*2.4. Surgical Operation*

The knee lesions were assessed arthroscopically. The defect size was measured through the arthroscope. For the Kartigen® implantation group, the required cell number of CPs was estimated before the collection of bone marrows. We aimed to implant <sup>2</sup> <sup>×</sup> <sup>10</sup><sup>6</sup> cells/cm<sup>2</sup> in the defect. The cartilage defect at the medial femoral condyle was approached either by mini-medial arthrotomy of 2 to 3 cm in length or by the arthroscope only. After debridement, the cartilage defect was filled with Kartigen® and then sealed with fibrin glue (TISSEEL, Baxter AG). For microfracture, we followed a standard arthroscopic procedure as reported by Steadman et al. [46]. The multiple holes (microfractures) were about 3–4 mm apart, to preserve the structure and function of the subchondral plate.
