Arthrogryposis Multiplex Congenita

The name does not even refer to muscle, but muscle is a key player.

arthro = joints
gryposis = twisted
multiplex = more than one (like the movie theaters)
congenita = seen at birth.
 

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At the heart of AMC is a failure to form a joint or to form it fully (arthrogryposis). Sometimes this defect is generalized (multiplex). Why someone stuck the really obvious congenita tag on an already overly lengthy name is anybody's guess as there are no acquired cases. It is always congenita. So just consider the first two terms as having significance.  In the developing limbs, the central mesoderm tissue that will become bone and muscle begins to sort out with a long tube of bone prone tissue wrapped in muscle bound tissue. At certain intervals, small fluid lakes begin to break the length of this into segments. The muscle development is running well ahead of the bone and even begins to do some twitching as nerves have dived in and linked up.

As the twitching moves the segments the spaces come alive with shape and detailed form steering
 qualities called joints. However, absent that twitching, FOR ANY REASON, the joint formation fails. If the interference is local, just one joint fails. If the interference is late, then some shaping may be amiss or maybe even things go well. Joints that do not form are not joints at all. Often we hear of this person or that having "fused" bones from birth in the foot or whatever. No. They are not fused as that means that they were once separate and then joined. They never succeeded in separating in the first place. But who wants to use this name - arthrogryposis? Nobody. So the bones are called fused - wink wink.

Even incompletely but somewhat formed joints, lacking proper shape and also lacking ligaments and connections in the right and only in the right places, wind up with the hallmark of this condition - rigidity. The typical arthrogrypotic joint is as stiff as oak. Often the skin has a dimple near the involved joint.

Neurologic failure, muscle failure, or muscle paralysis or weakness from any source can produce AMC. Therefore, a very large list of conditions may be at work. Because of this, many other organs or systems may also be abnormal according to the source.