system
Given the following post, respond as if you're commenting on Reddit:
human
With advances in many fields of Medicine including the transplant of synthetic hearts and 3d printing of various body parts making cheap prosthetics possible, why haven't we seen significant advances in prosthetic cartilage for damaged joints and herniated disks?

Something like cartilage seems like a simple enough structure to manufacture when we're printing heart valves and other much more complicated structures. 

And yet, I've been reading and talking with non-experts involved in fitness science that we just haven't found the right material, with the right type of properties to replace real cartilage. 

Doctors/medical researchers, what are the major hurdles faced by prosthetic cartilage today? 

Edit: please keep this as ELI5 as possible, I don't have a very scientific background. 

Edit2: Researchers : where is the research at now? What sort of time-frame are we looking at for general use of prosthetics, if you can provide one?
gpt
Ok, first things first :

The synthethic hearts are not too good. In most cases they are used as "bridge-to-transplant" solution, not as an endgame. People don't survive too long on them. Also 3d printing body parts is highly experimental and pretty far from being a standard.

Now, yes we do use mechanical heart valves. They are pretty good. You have to take anti-koagulation medication for the rest of your life, but otherwise you are not really handicapped.

Mechanical knee/hip/shoulder/etc replacements are pretty okay as well. You can't do sports with them, but they enable you to live a painless life, which is a pretty big deal for a patient with osteoporosis.

Now, when we replace joints, we don't manufacture human-like structures. We basically build something out of metal, that works like the original, without actually being *like* the original. 

Heart valves now are waaaaaaaaay simpler structures as joints or even cartilage. They are basically a few layers of cells that passively move. They don't have vessels, they don't regenerate, they have a very reduced metabolism (low turn over tissue). It's probably the easiest thing in the whole human to replace.

Cartilage on the other hand is a highly complicated tissue. It's capable of expanding and compressing, balancing the pressure of our whole body weight (and more, when you do movements like jumping). It's capable of storing water when under low pressure, and releasing it when the pressure rises, and various other things.

Manufacturing something like this is far beyond our current knowledge and technical abilities.