Two articles reporting on the same patient population at 2 different follow-up periods evaluated age and smoking status as potential patient-risk factors. Edwards et al39 after a 5.6-year follow-up, determined a statistically significant difference in age of patients who developed RASP versus those who did not develop RASP (40.8 vs. 48.0 yr, respectively; P = 0.03), suggesting that younger patients are at greater risk of degeneration. Kuhns et al41 reported on the same population after a 9-year follow-up and continued to find that younger patients were at greater risk for developing RASP than older patients (44.3 vs. 48.1 yr), although this was not statistically significant (P = 0.25). Edwards et al39 determined no significant difference with respect to smoking status and development of RASP (P = 0.76).
Edwards et al.39 and Kuhns et al41 also examined surgical risk factors for RASP. After a 9-year follow-up, Kuhns et al41 determined that patients who underwent longer fusions (T1–T7–L5) were 2.5 times more likely to develop RASP than patients who received shorter fusions (T8–T12–L5) (72% vs. 28%, respectively; P = 0.02). However, there was no significant difference (P = 0.17) in development of RASP based on shorter versus longer fusion in the study by Edwards etal.39 Although both the studies reported that patients who underwent a circumferential approach were more likely to develop RASP than patients who underwent a posterior approach alone, the difference was significant only in the study by Kuhns et al.41 Edwards et al39 also examined whether a deep-seated L5 was associated with RASP and determined no significant association (P = 0.40).
Multiple radiographical factors were identified as affecting the risk of developing RASP. Two studies reported that a preoperative disc degeneration of Grade 1 (mild degeneration) put the patient at a 2.5 to 3 times greater risk of developing moderate to advanced RASP (Grade 2 or 3) than patients with a preoperative disc degeneration Grade 0 (healthy-–no degeneration). Cho et al36 reported that 33% of Grade 0 patients and 85% of Grade 1 patients went on to develop moderate to advanced RASP (P = 0.136), whereas Edwards et al39 similarly determined that 27% of Grade 0 and 80% of Grade 1 patients went on to develop RASP (P = 0.007).
Four studies identified preoperative sagittal imbalance as a risk factor for developing moderate to advanced RASP after long TL fusions.35,36,39,41 Although all 4 studies reported a greater risk for developing RASP in patients with preoperative sagittal imbalance, only Edwards et al39 and Brown et al35 found this difference to be statistically significant (Tables 5 and 6). Cho et al36 and Kuhns et al41 reported a greater risk for developing RASP in patients with preoperative sagittal imbalance; however their findings were not statistically significant (73% vs. 62%; P = 0.679; and +3.2 vs. +1.3; P = 0.31, respectively).
Brown et al35 reported a statistically significant difference between postoperative L5–S1 disc space height between patients who developed RASP compared with those who did not (8.5 vs. 11 mm, respectively; P < 0.05), suggesting that narrowing of the L5–S1 disc space could present a greater risk for development of RASP.
To summarize, although younger age at TL fusion was reported to be associated with increased risk of RASP, the significance was lost at long-term follow-up. Preoperative disc degeneration was associated with higher chances of developing RASP in 2 studies; however, only 1 of them reported the difference to be significant. Preoperative sagittal imbalance was uniformly found to be associated with increased risk of developing RASP in 4 studies; only 2 of them found the difference to be significant. Only 1 postoperative parameter (postoperative L5–S1 disc space narrowing) was associated with increased risk of RASP.
Adult Versus Adolescent Populations
No studies meeting inclusion criteria were identified that reported on the frequency or risk factors of distal CASP that met our inclusion criteria.
Surgical Approach (TLIF Vs. ALIF)
No studies meeting inclusion criteria were identified that compared the effectiveness and safety of transforaminal lumbar interbody fusion versus anterior lumbar interbody fusion in the treatment of distal ASP after long TL fusion.
Treatment of Distal CASP
No studies meeting inclusion criteria were identified that reported on the treatment of distal CASP and whether to extend the fusion to the sacrum or pelvis.
Complications of Distal CASP Revision Surgery
No studies were identified that reported on the complications of distal CASP revision surgery that met our inclusion criteria.
The overall strength of evidence evaluating the frequency of distal clinical or RASP after long TL fusions is “low”; that is, we have low confidence in the absolute estimate and further research may change the estimate (Table 7). The overall strength of evidence evaluating risk factors for the development of CASP is “low” (low confidence in effect size) to “insufficient” (the evidence does not permit a conclusion). There was no evidence found to address key questions 3 to 6 (Table 7).
There have been a number of studies that have documented the occurrence of ASP after lumbar fusion.20,21,42 Most of these studies have focused on 2-level lumbar fusions and did not address the impact of long TL fusions, which are often performed for correction of spinal deformity. Adult spinal deformity often involves the lumbar spine and fusions that extend into the lower lumbar (L4 or L5) or sacral spine.2,4,13,17 Long fusions terminating in the distal lumbar spine can have resulting ASP and failure to maintain sagittal balance, likely due at least in part to subsequent L5–S1 disc degeneration and associated loss of disc space lordosis.36,38,39 Various studies have recommended extending long fusions, which would otherwise stop at L4–L5, to the sacrum to try to avoid this potential complication, but this remains controversial.22,38,39,43,44 Fusion to the L5 offers the theoretical benefits of preserved lumbosacral motion, smaller surgery, and a decreased likelihood of pseudarthrosis.43 On the contrary, the literature is also replete with poor results with use of S1 screws without distal augmentation, with fusion rates as low as 22%.45–48
The purpose of this systematic review was to determine the incidence and risk factors leading to distal ASP after performance of long TL instrumentation in adults and adolescents, and to compare the outcome of various approaches for management of distal ASP and associated complications. We initially sought studies that had reports of distal CASP/RASP after TL fusions for deformity in adults and adolescents. Finding no such study in the pediatric population, this systematic review is predominantly restricted to adults.
This review suggests that development of distal ASP after long TL fusion is not uncommon. CASP developed in 17.7% and 19.9% of patients after 2 to 6 years and 9-year mean follow-up, respectively. Reoperation due to CASP was reported in 15.6% of the patients after 2 to 6 years and 14.4% of patients after 9 years. RASP was more frequent, occurring in approximate 1 half of the patients.
The criteria for defining CASP and RASP were variable across different studies, which might account for differences in the reported incidences. For example, in the study by Brown et al,35 their definition of CASP required revision surgery, which likely underestimated the incidence of CASP because there may be patients who might be symptomatic but may not undergo revision surgery.35–41 Differences in definition for RASP between studies also likely impacted the reported incidences and comparisons across studies. Surprisingly, there was a decreased incidence of CASP in the study reporting 9 years of follow-up as compared with the study with 2 to 6 years of follow-up, although there was an increase in incidence of RASP, which exemplifies the complexity of patient-related factors and differences in the criteria used in different studies.35–41
There has been an increasing recognition of the importance of sagittal balance and clinical symptoms in adults with spinal deformity.1,3,4 Interestingly, patients with preoperative sagittal imbalance were 5 times more likely to develop CASP after long TL fusions.36 Patients with positive sagittal imbalance often have pain and disability, and it is possible that the increased risk of developing CASP in this population might partially reflect symptoms related to persistent positive sagittal balance rather than symptoms related to ASP. Studies that defined CASP based mainly on clinical factors and need of revision surgery may be most susceptible to this potential overlap of symptoms. However, in the study by Cho et al,36 most of the patients who had preoperative sagittal imbalance and developed CASP had restoration of their sagittal balance after surgery, suggesting that CASP can develop regardless of surgical correction of sagittal imbalance and may relate to preoperative sagittal imbalance. Apart from preoperative sagittal imbalance, unfavorable postoperative sagittal balance has been advocated as a risk factor for accelerated adjacent segment disease in in vitro studies.49,50 Whether coronal imbalance at the caudal level impacts the development of distal adjacent disease remains unclear due to a lack of literature relevant to this issue.
The presence of preoperative disc degeneration at the adjacent level has been suggested to increase the risk of subsequent ASP, which has been the basis for using magnetic resonance imaging and discography to decide whether to extend the fusion to the sacrum.43 However, magnetic resonance imaging assessment of hydration is more difficult for oblique discs and although discography has been suggested to reflect degeneration, the use of discography has been strongly debated, especially considering its potential to accelerate degeneration of normal healthy discs and its false positivity.51,52 Two studies in the present review demonstrated 2.5 to 3 times increased risk of RASP after long TL fusion in mild disc degeneration preoperatively as compared with patients with a healthy disc.36,39 However, in 1 of the 2 studies, there was no association between preoperative disc degeneration and symptomatic ASP (CASP). 36 Thus, although long fusions may lead to increased incidence of RASP in patients with mild preoperative disc degeneration, the clinical significance of this remains unclear. Hence, extension of fusion to the sacrum based on the presence of mild disc degeneration without presence of other predisposing factors for ASP is controversial.
There was an association between the development of CASP and higher postoperative fractional curve in the study by Brown et al.35 Considering the fact that presence of a lumbosacral fractional curve makes balancing the spine very difficult without extension to sacrum and that with an oblique takeoff, often the foramen is narrow on the concave side, the increased chances of CASP as reported by Brown et al35 in patients with higher postoperative fractional curve might be secondarily attributed to poor deformity correction or foraminal stenosis in the fractional curve, both of which can be clinically symptomatic.4,5,43 Interestingly, the diagnosis of CASP in their study was made on clinical criteria, rather than the need for revision surgery.
This review determined that patients who underwent a circumferential approach and received longer fusions (T1–T7–L5) were 2 to 2.5 times more likely to develop RASP than patients who underwent a posterior approach and shorter fusions, respectively. Although there is robust biomechanical evidence to support this, the clinical evidence is unclear, with studies documenting no risk versus increased risk of ASP after lumbar fusion, which precludes definitive conclusions.7,22,39,53–55 The clinical impact of this might be to consider extension of fusion to S1 if performing multilevel fusion or circumferential fusion at L4–L5 to prevent ASP at L5–S1. However, there is no strong clinical evidence to support or refute this approach.
Strengths of this study include the systematic approach to searching for and evaluating relevant studies to answer well-defined clinical questions. Combined with use of specified inclusion/exclusion criteria defined a priori, this approach enhances the validity of this report and facilitates identification of specific gaps in understanding the incidence, risk factors, and management of ASP after long TL instrumentation for spinal deformity.
The results of this review have several limitations. Although we were able to perform a thorough literature review, our conclusions are compromised somewhat due to the lack of studies addressing some of our key questions (key questions 4–6). This underscores the need for further studies to address these issues. Also, the included studies were all retrospective and are prone to the inherent limitations of retrospective design. The rate of ASP seen in this review is subject to bias secondary to variable follow-up times and varies according to the definition applied in different studies.35–41 Also, only 1 study identified the presence of preoperative sagittal imbalance as a risk factor for development of CASP.36 Although the differences between risk of developing CASP in patients with or without preoperative sagittal imbalance was striking in the study of Cho et al,36 the wide confidence interval in the study possibly indicates that the stability of the estimate may be questionable and is likely at least in part due to small sample size and it also being under powered to find a difference beyond chance precluding results from reaching statistical significance. The assessment of ASP in this review is restricted to disc degeneration and excludes other modes of distal junctional failure including pseudarthrosis at the most caudal fused level and implant failure with screw loosening/breakage/rod fracture at the caudal end of fixation, which might underestimate the overall risk of the distal construct failure and complications.27 Although there is typically no well-formed disc below S1 and thus essentially no chance of ASP, extension of fusions to the sacrum does not necessarily equate with improved outcomes and decreased complications, because the literature is replete with studies addressing poor outcome after TL fusion that include the sacrum.26,45–48 This should be considered when interpreting rates of distal ASP after long TL fusions.
Low-quality evidence suggests a cumulative rate of 18% to 20% for CASP and 45% to 65% for RASP after long TL fusion for spinal deformity during 9-year follow-up. Low-quality evidence suggests an association between preoperative sagittal imbalance and distal ASP with higher risk of distal ASP in patients with sagittal imbalance. Low-quality evidence suggests increased risk of CASP in patients with higher postoperative fractional curve and increased risk of RASP in younger patients and those with preoperative disc degeneration, longer fusions, circumferential procedures, and postoperative L5–S1 disc space narrowing.
- The risk of developing new symptoms secondary to distal ASP after long TL fusion for deformity is approximately 18% to 20% during a period of 9 years of follow-up, and most of these patients will require revision surgery.Strength of Statement: Weak
- The risk of developing distal ASP may be higher in those with preoperative sagittal imbalance, preoperative disc degeneration, longer fusions, circumferential procedures, and postoperative L5-S1 disc space narrowing.Strength of Statement: Weak
- Distal CASP developed in 17.7% at 2- to 6-year follow-up and 19.8% at 9-year follow-up, whereas reoperation due to CASP was reported in 15.6% at 2 to 6 years and 14.4% at 9 years.
- Distal RASP occurred in approximately one half of the patients (44.7%–65.5%).
- Preoperative sagittal imbalance and greater postoperative fractional curve are suggested as risk factors for development of distal CASP.
- There is increased risk of distal RASP in younger patients and in those with presence of preoperative disc degeneration, longer TL fusions, circumferential procedures, and postoperative narrowing of the L5–S1 disc space.
The authors thank Ms. Nancy Holmes, RN, for her administrative assistance. The authors M.K.K. and J.S.S. contributed toward data interpretation, manuscript preparation, and manuscript revision; C.I.S. and L.G.L. study design, data interpretation, manuscript preparation, and manuscript revision; J.R.D.: study design, data analysis and interpretation, manuscript preparation, and manuscript revision; and C.G.E.: data analysis and interpretation, manuscript preparation, and manuscript revision.
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adult; adolescent; complication; deformity; distal; adjacent segment degeneration; adjacent segment disease; management; pathology
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