Anatomy Question

[Zipper730]

This is going to sound silly as hell, but when it comes to the concept of joint dislocation, and how some people can basically dislocate their shoulders and take their hands from behind their back and go over the top and back down to their front.

The thing I get is this

• Joints can pop out of their sockets
• I get that bones are connected to bones by ligaments
• I get that muscles are connected by tendons

What I don't get is how you can effectively move the joint all the way around without effectively twisting the muscle into some kind of knot, or breaking the connective tissue?

I await an answer once everybody stops laughing

 

[mikewint]

The answer is simple, you can't. That being said all of these structures are living tissue and all respond to stress and all have some degree of elasticity. While there are no real "double" joints the correct term is benign hypermobility syndrome.
So in short if you keep applying stress to a joint, the joint will be modified over time by the stress as will the ligaments, tendons, and muscles. Contortionists simply keep forcing the movement, stretching all the above structures even tearing some until they have the movement they want.
The reverse is also true as anyone who has been in a cast and had an arm or leg immobilized can testify to. After weeks of non-movement just getting normal range of motion back is a challenge.
Some joints are easier to dislocate than others. The shoulder is not really much of a joint and the humerus is only held in place by the ligaments, tendons, and muscles. An injury or malformation can lead to a shoulder that can be dislocated at will. Houdini could easily dislocate both of his making his "straight-jacket" escapes fairly simple.

Anterior shoulder "dislocation"

The more common posterior "dislocation"

 

[Gnomey]

As possibly the most qualified here to answer this, here we go (though Mike has covered it well enough)...

Essentially some people have more elasticity in their tendons and ligaments than others which allows for the greater range of movement and the easier joint dislocation and relocation. There are a number of conditions that are known about that are hyper-mobility syndromes: Ehler-Danlos and hyper mobility (double jointed) syndrome.

Secondarily to this anyone who has previously dislocated a joint is more prone to do so again because of the laxity in the connective tissues that has been caused by the dislocation.

Now muscles don't get twisted into knots because there is enough play with them in their stretch to avoid this. That isn't to say that if you stretch them to far the won't break (partially or fully)and this is the cause of muscle strains/pulls/tears etc.

Now the connective tissues also can rupture, most people over the age of 70 will have some degree of rotator cuff tear in the shoulders. These tears happen in the same way as muscles with being over stretched e.g. Dislocations. They heal worse though because of poor blood supply.

 

[mikewint]

While there are no true "double" joints there are occasions where a bone breaks and then does not heal properly. Instead the two ends remain apart and form a pseudo-joint between them allowing some limited movement.
Pseudo-joint tibia (lower leg (shin) bone):

 

[GrauGeist]

I had my left shoulder violently dislocated in a wreck 5 years ago, and it's range of motion has actually become limited (and lets me know when I've reached the limit with savage pain).

And I concur with Mike's range of motion explanation. In the same wreck, my Talus was split in half. After successful surgery, it was just a matter of time (almost 6 months) before I could walk again. Walking was difficult, but it got better in time.

The interesting part, is the tiny change to the healed Talus was enought to limit the range of motion of my foot, as I cannot kneel down fully or do activities like skiing, surfing, etc. that require a kneeling action.

 

[pbehn]

I dislocated my hip in a violent motorcycle crash, my knee hit the car. The femur was dislocated and the acetabulum (the lip of the socket) was displaced (broken off) I was 6 weeks in a full body cast and then 6 weeks on crutches going from no weight bearing at the start to almost full at the end. It actually took a year to get full movement of my knee back and about another 6 months to get full use. The consultant said that the most important thing was to put no weight on the joint as the blood supply needed to sort itself out without the bone dying. Women say men cant imagine child birth simply because women usually havnt dislocated their hip lol

 

[Zipper730]

The shoulder is not really much of a joint

I'm not sure I understand this, last I checked it's a socket joint. The thing that I have trouble grasping is not that the ball can come out of the socket, but how this can be done without the muscles and tendon's not twisting like a twizzler...

I used to be able to do various things that involved putting one leg behind my head while standing, or putting both while sitting down (nearly broke my nose trying to walk on my hands): However in that case it was fairly obvious that the hip would be able to simply rotate through a wider range of travel and then would be brought back to it's original position after I was done.

Sometimes in the morning when I'd stretch, I'd feel/hear a firm ka-clunk as the bone would re-align (more like pop-in/out) itself

 

[GrauGeist]

The shoulder joint is not really a "ball and socket" type of joint, like the hip, for instance.

It's more of a tightly packed assembly of bone, cartilage, ligament, tendon and muscles that create a joint assembly.

Because of the configuration of the muscles, tendons and ligaments that are packed around the bone and cartilage, there is a natural range of motion. Also, the way the muscles are layered, it will prevent a knotting or twisting (unless you do something really wild, and then all bets are off - but it's really going to hurt) and if a range of motion is repeated, each time pushing the limits, then it can become much more flexible.

Go give an idea of what the joint looks like on a skeletal level, check out the illustration.

 

[Zipper730]

The shoulder joint is not really a "ball and socket" type of joint, like the hip, for instance.

It's more of a tightly packed assembly of bone, cartilage, ligament, tendon and muscles that create a joint assembly.

Ok

Go give an idea of what the joint looks like on a skeletal level, check out the illustration.

View attachment 499686

Does a dislocation of the shoulder revolve around the Glenoid cavity?

 

[GrauGeist]

Ok
Does a dislocation of the shoulder revolve around the Glenoid cavity?

Yes it does.
The Humerus is held in place against the Glenoid by the Bicep tendon and Rotator Cuff.

 

[Zipper730]

Yes it does.

It looks like the only thing that could rotate like that

The Humerus is held in place against the Glenoid by the Bicep tendon and Rotator Cuff.

The question is if one dislocated their shoulder like how Houdini and others did (I've seen people do it), how do the muscles not become twizzlered so to speak? Can the tendon and muscle slide around the bone?

 

[GrauGeist]

The Muscles and Tendons are "bundled", meaning they can't wander around too far.

Sort of like a Wiring Harness on a SUV's liftgate. They can move around to a certain extent, but they have a predetermined path.

There are actually several types of dislocations (Gnomey and Mike are well versed in Medics, so could provide better details) but since the shoulder joint is not an actual ball and socket, the shoulder can be forced out of it's "socket", which does increase it's range of motion, but only as far as the muscles and tendons will allow. If you force it beyond their limit, the muscle anchors and tendon attachment points will start tearing loose - so yes, Houdini could greatly increase his range without permanent damage for example, but if you try and rotate the shoulder (intentionally or otherwise) beyond the constraints of the attachment points, irreparable damage will result. Add to that a major artery and nerve bundle that passes through a sort of "sheath" in that area, and serious sh!t can start happening real fast.

 

[pbehn]

I had a friend who did a lot of body building, he dislocated his shoulder, then it happened again and again and again. In the end he was very restricted in what he could do, he had to give up the body building for a start, even stuff like throwing a stick for a dog. As others have said it isn't a ball socket joint.

 

[DerAdlerIstGelandet]

As possibly the most qualified here to answer this, here we go (though Mike has covered it well enough)...

Essentially some people have more elasticity in their tendons and ligaments than others which allows for the greater range of movement and the easier joint dislocation and relocation. There are a number of conditions that are known about that are hyper-mobility syndromes: Ehler-Danlos and hyper mobility (double jointed) syndrome.

Secondarily to this anyone who has previously dislocated a joint is more prone to do so again because of the laxity in the connective tissues that has been caused by the dislocation.

Now muscles don't get twisted into knots because there is enough play with them in their stretch to avoid this. That isn't to say that if you stretch them to far the won't break (partially or fully)and this is the cause of muscle strains/pulls/tears etc.

Now the connective tissues also can rupture, most people over the age of 70 will have some degree of rotator cuff tear in the shoulders. These tears happen in the same way as muscles with being over stretched e.g. Dislocations. They heal worse though because of poor blood supply.

The good Dr. is absolutely correct on this. When I was 16 I dislocated my shoulder. Most painful thing I have ever been through. Anyhow went to the emergency room, got some valium through an IV, and had my should reset.

Over the course of several months, I dislocated my shoulder 5 more times. Sometimes with no effort or apparant reason. I would just be laying in bed for instance, and I would move my arm behind my head and my shoulder would rotate out.

Had some ultrasounds and scans done, and it turned out everything in my shoulder was either torn or stretched from the original dislocation.

Had reconstructive surgery done, and have permanent "anchors" installed in my shoulder holding it all together.

 

[mikewint]

Dave is absolutely correct in everything that he posted. When I said that the shoulder was "not much of a joint" the meaning was that while it is a "ball and socket" type there is very little anatomically in the way of a "socket" to hold and anchor the "ball" in situ. Relatively speaking it is fairly easy to apply enough stress to the joint to cause a dislocation, i.e.: the head of the humerus is no longer within the glenoid cavity. As I posted earlier the most common dislocations are the anterior (to the front) and posterior. Less common is the inferior dislocation (down along the side)

As Chris posted, with zero exaggeration, the pain is intense due to the large amounts of damage to tendons, ligaments, muscles, the fibrous joint capsul, and even the cartilage pads along the bone contact surfaces. While healing does occur it is never totally complete and the joint is weaker making another dislocation easier with its attendant damage, et cetera. Again as Chris posted he had five more dislocations each one occurring with less stress on the joint. Eventually a surgical repair is required.
As to muscles, they are attached to bone at two points, the origin and the insertion. The Biceps (two heads),e.g.: the long head attaches to the supraglenoid tubercle of the scapula, which is just above the space where the humerus, or upper arm, enters the shoulder. The short head attaches to the coracoid process of the scapula. The insertion for both heads is the radial (radius bone) tuberosity. So within limits it is free to move laterally. In addition muscles are elastic, again within limits. The point being that the muscles of the shoulder joint can follow the dislocated humerus. Tendons and ligaments are more fibrous and much less elastic so tearing can occur. With time both of these can elongate allowing the "double jointed" motions noted earlier in this thread.

The anterior dislocation

 

[Gnomey]

In terms of the shoulder joint itself whilst being a ball and socket joint in literal terms the best thing is to compare it to the hip which is the other major ball and socket joint and the difference is compelling.

Whilst the hip has a deep bony capsule holding the femur in place as can easily be seen in the diagrams above there is barely any bony capsule in the shoulder. This is partly why the range of movement is so wide. The capsule of the shoulder is formed by the fibrous labrum of the glenoid fossa. In addition the rotator cuff which consist of cartilage and the muscles and tendons of teres minor, subscapularis, infraspinatus and supraspinatus.

The range of movement is then generated by the number of muscles acting on the joint and how the act in combination. It's truly a joint like no other.

 

[mikewint]

holding the femur in place

Don't forget (and I love this name) The Fovea Capitis Femoris and its associated ligament. Ya gotta love those old anatomists (Hole in the Head of the Femur)

 

[Gnomey]

Don't forget (and I love this name) The Fovea Capitis Femoris and its associated ligament. Ya gotta love those old anatomists (Hole in the Head of the Femur)

Of course which in itself is the remnant of the foveal artery which supplies the femoral head in youth and whose degeneration is why the elderly end up with a hemiarthroplasty if they fracture intracapsularly the femoral neck...

 

[pbehn]

Of course which in itself is the remnant of the foveal artery which supplies the femoral head in youth and whose degeneration is why the elderly end up with a hemiarthroplasty if they fracture intracapsularly the femoral neck...

So Dr Lahl wasn't telling me porkies then

 

[Gnomey]

Indeed he wasn't...