Introduction
In the human body, cartilage plays a very crucial role – it determines the growth of long bones and creates articular surfaces.[1] One of the most important and simultaneously, most frequently damaged human joints is the knee joint.[2] It is responsible for both horizontal (walking, running) and vertical (jumping) movements of the body. [3] The knee joint has a complex structure. Its primary function is to join the tibia with the femur and patella.[4] To alleviate the heavy burden, to which it is exposed, the knee joint is supported by additional elements, i.e. by ligaments and meniscus. The cartilaginous tissue plays one of the most critical functions in the knee joint – it forms the meniscus (fibrocartilage) and covers the surface of bones (hyaline cartilage).[4] The main task of this tissue is to intensify the movement, adjust articular surfaces, enable rotating movements in the bent knee (meniscus), and to cushion the movement (surfaces).[6] The cartilage is characterized by poor vascularisation – arteries, veins, and lymph vessels do not reach this type of tissue. Nutrients are transported by diffusion with synovial fluid and to a lesser extent, employing bones.[7] These limitations hinder self-regeneration of cartilages significantly and constitute a severe complaint affecting people of all ages. Therefore, regeneration of the cartilages constitutes a significant problem both to patients as well as to physicians [8]. Various factors may cause damages to joint cartilage.[9] They may be caused by direct injuries, repetitive micro-injuries, strains leading to gradual damages, abrasion of cartilage (degenerative lesions), and also by cartilage and bone necrosis.[10] Losses in the joint cartilage are manifested by pain, oedema, and inflammation, which are very troublesome to patients and limit his/her everyday functioning.[11] Previously used treatment methods aim mostly at minimization of symptoms and do not enable full remobilization of the patient [3, 9, 10].
Modern techniques of treatment the joint cartilage focused on biological methods that enable for self-regeneration potential of the body [3]. One such method is the technique of microfractures which is based on performing microdrillings in a subchondral layer of the joint where multipotent stem cells are released.[12] After some time, these cells differentiate into chondrocytes and then create the new surface of the joint. This method is often modified by using a collagen membrane (Autologous Matrix-Induced Chondrogenesis, AMIC) where cells released from the bone are settled. The microfracture technique has numerous advantages and is highly effective.[13] Unfortunately, they may not be used in cases of specific lesions located closely to each other. Moreover, one should keep in mind that newly formed cartilaginous tissue is only similar but not identical to the native tissue [14–16].
Another method is autogenic osteoarticular transplantation (Osteoarticular Transfer System, OATS).[15] In this method, damaged cartilage is removed along with the subchondral layer and bone lying underneath. Next, a cartilage part is taken (along with a subchondral layer and a bone fragment) out of a healthy surface of the bone, usually the femoral bone. The fragment of cartilage and bone is subsequently pushed in the place of the removed lesion. Healing of the lesion covered by the transplant occurs after 6–8 weeks.[17] This method is highly effective. However, it involves significant interference in the body of a patient. [18,19]
A technique using periosteal flaps and transplantation of bone marrow cells is implemented for more considerable cartilage damages.[20] After recessing damage and estimating its dimension, a fragment of the periosteum is taken from the femoral bone. It is then stitched to the site of the damage and sealed with the tissue glue forming a flap.[21] Furthermore, bone marrow drawn from the iliac bone or a suspension of cartilage cells cultivated in the laboratory is injected under the sewn and sealed flap. Like the previous techniques, this method is very effective but also involves significant interference, which diminishes the comfort of therapy for the patient. [22, 23]
A further method of cartilage regeneration is autologous chondrocyte implantation (ACI).[24] This technique is based on a biopsy of a small fragment of healthy cartilage. The chondrocytes are then multiplied for ca. 6–8 weeks.[25] After, the patient should undergo the next surgery. A fragment of the periosteum shinbone is collected and implanted in the place cleaned. Cultured cells of the cartilage tissue are then injected under the stitched periosteum which is multiplied once again and form a new fragment of the cartilage.[26] Unfortunately, this method requires two surgical procedures. [27–29]
Another way of cartilage regeneration involves modern biomaterials that are used for obtaining cellular scaffolds [30, 31]. They create a suitable spatial environment that enables the growth of chondrocytes retrieved from in vitro breeding or from stem cells of the bone marrow.[32] This scaffold should ensure the best distribution of cells, provide stability, and facilitate differentiation. The three-dimensional shape enables for forming of the structure that is very similar to natural cartilage.[33] Scaffolds are usually made of natural or synthetic polymers.[34–37] Synthetic and biodegradable polymers, i.e. polylactic acid (PLA), poly-ε-caprolactone (PCL), polyglycolide (PGA), and their copolymers constitute a popular and frequently used group of materials used for obtaining the scaffolds.[38–41] These compounds are characterized by biocompatibility, biodegradability, and degradation into products that are readily excreted from the body (CO2 and H2O). These polymers vary in degradation time which increases with increased length of the carbon chain of a polymer.[42] Among the mentioned polymers, the longest degradation time is attributed to PCL (over 2 years), followed by PLA (ca. 2 years) and PGA at the end (several months).[43] These polymers are characterized by simple processing (stability in variable temperature conditions) and excellent mechanical properties (especially in comparison to natural polymers – such as collagen or chitosan).[44,45] Moreover, by using biodegradable scaffolds, one improves the comfort of therapy because there is no necessity of removing an implant.[46] All the mentioned characteristics make biodegradable synthetic scaffolds very popular. [47–50]
The purpose of these studies was to obtain biodegradable scaffolds for the cultures of chondrocytes.[51] Scaffolds that were obtained should be characterized by a unique, three-dimensional spatial structure enabling proper differentiation of cells.[53] The strongly porous upper surface should allow for penetration of cells into scaffolds, and less porous bottom surface should prevent them from being ”excluded” from the structure.[54] The highly porous cross-section was supposed to contain large and interconnected pores with smaller pores (perforations), enabling intercellular communication and migration of nutrients and metabolites.[55]