If a dog has HD, he will pace certainly - no changing that.
Pacing is a sign of a physiological issue - it can be many things, including what has already been listed. So it can be structural (which can't be changed) or it can be injury or trauma and absolutely addressed.
Knowing what is causing it really helps to know the dog since a puppy - not everyone has this benefit. If the puppy had good movement and then changes, and depending on when this occurred during growth - there could be a correctable issue. My current Bernese Mtn. Dog had beautiful movement, and a correct gait...then at 1 1/2 years, started pacing. I knew this was an issue and we headed to an ortho specialist, chiropractor, and physical therapist and we worked on her for a year to correct the subtle injury that never showed any other signs - the injury was to her head actually and manifested down her spine....corre cting things one at a time resolved it.
Other dogs with HD, ED, or poor structure will pace inherently.