system
Given the following post, respond as if you're commenting on Reddit:
human
Seems like some joint injuries are the hardest to come back healthy from, like knees and ankles. Is it the complexity of the body part or the fact that it’s used the most in the body system?
gpt
Joint injuries being the hardest to come back from is subjective, as is rating it as one of the most complex or used parts of the body.  One could argue that brain or heart injuries are the hardest to come back from, and also suggest that it has something to do with their complexity or amount of use.
Instead, how much can be repaired (and how well) could actually be said to determine how easy an injury is to come back from.  Now, lets talk about joints. Joints are basically the ends of bones that move against each other, but friction and wearing down is prevented by them being covered by cartilage and lubricated by fluid. Without the cartilage and fluid, the bones of the joints would simply grind against each other in a very noisy and painful way.
When a joint is injured, any cartilage that is damaged cannot be repaired.  The body simply doesn't make new cartilage.  Instead, bone is commonly created instead, resulting in bone being where cartilage used to be. In addition, the amount of fluid there used to be is permanently affected.  On top of all of this, scar tissue can be made that just grows to cover anything that was injured, kind of like making a spiderweb out of duct tape to stick everything together. All these factors can result in a joint that doesn't move as much as it used to, is much more painful to use, or is weaker than it was before the injury.