Diagnosis and Classification of Proximal Junctional Kyphosis and Proximal Junctional Failure

Fig. 17.1

PJK without mechanical failure. (a, b) Preoperative AP and lateral radiographs demonstrating lumbar degenerative scoliosis with coronal imbalance and L4–5 spondylolisthesis. Her pelvic incidence (PI) measured 55° and lumbar lordosis (LL) measured 66°. (c, d) Postoperative AP and lateral radiographs demonstrating T10 to pelvis fusion. Her PI–LL mismatch remained within acceptable range postoperatively (PI = 55° and LL = 64°). Radiographs also illustrate kyphosis at the proximal adjacent segment. (e) Close-up lateral radiograph demonstrating proximal junctional kyphosis (PJK) measurement of about 18° between the upper instrumented vertebra (UIV) and the upper instrumented vertebra +2 (UIV +2) without evidence of implant failure or vertebral body fracture. This patient remains asymptomatic from her PJK and is being serially monitored with radiographs and clinically assessment

Sacramento-Dominguez et al. [36] evaluated the reproducibility of using the UIV+1 and UIV+2 to measure PJK. Although they demonstrated moderate to very high intra- and inter-rater reliability, the authors could not conclude which of the two vertebrae is the better landmark to use for measuring PJK [36]. Further work has recently shown that radiographic measurement of kyphosis from UIV to UIV+2 is highly repeatable, with or without the presence of PJF and at either upper thoracic or thoracolumbar junction [62].

Proximal Junctional Failure

PJF is more severe than PJK and is becoming increasingly recognized as one of the most frequent reasons for reoperation after adult spinal deformity surgery. It results, in some cases, in a higher need for revision surgery, a greater risk of neurologic injury, increased deformity, and pain [27–30]. Other terms used to describe this phenomenon have included “topping off syndrome,” “proximal junctional fracture,” and “proximal junctional acute collapse.” These terms highlight the associated structural failure and mechanical instability that distinguish this more severe form of proximal junctional pathology from its more common and more benign PJK counterpart. The estimated cost of revision surgery after PJF is $77,432, indicating a greater clinical and economic burden of this condition [18].

The structural failure that occurs with PJF can present as vertebral body fracture, implant pullout or breakage, and/or disruption of the posterior osseo-ligamentous complex [27, 29]. The development of a single definition and classification system for PJF remains ongoing. Yagi and colleagues defined PJF as a symptomatic PJK requiring any type of revision surgery [37]. Hostin et al. [29] and Smith et al. [30] defined acute PJF as 15° or more of PJK along with fracture of the UIV or UIV +1, failure of UIV fixation, or need for extension of instrumentation within 6 months of the index surgery. Hart and colleagues [28] described PJF on the basis of 10° or greater postoperative increase in kyphosis between the UIV and UIV + 2, along with one or more of the following features: fracture of the vertebral body of the UIV or UIV + 1, posterior osseo-ligamentous disruption, or pullout of instrumentation at the UIV (Fig. 17.2).

Fig. 17.2

Minimally symptomatic PJF. (a, b) Preoperative AP and lateral radiographs demonstrating flat back deformity and coronal and sagittal global imbalance. Preoperative PI–LL mismatch measured 56° (PI = 46°, LL = 10° kyphosis). (c, d) Immediate postoperative AP and lateral radiographs illustrating improved alignment after T11 to pelvis fusion and reconstruction. His PI–LL mismatch became normalized postoperatively (PI = 46° and LL = 55°). (e, f) Full-length and close-up lateral x-rays on their most recent follow-up illustrate classic features of proximal junctional failure (PJF), including compression deformity of the upper instrumented vertebra and proximal junctional kyphosis angle measuring 17°. This patient remains asymptomatic from his PJF and is being serially monitored with radiographs and clinical assessment

Epidemiology and Clinical Significance

It is difficult to ascertain the exact prevalence of these conditions in the adult population due to the varied definition of PJK and PJF. Different authors report the prevalence of PJK ranging from 20 to 39 % after spinal deformity fusion surgeries [16, 19, 38–41]. The prevalence of PJF is lower and has been reported to range between 1.4 and 35 % [29, 30, 37].

Experts continue to debate whether PJK is simply a radiographic diagnosis or has potential clinical implications for patient outcomes. Most studies have failed to demonstrate that PJK diminishes clinical outcomes [16, 19, 38, 39, 41]. Only when using 20° as the threshold for defining PJK did Bridwell et al. report a significant difference in self-image subscale scores of the SRS-22 [33]. In a large retrospective study, Kim et al. also demonstrated higher rates of pain in patients with PJK (29.4 %) compared to those without PJK (0.9 %) and that the presence of upper back pain had an odds ratio of 12.5 for prediction of PJK [42]. There is also an evidence that PJK can be progressive and that increased absolute PJK angles (in some cases likely an indication of structural failure) are directly correlated with pain and inversely correlated with function [42, 43].

Current literature suggests that separating PJK and PJF as two unrelated conditions may be overly simplistic. Rather, a more supported model is to conceptualize PJK and PJF as different clinical entities residing on the proximal junctional pathology spectrum. With worsening degrees of PJK, patients can develop the structural failures that define PJF. This may be accompanied by subsequent pain, neurologic deficit, gait difficulties, sagittal imbalance, and social isolation. While patients with PJK may be initially asymptomatic, Hart et al. [27] report that nearly half (47.4 %) of patients who developed acute PJF required revision surgery within 6 months of their index procedure.

Risk Factors

The etiologies of PJK and PJF are likely multifactorial as no study has elucidated a single variable that strongly and consistently predicts their development. However, several major risk factors for PJK and PJF have been described. The potentially modifiable risk factors include greater curvature correction [30, 33, 45, 47–50], combined anterior–posterior spinal fusion [19, 33, 41, 43, 51, 52], fusion to the sacro-pelvis [30, 34, 39–41, 43, 46], and residual sagittal imbalance [53]. Non-modifiable factors with clear correlation to PJK development include older age (>55 years) [19, 22, 33, 47] and severe preoperative sagittal imbalance [30, 41, 43–46, 50, 54, 55]. Other less well-established but likely risk factors include low bone density [43], the presence of a comorbidity [33], and high body mass index [22, 33].

There remains conflicting evidence regarding whether the type of instrumentation used at the UIV, the number of levels fused, or the location of the UIV influence the risk of PJK development. The use of hooks, wires, or pedicle screws at the proximal level has not been consistently shown to significantly affect the risk of PJK across studies [35, 39, 44–46, 51]. There are studies demonstrating that both a greater and lesser number of levels fused [33, 44] are risk factors for PJK. Similarly, both a UIV at the upper and lower thoracic level have been associated with the development of PJK [26, 33, 52].

Modes of Failure and Classification

Modes of Failure

Given that the prevalence of elevated thoracic kyphosis ranges between 20 and 40 % and is more common in geriatric patients, some authors posit that PJK represents a recurrence of deformity and/or natural history of aging rather than a postoperative complication. This assertion is supported by the fact that many of the radiographic features associated with the development of PJK correspond with those seen in the natural history of kyphosis observed with normal aging: osteopenia, facet joint degeneration, disc height loss and wedging, and compression deformities of vertebrae [16, 56]. The true etiology may be multifactorial, involving iatrogenic effects of altered mechanics and adjacent segment surgical injury, along with deformity progression and the processes of natural aging. Indeed, several authors have submitted evidence suggesting that surgical disruption of the posterior soft tissue tension band, construct stiffness, and correction forces may all play an important role in the pathogenesis of PJK [24, 26, 35, 41, 56–58].

Unlike PJK, the underlying pathology for PJF appears to be an acute structural event, most typically early in the postoperative period, although it can also include progressive deformity occurring over months to years [18, 22, 24, 28, 29]. Hostin and colleagues [29] reported that fracture was the most common mechanism of failure (47 %), followed by soft tissue disruption (44 %). They reported that 9 % of their patient cohort experienced PJF as a result of trauma and screw pullout accounted for approximately 9 % of failures. This variety in failure mechanisms accounts for the spectrum of severity in clinical presentations of PJF. Fracture subluxation and dislocation of the adjacent segment(s) has also been reported [22, 24, 29, 56, 59]. Hostin and colleagues [29] also reported that failure resulted more frequently from vertebral body fractures when the UIV ended in the thoracolumbar region, while when the UIV ended in the upper thoracic spine, soft tissue disruption and subluxation without fracture or instrumentation failure were the more common modes of failure [29].

Classification

Several studies have proposed a classification scheme for PJK and PJF [27, 37, 41, 60]. Yagi and colleagues initially presented their PJK classification scheme in 2011 and subsequently modified it in 2014 [37, 41]. While their modified classification system is simple and easy to use, it lacks prognostic information and does not guide management. The ideal classification scheme should both guide treatment and provide information regarding severity of the pathology. Recently, Hart et al. [60] and the International Spine Study Group (ISSG) proposed a Proximal Junctional Kyphosis Severity Scale (PJKSS) that assigns points to six different components thought to be important in the evaluation and management of PJK/PKF. Points are summed to give a total severity score. The PJKSS has been shown to strongly correlate with health-related quality of life (HRQOL) outcome scores and indication for revision surgery [61]. Demonstration of its reproducibility and reliability has also been completed [62] (Table 17.1).

Table 17.1

Hart-ISSG PJK Severity Scale (PJKSS)

Hart-International Spine Study Group (ISSG) Proximal Junctional Kyphosis Severity Scale (PJKSS)
Parameter	Qualifier	Severity score
Neurologic deficit	None	0
	Radicular pain	2
	Myelopathy/motor deficit	4
Focal pain	None	0
	VAS ≤4	1
	VAS ≥5	3
Instrumentation problem	None	0
	Partial fixation loss	1
	Prominence	1
	Complete fixation loss	2
Change in kyphosis/PLC integrity	0–10°	0
	10–20°	1
	>20°	2
	PLC failure	2
UIV/UIV + 1 fracture	None	0
	Compression fracture	1
	Burst/chance fracture	2
	Translation	3
Level of the UIV	Thoracolumbar junction	0
Level of the UIV	Upper thoracic spine	1

VAS visual analogue scale, PLC posterior ligamentous complex, UIV upper instrumented vertebra

Evaluation and Preoperative Planning

Evaluation

Failure to recognize and differentiate PJF from PJK and initiate the proper workup and treatment can put patients at risk of neurologic compromise. Unlike patients with PJK, patients with PJF can experience loss of neurologic function. Although pain can be substantial, some patients may have limited new complaints [18, 22, 24, 27, 29]. On physical examination, the patient’s gait and posture should be noted and compared to previous findings. Kyphotic deformity, tenderness to palpation at the proximal junction of instrumentation, and implant prominence and skin tenting should be assessed. If there are concerns, infection should be considered in the differential diagnosis, and the appropriate blood work should be ordered (CBC with differential, erythrocyte sedimentation rate, C-reactive protein). A thorough neurologic examination should be performed to evaluate for evidence of spasticity. Upright 36-inch-long cassette AP and lateral x-rays and, if indicated, advanced imaging such as computed tomography (CT) and magnetic resonance imaging (MRI) are essential in the complete assessment of symptomatic patients.

Preoperative Planning

When revision surgery is planned, performing a thorough history and physical exam and obtaining a complete imaging workup are mandatory. Full-length 36-in. standing anteroposterior and lateral radiographs allow for accurate assessment of segmental and global spinal alignment parameters. Inclusion of the femoral heads within the field of view is required for spinopelvic alignment parameter measurements. In addition, supine hyperextension lateral radiograph over a bolster can provide information regarding the flexibility of the kyphotic deformity. Preoperative CT with sagittal and coronal reconstructions is helpful in identifying anterior ankylosis, as well as delineating vertebral fractures and hardware fracture or pullout. CT can also be valuable in evaluating prior fusions and planning osteotomies. MRI or CT myelogram should be obtained if there is suspicion for neural element compression. Bone mineral density should be measured if it has not been done within the previous 6 months. Osteoporosis or osteopenia should be treated with teriparatide if possible, prior to consideration for elective revision surgery in order to reduce the chance of a second recurrence. If the procedure is more urgent, then it can be started postoperatively.

Treatment Concepts

Currently, there is no standard consensus to guide the surgeon in determining which patients with PJK would benefit most from revision surgery. In general, patients who are asymptomatic are managed with reassurance, education, and close monitoring (Figs. 17.1 and 17.2). On the other hand, those with significant symptoms and higher severity of deformity or instability can be considered for revision surgery. Some authors suggest that treatment may be essential even in the absence of symptoms in patients with disruption of the posterior column, due to risk of deformity progression and neurologic injury [19, 43].

Hart et al. [27] reported that about 47 % of patients with PJF underwent revision surgery within 6 months of their index procedure. The authors also elucidated several factors that may influence the surgeon to recommend revision surgery for PJF: traumatic etiology of PJF, severity of kyphosis, combined anterior/posterior approaches at the index surgery, female sex, and higher SVA [27]. Interestingly, mode of failure (soft tissue vs. bony), age and BMI, number of levels fused, and location of UIV did not statistically correlate with the decision to revise [27]. Smith et al. [30] also identified other factors affecting the decision to perform revision surgery, including the presence of hardware failure, uncontrolled pain, neurologic deficits, and myelopathy (Fig. 17.3). Of note, they also reported that the revision rates differed by the location of the UIV. In their patient cohort, the revision rate was much higher when the UIV was located in the lumbar or lower thoracic spine [30].