Rather than relying on the migration of bone marrow cells into the defect, chondral grafting relies on taking full thickness cores of bone and cartilage from a healthy part of the joint and grafting it into the defect.
This technique is relevant to defects in the cartilage between 1 and 2 centimetres squared.
A single graft goes by the name of OATS - osteochondral autograft transfer system. An autograft means that the graft was taken from the patient, but it is also possible to use donor tissue - which is called an allograft rather than an autograft. I wanted to find you a nice photo so you can see how OATS works. This one is on a German paper - scroll down to section 5 to the series of images marked A, B and C. Start with C. You can see two nice neat grafts that have been slotted into the two round holes cut out of the damaged cartilage to receive them. Now go back to A. This is how it looked before surgery. B is showing a non-weighbearing part of the joint surface being used as a donor site. A corer is harvesting the graft.
You can see that these cores are really quite large, so one of the problems with this technique is that there can be problems at the donor site. Because it is real cartilage that is being transplanted, rather than fibrocartilage being produced from altered stem cells, the healing is usually good and the cartilage strong.
If the lesion is too large for OATS, then a number of smaller cores can be harvested and packed side by side like a mosaic - mosaicplasty. I looked for a nice picture for you - see figure 13 and 14 on this French paper. Figure 13 shows the little plugs being harvested, and figure 14 shows them packed to fill the defect. You can see the two rows of harvested plugs, and it is not diffiicult to see that there might be residual pain problems here at the harvest sites.