He, Baorong MD; Yan, Liang MD; Guo, Hua MD; Liu, Tuanjiang MD; Wang, Xiaodong MD; Hao, Dingjun MD
Although efforts to preserve segmental motion during spinal surgery are now increasing, spinal fusion remains a standard method of surgical treatment of deformity, trauma, and degenerative disorders. Many previous studies have reported excellent surgical results with instrumented lumbar fusion.1,2 In contrast, numerous complications and problems related to instrumented fusion surgery have also been reported. Among these complications, the incidence of radiographical adjacent segment degeneration (ASD) has been reported to range from 8% to as high as 100% with symptomatic disease as 36% of cases, and most degenerative change occurred in the proximal segment to the fusion level.3–5 Abnormal loading and increased mobility in adjacent segments may explain the development of ASD, but it is still unclear whether it is caused by fusion sequelae or whether it is the result of natural degeneration. There have also been controversies about the exact incidence of ASD and its risk factors.6–8 Among the various relevant factors, the insertion angle and position of pedicle screw may be the important factor associated with ASD. Therefore, the purpose of this prospective randomized study was to test the hypothesis that different pedicle screw insertion positions would increase the likelihood of superior ASD.
MATERIALS AND METHODS
This prospective study included 210 patients with low-grade isthmic spondylolisthesis. From January 1999 to December 2003, patients were randomized underwent posterolateral fusion (PLF) using 2 different pedicle screw insertion positions. The study group was composed of 88 males (41.9%) and 122 females (58.1%), with a mean age of 45.5 ± 7.6 years (range, 19–54 yr). In terms of symptoms, 47 patients (22.4%) had predominantly back pain, 36 (17.1%) had predominantly radiculopathy, and 127 (60.5%) had both back pain and radiculopathy. The affected levels were L4–L5 in 28 patients (13.3%) and L5–S1 in 182 patients (86.7%). This study was approved by the Institutional Review Boards and the Ethics Committees of Hong Hui Hospital, Xi'an Jiaotong University.
The inclusion criteria were the presence of single-level unstable Meyerding grade 1 or 2 isthmic spondylolisthesis, chronic and persistent radiculopathy despite conservative treatment, progressive neurological deficits, persistent and unremitting low back pain for more than 6 months, loss of quality of life because of neurological claudication, 18 to 55 years of age, and a minimum follow-up period of 48 months. Exclusion criteria were as follows: revision surgery, grade 3 or more spondylolisthesis, concomitant scoliosis of more than 15°, patients who underwent either a discectomy or an interbody fusion during initial operation, and patients who underwent implant removal during the follow-up period.
After the patient had given consent, a nurse at the outpatient ward blindly chose 1 of 2 different notes indicating 1 of the 2 different pedicle screw insertion methods. In all patients, PLF and pedicle screw fixation were performed. After a midline surgical exposure, the soft tissues were dissected to the lateral tips of the transverse processes. Exposure for the superior facet joint required detachment of muscular insertions from the facet capsule, but care was taken to preserve the capsule itself. For the method of Du and Zhao,9 the accessory process and the peak of the “Λ”-shaped crest were exposed, but it was not necessary for Magerl's method.10 Decompressive surgery consisted of removal of the spinous process and bilateral laminectomy, and partial bilateral foraminotomy was performed prior to screw insertion and reduction maneuvers. Cancellous bone obtained from the posterior arch was packed over the decorticated surface of the transverse and articular processes. The patients were divided into 2 groups according to different pedicle screw insertion positions: group A (the method of Du and Zhao) with an entry point of where the pars interarticularis converges at the accessory process, described as a “Λ”-shaped crest, and group B (Magerl's method) using a starting point at the lateral border of the superior facet where it intersects the midportion of the transverse process (Figures 1 and 2).
Standard anteroposterior, lateral, dynamic lumbar radiographs, computed tomographic (CT) scans, and magnetic resonance (MR) images were available for all patients. The disc height was measured by the procedures of Miyakoshi et al.11 Disc degeneration on magnetic resonance imaging was rated from grades 1 to 5 using the classification system of Pfirrmann et al,12 and facet joint degeneration was rated from grades 1 to 4 according to the criteria of Weishaupt et al.13 Bone fusion was assessed on CT reconstruction scans and/or dynamic lumbar radiographs. Two orthopedic spine surgeons, who were not involved with the operation and blinded to all clinical information, performed radiological measurement using picture archiving and communication system (PACS; m-view 5.4; Marotec Medical System, Seoul, Korea). The measurements were performed twice for each parameter with an adequate time interval to prevent bias from distorting the results. To minimize the intraobserver error, the average values were used for statistical analyses. The average values obtained preoperatively, immediately after surgery, and at the last follow-up were compared and examined for any possible associations between the radiographical parameters. Radiographical ASD was diagnosed when plain radiographs, CT scans, or MR images demonstrated one or more of the following lesions at the segment adjacent to the fused segment that were not present preoperatively: more than 4 mm of anterolisthesis or retrolisthesis, more than 10° of angular motion between adjacent vertebral bodies, more than 50% loss of disc height, or more than 1 grade advancement of facet joint degeneration or disc degeneration or spinal canal stenosis on magnetic resonance imaging.
The Oswestry Disability Index (ODI) and visual analogue scale scores of low back pain and radiating pain, which were measured preoperatively, immediately after surgery, and at the last follow-up, were evaluated to assess the clinical outcomes. Diagnosis of clinical ASD was based on the presence of instability, radiculopathy, or spinal stenosis that was symptomatic enough for the patient to choose revision surgery at the adjacent segment.
Statistical analysis was performed using SPSS 17.0 statistical package (SPSS, Inc., Chicago, IL). The independent t test and χ2 test were performed to compare demographic, radiological, and clinical data between the 2 groups, and P value of less than 0.05 was accepted as significant.
Characteristics of the Patients
During the study period, 210 patients underwent PLF and pedicle screw fixations at our institution, 102 patients (48.6%) with group A and 108 (51.4%) with group B. No major surgery-related complications occurred, including wound infection, additional neurological dysfunction, or hardware failure. Ultimately, 178 of 210 (84.7%) patients were available for at least 9-year radiological and clinical follow-up data: 85.3% (87/102) patients in group A and 84.3% (91/108) patients in group B. There were no differences between the 2 groups for ages, sex, follow-up period, body mass index, clinical symptoms, and fusion level (Table 1).
Bone fusion was achieved in all patients at the last follow-up. ASD was proven in 110 (61.8%) of 178 patients. The incidences of radiographical and symptomatic ASD were 57.9% (103/178) and 3.9% (7/178), respectively. In terms of the fusion level, ASD after PLF at L4–L5 was observed in 4 of 22 patients (18.2%), and ASD after PLF at L5–S1 was observed in 106 of 156 patients (67.9%): the difference was significant (P < 0.001). ASD was noted in 63 (72.4%) of 87 patients in group A and 47 (51.6%) of 91 patients in group B, and the difference was significant (P = 0.004). ASD was diagnosed at a mean time of 62.8 ± 19.6 months after surgery.
The clinical and functional outcomes are summarized in Table 2. At the last follow-up, the ODI and the visual analogue scale scores demonstrated statistically significant differences between the preoperative and postoperative periods (P < 0.001). Postoperative ODI and visual analogue scale scores were not different between the ASD and non-ASD groups, nor were they different between the group A and the group B (Table 3). However, the difference was significant when ODI scores were compared in patients with ASD (P < 0.001) (Table 4). Clinical ASD occurred in 7 (8.0%) of 87 patients in group A and 0 in group B, and the occurrence of clinical ASD between groups was significantly different (P < 0.001).
Lumbar fusion surgery is a widely accepted treatment of lumbar diseases, such as lumbar stenosis, trauma, tumor, and spondylolisthesis. Fusion and clinical success rates have increased because of improvements in instrumentation and bone graft material. In contrast, numerous complications and problems of fusion surgery have been reported, with ASD being one of the most important.1,3–5,7 Hilibrand and Robbins14 used the term “adjacent segment degeneration” to describe radiographical changes seen at levels adjacent to a previous spinal fusion procedure. Clinical ASD has been used to designate the development of new clinical symptoms that are compatible with radiographical changes adjacent to the level of a previous spinal fusion. Although studies on the adjacent segment problem are now in active progress, most of them have utilized radiographical findings as a diagnostic tool rather than for symptoms.6,15 Many investigators have reported the degeneration at segment adjacent to a fusion. Park et al16 reviewed the results of 56 studies and found that the incidence of radiographical ASD ranged from 8% to 100%. Ghiselli et al17 reported that the rate of symptomatic ASD after posterior lumbar arthrodesis was predicted to be 16.5% at 5 years and 36.1% at 10 years. Sears et al18 reported that the mean annual incidence of symptomatic ASD was 2.5% in the first 10 years. There are few reports of the incidence of ASD requiring revision surgery. Aiki et al19 performed reoperations for symptomatic adjacent segment stenosis in 7.7% of patients who had undergone posterior lumbar fusion after at least a 2-year follow-up. Gillet20 reported that 20% of PLF patients needed a second operation for adjacent segment alteration after a minimum of 5 years. In this study, the incidences of radiographical and symptomatic ASD were 57.9% (103/178) and 3.9% (7/178), respectively, as close to many previous reports.
Many efforts have been made to find risk factors for ASD in order to predict and prevent this condition. These factors can be divided into 2 categories: patient factors, which are usually beyond the surgeon's control, and surgical factors, which can often be modified during the time of operation.5,7 The most commonly proposed patient factors include age, sex, obesity, menopause, and bone mineral density. Proposed surgical factors have included type of fusion, fusion length, instrumentation, coronal or sagittal alignment, disruption of facet joint (either during exposure or by the instrumentation), and loss of lumbar lordosis. ASD may be a time-related process. The interval of the follow-up period is an important factor. The longer the follow-up period is, the greater amount of ASD that develops. Because of prospective designed and small number of enrolled patients, there were no differences between the 2 groups of the patient-related factors. Among the surgical factors, authors chose to evaluate the presence of pedicle screw insertion position, because their significance has not yet been fully determined, and some studies also demonstrated that pedicle screw created greater mechanical stress at adjacent segment even after fusion.21,22 Most of the studies have reported that degenerative change occurred in the proximal segment to the fusion level. Accordingly, in this study, the superior segment to the fusion level was solely examined.3,8,13
There are a number of techniques described to determine the position to insert the lumbar pedicle screw. In this study, the 2 methods we chose are often discussed and used (Du and Magerl).9,10 Du and Zhao (group A) suggests that the entry point for inserting a pedicle screw should be based on a “Λ”-shaped crest. Magerl (group B) suggests that the starting point should be based on the anatomic relation of the facet joint and transverse process. Magerl's entry point is further lateral, located at the nape of the neck of the superior articular process. The insertion position, relative to the central portion and direction of the pedicle, leads to different angles of insertion of the screws. Some may have a straight trajectory down the pedicle, and others may require steeper angles for insertion. Different positions and angles of insertion can directly violate the facet joint or increase the corresponding facet joint contact force and disc stress. In this study, radiographical ASD was noted in 63 (72.4%) of 87 patients in group A and 47 (51.6%) of 91 patients in group B, and all clinical ASDs occurred in 7 (8.0%) of 87 patients in group A. Therefore, this study can be a step forward, demonstrating that degeneration of adjacent segment is closely related to the position of the pedicle screws during lumbar fusion surgery.
In this study, bone fusion was achieved in all patients at the last follow-up and no major surgery-related complications occurred, including wound infection or hardware failure. The reasons may include the following aspects: first, the patients in this series were of younger age. Second, enough cancellous bone was packed over the decorticated surface of the transverse and articular processes. Third, recombinant human bone morphogenetic protein-2 (rhBMP-2; Medtronic Sofamor Danek, Memphis, TN) was reconstituted according to the package labeling instructions and combined with local autograft in most cases. Finally, postoperative external immobilization was used for 12 weeks.
There were some limitations in this study. First, we did not have a control group that underwent conservative treatment, and there were a small number of enrolled patients. Second, some patients of solid fusion were assessed by dynamic lumbar radiographs and not by CT reconstruction scans. This could confound the results because if patients had a stable pseudarthrosis, ASD may not develop. Third, although this study included patients with a single homogeneous disease, there remained some heterogeneity in patient makeup with regard to the procedure performed. Finally, there is no consensus in assessing radiological or clinical ASD. Therefore, radiological and clinical results related with ASD can be greatly influenced by the definition of ASD.
In conclusion, our more than 9-year follow-up results indicate that the degeneration of superior adjacent segment is closely related to the position of the pedicle screws during lumbar fusion surgery. The position of the pedicle screw farther from the facet joint surface can reduce the degeneration of superior adjacent segment. Therefore, choosing the proper entry point is necessary during superior segment pedicle screw placement.
* Numerous complications and problems of lumbar fusion surgery have been reported, with ASD being one of the most important.
* Many efforts have been made to find risk factors for ASD in order to predict and prevent this condition.
* The position of the pedicle screw farther from the facet joint surface can reduce the degeneration of superior adjacent segment.
1. Harrop JS, Youssef JA, Maltenfort M, et al. Lumbar adjacent segment degeneration and disease after arthrodesis and total disc arthroplasty. Spine 2008;33:1701–7.
2. Kanayama M, Togawa D, Hashimoto T, et al. Motion-preserving surgery can prevent early breakdown of adjacent segments: comparison of posterior dynamic stabilization with spinal fusion. J Spinal Disord Tech 2009;22:463–7.
3. Leone A, Guglielmi G, Cassar-Pullicino VN, et al. Lumbar intervertebral instability: a review. Radiology 2007;245:62–77.
4. Ha KY, Son JM, Im JH, et al. Risk factors for adjacent segment degeneration after surgical correction of degenerative scoliosis. Indian J Orthop 2013;47:346–51.
5. Levin DA, Hale J, Bendo JA. Adjacent segment degeneration following spinal fusion for degenerative disc disease. Bull NYU Hosp Jt Dis 2007;65:29–36.
6. Gard AP, Klopper HB, Doran SE, et al. Analysis of adjacent segment degeneration with laminectomy above a fused lumbar segment. J Clin Neurosci 2013;20:1554–7.
7. Pellis F, Hernndez A, Vidal X, et al. Radiologic assessment of all unfused lumbar segments 7.5 years after instrumented posterior spinal fusion. Spine 2007;32:574–9.
8. Schmidt H, Galbusera F, Rohlmann A, et al. Effect of multilevel lumbar disc arthroplasty on spine kinematics and facet joint loads in flexion and extension: a finite element analysis. Eur Spine J 2012;21(suppl 5):663–74.
9. Du X, Zhao L. Anatomical study of the adjacent structures to the top point of the “Λ” shape crest and its relevance. Chin J Spine Spinal Cord 2001;11:89–91.
10. Magerl FP. Stabilization of the lower thoracic and lumbar spine with external skeletal fixation. Clin Orthop Relat Res 1984;189:125–41.
11. Miyakoshi N, Abe E, Shimada Y, et al. Anterior decompression with single segmental spinal interbody fusion for lumbar burst fracture. Spine 1999;24:67–73.
12. Pfirrmann CWA, Metzdorf A, Zanetti M, et al. Magnetic resonance classification of lumbar intervertebral disc degeneration. Spine 2001;26:1873–8.
13. Weishaupt D, Zanetti M, Boos N, et al. MR imaging and CT in osteoarthritis of the lumbar facet joints. Skeletal Radiol 1999;28:215–9.
14. Hilibrand AS, Robbins M. Adjacent segment degeneration and adjacent segment disease: the consequences of spinal fusion? Spine J 2004;4(6 suppl):190S–4S.
15. Chen Z, Zhao J, Xu H, et al. Technical factors related to the incidence of adjacent superior segment facet joint violation after transpedicular instrumentation in the lumbar spine. Eur Spine J 2008;17:1476–80.
16. Park P, Garton HJ, Gala VC, et al. Adjacent segment disease after lumbar or lumbosacral fusion: review of the literature. Spine 2004;29:1938–44.
17. Ghiselli G, Wang JC, Bhatia N, et al. Adjacent segment degeneration in the lumbar spine. J Bone Joint Surg Am 2004;86-A: 1497–1503.
18. Sears WR, Sergides IG, Kazemi N, et al. Incidence and prevalence of surgery at segments adjacent to a previous posterior lumbar arthrodesis. Spine J 2011;11:11–20.
19. Aiki H, Ohwada O, Kobayashi H, et al. Adjacent segment stenosis after lumbar fusion requiring second operation. J Orthop Sci 2005;10:490–5.
20. Gillet P. The fate of the adjacent motion segments after lumbar fusion. J Spinal Disord Tech 2003;16:338–45.
21. Kim HJ, Kang KT, Moon SH, et al. The quantitative assessment of risk factors to overstress at adjacent segments after lumbar fusion: removal of posterior ligaments and pedicle screws. Spine 2011;36:1367–73.
22. Kim HJ, Chun HJ, Kang KT, et al. The biomechanical effect of pedicle screws' insertion angle and position on the superior adjacent segment in 1 segment lumbar fusion. Spine 2012;37:1637–44.
posterolateral fusion; adjacent segment degeneration; pedicle screw insertion; prospective