My quest for a joint management routine

Leo's Lymes test came back negative so I thought long and hard about how to proceed. He's been on stall rest for 5 days and there is some improvement in his soundness but he's still isn't back to normal. Leo has always been stiff on his left hind and had a slight toe drag, but this is more severe than his "normal" imperfect movement. I know I need to do something, but what should I do and how much money should I spend??

Shockingly, Sally said she believes every horse has some degree of lameness. (There's an uncomfortable thought.) She told me last week that I need to figure out what Leo's joint management routine should be. I shouldn't immediately be going to direct joint injections every time I hit a problem. I need to determine what monthly or bi-monthly measures I can take to ensure his comfort and performance over time. I can't use up everything in a vet's arsenal on a training level horse, then find I hit the wall when I'm prepping for a 1-star. I need to leave something in the bank.

This year I've been in reaction mode. Leo was lame in the hocks, so I had his hocks x-rayed (they were clean) but I wanted immediate improvement so I had them injected. Problem was fixed. Leo put in a bad performance at Fair Hill. I had both stifles ultrasounded and one injected due to minor fluid buildup. Probably not necessary, but taking care of the legs is important. We all think it was really the ulcers that caused the problem at Fair Hill.

Liz has been talking to me about Adequan and Pentosan as maintenance options for quite some time but I wasn't really committed to the management at the time. Instead, I just went back to work with a focus on working on exercises to improve whatever weakness the lameness had uncovered. I've slowly realized I need to change my approach. Leo is 10 now. While that isn't old, my old approach of using training to strengthen his body and improve his movement is no longer working. So I have to do something else.

As of this week, there are a lot of options I can pursue. Each is a completely valid approach.

• Bone scan: Bone scan Leo's left hind and pelvis region and see where the "hot spots" are. This can easily cost $1200-$1800. Both Sally and Liz didn't recommend this. Bone scans are generally used to help diagnose multi-limb lameness or to help manage high-performing horses. A bone scan identifies a direction and then ultrasounds are required to pinpoint the problem. With Leo, all signs indicate that the issue in the SI region and the lameness is isolated in one limb. 
• Ultrasound pelvis and hip: Skip a bone scan and go straight to imaging the hip and SI region (which is done thru the rectum) - roughly $400. Liz suspects arthritis in the SI area because Leo works out of his stiffness. If it was a ligament or tendon tear, he would get worse with work - not better as he does. So the imaging would be used to confirm that Liz's hunch is correct. Is this truly necessary? Debatable. 
• Inject the SI Joint: I could go ahead and just inject the SI joint sans imaging. It's a chunk of change ($600 or so), but Liz thinks it will help substantially.
• Monthly or bi-monthly joint injections: Determine a schedule for IM or IV anti-flammatory, joint treatment drugs. Options include Adequan, Legend, or Pentosan with the addition of Estrone injections to help tighten ligaments. Pentosan averages about $16 a shot, Adequan about $36, and Legend about $70. 
• Feed-thru Supplements: To be honest, this really didn't come up in conversation with Liz or Sally, but it's an option. I think most people I have spoken with agree to invest the money in shots - not with feed-based supplements. Leo's currently on SmartCombo Ultra from SmartPak which includes SmartFlex III Resilience, a joint supplement, along with the digestion and hoof supplements I primarily got it for. Obviously, it didn't prevent our current problem.

I've decided to first try the monthly injections. I need to know how little I can do to manage the problem - particularly from a financial perspective. 

Liz taught me how to do IV injections and I gave Leo a shot of Legend, which can only be administered intravenously. She said I would notice a difference in a week if it works. His stiffness should disappear and he should be using his left hind better (less toe drag and tightness in his hip). If it works, I can switch to Pentosan or keep him on Legend, depending on how much I want to spend. If Legend doesn't work, we'll go ahead and inject the SI, but still put Leo on a regimen of Pentosan for management.