Instrument 11Coupled

Pre-op instrument · cervical spine

Every other page asks what the neighbour is doing. This one asks what you are about to do to it.

In the hip you measure whether the pelvis will rotate. In the ankle you find out whether the hindfoot gives the deformity back. In the elbow you pull on the radius and see whether the membrane holds. All of those are inputs. Here the adjacent segment is an output — something the fusion manufactures — and it is the only place in this programme where somebody has published a rate for it. Two point nine per cent per year. A quarter of patients within a decade.

Signature

66% vs 19%

Projected adjacent-segment exposure from identical films at 45 and at 75 after a one-level anterior fusion, assuming the published 2.9% yearly rate holds until age 82 (beyond ten years, an extrapolation).

Fig. 1 · Cumulative exposure against years remaining

The curve does not change with age — the distance you travel along it does. Everything right of the dashed line is extrapolated past the ten years anyone measured.

Measure this neck → See the same neck at 45 and at 75 →

T1 slope is this spine’s pelvic incidence · and the exposure is the patient’s remaining years

The extrapolation warning, because it is load-bearing here. The 2.9 per cent per year figure was measured over ten years. This instrument projects the same constant hazard further, because that is what a surgeon implicitly does when operating on a forty-five-year-old — but beyond ten years it is extrapolation, not evidence, and every number past that point is labelled. There is also a genuine, unsettled argument about whether fusion causes adjacent-segment disease or merely reveals the natural history of an already degenerating spine. This model assumes causation it cannot prove. Read it as a way of making an implicit assumption explicit, not as a risk score.

The envelope · standing lateral with T1 visible

T1 slope is the cervical spine’s pelvic incidence.

The hip page uses pelvic incidence as its fixed reference because it is morphology that nothing changes. The cervical spine has the same kind of parameter. T1 slope dictates how much lordosis this particular neck has to carry to hold a head over a base, and it correlates with cervical lordosis at r ≈ 0.89. A high T1 slope demands more lordosis. Judging every neck against one lordosis figure is the same error as one safe zone.

The base and what sits on it

From the standing lateral.

T1 slope42°

Fixed morphology. Your operation does not change it.

C2–C7 lordosis8°

The parameter you are going to change.

C2–C7 sagittal vertical axis55 mm

Beyond about 40 mm is the usual threshold for imbalance.

What you are about to manufacture

Levels fused, and how long the patient has to live with them.

Age45

Not a target here. An exposure.

Levels fused1

Motion-preserving construct instead of fusion

Projected adjacent-segment disease

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Tier D

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Alignment against this patient’s own base

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Log this neck →The loop this closes →

The finding

Same neck, same target, and a fifty-point difference in what it costs.

The lumbar page shows identical radiographs demanding different corrections at different ages, because the target itself moves. Here the target does not move at all — a T1 slope of forty-two demands the same lordosis at any age. What moves is how long the patient is exposed to the neighbour you just manufactured.

	45 years old	75 years old
T1 slope / lordosis / axis	42° / 8° / 55 mm	identical
Mismatch	34°	34°
Alignment target	the same	the same
Years of exposure ahead	≈ 37	≈ 7
Projected cumulative adjacent-segment disease assumes 2.9% a year, one level, until age 82	≈ 66%*	≈ 19%

*The sixty-six per cent figure extrapolates a ten-year hazard across thirty-seven years and is the least defensible number on this site. It is shown because the alternative — quietly assuming the hazard stops at ten years — is worse, and because the direction is not in doubt even if the magnitude is.

Age therefore matters twice in the spine, for two entirely unrelated reasons. In the lumbar spine it changes the target. Here it changes the exposure. Neither is captured by the other, and a surgeon reasoning about one has no particular reason to have thought about the second.

Three pages, one system

The hazard as input, the hazard being created, and the hazard priced.

Page	The question	The adjacent segment is
Hip	Will this pelvis rotate when the patient sits?	An input you measure and plan around
Lumbar spine	How much of that motion am I about to remove?	The thing the operation is creating
Cervical spine	At what rate will the neighbour fail because I fused?	An output, with a published rate

And it joins the elbow in a second class. Both are joints where the failure appears somewhere you did not operate — for the elbow, at the wrist; here, at the level above or below. That displacement, in anatomy and in time, is why both failures are so often not attributed to the operation that produced them.

The numbers

One firm rate, one firm reference, and one honest argument.

The rate

2.9% per year

Symptomatic adjacent-segment disease after anterior cervical arthrodesis, relatively constant across ten years

And at ten years

25.6%

Survivorship prediction, 95% confidence interval 20 to 32 per cent

The fixed reference

r ≈ 0.89

T1 slope against C0–C7 lordosis; 0.815 against C2–C7. It behaves as the cervical pelvic incidence

The mismatch parameter

T1S − CL

Post-operative mismatch associated with inferior outcomes after laminoplasty even without severe pre-operative kyphosis

The balance threshold

40 mm

C2–C7 sagittal vertical axis, alongside C2 slope and chin-brow vertical angle

This one IS in the mandate

TEAM

Spinal fusion is one of the five episode categories

What is assumed rather than found. The model does not raise the hazard with the number of levels fused: in the source series the adjacent-segment risk was significantly lower after a multilevel arthrodesis than after a single-level one (Hilibrand 1999), so the levels you enter are recorded, not scaled. Remaining-years estimation is crude and ignores comorbidity. Symptomatic disease and radiographic degeneration are different endpoints and this uses the symptomatic one. And the causation question above is real: this model assumes the fusion causes what follows, which is the strongest assumption anywhere on this site. Not a cleared device, not clinical advice.

Log this neck

Put the first row in.

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