Benthien and Behrens first described the AMIC technique [11, 12] with indications limited to lesions under 1.5 cm² [12]. Later, Piontek et al. [13] introduced a modification to the technique that allowed the insertion of the matrix under dry arthroscopy and fibrin glue fixation, after marrow stimulation of the defect. Kusano et al. [14] observed significant improvements in all clinical scores at 28 months follow-up. However, a retrospective review of 38 patients treated with a collagen-based matrix for full-thickness defects of the knee showed an incomplete defect filling observed at MRI in most of the cases. Furthermore, satisfactory clinical results were described by Schiavone Panni et al. [15] in 17 patients, with a 76.5% satisfaction rate at 36 months postoperatively. On the other hand, more than half of the MRI scans performed showed the presence of subchondral bone edema. Gille et al. [16] applied this technique on 57 patients with cartilage defects at various knee sites, confirming the safety of the procedure and showing significant clinical improvement at 24 months follow-up. They also reported 87% satisfaction rate in 27 patients at 36 months follow-up, paired with a moderate to complete filling of the defect at MRI, regardless of patient’s age or number of previous surgeries [17]. Finally, they recently updated their first series of 57 patients at 16 years follow-up [7]. Among the 18 patients evaluated at long term, 4 failed and underwent arthroplasty. The remaining cases showed a slight increase of clinical scores compared to mid-term evaluation. However, the reliability of these positive results is limited due to the low number of patients available for the final follow-up evaluation.

Other authors tried to improve the biological potential of these one-step procedures by adding a bone marrow concentrate (BMC) prepared during the same surgical session. The promising results showed by early reports [18, 19] have later been confirmed at longer follow-up [20, 21]. However, these augmentation procedures are outside the scope of this chapter and will be described elsewhere.

Finally, a further option to augment marrow-stimulation procedures for the treatment of chondral layer defects came from chitosan, a biodegradable hydrogel that showed optimal biocompatibility and the ability to support chondrogenesis in vitro and in the animal model [22]. BST-CarGel technique involves the use of chitosan hydrogel mixed with autologous whole blood, which is applied into a microfractured cartilage defect by arthroscopy, in order to stabilize the clot and, in the end, improve the repair process [23]. The results of a multicenter randomized controlled trial were first reported by Stanish et al., who compared 41 patients undergoing BST-CarGel with 39 undergoing microfractures alone. At 12 months, safety and clinical improvement offered by both procedures were comparable, but the treatment group achieved greater lesion filling and hyaline-like tissue at T2 mapping [24]. Later, a second study on the same series showed that these results were maintained up to 5 years after surgery [25]. The superior quality of the regenerative tissue offered by augmented microfractures was confirmed by macroscopic and histologic evaluations in nearly half of these patients, showing better features in most of the parameters for the treatment group, where a hyaline-like cartilage was observed [26].