SPINE Volume 39, Number 5, pp E339-E345 ©2014, Lippincott Williams & Wilkins
CLINICAL CASE SERIES
Risk Factors of Adjacent Segment Disease Requiring Surgery After Lumbar Spinal Fusion Comparison of Posterior Lumbar Interbody Fusion and Posterolateral Fusion Jae Chul Lee, MD, PhD,* Yongdai Kim, PhD,† Jae-Wan Soh, MD, PhD,‡ and Byung-Joon Shin, MD, PhD*
Study Design. A retrospective study. Objective. To determine the incidence and risk factors of adjacent segment disease (ASD) requiring surgery among patients previously treated with spinal fusion for degenerative lumbar disease and to compare the survivorship of adjacent segment according to various risk factors including comparison of fusion methods: posterior lumbar interbody fusion (PLIF) versus posterolateral fusion (PLF). Summary of Background Data. One of the major issues after lumbar spinal fusion is the development of adjacent segment disease. Biomechanically, PLIF has been reported to be more rigid than PLF, and therefore, patients who undergo PLIF are suspected to experience a higher incidence of ASD than those who underwent PLF. There have been many studies analyzing the risk factors of ASD, but we are not aware of any study comparing PLIF with PLF in incidence of ASD requiring surgery. Methods. A consecutive series of 490 patients who had undergone lumbar spinal fusion of 3 or fewer segments to treat degenerative lumbar disease was identified. The mean age at index operation was 53 years, and the mean follow-up period was 51 months (12–236 mo). The number of patients treated by PLF and PLIF were 103 and 387, respectively. The incidence and prevalence of revision surgery for ASD were calculated by Kaplan-Meier method. For risk factor analysis, we used log-rank test and Cox regression analysis with fusion methods, sex, age, number of fused segments, and presence of laminectomy adjacent to index fusion.
From the *Department of Orthopaedic Surgery, Soonchunhyang University Seoul Hospital, Seoul, Korea; †Department of Statistics, Seoul National University, Seoul, Korea; and ‡Department of Orthopaedic Surgery, Soonchunhyang University Cheonan Hospital, Cheonan, Korea. Acknowledgment date: August 26, 2013. First revision date: November 12, 2013. Second revision date: November 28, 2013. Acceptance date: December 8, 2013. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No relevant financial activities outside the submitted work. Address correspondence and reprint requests to Byung-Joon Shin, MD, Department of Orthopaedic Surgery, Soonchunhyang University Seoul Hospital, 657 Hannam-dong, Yongsan-gu, Seoul 140-743, Korea; E-mail: [email protected] DOI: 10.1097/BRS.0000000000000164 Spine
Results. After index spinal fusion, 23 patients (4.7%) had undergone additional surgery for ASD. Kaplan-Meier analysis predicted a disease-free survival rate of adjacent segments in 94.2% of patients at 5 years and 89.6% at 10 years after the index operation. In the analysis of risk factors, PLIF was associated with 3.4 times higher incidence of ASD requiring surgery than PLF (P = 0.037). Patients older than 60 years at the time of index operation were 2.5 times more likely to undergo revision operation than those younger than 60 years (P = 0.038). There were no significant differences in survival rates of the adjacent segment according to sex, preoperative diagnosis, number of fused segments, and concomitant laminectomy to adjacent segment. Conclusion. It was predicted that 10% of patients would undergo additional surgery for treating ASD within 10 years after index lumbar fusion. In this study, PLIF showed higher incidence of ASD than did PLF. Patient age greater than 60 years was another independent risk factor. Surgeons should carefully consider these factors at the time of surgical planning of lumbar fusion. Key words: adjacent segment disease, lumbar spinal fusion, risk factors, posterolateral fusion, posterior lumbar interbody fusion. Level of Evidence: 3 Spine 2014;39:E339–E345
R
ecently, the use of spinal fusion has become more frequent in the treatment of various lumbar diseases, but patients may experience secondary premature degeneration of adjacent segments and subsequent operation.1–4 For decades, posterolateral fusion (PLF) has been used widely in the treatment of the degenerative lumbar disease, and more recently, posterior lumbar interbody fusion (PLIF) has been gaining popularity because of biomechanical stability and high rates of successful fusion.5–9 Biomechanically, PLIF has been reported to be more rigid than PLF, and therefore, there has been some concern that PLIF may cause increased incidence of adjacent segment disease (ASD). To the best of our knowledge, however, no study has verified any difference in the incidence of ASD between the 2 fusion methods. Numerous studies have looked for the prevalence and risk factors of ASD after lumbar spinal fusion, but these studies have reported contradicting results.4 Insufficient follow-up rate was a practical concern in these studies and more recent www.spinejournal.com
E339
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. SPINE131009.indd E339
2/11/14 8:06 AM
CLINICAL CASE SERIES studies have used Kaplan-Meier survival analysis taking into account patients who were lost to follow-up.10–12 Cox regression analysis has been also used in risk factor analysis to control confounding variables.10,11 In this study, the authors have tried to estimate the true prevalence and incidence of ASD after lumbar spinal fusion using Kaplan-Meier analysis and to assess the risk factors for ASD by Cox regression analysis with special emphasis on comparison of PLIF versus PLF.
MATERIALS AND METHODS Study Population A single-institutional retrospective review of a period between August 1988 and December 2007 was performed and 490 consecutive patients who had undergone 3 or fewer segment lumbar spinal fusion supplemented by pedicle screw fixation were identified. All procedures had been performed by the senior author (B.J.S.) at a single institution. Five patients who had died within 1 year of index operation were excluded. Additionally, patients who had undergone subsequent anterior fusion or circumferential fusion were excluded from the study as well. None of the patients included in the analysis had received a spinal operation prior to the index procedure nor underwent the index procedure under the diagnosis of neoplasm, fracture, dislocation, or infection. Patients who underwent spinal fusion of more than 3 segments or who had a major deformity, such as scoliosis and kyphosis, were also excluded. Cases of pseudarthrosis were also excluded from the study. The study group consisted of 307 female and 183 male patients. The mean age at the time of index operation was 53 years (range, 20–81 yr), and the mean duration of followup was 51 months (range, 1–19 yr). PLF had been performed in 103 patients (21%), and PLIF in 387 patients (79%). In all patients receiving PLF, autogenous cancellous iliac bone was used as graft. PLIF was performed using autogenous iliac-crest strut graft in 114 patients between 1988 and 1999, and using 2 rectangular titanium cages filled with morselized local bone from neural decompression in 273 patients from 2000 to 2007. Single-segment fusion was performed in 321 patients (66%), 2-segment fusion in 147 patients (30%), and 3-segment fusion in 22 patients (4%). As dictated by the inclusion criteria, the surgical indication for index posterior lumbar fusion was severe low back pain and/or radiculopathy resistant to conservative treatment. In each patient, the index fusion had been performed for those segments that corresponded to clinical and radiological findings of neural compressive degenerative lesion. The etiological conditions were spinal stenosis in 245 patients, degenerative spondylolisthesis in 90, spondylolytic spondylolisthesis in 70, herniated intervertebral disc with segmental instability and/ or advanced disc degeneration in 65, and degenerative disc disease associated severe back pain in 20.
METHODS Hospital records, office charts, and radiographs were reviewed. Follow-up data, including follow-up length, survival period, E340
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Risk Factor Analysis of Adjacent Segment Disease • Lee et al
and history of subsequent operation for ASD, were searched for and recorded. Adjacent segments were defined as 2 cephalad and 2 caudad lumbar motion segments above and below the uppermost and lowermost levels of spinal fusion, respectively.10 A patient was considered to have ASD if the adjacent segments manifested degenerative lesions, such as spinal stenosis, radiculopathy, segmental instability, or deformity, causing clinically significant symptoms and requiring surgery. Clinical outcome was assessed at the final follow-up visit using Macnab criteria.13 Potential risk factors such as age, sex, preoperative diagnosis, fusion method (PLIF vs. PLF) at index operation, number of segments fused, and the presence of laminectomy adjacent to fusion were also reviewed in the medical records. For those patients who underwent revision surgery for ASD, the radiographs from the index surgery and at revision surgery were assessed by 2 spine surgeons (J.C.L and J.W.S.). For each patient, mean disc height was determined as an average value of anterior and posterior disc height. The relative disc height was calculated as a percentage of mean disc height to anterior body height of upper adjacent vertebral body. This was done to minimize magnification variance of each x-ray exposure condition. For assessment of intervertebral disc degeneration, the University of California Los Angeles scoring system was used.10
Statistical Analysis The annual incidence and prevalence of ASD requiring revision surgery were calculated by life-table methods, with the construction of a Kaplan-Meier survivorship curve. The annual incidence and prevalence of ASD requiring revision surgery was defined the same as previous reports.10–12 In risk factor analysis, a log-rank test was used for univariate analysis, and multivariate Cox proportional-hazards model was also performed to adjust for confounding variables. In both analyses, we verified the differences in survivorship according to variables such as fusion method, age, sex, number of segments fused, preoperative diagnosis, and presence of laminectomy adjacent to fusion. Interclass correlation coefficient was used to assess interobserver agreement for relative disc height measurement, and κ statistics was performed to assess that of disc degeneration grading. A paired t test was used to assess statistical significance of change in relative disc height, and Wilcoxon matched-pairs signed-rank test was performed to assess that of disc degeneration grade changes. SPSS 14.0 statistical software (SPSS, Chicago, IL) was used for the analyses, and a P value of less than 0.05 was considered statistically significant.
RESULTS Among the 490 patients, 24 patients required subsequent surgical management for clinically significant ASD for an overall prevalence of 4.9%. Of those, 15 patients underwent surgery for the treatment of upper ASD, 8 patients for lower ASD, and 1 patient for both upper and lower ASD. Causes of the revision surgery on adjacent segments were spinal stenosis March 2014
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. SPINE131009.indd E340
2/11/14 8:06 AM
CLINICAL CASE SERIES
Risk Factor Analysis of Adjacent Segment Disease • Lee et al
in 14 patients (12 central and 2 foraminal), herniated intervertebral disc in 8, and degenerative spondylolisthesis in 2. The choice of surgical intervention was extension of fusion in 15 patients, decompressive laminectomy without fusion extension in 1 patient, and discectomy in 8 patients. Clinical outcomes were considered excellent in 4 patients, good in 9, fair in 3, and poor in 8 patients. This represents 54% of these patients who experienced excellent or good functional outcome among patients who underwent secondary surgery due to ASD. This compares with the patients who did not develop adjacent segment disease requiring surgery, among whom outcome was excellent in 211 patients, good in 176, fair in 61, and poor in 18, for 83% of these patients having excellent or good functional outcome. Thus, the final clinical outcome was significantly worse for the patients who underwent secondary operation for ASD than for those patients who did not develop ASD (P < 0.001).
Incidence of ASD The annual incidence of ASD requiring surgery was relatively constant at 1.2% (95% confidence interval [CI], 0.8–1.7) for 10 years after the index spinal fusion (Table 1). We used Kaplan-Meier survivorship analysis to include all available data to the last follow-up point and predicted disease-free survivorship of the adjacent segments. Predicted survivorship of the adjacent segments was 94.2% (95% CI, 91.0–97.4) at postoperative 5 years and 89.6% (95% CI, 84.0–95.2) at 10 years (Figure 1). In other words, probability of undergoing a revision surgery for adjacent segment disease was 5.8% (95% CI, 2.6–9.0) at postoperative 5 years and 10.4% (95% CI, 4.8–16.0) at 10 years.
Risk Factor Analysis In the risk factor analysis, PLIF shows significantly lower survival rate than PLF in univariate analysis by log-rank test (P = 0.048). The incidence of ASD requiring surgery after
Figure 1. Kaplan-Meier survivorship curve. Survival time was scaled in months.
PLIF was 11.7% (95% CI, 4.7–18.3) at 10 years, but the incidence after PLF was significantly lower with 6.7% (95% CI, 2.6–10.8) at 10 years (Figure 2). Patients who were 60 years or older (≥60 yr) at the time of index surgery experienced significantly lower survival rate of adjacent segments than those who were younger than 60 years (P = 0.034). Prevalence of ASD requiring surgery among patients older than 60 years was 21.9% (95% CI, 0–39.4) at 10 years. These were significantly lower for patients younger than 60 years with 7.3% (95% CI, 2.5– 11.9) at 10 year (Figure 3). Other than age or the initial method of fusion (PLIF vs. PLF), no statistical significance was found for potential risk factor variables such as sex (P = 0.159), preoperative diagnosis (P = 0.274), number of segments fused (P = 0.796), and presence of laminectomy adjacent to fusion (P = 0.923). The results of univariate analysis were supported by multivariate Cox regression analysis (Table 2). In the analysis of possible risk factors, patients treated by PLIF experienced
TABLE 1. Incidence of Adjacent Segment Disease Requiring Surgery by Hazard Function Month
No. of Patients Entering Interval
No. of Patients Withdrawing
No. of Terminal Events
Incidence (%)
95% CI
12–24
490
151
5
1.3
1.1–1.6
24–36
334
97
4
1.6
1.2–2.0
36–48
233
53
2
1.1
0.8–1.4
48–60
178
36
3
2.0
1.3–3.0
60–72
139
25
0
0
0
72–84
114
19
2
2.0
1.2–3.5
84–96
93
16
0
0
0
96–108
77
8
1
1.4
0.8–2.4
108–120
68
14
1
1.7
1.0–3.1
1.2
0.8–1.8
Average annual incidence CI indicates confidence interval.
Spine
www.spinejournal.com
E341
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. SPINE131009.indd E341
2/11/14 8:06 AM
CLINICAL CASE SERIES
Risk Factor Analysis of Adjacent Segment Disease • Lee et al
and 0.762 at revision surgery, which also showed good agreement. Among the patients with ASD who underwent revision, the initial disc degeneration of adjacent segments were grade 1 in 12 adjacent segments, grade 2 in 5, grade 3 in 4, and grade 4 in 3. At the time of revision surgery, the patients had grade 1 in 3 adjacent segments, grade 2 in 11, grade 3 in 5, and grade 4 in 5. In all, the disc degeneration had worsened in 10 adjacent segments (1-grade increase in 7; 2-grade increase in 2; and 3-grade increase in 1). The changes of disc degeneration grading were significant (P = 0.004).
DISCUSSION
Figure 2. Kaplan-Meier survivorship curve of PLF versus PLIF. PLF indicates posterolateral fusion; PLIF, posterior lumbar interbody fusion.
3.4 times (95% CI, 1.1–10.7; P = 0.037) higher incidence of ASD requiring surgery than patients who were treated by PLF. Likewise, patients who were 60 years or older experienced 2.5 times (95% CI, 1.1–6.1; P = 0.038) higher incidence of ASD than did patients who were younger than 60 years at the time of index surgery. Again, factors such as sex, preoperative diagnosis, number of segments fused, or presence of laminectomy adjacent to fusion did not show statistical significance in the multivariate analysis.
Radiographical Analysis In the radiographical analysis of the 24 patients who had undergone revision surgical procedures for ASD, interclass correlation coefficient of relative disc height measurement was 0.954 at the time of index surgery and 0.999 at revision surgery—a result that showed excellent agreement across the 2 observers. Relative disc height had significantly decreased from an average of 36% (range, 20%–55%) at the time of index surgery to an average of 31% (range, 3%–55%) at revision surgery (P = 0.024). Kappa coefficient of the disc degeneration grading between observers was 0.685 at index surgery
Figure 3. Kaplan-Meier survivorship curve of old (≥60 yr) versus young (
CLINICAL CASE SERIES
Risk Factors of Adjacent Segment Disease Requiring Surgery After Lumbar Spinal Fusion Comparison of Posterior Lumbar Interbody Fusion and Posterolateral Fusion Jae Chul Lee, MD, PhD,* Yongdai Kim, PhD,† Jae-Wan Soh, MD, PhD,‡ and Byung-Joon Shin, MD, PhD*
Study Design. A retrospective study. Objective. To determine the incidence and risk factors of adjacent segment disease (ASD) requiring surgery among patients previously treated with spinal fusion for degenerative lumbar disease and to compare the survivorship of adjacent segment according to various risk factors including comparison of fusion methods: posterior lumbar interbody fusion (PLIF) versus posterolateral fusion (PLF). Summary of Background Data. One of the major issues after lumbar spinal fusion is the development of adjacent segment disease. Biomechanically, PLIF has been reported to be more rigid than PLF, and therefore, patients who undergo PLIF are suspected to experience a higher incidence of ASD than those who underwent PLF. There have been many studies analyzing the risk factors of ASD, but we are not aware of any study comparing PLIF with PLF in incidence of ASD requiring surgery. Methods. A consecutive series of 490 patients who had undergone lumbar spinal fusion of 3 or fewer segments to treat degenerative lumbar disease was identified. The mean age at index operation was 53 years, and the mean follow-up period was 51 months (12–236 mo). The number of patients treated by PLF and PLIF were 103 and 387, respectively. The incidence and prevalence of revision surgery for ASD were calculated by Kaplan-Meier method. For risk factor analysis, we used log-rank test and Cox regression analysis with fusion methods, sex, age, number of fused segments, and presence of laminectomy adjacent to index fusion.
From the *Department of Orthopaedic Surgery, Soonchunhyang University Seoul Hospital, Seoul, Korea; †Department of Statistics, Seoul National University, Seoul, Korea; and ‡Department of Orthopaedic Surgery, Soonchunhyang University Cheonan Hospital, Cheonan, Korea. Acknowledgment date: August 26, 2013. First revision date: November 12, 2013. Second revision date: November 28, 2013. Acceptance date: December 8, 2013. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No relevant financial activities outside the submitted work. Address correspondence and reprint requests to Byung-Joon Shin, MD, Department of Orthopaedic Surgery, Soonchunhyang University Seoul Hospital, 657 Hannam-dong, Yongsan-gu, Seoul 140-743, Korea; E-mail: [email protected] DOI: 10.1097/BRS.0000000000000164 Spine
Results. After index spinal fusion, 23 patients (4.7%) had undergone additional surgery for ASD. Kaplan-Meier analysis predicted a disease-free survival rate of adjacent segments in 94.2% of patients at 5 years and 89.6% at 10 years after the index operation. In the analysis of risk factors, PLIF was associated with 3.4 times higher incidence of ASD requiring surgery than PLF (P = 0.037). Patients older than 60 years at the time of index operation were 2.5 times more likely to undergo revision operation than those younger than 60 years (P = 0.038). There were no significant differences in survival rates of the adjacent segment according to sex, preoperative diagnosis, number of fused segments, and concomitant laminectomy to adjacent segment. Conclusion. It was predicted that 10% of patients would undergo additional surgery for treating ASD within 10 years after index lumbar fusion. In this study, PLIF showed higher incidence of ASD than did PLF. Patient age greater than 60 years was another independent risk factor. Surgeons should carefully consider these factors at the time of surgical planning of lumbar fusion. Key words: adjacent segment disease, lumbar spinal fusion, risk factors, posterolateral fusion, posterior lumbar interbody fusion. Level of Evidence: 3 Spine 2014;39:E339–E345
R
ecently, the use of spinal fusion has become more frequent in the treatment of various lumbar diseases, but patients may experience secondary premature degeneration of adjacent segments and subsequent operation.1–4 For decades, posterolateral fusion (PLF) has been used widely in the treatment of the degenerative lumbar disease, and more recently, posterior lumbar interbody fusion (PLIF) has been gaining popularity because of biomechanical stability and high rates of successful fusion.5–9 Biomechanically, PLIF has been reported to be more rigid than PLF, and therefore, there has been some concern that PLIF may cause increased incidence of adjacent segment disease (ASD). To the best of our knowledge, however, no study has verified any difference in the incidence of ASD between the 2 fusion methods. Numerous studies have looked for the prevalence and risk factors of ASD after lumbar spinal fusion, but these studies have reported contradicting results.4 Insufficient follow-up rate was a practical concern in these studies and more recent www.spinejournal.com
E339
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. SPINE131009.indd E339
2/11/14 8:06 AM
CLINICAL CASE SERIES studies have used Kaplan-Meier survival analysis taking into account patients who were lost to follow-up.10–12 Cox regression analysis has been also used in risk factor analysis to control confounding variables.10,11 In this study, the authors have tried to estimate the true prevalence and incidence of ASD after lumbar spinal fusion using Kaplan-Meier analysis and to assess the risk factors for ASD by Cox regression analysis with special emphasis on comparison of PLIF versus PLF.
MATERIALS AND METHODS Study Population A single-institutional retrospective review of a period between August 1988 and December 2007 was performed and 490 consecutive patients who had undergone 3 or fewer segment lumbar spinal fusion supplemented by pedicle screw fixation were identified. All procedures had been performed by the senior author (B.J.S.) at a single institution. Five patients who had died within 1 year of index operation were excluded. Additionally, patients who had undergone subsequent anterior fusion or circumferential fusion were excluded from the study as well. None of the patients included in the analysis had received a spinal operation prior to the index procedure nor underwent the index procedure under the diagnosis of neoplasm, fracture, dislocation, or infection. Patients who underwent spinal fusion of more than 3 segments or who had a major deformity, such as scoliosis and kyphosis, were also excluded. Cases of pseudarthrosis were also excluded from the study. The study group consisted of 307 female and 183 male patients. The mean age at the time of index operation was 53 years (range, 20–81 yr), and the mean duration of followup was 51 months (range, 1–19 yr). PLF had been performed in 103 patients (21%), and PLIF in 387 patients (79%). In all patients receiving PLF, autogenous cancellous iliac bone was used as graft. PLIF was performed using autogenous iliac-crest strut graft in 114 patients between 1988 and 1999, and using 2 rectangular titanium cages filled with morselized local bone from neural decompression in 273 patients from 2000 to 2007. Single-segment fusion was performed in 321 patients (66%), 2-segment fusion in 147 patients (30%), and 3-segment fusion in 22 patients (4%). As dictated by the inclusion criteria, the surgical indication for index posterior lumbar fusion was severe low back pain and/or radiculopathy resistant to conservative treatment. In each patient, the index fusion had been performed for those segments that corresponded to clinical and radiological findings of neural compressive degenerative lesion. The etiological conditions were spinal stenosis in 245 patients, degenerative spondylolisthesis in 90, spondylolytic spondylolisthesis in 70, herniated intervertebral disc with segmental instability and/ or advanced disc degeneration in 65, and degenerative disc disease associated severe back pain in 20.
METHODS Hospital records, office charts, and radiographs were reviewed. Follow-up data, including follow-up length, survival period, E340
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Risk Factor Analysis of Adjacent Segment Disease • Lee et al
and history of subsequent operation for ASD, were searched for and recorded. Adjacent segments were defined as 2 cephalad and 2 caudad lumbar motion segments above and below the uppermost and lowermost levels of spinal fusion, respectively.10 A patient was considered to have ASD if the adjacent segments manifested degenerative lesions, such as spinal stenosis, radiculopathy, segmental instability, or deformity, causing clinically significant symptoms and requiring surgery. Clinical outcome was assessed at the final follow-up visit using Macnab criteria.13 Potential risk factors such as age, sex, preoperative diagnosis, fusion method (PLIF vs. PLF) at index operation, number of segments fused, and the presence of laminectomy adjacent to fusion were also reviewed in the medical records. For those patients who underwent revision surgery for ASD, the radiographs from the index surgery and at revision surgery were assessed by 2 spine surgeons (J.C.L and J.W.S.). For each patient, mean disc height was determined as an average value of anterior and posterior disc height. The relative disc height was calculated as a percentage of mean disc height to anterior body height of upper adjacent vertebral body. This was done to minimize magnification variance of each x-ray exposure condition. For assessment of intervertebral disc degeneration, the University of California Los Angeles scoring system was used.10
Statistical Analysis The annual incidence and prevalence of ASD requiring revision surgery were calculated by life-table methods, with the construction of a Kaplan-Meier survivorship curve. The annual incidence and prevalence of ASD requiring revision surgery was defined the same as previous reports.10–12 In risk factor analysis, a log-rank test was used for univariate analysis, and multivariate Cox proportional-hazards model was also performed to adjust for confounding variables. In both analyses, we verified the differences in survivorship according to variables such as fusion method, age, sex, number of segments fused, preoperative diagnosis, and presence of laminectomy adjacent to fusion. Interclass correlation coefficient was used to assess interobserver agreement for relative disc height measurement, and κ statistics was performed to assess that of disc degeneration grading. A paired t test was used to assess statistical significance of change in relative disc height, and Wilcoxon matched-pairs signed-rank test was performed to assess that of disc degeneration grade changes. SPSS 14.0 statistical software (SPSS, Chicago, IL) was used for the analyses, and a P value of less than 0.05 was considered statistically significant.
RESULTS Among the 490 patients, 24 patients required subsequent surgical management for clinically significant ASD for an overall prevalence of 4.9%. Of those, 15 patients underwent surgery for the treatment of upper ASD, 8 patients for lower ASD, and 1 patient for both upper and lower ASD. Causes of the revision surgery on adjacent segments were spinal stenosis March 2014
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. SPINE131009.indd E340
2/11/14 8:06 AM
CLINICAL CASE SERIES
Risk Factor Analysis of Adjacent Segment Disease • Lee et al
in 14 patients (12 central and 2 foraminal), herniated intervertebral disc in 8, and degenerative spondylolisthesis in 2. The choice of surgical intervention was extension of fusion in 15 patients, decompressive laminectomy without fusion extension in 1 patient, and discectomy in 8 patients. Clinical outcomes were considered excellent in 4 patients, good in 9, fair in 3, and poor in 8 patients. This represents 54% of these patients who experienced excellent or good functional outcome among patients who underwent secondary surgery due to ASD. This compares with the patients who did not develop adjacent segment disease requiring surgery, among whom outcome was excellent in 211 patients, good in 176, fair in 61, and poor in 18, for 83% of these patients having excellent or good functional outcome. Thus, the final clinical outcome was significantly worse for the patients who underwent secondary operation for ASD than for those patients who did not develop ASD (P < 0.001).
Incidence of ASD The annual incidence of ASD requiring surgery was relatively constant at 1.2% (95% confidence interval [CI], 0.8–1.7) for 10 years after the index spinal fusion (Table 1). We used Kaplan-Meier survivorship analysis to include all available data to the last follow-up point and predicted disease-free survivorship of the adjacent segments. Predicted survivorship of the adjacent segments was 94.2% (95% CI, 91.0–97.4) at postoperative 5 years and 89.6% (95% CI, 84.0–95.2) at 10 years (Figure 1). In other words, probability of undergoing a revision surgery for adjacent segment disease was 5.8% (95% CI, 2.6–9.0) at postoperative 5 years and 10.4% (95% CI, 4.8–16.0) at 10 years.
Risk Factor Analysis In the risk factor analysis, PLIF shows significantly lower survival rate than PLF in univariate analysis by log-rank test (P = 0.048). The incidence of ASD requiring surgery after
Figure 1. Kaplan-Meier survivorship curve. Survival time was scaled in months.
PLIF was 11.7% (95% CI, 4.7–18.3) at 10 years, but the incidence after PLF was significantly lower with 6.7% (95% CI, 2.6–10.8) at 10 years (Figure 2). Patients who were 60 years or older (≥60 yr) at the time of index surgery experienced significantly lower survival rate of adjacent segments than those who were younger than 60 years (P = 0.034). Prevalence of ASD requiring surgery among patients older than 60 years was 21.9% (95% CI, 0–39.4) at 10 years. These were significantly lower for patients younger than 60 years with 7.3% (95% CI, 2.5– 11.9) at 10 year (Figure 3). Other than age or the initial method of fusion (PLIF vs. PLF), no statistical significance was found for potential risk factor variables such as sex (P = 0.159), preoperative diagnosis (P = 0.274), number of segments fused (P = 0.796), and presence of laminectomy adjacent to fusion (P = 0.923). The results of univariate analysis were supported by multivariate Cox regression analysis (Table 2). In the analysis of possible risk factors, patients treated by PLIF experienced
TABLE 1. Incidence of Adjacent Segment Disease Requiring Surgery by Hazard Function Month
No. of Patients Entering Interval
No. of Patients Withdrawing
No. of Terminal Events
Incidence (%)
95% CI
12–24
490
151
5
1.3
1.1–1.6
24–36
334
97
4
1.6
1.2–2.0
36–48
233
53
2
1.1
0.8–1.4
48–60
178
36
3
2.0
1.3–3.0
60–72
139
25
0
0
0
72–84
114
19
2
2.0
1.2–3.5
84–96
93
16
0
0
0
96–108
77
8
1
1.4
0.8–2.4
108–120
68
14
1
1.7
1.0–3.1
1.2
0.8–1.8
Average annual incidence CI indicates confidence interval.
Spine
www.spinejournal.com
E341
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. SPINE131009.indd E341
2/11/14 8:06 AM
CLINICAL CASE SERIES
Risk Factor Analysis of Adjacent Segment Disease • Lee et al
and 0.762 at revision surgery, which also showed good agreement. Among the patients with ASD who underwent revision, the initial disc degeneration of adjacent segments were grade 1 in 12 adjacent segments, grade 2 in 5, grade 3 in 4, and grade 4 in 3. At the time of revision surgery, the patients had grade 1 in 3 adjacent segments, grade 2 in 11, grade 3 in 5, and grade 4 in 5. In all, the disc degeneration had worsened in 10 adjacent segments (1-grade increase in 7; 2-grade increase in 2; and 3-grade increase in 1). The changes of disc degeneration grading were significant (P = 0.004).
DISCUSSION
Figure 2. Kaplan-Meier survivorship curve of PLF versus PLIF. PLF indicates posterolateral fusion; PLIF, posterior lumbar interbody fusion.
3.4 times (95% CI, 1.1–10.7; P = 0.037) higher incidence of ASD requiring surgery than patients who were treated by PLF. Likewise, patients who were 60 years or older experienced 2.5 times (95% CI, 1.1–6.1; P = 0.038) higher incidence of ASD than did patients who were younger than 60 years at the time of index surgery. Again, factors such as sex, preoperative diagnosis, number of segments fused, or presence of laminectomy adjacent to fusion did not show statistical significance in the multivariate analysis.
Radiographical Analysis In the radiographical analysis of the 24 patients who had undergone revision surgical procedures for ASD, interclass correlation coefficient of relative disc height measurement was 0.954 at the time of index surgery and 0.999 at revision surgery—a result that showed excellent agreement across the 2 observers. Relative disc height had significantly decreased from an average of 36% (range, 20%–55%) at the time of index surgery to an average of 31% (range, 3%–55%) at revision surgery (P = 0.024). Kappa coefficient of the disc degeneration grading between observers was 0.685 at index surgery
Figure 3. Kaplan-Meier survivorship curve of old (≥60 yr) versus young (
