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Imaging of the postoperative spine
Imaging of the Postoperative Spine William P. Sanders* and Eeric Truumees† With the widespread prevalence of back pain, and approximately 200,000 lumbar spine operations annually in the United States, imaging of the postoperative lumbar spine has become a major diagnostic issue. The many causes of the failed back surgery syndrome make the diagnosis of the etiology of the patient’s symptom complex very important to the treating physician. With the varied types of surgery, the increasing use of fusion hardware, and the myriad of complicating factors, the imaging strategy must be flexible enough to be comprehensive in the evaluation of these difficult patients. This article will address the normal and abnormal findings after lumbar spine surgery. Semin Ultrasound CT MRI 25:523-535 © 2004 Elsevier Inc. All rights reserved.
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ostoperative imaging of the lumbar spine is usually for one of two reasons. First, there is routine imaging to evaluate for progression of spinal fusion in patients with otherwise good clinical surgical outcome. The second common reason for postoperative imaging is for those patients who may have a complicating process after surgery or no improvement of their clinical symptoms. The modality that is used to evaluate both the routine and the nonroutine postoperative imaging depends on the type of surgery and the clinical information desired. With the progressively greater use of metallic instrumentation as part of the surgical procedure, which causes artifacts on computed tomography (CT) and magnetic resonance imaging (MRI), plain radiography and myelography have been assuming a greater role for the evaluation of these patients. A brief discussion of the operative procedures and their postoperative imaging needs will be presented, followed by examples of normal postoperative examinations, complicating processes, and a discussion of the modality of choice for the evaluation of these procedures.
Routine Assessment of Lumbar Surgery Simple laminectomy and/or discectomy is typically only imaged with intraoperative radiographs to assess the correct level of surgery before performing the surgery. If there is no fusion or instrumentation, no further imaging is required in *Division of Neuroradiology, William Beaumont Hospital, Royal Oak, and Henry Ford Hospital, Detroit, MI. †Gehring Biomechanics Laboratory, William Beaumont Hospital, Royal Oak, and Wayne State University, Detroit, MI. Address reprint requests to William P. Sanders, MD, Diagnostic Radiology, K-3, Henry Ford Hospital, Detroit, MI 48202; E-mail: Williams@ rad.hfh.edu
0887-2171/04/$-see front matter © 2004 Elsevier Inc. All rights reserved. doi:10.1053/j.sult.2004.09.007
the patients that have a good clinical outcome. Those patients with little or no relief or those with postoperative onset of new symptoms will be discussed in a later section. Routine imaging assessment of patients undergoing spinal fusion is performed on a regular basis to assess the degree of fusion and the stability of the fusion. The goals for this assessment include: 1. A guide for the timing of the elimination of restrictive bracing and the introduction of physical therapy as stable fusion progresses. 2. To evaluate for the possibility of pseudarthrosis. In North America today the majority of fusion procedures are performed with instrumentation, that is, the use of various screws, plates, and rods to effect early stability while the bone fusion matures to a stable osseous fusion. Initially, this spine instrumentation was used only for those patients thought to have marked lumbar instability requiring special consideration and included primarily trauma patients and in patients with tumors. In more recent years, instrumentation has now been added to the majority of lumbar fusions done for these above lesions, as well as for degenerative disease in an effort to increase the fusion rates and avoid the problems associated with postoperative bracing. Graft materials include iliac-crest autograft, local autograft from the decompression itself (cortical bone from the laminectomy), or cadaveric cancellous bone. Newer materials available at this time include physiologic stimulators of bone fusion such as bone morphogenetic proteins and bone marrow aspirates with osteocyte precursors. The relatively large number of various types of fusion and fusion stimulators raises the point that it is of great importance to achieve solid fusion, and also that there is difficulty in gaining this solid fusion in many patients. Solid fusion is particularly difficult to achieve in 523
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Figure 1 Dense corticated bone produces solid fusion between the transverse processes of L4 and L5 bilaterally (arrows).
patients who have had previous spine surgery and in those with certain medical comorbidities including those who smoke, those with diabetes, patients with vasculopathy, and those with fusions performed at certain levels of the spine, particularly the L5-S1 level. The most common spine fusion performed in the lumbar region is the posterior fusion, achieved by decortication of the transverse processes and placement of bone graft material and/or stimulators to cause consolidation and growth of bone mass between the transverse processes (Fig 1). This is typically performed with the
Figure 2 Bilateral pedicle screws and plates at L4 and L5 produce posterior fixation allowing osseous fusion to progress (arrows).
initial placement of pedicle screws with rods or plates to ensure initial stability while the bone fusion takes place (Fig 2). Interbody fusion is also being undertaken more frequently as the shortcomings of posterior-lateral fusion have been recognized. Particularly at L5-S1 or in the presence of significant spondylolisthesis, the interbody fusion is valuable as it restores foraminal height and offers a larger surface area for bone graft. Placement of this type of bone graft also has a favorable effect on compressive load as opposed to the lateral tension, seen in the posterolateral bone grafts. The imaging
Imaging of the postoperative spine
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Figure 3 Sagittal T1-weighted (A) and axial gradient echo (B) images with artifacts from stainless steel hardware rendering the study nondiagnostic, while titanium devices create less severe artifacts (C, sagittal; D, axial T1-weighted images).
assessment of all of these fusions typically relies on plain radiography. In the early stages, the radiographs are evaluated to assess resorption versus incorporation of the bone material. Flexion and extension radiographs are performed typically between 8 and 16 weeks if other signs of fusion are seen. The flexion and extension films are evaluated for intervertebral motion at the site of the fusion. Any change in the
degree of prior listhesis or scoliosis would suggest failure of fusion. Uncommonly the plates, rods, or screws may actually fracture because of the instability due to the excess strain placed on these metallic devices. In some cases, CT or MRI examination can be performed to better evaluate screw/rod placement and/or the possibility of good osseous fusion especially of the facet joints. There are limitations of CT and
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Figure 4 Sagittal T2-weighted image (A) in an asymptomatic patient 2 weeks post laminectomy show marked soft tissue swelling. Axial T1 (B) shows intermediate to low signal change in right lateral recess foramen. Axial contrast-enhanced T1weighted image (C) shows enhancing tissue in the right-sided operative bed, along with enhancement of the right S1 root posteriorly in the thecal sac (arrow), compared with the nonenhanced scan (B).
MRI due to artifacts produced by the metallic instrumentation, especially those made from stainless steel; whereas newer titanium constructs are less of a problem (Fig 3). There are also reconstruction algorithms for CT, which help to minimize streak artifact. Radionuclide scintigraphy may show continued abnormal uptake of tracer material at the site of pseudarthrosis or loosening of pedicle screws.
Imaging of Postoperative Complications Following spine surgery, patients may have little to no pain relief, may develop new problems, or their initial problem may recur at a time remote from the initial surgery. These patients often require more intensive imaging than simple radiographs to
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Figure 5 Three months post laminectomy, asymptomatic fluid is seen in the posterior disk on sagittal T2-weighted image (A), with focal enhancement at the curettage site (arrows in B and C).
evaluate the cause of their problems and to determine the best therapeutic options following the initial surgery. The term “failed back surgery syndrome” (FBBS) is a much generalized term that is often used in the literature to describe those patients with either recurrent problems or a failure to correct the problem with the initial surgical procedure.1,2 However, FBSS does not in any way describe the etiology for these complications. The complicating processes in the initial postoperative period include infection, hematoma, misplaced instrumentation, failure
to remove sufficient herniated disk material, and dural tear with cerebrospinal fluid (CSF) leak.2 New or recurrent symptoms occurring months or years after the initial surgery are most commonly due to recurrent degenerated/herniated disk, epidural fibrosis (scar), and degeneration or possible herniation of disks at levels above or below the previous fusion. A combination of routine radiographs with flexion and extension views, CT, myelography, and MRI are all modalities used for various reasons in this heterogeneous patient population. The modality required
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for optimal evaluation depends on the type of initial surgery as well as the type of symptoms the patient is now experiencing.
Early Postoperative Complications To evaluate for possible postoperative complications in the early phase, one needs to understand the typical appearance of the normal postoperative spine after successful back surgery. Again, one may use CT, CT myelography, or MRI for this evaluation. In current times, after simple discectomy, MRI is the most common modality used for this postoperative evaluation. The typical appearance of an uncomplicated discectomy patient in the early postoperative phase can be somewhat troublesome even in patients with very good clinical outcomes.3,4 In the first 30 to 60 days, there is often marked soft tissue swelling posteriorly, with apparent significant compression of the thecal sac.4 When one injects gadolinium, there is often marked abnormal appearing enhancement in the epidural soft tissues as well as the intraspinal nerve roots of the cauda equina (Fig 4). Following resolution of the initial postoperative swelling of the posterior soft tissues, the spinal canal returns to normal size. There is often a peripheral rim of enhancement of the operated parent disk.4 T2-weighted images may also show fluid within the disk although this is primarily at the site of the curettage and does not involve the entire disk. This area may also enhance in a linear fashion seen best on the sagittal and axial images (Fig 5). The presence of enhancement of the adjacent nerve roots may indicate irritation of those roots,2-4 although it can be seen in up to 20% of people with no recurrent clinical symptoms.4 In addition, edematous changes in the endplates at the operated level may also occur in the early postoperative period in patients with good clinical results (Fig 6). The presence of enhancement in the disk, edematous changes in the endplates, as well as the edematous changes of the soft tissues of the wound may make differentiation of normal postoperative changes difficult to separate from findings of early discitis or wound infection. In these cases, correlation with the clinical symptoms, white blood cell count, and possibly needle biopsy of the suspicious tissues is necessary to confirm or refute the existence of postoperative infection.5,6 Typical bacterial discitis shows intense enhancement of the disk and increased water content of the disks and endplates, usually to a more significant degree than the typical postoperative changes in the asymptomatic patient (Fig 7A and B). The presence of a paraspinal fluid collection, epidural fluid collection anteriorly adjacent to the operated disk (Fig 8), or enhancement of the psoas muscles (Fig 7C) is usually fairly specific for infection; however, these findings usually occur later in the course of the infection. The goal of postoperative imaging in complicated patients is to detect the infection at an early stage, before development of gross destructive changes of the disks and/or endplates.6
Figure 6 Sagittal T1-weighted image shows edema in the superior endplate of L5 after L4-5 laminectomy (same patient as in Fig 4).
Postoperative Fluid Collections Postoperative fluid collections in the operative bed may be either bland seromas, CSF collections from dural leaks, or postoperative abscess. The presence of a significant enhancing rim in these tissues on MRI with contrast would suggest an infectious component. These are often easily aspirated with either CT or ultrasound guidance. Even bland fluid collections may cause nerve root or thecal sac compression (Fig 9). In a patient who has recent spine surgery, the presence of a postural headache along with a fluid-filled area in the operative bed would strongly suggest a CSF leak (Fig 10). There is an importance to differentiating a CSF leak from other fluid collections, as aspiration or opening the wound to drain this collection may result in meningitis. Although usually not necessary, myelography or MR myelography7,8 could be performed with imaging in a supine position to identify the exact location of the leak. However, MRI may also show the location of the leak if the images are of diagnostic quality. Typically, the surgeon will expose the previously operated area and find the leak at the time of reoperation.
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Figure 7 T1-weighted (A) and T2-weighted (B) sagittal images show the edematous endplates at L4-5 1 week after surgery, with new pain, fever, and elevated white cell count. Axial enhanced T1-weighted image (C) shows enhancement in the left epidural space and psoas muscle, the latter fairly specific for infection.
Postoperative Hematoma Patients that present with worsening symptoms in the immediate postoperative period may have an intraspinal hematoma. These often cause severe compromise of neurologic function, causing an immediate cauda equina syndrome. These patients are best imaged with MRI if that is available, as this will show the exact nature of the symp-
tom causing process, as well as the location and extent of the hematoma.3,9,10 If the patient cannot undergo MR imaging, myelography, often from a cervical puncture, may be necessary. This will show the location of the lesion although hematoma versus postoperative swelling may be confused in some cases. Routine T1 and T2 sequences are usually sufficient to make this diagnosis, although a gradient echo sequence that is sensitive to the magnetic sus-
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Figure 8 T2-weighted sagittal image shows an epidural fluid collection at the S1 level (arrow); pre- (B) and postcontrast (C) images show enhancing phlegmon surrounding this liquefied epidural abscess.
ceptibility effects of iron can be conclusive. The presence of instrumentation may degrade images on MR scanning; however, a significantly sized hematoma will likely be accurately identified despite the artifacts created from the implanted metal. Plain CT imaging for intraspinal hematoma is of limited benefit due to the similar densities comparing muscle and hematoma. Myelography will demon-
strate the location of the lesion, but not the nature of the material. Patients may also present with hematoma in the operative site in the paraspinal soft tissues. Again, MR imaging is the preferred modality for evaluating this complication. CT would likely show a fluid collection but usually cannot differentiate hematoma from other forms of fluid.
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Figure 9 Axial T1-weighted, postcontrast images show the postoperative fluid collection compressing the thecal sac (arrow, A), and communication (arrows, B) of this intraspinal collection with the larger fluid component in the subcutaneous tissues.
Complications from Instrumentation Other causes of the acute onset of worsening neurologic deficit or pain could be due to misplaced instrumentation. Pedicle screws may be placed too medially or inferiorly, with
violation of the wall of the pedicle with nerve root damage. If the screw is misplaced, it may irritate or damage the adjacent nerve roots either in the lateral recess or the neural foramina (Fig 11), depending on the location of the misplaced screw. In uncommon situations, the screw placed may be too long and very rarely could injure significant vascular structures
Figure 10 Axial T2-weighted (A) and enhanced T1-weighted (B) images 1 week after surgery in a patient with a postural headache immediately following laminectomy show a significant fluid collection; reoperation revealed dural tear. Note the enhancing muscles in the operative bed; there was no clinical evidence of infection.
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Figure 11 This patient had an immediate left L4 radiculopathy following this extensive fusion surgery. One screw traverses the left L4 foramen. Axial CT image (A). Coronal reconstruction image (B); see screw medial to pedicle (arrow).
such as the iliac veins or arteries. This latter event usually presents in the operating room with significant bleeding. Sacral screws, if too long, may also irritate the lumbar nerve roots, especially L-5, where it is draped across the anterior surface of the sacrum (Fig 12). This would present with acute L-5 radiculopathy and/or weakness in the immediate perioperative period.
Imaging of Delayed Complications Those patients who have relatively good relief of their symptoms after their initial surgery may present at a later date with recurrent pain, new pain, or new onset of neurologic dysfunction. The causes for this are typically due to recurrent
Figure 12 Axial CT post fusion in a patient with L5 radiculopathy immediately after surgery. Note the proximity of the screw tip to the iliac vein (arrow).
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Figure 13 Axial T1-weighted pre- (A) and postcontrast (B) images show enhancing scar limited to surrounding the left S1 root in the lateral recess. (C and D) Another patient with circumferential enhancing scar 1 year after surgery.
disk herniation, epidural fibrosis, and scar formation or advanced degeneration of adjacent segments above or below the previous fusion site.2,3,10 Discerning the cause for this recurrent symptomatology is critical in planning further therapeutic endeavors.
Epidural Fibrosis One of the more common causes of FBSS is the formation of epidural fibrosis in the operative bed. This occurs most commonly in patients with complicated discectomy, although it can occur in any patient after spinal surgery, including simple laminectomy.2,11 Before MRI imaging, this was typically evaluated with myelography and CT, to evaluate for thickening of the nerve roots and adhesion of the nerve roots to the dural sac (“bald sac sign”) or thickened enhanced roots on MRI.12 However, this is only useful for patients with arachnoiditis along with epidural fi-
brosis. When the problem is limited to the epidural space, myelography may simply show nonfilling of nerve root sleeves, which could be due to recurrent disk or epidural fibrosis. MRI pre- and postcontrast is now the modality of choice for evaluating these patients. The typical appearance of epidural fibrosis is that of nonspecific enhancing soft tissue surrounding the nerve roots and thecal sac at the operated level (Fig 13). While this is a normal finding in the early postoperative period,4 this is considered to be pathologic after approximately 6 months after surgery.2,3,11 Following contrast administration, there is usually significant abnormal enhancement in the epidural tissue. Although there is some controversy regarding the correlation between the presence of enhancing scar and the patient’s symptomatology,13 a prospective multicenter study showed that there was good correlation with the recurrence of symptoms when there was diffuse scarring, but little correlation in those cases with a small amount of
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Figure 14 Sagittal T2-weighted image (A) and T1-weighted image (B) without contrast demonstrate a recurrent disk protrusion (confirmed at reoperation); peripheral enhancement (arrow, D) is demonstrated on the axial T1-weighted pre- (C) and postcontrast enhancement (D) studies.
focal scar.14 The presence of nerve root enhancement is poorly related to scar and may be seen normally in 30% of asymptomatic patients early after surgery, and in up to 10% of these patients at a later date.5 Recurrent disk herniation typically shows mass effect with displacement of the nerve roots and nonenhancing disk material in the spinal canal or lateral recess. However, after surgery, there is often peripheral enhancement of the
disk, which could be misconstrued for scar tissue (Fig 14). Careful evaluation of all of the images typically will be able to distinguish the recurrent disk versus scar tissue.15,16 One of the differentiating features of disk versus scar is that the disk material tends to displace and compress nerve roots into a more posterior location, whereas scar tissue will either simply surround the nerve or actually “pull” the root in an anterior direction.
Imaging of the postoperative spine
535 devices usually does not hinder the evaluation by either CT or MRI imaging.
Summary Effective imaging of the lumbar spine after surgery requires a thorough knowledge of the type of surgery performed and the patients’ symptoms. MRI is the most common modality used for the evaluation of postoperative complications; however, these difficult patients may require numerous modalities to arrive at the correct diagnosis.
References
Figure 15 Sagittal T2-weighted image 1 year after interbody fusion shows stenosis due to new protrusion of severely degenerated disk as well as facet hypertrophy (arrow).
One difficulty with the evaluation for recurrent symptoms is if the symptoms occur within months rather than years after the initial surgery. In these patients, there may be epidural enhancement, which is a normal situation and may persist for as long as 3 months. If the patient presents with new symptoms several months after surgery, this imaging finding may be due to normal postoperative findings and not true scar formation. Again, the recurrent herniation is usually well identified on postoperative MRI scanning. It is well known that after spinal fusion, because of added motion requirements at the adjacent disk levels, there is a tendency for patients to develop an accelerated degenerative process in the nonfused segments above or below the previously fused level.2 These findings include disk and endplate degeneration, disk herniation, facet degeneration, as well as ligamentous thickening (Fig 15). Again, these findings may be seen with CT, myelography, or MRI. As this is usually away from the level of instrumentation, the presence of these
1. Burton CV, Kirkaldy-Willis WH, Yong-Hing K, Heithoff KB: Causes of failure of surgery on the lumbar spine. Clin Orthop 157:191-199, 1981 2. Fritsch EW, Heisel J, Rupp S: The failed back surgery syndrome: Reasons, intraoperative findings, and long-term results: A report of 182 operative treatments. Spine 21:626-633, 1996 3. Van Goethem JWM, Parizel PM, Jinkins JR: Review article: MRI of the postoperative spine. Neuroradiology 44:723-739, 2002 4. Van Goethem JWM, Van de Kelft E, Biltjes IGGM, et al: MRI after successful lumbar discectomy. Neuroradiology 38:S90-S96, 1996 5. Yukawa Y, Kato F, Kajino G, Nakamura S: Serial gadolinium-enhanced MR imaging after lumbar disc resection: Observation of the affected root. J Spinal Disord 10:5;404-409, 1997 6. Kopecky KK, Gilmor RL, Scott JA, Edwards MK: Pitfalls of computed tomography in diagnosis of discitis. Neuroradiology 27:57-66, 1985 7. Grane P, Josephsson A, Seferlis A, Tullberg T: Septic and aseptic postoperative discitis in the lumbar spine: Evaluation by MR imaging. Acta Radiol 39:108-115, 1998 8. Phillips CD, Kaptain GJ, Razack N: Depiction of a postoperative pseudomeningocele with digital subtraction myelography. Am J Neuroradiol 23:337-338, 2002 9. Shafaie FF, Bundschuh C, Jinkins JR: The posttherapeutic lumbosacral spine, in Jinkins JR (ed): Posttherapeutic Neurodiagnostic Imaging. Philadelphia, Lippincott-Raven, pp 223-243, 1997. 10. Djukic S, Genant HK, Helms CA, Holt RG: Magnetic resonance imaging of the postoperative lumbar spine. Radiol Clin North Am 28:2;341360, 1990 11. Ross JS, Obuchowski N, Zepp R: The postoperative lumbar spine: Evaluation of epidural scar over a 1-year period. AJNR Am J Neuroradiol 19:183-186, 1998 12. Ross JS, Masaryk TJ, Modic MT, et al: MR imaging of lumbar arachnoiditis. AJR 149:1025-1032, 1987 13. Coskun E, Suzer T, Topuz O, et al: Relationships between epidural fibrosis, pain, disability, and psychological factors after lumbar disc surgery. Eur Spine J 9:218-223, 2000 14. Ross JS, Robertson JT, Frederickson RCA, et al: Association between peridural scar and recurrent radicular pain after lumbar discectomy: Magnetic resonance evaluation. Neurosurgery 38:4;855-860, 1996 15. Bundschuh CV, Stein L, Slusser JH, et al: Distinguishing between scar and recurrent herniated disk in postoperative patients: Value of contrast-enhanced CT and MR imaging. AJNR Am J Neuroradiol 11:949958, 1990 16. Haughton V, Schreibman K, De Smet A: Contrast between scar and recurrent herniated disk on contrast-enhanced MR images. AJNR Am J Neuroradiol 23:1652-1656, 2002
P
ostoperative imaging of the lumbar spine is usually for one of two reasons. First, there is routine imaging to evaluate for progression of spinal fusion in patients with otherwise good clinical surgical outcome. The second common reason for postoperative imaging is for those patients who may have a complicating process after surgery or no improvement of their clinical symptoms. The modality that is used to evaluate both the routine and the nonroutine postoperative imaging depends on the type of surgery and the clinical information desired. With the progressively greater use of metallic instrumentation as part of the surgical procedure, which causes artifacts on computed tomography (CT) and magnetic resonance imaging (MRI), plain radiography and myelography have been assuming a greater role for the evaluation of these patients. A brief discussion of the operative procedures and their postoperative imaging needs will be presented, followed by examples of normal postoperative examinations, complicating processes, and a discussion of the modality of choice for the evaluation of these procedures.
Routine Assessment of Lumbar Surgery Simple laminectomy and/or discectomy is typically only imaged with intraoperative radiographs to assess the correct level of surgery before performing the surgery. If there is no fusion or instrumentation, no further imaging is required in *Division of Neuroradiology, William Beaumont Hospital, Royal Oak, and Henry Ford Hospital, Detroit, MI. †Gehring Biomechanics Laboratory, William Beaumont Hospital, Royal Oak, and Wayne State University, Detroit, MI. Address reprint requests to William P. Sanders, MD, Diagnostic Radiology, K-3, Henry Ford Hospital, Detroit, MI 48202; E-mail: Williams@ rad.hfh.edu
0887-2171/04/$-see front matter © 2004 Elsevier Inc. All rights reserved. doi:10.1053/j.sult.2004.09.007
the patients that have a good clinical outcome. Those patients with little or no relief or those with postoperative onset of new symptoms will be discussed in a later section. Routine imaging assessment of patients undergoing spinal fusion is performed on a regular basis to assess the degree of fusion and the stability of the fusion. The goals for this assessment include: 1. A guide for the timing of the elimination of restrictive bracing and the introduction of physical therapy as stable fusion progresses. 2. To evaluate for the possibility of pseudarthrosis. In North America today the majority of fusion procedures are performed with instrumentation, that is, the use of various screws, plates, and rods to effect early stability while the bone fusion matures to a stable osseous fusion. Initially, this spine instrumentation was used only for those patients thought to have marked lumbar instability requiring special consideration and included primarily trauma patients and in patients with tumors. In more recent years, instrumentation has now been added to the majority of lumbar fusions done for these above lesions, as well as for degenerative disease in an effort to increase the fusion rates and avoid the problems associated with postoperative bracing. Graft materials include iliac-crest autograft, local autograft from the decompression itself (cortical bone from the laminectomy), or cadaveric cancellous bone. Newer materials available at this time include physiologic stimulators of bone fusion such as bone morphogenetic proteins and bone marrow aspirates with osteocyte precursors. The relatively large number of various types of fusion and fusion stimulators raises the point that it is of great importance to achieve solid fusion, and also that there is difficulty in gaining this solid fusion in many patients. Solid fusion is particularly difficult to achieve in 523
524
W.P. Sanders and E. Truumees
Figure 1 Dense corticated bone produces solid fusion between the transverse processes of L4 and L5 bilaterally (arrows).
patients who have had previous spine surgery and in those with certain medical comorbidities including those who smoke, those with diabetes, patients with vasculopathy, and those with fusions performed at certain levels of the spine, particularly the L5-S1 level. The most common spine fusion performed in the lumbar region is the posterior fusion, achieved by decortication of the transverse processes and placement of bone graft material and/or stimulators to cause consolidation and growth of bone mass between the transverse processes (Fig 1). This is typically performed with the
Figure 2 Bilateral pedicle screws and plates at L4 and L5 produce posterior fixation allowing osseous fusion to progress (arrows).
initial placement of pedicle screws with rods or plates to ensure initial stability while the bone fusion takes place (Fig 2). Interbody fusion is also being undertaken more frequently as the shortcomings of posterior-lateral fusion have been recognized. Particularly at L5-S1 or in the presence of significant spondylolisthesis, the interbody fusion is valuable as it restores foraminal height and offers a larger surface area for bone graft. Placement of this type of bone graft also has a favorable effect on compressive load as opposed to the lateral tension, seen in the posterolateral bone grafts. The imaging
Imaging of the postoperative spine
525
Figure 3 Sagittal T1-weighted (A) and axial gradient echo (B) images with artifacts from stainless steel hardware rendering the study nondiagnostic, while titanium devices create less severe artifacts (C, sagittal; D, axial T1-weighted images).
assessment of all of these fusions typically relies on plain radiography. In the early stages, the radiographs are evaluated to assess resorption versus incorporation of the bone material. Flexion and extension radiographs are performed typically between 8 and 16 weeks if other signs of fusion are seen. The flexion and extension films are evaluated for intervertebral motion at the site of the fusion. Any change in the
degree of prior listhesis or scoliosis would suggest failure of fusion. Uncommonly the plates, rods, or screws may actually fracture because of the instability due to the excess strain placed on these metallic devices. In some cases, CT or MRI examination can be performed to better evaluate screw/rod placement and/or the possibility of good osseous fusion especially of the facet joints. There are limitations of CT and
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Figure 4 Sagittal T2-weighted image (A) in an asymptomatic patient 2 weeks post laminectomy show marked soft tissue swelling. Axial T1 (B) shows intermediate to low signal change in right lateral recess foramen. Axial contrast-enhanced T1weighted image (C) shows enhancing tissue in the right-sided operative bed, along with enhancement of the right S1 root posteriorly in the thecal sac (arrow), compared with the nonenhanced scan (B).
MRI due to artifacts produced by the metallic instrumentation, especially those made from stainless steel; whereas newer titanium constructs are less of a problem (Fig 3). There are also reconstruction algorithms for CT, which help to minimize streak artifact. Radionuclide scintigraphy may show continued abnormal uptake of tracer material at the site of pseudarthrosis or loosening of pedicle screws.
Imaging of Postoperative Complications Following spine surgery, patients may have little to no pain relief, may develop new problems, or their initial problem may recur at a time remote from the initial surgery. These patients often require more intensive imaging than simple radiographs to
Imaging of the postoperative spine
527
Figure 5 Three months post laminectomy, asymptomatic fluid is seen in the posterior disk on sagittal T2-weighted image (A), with focal enhancement at the curettage site (arrows in B and C).
evaluate the cause of their problems and to determine the best therapeutic options following the initial surgery. The term “failed back surgery syndrome” (FBBS) is a much generalized term that is often used in the literature to describe those patients with either recurrent problems or a failure to correct the problem with the initial surgical procedure.1,2 However, FBSS does not in any way describe the etiology for these complications. The complicating processes in the initial postoperative period include infection, hematoma, misplaced instrumentation, failure
to remove sufficient herniated disk material, and dural tear with cerebrospinal fluid (CSF) leak.2 New or recurrent symptoms occurring months or years after the initial surgery are most commonly due to recurrent degenerated/herniated disk, epidural fibrosis (scar), and degeneration or possible herniation of disks at levels above or below the previous fusion. A combination of routine radiographs with flexion and extension views, CT, myelography, and MRI are all modalities used for various reasons in this heterogeneous patient population. The modality required
528
W.P. Sanders and E. Truumees
for optimal evaluation depends on the type of initial surgery as well as the type of symptoms the patient is now experiencing.
Early Postoperative Complications To evaluate for possible postoperative complications in the early phase, one needs to understand the typical appearance of the normal postoperative spine after successful back surgery. Again, one may use CT, CT myelography, or MRI for this evaluation. In current times, after simple discectomy, MRI is the most common modality used for this postoperative evaluation. The typical appearance of an uncomplicated discectomy patient in the early postoperative phase can be somewhat troublesome even in patients with very good clinical outcomes.3,4 In the first 30 to 60 days, there is often marked soft tissue swelling posteriorly, with apparent significant compression of the thecal sac.4 When one injects gadolinium, there is often marked abnormal appearing enhancement in the epidural soft tissues as well as the intraspinal nerve roots of the cauda equina (Fig 4). Following resolution of the initial postoperative swelling of the posterior soft tissues, the spinal canal returns to normal size. There is often a peripheral rim of enhancement of the operated parent disk.4 T2-weighted images may also show fluid within the disk although this is primarily at the site of the curettage and does not involve the entire disk. This area may also enhance in a linear fashion seen best on the sagittal and axial images (Fig 5). The presence of enhancement of the adjacent nerve roots may indicate irritation of those roots,2-4 although it can be seen in up to 20% of people with no recurrent clinical symptoms.4 In addition, edematous changes in the endplates at the operated level may also occur in the early postoperative period in patients with good clinical results (Fig 6). The presence of enhancement in the disk, edematous changes in the endplates, as well as the edematous changes of the soft tissues of the wound may make differentiation of normal postoperative changes difficult to separate from findings of early discitis or wound infection. In these cases, correlation with the clinical symptoms, white blood cell count, and possibly needle biopsy of the suspicious tissues is necessary to confirm or refute the existence of postoperative infection.5,6 Typical bacterial discitis shows intense enhancement of the disk and increased water content of the disks and endplates, usually to a more significant degree than the typical postoperative changes in the asymptomatic patient (Fig 7A and B). The presence of a paraspinal fluid collection, epidural fluid collection anteriorly adjacent to the operated disk (Fig 8), or enhancement of the psoas muscles (Fig 7C) is usually fairly specific for infection; however, these findings usually occur later in the course of the infection. The goal of postoperative imaging in complicated patients is to detect the infection at an early stage, before development of gross destructive changes of the disks and/or endplates.6
Figure 6 Sagittal T1-weighted image shows edema in the superior endplate of L5 after L4-5 laminectomy (same patient as in Fig 4).
Postoperative Fluid Collections Postoperative fluid collections in the operative bed may be either bland seromas, CSF collections from dural leaks, or postoperative abscess. The presence of a significant enhancing rim in these tissues on MRI with contrast would suggest an infectious component. These are often easily aspirated with either CT or ultrasound guidance. Even bland fluid collections may cause nerve root or thecal sac compression (Fig 9). In a patient who has recent spine surgery, the presence of a postural headache along with a fluid-filled area in the operative bed would strongly suggest a CSF leak (Fig 10). There is an importance to differentiating a CSF leak from other fluid collections, as aspiration or opening the wound to drain this collection may result in meningitis. Although usually not necessary, myelography or MR myelography7,8 could be performed with imaging in a supine position to identify the exact location of the leak. However, MRI may also show the location of the leak if the images are of diagnostic quality. Typically, the surgeon will expose the previously operated area and find the leak at the time of reoperation.
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Figure 7 T1-weighted (A) and T2-weighted (B) sagittal images show the edematous endplates at L4-5 1 week after surgery, with new pain, fever, and elevated white cell count. Axial enhanced T1-weighted image (C) shows enhancement in the left epidural space and psoas muscle, the latter fairly specific for infection.
Postoperative Hematoma Patients that present with worsening symptoms in the immediate postoperative period may have an intraspinal hematoma. These often cause severe compromise of neurologic function, causing an immediate cauda equina syndrome. These patients are best imaged with MRI if that is available, as this will show the exact nature of the symp-
tom causing process, as well as the location and extent of the hematoma.3,9,10 If the patient cannot undergo MR imaging, myelography, often from a cervical puncture, may be necessary. This will show the location of the lesion although hematoma versus postoperative swelling may be confused in some cases. Routine T1 and T2 sequences are usually sufficient to make this diagnosis, although a gradient echo sequence that is sensitive to the magnetic sus-
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Figure 8 T2-weighted sagittal image shows an epidural fluid collection at the S1 level (arrow); pre- (B) and postcontrast (C) images show enhancing phlegmon surrounding this liquefied epidural abscess.
ceptibility effects of iron can be conclusive. The presence of instrumentation may degrade images on MR scanning; however, a significantly sized hematoma will likely be accurately identified despite the artifacts created from the implanted metal. Plain CT imaging for intraspinal hematoma is of limited benefit due to the similar densities comparing muscle and hematoma. Myelography will demon-
strate the location of the lesion, but not the nature of the material. Patients may also present with hematoma in the operative site in the paraspinal soft tissues. Again, MR imaging is the preferred modality for evaluating this complication. CT would likely show a fluid collection but usually cannot differentiate hematoma from other forms of fluid.
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Figure 9 Axial T1-weighted, postcontrast images show the postoperative fluid collection compressing the thecal sac (arrow, A), and communication (arrows, B) of this intraspinal collection with the larger fluid component in the subcutaneous tissues.
Complications from Instrumentation Other causes of the acute onset of worsening neurologic deficit or pain could be due to misplaced instrumentation. Pedicle screws may be placed too medially or inferiorly, with
violation of the wall of the pedicle with nerve root damage. If the screw is misplaced, it may irritate or damage the adjacent nerve roots either in the lateral recess or the neural foramina (Fig 11), depending on the location of the misplaced screw. In uncommon situations, the screw placed may be too long and very rarely could injure significant vascular structures
Figure 10 Axial T2-weighted (A) and enhanced T1-weighted (B) images 1 week after surgery in a patient with a postural headache immediately following laminectomy show a significant fluid collection; reoperation revealed dural tear. Note the enhancing muscles in the operative bed; there was no clinical evidence of infection.
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Figure 11 This patient had an immediate left L4 radiculopathy following this extensive fusion surgery. One screw traverses the left L4 foramen. Axial CT image (A). Coronal reconstruction image (B); see screw medial to pedicle (arrow).
such as the iliac veins or arteries. This latter event usually presents in the operating room with significant bleeding. Sacral screws, if too long, may also irritate the lumbar nerve roots, especially L-5, where it is draped across the anterior surface of the sacrum (Fig 12). This would present with acute L-5 radiculopathy and/or weakness in the immediate perioperative period.
Imaging of Delayed Complications Those patients who have relatively good relief of their symptoms after their initial surgery may present at a later date with recurrent pain, new pain, or new onset of neurologic dysfunction. The causes for this are typically due to recurrent
Figure 12 Axial CT post fusion in a patient with L5 radiculopathy immediately after surgery. Note the proximity of the screw tip to the iliac vein (arrow).
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Figure 13 Axial T1-weighted pre- (A) and postcontrast (B) images show enhancing scar limited to surrounding the left S1 root in the lateral recess. (C and D) Another patient with circumferential enhancing scar 1 year after surgery.
disk herniation, epidural fibrosis, and scar formation or advanced degeneration of adjacent segments above or below the previous fusion site.2,3,10 Discerning the cause for this recurrent symptomatology is critical in planning further therapeutic endeavors.
Epidural Fibrosis One of the more common causes of FBSS is the formation of epidural fibrosis in the operative bed. This occurs most commonly in patients with complicated discectomy, although it can occur in any patient after spinal surgery, including simple laminectomy.2,11 Before MRI imaging, this was typically evaluated with myelography and CT, to evaluate for thickening of the nerve roots and adhesion of the nerve roots to the dural sac (“bald sac sign”) or thickened enhanced roots on MRI.12 However, this is only useful for patients with arachnoiditis along with epidural fi-
brosis. When the problem is limited to the epidural space, myelography may simply show nonfilling of nerve root sleeves, which could be due to recurrent disk or epidural fibrosis. MRI pre- and postcontrast is now the modality of choice for evaluating these patients. The typical appearance of epidural fibrosis is that of nonspecific enhancing soft tissue surrounding the nerve roots and thecal sac at the operated level (Fig 13). While this is a normal finding in the early postoperative period,4 this is considered to be pathologic after approximately 6 months after surgery.2,3,11 Following contrast administration, there is usually significant abnormal enhancement in the epidural tissue. Although there is some controversy regarding the correlation between the presence of enhancing scar and the patient’s symptomatology,13 a prospective multicenter study showed that there was good correlation with the recurrence of symptoms when there was diffuse scarring, but little correlation in those cases with a small amount of
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Figure 14 Sagittal T2-weighted image (A) and T1-weighted image (B) without contrast demonstrate a recurrent disk protrusion (confirmed at reoperation); peripheral enhancement (arrow, D) is demonstrated on the axial T1-weighted pre- (C) and postcontrast enhancement (D) studies.
focal scar.14 The presence of nerve root enhancement is poorly related to scar and may be seen normally in 30% of asymptomatic patients early after surgery, and in up to 10% of these patients at a later date.5 Recurrent disk herniation typically shows mass effect with displacement of the nerve roots and nonenhancing disk material in the spinal canal or lateral recess. However, after surgery, there is often peripheral enhancement of the
disk, which could be misconstrued for scar tissue (Fig 14). Careful evaluation of all of the images typically will be able to distinguish the recurrent disk versus scar tissue.15,16 One of the differentiating features of disk versus scar is that the disk material tends to displace and compress nerve roots into a more posterior location, whereas scar tissue will either simply surround the nerve or actually “pull” the root in an anterior direction.
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Summary Effective imaging of the lumbar spine after surgery requires a thorough knowledge of the type of surgery performed and the patients’ symptoms. MRI is the most common modality used for the evaluation of postoperative complications; however, these difficult patients may require numerous modalities to arrive at the correct diagnosis.
References
Figure 15 Sagittal T2-weighted image 1 year after interbody fusion shows stenosis due to new protrusion of severely degenerated disk as well as facet hypertrophy (arrow).
One difficulty with the evaluation for recurrent symptoms is if the symptoms occur within months rather than years after the initial surgery. In these patients, there may be epidural enhancement, which is a normal situation and may persist for as long as 3 months. If the patient presents with new symptoms several months after surgery, this imaging finding may be due to normal postoperative findings and not true scar formation. Again, the recurrent herniation is usually well identified on postoperative MRI scanning. It is well known that after spinal fusion, because of added motion requirements at the adjacent disk levels, there is a tendency for patients to develop an accelerated degenerative process in the nonfused segments above or below the previously fused level.2 These findings include disk and endplate degeneration, disk herniation, facet degeneration, as well as ligamentous thickening (Fig 15). Again, these findings may be seen with CT, myelography, or MRI. As this is usually away from the level of instrumentation, the presence of these
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