Proximal Junctional Kyphosis (PJK)

Proximal junctional kyphosis (PJK) is one of the most common mechanical complications following long-segment spinal fusion surgery, particularly in adult spinal deformity (ASD) correction. It represents excessive kyphosis development at the transition zone between the fused and unfused spine.

Definitions:

• PJK: Proximal junctional angle (PJA) more than 10 degrees compared to immediate postoperative values, measured between the UIV inferior endplate and UIV+2 superior endplate
• PJF (Proximal Junctional Failure): PJA more than 28 degrees OR change more than 22 degrees, often associated with fracture, ligament failure, or implant failure

Epidemiology:

Clinical Significance:

• PJK may be asymptomatic or cause significant morbidity
• PJF often requires revision surgery (40% revision rate)
• Healthcare costs substantially increased with PJK/PJF
• Most cases develop within first 18 months postoperatively

Mechanism of Development

PJK results from a mismatch between the mechanical demands at the proximal junction and the capacity of the adjacent tissues to withstand these loads.

Contributing Factors:

1. Abrupt stiffness transition: Rigid fused construct meets mobile unfused spine
2. Altered load distribution: Stress concentration at junctional level
3. Sagittal imbalance: Forward trunk shift increases moment arm at UIV
4. Tissue failure: Bone, ligament, or disc cannot withstand new loads

Anatomical Structures at Risk

Biomechanical Principles

Stress Concentration:

The junctional zone experiences increased stress due to:

• Transition from fused to mobile segments
• Lever arm effect of long constructs
• Loss of shock absorption from fused discs

Sagittal Compensation:

When PJK develops, the body attempts to compensate:

• Cervical hyperlordosis
• Pelvic retroversion
• Knee flexion

Bone Quality Impact:

• Osteoporotic bone cannot resist vertebral compression
• Reduced pull-out strength of UIV screws
• Cortical thinning increases fracture risk

History

Physical Examination

Clinical Indicators for Intervention

Surgical Indications:

1. Neurological deficit from compression
2. Progressive deformity on serial imaging
3. Intractable pain despite conservative measures
4. Documented instability on dynamic films
5. Skin breakdown risk from hardware prominence

Differential Diagnosis of New Junctional Symptoms After Fusion

New pain or kyphosis above a fusion is not always mechanical PJK. The following must be considered and excluded.

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Imaging Protocol

Step 1: Standing Full-Length Radiographs

• Compare to immediate postoperative films
• Measure PJA at UIV to UIV+2
• Assess global sagittal alignment (SVA, PI-LL)
• Evaluate hardware position

Step 2: CT Scan (If Indicated)

• Detect vertebral fracture at UIV/UIV+1
• Assess fusion mass (pseudarthrosis)
• Hardware evaluation (loosening, breakage)
• Bone quality assessment (HU values)

Step 3: MRI (If Neurological Symptoms)

• Cord compression assessment
• Soft tissue changes
• Disc pathology
• Posterior element integrity

Key Radiographic Measurements

Essential Measurements:

Dynamic Films (If Instability Suspected):

• Flexion-extension lateral radiographs
• Assess motion at UIV segment
• Document any subluxation

Bone Density Assessment

Complications of PJK/PJF

Direct Complications:

Complications of Revision Surgery

Risk Factors for Recurrence

Early Postoperative Phase

• Mobilisation: early supervised mobilisation once the construct is judged stable; a thoracolumbosacral orthosis (TLSO) may be used for comfort, though high-level evidence for brace prevention of PJK is lacking
• Neurological monitoring: regular neurological observation, especially after revision for PJF or after deformity correction
• Pain control: multimodal analgesia; expect a settling period after revision extension

Bone Health Optimisation

Because low bone mineral density is an independent predictor of proximal junctional failure (Yagi et al., pmid 28902106), bone health is a core part of postoperative and perioperative care:

• DEXA assessment and treatment of osteoporosis (bisphosphonates or denosumab)
• Calcium and vitamin D repletion
• In elective primary surgery, consider deferring 3-6 months for anabolic or antiresorptive optimisation in severe osteoporosis
• Anabolic agents (e.g. teriparatide) are of particular interest where bone quality is the limiting factor, though their specific role in PJK prevention remains under study

Surveillance Protocol

The first 3-6 months are the peak-risk window for acute failure (mean time to acute PJF 11.4 weeks; Hostin et al., pmid 22986834), so surveillance should be most intensive early.

Natural History

Revision Surgery Outcomes

Prognosis Factors

Global Epidemiology

Adult spinal deformity surgery is increasing worldwide as populations age, and PJK is consistently the most common mechanical complication of long-segment fusion across health systems. Reported incidence is broadly consistent internationally: radiographic PJK occurs in roughly a quarter to a half of patients fused to the pelvis (26% in the original Glattes series, 45.1% in a 679-patient multicentre cohort), while acute proximal junctional failure occurs in around 5-6% within the first 6 months (Hostin et al.). Female sex, advanced age and osteoporosis are recurrent risk factors in cohorts from North America, Europe and Asia, and large meta-analyses pooling these international series confirm osteoporosis (OR 1.58) and pelvic fixation (OR 2.08) as drivers.

Society and Guideline Guidance

There is no single randomised-trial-based clinical practice guideline that dictates PJK prevention; guidance is largely consensus and registry-informed:

Key practice principles with the strongest evidence are: individualised, age-adjusted alignment targets (avoid over-correction), proportional correction (GAP concept, acknowledging inconsistent external validation), proximal junction protection (ligamentous augmentation/hooks) and bone optimisation in osteoporotic patients.

Registry and Multicentre Evidence

Most high-quality PJK evidence comes from prospective multicentre deformity databases (e.g. the International Spine Study Group and European multicentre series) rather than national arthroplasty-style registries, because deformity volumes are comparatively low and constructs are heterogeneous. These collaboratives have produced the predictive models (Yagi), proportional scores (GAP) and age-adjusted target work that now guide practice globally. The lack of a dedicated high-volume PJK registry is itself a recognised gap.

Practice Variation

Practice varies internationally in the routine use of preventive adjuncts: prophylactic vertebroplasty/cement augmentation, transverse-process hooks and ligamentous "topping-off" devices are used selectively rather than universally, reflecting cost, availability and the absence of definitive randomised evidence. Thresholds for revising radiographic PJK also differ, with most centres reserving surgery for progressive deformity, intractable pain, instability or neurological compromise. Across health systems, complex deformity and PJK revision surgery is concentrated in tertiary specialist spine units with intraoperative neuromonitoring, advanced imaging and intensive care, with patients from regional or lower-resource settings typically transferred for complex revision.

Proximal junctional kyphosis (PJK) is defined as greater than 10° kyphosis at the upper instrumented vertebra (UIV) compared to preoperative alignment, occurring in 20-40% of adult spinal deformity surgeries. PJK must be distinguished from PJF (proximal junctional failure)—which includes vertebral fracture, ligament rupture, or implant failure at UIV and often requires revision (PJA greater than 28° or change greater than 22°). Risk factors include age over 60, osteoporosis, thoracic UIV, and critically over-correction of sagittal alignment. Most PJK occurs within 3-18 months postoperatively. Prevention strategies include age-adjusted alignment targets (avoid over-correction in elderly), cement augmentation at UIV and UIV+1 in osteoporotic bone, and soft tissue preservation at the proximal junction.