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Corresponding author: Department of Orthopedic Surgery, Asan Medical Center, University of Ulsan College of Medicine, 388-1, PungNap-2-dong, SongPa-gu, Seoul, Korea, Tel.: +82-230103549; fax: +82-230108555.
Overcorrected patients had a significantly higher PJK rate in the age-adjusted analysis (48.3%, 26.5%, and 13.3% for groups O, I, and U, respectively; p=.041).
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The degree of postoperative LL correction relative to the PI is a risk factor for the development of PJK (11.4° for PJK vs. 0.2° for non-PJK, p=.033).
•
Inferior clinical outcomes (back VAS and ODI) were found in patients with PJK.
•
To reduce the risk of PJK, surgeon's should take age-adjusted parameters into account and exercise caution not to overcorrect patients with low PI.
Abstract
BACKGROUND CONTEXT
The effect of the degree of lumbar lordosis (LL) correction on proximal junctional kyphosis (PJK) has not been analyzed in context of the age-adjusted sagittal alignment goal.
PURPOSE
To determine the effect of sagittal correction on the incidence of PJK after an age-adjusted analysis in patients with adult spinal deformity (ASD).
STUDY DESIGN/SETTING
Retrospective comparative study.
PATIENT SAMPLE
Seventy-eight ASD patients who underwent deformity correction.
OUTCOME MEASURES
Visual analog scale (VAS), Oswestry Disability Index (ODI), and imaging.
METHODS
This study included 78 ASD patients who underwent deformity correction and were followed-up more than 2 years. Patients were grouped according to the degree of LL correction relative to pelvic incidence (PI) by adjusting for age using the following formula: (age-adjusted ideal PI - LL) - (postoperative PI - LL). These were group U (undercorrection; <−10˚, N=15), group I (ideal correction; −10˚–10˚, N=34), and group O (over correction, >10˚, N=29). Various clinical and radiological parameters were compared among groups. The risk factors for PJK were also evaluated.
RESULTS
The overall incidence of PJK was 32.1% (25/78), with significantly higher PJK rate in group O (48.3%) compared with groups U (13.3%) and I (26.5%) (p=.041). The degree of postoperative LL correction relative to the PI by adjusting for age was a risk factor for the development of PJK (11.4° for PJK vs. 0.2° for non-PJK, p=.033). In addition, 2-year postoperative VAS (7.0 vs. 3.4, p<.001) and ODI (28.9 vs. 24.8, p=.040) scores were significantly higher in the PJK group than in the non-PJK group. A small PI (PI < 45°) was associated with a tendency of overcorrection (73.3%, P < 0.001) and thereby with the high incidence of PJK (53.3%, p=.005).
CONCLUSIONS
Overcorrection of LL relative to PI considering age-adjusted ideal sagittal alignment tends to increase the incidence of PJK. The incidence of PJK is expected to be high in patients with low PI (<45°) because of the tendency of overcorrection. To reduce the risk of PJK, surgeons should take age-adjusted parameters into account and exercise caution not to overcorrect patients with low PI, since this can result in suboptimal clinical outcomes.
The primary targets of deformity correction for adult spinal deformity (ASD) are achieving optimal sagittal alignment and improving clinical outcomes [
]. Furthermore, maintaining an ideal sagittal balance after corrective surgery is critical because of its association with the health-related quality of life in many studies [
Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery.
]. Otherwise, the incidence of mechanical complications increases, resulting in poor outcomes evidenced by frequent revisional surgeries. One such mechanical complication is proximal junctional kyphosis (PJK), which is frequently observed after long-level fusion surgery for deformity correction (17–61.7% of the cases) [
Proximal junctional kyphosis in adult spinal deformity following long instrumented posterior spinal fusion: incidence, outcomes, and risk factor analysis.
Analysis of the incidence and risk factors for the progression of proximal junctional kyphosis following surgical treatment for lumbar degenerative kyphosis: minimum 2-year follow-up.
Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up.
Type of anchor at the proximal fusion level has a significant effect on the incidence of proximal junctional kyphosis and outcome in adults after long posterior spinal fusion.
Analysis of the incidence and risk factors for the progression of proximal junctional kyphosis following surgical treatment for lumbar degenerative kyphosis: minimum 2-year follow-up.
Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up.
Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up.
Patients with proximal junctional kyphosis requiring revision surgery have higher postoperative lumbar lordosis and larger sagittal balance corrections.
Proximal junctional kyphosis in adolescent idiopathic scoliosis after 3 different types of posterior segmental spinal instrumentation and fusions: incidence and risk factor analysis of 410 cases.
Proximal junctional kyphosis in adult reconstructive spine surgery results from incomplete restoration of the lumbar lordosis relative to the magnitude of the thoracic kyphosis.
]. Thus, the effect of the degree of lumbar lordosis correction on PJK has remained controversial. Notably, a limitation of those studies was that they did not factor in age in the calculation for ideal sagittal alignment.
The age-related compensatory mechanism underlying sagittal balance has recently gathered attention [
]. Compensatory mechanisms such as pelvic retroversion and knee flexion prevent the anterior translation of the gravity line because of degenerative loss of lordosis. These changes maintain an erect posture, thus limiting muscle energy expenditure and biomechanical loads on intervertebral segments. Therefore, because age-related changes occur across the entire musculoskeletal and neurosensorial systems [
], patients manifest radiologic parameters of sagittal alignment appropriate for their age.
Taking this into consideration, the ideal target of correction for sagittal deformity should be adjusted according to age. Age has also been considered as mandatory for setting correction targets for sagittal alignment [
]. To the best of our knowledge, the effect of the degree of lumbar lordosis (LL) correction on PJK has not been analyzed by considering age-adjusted sagittal alignment goal. Therefore, the purpose of this study was to reveal the effect of sagittal correction on the incidence of PJK using the age-adjusted analysis.
Materials and methods
This study was approved by the Institutional Review Board of our institution (AMC IRB: 2021-0306). The requirement of obtaining informed consent was waived because of the retrospective nature of the study.
Patients and operative methods
This study retrospectively enrolled a consecutive cohort of patients with ASD who had undergone sagittal deformity corrective surgery at our institution from January 2012 to December 2017. All surgeries were performed by one of two surgeons (JHC and CJH), and all patients underwent fusion at more than five levels. The minimum follow-up duration was at least 2 years. We excluded any patients who had fusion at less than 5 levels, had less than 24 months of follow-up, or for those with missing/indeterminate relevant radiological measurements. The corrective surgeries were performed by either: (1) posterior only surgery combining Smith-Peterson osteotomy with or without pedicle subtraction osteotomy (PSO), or (2) combined anterior and posterior approach including oblique or anterior lumbar interbody fusion and Smith-Peterson osteotomy with or without PSO. All surgeries were conducted in an open manner. The upper instrumented vertebra (UIV) was frequently T2–T4 (upper thoracic) or T9–T12 (lower thoracic) according to the characteristics of the deformity. Bilateral iliac screw fixation or S2 alar iliac screw fixation was frequently performed. Rod constructs consisted of single or dual 5.5-mm titanium rods on each side. We used hooks at the UIV in a few cases.
We grouped the patients according to the degree of LL correction relative to pelvic incidence (PI) by adjusting for age, which uses following formula: (age-adjusted ideal PI - LL) - (postoperative PI - LL) [
]. The 3 groups were as follows: group U (undercorrection; <−10°), group I (ideal correction; −10° ± 10°), and group O (overcorrection; >10°). The prevalence of PJK, proximal junctional failure (PJF), implant failure, various radiologic parameters, and clinical outcomes were analyzed among three groups. In addition, the effect of the PI on PJK was also evaluated.
Study variables
Demographic data including age, sex, body mass index (BMI), and bone mineral density (BMD) were collected through an electronic chart review. We reviewed the number of fused spinal segments, the surgical approach, the UIV level, whether spinopelvic fixation was performed, whether a hook was used at the UIV, the rod type (double or multiple) used for instrumentation, and whether revision surgery was performed. We also evaluated whether PSO was performed for deformity correction.
Clinical outcomes were assessed in each group on a visual analog scale (VAS) on the back and bilateral legs (ie, the Oswestry Disability Index [ODI]) by comparing preoperative and 2-year postoperative scores.
Radiographic measurements of sagittal alignment were performed by an independent spinal fellowship-trained reviewer. Whole-spine 36-inch standing radiographs were obtained preoperatively and postoperatively with patients in the upright position in the lateral view and with their arms folded forward in the lateral view. We measured preoperative, 2-week postoperative, and 2-year postoperative PI (ie, the angle between the line joining the midpoint of the upper end plate of S1 and the center of the hip joint and the line vertical to the upper end plate of S1); pelvic tilt (PT) (ie, the angle between the line joining the midpoint of the upper end plate of S1 and the center of the hip joint and the vertical line); thoracic kyphosis (TK) (ie, the Cobb angle between the upper end plate of T1 and the lower end plate of T12); LL (ie, the Cobb angle between the upper end plates of L1 and S1); sagittal vertical axis (SVA) (ie, the distance between the posterosuperior corner of S1 and the vertical line from the center of the C7 body [C7 plumb line]); and the proximal junctional angle (PJA) (ie, the Cobb angle between the inferior endplate of UIV and the superior end plate of 2 vertebrae above the UIV). All measurements are illustrated in Fig. 1.
Fig. 1Radiographic measurement of sagittal alignment. Preoperative PI (°), TK (°), LL (°), PT (°), and SVA (mm); 2-week postoperative TK (°), LL (°), PT (°), SVA (mm), and PJA (°); 2-year postoperative TK (°), PT (°), SVA (mm), and PJA (°). PI, pelvic incidence; TK, thoracic kyphosis; LL, lumbar lordosis; PT, pelvic tilt; SVA, sagittal vertical axis; PJA, proximal junctional angle.
Proximal junctional kyphosis in adult spinal deformity following long instrumented posterior spinal fusion: incidence, outcomes, and risk factor analysis.
] as the presence of PJK with one or more of the following features: fracture of the vertebral body of the UIV or UIV+1, pulling out of screws at UIV, or posterior ligamentous disruption.
To analyze risk factors for PJK, patients were divided into 2 groups according to the presence or absence of PJK to analyze the risk factors for PJK. Surgical, radiographic, and patient-specific factors for PJK were assessed in each group. Furthermore, clinical outcomes were reviewed.
Statistical analyses
Independent t-test and ANOVA with Bonferroni correction were performed to compare continuous variables, and Chi-square test or Fisher's exact test was performed to compare categorical variables among the groups. Univariate and multivariate logistic regression analyses with the backward LR method were performed to identify the risk factors for PJK. Variables with p<.1 were entered in multivariate analyses. All statistical analyses were performed using SPSS software version 21.0 (SPSS, Inc., Chicago, IL, USA). Statistical significance was set at p<.05 were used to denote statistical significance.
Results
Comparison of demographic and clinical factors among groups
Overall, 78 patients (4 men and 74 women) were included, with 15, 34, and 29 patients in groups U, I, and O, respectively. The mean age was 67.7±6.8 years (63.4, 66.5, and 70.3 y for group U, I, and O, respectively, p=.003), and with the oldest patients belonging to group O. Demographic factors such as BMD, BMI, and clinical outcomes did not significantly differ among the groups (Table 1). Among 78 patients, 5 underwent revision surgeries, but not within 2 years postoperatively.
Table 1Comparison of demographic and clinical factors among groups
Statistically significant (p<.05). N, number; BMD, bone marrow density; BMI, body mass index; Preop, preoperative; 2Y Postop, 2-year postoperative; VAS, visual analog scale; ODI, Oswestry disability index.
Statistically significant (p<.05). N, number; BMD, bone marrow density; BMI, body mass index; Preop, preoperative; 2Y Postop, 2-year postoperative; VAS, visual analog scale; ODI, Oswestry disability index.
Female
15 (19.2)
34 (43.6)
25 (32.1)
BMD T-score (gm/cm2)
−1.6±1.8
−2.1±0.8
−1.5±1.3
.127
BMI (kg/m2)
26.0±7.1
26.6±3.9
26.0±3.5
.819
Preop VAS
5.5±2.8
7.3±2.1
5.2±3.4
.113
2Y Postop VAS
4.1±2.3
4.6±2.7
4.9±3.1
.668
Preop ODI
35.2±5.3
34.8±7.9
29.1±10.3
.165
2Y Postop ODI
26.3±8.9
26.7±8.4
24.9±7.9
.604
Data are expressed as the mean value and standard deviation for continuous variables and numbers for categorical variables.
Statistically significant (p<.05).N, number; BMD, bone marrow density; BMI, body mass index; Preop, preoperative; 2Y Postop, 2-year postoperative; VAS, visual analog scale; ODI, Oswestry disability index.
The group U had higher preoperative PI than groups I and O (64.1°, 58.1°, and 46.6°, respectively; p<.001) (Table 2). Two weeks postoperatively, TK was significantly greater in group O than in group U (36.1° vs. 28.7°, p=.041) (Table 2). The SVA tended to significantly shift posteriorly as the degree of correction increased at 2 weeks and 2 years postoperatively (p=.002 and .012, respectively) (Table 2). The larger the angle of correction, the smaller the PT angle in groups U, I, and O at each postoperative period (28.4°, 19.9°, and 13.2°, respectively at 2 weeks postoperatively; p<.001) (Table 2). The preoperative PI-LL mismatch was greater in group U (68.6°) than in in groups I (47.7°) or O (45.8°) (p<.001). This trend was also continued at 2 weeks and 2 years postoperatively (25.0°, 8.9°, and −8.4°; 32.3°, 13.1°, and −6.3° for group U, I, and O, respectively, p<.001 for both). The degree of change in PJA was greater in group O than in group U (8.8° vs. 3.1°; p=.010) (Table 2).
Table 2Comparison of radiological factors among groups
The overall incidence of PJK was 32.1% (25/78). The incidence of PJK differed significantly among groups U, I, and O (13.3%, 26.5%, and 48.3%, respectively; p=.041) (Table 2). Representative cases of PJK are illustrated in Fig. 2, Fig. 3. The incidence of PJF or implant failure did not differ significantly among the groups (Table 2). The main cause of PJF was fracture of the UIV (16 cases, 84.2%), followed by pullout of screws (3 cases, 15.8%).
Fig. 2Group O. A 72-year-old woman with flatback deformity (PI: 43°, TK: 17°, LL: −18°, PT: 43°, SVA: +175.94 mm) underwent oblique lateral interbody fusion from L2-L5, posterior lateral interbody fusion at L5-S1 and posterior instrumentation at T10-pelvis. This patient's BMD was −3.3. The 2-week postoperative PI-LL was −4° (TK: 27°, LL: 47°, PT: 26°, SVA: −7.15 mm, PJA: 11°), and the correction seemed appropriate according to the SRS-Schwab modifiers. However, this patient's age-adjusted PI-LL was 11.5°. Proximal junctional kyphosis occurred, and the PJA increased from 6° to 43° with a T10 (upper instrumented vertebra) fracture. At 2 years postoperatively, optimal sagittal balance (SVA: +45.16 mm) was well-maintained due to the patient's compensatory mechanism, but the visual analogue scale score increased from 2 to 9. PI, pelvic incidence; TK, thoracic kyphosis; LL, lumbar lordosis; PT, pelvic tilt; SVA, sagittal vertical axis; BMD, bone mass density; PJA, proximal junctional angle.
Fig. 3Group I. A 68-year-old woman with flatback deformity (PI: 61°, TK: 24°, LL: −13°, PT: 30°, SVA: +228.67 mm) underwent oblique lateral interbody fusion from L2-L5, posterior lateral interbody fusion at L5-S1, and posterior instrumentation at T10-pelvis. This patient's BMD was −3.3. The 2-week postoperative PI-LL was 6° (TK: 51°, LL: 55°, PT: 28°, SVA: +69.37 mm, PJA: 9°). The patient's age-adjusted PI-LL was 4.5°. The PJA increased from 9° to 14°. At 2 years postoperatively, optimal sagittal balance (SVA: +60.77 mm) was maintained, and the visual analogue scale score decreased from 5 to 1.PI, pelvic incidence; TK, thoracic kyphosis; LL, lumbar lordosis; PT, pelvic tilt; SVA, sagittal vertical axis; BMD, bone mass density; PJA, proximal junctional angle.
The number of fused spinal segments ranged from 5 to 14 (7.8±1.9). There were no significant differences in the following surgical factors: approach side, the UIV level, whether spinopelvic fixation was performed, whether hooks were used, or whether revision surgery was performed, except for rod type and PSO. Although PSO was more frequently performed, the degree of correction was relatively lower in group U (53.3%, 20.6%, and 20.7% in groups U, I, and O, respectively; p=.037). In addition, the dual rod system was more frequently performed in group O (44.8%), compared to groups U (6.7%) and I (35.3%) (p=.037) (Table 3).
Table 3Comparison of surgical factors among groups
Preoperative PI was smaller in the PJK group than in the non-PJK group (48.1° vs. 58.3°, p=.001), whereas 2-week postoperative TK was greater in the PJK group than in non-PJK group (37.6° vs. 31.9°, p=.027) (Table 4). The PJK group had a significantly greater degree of sagittal correction using the age-adjusted ideal PI-LL than the non-PJK group (11.4° vs. 0.2°, p=.004) (Table 4). Two-week postoperative LL or SVA did not relate with PJK (Table 4).
Table 4Univariate and multivariate regression analysis on risk factors of PJK
Between the two groups, there were no significant differences found in the approach side, the UIV level, whether spinopelvic fixation was performed, the technique of correction, whether hooks were used at the UIV, the rod type, and whether revision surgery was performed (Table 4). However, 2-year postoperative VAS (7.0 vs. 3.4, p<.001) and ODI (28.9 vs. 24.8, p=.040) scores were significantly higher in the PJK group than in the non-PJK group (Table 4).
Risk factor analysis of PJK
On multivariate regression analysis, the only risk factor for the development of PJK was the degree of postoperative LL correction relative to the PI adjusted for age (p=.033) (Table 4).
Comparisons according to the PI
We divided the patients into three subgroups according to PI size: group L (PI<45°), group M (PI 45°–65°), and group H (PI>65°). A large PI was associated with fewer cases of overcorrection (p<.001) and thereby with lower incidence of PJK (p=.005) (Table 5).
Table 5Analysis between subgroups according to pelvic incidence
Although surgical deformity correction is a valuable treatment for ASD, many of its complications associated with old age, longer operative time, surgical invasiveness, and comorbidities (ie, osteoporosis or frailty) could be problematic. Among those complications, PJK is one of the most common mechanical complications preventing the maintenance of adequate sagittal alignment and is associated with suboptimal clinical outcomes [
Type of anchor at the proximal fusion level has a significant effect on the incidence of proximal junctional kyphosis and outcome in adults after long posterior spinal fusion.
Patients with proximal junctional kyphosis requiring revision surgery have higher postoperative lumbar lordosis and larger sagittal balance corrections.
]. However, despite many studies on this phenomenon, the etiology or mechanism of PJK is not yet fully understood. It has been reported that both sagittal plane under- and overcorrection can cause mechanical complications [
Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up.
Patients with proximal junctional kyphosis requiring revision surgery have higher postoperative lumbar lordosis and larger sagittal balance corrections.
Proximal junctional kyphosis in adolescent idiopathic scoliosis after 3 different types of posterior segmental spinal instrumentation and fusions: incidence and risk factor analysis of 410 cases.
Proximal junctional kyphosis in adult reconstructive spine surgery results from incomplete restoration of the lumbar lordosis relative to the magnitude of the thoracic kyphosis.
]. Furthermore, mechanical complications can occur even after ideal correction of all Schwab modifiers (Fig. 2). On the other hand, some reports have reported no relationship between the development of PJK and overcorrection [
Patients with proximal junctional kyphosis requiring revision surgery have higher postoperative lumbar lordosis and larger sagittal balance corrections.
] also reported that patients requiring revision for PJK had a larger or higher LL angle than the PI. These findings were in agreement with our study; we found that postoperative correction of LL relative to PI adjusted for age was greater in the PJK group than that in the non-PJK group (p=.004). By the compensatory mechanism of lumbar overcorrection, higher TK and less PT was observed in postoperative period. We could not find any difference of C7-S1 SVA in the postoperative period between the PJK and non-PJK groups (p=.764), but a few reports have suggested a correlation between larger SVA correction and PJK [
Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up.
Proximal junctional kyphosis in adult reconstructive spine surgery results from incomplete restoration of the lumbar lordosis relative to the magnitude of the thoracic kyphosis.
]. We thought there could be a compensatory mechanism such as increased TK to restore adequate SVA, which might be stressful to the UIV.
We also found that PI was greater in group U than in group O (P < 0.001), whereas PSO was more frequently performed in group U (p=.037). This means that ASD patients with higher PI showed a trend of frequent undercorrection regardless of PSO. Interestingly, clinical outcomes (VAS and ODI) were inferior in patients with PJK at 2 years postoperatively, although they did not differ according to the degree of sagittal correction. Although many studies revealed risk factors for PJK, its clinical importance has not yet been widely researched. One systematic review found that PJK had no detrimental effects, but the level of evidence was weak [
]. On the other hand, inferior outcomes have also been suggested with regards to back VAS, ODI, and SRS-22 by one recent study about the long-term fate of PJK. [
]. These conflicting findings could be attributed to the broad definition PJK, which may include many asymptomatic cases. The comparisons of clinical outcomes according to the degree of correction have also been widely researched. Many studies have showed the inferiority of undercorrection compared to ideal correction or overcorrection [
Under correction of sagittal deformities based on age-adjusted alignment thresholds leads to worse health-related quality of life whereas over correction provides no additional benefit.
Rigorous correction of sagittal vertical axis is correlated with better ODI outcomes after extensive corrective fusion in elderly or extremely elderly patients with spinal deformity.
]. However, this is hard to conclude, because these studies used different definitions of the ideal correction.
The ideal sagittal alignment should take into consideration age as well as PI. Age-related changes of the spine occur because of progressive degenerative changes of the discs and facet joints. The degenerative loss of lordosis causes anterior translation of the gravity line. To shift the line of gravity backward, compensatory mechanisms (ie, pelvic retroversion, knee flexion, and ankle extension) cause a posterior pelvic shift. If the thoracic erector muscles remain efficient, TK decreases to restore balance. In addition, cervical hyperlordosis results from contraction of the cervical erector muscles to keep the gaze horizontal. The global sagittal alignment also becomes unbalanced as degenerative changes become more severe (Fig. 4) [
The mechanism in junctional failure of thoraco-lumbar fusions. Part I: Biomechanical analysis of mechanisms responsible of vertebral overstress and description of the cervical inclination angle (CIA).
]. Due to these reasons, the sagittal parameters of normal populations could vary according to age. Thus, the ideal target LL relative to PI could be lower in older patients. Originally, the target PI-LL ranged from −10° to +10° [
Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery.
]. Postoperative PI-LL mismatch without consideration of age was different between PJK (-1.3˚) and non-PJK (8.8˚) group (Table 4). However, it is not reasonable that postoperative PI-LL was around 0˚ (ideal target) in PJK group. Thus, we thought that many proportions of patients who underwent corrective surgery according to the previous target could have been overcorrected according to the new age-adjusted targets. Indeed, we could find postoperative PI-LL in non-PJK group was almost same with age-adjusted ideal PI-LL (0.2˚), compared to that (11.4˚) in PJK group. This means definite overcorrection in PJK group in consideration of age. Interestingly, a trend of overcorrection was seen in older in the present study (63.4 years for group U vs. 70.3 years for group O), this revealed that we did not consider the age-adjusted correction in the past. Given this context, we attempted to determine the effect of overcorrection in ASD older patients, because we hypothesized that appropriate age-related consideration in the correction of sagittal malalignment is critical to achieve ideal alignment and prevent mechanical complications (Fig. 3) [
Fig. 4Compensatory mechanisms for age-related sagittal imbalance, such as degenerative loss of lumbar lordosis, pelvic retroversion, knee flexion, ankle extension, decreased thoracic kyphosis, and cervical hyperlordosis. The global sagittal alignment becomes unbalanced upon worsening.
The mechanism behind PJK has remained inconclusive until now. However, believe that stress concentration at the proximal junction is one of the reasons for PJK, based on the results of our study. After multilevel spinal arthrodesis, the fused segments act like a lever arm which is affected by the force of gravity. Posterior shifting of the proximal portion of fused segments induces an effort of physiologic global sagittal balance. This leads to an anterior shift of the center of gravity of the proximal junction through the lever-arm effect, which results in increased kyphotic stresses on the proximal junction (Fig. 5) [
The mechanism in junctional failure of thoraco-lumbar fusions. Part II: analysis of a series of PJK after thoraco-lumbar fusion to determine parameters allowing to predict the risk of junctional breakdown.
]. Thus, we have been trying not to overcorrect LL, especially in patients with older age to prevent PJK, since these results in suboptimal clinical outcomes.
Fig. 5The center of gravity (red arrows) results in a posterior shift according to the degree of correction. The fused spinal segments act like a lever arm (green section). In overcorrection, the lever arm of the bending moment at the proximal junction increased to shift the center of gravity anteriorly (blue arrow).
PI has been considered an important parameter in the goals of deformity surgery. The PI is an anatomical (morphologic) parameter unique to each patient, with a small PI implying less potential for pelvic retroversion compensatory to the LL angle [
]. Interestingly, overcorrection was more frequently seen in the smaller PI group, which resulted in a higher incidence of PJK (Table 5). Thus, there should be a focus on achieving sufficient correction in high PI patients, whereas surgeons should be wary of overcorrection in low PI patients because of its higher correlation with PJK.
Several studies have identified certain surgical risk factors (ie, combined anterior–posterior approach, UIV level, extension of fusion to the sacropelvis, and stiffness of the construct) which contribute to the development of PJK [
Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up.
Proximal junctional kyphosis and failure after spinal deformity surgery: a systematic review of the literature as a background to classification development.
]. In this study, we could not find any significant association between the aforementioned surgical risk factors and the prevalence of PJK. This was likely because these surgical factors did not account for physiologic variations among patients. Many recent studies have suggested strategies for preventing PJK or PJF, such as meticulous dissection and ligamentoplasty at the UIV, use of hooks at the proximal level, and prophylactic 1- or 2-level vertebral augmentation above long fusions [
Type of anchor at the proximal fusion level has a significant effect on the incidence of proximal junctional kyphosis and outcome in adults after long posterior spinal fusion.
Proximal junctional kyphosis and failure after spinal deformity surgery: a systematic review of the literature as a background to classification development.
Preliminary results of the effect of prophylactic vertebroplasty on the incidence of proximal junctional complications after posterior spinal fusion to the low thoracic spine.
]. We found that the use of hooks was not significantly effective in preventing PJK, although it not used in many cases. Regretfully, additional vertebroplasty or ligamentoplasty could not be studied due to the problem of medical insurance in our country.
This study has a few limitations, similar to those of other retrospective studies. First, the type of operation and operative levels were not standardized. Second, BMD was assessed at the lumbar level (L1-L4), and this did not reflect the bone quality of proximally adjacent vertebra. Third, the long-term outcomes of patients with PJK who do not undergo revision surgery were not assessed. Regardless of the aforementioned limitations, this study is meaningful because this is the first study to assess PJK risk based on the degree of sagittal correction by adjusting for age, which has recently been regarded as an important parameter.
Conclusions
Overcorrection of LL relative to PI considering age-adjusted ideal sagittal alignment tends to increase the incidence of PJK. The incidence of PJK is expected to be higher in patients with low PI (<45°) because of the tendency of overcorrection. To reduce the risk of PJK, surgeons should take age-adjusted parameters into account and exercise caution not to overcorrect patients with low PI, which can result in suboptimal clinical outcomes.
Declaration of competing interests
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgments
This study was approved by the Institutional Review Board of the Asan Medical Center (IRB No. 2021-0306).
References
Kim YJ
Bridwell KH
Lenke LG
Rhim S
Cheh G
An analysis of sagittal spinal alignment following long adult lumbar instrumentation and fusion to L5 or S1: can we predict ideal lumbar lordosis?.
Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery.
Proximal junctional kyphosis in adult spinal deformity following long instrumented posterior spinal fusion: incidence, outcomes, and risk factor analysis.
Analysis of the incidence and risk factors for the progression of proximal junctional kyphosis following surgical treatment for lumbar degenerative kyphosis: minimum 2-year follow-up.
Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up.
Type of anchor at the proximal fusion level has a significant effect on the incidence of proximal junctional kyphosis and outcome in adults after long posterior spinal fusion.
Patients with proximal junctional kyphosis requiring revision surgery have higher postoperative lumbar lordosis and larger sagittal balance corrections.
Proximal junctional kyphosis in adolescent idiopathic scoliosis after 3 different types of posterior segmental spinal instrumentation and fusions: incidence and risk factor analysis of 410 cases.
Proximal junctional kyphosis in adult reconstructive spine surgery results from incomplete restoration of the lumbar lordosis relative to the magnitude of the thoracic kyphosis.
Under correction of sagittal deformities based on age-adjusted alignment thresholds leads to worse health-related quality of life whereas over correction provides no additional benefit.
Rigorous correction of sagittal vertical axis is correlated with better ODI outcomes after extensive corrective fusion in elderly or extremely elderly patients with spinal deformity.
The mechanism in junctional failure of thoraco-lumbar fusions. Part I: Biomechanical analysis of mechanisms responsible of vertebral overstress and description of the cervical inclination angle (CIA).
The mechanism in junctional failure of thoraco-lumbar fusions. Part II: analysis of a series of PJK after thoraco-lumbar fusion to determine parameters allowing to predict the risk of junctional breakdown.
Proximal junctional kyphosis and failure after spinal deformity surgery: a systematic review of the literature as a background to classification development.
Preliminary results of the effect of prophylactic vertebroplasty on the incidence of proximal junctional complications after posterior spinal fusion to the low thoracic spine.
Statistically significant (p<.05).N, number; BMD, bone marrow density; BMI, body mass index; Preop, preoperative; 2Y Postop, 2-year postoperative; VAS, visual analog scale; ODI, Oswestry disability index.
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