If it would have been me, I would have called the police. This dog may have attacked before, and if so is dangerous. Also, this way you would have some recourse for restitution.

Our local independent drugstore sells staple remover kits. Similar to an office staple remover. If you can't find one, you could sterilize one from home to remove the staples.

So sorry to hear your dog was injured but this is my take on this..........

If I see a dog running off lead then I assume it's friendly to people and other dogs (especially in a dog park) but If someone has their dog on a lead then I assume it's not so friendly to other dogs or people so I keep myslef and my dogs away. If I am unable to control my dogs (which thankfully I am) and keep them away whilst off lead I have to put their leads on until the dog has gone.

If my dogs are running off lead and I can't keep them away from the assumed aggressive dog then it's my fault if my dog gets attacked.

Now the question is then raised as to whether or not the aggressive dog should have been muzzled as it was in a dog park and would likely come in contact with dogs running off lead.

So it could be argued that the dog should of had a muzzle on but it can also be argued that you should have kept control of your dog and kept him away.

Is there any signs saying that aggressive dogs must be muzled?

I think you and the other dog owner are equally responsible. He has just as much right to be in the park as you. 

Please don't think this is a dig at how well your dog is or is not trained, that's not what I am trying to point out so please don't be offended. My dogs are very well behaved and come to me instantly when called but if they got too close to an aggressive dog, that was being kept on a lead, because either myself or my dog had a momentary lapse of concentration then I could not entirely blame the aggressive dogs owner.

I had an incedent a few months ago in the local park (not a dog park just a large wooded area where everyone walks their dogs) when my boy ran off towards a dog that was on a lead and I didn't notice the other dog or Leos running over to say hi. I was alerted to it when the aggressive dog started to bark. I called Leos and he turned round immediatey but if he hadn't it would have been my fault if he'd been attacked for not having him under control. I appologised to the man with the aggressive dog and he waved that all was OK.

Place to place, there are very different laws, rules, customs & compliance regarding dogs.  And there's often more to the story than meets the eye as well.

This county has a Leash Law.  Outside of fenced personal property there are few legal options for letting any dog Off Leash.  This is a free County Dog Park, there's a membership Dog Park & one provided by electrician's union nearby-all designated Off Leash.  There are at least a couple of signs posting rules & a LOT more Fine Print in the law.  Free running dogs that run amuck were automatically at fault, an escaped dog is a liability.

In Kal's case the couple carried extremely attractive treats in Dog Park.  Bits of hot dog crisped in microwave & had given treats to Kal.  The attack happened when she was giving treats to Duncan & Kal was looking for more.  We'd been trying to help socialize their other dog.  Find it ominous that they haven't been seen again at Dog Park.  Still don't know if Duncan was hurt?

Took the SS staples out myself by using alcohol swab, small diagonal wire cutter to snip staples in middle & forceps to curl the ends out.  Only complication was Kal finally managed to rip at 1 of the staples kick scratching his ear w/hind foot!  So more antibiotic oinment & waited 12 days, instead of 10.

Didn't see how well aligned when staples went in, but as healed, Kal has a notch.  It'll be his Frankenstein ear for awhile.  That's 1 big beautiful velvet ear that's scarred for life.