Proximal Junctional Kyphosis in Modern Spine Surgery: Why Is it So Common?

Abstract

In the past few decades, proximal junctional kyphosis (PJK) has emerged as a new complication after instrumented spinal fusion in adult and pediatric spinal deformities. This phenomenon has occurred concomitantly with the rise of robust instrumentation techniques and enhancement of our abilities to obtain greater spinal deformity correction. The goal of this paper is to review the mechanical and biological causes of PJK and recommend prevention strategies.

Introduction

In the past few decades, proximal junctional kyphosis (PJK) has emerged as a new complication after instrumented spinal fusion in adult and pediatric spinal deformities^1-3). This phenomenon has occurred concomitantly with the rise of robust instrumentation techniques and enhancement of our abilities to obtain greater spinal deformity correction^4,5). The goal of this paper is to review the mechanical and biological causes of PJK and recommend prevention strategies.

Definition

What is PJK?

PJK is an abrupt kyphotic change between two spinal units in the sagittal plane of the spine, occurring above the upper part of the instrumentation (Fig. 1). While the radiographic definition of PJK is variable, it is widely accepted that PJK is a kyphotic change of at least 10° cranial to the upper instrumented vertebra using the Cobb angle method⁶⁾. PJK may occur at any part of the spine but is most frequently observed at the thoracolumbar junction, the upper thoracic level, and in the cervical spine in the setting of recurrent PJK. It is sometimes associated with junctional vertebral fracture, not always secondary to osteoporosis. PJK deformity appears acutely or progressively with or without pain but often with implant failure or vertebral listhesis that could be palpable under the skin at the uppermost instrumented levels. Mild PJK may be well tolerated by patients if the sagittal alignment is not too disrupted. However, severe PJK cases worsen with time and lead to revision surgery that often includes an extension of the fusion, instrumentation, and sometimes revision of realignment^2,5).

Figure 1.

Example of PJK in pediatric (left) and adult (right) patients.

The Sagittal Alignment and Balance of the Human Body

To better understand the emergence of PJK in modern spine surgery, it is important to go over three major biomechanical concepts explaining the erect posture of humans and its relationship with gravity in three dimensions:

Concept 1: Chain of alignment

From the polygon of support (a surface created by the plantar surface area of bilateral feet in the standing position and by both buttocks and the posterior aspect of the thighs in the sitting position) up to the head, the regional masses of the entire body are harmoniously distributed in Three dimension (3D) along the gravity line. As illustrated in Fig. 2, the various parts of the skeleton and joints create a “chain of alignment” where the entire pelvis can be considered a “pelvic vertebra” playing a major role as an intercalary and adaptive bone between the spine and lower limbs, and the head can be considered a “cephalic vertebra” playing a special role in the reverse pendulum (sagittal alignment) of the spine due to its own mass^7,8).

Figure 2.

Schematic representation of the chain of alignment. Reproduction from “Cone of Economy with the Chain of Balance -Historical Perspective and Proof of Concept” (https://doi.org/10.22603/ssrr.2022-0038).

While all these elements are perfectly aligned (i.e., straight) in the frontal plane, the chain of alignment is more complex in the sagittal plane. First, the orientation of the head is aimed at maintaining a horizontal gaze; then successively, the vertebral column is arranged in cervical lordosis, thoracic kyphosis, lumbar lordosis, and a slight pelvic retroversion in relation to the lower limbs. A good sagittal alignment in the standing position includes a gravity line (from the center of the polygon of support up to the head's center of mass) located just behind the center of femoral head projections on X-rays^7,9). Each spinal region has a harmonious transition to the adjacent segment, especially at the junctional zones of the alignment chain.

X-ray projections permit the measurement of angles at numerous levels of the alignment chain in asymptomatic populations and patients. Numerous measurements have been described and used to classify spinal pathologies or help in planning surgical interventions¹⁸⁾. One of the fundamental parameters described by Mrs. Duval Beaupère is the pelvic incidence (PI), an anatomical parameter of the pelvis, which in reality measures the position of the sacroiliac joint within the ilium and was found to be highly determinant of lumbar lordosis and subsequent thoracic kyphosis¹⁰⁾. This led to a classical formula: PI=SS+PT (SS and PT are postural parameters) where SS is the sacral slope or inclination of the sacral plateau versus the horizontal line and pelvic tilt (PT) is the measurement of the pelvic version¹¹⁾. PI is acquired progressively during childhood in response to the evolution of the standing posture and is supposed to remain fixed once growth is completed. PI, however, is a unique measure between individuals; this characteristic served as a basis for Dr. Pierre Roussouly's classification of spinal shapes in a “normal” population where the “ideal” lordosis curvature is defined based on four groups of PI angles¹²⁾.

Concept 2: Alignment is not balance

One fundamental concept is that alignment is a static concept, whereas balance is dynamic and relates to the body's motion around the gravity line in 3D. It is not possible to evaluate balance solely based on X-ray projections on the screen, as we see in “scientific presentations” and meetings all over the world. When we say motion, it means that balance is under the neuromuscular system's dependence and can be evaluated by measuring the amplitude of motion for each body region throughout the alignment chain in each of the three directions of space. Those body masses are under the influence of muscle function modulated by the nervous system. One could say that balance is “stability within the movement,” remembering the words of Albert Einstein―“Life is like riding a bicycle: to keep balance you must keep moving.”

When studying alignment from an anatomopathological point of view, it is essential to assess the regional alignment between two consecutive spinal units as well as the global alignment of the spine. This is important because regional and global spinal alignment are governed by two types of soft tissue elements at the local and global levels:

- Passive function at the local level: soft tissue structures such as discs, ligaments, capsules, and aponeurosis (all “white structures” as defined by François Bonnel¹³⁾) have a critical function of tensegrity (tension/compression) and can release the energy created by their tension passively.

- Active function at the global level: muscles surrounding the spine and its related structures (ribs, limb girdles, and upper/lower extremity) have direct and indirect impacts on spinal alignment with active muscle function.

These two concepts are the foundation of the 3D balance of the human body in the standing or sitting positions and served as the basis of the Cone of Economy concept (Fig. 3)^7,8). The cone starts from the polygon of support and represents the maximum amplitude of movements the body can tolerate in each direction away from the gravity line, without falling. If the patient is aligned within the conus of economy, minimum muscle movements exist, and erect posture requires almost no muscular expenditure. In contrast, when the body drifts away from the center of the conus, a larger amplitude of motion and muscle recruitment are required to avoid falling. This theory from 1975 has recently been supported by the work of Drs. Isador Lieberman and Ram Haddas aiming to quantify the “Conus of Economy” based on objective data¹⁴⁾. Using a force plate, the authors found an increased amount of sway demonstrated by mapping the center of mass in the horizontal plane in patients with spinal pathologies. In addition, using gait analysis and concomitant Electromyography (EMG), the authors found an increased duration of muscle activities such as those of the rectus femoris, tibialis anterior, semitendinosus, and medial gastrocnemius during gait in patients with spinal deformities^15,16). Gait analysis ensures a proper evaluation of the body balance in 3D with precise measurement of active muscle function in any direction but essentially in the sagittal plane and against the gravity forces.

Figure 3.

Cone of Economy concept as described by Dr. Dubousset in 1975. Reproduction from “Cone of Economy with the Chain of Balance -Historical Perspective and Proof of Concept” (https://doi.org/10.22603/ssrr.2022-0038).

While the balance evaluation can be conducted in the setting of a research laboratory and gait analysis, it is also accessible with clinical measurements of these movements or via dynamic X-rays. In addition to the evaluation of movement, it is crucial to also evaluate the peripheral and central nervous systems and the cognition of every patient. In this context, the Dubousset Functional Test (DFT, Fig. 4) consisting of four simple tasks has been utilized by Dr. Dubousset in his practice for numerous years, but this test was only recently published for the spine and orthopedic community¹⁷⁾. DFT is a practical test that is applicable in a clinic setting and measures the time needed by the patient to perform the following individual tasks: Up and Walking Test (unassisted sit-to-stand from a chair, walk forward/backward 5 meters [no turn], and then unassisted stand-to-sit), Steps Test (ascend three steps, turn, and descend three steps), Down and Sitting Test (stand-to-ground, followed by ground-to-stand, with assistance as needed), and Dual-Tasking Test (walk 5 meters forwards and back while counting down or talking over the phone). DFT is scored by the time (in seconds) consumed by the patient to perform each test. When DFT is performed on an annual or bi-annual basis, the impact of aging or spinal surgery on a patient's function can be estimated. If a significant anomaly is noted during DFT, then a neurological consultation is required. Since PJK is technically a complication of spinal realignment, which is, in part, a reversal of aging, the ability of the patients to tolerate the new alignment from a neuromuscular perspective plays an important role in the surgical outcomes. DFT may give the surgeon a better understanding of how much correction a given patient can tolerate. Time to perform DFT is expected to be an important indicator of the patient's musculoskeletal state, frailty, neurosensorial condition, and coordination.

Figure 4.

The four components of the Dubousset Functional Test.

Concept 3: The compensation phenomenon

All muscles and joints in the chain of alignment adapt to dynamic changes in one region with compensatory mechanisms in different regions to keep the gravity line centered with the polygon of support. For example, it is well known that a knee flexion will compensate for a hip flexion contracture or a pelvic retroversion will compensate for lumbosacral kyphosis, as observed in some high-grade spondylolisthesis or flatback syndromes. With aging or mild forms of deformity, compensatory phenomena allow the patient to maintain an acceptable function for many years and sometimes all life long¹⁸⁾. Therefore, it is essential to recognize compensations and differentiate them from the drivers of malalignment. Surgical intervention on one region of the chain should not compromise the other compensatory segments to avoid functional impairment¹⁹⁾. For example, full-body assessment of alignment and compensation helps in recognizing when hip flexion contracture eliminates the patient's hip extension reserve generating lumbar kyphosis and pain; in such cases, addressing hip flexion contracture with a total hip (for example) re-establishes a good sagittal alignment, restores the lordosis, and decreases disability²⁰⁾. Even a pathology at a single spinal level (local kyphosis driven by a disc herniation or degenerative disc disease) is compensated by hyperextension of adjacent segments and may result in a global spinal malalignment¹⁸⁾. Addressing this localized spinal pathology without respecting the local and regional alignment may result in local or iatrogenic kyphosis. In extreme cases, the situation may get complicated by PJK and subsequent catastrophic operation that may end with a fusion from the pelvis to the cervical spine, rendering the “spine like a statue” (Fig. 5).

Figure 5.

Case example of multiple recurrent PJK. Preoperative image (A), followed by radiographs after the index procedure (B). The patient developed PJK (C), subsequently underwent revision (D), and developed recurrent PJK (E). Reprinted with permission from Kim et al..

Cause of Proximal Junctional Kyphosis

PJK occurs after short or long spinal fusions and lies at the junction between the instrumented (stiff) and un-instrumented (mobile) spine. We can divide the causes of PJK into two groups, mechanical and biological, at local and global levels.

Mechanical causes of PJK

The mechanical causes of PJK can be categorized as local or global, keeping in mind that in spinal alignment, there is always an association between local and global pathologies.

1. Local mechanical causes of PJK

a) Soft tissue disruption secondary to the surgical exposure at the upper level of fixation: Surgical exposure often requires soft tissue stripping (muscles, ligaments, and joint capsules) to expose the subperiosteum around the laminae or facet joints. This step is important to properly visualize anatomical landmarks for pedicle screw insertion or hook application (laminar or pedicular). However, such soft tissue destruction destabilizes each vertebral unit. A few decades ago, Dr. Dubousset and Dr. Henri Robert aimed to investigate the consequences of laminectomies. Data showed that removal of the interspinous ligament at one spinal level initiated kyphotic deformity, and removal of each posterior facet joint of the same level produced 100% kyphosis at that level^21-23). Similar findings were observed in the clinical practice of the first author.

b) Weakness of the posterior instrumentation to pullout: Biomechanical studies demonstrated that the pullout strength of pedicle screws with or without cement augmentation is inferior to that of vertebral claws (i.e., supra laminar + infra pedicular hooks) placed on two consecutive vertebrae. As such, the systematic use of all-screw constructs may not be optimal²⁴⁾. Recent work by Viswanathan et al. highlighted the potential benefit of supplementing pedicle screw constructs with sublaminar banding as a means for a gradual transition between fused and unfused segments²⁵⁾.

c) Lack of anterior vertebral body resistance to compression due to multiple reasons, including osteoporosis or metabolic bone diseases. In such conditions, gradual progressive kyphosis or sudden hyperflexion of the cephalad adjacent levels leads to bending moments at the upper instrumented vertebra and ultimately fatigue failure and possibly compression fracture with a subsequent kyphotic deformity.

2. Global mechanical causes of PJK

a) Displacement of the center of gravity of the head is a common situation occurring secondary to numerous iatrogenic factors. The most frequent is that the uppermost vertebra chosen to be instrumented is an unstable one, such as the junctional zone. Another example is in the setting of thoracic scoliosis, which frequently presents with a mild structural curve between T1 and T5. In these cases, the power of instrumentation encourages the surgeon to achieve the greatest amount of correction and aim for a 0° Cobb angle in the coronal plane. To achieve that, and often out of fear of residual shoulder imbalance, the surgeon extends the instrumentation to T1 or T2. It is difficult, with modern segmental instrumentation with screws at every level, to bend the rods adequately to achieve physiological upper thoracic kyphosis. Therefore, extending the fusion often leads to pulling back the upper thoracic spine and subsequently the head posteriorly, which may potentially increase the risk of PJK. Similarly, in the adult spine, flattening of thoracic kyphosis with all-screw posterior instrumentation leads to posterior translation of the body mass above the instrumented spine including the head. This will necessitate a compensatory cervicothoracic flexion to maintain the gravity line and horizontal gaze, which potentially increases the loading moment at the upper instrumented vertebra and leads to PJK. A similar phenomenon occurs with surgically induced hyperlordosis, especially in long instrumentations to the pelvis²⁶⁾, as the upper thoracic spine translates posteriorly, setting the stage for PJK²⁷⁾. Of note, with the old hybrid Cotrel-Dubousset (CD) instrumentation, PJK was less frequently observed²⁸⁾. Finally, PJK observed at the cervical level is quite peculiar; it is often severe and progressively includes level after level, leading to a fusion of the entire cervical spine. These successive surgeries are often the result of insufficient intervertebral lordosis. We must remember that the greatest amount of cervical lordosis lies at the Occiput-C2 level.

Biological causes of PJK

a) In adult patients, aging is associated with decreased muscle mass and therefore a soft tissue compromise, especially within the postural muscles such as the glutei, lumbar spine extensors, and quadriceps. This leads to poor stabilization of the pelvis, predisposes patients to global malalignment of the spine, and has recently been associated with an increased risk of PJK in the setting of adult spinal deformity^29-31).

b) Undiagnosed neuromuscular pathology either central or sometimes secondary to nerve root compression in one or more foramina. PJK often reoccurs in those patients despite multiple revision surgeries³²⁾.

c) Metabolic bone diseases: The state of bone health plays an important role in the etiology of PJK. Patients with osteoporosis have a higher risk of implant-related complications, including screw loosening, subsidence of interbody devices, and compression fracture at the junction with subsequent PJK³³⁾.

Prevention Strategies

In addition to the innovative efforts to improve prophylactic surgical techniques against PJK, a comprehensive approach to treating spinal deformities should include a thorough clinical evaluation, detailed surgical planning, and proper surgical execution.

Comprehensive clinical and patient-tailored assessment

After a thorough history and physical examination, it is important to understand the patient's motivation for seeking medical care. Pain, cosmesis, difficulty walking, frequent falls, and patients' expectations are aspects that need to be addressed before indicating the patient for spine surgery. Preoperative screening for osteoporosis by evaluation of bone mineral density and history of any metabolic bone disease is of utmost importance.

The spinal deformity needs to be assessed on 2D imaging, including the flexibility of the spine in supine or bending X-ray views³⁴⁾. Full-body imaging and 3D evaluation of the deformity have become possible using recent technology. This is important to radiographically evaluate the chain of alignment and compensation from the polygon of support to the head. In addition, the range of motion of the entire chain (head position, cervical to lumbar spine, pelvic vertebra, and lower limb motion including hip extensor reserve) should mandatorily be assessed to estimate the amount of correction that can be tolerated by the patient. Neuromuscular patients, especially those with dystonia, are uniquely challenging when evaluating active and passive range of motion. Precise examination of the range of motion and function of the full body requires a full gait analysis in a dedicated laboratory to complement the full-body static radiographic assessment. Alternatively, in a clinic setting, the functional abilities and disabilities of the patient can be examined using DFT, a simplified method described earlier in this paper¹⁷⁾. The “Down and Sitting” test is expected to be the most discriminant of patients' function, whereas the “Dual Tasking” test aims to provide a rough estimate of the patient's coordination.

Surgical planning

Detailed surgical planning should include a decision on the approach, type, and extent of instrumentation, as well as patient positioning.

a) Approach and instrumentation: All approaches, anterior vs. posterior, open vs. minimally invasive, have their limitations and strengths. Posterior percutaneous transpedicular cement injection can be easily indicated for proximal junctional failure at the thoracolumbar junction. However, the choice of approach and instrumentation for posterior spinal fusion up to T1 is more challenging. While not commonly used, the choice of hooks placed on the two consecutive most cephalad vertebrae (supralaminar at T1 and subpedicular at T2), is more resistant to pullout when compared to all-screw constructs³⁵⁾, especially if the insertion is performed with minimal stripping of the ligamentum flavum ligament, capsules, and periosteum³⁵⁾. If a combined approach is to be utilized, an anterior approach with or without instrumentation is recommended first, followed by a posterior approach either the same day or staged. The rationale behind this is that releasing the anterior column first reduces the correction forces needed during the posterior approach (and therefore decreases the risk of pullout).

b) Preoperative traction in severe spinal deformities can be useful. Despite its external appearance, halo traction is one of the less aggressive options and provides good access for the anesthesiology team. Lower limb tractions (skin traction from thigh to foot) with excellent padding of bony prominences is another option that can be asymmetrical to address pelvic obliquity. Total traction must remain at a maximum of 1/3 to 1/4 of the total body weight and can be assessed using a dynamometer. Preoperative traction can partially correct the deformity, leading to a less aggressive correction, less instrumental reduction, and subsequently a lower risk of PJK.

c) Draw your surgical plan: Whether manually or through established software solutions, it is recommended to draw a schematic representation of the surgical plan, including the choice of implants, on the radiograph of the spine (hooks, pedicle screws “mono- or poly-axial,” rods, plates, cages, etc). This plan should be visible and accessible at any time to the OR staff, especially the circulating nurse and scrub technician.

d) Software simulation: Surgical correction can be simulated using various dedicated software available today to every surgeon. Some sophisticated software solutions can predict the postoperative alignment to a certain degree^36,37). This has been an academic exercise for researchers in the field of biomechanics. However, the surgeon needs to be able to see and adjust her/his surgical plan based on the software prediction. Some software solutions are able to incorporate different data points (bending films, flexibility/rigidity of the spine, and traction films), enabling the surgeon to modify the plan accordingly. Finally, this simulation can be an educational tool for the patient herself/himself to see their potential postoperative alignment and deformity correction, which helps ease their anxiety and manage their expectations.

Intraoperative measures

a) OR table positioning: Proper positioning is required to preserve spinal alignment, especially the cervical spine, which is best positioned in a neutral alignment. Using thoracic and pelvic support to minimize pressure on the abdomen also reduces the variability of intraoperative spinal alignment. Finally, this alignment should be confirmed radiographically before making an incision.

b) Soft tissue preservation: It is always beneficial to place an implant with a less invasive technique, especially in the upper thoracic and thoracolumbar junction. In addition, respecting the soft tissues (facet joints and interspinous ligament) of the vertebra adjacent to the uppermost instrumented level is of great importance³⁸⁾.

c) Rod bending and rotation: Rod placement should be meticulously planned, including insertion order and rotation direction. In terms of the rod rotation's technique for scoliosis correction, the surgeon can consider the backward/forward method, which ensures gentle and progressive stretching of the soft tissue (discs, ligaments, capsules). Abrupt or one-step rod insertion not only is dangerous for the neural elements but also disrupts the sagittal alignment of the patient by placing greater stress on the distal and proximal fixation sites and increasing the risk of mechanical failure and future PJK. The gentle backward/forward method also allows the surgeon to visualize the impact of rotation/bending on the entire construct, including bone-implant interfaces, and provides feedback on the fixation quality. These principles apply to every fixation at any spinal region.

d) Transition from instrumented to un-instrumented spine: Mechanically, a fused spine is extremely rigid compared with an un-instrumented one. The change from a stiff to a flexible spine induces increased stresses at the junction, potentially leading to failure in the setting of poor bone and/or muscle quality. Recent studies have demonstrated the potential effect of prophylactic techniques, such as tether or sublaminar band, used proximally to ease this transition from an instrumented to uninstrumented spine³⁹⁾.

e) Transferring the patient off the OR table and releasing traction should be performed gently with specific attention to sudden head flexion or thoracolumbar motion to protect the construct and the spinal realignment obtained.

f) Postoperative radiographs a few days after surgery provide excellent feedback on the new spinal alignment and global body posture. Ideally, the patient should maintain a horizontal gaze without cervical kyphosis or protraction of the head. In addition, lower limb alignment must be assessed to identify residual compensatory mechanisms such as hip flexion/extension or knee flexion.

Conclusion

Our critical analysis of the literature is shown in (Table 1 and Table 2). We conclude that the increased power of instrumentations in modern spine surgery may explain the higher frequency of PJK in the past few decades. In addition to emerging prophylactic surgical techniques, soft tissue preservation and emphasis on comprehensive preoperative assessments of spine patients including static alignment, functional capacity, full-body balance, and 3D analysis of the spine may play significant roles in reducing the incidence of PJK.

Table 1.

Recommendations for Care*.

Recommendations for Care	Quality of Evidence
PJK is an emerging complication in spinal deformity surgery due to the rise of robust instrumentation techniques.	B
The mechanical causes of PJK include soft tissue disruption at the junctional level, displacement of the center of gravity of the head due to over- or under-correction of the deformity in the sagittal plane.	B
The biological causes of PJK include suboptimal para-spinal muscle mass or fatty infiltration, undiagnosed neuromuscular pathology, or metabolic bone disease.	B
Full-body 3D assessment of the spinal deformity and compensatory mechanisms associated with it is of utmost importance for surgical planning.	C
Preoperative functional assessments using gait analysis or the Dubousset Functional Test are important to objectively evaluate the level of functionality and coordination for each patient undergoing spinal deformity correction.	B
Surgical planning must include drawing of the surgical plan, preoperative software simulation, intraoperative radiographic verification, and postoperative full-body standing radiographs for feedback.	C
Soft tissue preservation and gradual transition from fused to un-fused spinal segments are possible methods to aid in PJK prevention.	B

*Grade A=Good evidence (Level I studies with consistent findings) for or against recommending intervention. Grade B=Fair evidence (Level II or III studies with consistent findings) for or against recommending intervention. Grade C=Conflicting or poor-quality evidence (Level IV or V studies) not allowing a recommendation for or against intervention. Grade I=There is insufficient evidence to make a recommendation.

Table 2.

Peer-Reviewed Publications Supporting Recommendations for Care.

Publication	Take-Home Message
Treatment of adult thoracolumbar spinal deformity: past, present, and future. Smith et al., Journal of Neurosurgery, 2019.	PJK arguably remains the greatest unsolved problem in modern spinal deformity surgery.
Alignment targets, curve proportion and mechanical loading: preliminary analysis of an ideal shape toward reducing proximal junctional kyphosis. Katsuura et al., Global Spine Journal, 2021.	Spinopelvic over-correction, under-correction of TK (flattening), and under-loading of the UIV (decreased bending moment) were associated with PJK and PJF.
Sagittal balance is more than just alignment: why PJK remains an unresolved problem. Glassman et al., Scoliosis Spinal Disorder, 2016.	Neuromuscular disorders were found in 76% of patients undergoing revision surgery for PJK.
Recruitment of compensatory mechanisms in sagittal spinal malalignment is age and regional deformity dependent: a full-standing axis analysis of key radiographical parameters. Diebo et al., SPINE, 2015.	Regional spinal deformity correlates with full-body recruitment of compensatory mechanisms to maintain an erect posture.
The Dubousset Functional Test is a novel assessment of physical function and balance. Diebo et al., CORR, 2019.	The Dubousset Functional Test provides objective metrics that are easy to obtain to assess function and coordination, and the test requires no special equipment.
What is actually happening inside the “cone of economy”: compensatory mechanisms during a dynamic balance test. Haddas et al., European Spine Journal, 2020.	Utilizing 3D video kinematics and electromyography, the Dubousset conus of economy is quantifiable using a human motion capture system.
Surgical planning for adult spinal deformity: anticipated sagittal alignment corrections according to the surgical level. Lafage et al., Global Spine Journal, 2021.	Planned restoration of the caudal lordosis can predict postoperative global alignment and pelvic tilt corrections.
The hybrid open muscle-sparing approach in adult spinal deformity patients undergoing lower thoracic fusion to the pelvis. Park et al., Neurospine, 2020.	A novel technique that allows for decortication of bony surfaces as well as clear exposure of anatomic landmarks for freehand pedicle screw placement, while protecting the posterior soft tissue structures to reduce the risk of PJK.

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

Sources of Funding: None

Author Contributions: JD wrote the first draft, BD provided critical editing, and JD and BD reviewed the revised version and approved it for submission.

Ethical Approval: Ethical approval was waived by the ethics committee due to the review study design.

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