Yes it can be repaired and replaced to varying extents. The core problem with cartilage is its blood supply makes grafts and localised repairs difficult - but not impossible and there are new techniques being developed constantly at the moment.

The other option is a total knee replacement or even a partial knee replacement. That is basically replacing the articular surfaces with synthetic surfaces in the whole knee or one compartment.

Silicone injections doesnt make sense for this joint - its generally not so much a lubrication issue (the knee has its own lubricanta) and more an issue of pain from the carrilage or bone grinding on bone. Silicone would not gove a lasting benefit - the surface needs repairing.

Knee replacements work well, especially in younger people. Pain relief is a given and aim for this, the real issue is how much function is wanted and achieved The difficult end is restoring function for athletes. If the expectation is pain free walking or climbing stairs that is very achievable. Running and sports is doable but more difficult and a big part of all of it is the post op physiotherapy, and compliance & motivaton by the person having the op to do the work needed.

The other issue of course is cost. Its expensive and even in "free" healthcare systems there is usually a mininum threshold of pain or dysfunction that needs to be met before theyll pay for it.