Proximal Junctional Kyphosis (PJK)

To make sense of PJK it helps to picture what a long deformity fusion actually does. In adult spinal deformity, the spine has lost its normal balance: it may curve sideways, as in scoliosis, or tip forwards, losing the inward curve of the lower back that normally keeps a person standing upright over their hips. To correct this, a surgeon joins a long series of vertebrae together with pedicle screws and rods, often extending from the upper back down to the pelvis. The fused segment becomes a single rigid column. The vertebra at the very top of that column, the highest one holding a screw, is known as the upper instrumented vertebra, or UIV. PJK is, by definition, a problem of the junction immediately above it.

A fusion changes the mechanics of the spine in a fundamental way. Below the top of the construct, the rigid rods prevent movement and share load between the vertebrae. Above it, the spine is once again mobile and flexible. The point where stiff metal-reinforced bone meets soft, moving spine is a transition zone, and it is here that stress concentrates. The body weight of the upper trunk, which previously passed through a long, gently curved spine, must now pivot over the top of a stiff lever. The vertebrae, discs and ligaments just above the UIV bear this concentrated load, and over time the segment can tip forwards into kyphosis, the medical word for a forward-bending curve.

This is why PJK is described as a junctional problem. It is not a disease of the fused bone itself, which usually heals solidly, but of the boundary between the fused and the unfused spine. The junction is the weak link in an otherwise strong chain, and proximal junctional kyphosis is the name for what happens when that link starts to bend.

The reason PJK can be discussed with any precision at all is that it has a measurable definition, set out by Glattes and colleagues in 2005. They described an angle, the proximal junctional angle, measured on a standing side-on X-ray. The angle is drawn between the lower endplate of the upper instrumented vertebra and the upper endplate of the vertebra two levels above it. In plainer terms, it captures how sharply the spine bends forwards across the two segments immediately above the top of the fusion.

The definition then sets two conditions, both of which must be met. The proximal junctional angle must be at least ten degrees, and it must also be at least ten degrees greater than it was before the operation. The first condition rules out trivial angles; the second ensures that the change has genuinely arisen at the junction rather than simply reflecting a curve that was always there. It is the combination — ten degrees of angle and ten degrees of change — that defines proximal junctional kyphosis. This careful, numerical definition is precisely what makes PJK a radiographic entity, identified by drawing lines on an image rather than by how a person feels.

Here lies the heart of the matter. Proximal junctional kyphosis is a radiographic measurement. Proximal junctional failure (PJF), by contrast, is a structural and clinical event. The two are related but they are not the same, and confusing them is the commonest source of misunderstanding about this topic.

PJK is the angle. It is often gradual, frequently mild and very commonly silent. A person may have a proximal junctional angle that has increased past the defined threshold and yet have no pain, no visible change in posture and no functional limitation. PJF is the failure: a fracture of the upper instrumented vertebra or the vertebra just above it, a pulling-out of the screws, a slipping of one vertebra on another, a rupture of the ligaments, or pressure on the spinal nerves producing a neurological deficit. Researchers have defined acute PJF using a sharper angular threshold, an increase of fifteen degrees or more, combined with these structural features or the need for further surgery. PJF, in short, is what happens when the junction does not merely bend but gives way.

The relationship is best thought of as a spectrum of severity. At the mild end sits asymptomatic radiographic PJK, which vastly outnumbers the rest. At the severe end sits PJF, which is far less common but far more consequential. A useful tool is the proximal junctional kyphosis severity scale, which scores pain, neurological involvement, problems with the metalwork, the degree of kyphosis and disruption of the ligaments, the presence of fracture and the level of the top of the fusion. The scale exists precisely because the bare angle, on its own, says little about whether anything needs to be done.

In the world of shorter lumbar fusions done for degenerative conditions, surgeons distinguish between adjacent segment degeneration (the radiographic wear-and-tear next to a fusion, often without symptoms) and adjacent segment disease (the clinical syndrome that arises when that degeneration produces new pain or nerve symptoms). PJK and PJF are the mirror image of this pair, but located at the top of a long deformity construct. PJK is the radiographic finding, the counterpart of adjacent segment degeneration. PJF is the clinical failure, the counterpart of adjacent segment disease.

It is worth being clear how PJK differs from ordinary adjacent segment disease. Adjacent segment disease typically follows short fusions of one or two levels for degenerative problems, and is driven by gradual wear at the neighbouring disc. PJK arises after long fusions that correct a whole-spine deformity, and is driven principally by the mechanical stress at the top of a long stiff construct. The two also differ from global sagittal imbalance, which is a failure of the whole spine to stay balanced over the pelvis rather than a localised problem at one junction.

Several distinct mechanisms can produce PJK and PJF. The first is disruption of the posterior ligamentous complex, the strong band of ligaments along the back of the spine that act as a tension band resisting forward bending. Any weakening of this tension band at or just above the UIV removes part of what normally holds the segment upright, allowing it to tip forwards. This is the soft-tissue or ligamentous mode, which tends to predominate when the top of the fusion sits high in the upper back.

The second is fracture of the bone itself, either of the upper instrumented vertebra or of the vertebra immediately above it. The concentrated load at the junction simply exceeds what the bone can bear. This bone mode is more common when the top of the fusion lies at the thoracolumbar junction and is strongly linked to weak, osteoporotic bone. The third is failure of the metalwork, in which the screws pull out of the bone or the implant loses its grip. The fourth involves the disc and ligaments above the construct giving way over time. In practice these mechanisms overlap, and a single failure may combine fracture, ligament disruption and forward slippage.

Older age is a recurring factor, partly because it travels with weaker bone and stiffer tissues. Osteoporosis and low bone density stand out as some of the most important contributors, because they govern whether the vertebrae at the junction can withstand the concentrated load. Over-correction of the spine is another major theme: an aggressive correction can leave the junction over-stressed, and the modern understanding is that correcting an older spine all the way to the alignment of a young adult is itself a risk.

The level at which the fusion ends matters too. Ending the construct at the thoracolumbar junction is associated with a different and in some respects higher-risk profile than ending higher in the upper back. Extending the fusion all the way to the sacrum and pelvis also raises the risk at the top. Greater pre-operative imbalance, a larger pre-existing thoracic curve, and the use of major bone-cutting procedures such as three-column osteotomies have each been linked to higher rates. What unites most of these factors is that they either weaken the junction or increase the load it must carry.

Because PJK is so common, a great deal of surgical effort has gone into trying to prevent it. None of the strategies is a guaranteed solution. One group of strategies aims to soften the abrupt change in stiffness at the top of the construct: using hooks rather than pedicle screws at the very top level provides a more flexible anchor, and transition or tapered rods work on the same principle of easing the mechanical step between rigid and mobile spine. A second group aims to reinforce the tension band: junctional tethers and ligament augmentation, in which strong bands reconstruct or supplement the posterior ligaments above the construct, have been reported to reduce the change in the junctional angle and, in some series, the rate of failure.

A third group addresses the strength of the bone. Injecting bone cement into the upper instrumented vertebra and the one above it — prophylactic vertebroplasty — has been studied as a way to prevent fracture at the junction, and the protective effect appears to depend on treating both levels rather than one. Alongside these technical measures sits the broader strategy of getting the alignment right in the first place: avoiding over-correction, aiming for an age-appropriate posture, and sparing the soft tissues at the top of the construct. Optimising bone health before surgery is a further strand, and one trial of a bone-building medication reported a lower rate of failure in patients with osteoporosis.

For the great majority, who have radiographic PJK that is mild, stable and causing no symptoms, the reported approach is observation. The angle is noted and monitored over time on follow-up imaging, because the evidence indicates that mild PJK of this kind does not generally harm a person's quality of life or progress to anything worse. The recognition that an X-ray finding is not in itself a problem to be treated is central to sensible management.

Proximal junctional failure that causes significant symptoms is treated more actively. The features that move a case towards intervention are a neurological deficit, severe and persisting pain, mechanical instability, broken or pulled-out metalwork, and a deformity that is progressing rather than holding steady. The reported treatment is revision surgery, most often extending the fusion further up the spine to capture the failed segment and re-establish a stable top to the construct, and relieving pressure on the nerves where needed. It is the constellation of pain, nerve involvement, fracture and instrumentation problems — not the bare angle — that distinguishes a finding to be watched from a failure to be addressed.

For the large majority with mild radiographic PJK, the picture reported in the literature is reassuring: the angle is often stable, symptoms are typically absent, and the finding does not detract from the result of the original deformity correction. For these patients, PJK is a measurement on a scan rather than an event in their lives. For the smaller number who develop PJF, the outlook depends on the nature of the failure and how it is managed. Most failures occur early, with a large proportion within the first few months and the majority within the first couple of years, which is why the period soon after the original operation is watched most closely.

This page is general medical education and is not a substitute for individual advice. New or worsening weakness, numbness, saddle numbness or any disturbance of bladder or bowel control requires urgent assessment.

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This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.