I am going to try to keep this as far away from a 'how-to' as possible, in light of recent ana% arguing.
I am not a doctor, and all I haveto say is from personal experience, which to some people may translate as me lying, in anycase its a free world, and I will exercise my right to reply
I dont know from personal experience whether knuckle conditioning by hitting hard objects repeatedly makes any difference in calcium deposits in the knuckles. I know that it does build calcium deposits in the forearms because I have done forearm conditioning (of a fairly hard nature) and I have seen the results first hand. Note: I have not killed my nerves as is the popular view of conditioning.
I have come to the conclusion that it MUST create problems if you do a similar type of training on a joint since I ASSUME that the calcium builds on joints also, and will invariably lead to deformation of the joint and therefore potential problems.
I dont know what the balance is. I know that hitting marble slabs repeatedly will lead to BIG problems, and I know that wearing massive gloves and punching dough will not benefit the practitioner greatly. I dont know what the long term effects of makiwara training are because I dont do it, or personally know anyone who has done it for a prolonged period of time.
Some conditioning is needed, because a trader always keeps his tools sharp.
There are A LOT of different methods of conditioning. Some rely on building the calcium in the bones, and others depend on strengthening the muscle inorder to protect the bone.
The makiwara/ice/breaking/heavy bag are of the former discipline.
Iron palm, on ball bearing are of the later discipline.
That is why I tell people not to condition their knuckles if they are doing iron palm. Because there are no muscles in the knuckles and therefore are better off hitting the heavy bag.
I know first hand that if done properly (it takes three years to do iron palm, and it begins by letting your hands drop on dried peas) iron palm training, does indeed strengthen your PALM and KNIFE. This is a 'safe' method because you are not hitting any bones just muscle. At the very worst it just wont work and you ll be just as weak as you first started.
So in conclusion.
1. there are two types of conditioning (bone/muscle) and it is best to know which one you are doing.
2. forearms build calcium and I assume in the same way so do knuckles and all other joints.
3. IN MY OPINION its ok to practice calcium building (making bones denser) on a bone but not on a joint.
4. If you are strengthening muscle its a whole other ball game, and IN MY OPINION a safer bet than the makiwara training.
5. Some conditioning of the fist is needed otherwise you will get injured from the first encounter. But there is a fine line between trying to prevent injury by conditioning and sustaining injury from conditioning.
6. There are VERY FEW qualified instructors for this type of stuff.
7. There is not much data availiable, because: What is the average maximum clacium that the knuckle can take before it causes problems? How many times a day of makiwara training and with how much force? All these things are hard to measure and hard to control.
Will these lead to arthritis or osteoperosis, calcification or other effects?
It depends. What you are doing, how you are doing it, and how often you do it.
If you dont have an instructor that knows his stuff...then DONT DO IT.