Proximal junctional kyphosis (PJK) is a well-recognized complication after long-segment spinal fusion. The definition varies and likely falls on a spectrum that ranges from asymptomatic radiographic features to severe, symptomatic PJK with hardware failure requiring reoperation, also known as “proximal junctional failure” (PJF).1,2 The radiographic definitions of PJK generally require an increase of 10° or more in the sagittal Cobb angle from the upper instrumented vertebra (UIV) to UIV+2 compared to the preoperative baseline, but there is no standardized definition.3–5 Several authors have published scales to grade and classify PJK including the Boachie-Adjei classification and the PJF severity scale by Hart et al. and the International Spine Study Group.6,7
Several factors contribute to PJK including age-related degeneration, disruption of the posterior ligamentous complex or ligamentous fatigue, vertebral body fractures at the UIV, and instrumentation failure.1,3,8–10 Additional risk factors include older age,11–14 greater preoperative sagittal imbalance,5,15–20 significant curve correction,5,17,19 disruption of posterior elements,12,13,16,19 and fusion to lower lumbar vertebrae or the sacrum.15,17,18,21–23 Rates of PJK vary by definition, but most reports range from 17% to 39%.3,11,15,18,21,22,24 The majority of cases occur early in the postoperative course, with 66% in the first 3 months and 80% in the first 18 months.17,18
Ligament augmentation is a novel strategy for PJK/PJF prevention that provides junctional strength between the UIV and UIV+1. A flexible tether is passed between the spinous processes at the UIV, UIV+1, and UIV−1 and then secured to the rod under moderate tension. The technique is technically straightforward and has been shown by several groups to be effective in preventing PJK and PJF;25–28 however, these results are not universal, likely because of differences in technique.29,30 By preventing revision surgery for PJF, ligament augmentation has the potential to be highly cost-effective.31 In this study, we sought to determine the effect of ligament augmentation on rates of PJF in a cohort of adult spinal deformity patients with at least 12 months of follow-up.
Methods
Data Collection
A consecutive series of adult spinal deformity patients who had undergone instrumented fusion was queried for this study. Patients with deformity related to infection, tumor, or trauma were excluded. Surgeries had been performed from 2010 to 2018 at a single institution by two surgeons (V.D. and C.P.A). Ligament augmentation had been implemented in 2015 and was applied to all subsequent patients. Data on patient characteristics including age, gender, indication for surgery, number of levels fused, use of bone morphogenetic protein (BMP), patient-specific rods, and three-column osteotomy were collected. For patients with upper thoracic UIVs, we assessed the use of transverse process hooks. For patients with lower thoracic UIVs, we assessed the use of vertebroplasty at the UIV and UIV+1. All patients included in the analysis had constructs terminating in the pelvis with either upper thoracic (T1–6) or lower thoracic (T7–12) UIVs. Patients without adequate follow-up data or neuromuscular/neurodegenerative disorders were excluded from analysis. All patients had a minimum of 12 months of follow-up from the time of the index surgery, including those with PJF. Patients treated without ligament augmentation from 2010 to 2015 are referred to as the “historical cohort” for this study. All activities were approved by the Committee on Human Research, our institutional review board.
Ligament Augmentation Technique
We previously reported our technique for ligament augmentation.32 In brief, a matchstick burr is used to drill holes through the center of the spinous processes at the UIV, UIV−1, and UIV+1. Two sublaminar cables are passed through these holes in a mirrored fashion (one per side) and then secured to a connector attached to each rod (Medicrea USA). The spinous processes are loaded in slight extension, and after the desired tension is provided, the cable is secured in its final configuration.
Statistical Analysis
Statistical analysis was performed using SPSS version 26 (IBM Corp.). Continuous variables were compared using the Student t-test or analysis of variance. Categorical variables were compared using the chi-square or Fisher’s exact test.
Variables were included in the initial multivariate model if they demonstrated a statistically significant relationship on univariate analysis, defined as p < 0.100. Multivariate analysis was performed using binary logistic regression with backward elimination so that only statistically significant variables remained in the final model. Statistical significance was defined as p < 0.050.
Results
Patient and Surgical Characteristics
A total of 242 patients with ligament augmentation were included in this analysis. The mean age was 66 years (range 41–87 years), and there were 166 women (68.6%). One hundred four patients (43.0%) had undergone prior spine fusions. The mean number of levels fused was 10 (range 6–16), and the UIV distribution included 90 upper thoracic UIVs (37.2%) and 152 lower thoracic UIVs (62.8%). Pelvic satellite rods were used in 94 patients (38.8%), patient-specific rods in 96 (39.7%), anterior or lateral interbody grafts in 155 (64.0%), three-column osteotomies in 64 (26.4%), and posterior BMP in 196 (81.0%). Among the 90 patients with upper thoracic UIVs, transverse process hooks were used in 80 (88.9%). Among the 152 patients with lower thoracic UIVs, vertebroplasty was used in 139 (91.4%). The mean follow-up was 19 months (range 12–54 months). Patient and surgical characteristics are summarized in Table 1.
Summary of characteristics among 242 patients who underwent ligament augmentation
| Variable | Value |
|---|---|
| Age in yrs (range) | 66 (41–87) |
| Female gender | 166 (68.6%) |
| Prior spine fusion | 104 (43.0%) |
| No. of levels fused (range) | 10 (6–16) |
| UIV | |
| Upper thoracic | 90 (37.2%) |
| Lower thoracic | 152 (62.8%) |
| Ligament augmentation | 242 (100%) |
| Pelvic fixation | 242 (100%) |
| Pelvic satellite rod | 94 (38.8%) |
| Patient-specific rod | 96 (39.7%) |
| Anterior/lateral interbody graft | 155 (64.0%) |
| 3-column osteotomy | 64 (26.4%) |
| Posterior BMP | 196 (81.0%) |
| UIV transverse process hooks* | 80/90 (88.9%) |
| UIV vertebroplasty† | 139/152 (91.4%) |
| FU in mos (range) | 19 (12–54) |
FU = follow-up.
Values are reported as number (%) or mean (range).
For upper thoracic UIV only.
For lower thoracic UIV only.
Comparison to Historical Cohort Without Ligament Augmentation
We compared patients with and without ligament augmentation within our cohort of spinal deformity patients. Both groups included patients with pelvic fixation and UIV at the upper or lower thoracic spine. The ligament augmentation group was slightly older (66 vs 63 years, p = 0.010), but there were no significant differences in gender or number of levels fused. The historical cohort had a higher rate of prior fusions (57.1% vs 43.0%, p = 0.030). The ligament augmentation cohort had a higher rate of pelvic satellite rods (38.8% vs 5.2%, p < 0.001), patient-specific rods (39.7% vs 5.2%, p < 0.001), anterior or lateral interbody grafts (64.0% vs 36.4%, p < 0.001), BMP use (81.0% vs 46.8%, p < 0.001), transverse process hooks for upper thoracic UIVs (88.9% vs 60.0%, p < 0.001), and vertebroplasty for lower thoracic UIVs (91.4% vs 40.4%, p < 0.001). Use of three-column osteotomy was lower in the ligament augmentation group (26.4% vs 44.2%, p = 0.003). The rate of reoperation for PJF was significantly lower in the ligament augmentation cohort (3.3% vs 15.6%, p < 0.001). These data are summarized in Table 2. Rates of PJF were compared by UIV. Among patients with upper thoracic UIV, the rate of PJF decreased from 6.7% to 0% (p = 0.014). Among patients with lower thoracic UIV, the PJF rate decreased from 21.3% to 5.3% (p = 0.001). These data are summarized in Table 3.
Comparison of patients with ligament augmentation to historical cohort
| Variable | Historical Cohort | Ligament Augmentation | p Value |
|---|---|---|---|
| No. of patients | 77 | 242 | |
| Age in yrs | 63 | 66 | 0.010 |
| Female gender | 56 (72.7%) | 166 (68.6%) | 0.492 |
| Prior spine fusion | 44 (57.1%) | 104 (43.0%) | 0.030 |
| No. of levels fused | 10 | 10 | 0.734 |
| UIV | |||
| Upper thoracic | 30 (39.0%) | 90 (37.2%) | 0.780 |
| Lower thoracic | 47 (61.0%) | 152 (62.8%) | 0.780 |
| Pelvic satellite rod | 4 (5.2%) | 94 (38.8%) | <0.001 |
| Patient-specific rod | 4 (5.2%) | 96 (39.7%) | <0.001 |
| Anterior/lateral interbody graft | 28 (36.4%) | 155 (64.0%) | <0.001 |
| 3-column osteotomy | 34 (44.2%) | 64 (26.4%) | 0.003 |
| Posterior BMP | 36 (46.8%) | 196 (81.0%) | <0.001 |
| UIV transverse process hooks* | 18/30 (60.0%) | 80/90 (88.9%) | <0.001 |
| UIV vertebroplasty† | 19/47 (40.4%) | 139/152 (91.4%) | <0.001 |
| Reoperation for PJF | 12 (15.6%) | 8 (3.3%) | <0.001 |
| FU in mos | 30 | 19 | <0.001 |
Values are reported as number (%) unless otherwise indicated. Boldface type indicates statistical significance.
For upper thoracic UIV only.
For lower thoracic UIV only.
PJF rate by UIV
| Variable | Historical Cohort | Ligament Augmentation | p Value |
|---|---|---|---|
| No. of patients | 77 | 242 | |
| UIV | |||
| Upper thoracic | 2/30 (6.7%) | 0/90 (0%) | 0.014 |
| Lower thoracic | 10/47 (21.3%) | 8/152 (5.3%) | 0.001 |
Values are reported as number (%) unless otherwise indicated. Boldface type indicates statistical significance.
Propensity matching was used to generate a well-matched cohort of patients with and without ligament augmentation. Propensity scores were generated using covariates including age, gender, UIV, transverse process hooks, vertebroplasty, and magnitude of sagittal vertical axis (SVA) and pelvic incidence–lumbar lordosis (PI-LL) mismatch correction. Scores were matched, resulting in a cohort of 80 patients composed of 40 patients with and 40 without ligament augmentation. There were no significant differences in age, gender, prior spine fusion, number of levels fused, UIV, magnitude of SVA or PI-LL mismatch correction, vertebroplasty, or transverse process hooks. The rate of reoperation for PJF was 22.5% without ligament augmentation compared to 2.5% for those with ligament augmentation. These data are summarized in Table 4.
Propensity-matched comparison of ligament augmentation for PJF prevention
| Variable | w/o Ligament Augmentation | w/ Ligament Augmentation | p Value |
|---|---|---|---|
| No. of patients | 40 | 40 | |
| Age in yrs | 65 | 65 | 0.772 |
| Female gender | 32 (80.0%) | 26 (65.0%) | 0.133 |
| Prior spine fusion | 19 (47.5%) | 19 (47.5%) | 0.999 |
| No. of levels fused | 10 | 10 | 0.637 |
| UIV | |||
| Upper thoracic | 17 (42.5%) | 14 (35.0%) | 0.491 |
| Lower thoracic | 23 (57.5%) | 26 (65.0%) | 0.491 |
| Magnitude of SVA change in cm | 4.0 | 3.9 | 0.959 |
| Magnitude of PI-LL mismatch change in ° | 8.3 | 13.0 | 0.177 |
| UIV vertebroplasty* | 17/23 (73.9%) | 19/26 (73.1%) | 0.947 |
| UIV transverse process hooks† | 12/17 (70.6%) | 10/14 (71.4%) | 0.959 |
| Reoperation for PJF | 9 (22.5%) | 1 (2.5%) | 0.007 |
| FU in mos | 30 | 21 | 0.009 |
Values are reported as number (%) unless otherwise indicated. Boldface type indicates statistical significance.
Among patients with lower thoracic UIV.
Among patients with upper thoracic UIV.
Comparison of Variables Associated With PJF
We combined the pre–ligament augmentation and ligament augmentation cohorts to assess variables associated with reoperation for PJF. There were no significant differences in age, gender, or prior spine fusion. Patients with PJF had fewer levels fused (8 vs 10, p = 0.001) and a higher rate of lower thoracic UIVs (90.0% vs 60.5%, p = 0.008). Use of transverse process hooks was lower in the PJF group (10.0% vs 33.4%, p = 0.030). Use of ligament augmentation was also lower in the PJF group (40.0% vs 78.3%, p < 0.001). There were no significant differences in the use of pelvic satellite rods, patient-specific rods, anterior or lateral interbody grafts, 3-column osteotomies, and BMP. These data are summarized in Table 5.
Comparison of patient variables associated with PJF
| Variable | No PJF | Reoperation for PJF | p Value |
|---|---|---|---|
| No. of patients | 299 | 20 | |
| Age in yrs | 66 | 65 | 0.753 |
| Female gender | 206 (68.9%) | 16 (80.0%) | 0.296 |
| Prior spine fusion | 139 (46.5%) | 9 (45.0%) | 0.897 |
| No. of levels fused | 10 | 8 | 0.001 |
| UIV | |||
| Upper thoracic | 118 (39.5%) | 2 (10.0%) | 0.008 |
| Lower thoracic | 181 (60.5%) | 18 (90.0%) | 0.008 |
| Pelvic satellite rod | 95 (31.8%) | 3 (15.0%) | 0.115 |
| Patient-specific rod | 96 (32.1%) | 4 (20.0%) | 0.258 |
| Anterior/lateral interbody graft | 171 (57.2%) | 12 (60.0%) | 0.806 |
| 3-column osteotomy | 92 (30.8%) | 6 (30.0%) | 0.942 |
| Posterior BMP | 219 (73.2%) | 13 (65.0%) | 0.423 |
| UIV transverse process hooks* | 100 (33.4%) | 2 (10.0%) | 0.030 |
| UIV vertebroplasty† | 147 (49.2%) | 13 (65.0%) | 0.170 |
| Ligament augmentation | 234 (78.3%) | 8 (40.0%) | <0.001 |
| FU in mos | 20 | 41 | <0.001 |
Values are reported as number (%) unless otherwise indicated. Boldface type indicates statistical significance.
For upper thoracic UIV constructs.
For lower thoracic UIV constructs.
Multivariate Analysis
We performed multivariate analysis using binary logistic regression with backward elimination. Variables were included in the model based on significance of p < 0.100 on univariate analysis. These variables included number of levels fused, UIV (upper thoracic or lower thoracic), transverse process hooks, and ligament augmentation. The only variables that remained statistically significant in the final model were ligament augmentation (OR 0.184, 95% CI 0.071–0.478, p = 0.001) and number of levels fused (OR 0.762, 95% CI 0.620–0.937, p = 0.010). Upper thoracic UIV (OR 0.621, p = 0.811) and transverse process hooks (OR 1.456, p = 0.763) were not associated with reoperation for PJF. Significant variables associated with a reduction in reoperation for PJF are summarized in Table 6.
Significant variables associated with reduced reoperation for PJF in a multivariate analysis
| Variable | OR (95% CI) | p Value |
|---|---|---|
| No. of levels fused | 0.762 (0.620–0.937) | 0.010 |
| Ligament augmentation | 0.184 (0.071–0.478) | 0.001 |
Boldface type indicates statistical significance.
Illustrative Cases
Case 1: Ligament Augmentation Without PJF
A 69-year-old male presented to our institution with back pain, iatrogenic flat back deformity, and sagittal imbalance after undergoing an L1–S1 fusion at an outside hospital (Fig. 1A and B). He was taken to the operating room for revision T10–pelvis fusion with L4 pedicle subtraction osteotomy, T9–10 vertebroplasty, and ligament augmentation, followed by an L5–S1 anterior lumbar interbody fusion. At the 12-month follow-up, he was doing well with a good radiographic result (Fig. 1C and D).
Case 1. Preoperative anteroposterior (A) and lateral (B) standing radiographs demonstrate a prior L1–S1 fusion with flat back deformity and sagittal imbalance. The patient was taken for revision T10–pelvis fusion with L4 pedicle subtraction osteotomy, T9–10 vertebroplasty, and ligament augmentation. Postoperative standing anteroposterior (C) and lateral (D) standing radiographs demonstrate good alignment without evidence of PJF at 12 months.
Case 2: Ligament Augmentation With PJF
An 80-year-old woman presented to our institution with progressive leg weakness and adjacent-segment disease after undergoing L3–5 fusion at an outside hospital (Fig. 2A and B). She underwent a T10–pelvis fusion with ligament augmentation. Postoperative radiographs showed good alignment (Fig. 2C and D); however, she presented 5 months later with progressive leg weakness and radiographic PJK (Fig. 2E and F). She was taken to the operating room for extension of fusion to T3, demonstrating a good radiographic outcome at 24 months (Fig. 2G and H).
Case 2. Preoperative anteroposterior (A) and lateral (B) standing radiographs demonstrate prior L3–5 instrumented fusion with adjacent-segment disease. The patient underwent a successful T10–pelvis fusion with ligament augmentation with a good result on anteroposterior (C) and lateral (D) radiographs. She presented 5 months later with leg weakness and radiographic PJK on anteroposterior (E) and lateral (F) radiographs. She was taken to the operating room for extension of fusion to T3, demonstrating a good clinical and radiographic result at 24 months. Postoperative anteroposterior (G) and lateral (H) radiographs show good alignment without implant failure.
Discussion
Prevention strategies for PJK and PJF are critically important in the treatment of adult spinal deformity. Ligament augmentation strengthens the junction between the UIV and adjacent noninstrumented levels without violating facet joints or significantly increasing blood loss or operative time. Over the past 10 years, our PJK/PJF prevention techniques have evolved to now include hook fixation for upper thoracic UIVs to minimize soft tissue and facet dissection, vertebroplasty for lower thoracic UIVs to prevent compression fractures, patient-specific rods to prevent overcorrection, and terminal kyphosis at the UIV to ease the transition between fused and nonfused segments. In this study, we present a large cohort of adult spinal deformity patients with a minimum 1-year follow-up to demonstrate significant reductions in reoperation for PJF in patients treated with ligament augmentation, particularly among those with upper and lower thoracic UIVs.
The abrupt change in biomechanical rigidity between instrumented vertebrae and relatively mobile noninstrumented segments is a potential contributor to PJK and PJF. Various techniques have been evaluated in an attempt to mitigate the biomechanical stress associated with this acute change. Ligament augmentation with polyethylene tethers produces a smoother transition between instrumented and noninstrumented levels. A finite element analysis by Bess et al. highlighted the ability of posterior polyester tethers extending from the uppermost instrumented vertebra (UIV) to the UIV+1, +2, and +3 to provide a more gradual transition in biomechanical stress from instrumented to noninstrumented vertebrae.28 Buell et al. assessed configurations of posterior polyester tethers and found that extending to the UIV+2 in a loop or weave configuration most effectively mitigated adjacent-segment stress in long-segment instrumented constructs.27
These biomechanical data provided impetus for clinical studies investigating the efficacy of ligament augmentation for PJK and PJF prevention. Iyer et al. reviewed a series of 108 adult deformity patients with 5-level fusions terminating in the pelvis.30 This cohort contained only 31 patients with augmentation of posterior ligamentous structures. Ligament augmentation patients were older and had greater sagittal correction than the control cohort. The authors found no significant difference in the rate of radiographic PJK (27.3% vs 28.6%). Buell et al. reviewed 184 adult deformity patients treated with no posterior ligamentous tethers, tethers secured to the spinous process, and tethers secured to a crosslink.26 These authors found significant reductions in radiographic PJK for patients with tethers compared to those in patients without tethers (26.7% vs 45.3%, p = 0.011). In their study, older age and a greater change in LL were independent predictors of PJK. Our group reviewed a series of 100 consecutive patients treated with ligament augmentation and found significant reductions in the proximal junctional angle (6° vs 14°, p < 0.001).25 In a multivariate model, only ligament augmentation was associated with a reduction in PJF (OR 0.193, p = 0.012).
Although the Iyer and Buell studies utilized radiographic PJK as an outcome, our group also included PJF as an outcome measure.26,30 In both our previous and our current studies, now with a minimum 12-month follow-up, we showed that ligament augmentation is associated not only with reductions in early radiographic PJK, but also with clinical outcome as defined as reoperation for PJF. Given the high cost of revision surgery for PJF combined with the relatively low cost and morbidity of ligament augmentation, we have shown that this technique is also very cost-effective.31 The current study reproduces our previous findings of reduced PJF in patients receiving ligament augmentation in a large cohort of adult spinal deformity patients with a longer follow-up, which provides additional evidence to support this intervention as a valuable technique in appropriately selected patients. Nevertheless, additional prospective, multicenter studies are needed to more comprehensively evaluate the utility of ligament augmentation for PJF prevention.
Our experience suggests that ligament augmentation provides significant benefit for both upper and lower thoracic UIV constructs; however, the reduction in PJF was most pronounced for lower thoracic UIV (21.3% to 5.3%). Since most PJF at these levels is due to compression fractures at the UIV or UIV+1 rather than ligamentous fatigue for upper thoracic UIV constructs, we were surprised that ligament augmentation had such a dramatic effect. To be clear, ligament augmentation still provided a benefit in PJF reduction for upper thoracic UIV constructs (6.7% to 0%, p = 0.014); however, the effect seems to be most pronounced for lower thoracic UIVs. We suspect this is attributable to the biomechanical advantage provided by easing the transition at the thoracolumbar junction and by loading the vertebrae in slight extension. Additional studies and biomechanical models are needed to confirm these findings and determine if the true benefit of ligament augmentation is location dependent.
Limitations of our study include its retrospective design and homogeneous patient population that is limited to adult spinal deformity patients treated by two attending surgeons with similar practice patterns and techniques. Future studies investigating ligament augmentation should utilize a prospective, multicenter design to avoid such potential biases. It is important to note that our PJK/PJF prevention strategies have continuously evolved. This was most evident in the increasing use of BMP, transverse process hooks, vertebroplasty, pelvic satellite rods, patient-specific rods, and anterior or lateral interbody grafts over time. We attempted to control for these factors in a multivariate model, but future studies should attempt to isolate the specific benefit of ligament augmentation alone. Despite its limitations, the present study is the largest to investigate the effect of ligament augmentation on the incidence of PJF in adult spinal deformity with a minimum 12-month follow-up. While additional modalities exist to prevent PJF,32 this study demonstrates the long-term efficacy of ligament augmentation in PJF prevention. Thus, our results suggest that ligament augmentation should be considered for use as a method to prevent PJF in select adult deformity patients undergoing surgical correction.
Conclusions
PJF is a feared complication for all long-segment spinal fusions, particularly in cases of adult spinal deformity. Ligament augmentation is a novel technique for PJK/PJF reduction that requires minimal operative time with virtually no increase in blood loss and a relatively low financial cost. In a cohort of adult patients with a minimum 1-year follow-up, we showed significant reductions in reoperation for PJF among patients with ligament augmentation. In a multivariate model, ligament augmentation (OR 0.184, 95% CI 0.071–0.478, p = 0.001) was associated with decreased reoperation for PJF. In appropriately selected patients, ligament augmentation may prove to be a powerful adjunct for early PJF prevention, although long-term data are still needed.
Acknowledgments
Dr. Ames receives grant funding from the Scoliosis Research Society.
Disclosures
Dr. Safaee has received travel support from Medicrea. Dr. Deviren is a consultant for NuVasive, Seaspine, Alphatec Spine/Atec Spine, Medicrea, and Biomet/Zimmer; has received institutional fellowship grants from NuVasive and Omega; and receives royalties from NuVasive and Medicrea. Dr. Ames receives royalties from Stryker, Biomet Zimmer Spine, DePuy Synthes, NuVasive, Next Orthosurgical, K2M, and Medicrea; is a consultant for DePuy Synthes, Medtronic, Medicrea, and K2M; conducts research for Titan Spine, DePuy Synthes, and ISSG; serves on the editorial board of Operative Neurosurgery; serves on the Executive Committee of the ISSG; and serves as a director of Global Spinal Analytics.
Author Contributions
Conception and design: Ames, Safaee, Deviren. Acquisition of data: Safaee, Haddad, Fury, Maloney, Scheer, Lau. Analysis and interpretation of data: Ames, Safaee, Haddad, Scheer, Lau. Drafting the article: Safaee, Lau. Critically revising the article: Ames, Safaee, Lau, Deviren. Reviewed submitted version of manuscript: Ames, Safaee, Deviren. Approved the final version of the manuscript on behalf of all authors: Ames. Statistical analysis: Ames, Safaee, Haddad. Administrative/technical/material support: Ames, Deviren. Study supervision: Deviren.
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