thus, there is a trend toward minimally invasive surgery designed for lower complication rates. For example, extreme lateral interbody fusion (XLIF, NuVasive) is used to access the disk space from a far lateral approach, thus with no peritoneal disruption or mobilization [5, 6]. Diskectomy is performed with an intact posterior annulus. A cage or ramp with bone graft is placed in the disk space. The implant has a characteristic long rectangular shape designed to maximize the surface area (Fig. 3). Potential complications of use of this route include thigh paresthesia or dysesthesia, subsidence, and hardware failure or migration [5–7]. The XLIF and other minimally invasive techniques are also used to treat scoliosis [8]. Surgical therapy is offered for patients with symptomatic curvature greater than
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Ha and Petscavage-Thomas 45°. Traditionally, surgery involved fusion by internal fixation with Harrington rods. Although these procedures improve spinal alignment and function, fusion extending to the lumbar spine has been highly associated with flat back syndrome [9] (Fig. 4). This syndrome, characterized by pain and loss of normal lumbar lordosis, is much less common with more modern segmental spinal fusion techniques, such as the XLIF device. An anterior surgical approach is used primarily if pain is diskogenic and posterior decompression is not required [2]. An anterior low-profile device was developed for the lumbar spine to prevent disruption of normal muscle activity, to be inserted with minimal operative morbidity, and to cause little risk of damage to adjacent neural and vascular structures [10]. The device consists of a radiolucent plate that does not extend anterior to the vertebral body endplate. It is anchored by two locking screws with a polyether ether ketone implant in the disk space (Fig. 5A). However, the polyether ether ketone or graft material within the cage can cause a foreign body reaction with imaging findings of osteolysis, softtissue masses, and loosening (Fig. 5B). Dynamic Posterior Stabilization In response to hardware-related complications and the high risk of adjacent segment degeneration seen with posterior lumbar fusion, dynamic posterior stabilization was introduced. The goal of dynamic stabilization is to distribute stress throughout the lumbar segments, lowering the risk of adjacent segment degeneration [11]. The Dynesys device (Zimmer) is the most extensively used of these posterior dynamic stabilization systems. The Dynesys device is a semirigid artificial ligament system composed of titanium alloy pedicle screws and polycarbonate urethane spacers connected by polyester cords placed under tension [2, 12] (Fig. 6). Reported complications include incorrect screw placement, loosening of screws, and infection [12]. Another dynamic posterior stabilization product is the Graf artificial ligament system, a nonelastic polyester ligament looped around pedicle screws and placed under tension to prevent rotation while allowing some flexion [13] (Fig. 7). However, placement of too much tension increases the risk of foraminal narrowing, nerve root impingement, and hyperlordosis [2]. The Stabilimax NZ system (Applied Spine Technologies) is a metallic posterior dynamic stabilization device that works with a dual spring mechanism (Sengupta D, presented
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TABLE 1: Summary of Complications Related to Lumbar Spine Instrumentation Device Name
Complications
Lumbar fusion devices Traditional posterior fusion and decompression, Adjacent segment degeneration including Aspen (Lanx) device Hardware loosening and failure Hardware irritation of canal and adjacent soft tissues Extreme lateral interbody fusion (XLIF, NuVasive)
Thigh paresthesia or dysesthesia Subsidence Hardware failure and migration
Anterior low-profile fusion devices
Foreign body reaction of polyether ether ketone /graft Hardware loosening and failure
Dynamic stabilization Dynesys (Zimmer)
Incorrect screw placement Screw loosening Infection
Graf artificial ligament system
Foraminal narrowing Nerve root impingement Hyperlordosis
Stabilimax NZ (Applied Spine Technologies)
Hardware loosening and failure
Disk replacement ProDisc-L (Synthes), Charité (Depuy Spine)
Polyethylene wear or migration Subsidence Adjacent disk degeneration Facet joint degeneration Compression fracture Wire breakage in Charité
Maverick (Medtronic), Kineflex (SpinalMotion)
Metal on metal wear Subsidence Adjacent disk degeneration Facet joint degeneration Compression fracture
Interspinous distraction X-Stop (Medtronic)
Displacement Fracture Adjacent segment degeneration Erosion adjacent bone
Wallis (Zimmer)
Polyether ether ketone foreign body reaction Loosening
Coflex (Paradigm Spine)
Spinous process resorption
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Lumbar Spine Hardware at the 2005 annual global symposium of the Spine Arthroplasty Society). The Stabilimax NZ device consists of fixed titanium posterior pedicle screws connected by parallel rods (Fig. 8). The rods have a ball-and-socket joint surrounded by springs, allowing limited motion. Clinical trials are being conducted to compare Stabilimax NZ stabilization with nondynamic fusion. In vitro studies have shown dynamic stabilization capable of decompressing the spinal level effectively in the sagittal and frontal planes while allowing a good amount of normal rotation, but not in torsion [14]. Lumbar Disk Replacement Lumbar disk replacement was designed to decrease the risk of adjacent segment degeneration that occurs with lumbar fusion while controlling symptoms of degenerative disk disease. Several devices are available for use in the lumbar spine. Total disk replacement was found to have clinical outcomes similar to those of lumbar fusion without a higher complication rate [15, 16]. The ProDisc-L device (Synthes) for the lumbar spine is a ball-and-socket mechanism with metal endplates embedded into the bone with keels (Fig. 9). A polyethylene inlay is attached to the inferior endplate, making the device semiconstrained [17]. The lumbar design differs from the cervical design only in the presence of a spike on the lateral aspect of each side of the keel to enhance stability [17] (Fig. 9). The Maverick (Medtronic) (Fig. 10) and Kineflex (SpinalMotion) devices are metal-on-metal ball-and-socket semiconstrained designs with keels embedding the endplates [17]. The Charité device (Depuy Spine) is an unconstrained device consisting of cobaltchromium endplates and a polyethylene inlay not fixed to the endplates [17] (Fig. 11). The inlay is held in place by compressive forces. A wire around the inlay enables localization. Three small stability spikes are present along the endplates. Reported complications of lumbar total disk replacement include subsidence, adjacent disk degeneration, facet joint degeneration, migration, breakage of the Charité metal wire, wear, severe osteolysis, macrophage-related particle disease [18], osteoporotic vertebral body fracture, and subluxation of the polyethylene core [18]. Metal wear is a concern with the metal-on-metal designs. Systemic release of cobalt and chromium ions was found in the serum of 10 patients who received Maverick disk replacements [19, 20].
All total disk replacements require anterior access for implantation by a transperitoneal or retroperitoneal approach. Complications related to this access include damage to adjacent structures and infection and occur in approximately 1–12% of cases [21]. Lumbar Interspinous Distraction Devices The indication for an interspinous distraction device is position-dependent intermittent claudication related to spinal stenosis [5] (Sengupta D, 2005 Spine Arthroplasty Society symposium). The principle of these devices is that flexion often relieves symptoms by decreasing epidural pressure, increasing the cross-sectional area of the foramina and central canal, and decreasing nerve root compression. Thus these distraction devices mimic a flexed position. Studies have shown fewer complications for the index operation in patients with interspinous distraction but higher rates of revision surgery at follow-up [22]. Four main types of interspinous distraction devices are in clinical use. The X-Stop device (Medtronic) consists of two metal parallel lateral wings connected by a titanium or polyether ether ketone spacer (Fig. 12) placed between adjacent spinous processes to hold the lumbar spine in flexion. The wings prevent lateral migration of the device. Treatment with the X-Stop device is less invasive than total disk replacement or fusion because the longitudinal ligaments are left intact, the procedure can be performed with local anesthesia, and the device does not preclude use of other devices and therapies [2, 23]. Potential complications include displacement, fracture (Fig. 13), adjacent segment degeneration, and erosion of adjacent bone [24]. The Wallis device (Zimmer) is composed of a block of polyether ether ketone held in place with flat Dacron polyester bands wrapped around the spinous processes [2]. On radiographs, two metal pins are seen in the spacer and two metal straps secure the tape (Fig. 14). Studies have shown an incidence of adjacent segment degeneration as low as 4.1% [25]. The Coflex device (Paradigm Spine) is a U-shaped titanium implant with vertical wings crimped onto the spinous process to hold it in place (Fig. 15A). It is a more invasive implantation because both interspinous and supraspinous ligaments are resected [2, 23]. The indications for use of a Coflex device are to unload the facet joints,
restore foraminal height, and provide stability after decompressive surgery to improve the clinical outcome [26]. A unique complication is spinous process resorption along the implant, which occurs near the site of crimping and may be due to an ischemic phenomenon [27] (Fig. 15B). Conclusion The last 5–10 years have seen considerable development in lumbar spine hardware. This hardware has been designed to provide clinical outcomes similar to those of spinal fusion while improving segmental motion and decreasing adjacent segment degeneration. Radiologists will likely encounter these devices in practice and should understand the normal imaging appearances and unique complications (Table 1) related to hardware materials and designs. References 1. Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical versus nonsurgical treatment for lumbar degenerative spondylolisthesis. N Engl J Med 2007; 356:2257–2270 2. Rutherford EE, Tarplett LJ, Davies EM, Harley JM, King LJ. Lumbar spine fusion and stabilization: hardware, techniques, and imaging appearances. RadioGraphics 2007; 27:1737–1749 3. Eisner W. Lanx’s Aspen system achieves 94% fusion rate. Orthopedics This Week website. ryortho.com/breaking/lanx039s-aspen-systemachieves-94-fusion-rate. Published September 19, 2012. Accessed April 28, 2013 4. Aspen® MIS Fusion System. Lanx website. www. lanx.com/products-aspen-mis-fusion-system. aspx. Accessed April 28, 2013 5. Ross JS. Specialty imaging: postoperative spine. Philadelphia, PA: Lippincott Williams & Wilkins, 2012 6. Ozgur BM, Aryan HE, Pimenta L, Taylor WR. Extreme lateral interbody fusion (XLIF): a novel surgical technique for anterior lumbar interbody fusion. Spine J 2006; 6:435–443 7. Rodgers WB, Gerber EJ, Patterson J. Intraoperative and early postoperative complications in extreme lateral interbody fusion: an analysis of 600 cases. Spine 2011; 36:26–32 8. Sarwahi V, Wollowick AL, Sugarman EP, et al. Minimally invasive scoliosis surgery: an innovative technique in patients with adolescent idiopathic scoliosis. Scoliosis 2011; 6:16 9. Cochran T, Irstam L, Nachemson A. Long-term anatomic and functional changes in patients with adolescent idiopathic scoliosis treated by Harrington rod fusion. Spine 1983; 8:576–584 10. Cain CM, Schleicher P, Gerlach R, et al. A new stand-alone anterior lumbar interbody fusion de-
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Ha and Petscavage-Thomas vice: biomechanical comparison with established fixation techniques. Spine (Phila Pa 1976) 2005; 30:2631–2636 11. Sengupta DK, Herkowitz HN. Lumbar spinal stenosis: treatment strategies and indications for surgery. Orthop Clin North Am 2003; 34:281–295 12. Grob D, Benini A, Junge A, Mannion AF. Clinical experience with the Dynesys semirigid fixation system for the lumbar spine: surgical and patient-oriented outcome in 50 cases after an average of 2 years. Spine 2005; 30:324–331 13. Graf H. Lumbar instability: surgical treatment without fusion. Rachis 1992; 412:123–137 14. Panjabi MM, Henderson G, James Y, Timm JP. StabilimaxNZ versus simulated fusion: evaluation of adjacent-level effects. Eur Spine J 2007; 16:2159–2165 15. Zigler J, Delamarter R, Spivak JM, et al. Results of the prospective, randomized, multicenter Food and Drug Administration investigational device exemption study of the ProDisc-L total disc replacement versus circumferential fusion for the treatment of 1-level degenerative disc disease. Spine (Phil Pa 1976) 2007; 32:1155–1162 16. Jacobs W, Van der Gaag NA, Tuschel A, et al. To-
tal disc replacement for chronic back pain in the presence of disc degeneration. Cochrane Database Syst Rev 2012; 9:CD008326 17. Murtagh RD, Quencer RM, Cohen DS, Yue JJ, Sklar EL. Normal and abnormal imaging findings in lumbar total disc replacement: devices and complications. RadioGraphics 2009; 29:105–118 18. Punt IM, Visser VM, van Rhijn LW, et al. Complications and reoperations of the SB Charité lumbar disc prosthesis: experience in 75 patients. Eur Spine J 2008; 17:36–43 19. Cavanaugh DA, Nunley PD, Kerr EJ 3rd, Werner DJ, Jawahar A. Delayed hyper-reactivity to metal ions after cervical disc arthroplasty: a case report and literature review. Spine 2009; 34:E262–E265 20. Zeh A, Planert M, Siegert G, et al. Release of cobalt and chromium ions into the serum following implantation of the metal-on-metal Mavericktype artificial lumbar disc (Medtronic Sofamor Danek). Spine 2007; 32:348–352 21. Beiner JM, Grauer J, Kwon BK, Vaccaro AR. Postoperative wound infections of the spine. Neurosurg Focus 2003; 15:E14 22. Deyo RA, Martin BI, Ching A, et al. Interspinous spacers compared to decompression or fusion for
lumbar stenosis: complications and repeat operations in the Medicare population. Spine (Phila Pa 1976) 2013; 38:865–872 23. Kabir SM, Gupta SR, Casey AT. Lumbar interspinous spacers: a systematic review of clinical and biomechanical evidence. Spine 2010; 35:E1499–E1506 24. Sobottke R, Schlüter-Brust K, Kaulhausen T, et al. Interspinous implants (X Stop, Wallis, Diam) for the treatment of LSS: is there a correlation between radiological parameters and clinical outcome? Eur Spine J 2009; 18:1494–1503 25. Korovessis P, Repantis T, Zacharatos S, Zafiropoulos A. Does Wallis implant reduce adjacent segment degeneration above lumbosacral instrumented fusion? Eur Spine J 2009; 18:830–840 26. Richter A, Schutz C, Hauck M, Halm H. Does an interspinous device (Coflex) improve the outcome of decompressive surgery in lumbar spinal stenosis? One-year follow up of a prospective case control study of 60 patients. Eur Spine J 2010; 19:283–289 27. Park SC, Yoon SH, Hong YP, Kim KJ, Chung SK, Kim HJ. Minimum 2-year follow-up result of degenerative spinal stenosis treated with interspinous U (Coflex). J Korean Neurosurg Soc 2009; 46:292–299
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Fig. 1—48-year-old man with interbody spacer migration. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show interbody fusion device (arrow) dislocated to left and posteriorly, compressing neural foramen and central canal. Radiopaque marker normally should be at least 2 mm anterior to posterior margin of vertebral body. Also present is interspinous distraction device. C, Sagittal CT image in bone window shows full extent of posterior migration of interbody device (arrow).
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Lumbar Spine Hardware
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Fig. 2—67-year-old man treated with Aspen device. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show Aspen device (Lanx). Two locking metal plates are located on sides of spinous process (arrow, B) and are connected by spikes (arrow, A) to spinous process. Bone graft material is present in radiolucent hole of plate. C, Coronal CT image in bone window shows plates on each side of spinous process and connecting spikes.
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Fig. 3—56-year-old man treated with extreme lateral interbody fusion (XLIF device, NuVasive). A and B, Anteroposterior radiograph (A) and axial CT image in bone algorithm (B) of lumbar spine show XLIF device at L3–L4. Implant has long, rectangular shape to maximize surface area.
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Ha and Petscavage-Thomas
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Fig. 4—50-year-old woman with back pain who as adolescent underwent surgical correction of idiopathic scoliosis. A and B, Frontal (A) and lateral (B) radiographs obtained at age 13, when thoracolumbar fusion with Harrington rods (compression on right and distraction on left) was performed after 6 months of bracing therapy failed. Increased lumbar lateral scoliosis and kyphosis are associated with foraminal stenosis at L4–L5 and L5–S1 (flat back syndrome). C, Frontal radiograph shows appearance after corrective surgery.
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Fig. 5—49-year-old man treated with low-profile anterior fusion. A and B, Lateral radiograph (A) and sagittal CT image in bone window (B) at L5–S1 show zero-profile system for anterior interbody fusion 18 months after initial surgery. Osteolysis (arrow) is present around anterior margins and S1 screw. Loosening of hardware and nonunion across interbody fusion are evident. Total disk replacement of L4-5 also is evident.
Fig. 6—70-year-old man treated with dynamic stabilization system. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show Dynesys (Zimmer) rod and screw system at L3–L4 with two titanium alloy pedicle screws connected by nonradiopaque cylindric polycarbonate urethane spacer through which polyester cord is strung. (Courtesy of Hunter J, University of California, Davis, Sacramento, CA)
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Lumbar Spine Hardware
Fig. 7—Drawing shows Graf dynamic stabilization. Nonelastic polyester ligaments (yellow) are looped around pedicle screws (red).
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Fig. 8—39-year-old woman treated with dynamic posterior stabilization. A and B, Sagittal (A) and coronal (B) CT images of lumbar spine show Stabilimax NZ (Applied Spine Technologies) dynamic posterior stabilization hardware. Parallel rods connect fixed titanium posterior pedicle screws. Rods have ball-and-socket joint surrounded by springs that allow limited motion.
Fig. 9—44-year-old woman treated with lumbar disk replacement. A and B, Lateral (A) and anteroposterior (B) radiographs of lumbar spine show ProDisc-L device (Synthes) at L4–L5. Device consists of radiolucent polyethylene inlay attached to inferior endplate. Keels (white arrow) embed metal endplates into bone, and spike (black arrow) on lateral aspect of each side of keel enhances stability.
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Fig. 10—52-year-old man treated with lumbar disk replacement. Sagittal CT myelogram of lumbar spine in bone algorithm shows Maverick (Medtronic) disk replacement at L5–S1. Keels embed metal endplates to bone. There is no polyethylene inlay, thus this is metal-on-metal design.
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Ha and Petscavage-Thomas
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Fig. 11—45-year-old woman treated with lumbar disk replacement. A–C, Lateral (A) and anteroposterior (B) radiographs and sagittal CT image in bone window (C) show Charité DePuy Spine disk replacement at L4–L5. In this unconstrained design, radiolucent polyethylene marked by wire (arrow, A) is held in place only by compressive forces. Three small stability spikes (arrow, B) are present along superior and inferior endplates.
A Fig. 12—66-year-old man treated with interspinous distraction. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show X-Stop (Medtronic) interspinous distraction device. Two parallel wings (arrow, A) are connected by titanium spacer that places patient in slight flexion. Wings prevent lateral migration of device.
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Fig. 13—55-year-old man with complications of interspinous distraction. Lateral radiograph of lumbar spine shows fracture (arrow) of L5 spinous process adjacent to X-Stop Medtronic device.
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Lumbar Spine Hardware Fig. 14—64-year-old man treated with interspinous distraction. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show Wallis device (Zimmer) at L4–L5 level. Block of radiolucent peek identified with two metal pins (white arrow, B) is held in place with flat Dacron polyester band wrapped around spinous process. Two metal straps (black arrow, B) secure tape.
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B Fig. 15—Interspinous distraction. A, 68-year-old woman. Lateral radiograph of lumbar spine shows Coflex (Paradigm Spine) interspinous distraction device is U-shaped titanium implant with vertical wings crimped onto spinous process to hold it in place. B, 44-year-old man. Sagittal CT image of lumbar spine in bone window shows resorption of inferior spinous process (arrow) around site of crimping of Coflex device. This occurs in as many as 57% of cases.
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F O R YO U R I N F O R M AT I O N
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Musculoskeletal Imaging Review
Alice S. Ha1 Jonelle M. Petscavage-Thomas 2 Ha AS, Petscavage-Thomas JM
Keywords: disk replacement, fusion, lumbar spine, spinous process distraction DOI:10.2214/AJR.13.12217 Received November, 10, 2013; accepted after revision February 18, 2014. 1 Department of Radiology, University of Washington, Seattle, WA. 2 Department of Radiology, Penn State Hershey Medical Center, 500 University Dr, Hershey, PA 17033. Address correspondence to J. M. Petscavage-Thomas ([email protected]).
This article is available for credit. AJR 2014; 203:573–581 0361–803X/14/2033–573 © American Roentgen Ray Society
Imaging of Current Spinal Hardware: Lumbar Spine OBJECTIVE. The purposes of this article are to review the indications for and the materials and designs of hardware more commonly used in the lumbar spine; to discuss alternatives for each of the types of hardware; to review normal postoperative imaging findings; to describe the appropriateness of different imaging modalities for postoperative evaluation; and to show examples of hardware complications. CONCLUSION. Stabilization and fusion of the lumbar spine with intervertebral disk replacement, artificial ligaments, spinous process distraction devices, plate-and-rod systems, dynamic posterior fusion devices, and newer types of material incorporation are increasingly more common in contemporary surgical practice. These spinal hardware devices will be seen more often in radiology practice. Successful postoperative radiologic evaluation of this spinal hardware necessitates an understanding of fundamental hardware design, physiologic objectives, normal postoperative imaging appearances, and unique complications. Radiologists may have little training and experience with the new and modified types of hardware used in the lumbar spine.
Lumbar Spinal Fusion and Instrumentation Lumbar spinal stenosis due to degenerative change is the fastest growing reason for spinal fusion surgery in adults older than 65 years [1]. The procedure involves bilateral partial laminectomies followed by diskectomy. As in the cervical spine, fusion consists of placement of graft material and an interbody spacer in the disk space by use of stabilization hardware. The location of the spacer can be assessed on radiographs with two to three radiopaque markers. The markers should be no less than 2 mm from the posterior vertebral body margin to prevent protrusion into the spinal canal [2] (Fig. 1). Posterior instrumentation provides support until bone fusion occurs. The posterior pedicle screws should not extend beyond the anterior margin of the vertebral body or lateral or medial to the pedicle [2]. Metal or radiolucent rods or plates may connect the pedicle screws. The Aspen device (Lanx) is a less invasive alternative to pedicle screws that requires little or no bone removal and has rates of fusion as high as 94% [3, 4] (Fig. 2). It is indicated for posterior stabilization with interlaminar fusion; anterior, transforaminal, and posterolateral interbody
fusion; and revision procedures. The device consists of two locking metal plates placed on the sides of the spinous process and connected by spikes into the spinous process [3, 4]. Bone graft material is placed in the radiolucent hole of the plate. Despite high rates of fusion and clinical success, complications related to dural exposure and canal disruption continue to be a problem with posterior fusion. Thus, there is a trend toward minimally invasive surgery designed for lower complication rates. For example, extreme lateral interbody fusion (XLIF, NuVasive) is used to access the disk space from a far lateral approach, thus with no peritoneal disruption or mobilization [5, 6]. Diskectomy is performed with an intact posterior annulus. A cage or ramp with bone graft is placed in the disk space. The implant has a characteristic long rectangular shape designed to maximize the surface area (Fig. 3). Potential complications of use of this route include thigh paresthesia or dysesthesia, subsidence, and hardware failure or migration [5–7]. The XLIF and other minimally invasive techniques are also used to treat scoliosis [8]. Surgical therapy is offered for patients with symptomatic curvature greater than
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Ha and Petscavage-Thomas 45°. Traditionally, surgery involved fusion by internal fixation with Harrington rods. Although these procedures improve spinal alignment and function, fusion extending to the lumbar spine has been highly associated with flat back syndrome [9] (Fig. 4). This syndrome, characterized by pain and loss of normal lumbar lordosis, is much less common with more modern segmental spinal fusion techniques, such as the XLIF device. An anterior surgical approach is used primarily if pain is diskogenic and posterior decompression is not required [2]. An anterior low-profile device was developed for the lumbar spine to prevent disruption of normal muscle activity, to be inserted with minimal operative morbidity, and to cause little risk of damage to adjacent neural and vascular structures [10]. The device consists of a radiolucent plate that does not extend anterior to the vertebral body endplate. It is anchored by two locking screws with a polyether ether ketone implant in the disk space (Fig. 5A). However, the polyether ether ketone or graft material within the cage can cause a foreign body reaction with imaging findings of osteolysis, softtissue masses, and loosening (Fig. 5B). Dynamic Posterior Stabilization In response to hardware-related complications and the high risk of adjacent segment degeneration seen with posterior lumbar fusion, dynamic posterior stabilization was introduced. The goal of dynamic stabilization is to distribute stress throughout the lumbar segments, lowering the risk of adjacent segment degeneration [11]. The Dynesys device (Zimmer) is the most extensively used of these posterior dynamic stabilization systems. The Dynesys device is a semirigid artificial ligament system composed of titanium alloy pedicle screws and polycarbonate urethane spacers connected by polyester cords placed under tension [2, 12] (Fig. 6). Reported complications include incorrect screw placement, loosening of screws, and infection [12]. Another dynamic posterior stabilization product is the Graf artificial ligament system, a nonelastic polyester ligament looped around pedicle screws and placed under tension to prevent rotation while allowing some flexion [13] (Fig. 7). However, placement of too much tension increases the risk of foraminal narrowing, nerve root impingement, and hyperlordosis [2]. The Stabilimax NZ system (Applied Spine Technologies) is a metallic posterior dynamic stabilization device that works with a dual spring mechanism (Sengupta D, presented
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TABLE 1: Summary of Complications Related to Lumbar Spine Instrumentation Device Name
Complications
Lumbar fusion devices Traditional posterior fusion and decompression, Adjacent segment degeneration including Aspen (Lanx) device Hardware loosening and failure Hardware irritation of canal and adjacent soft tissues Extreme lateral interbody fusion (XLIF, NuVasive)
Thigh paresthesia or dysesthesia Subsidence Hardware failure and migration
Anterior low-profile fusion devices
Foreign body reaction of polyether ether ketone /graft Hardware loosening and failure
Dynamic stabilization Dynesys (Zimmer)
Incorrect screw placement Screw loosening Infection
Graf artificial ligament system
Foraminal narrowing Nerve root impingement Hyperlordosis
Stabilimax NZ (Applied Spine Technologies)
Hardware loosening and failure
Disk replacement ProDisc-L (Synthes), Charité (Depuy Spine)
Polyethylene wear or migration Subsidence Adjacent disk degeneration Facet joint degeneration Compression fracture Wire breakage in Charité
Maverick (Medtronic), Kineflex (SpinalMotion)
Metal on metal wear Subsidence Adjacent disk degeneration Facet joint degeneration Compression fracture
Interspinous distraction X-Stop (Medtronic)
Displacement Fracture Adjacent segment degeneration Erosion adjacent bone
Wallis (Zimmer)
Polyether ether ketone foreign body reaction Loosening
Coflex (Paradigm Spine)
Spinous process resorption
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Lumbar Spine Hardware at the 2005 annual global symposium of the Spine Arthroplasty Society). The Stabilimax NZ device consists of fixed titanium posterior pedicle screws connected by parallel rods (Fig. 8). The rods have a ball-and-socket joint surrounded by springs, allowing limited motion. Clinical trials are being conducted to compare Stabilimax NZ stabilization with nondynamic fusion. In vitro studies have shown dynamic stabilization capable of decompressing the spinal level effectively in the sagittal and frontal planes while allowing a good amount of normal rotation, but not in torsion [14]. Lumbar Disk Replacement Lumbar disk replacement was designed to decrease the risk of adjacent segment degeneration that occurs with lumbar fusion while controlling symptoms of degenerative disk disease. Several devices are available for use in the lumbar spine. Total disk replacement was found to have clinical outcomes similar to those of lumbar fusion without a higher complication rate [15, 16]. The ProDisc-L device (Synthes) for the lumbar spine is a ball-and-socket mechanism with metal endplates embedded into the bone with keels (Fig. 9). A polyethylene inlay is attached to the inferior endplate, making the device semiconstrained [17]. The lumbar design differs from the cervical design only in the presence of a spike on the lateral aspect of each side of the keel to enhance stability [17] (Fig. 9). The Maverick (Medtronic) (Fig. 10) and Kineflex (SpinalMotion) devices are metal-on-metal ball-and-socket semiconstrained designs with keels embedding the endplates [17]. The Charité device (Depuy Spine) is an unconstrained device consisting of cobaltchromium endplates and a polyethylene inlay not fixed to the endplates [17] (Fig. 11). The inlay is held in place by compressive forces. A wire around the inlay enables localization. Three small stability spikes are present along the endplates. Reported complications of lumbar total disk replacement include subsidence, adjacent disk degeneration, facet joint degeneration, migration, breakage of the Charité metal wire, wear, severe osteolysis, macrophage-related particle disease [18], osteoporotic vertebral body fracture, and subluxation of the polyethylene core [18]. Metal wear is a concern with the metal-on-metal designs. Systemic release of cobalt and chromium ions was found in the serum of 10 patients who received Maverick disk replacements [19, 20].
All total disk replacements require anterior access for implantation by a transperitoneal or retroperitoneal approach. Complications related to this access include damage to adjacent structures and infection and occur in approximately 1–12% of cases [21]. Lumbar Interspinous Distraction Devices The indication for an interspinous distraction device is position-dependent intermittent claudication related to spinal stenosis [5] (Sengupta D, 2005 Spine Arthroplasty Society symposium). The principle of these devices is that flexion often relieves symptoms by decreasing epidural pressure, increasing the cross-sectional area of the foramina and central canal, and decreasing nerve root compression. Thus these distraction devices mimic a flexed position. Studies have shown fewer complications for the index operation in patients with interspinous distraction but higher rates of revision surgery at follow-up [22]. Four main types of interspinous distraction devices are in clinical use. The X-Stop device (Medtronic) consists of two metal parallel lateral wings connected by a titanium or polyether ether ketone spacer (Fig. 12) placed between adjacent spinous processes to hold the lumbar spine in flexion. The wings prevent lateral migration of the device. Treatment with the X-Stop device is less invasive than total disk replacement or fusion because the longitudinal ligaments are left intact, the procedure can be performed with local anesthesia, and the device does not preclude use of other devices and therapies [2, 23]. Potential complications include displacement, fracture (Fig. 13), adjacent segment degeneration, and erosion of adjacent bone [24]. The Wallis device (Zimmer) is composed of a block of polyether ether ketone held in place with flat Dacron polyester bands wrapped around the spinous processes [2]. On radiographs, two metal pins are seen in the spacer and two metal straps secure the tape (Fig. 14). Studies have shown an incidence of adjacent segment degeneration as low as 4.1% [25]. The Coflex device (Paradigm Spine) is a U-shaped titanium implant with vertical wings crimped onto the spinous process to hold it in place (Fig. 15A). It is a more invasive implantation because both interspinous and supraspinous ligaments are resected [2, 23]. The indications for use of a Coflex device are to unload the facet joints,
restore foraminal height, and provide stability after decompressive surgery to improve the clinical outcome [26]. A unique complication is spinous process resorption along the implant, which occurs near the site of crimping and may be due to an ischemic phenomenon [27] (Fig. 15B). Conclusion The last 5–10 years have seen considerable development in lumbar spine hardware. This hardware has been designed to provide clinical outcomes similar to those of spinal fusion while improving segmental motion and decreasing adjacent segment degeneration. Radiologists will likely encounter these devices in practice and should understand the normal imaging appearances and unique complications (Table 1) related to hardware materials and designs. References 1. Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical versus nonsurgical treatment for lumbar degenerative spondylolisthesis. N Engl J Med 2007; 356:2257–2270 2. Rutherford EE, Tarplett LJ, Davies EM, Harley JM, King LJ. Lumbar spine fusion and stabilization: hardware, techniques, and imaging appearances. RadioGraphics 2007; 27:1737–1749 3. Eisner W. Lanx’s Aspen system achieves 94% fusion rate. Orthopedics This Week website. ryortho.com/breaking/lanx039s-aspen-systemachieves-94-fusion-rate. Published September 19, 2012. Accessed April 28, 2013 4. Aspen® MIS Fusion System. Lanx website. www. lanx.com/products-aspen-mis-fusion-system. aspx. Accessed April 28, 2013 5. Ross JS. Specialty imaging: postoperative spine. Philadelphia, PA: Lippincott Williams & Wilkins, 2012 6. Ozgur BM, Aryan HE, Pimenta L, Taylor WR. Extreme lateral interbody fusion (XLIF): a novel surgical technique for anterior lumbar interbody fusion. Spine J 2006; 6:435–443 7. Rodgers WB, Gerber EJ, Patterson J. Intraoperative and early postoperative complications in extreme lateral interbody fusion: an analysis of 600 cases. Spine 2011; 36:26–32 8. Sarwahi V, Wollowick AL, Sugarman EP, et al. Minimally invasive scoliosis surgery: an innovative technique in patients with adolescent idiopathic scoliosis. Scoliosis 2011; 6:16 9. Cochran T, Irstam L, Nachemson A. Long-term anatomic and functional changes in patients with adolescent idiopathic scoliosis treated by Harrington rod fusion. Spine 1983; 8:576–584 10. Cain CM, Schleicher P, Gerlach R, et al. A new stand-alone anterior lumbar interbody fusion de-
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Ha and Petscavage-Thomas vice: biomechanical comparison with established fixation techniques. Spine (Phila Pa 1976) 2005; 30:2631–2636 11. Sengupta DK, Herkowitz HN. Lumbar spinal stenosis: treatment strategies and indications for surgery. Orthop Clin North Am 2003; 34:281–295 12. Grob D, Benini A, Junge A, Mannion AF. Clinical experience with the Dynesys semirigid fixation system for the lumbar spine: surgical and patient-oriented outcome in 50 cases after an average of 2 years. Spine 2005; 30:324–331 13. Graf H. Lumbar instability: surgical treatment without fusion. Rachis 1992; 412:123–137 14. Panjabi MM, Henderson G, James Y, Timm JP. StabilimaxNZ versus simulated fusion: evaluation of adjacent-level effects. Eur Spine J 2007; 16:2159–2165 15. Zigler J, Delamarter R, Spivak JM, et al. Results of the prospective, randomized, multicenter Food and Drug Administration investigational device exemption study of the ProDisc-L total disc replacement versus circumferential fusion for the treatment of 1-level degenerative disc disease. Spine (Phil Pa 1976) 2007; 32:1155–1162 16. Jacobs W, Van der Gaag NA, Tuschel A, et al. To-
tal disc replacement for chronic back pain in the presence of disc degeneration. Cochrane Database Syst Rev 2012; 9:CD008326 17. Murtagh RD, Quencer RM, Cohen DS, Yue JJ, Sklar EL. Normal and abnormal imaging findings in lumbar total disc replacement: devices and complications. RadioGraphics 2009; 29:105–118 18. Punt IM, Visser VM, van Rhijn LW, et al. Complications and reoperations of the SB Charité lumbar disc prosthesis: experience in 75 patients. Eur Spine J 2008; 17:36–43 19. Cavanaugh DA, Nunley PD, Kerr EJ 3rd, Werner DJ, Jawahar A. Delayed hyper-reactivity to metal ions after cervical disc arthroplasty: a case report and literature review. Spine 2009; 34:E262–E265 20. Zeh A, Planert M, Siegert G, et al. Release of cobalt and chromium ions into the serum following implantation of the metal-on-metal Mavericktype artificial lumbar disc (Medtronic Sofamor Danek). Spine 2007; 32:348–352 21. Beiner JM, Grauer J, Kwon BK, Vaccaro AR. Postoperative wound infections of the spine. Neurosurg Focus 2003; 15:E14 22. Deyo RA, Martin BI, Ching A, et al. Interspinous spacers compared to decompression or fusion for
lumbar stenosis: complications and repeat operations in the Medicare population. Spine (Phila Pa 1976) 2013; 38:865–872 23. Kabir SM, Gupta SR, Casey AT. Lumbar interspinous spacers: a systematic review of clinical and biomechanical evidence. Spine 2010; 35:E1499–E1506 24. Sobottke R, Schlüter-Brust K, Kaulhausen T, et al. Interspinous implants (X Stop, Wallis, Diam) for the treatment of LSS: is there a correlation between radiological parameters and clinical outcome? Eur Spine J 2009; 18:1494–1503 25. Korovessis P, Repantis T, Zacharatos S, Zafiropoulos A. Does Wallis implant reduce adjacent segment degeneration above lumbosacral instrumented fusion? Eur Spine J 2009; 18:830–840 26. Richter A, Schutz C, Hauck M, Halm H. Does an interspinous device (Coflex) improve the outcome of decompressive surgery in lumbar spinal stenosis? One-year follow up of a prospective case control study of 60 patients. Eur Spine J 2010; 19:283–289 27. Park SC, Yoon SH, Hong YP, Kim KJ, Chung SK, Kim HJ. Minimum 2-year follow-up result of degenerative spinal stenosis treated with interspinous U (Coflex). J Korean Neurosurg Soc 2009; 46:292–299
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Fig. 1—48-year-old man with interbody spacer migration. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show interbody fusion device (arrow) dislocated to left and posteriorly, compressing neural foramen and central canal. Radiopaque marker normally should be at least 2 mm anterior to posterior margin of vertebral body. Also present is interspinous distraction device. C, Sagittal CT image in bone window shows full extent of posterior migration of interbody device (arrow).
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Lumbar Spine Hardware
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Fig. 2—67-year-old man treated with Aspen device. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show Aspen device (Lanx). Two locking metal plates are located on sides of spinous process (arrow, B) and are connected by spikes (arrow, A) to spinous process. Bone graft material is present in radiolucent hole of plate. C, Coronal CT image in bone window shows plates on each side of spinous process and connecting spikes.
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Fig. 3—56-year-old man treated with extreme lateral interbody fusion (XLIF device, NuVasive). A and B, Anteroposterior radiograph (A) and axial CT image in bone algorithm (B) of lumbar spine show XLIF device at L3–L4. Implant has long, rectangular shape to maximize surface area.
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Fig. 4—50-year-old woman with back pain who as adolescent underwent surgical correction of idiopathic scoliosis. A and B, Frontal (A) and lateral (B) radiographs obtained at age 13, when thoracolumbar fusion with Harrington rods (compression on right and distraction on left) was performed after 6 months of bracing therapy failed. Increased lumbar lateral scoliosis and kyphosis are associated with foraminal stenosis at L4–L5 and L5–S1 (flat back syndrome). C, Frontal radiograph shows appearance after corrective surgery.
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Fig. 5—49-year-old man treated with low-profile anterior fusion. A and B, Lateral radiograph (A) and sagittal CT image in bone window (B) at L5–S1 show zero-profile system for anterior interbody fusion 18 months after initial surgery. Osteolysis (arrow) is present around anterior margins and S1 screw. Loosening of hardware and nonunion across interbody fusion are evident. Total disk replacement of L4-5 also is evident.
Fig. 6—70-year-old man treated with dynamic stabilization system. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show Dynesys (Zimmer) rod and screw system at L3–L4 with two titanium alloy pedicle screws connected by nonradiopaque cylindric polycarbonate urethane spacer through which polyester cord is strung. (Courtesy of Hunter J, University of California, Davis, Sacramento, CA)
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Lumbar Spine Hardware
Fig. 7—Drawing shows Graf dynamic stabilization. Nonelastic polyester ligaments (yellow) are looped around pedicle screws (red).
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Fig. 8—39-year-old woman treated with dynamic posterior stabilization. A and B, Sagittal (A) and coronal (B) CT images of lumbar spine show Stabilimax NZ (Applied Spine Technologies) dynamic posterior stabilization hardware. Parallel rods connect fixed titanium posterior pedicle screws. Rods have ball-and-socket joint surrounded by springs that allow limited motion.
Fig. 9—44-year-old woman treated with lumbar disk replacement. A and B, Lateral (A) and anteroposterior (B) radiographs of lumbar spine show ProDisc-L device (Synthes) at L4–L5. Device consists of radiolucent polyethylene inlay attached to inferior endplate. Keels (white arrow) embed metal endplates into bone, and spike (black arrow) on lateral aspect of each side of keel enhances stability.
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Fig. 10—52-year-old man treated with lumbar disk replacement. Sagittal CT myelogram of lumbar spine in bone algorithm shows Maverick (Medtronic) disk replacement at L5–S1. Keels embed metal endplates to bone. There is no polyethylene inlay, thus this is metal-on-metal design.
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Fig. 11—45-year-old woman treated with lumbar disk replacement. A–C, Lateral (A) and anteroposterior (B) radiographs and sagittal CT image in bone window (C) show Charité DePuy Spine disk replacement at L4–L5. In this unconstrained design, radiolucent polyethylene marked by wire (arrow, A) is held in place only by compressive forces. Three small stability spikes (arrow, B) are present along superior and inferior endplates.
A Fig. 12—66-year-old man treated with interspinous distraction. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show X-Stop (Medtronic) interspinous distraction device. Two parallel wings (arrow, A) are connected by titanium spacer that places patient in slight flexion. Wings prevent lateral migration of device.
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Fig. 13—55-year-old man with complications of interspinous distraction. Lateral radiograph of lumbar spine shows fracture (arrow) of L5 spinous process adjacent to X-Stop Medtronic device.
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Lumbar Spine Hardware Fig. 14—64-year-old man treated with interspinous distraction. A and B, Anteroposterior (A) and lateral (B) radiographs of lumbar spine show Wallis device (Zimmer) at L4–L5 level. Block of radiolucent peek identified with two metal pins (white arrow, B) is held in place with flat Dacron polyester band wrapped around spinous process. Two metal straps (black arrow, B) secure tape.
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B Fig. 15—Interspinous distraction. A, 68-year-old woman. Lateral radiograph of lumbar spine shows Coflex (Paradigm Spine) interspinous distraction device is U-shaped titanium implant with vertical wings crimped onto spinous process to hold it in place. B, 44-year-old man. Sagittal CT image of lumbar spine in bone window shows resorption of inferior spinous process (arrow) around site of crimping of Coflex device. This occurs in as many as 57% of cases.
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F O R YO U R I N F O R M AT I O N
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