1. Introduction
Due to the aging of the population, adult spinal deformity (ASD) is increasing over time, affecting the quality of life in adults [1]. Many spinal deformities in adults become symptomatic with intractable pain, gait disturbances, and even neurological deficits that require surgical intervention [2].
ASD is associated with several factors, these factors include radiological deformity, spinal canal stenosis, radiculopathy, and patient comorbidities related to age, obesity, and osteoporosis [3, 4]. ASD has an economic impact and patients with pain and ambulatory difficulties become socially isolated and reduce their quality of life [1, 5]. Complications from ASD surgery include proximal junctional kyphosis (PJK) (the most common), distal junctional failure, screw loosening, disc herniation, rod breakage, and pseudoarthrosis [3, 6, 7, 8].
One of the most common complications in ASD surgery is PJK [5, 9-17]. The prevalence of PJK varies from 6% to 60% in different studies, but it is between 30% and 40% in most studies [18, 19, 20, 21]. As the patient’s condition worsens, severe pain and structural failure occur, which is mentioned as proximal junctional failure (PJF); therefore, PJF is on the end of the same spectrum [22].

2. Methods and Materials/Patients
This study was a retrospective review. The keywords for ASD, PJK, PJF, upper instrumented vertebra (UIV), range of motion (ROM), revision surgery, and transitional zone were used as search terms and most related articles were reviewed. The setting of this study was Hazrat Rasoul Hospital. Data was collected from March 2019 to September 2022 for about three and a half years. 

3. Results
Fifteen consecutive patients who had previous ASD surgery and needed revision surgery were included in this research. The average age ranged from 41 to 65 and the mean age was 53 years. Six men (40%) and nine women (60%) were included in the study. We used spinal x-rays and computerized tomography (CT) scans and magnetic resonance imaging (MRI) to examine the degenerative spine, and canal stenosis, and measure Cobb angle (Tables 1 and Table 2). 




Cobb angle of more than 10° was considered proximal junctional kyphosis in these symptomatic patients. Table 1 presents complications of ASD surgery that need revision surgery. Table 2 presents the age and gender ratio in patients. PJK was the most common complication (6 patients=40%), other complications included disc herniation and canal stenosis (4 patients=26.6%), screw loosening (2 patients=13.3%), rod breakage (one patient=6.6%), and distal junctional failure (2 patients=13.3%). Figure 1 shows different complications of ASD surgery that require revision (Figure 1).

Based on the Cobb angle index, we evaluated the patients with PJK. We investigated the types of failure in patients with PJK as well as the level of UIV and the presence of gradual transitional zone-using hooks. 
In this article, we examined patients with ASD who underwent revision surgery. As a result, of these fifteen patients, six people (40%) were men and nine people (60%) were women. Also in terms of age ratio, the mean age was 53 years. We measured Cobb angle in radiographic and CT-scan parameters to separate patients with Cobb angle equal or greater than 10° for PJK complication in our research. Complications after ASD surgery that needed revision surgery in our study involved PJK, disc herniation and canal stenosis, screw loosening, rod breakage, and distal junctional failure.
Six patients out of fifteen patients who required revision surgery had PJK (40%). We determined:
1. UIV level in three categories:
1-1 Upper thoracic 
1-2 Lower thoracic and thoracolumbar junction
1-3 Lumbar region
Also,
2. Existence of gradual transitional zone-using hooks
Analysis showed that an upper instrumented vertebra in the lower thoracic region, especially in the thoracolumbar junction and lumbar area were mostly associated with developing PJK.
Regarding the level of UIV, no patient had UIV in the upper thoracic and 4 patients (66.6%) in the lower thoracic and thoracolumbar junction, and 2 patients (33.3%) in the lumbar area (Table 3).


Regarding the transitional zone, all these patients with PJK had a rigid pedicular screw on the top of the construct. We also noticed that pedicular screws on the proximal end of the construct in most of these patients were the largest size, which can produce a good fusion, but unfortunately can lead to adjacent segment disease and PJK.
Also, according to Table 4, bone failure is at the top of PJK-related failures (three patients=50%), two patients (33.3%) had implant/bone interface failure and one patient (16.6%) had a disc-ligamentous failure (Figure 2) (Table 4).

4. Discussion
There is no consensus on the definition, classification, and indication of PJK/PJF for revision surgery but there is consensus on treatment and prevention strategies [19, 23]. Hart et al. defined PJF as increased kyphosis of more than 10° at UIV or UIV+2 related to disruption of osseo-ligamentous or pull-out of instrumentation at UIV [9]. Hostin et al. described PJF as 15° or more PJK related to fracture of UIV or UIV+1 or the need for an extension instrument [23]. However, Bridwell et al. investigated the proximal junctional angle of more than 20° as a threshold for PJK outcome [11]. Therefore the definition of PJK/PJF and clinical information can significantly vary in different studies [25]. Three symptoms are involved with severe PJK or PJF, that is, severe pain, neurological deficit, and progressive trunk deformity [19]. Effective factors in deciding on revision surgery in PJK include traumatic etiology, the severity of angulation, high sagittal vertical axis, and female gender [26]. Yagi et al. classified PJK/PJF into three categories, disc-ligamentous failure, bone failure, and implant-bone interface failure [27]. Failure in the thoracolumbar and lumbar is mostly due to bony fracture; however, in the upper-thoracic, it is more common with soft tissue failure and failure in the thoracolumbar region [24]. Risk factors for PJK include age, low bone mineral density, shorter instrumentation below L2, and inadequate restoration of sagittal balance [18]. In clinical outcomes measured by the Scoliosis Research Society (SRS), the pain has a strong predictive value for PJK [20], also, old age, obesity, and osteopenia can increase the risk of fracture at the top of the long segmental pedicle screw construct [7, 8]. Muscle atrophy and relative reduction of muscle mass in obese patients affect the developing PJK. Pennington et al. demonstrated para-spinal muscle size as a risk factor for PJK [28]
Risk factors for PJK are classified as patient-specific, surgical, and radiographic parameters [1, 8, 10, 17, 29]. 
Some PJK with minimal deformity are asymptomatic with only benign radiologic findings and deserve follow-up without any surgical intervention, but severe cases are extremely debilitating and require aggressive and extensive revision surgery. They are a common reason for returning to the operating room as well as a common source of poor postoperative outcomes. Reducing junctional complications has a potential impact on the patient’s quality of life as well as the cost-effectiveness of surgery, which should not be ignored.
Protective factors to prevent PJK are discussed a lot, including soft tissue protection, cement augmentation, ligamentous augmentation, hybrid instrumentation, such as hook, and adequate selection of UIV [2, 3, 30]. In most studies, a significant reduction of PJK by gradual transitional zone at the top of the construct is observed and overall data showed that the use of hooks, tension bands, and tethers can make a softer loading zone, create less rigidity, and in comparison, hooks are the best technique to distribute forces at the proximal end of the instrument and reduce the risk of PJK eventually [30-37]. It can improve ROM and flexibility within the UIV and make a smooth transitional zone.
On the other hand, careful selection of UIV can reduce the risk of PJK after ASD surgery [24, 31, 33].
In this study, we focus on choosing the UIV level pre-operatively and figuring out where to stop the construct and the goal is always to try to avoid stopping in the kyphotic segment. We also emphasize that stopping the UIV at the lumbar region and thoracolumbar junction level between T11 and L1 is considered a high-risk factor for developing PJK [24]. This study aims to provide attention to protective factors against developing PJK, especially two crucial aspects by careful selection of UIV, and gradual transitional zone using hooks in terms of the ROM.
Elderly people are prone to degenerative spinal disease with a wide spectrum and related complications. ASD is one crucial part of this range. In some cases, no severe situation is observed and the patient can be treated conservatively with the following methods:
1. Restriction of high physical activity 
2. Getting more rest
3. Using an appropriate brace
4. Losing weight
5. Physiotherapy with the functional exercise of spinal muscles
6. Taking analgesic and osteoporotic medication such as teriparatide 
7. Having a diet with more calcium and protein
8. Injections (corticosteroids) to help reduce pain and swelling [35, 37].
However, despite all these conditions, some of these patients end up with surgery, and even though many successful results for ASD surgery exist, a significant percentage of patients experience post-operative complications, the most common of which is PJK [1, 2, 5, 9]. Minimizing the risk of PJK after ASD surgery is still very challenging because different risk factors related to the patient-specific/surgical techniques and radiological parameters are involved [17, 29].
In this study, we reviewed protective factors to minimize the risk of PJK and focused on two crucial aspects: 
1) Careful selection of UIV
2) Gradual-transitional zone on the proximal end of construct-using hooks.
1. Careful selection of UIV:
It is essential to determine the location of UIV by pre-operative planning on the x-ray, CT scan, and MRI.
Stopping the instrument in the kyphotic segment should be avoided to prevent post-operative PJK. Also, placing UIV at the thoracolumbar junction (T11 to L1) or lumbar region is more likely to develop PJK, especially in elderly patients with poor bone quality and osteoporosis [1, 24, 33].
2. Gradual-transitional zone on the proximal end of instrumented construct:.
Many studies have been conducted to gradually reduce the forces on the top of the instrumentation using hooks and diminish the stiffness between rigid-instrumented-spine and mobile-non-instrumented-spine on the top [36, 37].
Almost any smooth passage from the fused spine to the mobile unfused spine on the top decreases the risk of PJK post-operatively, thus even though pedicle screw fixation gives us a better fusion system, but still ends up with significant motion at the top of the construct and eventually leads to disruption of posterior ligamentous complex, instability and eventually PJK [33-35]. 
In these patients, most flexibility of the spine ends up just above UIV and as a result, the patient fails at the top.
By using hooks, we share loading on the top of the construct and distribute the forces by gradually transitioning toward the mobile segment above the UIV. Most studies in this field exist and this challenge still needs more research.

5. Conclusion
We studied ASD patients who needed revision surgery for the prevalence of common complications and the development of PJK as the most common complication as well as the most common cause of revision surgery. In our study, 40% of patients with ASD who needed revision surgery suffered from PJK. We also found out that in the majority of these PJK patients, UIV was placed in the lower thoracic or thoracolumbar junction or lumbar area. Besides, in patients with PJK, no gradual transitional zone in the proximal part of the construct is observed and more studies are needed to show using hooks either transverse process hooks or sub-laminar hooks for making smooth passage on the proximal end of the construct and reducing risk for PJK.

Ethical Considerations
Compliance with ethical guidelines
This article is a review with no human or animal samples.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.

Authors' contributions
All authors equally contributed to preparing this article. 

Conflict of interest
The authors declared no conflict of interest.

Acknowledgements
We appreciate the stuff of artificial intelligence and MISS center at Hazrat Rasool Akram Hospital for their help.


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