4.5. Lesion Preparation

Lesion preparation is the process of producing stable cartilage lesion areas and stimulating the bone marrow of the subchondral bone, where MSCs are transplanted. Subchondral bone plays a key role in providing the deepest layers of articular cartilage with nutrient supply and removal of waste products [72]. The widely used preparation methods include chondroplasty, microfracture, and subchondral drilling. Chondroplasty removes all damaged and unstable cartilages, such as articular cartilage fragments, chondral flaps, or osteophytes, to form stable cartilage [35,58]. Chondroplasty can be performed alone to make the subchondral region uniform and visible. This process is accompanied by microfractures and subchondral drilling. Microfractures are performed by penetrating holes 3–4 mm apart, deep enough to develop the “fat-pearls” of the subchondral bone [34]. Subchondral drilling is processed by multiple drillings of 2.00 mm diameter at a depth of 4–6 mm after meticulous removal of the calcified cartilage layers of the subchondral bone and formation of a stable edge of good cartilage [62]. Multiple drilling techniques use a high speed of approximately 10,000–400,000 rpm [73], which can cause damage to the surrounding tissue due to heat generation and frictional forces. Microfracture and subchondral drilling are widely used bone marrow stimulation techniques for articular cartilage repair in clinical circumstances, and they have therapeutic effects in treating OA. The damaged environment releases signals to recruit dormant pericytes to the sites, and the endogenous progenitor secretes the profile of chemical factors to create a regenerative environment [8].