Spinal arthrodesis is the current gold standard in surgical management of lumbar spinal instability,1–4 but it increases mechanical stress at the segments adjacent to the fusion level5,6 and might accelerate disc degeneration.7–9 Limitations and problems with spinal fusion have led some investigators to explore motion-preserving technologies including disc arthroplasty10–13 and posterior ligamentoplasty.14–17
These technologies may have a potential advantage to reduce the risk of adjacent-segment disease, but only a few studies have investigated the adjacent-segment morbidity after motion-preserving surgeries.18–20 Also, the methods of spinal fusion—posterior lumbar interbody fusion (PLIF) versus posterolateral lumbar fusion (PLF)—may affect the prevalence of adjacent-segment disease, but it remains unclear.
The objective of this study is to determine the prevalence and nature of adjacent-segment deterioration after single-level PLIF, PLF versus posterior ligamentoplasty.
From January 1997 to November 2004, a total of 224 consecutive patients had undergone single-level posterior L4-5 fusion or ligamentoplasty in our hospital; 218 patients (97.3%) were reviewed at a minimum 2-year follow-up. Degenerative scoliosis was excluded from the current investigation because the remaining adjacent-level alignment may affect the results. They were 91 males and 127 females with mean age of 62 years. The pathologies included degenerative spondylolisthesis in 185 patients, disc herniation in 13, isthmic spondylolisthesis in 10, spinal stenosis in 5, and foraminal stenosis in 5 patients. Preoperatively, all the patients had low back pain and sciatic symptoms and failed nonoperative treatment. Follow-up period was averaged 41 months (24 to 81 mo). All the fusion cases had undergone pedicle-screw fixation, and Brantigan carbon-fiber I/F cages (DePuy Acromed Corporation, Raynham, MA) had been used in PLIF. Ligamentoplasty had been performed using Graf artificial ligament stabilization system (SEM Co, Mountrouge, France). There were 78 cases of the PLIF group, 75 in the PLF group, and 65 in the Graf ligamentoplasty group. In the PLIF group, the pathologies were degenerative spondylolisthesis in 60 patients, isthmic spondylolisthesis in 9, disc herniation in 5, and foraminal stenosis in 4 patients. In the PLF group, the pathologies were degenerative spondylolisthesis in 68 patients, disc herniation in 5, isthmic spondylolisthesis in 1, and foraminal stenosis in 1 patient. In the Graf ligamentoplasty group, the pathologies were degenerative spondylolisthesis in 57 patients, spinal stenosis in 5, and disc herniation in 3 patients. Demographics including age at the surgery, follow-up period, and follow-up rate were not statistically different among the 3 groups (Table 1). Radiographic fusion rate was 96.1% (75/78 patients) in the PLIF group and 93.3% (70/75 patients) in the PLF group.
Segmental lordosis at the operative segment (L4-5) was radiographically evaluated at the final follow-up. Rate of kyphotic fusion/stabilization was also investigated. It was recorded whether or not decompression procedures were performed to the adjacent segments at the initial surgery. Facet tropism was evaluated by facet angle using preoperative computed tomographic scans. The facet angle was defined as an angle between midsagittal line and facet articular tangent line (Fig. 1).
Adjacent-segment disease was defined as radiculopathy associated with newly developed pathologies at L1-2, L2-3, L3-4, and L5-S1. It was diagnosed on the basis of clinical presentation, not only radiographic findings. “Asymptomatic” adjacent-segment pathologies were not included. Low back pain also was not a form of adjacent-segment disease in this study.
Prevalence of adjacent-segment disease, time to represent symptom, and the rate for requiring additional surgery were compared between the 3 groups. Pathologies of adjacent-segment lesions and the type of additional surgery required were also investigated.
The prevalence of adjacent-segment disease and the rate of additional surgery were statistically compared between the groups using χ2 test. Postoperative segmental lordosis and time to represent adjacent-segment disease were analyzed among the 3 groups using 1-way analysis of variance (ANOVA) and Fisher PLSD.
Postoperative Spinal Alignment
Radiographic assessment at the finial follow-up showed that segmental lordosis was averaged 11.2 degrees in the PLIF group, 14.6 degrees in the PLF group, and 14.5 degrees in the Graf ligamentoplasty group. Kyphotic fusion was found in 1.4% of all fusion cases (1/78 patients in PLIF and 1/75 patients in PLF); no cases in the Graf ligamentoplasty group showed kyphotic stabilization. Although the rate of kyphotic fusion/stabilization was minimal and not statistically different, postoperative segmental lordosis was significantly larger in PLF and Graf ligamentoplasty groups than PLIF group (P<0.05 in 1-way ANOVA and Fisher PLSD).
Adjacent-level Decompression and Facet Tropism
Multilevel laminoplasty was performed at the initial surgery in 22 patients in the PLIF group (28.2%), 23 patients in the PLF group (30.7%), and 18 patients in the Graf ligamentoplasty group (27.7%). In addition, 6 patients in the PLIF group (7.7%), 4 patients in the PLF group (5.3%), and 7 patients in the Graf ligamentoplasty group (10.8%) had undergone posterior herniotomy at the adjacent level. No patients had undergone laminectomy and/or facetectomy. Thus, the rate of adjacent-level decompression was equivalent among the 3 groups.
Facet tropism at the L1/2, L2/3, L3/4, and L5/S1 was listed in Table 2; there were no statistical difference among the 3 groups.
Prevalence of Adjacent-segment Disease
Prevalence of adjacent-segment disease was 14.1% (11/78 patients) in the PLIF group, 13.3% (10/75 patients) in the PLF group, and 9.2% (6/65 patients) in the Graf ligamentoplasty (Fig. 2). Although the prevalence of adjacent-segment disease seemed to be lower in the Graf ligamentoplasty group than the fusion groups, the difference was not significant (P>0.05 in χ2 test).
The time to represent adjacent-segment disease was averaged 25.2 postoperative months in the PLIF group, 39.3 months in the PLF group, and 51.8 months in the Graf ligamentoplasty group. Adjacent-segment disease related to Graf ligamentoplasty occurred significantly later than the fusion surgeries (P<0.05 in 1-way ANOVA and Fisher PLSD). There was no statistical difference between the PLIF and PLF groups.
Of those with adjacent-segment morbidity, 5 in the PLIF group (45.5%), 2 in the PLF group (20.0%), and 1 in the Graf ligamentoplasty group (16.6%) developed instability at the adjacent levels. Adjacent-segment disease after PLIF highly accompanied instability than PLF and Graf ligamentoplasty.
Surgery for Adjacent-segment Disease
Six cases in the PLIF group (7.6%), 5 in the PLF group (6.7%), and 1 in the Graf ligamentoplasty group (1.5%) finally underwent additional surgeries for adjacent-segment pathologies (Fig. 2). Although the rate of additional surgery seemed to be lower in the Graf ligamentoplasty group than the fusion groups, the difference was not significant (P>0.05 in χ2 test).
Regarding the type of required surgery, 4 of 6 PLIF cases (66.7%) required supplemental fusion for progression of adjacent-segment instability (Fig. 3), whereas all the PLF cases needed only decompression surgeries (Fig. 4).
Clinically, the results of several long-term follow-up studies have suggested that spinal fusion might cause deterioration of the adjacent segment.7–9 Lehmann et al8 reviewed the data obtained in 33 patients with a minimum 21-year follow-up after posterior spinal fusion, and reported a 30% incidence of spinal stenosis that developed immediately above the fusion site. Leong et al9 also reported that, in a study of 40 patients with a minimum 10-year follow-up after anterior spinal fusion, radiographic evidence of disc degeneration adjacent to the fused segment was demonstrated in 52.5% of the patients. In the literature review by Park et al,21 adjacent-segment degeneration was observed radiographically in 5.2% to 100% of patients who underwent spinal arthrodesis, and 5.2% to 18.5% of the patients eventually required surgical interventions. Thus, accelerated degenerative change of motion segments above or below spinal fusion site has been a concern of spine surgeons for many years.
Biomechanical alterations likely play a primary role in causing adjacent-segment morbidity after spinal fusion. Cunningham et al5 developed a human cadaveric biomechanical model, showing that rigid spinal instrumentation increased proximal adjacent disc pressure up to 45% on axial compression and flexion loading. Motion-preserving technologies have been explored and developed to reduce such mechanical stress at the adjacent segment and avoid adverse events related to spinal fusion.22,23
Recently, several long-term follow-up results of motion-preserving surgeries were published. Gardner and Pande24 reported in a 7.4-year follow-up study that the longevity of Graf ligamentoplasty was acceptable despite the presence of an established degenerative process. Markwalder and Wenger25 also demonstrated favorable long-term results of Graf ligamentoplasty, but did not show the prevalence of adjacent-segment morbidity. Potential advantages of motion-preserving technologies included minimizing the risk of adjacent-segment morbidity, but it remains unclear how they contribute to circumvent adjacent-segment disease.
The current study investigated adjacent-segment morbidity after spinal fusion and motion-preserving surgery. Prevalence of adjacent-segment morbidity and reoperation rate seemed to be lower in the posterior ligamentoplasty than fusion surgeries, but the difference was not significant. Posterior ligamentoplasty circumvented adjacent-segment disease for longer period than fusion surgeries. Thus, ligamentoplasty favored reducing the risk of adjacent-segment deterioration and was effective as a time-saving procedure.
PLIF developed adjacent-segment instability more frequently compared with PLF; the rate for requiring supplemental fusion was significantly higher. Thus, the addition of interbody fusion with cages should be another risk for the early development of adjacent-segment disease. Specifically, the interval of occurrence of adjacent-segment disease after PLIF was shorter than PLF. From biomechanical standpoint, the immediate rigidity produced by PLIF may cause more stress leading to accelerated degeneration at neighboring levels. Sudo et al26 investigated the effect of interbody cage on the stress distribution at the adjacent segment using a calf spine model. They demonstrated that PLIF or interbody cage insertion created higher intradiscal pressure at the superior adjacent segment when compared with PLF.
Wang et al27 commented in the literature review article that surgical success of spinal fusion depends on appropriate patient selection. They recommended lumbar interbody fusion for those who had risk factors of pseudarthrosis (tobacco use, rheumatologic disease, diabetes, etc.), axial load-bearing pain, need to restore disc space height, and need to correct coronal/sagittal imbalance; posterolateral fusion was indicated for those who had no risk factors for pseudarthrosis, osteopenia, grade III/IV spondylolisthesis, and alignment/balance preserved. Also, Kanayama et al28 examined lumbar instability through an intraoperative biomechanical measurement, and suggested that segmental kyphosis in flexion was the key parameter predicting anterior column insufficiency. On the basis of the results of the current study, surgical indication for PLIF should be limited to the cases with anterior column insufficiency to reduce the risk of adjacent-segment disease.
Several limitations of the current study should be addressed. First, this is a retrospective comparative study and different surgical indications were applied for each surgical procedure. We performed Graf ligament stabilization for flexion instability or degenerative spondylolisthesis with preserved disc height and coronally oriented facet articulation. PLIF was indicated for the patients who required total facetectomy, or those who had severe spinal instability with anterior column insufficiency. PLF was performed for the remaining cases who had spinal instability. Appropriate surgical indication resulted in favorable clinical outcome in each surgical procedure.15,17,29 Thus, the pathologies at the operative segment were different among the 3 groups. However, the demographic features had no difference among the 3 groups (Table 1). With regards to preoperative adjacent-segment status, no statistical differences were observed as listed in Table 2. Therefore, regarding preoperative adjacent-segment condition, we believe the bias was minimal among the 3 groups. Second, averaged 41-month follow-up period may be relatively short for assessment of adjacent-segment morbidity. Further follow-up investigation is required to report the long-term results of adjacent-segment pathologies.
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Keywords:© 2009 Lippincott Williams & Wilkins, Inc.
posterior dynamic stabilization; Graf ligamentoplasty; posterior lumbar interbody fusion; posterolateral lumbar fusion; adjacent segment