The Spine Journal
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(2014)
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Clinical Study
Imaging characteristics of ‘‘dynamic’’ versus ‘‘static’’ spondylolisthesis: analysis using magnetic resonance imaging and flexion/extension films Jesse L. Even, MD*, Antonia F. Chen, MD, MBA, Joon Y. Lee, MD Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, 3471 Fifth Ave, Pittsburgh, PA 15213, USA Received 11 October 2012; revised 11 November 2013; accepted 26 November 2013
Abstract
BACKGROUND CONTEXT: Traditionally, the ‘‘dynamic’’ and ‘‘static’’ types of spondylolisthesis have been lumped into a single group in the literature. The goal of this study was to define the radiographic characteristics of ‘‘dynamic’’ and ‘‘static’’ spondylolisthesis with the use of magnetic resonance imaging (MRI) and flexion/extension radiographs. PURPOSE: Describe the characteristic findings present on MRI and flexion/extension radiographs that are associated with dynamic versus static spondylolisthesis. STUDY DESIGN: Retrospective radiographic/imaging study. METHODS: From 2009 to 2011, patients who underwent elective primary posterior spinal fusion for the diagnosis of spondylolisthesis had their plain films assessed for the degree of spondylolisthesis and were designated ‘‘dynamic’’ or ‘‘static,’’ as defined by historical measures. Axial and sagittal T2 MRIs were evaluated for associated facet fluid (FF), facet cysts, interspinous fluid (ISF), and facet hypertrophy. These finding were then statistically evaluated for associations between dynamic and static spondylolisthesis on flexion/extension radiographs and characteristic MRI findings. RESULTS: Ninety patients were included in the study with 114 levels examined for spondylolisthesis. Patients with greater than 3 mm of instability on flexion/extension films were more likely to have FF (p5.018) and ISF (p!.001). Of the patients who had a greater than 3 mm of instability, 39.5% did not demonstrate spondylolisthesis on the sagittal MRI reconstruction. If ISF was present on MRI, there was a positive predictive value of 69.0% that there would be greater than 3 mm instability on flexion/extension films. Absence of FF on MRI had a positive predictive value of 75.6% for instability less than 3 mm on flexion/extension films. In the presence of ISF on MRI, the likelihood ratio of finding more than 3 mm of instability on flexion/extension films was 3.68. The presence of FF on MRI had a likelihood ratio of 1.43 for instability. A total of 36.8% of all spondylolisthesis reduced when supine on MRI. CONCLUSIONS: The presence of FF and/or ISF is associated with instability greater than 3 mm in flexion/extension radiographs. Ó 2014 Elsevier Inc. All rights reserved.
Keywords:
Dynamic; Static; Spondylolisthesis; MRI; Flexion/extension; Facet fluid; Interspinous fluid
Introduction Degenerative spondylolisthesis is one of the most common diagnoses prompting surgical intervention in the lumbar spine. Laminectomy may be performed as a stand-alone procedure or may be performed in conjunction with arthrodesis when instability in between the lumbar segments FDA device/drug status: Not applicable. Author disclosures: JLE: Nothing to disclose. AFC: Nothing to disclose. JYL: Nothing to disclose. * Corresponding author. Kaufmann Medical Building, Suite 1010, 3471 Fifth Ave., Suite 1010, Pittsburgh, PA 15213, USA. Tel.: (817) 845-7821. E-mail address: [email protected] (J.L. Even) 1529-9430/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.11.057
is suspected [1–4]. In the past, the determination of instability (and thus the decision to include arthrodesis) was evaluated using weight-bearing lateral flexion/extension radiographs with greater than 3 mm translation being defined as instability [5–8]. The 3-mm threshold was defined by Boden et al. in 1990 using 40 asymptomatic patients while measuring their olisthesis on flexion and extension radiographs. The mean translation in this asymptomatic population was 1.3 mm with no patient registering with more than 3 mm of translation. Based on this, Boden et al. concluded that greater than 3 mm of olisthesis was pathologic and represented the presence of dynamic instability [6]. However, with the advent of modern imaging techniques, namely magnetic resonance imaging (MRI),
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the use of dynamic radiographs (flexion/extension) has become underutilized in the clinical setting. In a recent article by Rhin et al., the presence of facet fluid (FF) on T2-weighted MRI had a positive predictive value (PPV) of 82% for instability on flexion/extension films [4]. Chaput et al. also examined the significance of FF on MRI and its association with instability [9]. They noted that O1.5 mm of FF on axial MRI imaging is associated with spondylolisthesis in the absence of measurable anterolisthesis on supine MRI. Moreover, the literature to date has not distinguished the difference between ‘‘dynamic’’ and ‘‘static’’ subtypes of spondylolisthesis. The ‘‘dynamic’’ type, as defined as spondylolisthesis that has greater than 3 mm of instability on flexion/extension films, may be a different clinical entity than the ‘‘static’’ sub- type, which does not show instability on flexion/extension films. To our knowledge, there has not been a comprehensive imaging analysis of dynamic and static subtypes of spondylolisthesis. Thus, our goal was to define imaging characteristics of dynamic and static spondylolisthesis using both MRI as well as flexion/extension plain radiographs. Materials and methods After obtaining Institutional Review Board approval, the clinical database was deidentified and collected for patients who underwent a primary laminectomy and arthrodesis between the years of 2009 to 2011 to determine study eligibility. Patients who had preoperative MRIs and weight-bearing flexion/extension radiographs were included in the study. Patients who had a previous history of lumbar surgery or with a diagnosis of trauma, infection, or tumor were excluded from the study. Subjects were also excluded if the plain films and MRI were taken more than 6 months apart. Those with lateral listhesis or scoliosis were also excluded from the study. A total of 90 patients met the criteria for the study with 114 levels being evaluated. We defined instability as the radiographic finding of anterior translation of the cranial vertebral body on the caudal vertebral body in the sagittal plane, as determined on flexion/extension lumbar radiographs. The amount of slippage was calculated using the posterior cortices of the cranial and caudal vertebral bodies, as previously described [4]. Percentage of slippage (% mvt) was calculated as percentage of movement in relation to the caudal vertebral body width (Fig. 1). The spondylolisthesis was considered ‘‘dynamic’’ if there were greater or equal to 3 mm of instability on flexion/ extension films as described by Boden et al. [6]. In contrast, if the adjacent segments showed less than 3 mm change in flexion/extension films, the spondylolisthesis was considered ‘‘static.’’ Sagittal T2-weighted MRI sequences were evaluated for evidence of spondylolisthesis. Axial T2-weighted MRI sequences were evaluated for FF (Fig. 2), FF thickness,
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Fig. 1. Lateral lumbar spine radiograph demonstrating slip percentage measurement method.
facet cysts, interspinous fluid (ISF) (Fig. 3), and facet hypertrophy. Official radiology interpretations were also recorded and omissions in the report of any of the stated previous findings were recorded. All radiographic measurements, plain films, and MRIs were evaluated by an independent examiner who was not directly involved with the care or surgery of the patient. Interobserver reliability using these methods has already been established by several previous studies [4,10]. Statistical methods All data statistical analyses for categorical variables were performed by chi-squared tests. Positive predictive value and likelihood ratios were calculated for categorical values, as well. Continuous variables were analyzed using Students t tests. Statistical significance was defined as a p value !.05. All statistical analyses were performed using Predictive Analytics Software Statistics (PASW) version 18.0 (SPSS, Chicago, IL, USA).
Results A total of 90 patients were included in the study, with 114 levels of spondylolistheses being evaluated. There were 22 (24.4%) males and 68 (65.6%) females. The average age of the patients was 66.7 years (range, 42.7–85.8). There was a statistical difference between degree of instability on flexion/extension films with regard to the presence of FF and ISF. A 74.4% (32/43) incidence of FF was noted with spondylolisthesis that was greater or equal to 3 mm of instability on flexion/extension films (dynamic), compared with only a 52.1% (37/71) incidence of FF on those with less than 3 mm of instability (static, p5.018). Interspinous fluid was noted on 67.4% (29/43) of dynamic spondylolisthesis and only 18.3% (13/71) of static spondylolisthesis (p!.001). Facet cysts were present in 32.6% (14/ 43) of the dynamic spondylolisthesis and 19.7% (14/71) of the static spondylolisthesis, which approached statistical significance (p5.123). There was a statistically significant
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Table 1 Clinical findings on MRI comparing static and dynamic spondylolisthesis Clinical variables
Dynamic (%) Static (%)
FF 32/43 ISF 29/43 Facet cysts 14/43 ISF and FF 22/43 Facet hypertrophy 42/43 Spondylolisthesis 26/43 on MRI Percent movement 9.336
(74.4) (67.4) (32.6) (51.2) (97.7) (60.5)
37/71 13/71 14/71 10/71 71/71 46/71
p Value
LR predicting dynamic
(52.1) .018* 1.48 (18.3) !.001* 3.68 (19.7) .123 n/a (14.1) !.001* 3.63 (100) .197 n/a (64.8) .643 n/a
(2.49) 2.626 (2.04) !.001* n/a
MRI, magnetic resonance imaging; LR, likelihood ratio; FF, facet fluid; ISF, interspinous fluid; n/a, not applicable. Note: There are statistically significant differences in MRI findings between static and dynamic spondylolisthesis with regard to FF, ISF, FF and ISF combined, and percent movement.
Fig. 2. Facet fluid seen on magnetic resonance imaging.
difference in slip percentage (defined by movement of slip/ caudal vertebral body width), with static slips only moving 2.62%62.04% and dynamic slips moving 9.33%62.49% (p#.001). When viewing supine dynamic spondylolistheses (as defined on plain films), 39.5% did not show a spondylolisthesis on the sagittal MRI reconstruction. Positive predictive value for ISF on MRI for a dynamic spondylolisthesis was 69.0%. Positive predictive value for the absence of FF for static spondylolisthesis was 75.6%. Likelihood ratio for dynamic slips in the presence of ISF on MRI was 3.68. Likelihood ratio for dynamic
spondylolisthesis in the presence of FF had a likelihood ratio51.43 (Tables 1–3). When evaluating reduction of spondylolisthesis on MRI, 36.8% of all spondylolisthesis reduced on supine MRI. Radiologists neglected to comment on the findings of FF 79.2% (57/72), facet cysts 65.5% (19/29), and ISF 86.7% (39/45) of the time, respectively. They also neglected to note spondylolisthesis 25.4% (29/114) of the time (Table 4).
Discussion Weight-bearing lumbar flexion/extension radiographs have been advocated in the literature to diagnose the presence of lumbar degenerative instability. Although MRIs are extremely sensitive and specific for the evaluation of many lumbar conditions, traditional MRIs are performed in the supine position and have been shown to mask lumbar degenerative lumbar instability that is usually present in the upright position [10–12]. Indeed, there are case reports in the literature of missed L4/L5 spondylolisthesis because plain radiographs were not obtained before surgery and preoperative MRIs did not demonstrate any spondylolisthesis [5]. Literature to date has not differentiated between the dynamic and static sub-types of spondylolisthesis. Clinically, it may be important to distinguish the difference between Table 2 Positive predictive values of MRI findings that are associated with static and dynamic spondylolisthesis
Fig. 3. Interspinous fluid seen on magnetic resonance imaging.
PPVs
Static (%)
Dynamic (%)
PPV of FF PPV of no FF PPV of ISF PPV of no ISF PPV for ISF and FF PPV for no ISF and FF
37/69 34/45 13/42 58/72 10/32 61/82
32/69 11/45 29/42 14/72 22/32 21/82
(53.6) (75.6) (31.0) (80.6) (31.2) (74.4)
(46.4) (24.4) (69.0) (19.4) (68.8) (25.6)
MRI, magnetic resonance imaging; PPV, positive predictive value; FF, facet fluid; ISF, interspinous fluid.
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Table 3 Lumbar levels of spondylolisthesis studied on MRI Levels
Number
Percentage
L2/L3 L3/L4 L4/L5 L5/L1 Total
1 25 81 7 114
0.88 21.93 71.05 6.14 100
MRI, magnetic resonance imaging.
these two subtypes. It has been shown in the past that arthrodesis, in conjunction with decompression, improves clinical outcomes in patients with pre-existing spondylolisthesis [2,13,14]. However, there still is a debate as to whether the use of instrumentation in this setting is beneficial [14–21]. Some of this controversy may be because dynamic and static sub-types of spondylolisthesis are not differentiated in the current literature. The surgical options and outcomes may be significantly different for these two subtypes if the dynamic and static types behave as separate pathologic entities. For example, static spondylolisthesis type may not necessarily need instrumentation or require a fusion, whereas dynamic subtypes may require additional fixation for a better outcome. To our knowledge, this is the first study in the literature to comprehensively analyze MRI findings in association with flexion/extension radiographs for dynamic and static sub-types of spondylolisthesis. Our study confirms previous findings that FF is highly suggestive of dynamic spondylolisthesis. In addition, presence of ISF and interspinous cysts also are indicative of dynamic spondylolisthesis. In our study, the absence of FF has a PPV of 75.6% for static spondylolisthesis, while the presence of FF does not distinguish between static and dynamic spondylolisthesis with a PPV of 53.6% and 46.4%, respectively. This indicates that FF alone cannot determine whether greater than 3 mm of instability will exist between the lumbar segments in flexion/extension films. However, the presence of ISF on an MRI is a strong predictor of dynamic spondylolisthesis (PPV 69%), as is absence of ISF, which was an even stronger predictor of static spondylolisthesis (PPV 80.6%). When both FF and ISF were present on MRI, dynamic spondylolisthesis was highly predictive (PPV 68.8%). When both FF and ISF
Table 4 Pathology missed by radiologists with regard to MRI findings associated with static and dynamic spondylolisthesis Missed by radiology
Number
Percentage
FF ISF FC Static spondylolisthesis Dynamic spondylolisthesis Total spondylolistheses
57/72 39/45 19/29 16/71 13/43 42/114
79.17 86.67 65.52 22.54 30.23 36.84
MRI, magnetic resonance imaging; FF, facet fluid; ISF, interspinous fluid; FC, facet cyst.
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were absent, static spondylolisthesis was predictive 74.4% of the time (Table 2). The radiologists’ reports were analyzed to emphasize that many of the characteristics of pathologic olisthesis were not reported by the attending radiologists (8 of 10 reads did not mention the pathologic signs [79.17%]). This finding highlights the importance of the attending surgeon viewing the radiographic studies themselves for ultimate surgical planning. Weaknesses of our article include the fact that we did not take into account rotational instability. This may underestimate rare instability patterns where only a unilateral facet joint exhibits instability. Also, computed tomography findings were not included in our study, and significant bony changes such as facet hypertrophy could not be fully analyzed due to the insufficient resolution of MRI to exhibit bony anatomy. Finally, our findings do not relate to iatrogenic or coronal instability, as patients with previous surgical history or scoliosis were excluded from our study. ‘‘Dynamic’’ radiographs should still be regarded as the gold standard to determine the presence of instability. However, the delineation of ‘‘stable’’ versus ‘‘dynamic’’ spondylolisthesis may allow more specific analysis of surgical outcomes. A more specific analysis of outcomes will in turn better define roles for the various techniques of surgical treatments (instrumented vs. non-instrumented, posterior lateral fusion, posterior lumbar interbody fusion, and so on). For example, it may be that a stable subclass of spondylolisthesis may not require as rigid arthrodesis and in fact may have an equivalent outcome with non-instrumented fusion or a stand-alone decompression. However, even if patients do have O3 mm of motion on flexion extension films and have MRI findings described above in our study, these findings do not mandate that a fusion is necessary but they should be taken into the overall consideration for the patient’s personal treatment plan. No recommendation regarding decompression without fusion versus decompression with fusion can be made from the findings in this study. Conclusion The differentiation between static versus dynamic spondylolistheses has been neglected in the literature. The findings presented here may be significant in stratifying future study population to determine the benefits of certain surgical strategies. Namely, sub-stratification of patients to static and dynamic spondylolisthesis should be performed to resolve the necessity of non-instrumented fusion, instrumented fusion, interbody fusion, stand-alone decompression, and multitudes of other surgical techniques currently used to treat degenerative lumbar spondylolisthesis. Acknowledgments The authors acknowledge James D. Kang, MD, for contributions to clinical data collection.
J.L. Even et al. / The Spine Journal
References [1] Bridwell KH, Sedgewick TA, O’Brien MF, et al. The role of fusion and instrumentation in the treatment of degenerative spondylolisthesis with spinal stenosis. J Spinal Disord 1993;6:461–72. [2] Herkowitz HN, Kurz LT. Degenerative lumbar spondylolisthesis with spinal stenosis. A prospective study comparing decompression with decompression and intertransverse process arthrodesis. J Bone Joint Surg Am 1991;73:802–8. [3] Mardjetko SM, Connolly PJ, Shott S. Degenerative lumbar spondylolisthesis. A meta-analysis of literature 1970-1993. Spine 1994;19(20 Suppl):2256S–65S. [4] Rihn JA, Lee JY, Khan M, et al. Does lumbar facet fluid detected on magnetic resonance imaging correlate with radiographic instability in patients with degenerative lumbar disease? Spine 2007;32: 1555–60. [5] Bendo JA, Ong B. Importance of correlating static and dynamic imaging studies in diagnosing degenerative lumbar spondylolisthesis. Am J Orthop (Belle Mead NJ) 2001;30:247–50. [6] Boden SD, Wiesel SW. Lumbosacral segmental motion in normal individuals. Have we been measuring instability properly? Spine 1990;15:571–6. [7] Quinnell RC, Stockdale HR. Flexion and extension radiography of the lumbar spine: a comparison with lumbar discography. Clin Radiol 1983;34:405–11. [8] Wood KB, Popp CA, Transfeldt EE, Geissele AE. Radiographic evaluation of instability in spondylolisthesis. Spine 1994;19:1697–703. [9] Chaput C, Padon D, Rush J, et al. The significance of increased fluid signal on magnetic resonance imaging in lumbar facets in relationship to degenerative spondylolisthesis. Spine 2007;32:1883–7. [10] Schinnerer KA, Katz LD, Grauer JN. MR findings of exaggerated fluid in facet joints predicts instability. J Spinal Disord Tech 2008;21: 468–72. [11] Ben-Galim P, Reitman CA. The distended facet sign: an indicator of position-dependent spinal stenosis and degenerative spondylolisthesis. Spine J 2007;7:245–8.
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[12] McGregor AH, Anderton L, Gedroyc WM, et al. The use of interventional open MRI to assess the kinematics of the lumbar spine in patients with spondylolisthesis. Spine 2002;27:1582–6. [13] Fischgrund JS, Mackay M, Herkowitz HN, et al. 1997 Volvo Award winner in clinical studies. Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective, randomized study comparing decompressive laminectomy and arthrodesis with and without spinal instrumentation. Spine 1997;22:2807–12. [14] Fischgrund JS. The argument for instrumented decompressive posterolateral fusion for patients with degenerative spondylolisthesis and spinal stenosis. Spine 2004;29:173–4. [15] Abdu WA, Lurie JD, Spratt KF, et al. Degenerative spondylolisthesis: does fusion method influence outcome? Four-year results of the spine patient outcomes research trial. Spine 2009;34:2351–60. [16] Bjarke Christensen F, Stender Hansen E, Laursen M, et al. Long-term functional outcome of pedicle screw instrumentation as a support for posterolateral spinal fusion: randomized clinical study with a 5-year follow-up. Spine 2002;27:1269–77. [17] Detwiler PW, Porter RW, Han PP, et al. Surgical treatment of lumbar spondylolisthesis. Adv Tech Stand Neurosurg 2000;26:331–46. [18] Kleinstueck FS, Fekete TF, Mannion AF, et al. To fuse or not to fuse in lumbar degenerative spondylolisthesis: do baseline symptoms help provide the answer? Eur Spine J 2012;21:268–75. [19] Nork SE, Hu SS, Workman KL, et al. Patient outcomes after decompression and instrumented posterior spinal fusion for degenerative spondylolisthesis. Spine 1999;24:561–9. [20] Thomsen K, Christensen FB, Eiskjaer SP, et al. 1997 Volvo Award winner in clinical studies. The effect of pedicle screw instrumentation on functional outcome and fusion rates in posterolateral lumbar spinal fusion: a prospective, randomized clinical study. Spine 1997;22: 2813–22. [21] Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical compared with nonoperative treatment for lumbar degenerative spondylolisthesis. Four-year results in the Spine Patient Outcomes Research Trial (SPORT) randomized and observational cohorts. J Bone Joint Surg Am 2009;91:1295–304.
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Clinical Study
Imaging characteristics of ‘‘dynamic’’ versus ‘‘static’’ spondylolisthesis: analysis using magnetic resonance imaging and flexion/extension films Jesse L. Even, MD*, Antonia F. Chen, MD, MBA, Joon Y. Lee, MD Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, 3471 Fifth Ave, Pittsburgh, PA 15213, USA Received 11 October 2012; revised 11 November 2013; accepted 26 November 2013
Abstract
BACKGROUND CONTEXT: Traditionally, the ‘‘dynamic’’ and ‘‘static’’ types of spondylolisthesis have been lumped into a single group in the literature. The goal of this study was to define the radiographic characteristics of ‘‘dynamic’’ and ‘‘static’’ spondylolisthesis with the use of magnetic resonance imaging (MRI) and flexion/extension radiographs. PURPOSE: Describe the characteristic findings present on MRI and flexion/extension radiographs that are associated with dynamic versus static spondylolisthesis. STUDY DESIGN: Retrospective radiographic/imaging study. METHODS: From 2009 to 2011, patients who underwent elective primary posterior spinal fusion for the diagnosis of spondylolisthesis had their plain films assessed for the degree of spondylolisthesis and were designated ‘‘dynamic’’ or ‘‘static,’’ as defined by historical measures. Axial and sagittal T2 MRIs were evaluated for associated facet fluid (FF), facet cysts, interspinous fluid (ISF), and facet hypertrophy. These finding were then statistically evaluated for associations between dynamic and static spondylolisthesis on flexion/extension radiographs and characteristic MRI findings. RESULTS: Ninety patients were included in the study with 114 levels examined for spondylolisthesis. Patients with greater than 3 mm of instability on flexion/extension films were more likely to have FF (p5.018) and ISF (p!.001). Of the patients who had a greater than 3 mm of instability, 39.5% did not demonstrate spondylolisthesis on the sagittal MRI reconstruction. If ISF was present on MRI, there was a positive predictive value of 69.0% that there would be greater than 3 mm instability on flexion/extension films. Absence of FF on MRI had a positive predictive value of 75.6% for instability less than 3 mm on flexion/extension films. In the presence of ISF on MRI, the likelihood ratio of finding more than 3 mm of instability on flexion/extension films was 3.68. The presence of FF on MRI had a likelihood ratio of 1.43 for instability. A total of 36.8% of all spondylolisthesis reduced when supine on MRI. CONCLUSIONS: The presence of FF and/or ISF is associated with instability greater than 3 mm in flexion/extension radiographs. Ó 2014 Elsevier Inc. All rights reserved.
Keywords:
Dynamic; Static; Spondylolisthesis; MRI; Flexion/extension; Facet fluid; Interspinous fluid
Introduction Degenerative spondylolisthesis is one of the most common diagnoses prompting surgical intervention in the lumbar spine. Laminectomy may be performed as a stand-alone procedure or may be performed in conjunction with arthrodesis when instability in between the lumbar segments FDA device/drug status: Not applicable. Author disclosures: JLE: Nothing to disclose. AFC: Nothing to disclose. JYL: Nothing to disclose. * Corresponding author. Kaufmann Medical Building, Suite 1010, 3471 Fifth Ave., Suite 1010, Pittsburgh, PA 15213, USA. Tel.: (817) 845-7821. E-mail address: [email protected] (J.L. Even) 1529-9430/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.11.057
is suspected [1–4]. In the past, the determination of instability (and thus the decision to include arthrodesis) was evaluated using weight-bearing lateral flexion/extension radiographs with greater than 3 mm translation being defined as instability [5–8]. The 3-mm threshold was defined by Boden et al. in 1990 using 40 asymptomatic patients while measuring their olisthesis on flexion and extension radiographs. The mean translation in this asymptomatic population was 1.3 mm with no patient registering with more than 3 mm of translation. Based on this, Boden et al. concluded that greater than 3 mm of olisthesis was pathologic and represented the presence of dynamic instability [6]. However, with the advent of modern imaging techniques, namely magnetic resonance imaging (MRI),
2
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the use of dynamic radiographs (flexion/extension) has become underutilized in the clinical setting. In a recent article by Rhin et al., the presence of facet fluid (FF) on T2-weighted MRI had a positive predictive value (PPV) of 82% for instability on flexion/extension films [4]. Chaput et al. also examined the significance of FF on MRI and its association with instability [9]. They noted that O1.5 mm of FF on axial MRI imaging is associated with spondylolisthesis in the absence of measurable anterolisthesis on supine MRI. Moreover, the literature to date has not distinguished the difference between ‘‘dynamic’’ and ‘‘static’’ subtypes of spondylolisthesis. The ‘‘dynamic’’ type, as defined as spondylolisthesis that has greater than 3 mm of instability on flexion/extension films, may be a different clinical entity than the ‘‘static’’ sub- type, which does not show instability on flexion/extension films. To our knowledge, there has not been a comprehensive imaging analysis of dynamic and static subtypes of spondylolisthesis. Thus, our goal was to define imaging characteristics of dynamic and static spondylolisthesis using both MRI as well as flexion/extension plain radiographs. Materials and methods After obtaining Institutional Review Board approval, the clinical database was deidentified and collected for patients who underwent a primary laminectomy and arthrodesis between the years of 2009 to 2011 to determine study eligibility. Patients who had preoperative MRIs and weight-bearing flexion/extension radiographs were included in the study. Patients who had a previous history of lumbar surgery or with a diagnosis of trauma, infection, or tumor were excluded from the study. Subjects were also excluded if the plain films and MRI were taken more than 6 months apart. Those with lateral listhesis or scoliosis were also excluded from the study. A total of 90 patients met the criteria for the study with 114 levels being evaluated. We defined instability as the radiographic finding of anterior translation of the cranial vertebral body on the caudal vertebral body in the sagittal plane, as determined on flexion/extension lumbar radiographs. The amount of slippage was calculated using the posterior cortices of the cranial and caudal vertebral bodies, as previously described [4]. Percentage of slippage (% mvt) was calculated as percentage of movement in relation to the caudal vertebral body width (Fig. 1). The spondylolisthesis was considered ‘‘dynamic’’ if there were greater or equal to 3 mm of instability on flexion/ extension films as described by Boden et al. [6]. In contrast, if the adjacent segments showed less than 3 mm change in flexion/extension films, the spondylolisthesis was considered ‘‘static.’’ Sagittal T2-weighted MRI sequences were evaluated for evidence of spondylolisthesis. Axial T2-weighted MRI sequences were evaluated for FF (Fig. 2), FF thickness,
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Fig. 1. Lateral lumbar spine radiograph demonstrating slip percentage measurement method.
facet cysts, interspinous fluid (ISF) (Fig. 3), and facet hypertrophy. Official radiology interpretations were also recorded and omissions in the report of any of the stated previous findings were recorded. All radiographic measurements, plain films, and MRIs were evaluated by an independent examiner who was not directly involved with the care or surgery of the patient. Interobserver reliability using these methods has already been established by several previous studies [4,10]. Statistical methods All data statistical analyses for categorical variables were performed by chi-squared tests. Positive predictive value and likelihood ratios were calculated for categorical values, as well. Continuous variables were analyzed using Students t tests. Statistical significance was defined as a p value !.05. All statistical analyses were performed using Predictive Analytics Software Statistics (PASW) version 18.0 (SPSS, Chicago, IL, USA).
Results A total of 90 patients were included in the study, with 114 levels of spondylolistheses being evaluated. There were 22 (24.4%) males and 68 (65.6%) females. The average age of the patients was 66.7 years (range, 42.7–85.8). There was a statistical difference between degree of instability on flexion/extension films with regard to the presence of FF and ISF. A 74.4% (32/43) incidence of FF was noted with spondylolisthesis that was greater or equal to 3 mm of instability on flexion/extension films (dynamic), compared with only a 52.1% (37/71) incidence of FF on those with less than 3 mm of instability (static, p5.018). Interspinous fluid was noted on 67.4% (29/43) of dynamic spondylolisthesis and only 18.3% (13/71) of static spondylolisthesis (p!.001). Facet cysts were present in 32.6% (14/ 43) of the dynamic spondylolisthesis and 19.7% (14/71) of the static spondylolisthesis, which approached statistical significance (p5.123). There was a statistically significant
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Table 1 Clinical findings on MRI comparing static and dynamic spondylolisthesis Clinical variables
Dynamic (%) Static (%)
FF 32/43 ISF 29/43 Facet cysts 14/43 ISF and FF 22/43 Facet hypertrophy 42/43 Spondylolisthesis 26/43 on MRI Percent movement 9.336
(74.4) (67.4) (32.6) (51.2) (97.7) (60.5)
37/71 13/71 14/71 10/71 71/71 46/71
p Value
LR predicting dynamic
(52.1) .018* 1.48 (18.3) !.001* 3.68 (19.7) .123 n/a (14.1) !.001* 3.63 (100) .197 n/a (64.8) .643 n/a
(2.49) 2.626 (2.04) !.001* n/a
MRI, magnetic resonance imaging; LR, likelihood ratio; FF, facet fluid; ISF, interspinous fluid; n/a, not applicable. Note: There are statistically significant differences in MRI findings between static and dynamic spondylolisthesis with regard to FF, ISF, FF and ISF combined, and percent movement.
Fig. 2. Facet fluid seen on magnetic resonance imaging.
difference in slip percentage (defined by movement of slip/ caudal vertebral body width), with static slips only moving 2.62%62.04% and dynamic slips moving 9.33%62.49% (p#.001). When viewing supine dynamic spondylolistheses (as defined on plain films), 39.5% did not show a spondylolisthesis on the sagittal MRI reconstruction. Positive predictive value for ISF on MRI for a dynamic spondylolisthesis was 69.0%. Positive predictive value for the absence of FF for static spondylolisthesis was 75.6%. Likelihood ratio for dynamic slips in the presence of ISF on MRI was 3.68. Likelihood ratio for dynamic
spondylolisthesis in the presence of FF had a likelihood ratio51.43 (Tables 1–3). When evaluating reduction of spondylolisthesis on MRI, 36.8% of all spondylolisthesis reduced on supine MRI. Radiologists neglected to comment on the findings of FF 79.2% (57/72), facet cysts 65.5% (19/29), and ISF 86.7% (39/45) of the time, respectively. They also neglected to note spondylolisthesis 25.4% (29/114) of the time (Table 4).
Discussion Weight-bearing lumbar flexion/extension radiographs have been advocated in the literature to diagnose the presence of lumbar degenerative instability. Although MRIs are extremely sensitive and specific for the evaluation of many lumbar conditions, traditional MRIs are performed in the supine position and have been shown to mask lumbar degenerative lumbar instability that is usually present in the upright position [10–12]. Indeed, there are case reports in the literature of missed L4/L5 spondylolisthesis because plain radiographs were not obtained before surgery and preoperative MRIs did not demonstrate any spondylolisthesis [5]. Literature to date has not differentiated between the dynamic and static sub-types of spondylolisthesis. Clinically, it may be important to distinguish the difference between Table 2 Positive predictive values of MRI findings that are associated with static and dynamic spondylolisthesis
Fig. 3. Interspinous fluid seen on magnetic resonance imaging.
PPVs
Static (%)
Dynamic (%)
PPV of FF PPV of no FF PPV of ISF PPV of no ISF PPV for ISF and FF PPV for no ISF and FF
37/69 34/45 13/42 58/72 10/32 61/82
32/69 11/45 29/42 14/72 22/32 21/82
(53.6) (75.6) (31.0) (80.6) (31.2) (74.4)
(46.4) (24.4) (69.0) (19.4) (68.8) (25.6)
MRI, magnetic resonance imaging; PPV, positive predictive value; FF, facet fluid; ISF, interspinous fluid.
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Table 3 Lumbar levels of spondylolisthesis studied on MRI Levels
Number
Percentage
L2/L3 L3/L4 L4/L5 L5/L1 Total
1 25 81 7 114
0.88 21.93 71.05 6.14 100
MRI, magnetic resonance imaging.
these two subtypes. It has been shown in the past that arthrodesis, in conjunction with decompression, improves clinical outcomes in patients with pre-existing spondylolisthesis [2,13,14]. However, there still is a debate as to whether the use of instrumentation in this setting is beneficial [14–21]. Some of this controversy may be because dynamic and static sub-types of spondylolisthesis are not differentiated in the current literature. The surgical options and outcomes may be significantly different for these two subtypes if the dynamic and static types behave as separate pathologic entities. For example, static spondylolisthesis type may not necessarily need instrumentation or require a fusion, whereas dynamic subtypes may require additional fixation for a better outcome. To our knowledge, this is the first study in the literature to comprehensively analyze MRI findings in association with flexion/extension radiographs for dynamic and static sub-types of spondylolisthesis. Our study confirms previous findings that FF is highly suggestive of dynamic spondylolisthesis. In addition, presence of ISF and interspinous cysts also are indicative of dynamic spondylolisthesis. In our study, the absence of FF has a PPV of 75.6% for static spondylolisthesis, while the presence of FF does not distinguish between static and dynamic spondylolisthesis with a PPV of 53.6% and 46.4%, respectively. This indicates that FF alone cannot determine whether greater than 3 mm of instability will exist between the lumbar segments in flexion/extension films. However, the presence of ISF on an MRI is a strong predictor of dynamic spondylolisthesis (PPV 69%), as is absence of ISF, which was an even stronger predictor of static spondylolisthesis (PPV 80.6%). When both FF and ISF were present on MRI, dynamic spondylolisthesis was highly predictive (PPV 68.8%). When both FF and ISF
Table 4 Pathology missed by radiologists with regard to MRI findings associated with static and dynamic spondylolisthesis Missed by radiology
Number
Percentage
FF ISF FC Static spondylolisthesis Dynamic spondylolisthesis Total spondylolistheses
57/72 39/45 19/29 16/71 13/43 42/114
79.17 86.67 65.52 22.54 30.23 36.84
MRI, magnetic resonance imaging; FF, facet fluid; ISF, interspinous fluid; FC, facet cyst.
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were absent, static spondylolisthesis was predictive 74.4% of the time (Table 2). The radiologists’ reports were analyzed to emphasize that many of the characteristics of pathologic olisthesis were not reported by the attending radiologists (8 of 10 reads did not mention the pathologic signs [79.17%]). This finding highlights the importance of the attending surgeon viewing the radiographic studies themselves for ultimate surgical planning. Weaknesses of our article include the fact that we did not take into account rotational instability. This may underestimate rare instability patterns where only a unilateral facet joint exhibits instability. Also, computed tomography findings were not included in our study, and significant bony changes such as facet hypertrophy could not be fully analyzed due to the insufficient resolution of MRI to exhibit bony anatomy. Finally, our findings do not relate to iatrogenic or coronal instability, as patients with previous surgical history or scoliosis were excluded from our study. ‘‘Dynamic’’ radiographs should still be regarded as the gold standard to determine the presence of instability. However, the delineation of ‘‘stable’’ versus ‘‘dynamic’’ spondylolisthesis may allow more specific analysis of surgical outcomes. A more specific analysis of outcomes will in turn better define roles for the various techniques of surgical treatments (instrumented vs. non-instrumented, posterior lateral fusion, posterior lumbar interbody fusion, and so on). For example, it may be that a stable subclass of spondylolisthesis may not require as rigid arthrodesis and in fact may have an equivalent outcome with non-instrumented fusion or a stand-alone decompression. However, even if patients do have O3 mm of motion on flexion extension films and have MRI findings described above in our study, these findings do not mandate that a fusion is necessary but they should be taken into the overall consideration for the patient’s personal treatment plan. No recommendation regarding decompression without fusion versus decompression with fusion can be made from the findings in this study. Conclusion The differentiation between static versus dynamic spondylolistheses has been neglected in the literature. The findings presented here may be significant in stratifying future study population to determine the benefits of certain surgical strategies. Namely, sub-stratification of patients to static and dynamic spondylolisthesis should be performed to resolve the necessity of non-instrumented fusion, instrumented fusion, interbody fusion, stand-alone decompression, and multitudes of other surgical techniques currently used to treat degenerative lumbar spondylolisthesis. Acknowledgments The authors acknowledge James D. Kang, MD, for contributions to clinical data collection.
J.L. Even et al. / The Spine Journal
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