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]