What does it mean when fluid comes out when they place the needle in the joint?
Conversely, what does it mean when the needle is increasingling more difficult to place? (Hocks specifically) Assuming that they are possibly on their way to fusing?

If they are fusing... what does that process look like? Are injections no longer needed once that stage has completed? To my knowledge there is only one spot in the hock that actually fuses? 

Fluid coming out is generally good, thats their natural joint fluid. It must be removed partly so space is made for the injected product. If you have no fluid they have dry joints, which is not ideal at all. More difficult to place could be a variety of factors & which joint they're trying to enter, at that point I'd be doing fresh xrays. Their lower hock might be fusing, its a long process sometimes and painful. A lot of people will chemically speed it up to reduce layoff time. Typically once thats done fusing injections aren't needed, unless you have other things going on besides that. You may still have to have the upper injected, I've never just done lower. 

Fluid coming out of a joint is normal. Large amounts of fluid indicates a lot of inflammation. Color of fluid should be light straw color and should be stringy, not watery. The thinner the fluid, the more inflammation is indicated. 

there are 3 parts of the hock joint to inject : upper  (tibiotarsal) , middle (distal intertarsal)and lower (tarsal metatarsal). Lower and lower/middle joints are usually injected , upper joint usually isn't injected unless there's obvious radiographic changes.

Trouble inserting the needle in the lower joint or tighteness in the joint that prevents all of the meds to be injected indicates cartilage loss between the joints, "fusing". 

yes, once hocks are fused, there won't be a need for injections. The natural fusion process can take a very long time or may not happen on its own. Laser or chemical fusion is sometimes done to speed the process but l the results aren't predictable.