The thumb base fails quietly. The joint above it does not.
As the carpometacarpal joint collapses into adduction the thumb cannot get out of the palm, so the joint above hyperextends to open the web. That compensation is invisible on the radiograph everyone looks at, and past roughly thirty degrees it becomes a decision in its own right. Rebuild the base without addressing it and the reconstruction inherits a problem it did not create.
The adjacent segment is the metacarpophalangeal joint.
In the ankle the adjacent segment is the subtalar joint and it either shares the deformity or refuses to. In the thumb it is the joint immediately above the failing one, and it is not sharing anything — it is compensating, and the compensation is what you inherit.
The base
What the thumb is doing about it
How far past the base of the thumb the operation has to reach.
Not yet an operation.
Either non-operative care has not been exhausted or the disease is early and the joint above is quiet. The indication is failed non-operative management, not the radiographic stage.
Osteoarthritis of the thumb, ASSH →The base is the whole operation.
Established arthrosis, a joint above that is behaving, a web that opens. Nothing here argues for reaching further.
The Eaton–Littler classification →Both, in the same sitting.
The joint above is compensating, or the web has contracted. Correcting the base alone hands the reconstruction the compensation it inherited.
Managing the MCP joint in thumb CMC arthritis →The compensation needs its own procedure.
Hyperextension past the published decision point, or scaphotrapezial involvement that takes the disease outside the CMC joint altogether. What to do about the MCP at that point is genuinely unsettled — the cited review lists everything from doing nothing to arthrodesis.
The published options, and the disagreement →This instance is structural, not kinematic.
The hip has a seated film. The ankle has a block. The knee has a stress examination at each flexion angle. This page has no second position at all — the MCP hyperextension is measured once, statically. The adjacent-segment logic is exactly the same as the ankle’s and it is real, but it is a structural argument rather than a measured envelope, which makes it a weaker member of the set than the joints that are actually tested in two states.
The adjacent segment shows up three times in this programme.
| Joint | Primary | Adjacent segment | What the adjacent segment does |
|---|---|---|---|
| Hip | Acetabulum | Pelvis | Rotates when the patient sits, handing the cup anteversion for free — or does not |
| Ankle | Tibiotalar joint | Subtalar joint | Gives the deformity back on a block, or refuses to and forces a fusion |
| Hand | Carpometacarpal joint | Metacarpophalangeal joint | Compensates by hyperextending, and past 30° you inherit it |
Put the first row in.
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