Does any of you have any experience with injecting the TMJ's? 

Bump

Personally there would have to be something seriously wrong (with diagnostic images to show abnormalities ) to warrant injecting the TMJ on a horse.  From my personal experience of having my own TMJ joint injected (due to major joint issues) I wouldn't want to subject a horse to it.  

Whats the reason for injecting? 

Head shaking violenty when stopping.  And no, it is not the bit or teeth.  

This may sound obvious but have you tried different bits or tried a hack? Has the horse been fully vetted to make sure there isnt a pain issue somewhere else?

The horse has been fully vetted, teeth done, tried in different bits.  He only does it when we put the breaks after a run, never during slow work.  The vet is the one that suggested it.  Just wondering if anyone has done it before and if it helped any.  

What type of bits have you tryed? The only time I see a horse shaking its head really bad is when the rider is in its face to much and heavy handed, do you have a video? 

If the shaking is only during a hard stop after a run then something is going related to the run.  If the headshaking was all the time when pressure was applied then it may be something related to the mouth/joint.  I doubt injecting will make a difference in this case. 

If you breeze the horse and ask it to come back down, does it shakes it's head then?  If you lope and ask for a halt does it do it then?  If it's just during a run maybe have someone video you and the horse with specific focus on when you're stopping and see if you can find something that may set them off.  

Vet here. I do inject quite a few TMJ joints. I find it really helps horses who have had some pain related to an unbalanced mouth, or horses with conformational defects such as parrot mouth, which tends to put an increased amount of pressure on the TMJ joint. It is a pretty simple process- the horses don’t seem to mind it very much (I always do sedate, as with all joint injections. 

   There is a specific acupressure point that also helps me determine if the horse is having significant TMJ pain.

X-ray his poll to see if any vertebrae bones have been fractured. Common issue if your horse has ever flipped over.