These procedures are reserved for completely detached talar osteochondral lesions that are not amenable by internal fixation.

Human articular cartilage has a limited reparative capability due to its avascularity. To address this problem, arthroscopic techniques have been developed to stimulate bone marrow reparative properties, including drilling and microfractures [14].

These techniques promote the development of a fibrocartilaginous formation over the defect, as frequently occurs in the case of small lesions.

The principal goal consists in breaching the subchondral plate at multiple intervals to stimulate chondroprogenitor cells of the bone marrow into the lesion site; the release of fatty drops from the new fractures provides a clinical indicator that the depth of the microfracture is adequate. This procedure stimulates the formation of fibrin clots and subsequently fibrocartilage (type I collagen) in the defect.

The technique is supplemented by excision and curettage of the damaged tissue for better results [14].

It is well documented that arthroscopic excision, curettage and bone marrow stimulation should be the first treatment of choice for primary osteochondral talar lesions: this procedure is relatively inexpensive; there is low morbidity, a quick recovery and a high success rate [1].

The best indications for bone marrow stimulation techniques are a lesion smaller than 1,5 cm² with frayed cartilage, primary surgery, patients younger than 50 years old, a traumatic etiology, lateral lesions and low body mass index.

When advanced fraying of the lesion is present, whether or not the lesion is stable, curettage is indicated and can be eventually combined with penetration of the subchondral bone.

Microfracturing and microdrilling are mini-invasive techniques which require a short period of hospitalization, avoiding risks of thermal damage with easier access to the defects without more invasive steps such as transtibial drilling or osteotomy of the medial malleolus [15].

Drilling, first introduced by Pridie in 1959, was used for decades before microfractures were advocated in the 1990s [16]. These two techniques are thought to be very similar; nevertheless, some differences exist. Chen et al. were the first to compare microfractures (MF) to microdrilling (MD): in particular, they compared acute osteochondral characteristics 24 h after MF or MD procedures in a rabbit model. They found that MF produced a more compact bone around holes than MD did, avoiding marrow cells to reach the new holes and thus limiting the repairing process. On the other hand, MD did not produce the expected heat necrosis: this result was reached using a properly thin drill bit under cooled irrigation [16].

For Takao [17], arthroscopic drilling is less invasive than microfractures. It must be said that no reports of long-term follow-up results have been published yet; therefore, the incidence and degree of complications are not clear.

On the other hand, a disadvantage of the microfracture technique is that the repaired cartilage is fibrous, with inferior mechanical qualities when compared to hyaline cartilage, and it can often degenerate eventually causing osteoarthritis [18].

For this reason, if the cartilage is intact, a retrograde drilling of the subchondral bone without damaging the overlying cartilage can be considered [19].

Retrograde drilling was first described in 1999 by Taranow et al.: it avoids removal of healthy articular cartilage [12]. It has been reported that there is often no difference in functional outcomes between the transmalleolar and retrograde drilling. However, retrograde drilling avoids damage to healthy articular structures [20].