Relatedly! I found out today that http://www.autisticzebra.wordpress.com is doing an EDS A to Z as well, so go and have a look!
I’ve mentioned before that not all joints are created equal, and that not all joints are even things that a healthy person would think of as a joint. And, as is traditional in hypermobility syndrome, there are a dozen fascinating ways that each one can cause different kinds of problems.
Hinge joints are probably the simplest and easiest-to-understand joints in the body, as well as being pretty common. Three in each finger, three in each toe, elbows, and slightly complicated examples in the knees and ankles. In each case, there is one degree of freedom – The joint can fold open or closed, like a door, and like a door, there is supposed to be a point at which it won’t open any further. In hypermobility, that point is much further than is usual. Six of the points on the ten-point Beighton criteria relate to hyperextensions of hinge joints – Bending the little finger back at 90 degrees past “flat”, and hyperextending the elbows and knees past straight – and thus it’s fair to say that hyperextending hinge joints is an important clinical feature of EDS-hypermobility in otherwise uninjured patients (It’s important to note, however, that in older patients or patients who’ve repeatedly injured their hinge joints, they might no longer be able to hyperextend them due to a build-up of scar tissue or a shortening and tightening of the muscles and connective tissues around them). Luxations of the hinge joints tend to involve a slipping-sideways, in any other axis but the one that they’re supposed to move in – Especially notable in the fingers and toes, which can slide off and past their correct orientations due to the surfaces of the bones being so flat and smooth and relying so much on the joint capsule to make them articulate. In the elbow, the olecranon (the hook of bone which locks the elbow in place when it’s straight) prevents luxations when the elbow is extended, but not when it’s bent, and the knee (Being a complicated hinge with four bones articulating at it) often ends up with the patella luxating, even if the rest of the joint is still basically intact. Despite the ankle being technically a hinge, in hypermobility is has so many degrees of freedom that it almost moves more like a wrist.
Wrists, and the joints between the carpus and tarsus and their phalanges, are condyloid joints – Elliptical sockets with elliptical “balls” which allow movements from side-to-side and up-and-down but without allowing rotation. Confusingly, the wrist itself is actually two of these joints (The distal and proximal radioulnar joints) and also two pivot joints, which do allow rotation. These joints partially luxate by sliding around within the sockets, and fully luxate when the ball comes completely out of the socket. In the case of the wrists, the sockets themselves can become deformed from being move beyond their natural range (since the multiple bones making up each one move apart from each other) causing a luxation. Even before getting to the point of luxation, the hypermobile person has more movement on each of these articulations than a non-hypermobile person.
Ball-and socket joints allow the greatest degree of freedom, and are best typified by the hip itself, where the femoral head sits in the acetabulum, locked in by a sturdy joint capsule which isn’t all that sturdy in a hypermobile person. These joints rotate freely on all axes (so can pivot and hinge in any direction) and, unfortunately, in hypermobility they also stretch – The joint capsule stretches, letting the bones pull apart from each other and causing luxations.
It’s not just the stretchy collagen though, which causes hypermobility – In many cases, especially in the other hypermobility syndromes like Marfan’s or Osteogenesis Imperfecta, the joints themselves are an unusual shape – The classic one being that the sockets of ball-and-socket joints are too shallow, meaning that less force is needed to make them luxate, or the contact surfaces of condyloid or pivot joints aren’t quite matched to each other, making them slippery. Another problem, when repeated luxations have happened, is the formation of a false joint – This basically means that instead of there being one stable “correct” configuration for the joint, a second “socket” has been worn into the bone by the pressure from repeated luxations, meaning that the joint can be stable, but in the wrong place, which might reduce function or cause pain.
Detecting all these problems in the joints really needs long-term, serious observation, not just single x-rays every time the hypermobile person presents at A+E with an acute injury. Joints are, generally, fairly amorphous, imprecise things, and the difference between “in the right place” and “so far wrong that the limb doesn’t move” can be a few millimetres, depending on the joint, and without a “before” picture, the “after” picture often looks quite complete and correct – Much like finding a Neolithic arrow-head and surmising that they never used shafts for their arrows, because you’d never seen one. Unfortunately, long-term serious observation costs money, and requires medics at all levels to just believe the patient, which seems to be par for the course for high-ranking specialists, but completely beyond the wit of the average A+E locum.
And this is why we’re having to try to increase awareness, amongst other things.
(Notable runners-up to be “J is for…”; Jellyfish, jejunum)