ELI5: Why can't doctors replace damaged joint cartilage with synthetic material instead of replacing the whole joint?

Doctors cannot easily replace just the cartilage because it is nearly impossible to permanently glue a synthetic material to slippery, wet bone under the crushing pressure of your everyday movements.

Think of your joint like a well-paved road:

• Healthy cartilage is like smooth, fresh asphalt laid on top of solid ground (the bone). It is slick, absorbs shocks when you jump, and allows your bones to glide past each other without grinding.
• Damaged cartilage is like a giant pothole.

• The Glue Problem: Your joints are filled with natural, slippery fluid. Imagine trying to use superglue on a greasy frying pan while running water over it. Nothing synthetic sticks well to wet, living bone for very long.
• Extreme Crushing Forces: Every time you take a step, run, or climb stairs, your knees and hips absorb forces equal to several times your body weight. A small synthetic patch gets squished, knocked loose, or torn away very quickly under that kind of pounding.
• The Edge Effect: If you patch a pothole in a road, the seam where the patch meets the old asphalt often cracks first. In a joint, the edge of a synthetic patch rubs against the delicate remaining natural cartilage, quickly shredding it like sandpaper.
• Living vs. Non-Living: Natural cartilage is connected deeply into the bone by tiny living roots. Synthetic materials don't heal, grow, or integrate into the bone naturally.

Instead of patching one pothole on a ruined street, the surgeon repaves the entire intersection. They cap the ends of the bones with strong metal parts and snap a durable, medical-grade plastic spacer between them, anchoring everything deep inside the bone so it can never slide out of place.