## 1 Introduction  
Osteoarthritis [OA] is a complex process, in which degenerative changes appearing in articular cartilage [AC], subchondoral bone, or synovial membrane can lead to joint failure [1]. Hyaline cartilage is the main component of every synovial joint; however, due to its function and structure, it has a very limited capacity for healing [2]. Once the cartilage is damaged, the joint is predisposed to rapid osteoarthritis changes, which lead to irreversible joint failure [2]. OA is mainly considered as a disease of older people, as over one-third of the population over 65 years old present OA in at least one joint [3]; however, factors such as obesity, sports, genetic factors, work, previous injuries or socio-economic status can influence osteoarthritic changes in the joint [4, 5] in younger populations. Therefore, researchers seek methods for effective treatment at every disease stage that could decrease disease progress and the need for total joint replacements in the end-stage disease. A range of conservative treatment strategies is implemented in the treatment process, including weight loss, daily activity modulation, pharmacological or surgical treatment. Although each of these methods is carried out in the hope of prolonging native joint survival, they are incapable of curing the cartilage defect itself [2]. Total knee replacement is a gold standard in end-stage disease; however, every total joint replacement procedure has its limits in regard to prostheses survival time [6]. The younger the patient is at the time of total knee replacement, the higher risk of revision surgery is expected since a 10-year survival rate of total knee replacement reaches 81-92% [6; 7]. One of the surgical procedures widely used for treating cartilage defects is microfracture [MFx] of the subchondral bone, introduced by [PERSON] [8]. The technique in question is relatively easy and inexpensive, while it uses body healing capacity on the ground of clot formed in the cartilage defect. Over the years, the technique has been improved and now steps like removing a calcified subchondral bone, creating a straight even cartilage edges, and performing MFx as close to each other as possible [9] are considered essential. As a result, a rough surface is created onto which a blood clot can attach [10]. The effectiveness of MFx depends on multiple factors such as the size of the defect, age, comorbidities, or genetic factors. Age and the size of the defect are the biggest issues influencing the final effect of the procedure. During MFx, mesenchymal stem cells [MSC] are released from bone marrow. However, after the age of 35, the number of available MSC decreases significantly [11], which is among the key reasons for recommending MFx in individuals below 40 years of age [12]. Moreover, fibrocartilage which fills the defect after MFx is shown to have inferior mechanical properties in comparison with hyaline cartilage [13]. Nevertheless, MFx remains one of the most common procedures carried out by orthopedic surgeons in cartilage defects on account of its simplicity, low cost, or fast recovery; however, methods of augmentation of MFx to broaden its indications are actively sought. The effect of the treatment is the creation of fibrous cartilage, which covers the hyaline cartilage defect. The fibrous cartilage is shown to have inferior mechanical properties in comparison with hyaline cartilage [14; 15]. Moreover, the best therapeutic effect is reached when defects smaller than 4 cm are treated with MFx [16]. Based on animal models, it is proposed that MFx outcomes can be enhanced by augmentation of platelet-rich plasma [PRP] [17]. Since the early eighties, platelet mixtures have been used in medicine to improve skin healing [18]. The interest in PRP has grown rapidly since the first publications of its usage in maxillomandibularw surgery [19; 20].