Cage Subsidence

When a worn or damaged disc is removed during a spinal fusion, the space it leaves behind is filled with an interbody cage. This cage has several jobs at once. It props the two vertebrae apart to restore the natural height of the disc, which in turn opens up the small bony tunnels, the foramina, through which the nerve roots leave the spine. It helps to restore the gentle forward curve of the lower spine, the lordosis. And, packed with bone graft, it provides a stable scaffold across which the two vertebrae can knit together into a single block of bone, the fusion.

The surfaces of the vertebrae that the cage rests against are called the endplates. Each endplate is a thin layer of dense bone covering the softer, spongy bone of the vertebral body beneath. Cage subsidence is the process by which this endplate gives way, allowing the cage to sink, or telescope, into the vertebral body. In effect, the implant presses into the bone and the bone yields. Subsidence is a failure of the bone, not of the metal. The cage itself usually remains structurally intact; what changes is its position, because the bony platform it stands on has given way beneath it.

That framing explains why bone quality dominates the whole subject. A cage of a given size and material will behave very differently when it rests on the strong, dense bone of a younger or healthier spine than when it rests on the thinned, weakened bone of an osteoporotic one.

It would be a mistake to regard all subsidence as a complication. A degree of early, minor settling is extremely common after interbody fusion, frequently causes no symptoms at all, and is widely understood as part of the way the construct beds in. Immediately after surgery, a relatively stiff cage sits on a relatively thin endplate, and the load passing through the spine is concentrated at that interface. As the cage settles a small amount into the bone, the contact area broadens and the load is shared more evenly between the implant and the surrounding bone, rather than being borne at a few sharp points.

The literature describes this benign settling as a process that tends to occur early, often within the first weeks, and then to stop. Once the fusion begins to mature and new bone can carry the load across the cage-to-endplate interface, the settling no longer progresses. It is the progressive, larger-magnitude subsidence, not the small early settle, that carries the clinical concern.

It is not hardware failure. Hardware failure means the metal itself failing — a screw fracturing or loosening, a rod snapping, a cage cracking or backing out. Subsidence is different in kind: the metalwork is usually intact, and it is the bone that has yielded around it.

It is not the same as pseudarthrosis, although the two are linked. Pseudarthrosis, or non-union, means that the two vertebrae have failed to knit together into solid bone. Subsidence can contribute to non-union, because a sinking cage creates ongoing micromovement; yet a good deal of minor subsidence happens in segments that go on to fuse perfectly well. The two are best held in mind as overlapping but distinct.

It is not adjacent segment disease. Adjacent segment disease is wear at the level next to the fusion, developing over years. Subsidence happens at the operated level itself.

Most subsidence is minor and silent. The clinically important entity is severe or progressive subsidence, and it matters because it can quietly reverse the very things the operation set out to achieve. The first consequence is loss of correction: as the cage sinks, the disc height it was holding open is lost, and with it some of the restored lordosis. The segment can settle into a more forward-bent, kyphotic position. In operations that rely on opening the disc space to take pressure off the nerves indirectly, particularly the lateral approaches, this loss of height directly undermines the decompression that was gained.

The second consequence follows from the first. As height is lost, the foramina narrow once more, and this recurrent foraminal narrowing can bring back the radiating nerve pain that the surgery was intended to relieve. The third consequence is the link to non-union: severe subsidence and the micromovement that accompanies it are associated, in some series, with higher rates of failure to fuse. It is important to keep these consequences in proportion: across the literature, the majority of subsidence is focal and minor, and large reviews have repeatedly found that radiographic subsidence does not reliably translate into worse symptoms.

Subsidence is one of the most commonly reported radiographic findings after interbody fusion, with figures that vary widely. In anterior cervical fusion of the neck, a large systematic review reported a mean incidence of around twenty-one per cent, with individual studies ranging from none at all to over eighty per cent. In lateral fusion of the lower back, reviews report subsidence in roughly a quarter to a third of levels, with stand-alone constructs at the higher end. In anterior lumbar fusion, rates of around a fifth to a quarter are described. The wide spread reflects how differently subsidence is defined from study to study. The location is also patterned: the upper, or cranial, endplate is mechanically weaker than the lower one, and subsidence and endplate injury are observed most often there.

There is no single agreed definition of cage subsidence. The commonest approach is to measure the loss of disc or interbody height, comparing a scan taken immediately after surgery with a later one, and to call it subsidence once the loss crosses a set threshold. The most frequently used thresholds are a loss of two millimetres or more, with some studies using a stricter cut-off of three millimetres. A widely used refinement labels a two-millimetre loss as minor and a three-millimetre loss as major. Other studies, particularly of lateral fusion, express subsidence as a percentage of the disc height lost, and many take a loss of a quarter of the height as the point at which subsidence is declared. The number alone means little without knowing how it was defined.

Bone quality is the dominant factor. Low bone mineral density, and osteoporosis in particular, are the strongest and most consistent risks, with several analyses placing the risk in osteoporotic bone at roughly double that of normal bone. A growing body of work shows that bone quality measured specifically at the endplate, rather than averaged across the whole vertebra, predicts subsidence best.

The endplate and its preparation matter greatly. The dense layer of subchondral bone just beneath the cartilage is far stronger than the spongy bone below it. When the endplate is breached, or too much of this strong layer is removed during preparation, the cage is left resting on weak bone and subsides more readily. Preserving as much of the strong subchondral bone as possible is repeatedly identified as protective.

Cage design and position contribute. A larger footprint that rests on the strong peripheral rim of the endplate, the apophyseal ring, spreads load and resists subsidence, while a small or poorly matched footprint concentrates it. Excessive cage height and over-distraction of the disc space over-tension the endplates and raise the risk. Cage material — PEEK plastic, whose stiffness is close to that of bone, or titanium, which is stiffer — has been studied extensively, but the evidence is mixed.

Standing alone versus being supported makes a difference. A cage placed by itself, without supplementary screws and rods, allows more micromovement and subsides more often than one supported by posterior fixation. The approach also shapes the pattern: lateral and oblique fusions place a broad cage spanning the strong outer rim of the endplate, but because these operations rely on maintaining height to decompress indirectly, subsidence directly threatens that decompression.

The assessment rests on serial imaging, comparing the position and height achieved immediately after surgery with later studies. Plain X-rays, including views taken in bending forwards and backwards, are the standard means of measuring loss of disc and segmental height over time. Computed tomography is the most detailed way of examining the interface between the cage and the bone, and the density of the bone on CT, expressed in Hounsfield units, serves as a measure of bone quality that has been shown to predict both subsidence and the likelihood of further surgery. Magnetic resonance imaging is used to assess bone marrow and endplate changes and to look at whether the nerves are being compressed again. Bone density testing, such as a DEXA scan, formally measures bone mineral density.

Observation is the course described for the great majority of subsidence, which is minor, stable and without symptoms. Such settling is frequently understood as part of normal cage incorporation and load-sharing, and most published series report that no revision surgery was needed for subsidence alone. Attention to bone health is a recurring theme on the preventive side: because low bone density is the dominant risk, the assessment and treatment of osteoporosis before surgery, together with endplate-sparing technique, careful cage sizing and the use of supplementary fixation where appropriate, are described as the principal levers for reducing subsidence in the first place.

Revision surgery is reserved for the minority of cases where subsidence is severe or progressive and is causing problems: significant loss of correction with developing kyphosis, recurrent narrowing of the nerve channels producing radicular pain, an established and symptomatic failure to fuse, or instability. The reconstructive options described include adding or revising posterior instrumentation to provide stability, exchanging the cage for one with a larger footprint or an expandable design, restoring the lost height and alignment, and addressing the underlying bone quality.

The common, minor form of subsidence tends to be self-limiting, stabilising once the fusion matures, is compatible with a solid fusion and a good result, and large reviews have repeatedly failed to find a reliable link between this kind of radiographic settling and worse symptoms. The concern lies with the severe and progressive form, which can undo the height, alignment and indirect decompression that the operation achieved, bring back nerve and back or neck pain, and sometimes accompany a failure of the fusion to consolidate. Even here, much depends on the surrounding circumstances: the quality of the bone, the magnitude and progression of the settling, the type of operation, whether the cage was supported by additional fixation, and, above all, whether the fusion ultimately takes.

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.