How bones grow?

All information is from the, 'How Bones Grow' video.

Osteoblasts, the makers, have a counterpart called osteoclasts, the recyclers. Osteoclasts break down the unneeded mineral lattice using acid and enzymes so that osteoblasts can then add more material. As calculated by Wolff’s Law, that makes osteoclasts more active than osteoblasts, ending in a loss of bone mass and strength. When bones break, the body has an amazing ability to repair the bone as if it never happened.

First, doctors extract stem cells from the patient’s fat tissue and take CT scans to find the exact dimensions of the missing bone. Next, they model the exact shape of the hole, either with 3D printers or by carving with decellularized cow bones. Those are the bones where all of the cells have been stripped away, leaving only the sponge-like mineral lattice. They then add the patient's stem cells to this lattice and place it in a bioreactor, a device that will simulate all of the conditions found inside the body. Temperature, humidity, acidity and nutrient composition all need to be just right for the stem cells to differentiate into osteoblasts and other cells, colonise the mineral lattice, and remodel it with living tissue. Remember Wolff’s Law? An artificial bone needs to experience real stress, or else it will come out weak and brittle, so the bioreactor constantly pumps fluids around the bone, and the pressure tells the osteoblasts to add bone density. Put all of this together, and within three weeks, the now living bone is ready to come out of the bioreactor and to be implanted into the patient's body.