A comprehensive review of risk factors and prevention strategies: how to minimize mechanical complications in corrective surgery for adult spinal deformity

Article information

Asian Spine J. 2025;.asj.2024.0505

Publication date (electronic) : 2025 March 4

doi : https://doi.org/10.31616/asj.2024.0505

Jin-Sung Park ¹^,^*

, Hyun-Jun Kim ²^,^*

, Se-Jun Park ¹

, Dong-Ho Kang ¹

, Chong-Suh Lee ³

¹Department of Orthopaedics, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea

²Department of Orthopaedic Surgery, Hanyang University Guri Hospital, Hanyang University College of Medicine of Medicine, Guri, Korea

³Department of Orthopaedics, Haeundae Bumin Hospital, Busan, Korea

Corresponding author: Jin-Sung Park, Department of Orthopaedics, Samsung Medical Center, Sungkyunkwan University School of Medicine, 81 Irwon-ro, Gangnam-gu, Seoul 06351, Korea, Tel: +82-2-3410-1583, Fax: +82-2-3410-0061, E-mail: paridot@hanmail.net, jinsungosspine.park@samsung.com

*These authors contributed equally to this work as the first authors.

Received 2024 November 27; Revised 2024 December 10; Accepted 2024 December 12.

Abstract

Adult spinal deformity (ASD) surgery aims to correct abnormal spinal alignment in both the sagittal and coronal planes to alleviate pain and improve functional activities of daily living. Despite the advancements in surgical techniques that have led to better clinical outcomes, postoperative mechanical complications remain. These complications include instrumentation failure, with proximal junctional kyphosis (PJK), proximal junctional failure (PJF), and rod fractures (RFs) being the most common. Such complications deteriorate clinical outcomes and often require revision surgery, which can be more burdensome for surgeons and patients, than index surgery. Thus, the risk factors for mechanical complications must be identified, and effective preventive strategies established. Therefore, this study aimed to review the risk factors for mechanical complications, focusing on PJK, PJF, and RF, and explore prevention strategies for these complications in ASD surgery, drawing upon recent literature.

Keywords: Adult spinal deformity; Mechanical complication; Proximal junctional complication; Risk factors; Prevention strategy

Introduction

Increasing life expectancy has led to the growing need to maintain an active lifestyle among older patients, resulting in an increased number of surgical treatments for adult spinal deformity (ASD) [1]. Despite the outcomes promising of surgical management [2], mechanical complications such as proximal junctional kyphosis (PJK), proximal junctional failure (PJF), and rod fractures (RFs) remain significant challenges [3], occurring in up to 46% of ASD surgeries, with many requiring revisions [4]. This has led to extensive studies identifying risk factors across patient, surgical, and radiological factors. However, the literature presents conflicting evidence regarding these risk factors, necessitating a comprehensive understanding of their complex interrelationships [5].

The global alignment and proportion (GAP) score was introduced as an innovative scoring system to predict mechanical complications based on patient-specific pelvic incidence (PI). Subsequent studies evaluating the validity of the GAP score have reported mixed data, with some demonstrating adequate predictive power, whereas others reporting limited correlation [6]. Subsequently, additional methods, including the Roussouly sagittal profile and sagittal age-adjusted score (SAAS), were introduced, showing their potential ability to predict mechanical complications. In addition, the Scoliosis Research Society (SRS)–Schwab classification and age-adjusted alignment goals, which were originally introduced to evaluate clinical outcomes, have demonstrated predictive value for mechanical complications [7].

Thus, through a comprehensive review of the current literature, this study aimed to explore effective strategies for minimizing mechanical complications and improving surgical outcomes by examining the risk factors and scoring systems for optimal global alignment. In addition, this study sought to introduce preventive methods currently used in clinical practice.

Definition and Incidence of Proximal Junctional Kyphosis and Proximal Junctional Failure

PJK is a radiographic diagnosis characterized by an increase in the kyphotic angle between the lower endplate of the upper instrumented vertebra (UIV) and the upper endplate of the vertebra, two levels above the UIV. A diagnostic threshold for PJK is an increase in the kyphotic angle by 10°–20° or more [8]. Although it is typically considered a radiographic change without symptoms, a kyphotic angle increase of ≥20° is associated with a higher likelihood of negative clinical outcomes [9].

PJF is considered severe PJK, defined by the presence of one or more of the following: (1) fracture of the UIV or UIV+1, (2) subluxation or instability between the UIV and UIV+1, (3) fixation failure, (4) neurological deficit, and (5) clinical deterioration requiring revision surgery. PJF and PJK are now considered part of the spectrum, and PJF is defined as any form of PJK requiring surgical intervention [10].

The incidence of PJK varies depending on its definition, follow-up period, and patient population, with reported rates ranging from 20% to 40% [11]. In contrast, the incidence of PJF was lower than that of PJK, ranging from 11.5% to 23.7% in various studies [12]. However, the occurrence of PJF is clinically important, with a revision rate of up to 47% reported in the literature [13].

Risk Factors and Preventive Strategies for Preventing Mechanical Complications

To minimize mechanical complications, the risk factors must be identified and clarified. These factors are categorized as modifiable or unmodifiable. Unmodifiable risk factors should be thoroughly evaluated during patient selection to assess the risk-benefit ratio preoperatively. For modifiable factors, preoperative evaluation should address them thoroughly to ensure informed decision-making by balancing surgical benefits and risks (Table 1).

Table 1

Patient, radiological, and surgical variables influencing PJK, PJF, and RF, with prevention strategies

Patient factors

Demographic factors

Patient factors such as age, sex, American Society of Anesthesiologists grade, and body mass index (BMI) are critical in assessing the risk of PJK/PJF [9,14–25]. Older age is a major risk factor, with thresholds commonly reported at 55, 65, and 70 years [9,14–16]. This increased risk is primarily attributed to differences in bone density and muscle degeneration rather than a strict age cutoff [26]. Bridwell et al. [9] reported that a high BMI (median of 25.6 kg/m²) and the presence of comorbidities were associated with PJK >20°. However, conflicting findings exist, as a meta-analysis reported that BMI did not significantly affect the risk of PJK/PJF [14]. These findings underscore the importance of evaluating BMI alongside other physiological and structural factors to fully assess PJK and PJF risk.

Attempts have been made to develop an index that integrates various aspects of physiological age, effectively applying the concept of “fragility.” This has led to the development of the ASD fragility index, which incorporates BMI, muscle function, and comorbidities to comprehensively evaluate fragility and its association with mechanical complications [27]. Preoperative evaluation should include a thorough assessment of these factors to clearly understand the potential risks of mechanical complications. This approach ensures the balance between the benefits and risks of surgery, allowing for informed decision-making.

Bone quality

Low bone mineral density (BMD) and osteoporosis are among the most critical risk factors for mechanical complications [14,18,24,25]. A study reported patients with low BMD have a higher PJK risk than those with normal BMD because of increased fracture and subluxation at the UIV [28]. Osteopenia or osteoporosis doubles the risk of PJK, whereas patients with lower BMD exhibit a 6.4-fold higher odds ratio for PJF than those with normal BMD [14,28]. Effective management of bone quality is essential to improve surgical outcomes. The use of bisphosphonates, which are widely used antiresorptive medications, in spinal surgery have been extensively studied. However, they have shown an equivocal effect on lumbar fusion rates, with no significant evidence [29]. Although some studies have reported that perioperative bisphosphonate use reduces the incidence of screw loosening and adjacent vertebral body fractures, the evidence level remains low, with a recommended level of grade C [29,30]. Conversely, teriparatide, an anabolic agent, is supported by high-grade evidence (grade B) for enhancing fusion rates and reducing pedicle screw-related complications [30]. Denosumab, a potent antiresorptive agent, has demonstrated high-level evidence for improving fusion rates and increasing pedicle screw pullout strength, making it an effective option for patients with poor bone quality. In cases where teriparatide is contraindicated, denosumab is recommended as a grade B alternative [29,31]. In addition, combination therapy involving denosumab and teriparatide has demonstrated rapid improvements in spinal BMD, highlighting its potential for patients requiring intensive bone health management [32]. Despite the lack of definitive consensus on the optimal duration of perioperative and postoperative teriparatide use, a regimen of at least 3 months preoperatively and 6 months postoperatively is generally accepted [30].

Sarcopenia

Recent studies have emphasized the role of muscle condition in predicting mechanical complications [21]. Specifically, the volume of the paraspinal muscles and the fat infiltration index were identified as potential predictors of PJK/PJF [21]. In this context, sarcopenia, reflecting muscle function, has been identified as an independent risk factor for proximal junctional problems following ASD surgery [18,21].

Radiological factors

Surgical treatment in patients with ASD aims to optimally restore spinal alignment and mitigate mechanical complications. Numerous studies have investigated the radiological risk factors associated with mechanical complications. The reported factors include PI >55°, lower lumbar lordosis (LL), severity of preoperative sagittal imbalance, correction magnitude (including a large change in LL, thoracic kyphosis [TK], sagittal vertical axis [SVA], and overcorrection), PI–LL mismatch, pseudoarthrosis, and a high GAP score (Table 1) [14,16,17,25,33–45]. Several alignment schemes have been proposed, including the SRS–Schwab classification [46], GAP score [47], Roussouly classification [48], age-adjusted alignment [7], and SAAS [49]. These schemes are composed of fundamental spinal alignment parameters and/or age considerations and offer clinical relevance despite certain limitations (Table 2). Although the individual components (e.g., PI, LL, sacral slope [SS], and SVA) of the above-mentioned alignment schemes are independently associated with radiologic and mechanical complications, no single scheme fully predicts these complications, and their preventive efficacy remains controversial. The complex nature of mechanical complications makes it difficult to rely exclusively on alignment schemes for accurate prediction and prevention. Therefore, optimal preoperative planning requires not only consideration of each alignment scheme but also a thorough evaluation of various patient-specific, radiological, and surgical risk factors.

Table 2

Comparison of alignment schemes with components, clinical relevance, and limitations

SRS–Schwab classification

This classification was introduced to describe the severity of spinal deformities based on patient-reported clinical outcomes. It consists of four coronal curve types and three sagittal modifiers classified as “0,” “+,” and “++” according to PI–LL, SVA, and pelvic tilt (PT). Previous studies have validated that achieving the recommended targets in this scheme (SVA <50 mm, PT <20°, and PI–LL=±9°) can positively affect clinical outcomes [50]; however, its role in PJK prevention remains controversial [11]. Although SVA, PT, and PI–LL influence the risk of mechanical complications, PI has substantial individual variability, and PT >20° may still fall within the normal range in cases of a large PI. Moreover, SVA can be masked by compensatory pelvic retroversion [51]. Therefore, the application of this scheme alone may have limitations in planning optimal postoperative alignment.

GAP score

The GAP score is a PI-based proportional scoring system that can predict mechanical complications [47]. It is expressed as the total score of the relative pelvic version (measured SS minus ideal SS [ideal SS=PI×0.59+9]), relative LL (measured LL minus ideal LL [ideal LL=PI×0.62+29]), lordosis distribution index (LDI, L4–S1/L1–S1×100), relative spinopelvic alignment (measured global tilt [GT] minus ideal GT [ideal GT=PI×0.48–15]), and age, ranging from 0 to 13 points. Based on the total score, patients were classified into three groups: proportioned (0–2 points), moderately disproportioned (3–6 points), and severely disproportioned (≥7 points). Although some studies have supported the ability of the GAP score to predict PJK development [45], others have questioned its utility [6]. Moreover, the GAP score includes young patients aged >18 years who have undergone fusion surgery involving >4 levels, which often leads to overcorrection. In 2020, a modified scoring system called GAPB was proposed to include BMI and BMD [52]; however, it must be further validated to confirm its effectiveness.

Roussouly classification

Since the introduction of the Roussouly classification in 2005, Laouissat et al. [48] added a fifth type characterized by a low PI and an anteverted pelvic shape, leading to a classification with five types. This classification defines normal sagittal alignment in asymptomatic patients based on the PI, SS, PT, and apex of the lordotic curve [48]. Several studies have validated that postoperative spinal alignment using this scheme can reduce the risk of mechanical complications [53]. However, other studies have suggested that its predictive power is inferior to that of other schemes [54]. More importantly, it has limitations in practical application owing to the challenges in identifying predegenerative shapes and the lack of quantitative measurements for assessment [55]. In addition, it does not inherently include predictive elements for mechanical complications [53].

Age-adjusted alignment

Age-adjusted alignment is intended to individualize conventional PI–LL, PT, and T1 pelvic angle (TPA), considering patient age. Lafage et al. [7] suggested that spinal correction surgery should account for the natural changes in spinal alignment that occur with age. The age-adjusted model was validated in terms of the incidence of PJF and clinical outcomes. In this scheme, the ideal targets are calculated as follows: PI–LL=(age–55)/2+3, PT=(age–55)/3+20, and TPA=(age–55)/2+16. Patients are classified into three groups based on the offset between the actual and ideal PI–LL values: undercorrection (offset >10°), matched correction (offset within ±10°), and overcorrection (offset <−10°) [7]. Selecting an age-appropriate alignment helps minimize overcorrection, which can be valuable in preventing mechanical complications. Matched correction using this scheme reduces the risk of mechanical complications compared with overcorrection [12,13,43].

SAAS

SAAS is an age-adjusted scheme that incorporates global spinal deformity (represented by TPA), pelvic compensation (represented by PT), and PI–LL [49]. Matched corrections indicate values within a ±10-year range of the calculated ideal target. Undercorrection and overcorrection are defined as values above the +10-year and below the −10-year limit, respectively [56]. To calculate SAAS, starting from zero points for matched correction, one point is added for every 20-year increase above the age-adjusted target (e.g., +1 point for +10 to +30 years, +2 points for +30 to +50 years), and one point is subtracted for every 20-year decrease below the target (e.g., −1 point for −10 to −30 years, −2 points for −30 to −50 years). The SAAS is the sum of all three components. Based on this total, sagittal correction is categorized as “undercorrection” (<−1), “matched” (−1 to +1), or “overcorrection” (>+1). Lafage et al. [49] reported that SAAS predicts surgical outcomes and PJK more effectively than previous alignment schemes. However, according to Park et al. [57], this predictive ability is largely driven by the PI–LL component, which better predicts PJK/PJF risk and clinical outcomes than the overall SAAS.

Surgical factors

A surgeon must consider surgical factors such as the selection of the UIV level, stiffness or rigidity of the construct, bone quality, screw direction at the UIV, LL shape, fusion levels, whether to perform anterior column realignment (ACR) and/or pedicle subtraction osteotomy (PSO), rod characteristics, and meticulous soft tissue dissection (Table 1) [4,11,14,16,20,35,58–71].

UIV selection

The choice of UIV significantly influences the PJK/PJF risk [4,16,58]. Therefore, when performing ASD surgery, the extent to which the fusion should be extended must be determined. Generally, a neutral and stable vertebra for the UIV is recommended [3]. Special consideration is required for the thoracolumbar junction, which is a transitional area from the highly mobile lumbar spine to the less mobile thoracic spine and has a higher risk of mechanical complications [16]. Park et al. [16] reported that risk factors for PJF when stopping fusion at the thoracolumbar junction include age >70 years, osteoporosis, and preoperative proximal junctional angle ≥0°, suggesting that a UIV at T10 or higher may be advisable in such cases. In cases of thoracic hyperkyphosis, extending the fusion to the upper thoracic region may be considered; however, more studies are required to establish precise guidelines [72]. Moreover, the condition of the disc and facet joints, including the degree of degeneration and instability in the segments immediately above the UIV, may also warrant extending the fusion by one or two additional segments [58].

Construct stiffness/rigidity

The UIV level is the area that bears the greatest load after long fusion surgery [59]. To reduce the stress on the proximal junction, a less rigid construct such as transverse hooks, tethers, or sublaminar tape can be used at the UIV+1 level instead of a rigid fixation with a pedicle screw [14,20,59,73–75]. Biomechanical studies have shown that using a hybrid-form construct, such as hooks, can reduce the loading stress at the proximal junction compared with using bilateral screw fixation [73]. Although some clinical studies have reported that the use of hooks reduces the incidence of PJK [73,74], others have suggested that hooks decrease loading stress by reducing rigidity but may increase the intradiscal pressure at the adjacent segment, potentially increasing the PJK risk [76]. Therefore, cautions must be exercised when using hooks, and further research may clarify these findings.

Bone quality and augmentation

Bone quality at the UIV is crucial for PJF prevention [24,60,77]. Mikula et al. [60] reported that lower Hounsfield units at the UIV and UIV+1 were the sole risk factors for PJF. The use of cement-augmented pedicle screws at the UIV is highly effective in reducing the incidence of PJF in patients with low BMD or osteoporosis by increasing the pullout strength of the pedicle screws [78]. A study showed that in the thoracic spine, using approximately 1 mL of cement provides optimal fixation strength without overly increasing the vertebral stiffness [79].

Trajectory of the screw at the UIV

The trajectory of the screw is another critical consideration at the UIV level. Recent studies have indicated that a cranially directed UIV screw angle is associated with increased PJK/PJF risk [61,62]. To reduce the likelihood of mechanical complications, the UIV screw must be positioned at a caudally directed angle [61,62,80]. Thus, the screw must be inserted in a cranial direction with cement applied only around the screw, leaving as much normal bone as possible between the screw and the upper endplate of the UIV.

Appropriate shape of the LL

The shape of the LL is also crucial. During surgical correction, improper load distribution between the lower lumbar (L4–S1) and upper lumbar (L1–L3) regions can lead to mechanical failure [63,64]. Planning for the lower lumbar spine to account for at least two-thirds of the total LL helps prevent a posterior leaning posture and makes it easier to perform kyphotic rod bending on the proximal portion [64].

Rod characteristics

Rod characteristics are also a risk factor for mechanical complications [20,59,68]. Although stiffer rods, such as cobalt-chromium, enhance spinal construct stability, their higher rigidity has been associated with a higher incidence of PJK than titanium alloy rods [81]. In addition, proximal rod contouring is important because it can help minimize junctional stress and screw pullout [68]. Therefore, titanium alloy rods are recommended, and meticulous kyphotic bending of the proximal portion is necessary. If additional rod strength is required at the surgical site, a satellite rod should be considered [82].

Soft tissue damage

Reducing mechanical complications primarily rely on minimizing posterior soft tissue damage. Several studies have reported that an important risk factor for mechanical complications is damage to the proximal-level posterior ligament complex, paraspinal muscle, and facet joints [11,69–71]. Therefore, when approaching proximal levels, iatrogenic injury to normal tissue must be avoided, and damage to normal stabilizing structures must be minimized as much as possible. Recently, efforts to replace open surgery with minimally invasive techniques in deformity correction surgery have introduced the minimally invasive spinal deformity surgery algorithm, which provides guidelines for minimally invasive surgery (MIS) [83]. Although the use of MIS techniques for deformity correction is expected to reduce mechanical complications by minimizing damage to normal structures, related research remains limited. Ongoing studies are warranted to expand the potential of MIS approaches and address their current limitations.

Risk factors and preventive strategies for rod fracture

RF is a common implant-related complication of ASD surgery, leading to pain and deterioration of the spinal alignment [84]. In a meta-analysis of postoperative RF in ASD, Noh et al. [22] reported an overall incidence of 12%, typically occurring at an average of 23.2 months postoperatively. Regarding surgical factors, PSO, smaller rod diameter, use of cobalt–chromium rods, and a larger number of fusion segments have been reported to significantly increase the RF risk [20,35,67]. PSO is a strong risk factor for RF [20]; therefore, interbody fusion to enhance anterior support or the use of multiple rods is recommended [85]. Studies comparing rod diameters of 5.5, 6.0, and 6.35 mm have reported that smaller rods (5.5 mm) are associated with higher RF rates [86–88]. In addition, cobalt–chromium rods have been reported to be associated with a higher RF risk than stainless steel and titanium alloy rods [86,87]. Therefore, the use of thick rods could be considered, taking into account construct stiffness. Regarding radiological factors, a larger preoperative TK was reported as a risk factor [86]. Reducing pseudoarthrosis is critical for minimizing RF, and complete bone fusion in areas such as L5–S1, circumferential arthrodesis, and use of recombinant human bone morphogenetic protein could be considered [86]]. However, even with solid radiographic fusion, RF can occur in up to 9.5% of cases, with 21.1% requiring surgery [87]. Therefore, to enhance stability and reduce symptomatic pseudoarthrosis, the use of a multirod structure rather than a standard two-rod structure is recommended [85].

Summary of the Current Strategies to Minimize Mechanical Complications

Table 3 summarizes the strategies used at Samsung Medical Center to prevent mechanical complications during the preoperative, intraoperative, and postoperative phases of ASD surgery. Preoperatively, a thorough assessment of patient suitability is essential, including patient-specific factors. Proactive use of anabolic agents such as teriparatide for at least 3 months is recommended for patients with osteoposis. Resistance exercises such as hyperextension movements are encouraged to strengthen the back muscles. Spinopelvic alignment parameters and LL correction targets should be evaluated based on an age-adjusted alignment scheme. The flexibility of the deformity should be assessed using fulcrum hyperextension radiographs. Radiographic and magnetic resonance imaging findings may aid in making decisions can be made regarding the surgical approach—whether to use a combined anterior and posterior approach or a posterior-only approach—as well as the need for ACR or PSO. UIV selection should consider risk factors for failure at the thoracolumbar junction. Intraoperatively, efforts should focus on minimizing soft tissue damage, and UIV screws should be oriented caudally, and cement augmentation must be considered depending on bone quality. In addition, hooks can be utilized. At the L5–S1 level, anterior lumbar interbody fusion is preferred for LDI. Indirect decompression can also be achieved using a high-height cage, which facilitates circumferential fusion to prevent pseudarthrosis. Appropriate rod contouring requires careful attention to proximal kyphotic bending and LDI, avoiding overcorrection during rod application. Titanium alloy rods should be employed, and additional rods may be used for reinforcement. Postoperatively, patients must continue osteoporosis medications, including teriparatide. Patient education regarding daily activity modification is effective in reducing the load on the proximal junction after long fusion surgeries. Regular outpatient follow-ups with whole-spine radiographs are necessary to monitor changes in the global alignment and proximal junctional angle.

Table 3

Pre-, intra-, and postoperative considerations to prevent mechanical complications in ASD surgery

Conclusions

Recent advances in implant technology and surgical techniques have enhanced outcomes in long-term ASD surgery. However, mechanical complications such as PJK, PJF, and RF remain challenging. Preventing these complications require addressing unmodifiable risk factors and optimizing modifiable ones. Although alignment schemes offer useful guidance, mechanical complications are complex and demand consideration of patient and surgical factors beyond alignment alone. Future studies should aim to develop predictive models that integrate diverse risk factors to enhance accuracy and outcomes.

Key Points

Various patient, radiological, and surgical factors contribute to mechanical complications.
Alignment schemes are helpful but insufficient alone; other risk factors must be considered.
Prevention strategies focus on optimizing modifiable risks and accounting for unmodifiable ones at all surgical stages.

Notes

Conflict of Interest

JSP, SJP, and CSL serve as Editorial Board members of the Asian Spine Journal but have no role in the decision to publish this article. Except for that, no potential conflict of interest relevant to this article was reported.

Author Contributions

Conceptualization: JSP, HJK, CSL. Data curation: JSP, HJK, SJP, DHK. Formal analysis: SJP, DHK. Methodology: JSP, HJK, SJP, CSL. Project administration: JSP, HJ K, SJP. Visualization: JSP, HJK. Writing–original draft: JSP, HJK. Writing–review & editing: JSP, HJK. Final approval of the manuscript: JSP, HJK, SJP, DHK, CSL.

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Associated factors	PJK	PJF	RF	Preventative strategy
Patient-related factors
Non-modifiable
Older age [14] (> 55 [9], > 65 [15], >70 [16])	Ο [14,15]	Ο [16]
Female sex	Ο [15]	Ο [15]
Comorbidities	Ο [9]	Ο [19]
ASA grade	Ο [17]		Ο [17]
Previous spine surgery			Ο [20]
Modifiable
High BMI [22,23] (median 25.6 kg/m² [9])	Ο [9]	Ο [23]	Ο [22]	Weight reduction (controversy) [14,22,23]
Sarcopenia	Ο [21]	Ο [18]		Resistance exercise regularly [21]
Osteopenia [18], osteoporosis [14,24,25]	Ο [18,24]	Ο [25]	Ο [26]	Teriparatide and/or denosumab before/after surgery (grade B recommended) [29–32], bisphosphonate (grade C recommended) [29,30], cement augmentation on UIV/UIV+1 [60] (1 mL in T-spine [79])
Radiological factors
Non-modifiable (preoperative)
Imbalance of spinal alignment parameters
Larger PI (>55° [33])	Ο [33]
Lower LL	Ο [14]
Larger SVA [14] (>5 cm [11]), >9.5 cm [17])	Ο [11,14,17	Ο [14]	Ο [17]
Larger TK [86] (>30° [33], >40° [34])	Ο [33,34]		Ο [86]
Larger PJA (>0° [16], >10° [33])	Ο [33]	Ο [16]
Pseudoarthrosis after 1 year			Ο [35]	Consider the use of rh-BMP [86], multi-rods [85]
Pre-existing vertebra fracture			Ο [25]
Modifiable (postoperative)
Larger change in spinal alignment parameters
LL [25,36,37] (>30° [33])	Ο [33,37]	Ο [36]	Ο [25]	Avoid excessive correction of LL [12]
SVA [38] (>50 mm [34])	Ο [34]	Ο [38]		Appropriate correction of sagittal alignment [33]
TK (>10° [39], ≥40° [40])	Ο [39,40]			Appropriate kyphotic rod bending [68], consider fusion extension to the UT [72]
Larger PJA [41] (>5° [36])	Ο [41]	Ο [36]		Optimal correction of PJA [41]
PI-LL mismatch (age-adjusted overcorrection)	Ο [42,43]	Ο [42]		Avoid overcorrection considering age-adjusted goal [12,13,43]
Higher GAP score [45] (≥5 points) [44]	Ο [44,45]	Ο [44,45]	Ο [45]	Attempt adjusting GAP score [45]
Surgical factors
Modifiable
Selection of UIV	Ο [4,58]	Ο [4,16]		Neutral and stable vertebra recommended [3], UT (controversy), be careful T-L junction [16], fusion extension to UT (if, hyper kyphosis) [72], extension above T10 [16], including 1 or 2 additional segments (if, UIV/UIV+1 present degeneration or instability) [58]
Stiffness, rigidity of construct	Ο [14]	Ο [59]	Ο [20]	Soft landing procedure (hooks, hybrid constructs, sublaminar tape, tethering) [14,20,59,73–75], avoid pedicle screw at the UIV [73]
Lower HU on CTa)	Ο [24,60,77]	Ο [60,77]		Cement augmented pedicle screw on UIV [60]
Cranially directed screw at UIV	Ο [61]	Ο [62]		Caudally directed screw trajectory at UIV [61,62,80]
Shape of lumbar lordosis (LDI)	Ο [63]	Ο [64]		At least 2/3 of lordosis to be in the lower lumbar [64]
Including sacrum, pelvis fusion	Ο [65]	Ο [66]	Ο [35]	Be cautious about fusion to the sacrum/ilium [65,66]
Fewer fusion levels	Ο [67]		Ο [67]	Appropriate decision of fusion extent [67]
Pedicle subtraction osteotomy			Ο [20]	Consider multiple rods [85], larger rod diameter (6.0 mm) [86–88], stainless steel or titanium alloy (controversy)
Rod characteristics	Ο [68]	Ο [59]	Ο [20]	Titanium alloy [86,87], proximal rod kyphotic bending [68], satellite rod (if, necessary) [82]
Posterior soft tissue damage	Ο [11,69–71]	Ο [71]		Minimize soft tissue damage (paraspinal muscle, facet capsule, posterior tension band, posterior ligament complex) [11,69–71], avoid overexposure [71], minimally invasive surgery [83]

	PI	LL	SS	SVA	PT	TPA	Age	Clinical relevance	Limitations
S_RS-Schwab classification	v	v		v	v			Achieving recommended targets may improve clinical outcomes	Controversial role in PJK prevention Individual PI variability SVA can be masked by pelvic retroversion.
GAP score	v	v	v				v	Predicting mechanical complications	Inconsistent predictive value among studies Risk of overcorrection
R_oussouly classification	v		v		v		v	May reduce the risk of mechanical complications	Difficult to identify the pre-degenerative shape Lacks quantitative measurement
A_ge-adjusted alignment	v	v			v	v	v	Provides individualized alignment targets, especially for elderly patients (less rigorous correction) Overcorrection is highly related to mechanical complications.	Limited ability to fully predict or explain mechanical complications even if matched correction
SAAS	v	v			v	v	v	Effective ability for predicting surgical outcomes and PJK	Predictive ability of mechanical complications is mainly influenced by PI–LL

	Patient factors	Radiological factors	Surgical factors
Preoperative	Assessment of the patient’s tolerance for ASD surgery Active use of anabolic agents, including teriparatide, for at least 3 months before surgery Strengthening the back muscle with resistance and extension exercises	Assessing sagittal imbalance severity by evaluating spinopelvic parameters Calculating target lumbar lordosis based on an age-adjusted alignment scheme Flexibility assessment of the deformity using a fulcrum extension Identifying major vessel locations on MRI Identifying PJA and adjacent degeneration at the expected UIV level	Determining whether to use an A-P or posterior-only approach Assessing the need for ACR and PSO Careful selection of UIV (T-L junction versus usually T10)
Intraoperative	Consider cement augmentation at UIV based on bone quality	Ideal correction based on an age-adjusted alignment scheme (especially avoiding overcorrection) At least 2/3 of lordosis in the lower lumbar region	Meticulous dissection to minimize soft tissue damage The caudal direction of the screw at UIV Consider hook at UIV+1 Consideration for L5–S1: prefer to anterior lumbar interbody fusion for LDI, circumferential fusion to prevent pseudarthrosis, lumbosacral fixation with ≥4 levels fusion Use titanium alloy rod Proximal rod bending (kyphotic) Consider additional rods
Postoperative	Active management of anabolic agents, including teriparatide, for at least 6 months after surgery, followed by osteoporosis treatment Education on daily activities to minimize adjacent segment strain	Regularly evaluate global alignment with standing whole-spine radiograph