Key Considerations for the Prevention of Proximal Junctional Kyphosis following Adult Spinal Deformity Surgery: A Literature Review

Abstract

Introduction

Proximal junctional kyphosis (PJK) in patients undergoing instrumented deformity correction surgery for adult spinal deformity (ASD) is found to be multifactorial. This review aims to provide comprehensive information on which factors affect PJK in ASD correction surgery including prevention strategies.

Materials and Methods

A literature review was conducted through a web search on PubMed with the following combination keywords: “proximal junctional kyphosis,” “adult spinal deformity,” and “risk factor” between January 2001 and June 2024. Primary outcomes of interest were divided into two groups: non-radiological parameters including patient characteristics and surgical techniques, and radiological parameters.

Results

The non-radiological parameters associated with PJK included age, body mass index, comorbidities, low bone quality, muscle degeneration, combined anterior-posterior surgical approach, rigid proximal instrumentation, upper instrumented vertebrae (UIV) selection in the junctional zone, long-segment fusion, and overcorrection. Moreover, lumbar lordosis, spinopelvic parameter, thoracic tilt, upper instrumented vertebra-femoral angle, fused spinopelvic angle, and UIV inclination were found to be the radiological parameters that influence the incidence of PJK in patient with ASD correction surgery.

Conclusions

Understanding the multifactorial aspects of PJK could aid in the preoperative planning and assessment for patients with ASD. Furthermore, the proposed correction should be based on an individualized approach.

Introduction

The prevalence of adult (de novo) scoliosis and adult spinal deformities (ASD) has risen in parallel with the increase in average life expectancy in recent decades. These conditions often lead to low back discomfort as a result of asymmetric spinal degeneration. ASD comprise a diverse range of anomalies that occur in the thoracic or thoracolumbar spine and have a major impact on health-related quality of life¹⁾. The use of long-segment stabilization, advanced technology, and various types of implants in the management of ASD has become feasible as a result of advancements in surgical techniques and expertise²⁾. Conversely, these developments have also resulted in the emergence of new difficulties, such as the occurrence of proximal junctional kyphosis (PJK). Several factors have been found to contribute to the development of PJK in patients who have instrumented fusion surgery for ASD^3-5).

Complications at the proximal segment following balancing and deformity correction surgery in ASD may arise in several forms, including kyphotic deformity or adjacent vertebral fractures at the upper instrumented vertebrae (UIV) after posterior spinal fusion^6,7). Glattes et al.⁸⁾ define PJK after instrumentation surgery as a proximal junctional sagittal Cobb angle of 10° or more, exceeding the presurgery measurement by at least 10°. This angle is formed by the line at the lower endplate of the UIV and the upper endplates of the two adjacent vertebrae. Conversely, other considerations in PJK, such as the postoperative damage of soft tissue, paraspinal muscle, facet capsule, and the interspinous ligament above the instrumented level, were not considered^6,9). Helgeson et al.¹⁰⁾ revised the definition of PJK by an angle of at least 15° proximal to the upper endplate of one or more vertebrae above. Moreover, on the basis of its correlation with patient-reported outcomes, O'Shaughnessy et al.¹¹⁾ and Bridwell et al.¹²⁾ used 20° as the cutoff value. However, when making the ultimate decision on revision surgery, spine surgeons should still take into consideration the patient's condition, the extent of the re-surgery, radiological measurement, and proposed clinical outcomes. This consideration remains important even though a cutoff value of 15° or 20° could reflect the physiological components and clinical outcomes for PJK^13,14).

A better understanding of the clinical and radiological parameters that influence the incidence of PJK following ASD surgery could assist surgeons in preventing issues linked to the upper segment both before surgery and during surgical evaluation. This literature review aims to provide a comprehensive analysis of the existing knowledge on the factors influencing PJK in ASD corrective surgery and prevention techniques, with the goal of serving as a guideline in the evaluation of those patients.

Materials and Methods

A literature review was conducted through web search on PubMed with the following combination keywords: “proximal junctional kyphosis,” “adult spinal deformity,” and “risk factor.” Between January 2001 and June 2024, 945 articles were found, which were then thoroughly reviewed by all authors in accordance with PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) criteria (Fig. 1). The inclusion criteria encompassed articles pertinent to the research objectives, published from January 2001 onward, written in English, and addressing exposures related to the broad spectrum of PJK following instrumentation in patients with ASD. The exclusion criteria included studies with methodological flaws, irrelevant focuses of discussion, and non-peer-reviewed sources (such as conference abstracts, opinions, or unpublished work). The primary outcomes of interest in this review were factors associated with the incidence of PJK in patients with ASD who had deformity correction surgery and instrumented fusion. We divided the factors into two categories: radiological and non-radiological parameters, including patient characteristics and surgical techniques.

Figure 1.

Flow diagram (PRISMA format) of the screening and selection process used in this study.

PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses

Results

Non-Radiological Parameters

Recent studies indicate that patient-related variables have emerged as a risk factor for PJK following ASD surgery, as shown in Table 1^6,15,16). Age should be regarded as an important consideration in the preoperative planning process by surgeons. Older adults comprised the vast majority of patients with symptomatic ASD, and they had degenerative comorbidities. Patients who undergo surgery for ASD have been found to have pulmonary circulation issues, congestive heart failure, and neurological abnormalities as factors contributing to the need for revision surgery^17-19). Elevated body mass index is another possible risk factor given the additional load on the spine due to increased body weight^20-22).

Table 1.

Non-radiological Risk Factors for PJK in Patients with ASD Surgery.

Risk factor	Prevention measures
Patient-related factor
Age >55 year old6,16)	Non-modifiable
High BMI20-22)	Counseling for overweight management
Comorbidity(pulmonary circulation, congestive heart failure, and neurological disorders)18,19)	Careful preoperative screening and preventive measures
Low bone quality6,28,77)	Routine evaluation of bone quality preoperatively in patients with adult spinal deformity (DEXA scan, FRAX score, S1 VBQ score, Hounsfield units based on CT scan)26-28,30)
	Teriparatide before/after surgery29,52)
Preexisting vertebral fracture49,50,78)	Increase bone quality preoperatively Consider cement augmentation in eligible patients79)
Back muscle degeneration22,57)	Preoperative and postoperative physiotherapy is recommended59,60)
Surgical technique procedure
Combined AP approach56)	Minimally invasive approach for proximal segment37,38,80) Combined AP approach only in selected cases Ligament augmentation in patients at high risk of PJK9,35) Preserve neuromuscular envelope as much as possible1,38)
Rigid proximal instrumentation10)	Hybrid construct in UIV81,82)
Junctional zone UIV selection83,84)	Avoid UIV at T12-L1
Long-segment fusion40,78)	Minimized fusion level as needed Minimize fix fusion to the pelvis (only in selected cases)85)
Overcorrection6,86)	There is no definitive target; considering age-adjusted alignment goals has the potential to reduce PJK73)

AP: Antero-posterior; ASD: adult spinal deformity; BMI: body mass index; CT: computed tomography; DEXA: Dual-energy X-ray absorptiometry; FRAX: Fracture Risk Assesment; PJK: proximal junctional kyphosis; UIV: upper instrumented vertebrae; VBQ: vertebral bone quality

In spinal instrumentation surgery, the quality of the bone plays an important role. Assessing bone quality has become one of the preventive measures in preventing PJK. Dual-energy X-ray absorptiometry (DEXA) is universally recognized as the most reliable method for assessing bone mineral density and the quality of bone^23-25). Nevertheless, DEXA has its limitations. The findings can be influenced by operator expertise and spinal degeneration, resulting in an imperfect assessment of bone integrity in the affected region. Deng et al.²⁶⁾ introduced a different method for assessing vertebral bone using non-contrast T1-weighted lumbar magnetic resonance imaging. By measuring the vertebral bone quality (VBQ) score of the S1 vertebra, the surgeon can predict the risk of PJK after the surgery. There is a direct correlation between the S1 VBQ score and PJK risk, such that a higher S1 VBQ score corresponds to an increased likelihood of PJK. Computed tomography (CT) scans aid in surgical planning for patients with ASD who are unable to undergo a DEXA scan^23,27,28). By analyzing the Hounsfield unit (HU) values on a preoperative CT scan at the UIV, we may evaluate the strength of the bone in specific regions. Additionally, assessing the HU value along the anticipated screw route can help predict the strength of the contact between the bone and the screw before inserting the screw^27,29). The FRAX score is also a valuable tool for evaluating bone quality, aiding surgeons in preoperative planning for patients with ASD³⁰⁾.

Paraspinal muscle degeneration might be a risk factor for sagittal decompensation and proximal junctional complications^22,31-33). Maintaining essential spinal function and substantial quality of paraspinal muscle are the keys to long-lasting favorable outcomes following fusion surgery in ASD cases³⁴⁾. Soft-tissue preservation techniques during dissection are crucial for preventing PJK after the surgery. A biomechanical study in a calf model showed that dissection of the posterior ligament at UIV/UIV+1 leads to a significant increase in range of motion at this level, which can be considered a risk factor for PJK and should be avoided during surgery³⁵⁾. Furthermore, the supraspinous ligament's fibers attach multisegmentally to the spinous processes; maintaining these ligaments and spinous processes distal to the UIV is considered beneficial for maintaining sagittal stability. Other advantages of performing minimally invasive surgery (MIS) for ASD include less scarring, shorter hospital stays, and decreased intraoperative blood loss³⁶⁾. It is very highly likely that MIS techniques could help to reduce the development of PJK because they preserve the integrity of soft tissue³⁷⁾. Park et al.³⁸⁾ described a hybrid technique for cases where the surgeon opts for open surgery. In this approach, most of the exposure is performed using an open technique, and the muscular and posterior parts of the topmost levels are left intact³⁸⁾. The use of three intact layers of the upper muscle is recommended to prevent PJK following surgical procedures. Another surgical procedure that can be performed is ligament augmentation. Implementing this procedure was proven to significantly enhance spinal stability in patients with ASD and a concomitant risk of developing PJK⁹⁾.

The type of instrument used to achieve fusion also plays a role in the occurrence of PJK. The goal is to create a semi-rigid transition proximal to the instrumented vertebrae to reduce junctional-level stresses or reinforce the anterior column to increase the fracture resistance of vertebrae²⁾. Although it may compromise construct rigidity and PJK still occurs with hybrid instrumentation, using a more rigid instrument at the UIV, such as a rigid pedicle screw, has been associated with higher risk compared with the use of hooks^10,39). Another study conducted by Kim et al.⁴⁰⁾ showed that hybrid instrumentation (proximal hooks and distal pedicle screws) has a higher risk of PJK compared with hooks only. Cammarata et al.⁴¹⁾ found that in addition to hooks at the UIV and tapered transition rods, sagittal preoperative rod curvature allowed a decrease in loads and tension in the upper segment of the UIV. Finite element analysis has shown that the semi-rigid fixation increases the mobility at the upper instrumented segment. Semi-rigid instrumentation and a more gradual transition in the upper instrumented segment provide benefits due to decreasing the screw loads at the UIV level, which in turn may assist in lowering the risk of PJK⁴²⁾. However, contradictory data suggest that the clinical significance of adjacent proximal kyphosis following ASD surgery remains unclear¹⁰⁾.

Although adjacent-level proximal kyphosis has been shown to significantly increase when using pedicle screws, the clinical significance of this is unclear¹⁰⁾. When pedicle screws alone are used, optimal rod contouring has also been proposed as a method to decrease PJK incidence. Another attempt to decrease the risk of developing PJK is to aim the pedicle screw trajectory at UIV toward the caudal direction. An angle of <3° between the UIV superior endplate and the screw trajectory could minimize the risk of PJK⁴³⁾. Soft-tissue-preserving methods with less rigid instrumentation and/or sagittal rod contouring in the upper segment of the fusion are believed to be beneficial for preventing the incidence of PJK.

Longer fusion levels have been associated with a higher risk of PJK postoperatively. Kim et al.⁴⁰⁾ demonstrated that more than 11 levels of fusion statistically significantly increased PJK risk compared with shorter fusion levels. The upper level of fusion is particularly important given that the highest incidence of PJK has been found among patients fused from the upper thoracic spine (above T7) to the sacrum⁴⁴⁾. Defining the UIV is crucial; however, the lower aspect of fusion has been equally significant. A significantly higher rate of fusion to the sacrum has been found among patients with PJK compared with those without⁵⁾. However, in some cases, extending the fusion to the sacrum is beneficial, such as in patients with adult scoliosis who aim to prevent early degeneration of the lumbosacral disc or older patients at risk of sagittal decompensation after long spinal fusion⁴⁰⁾. One study supports the assumption that instrumentation to the sacrum, with or without iliac extension, does not increase the overall risk of PJK⁴⁵⁾. Therefore, when the benefits of lumbosacral fusion outweigh those of lumbosacral non-fusion, the final alignment and balance should be normalized, leading to the normalization of global spine alignment and balance. Furthermore, to date, there is no consensus on the ideal upper or lower instrumented vertebra to prevent PJK in ASD. Thoroughly considering which levels to fuse in cases of severe ASD is essential, as it will have an impact on both clinical outcomes and budgetary considerations^46,47).

The alignment imbalance can be more extensive after surgery if there is a fracture in the proximal part of the fusion level. Proximal junctional fracture (PJF) is a known problem that usually occurs after fusion surgery on long segments (at least four spinal levels) in people with ASD. Fractures typically occur at the UIV or UIV+1 level, with fractures at the UIV level tending to appear earlier than those at the UIV+1 level⁴⁸⁾. Having a pre-existing vertebral fracture increases the likelihood of developing a PJF after corrective surgery in adults with spinal deformity. From the clinical point of view, multiple investigations have demonstrated a correlation between severe vertebral fracture (Grade 2 or 3) and low bone mineral density^49,50). The severe vertebral fracture group had a higher occurrence of PJF due to the added strain on the upper adjacent segments of the UIV caused by the lengthy spinal fusion and alignment correction. The decline of the frontal osseous structure due to inadequate bone quality would ultimately result in the incapacity of the bone to withstand the heightened pressure resulting from the transition of rigid and mobile segments of the spinal column⁵¹⁾.

Given the significant impact on the PJF, comprehending the pathophysiology of PJF following spinal deformity may be advantageous. The risk factors are multifactorial, encompassing a combination of mechanical, biological, and surgical elements. Table 2 presents several factors that may contribute to PJF following surgery. The significant mechanical pressures exerted on the proximal junction following spinal deformity surgery, together with variables such as osteoporosis and diminished spinal flexibility, contribute to the incidence of PJF. Preventive strategies, such as optimizing surgical methods, preserving bone health, and ensuring proper postoperative alignment, are essential for reducing the risk of PJF. Surgeons must prioritize the prevention of postoperative proximal neighboring fractures in patients with substantial vertebral fractures undergoing corrective surgery for ASD. Teriparatide can be administered both before and after surgery as part of a treatment program to enhance bone quality^29,52,53).

Table 2.

Risk Factors for PJF in Adult Spinal Deformity Surgery.

Mechanical factor	Increased mechanical stress87)
	Loss of motion segment flexibility88)
	Spinal imbalance89)
	Altered loading pattern90)
Biological	Bone quality28,91)
	Preoperative condition92)
Surgical	Muscle weakness93)
	Postsurgical inflammation and vascular changes in the adjacent soft tissues92)

PJF: proximal junctional fracture

The use of cement augmentation screws in ASD has been associated with lower rates of PJK. However, no significant differences have been found in the rates of surgeries related to complications such as vertebral column fracture, screw loosening and pullout, implant failure, and pseudarthrosis. Moreover, the same study showed that there were several cases of serious cardiopulmonary complications following cement augmentation^54,55). The use of cement augmentation might be beneficial in reducing PJK. Nevertheless, the decision is determined by the surgeon's clinical assessment, taking into account the potential escalation of certain severe complications.

Back muscle degeneration and muscular atrophy are often observed in patients with ASD^56,57). The condition is influenced by a variety of variables, ranging from the natural aging process to persistent ailments that hinder the patient's ability to engage in typical activities. Both preoperative and postoperative physiotherapy have been shown to reduce pain, enhance psychological well-being, and increase overall quality of life and levels of physical activity. However, it is important to note that this treatment option can be costly^47,58,59). Supervised exercise under the guidance of a health professional is more beneficial in improving a patient's pain and disability after spine surgery⁶⁰⁾. Implementing a well-structured and gradual training regimen that specifically targets the muscles will result in notable improvements in both muscle strength and muscle mass. Increasing muscle mass and reducing the amount of fat infiltration in the paraspinal muscles can assist the patient in maintaining proper postoperative alignment³¹⁾.

Radiological parameters

Degenerative processes of the spine, including the weakening of the soft tissue around the spine, enlargement of the facet joints, age-related issues with the discs, and poor bone quality, can significantly affect the spine alignment in the sagittal plane. Consequently, inadequate surgical planning can result in excessive correction. Table 3 shows several radiological measurements that should be considered in ASD surgery. Greater degrees of deformity adjustments or excessive correction are linked to a higher likelihood of PJK¹⁾. Lafage et al.^61,62) demonstrated that patients with PJK experienced a more significant alteration in the curvature of the lower back (lumbar lordosis [LL]) and upper back (thoracic kyphosis [TK]) compared with patients without PJK. While older adults are susceptible to independent risk factors for PJK, customizing the surgical alignment correction can enhance the outcomes. Surgeons must include age-specific alignment targets in the usual preoperative planning process⁶³⁾.

Table 3.

Radiographic Parameters Associated with PJK Occurrence in Patients Undergoing ASD Surgery.

Radiographic parameters	Definition	Proposed correction
Preoperative low LL72)		Be careful of overcorrection
Higher SVA, overcorrection of spinopelvic parameters69,72)		Proposed to aim for lower SVA postoperatively
Thoracic tilt (TT)64)	Angle is formed by the line of anterior edge of T1 superior endplate, T12 inferior endplate, and the plumb line. Positive value if T1 is anterior to the anterior aspect of T12 and negative value if it is posterior to T12.	Rebalancing the TT in a sagittal neutral position
Upper instrumented vertebra-femoral angle (UIV-FA)68)	The angle subtended by a line from the UIV center to the center of the femoral heads to a vertical line.	Position the UIV center as close as possible to the center of femoral head (<5° is preferable)
Fused spinopelvic angle (FSPA)65)	The angle is formed by the line connecting the center of the lower endplate of the UIV to the midpoint of the bicoxofemoral axis, and the line connecting the center of the sacral endplate to the midpoint of the bicoxofemoral axis.	Improving the FSPA by modifying the PI-LL and lordosis distribution index
UIV inclination61,66,68)	The angle between the vertical and the best-fit line crosses the center of the UIV, UIV-1, and UIV-2.	Make the alignment on the three upper levels of fusion approaching the virtual vertical line.

ASD: adult spinal deformity; LL: lumbar lordosis; PJK: proximal junctional kyphosis; SVA: sagittal vertical axis; UIV: upper instrumented vertebrae

Qiu et al.⁶⁴⁾ found that thoracic tilt (TT) can be used as a radiological parameter to predict PJK. TT is defined as the angle created by the line connecting the front edge of the T1 superior endplate and the T12 inferior endplate, and the vertical plumb line, as depicted in Fig. 2. If T1 is located in front of the anterior aspect of T12, it will have a positive value. Conversely, if T1 is positioned behind T12, it will have a negative value. A higher preoperative TT may indicate an unbalanced TK. The fused spinopelvic angle (FSPA) is a fixed parameter that is not dependent on the patient's position. A decrease in the FSPA increases the risk of PJK. The FSPA can be changed using pelvic incidence-lumbar lordosis (PI-LL) mismatch and the lordosis distribution index. Surgeons should improve the FSPA by modifying the PI-LL and lordosis distribution index during ASD surgery to prevent PJK⁶⁵⁾. PJK is more likely to occur in patients with smaller UIV inclination angles. The UIV inclination angle is the angle between the vertical and the best-fit line crossing the center of the UIV, UIV-1, and UIV-2^61,66). Another radiological parameter is the extent of dorsal displacement of L1 relative to the lumbar gravity line. During the surgery, it may be important to take into account the postoperative distance of the L1-lumbar gravity line⁶⁷⁾. In addition, the measurement of the upper instrumented vertebra-femoral angle (UIV-FA) is recommended to be evaluated. This parameter has been shown to correlate with other parameters, such as pelvic tilt, sagittal vertical axis (SVA), TK, and lumbar distribution index⁶⁸⁾.

Figure 2.

A. The illustration of TT, SVA, and LL. B. The illustration of UIV inclination, UIV-FA, and FSPA.

FSPA: fused spinopelvic angle; LL: lumbar lordosis; SVA: sagittal vertical axis; TT: thoracic tilt; UIV: upper instrumented vertebrae; UIV-FA: upper instrumented vertebra–femoral angle

Kim et al. showed that patients with PJK had a higher preoperative and postoperative SVA compared with the non-PJK group⁷²⁾. Those findings suggest that these patients had worse radiographic sagittal malalignment preoperatively; when the surgeon made an overcorrection, assuming it would produce a better outcome because spinal alignment was achieved, the risk of PJK eventually increased. Increasing SVA is closely associated with an increase in the PI-LL mismatch. Furthermore, spinopelvic overcorrection may result in undercorrection of TK (flattening), which increases the risk of PJK or PJF following surgery^68-70). Accordingly, overcorrection of SVA and lumbar lordosis has been found to be significantly more common in patients with PJK^71,72). There is no definitive target for correction, but age-adjusted alignment correction could help reduce PJK⁷³⁾. However, spinal degeneration over time is difficult to quantify and varies among patients⁷⁴⁾. In terms of aging, Roussouly et al.⁷⁵⁾ state that maintaining sagittal plane alignment preserves the physiological configuration and also prevents the development of further deformity. Prasse et al.⁴⁶⁾ recommend using anatomical reconstruction of the sagittal spinal profile following the Roussouly types as a reference. They generated the formula “L4S1=0.66×L1S1” for the calculation of the proposed angle for correcting deformity⁷⁶⁾. However, the target correction indeed needs to be personalized according to the patient's condition and postoperative goals.

Conclusion

The occurrence of PJK at adjacent levels following deformity correction surgery in patients with ASD was found to be multifactorial. Understanding these factors can aid in the preoperative planning and assessment for patients with ASD. Furthermore, the proposed correction should be based on an individualized approach.

Author Contribution: Z.A.L. contributed to the conception, design, drafting, and writing of the article. Y.M.S. and A.J.R. contributed to the conception and revising of the article. K.N. contributed to the drafting and revising of the article.

Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.

Ethical Approval: This study did not require approval from the relevant institutional ethical review board because this study is a literature review.

Informed Consent: Consent was not required because this study involved no human participants.