Polylactide-based biomaterial and its use as bone implants ( analytical literature review )

In many areas of medicine, implants from various synthetic and natural biomaterials are widely used. One of the materials that are most often used to create implants is polylactide (PLA), a feature of which is biodegradation in implantation sites, osseointegration, the ability to induce bone formation and high biocompatibility with the body. Aim of the review is to analyze and summarize data on the rearrangement in the bone of biodegradable biomaterials based on polylactide and to identify trends in the development of the problem. The review of the literature presents a general description of PLA and identifies historical milestones in the development of the problem and the use of biodegradable polymers in bone surgery. The data on the factors affecting the biodegradation of this biomaterial in the bones are presented and the peculiarities of its osseointegration are determined depending on composition. The data on the use of PLA and copolymers in bone surgery and regenerative medicine are given. An important direction for future researches will be the development of composite biomaterials based on PLA with the desired qualities of osseointegration and controlled biodegradation. New trends in the use of implants based on composite materials made from PLA in bone surgery and creation of implants using 3D printing are presented.


Introduction
Implants made of various synthetic and natural biomaterials are widely used in numerous fields of medicine. Their list is ever increasing due to the new ones being developed annually, and their properties being optimized for specific uses [17,21,25,43,46].
The biomaterials are divided into two groups depending on their properties: bioinert and biodegrading in the implantation areas. Bioinert materials are popular in orthopedics and traumatology [27]; however, it is the biodegrading ones that look most promising due to their complete degradation obliterating the necessity of an additional removal operation and a stable fixation for a certain period of time in case of bone implantation [33,70,69].
Developing biodegrading materials for the fixing tools and bone cavity fillers is a complicated task which requires a collaboration of expert material developers and researchers in the fields of biology and medicine.
Studies of human body reacting to various artificial materials become prominent at the time when a range of implants for bone surgery and regenerative medicine expands.
Human tissues are prone to react to the insertion of a foreign body [16,38]. In this regard, every new biomaterial or biomaterial-based composite is severely tested to prove its biocompatibility and explore features of its remodeling at the implantation sites.

General information on PLA
Polylactide is a semi-crystalline polymer synthesized during polymerization or polycondensation. Its molecular weight range is from 180000 to 530000, melting point being set approximately at 174°C and glass transition -at 57°С. At the implantation sites, PLA is degraded by hydrolysis. Depending on its configuration, PLA may act as two different stereoisomers, poly-L-lactide (PLLA) and poly-D-lactide (PLDA), of divergent properties: PLLA's slow degradation takes 2 to 5 years while PLDA's mechanical strength is being lost much faster [23]. For this reason, a lot of orthopedic implants are made of PLLA [6].

Historical aspect
The first animal-based experimental study of PLLA was published in 1966, proving the biomaterials biocompatibility, absence of toxicity and slow degradation at the implantation sites [30]. When the PLLA plates and screws were used to fix the canine lower mandible, there was no side effect revealed in the tissues [29]. In 1971, the clinical study of humans with lower mandible fractures evidenced the prospects of this biomaterial being used in clinical practice [10]. However, the wide-ranging application was still pending as the first PLLA implants tested from 1990 to 1996 triggered inflammation and swelling at the implantation sites in 47 % of patients [6]. Moreover, certain aspects of PLLA implants, namely their fast destabilization and breakage, lack of stiffness and strength compared to the metal counterparts, impeded further use [15].
Progress of material engineering paving the way for implants being made of PLA stereoisomers, removal of the material's deficiencies, and development of sterilization methods resulted in a gradual change of the scientific attitude towards implants [25]. The next stage heralded creation of PLA copolymers, PLA-based composites, and improvement of production technologies and manifested itself in an optimized biodegradation and implant mechanical properties.

Factors affecting implant biodegradation
Biodegradation is a process of polymer being reduced to water and CO 2 under the influence of biologic environment [21]. In case of a complete biodegradation, PLA turns into gaseous products filling the implantation sites with bone tissue.
Mechanical properties, biological mechanisms and biodegradation vary as far as PLA-based composites are concerned, requiring further exploration and optimal surgery application. Biodegrading materials should be biocompatible without any inflammatory or immune response; and the degradation products should not be toxic.
Implant's biodegrading rate depends on their localization at the skeletal sites. If those sites aren't vascularized enough, there is a risk of adverse reaction due to acid debris accumulating and affecting the tissues. Moreover, implants at the increased load sites get degraded a lot quicker than the ones at the reduced load sites [39]. Other sources of influence include the bone quality (trabecular, cortical), thickness of soft tissue layer over the implant, bloodstream, osteosynthetic type (intra-or supra-osseous), and individual features [2,8]. Total degradation period is a combination of various factors, namely the material quality (hydrophilic or hydrophobic), its structure -crystallinity, original molecular weight, polymer component ratio, implant size and design, production and sterilization technology [8,9,40,50]. Recent studies are targeted at reducing implant's reaction to a foreign body, polymer crystallinity and pH-control under biodegradation.
PLA implant degradation starts with biological substances penetrating the implant crevices and breaking the polymer chains into a multitude of fragments. This process reduces viscosity and impacts mechanical density. Cracks and crevices of the hydrophobic surface may incite an inflammatory reaction, while a biodegradation results in a reduced adhesion and cell proliferation [21]. However, the frequency of side effects at the PLA implantation sites (if the implant is a state-of-the-art technology) is quite low: only 0 to 1 % [6]. Actual weight loss is provoked by the lactic acid dissolution, release of the degraded products through osteoclast and macrophage-assisted phagocytosis and, as a result of Krebs cycle, final reduction to the dual metabolic products: carbon dioxide and water secreted with air and urine [55,57]. Thus, a polylactide biodegradation ends with metabolic derivatives devoid of toxicity, which makes the material totally applicable in medicine [51].
In order to create stiff supra-osseous plates and improve mechanical qualities, researchers suggest a PLA-based composite with tricalcium phosphate [58]. Bioactive supplements, such as ceramics made of calcium phosphate, may improve its biological properties and osteointegration, while under biodegradation calcium phosphate ceramics promote bone regeneration. PLA, combined with hydroxylapatite, increases strength of osteosynthetic plates (OSTEO-TRANS MX®) [41].
Plates and intra-osseous implants made of PLLA, hydroxylapatite and tricalcium phosphate with a ratio of 70:10:20 provide better osteointegrative properties and improve the biomaterial's strength [42]. There is also a PLA-based composite with carbon fibers and hydroxylapatite [41].
When the deteriorated bone state (osteoporosis, osteopenia) was taken into account, a PLA-based composite with apatite and strontium was offered [33].
PLA's osteointegrative and reparative properties increase with spongy structures where the pores have optimal sizes (about 100-200 µm) promoting bone tissue penetration and functional remodeling [35].
PLA and PLA-based composites are recently viewed as the most promising biopolymers for the bone surgery [13]. PLA is well researched and proved to be a safe biomaterial for the clinical practice, meet the demands, and draw maximum attention of the scientific community [59]. However, the search for beneficial PLA-based composites goes on.

Innovations in the field of PLA and PLA-based composite production
Major segmental bone defects caused by trauma, infection, tumors seem to be a topical issue for traumatologists. Due to patients' individual and complex defects, material and implants should meet their individual demands, and thus even the most convoluted implant molds easily recreated.
In 1986, Charles Hull used 3D printing for the first time, and since then this technology is on the rise [54]. 3D printing is a novel approach to the production of implants with various sizes and properties for the tissues and organs. 3D printing in the field of medicine will enable a customized implant to replace a bone defect of a specific form and volume at the traumatized skeletal site. However, the method raises a lot of questions as to the printing materials, opportunities for the newly created biocomposites to be offered depending on their biocompatibility, implanation sites in terms of their load and blood supply.
3D printed biodegradable bone implants have a range of advantages, among them, a relatively non-invasive treatment, no need of a repeat operation to remove the implant, creating a customized implant version for every patient. In 3D printing, only the most innovative biomaterials, i.e. ceramics, various polymers (mostly PLAs and PGАs, as well as their composites) are used [19], thus opening new vistas for bone surgery [37] and enabling surgeons to perform the previously impossible operations. 3D printed PLA implants guarantee mechanical stability, possess a high biocompatibility and osteoconductivity [9].
Studies on PLA endotoxin pollution of 3D printed implants reveal a low margin level the FDA prescribed [49].
These data expand PLA use, especially among the 3D printed bone implants. These implants could also be used as scaffolds in the regenerative medicine, in particular for bone defects.
However, it should be mentioned that PLAs by various producers could affect the printing conditions and quality, as well as they get remodeled in the bone tissue differently, and thus require additional clinical and experimental studies [44].

Experimental studies of PLAs and PLA-composites
Experimental studies among the cell cultures and animals augment the range of biomaterial testing, including biocompatibility, osteointegration, strength and degradation rates [18].
PLA in regenerative medicine. This biomaterial demonstrates its great promise for the tissue engineering, in particular scaffolds of various shapes and volumes for cell cultivation and further implantation into organs and tissues [1,64]. The cell culture studies prove PLA's biocompatibility, absence of immune response, and cell integration.
In vitro study of cell cultures was performed to compare cell adhesion and proliferation depending on various cell-cultivating scaffolds [64]. Titanium disks and polystyrol scaffolds were tested because of their wide use in bone surgery. Cellular viability was found to be higher on the PLA scaffold (95,3 ± 2,1 %) as compared with polystyrol (91,7 ± 2,7 %). Cellular proliferation, by contrast, was more significant on polystyrol disks. However, when PLA and titanium disks were compared, PLA disks were proved to promote greater proliferation. During the scanning electron microscopic analysis, it was found that all the disks were covered with a homogeneous cellular layer; nonetheless, PLA and titanium disks had a higher density of cells.
Comparative analysis of cell cultures was also performed on solid and spongy PLA disks, covered and filled with collagen [49]. Biosamples were 3D printed. Their microscopic analysis showed that various cell types (preosteoblasts, osteoblasts, fibroblasts and endothelial cells) grow and spread on PLA printed disks, proliferating as the biomaterial is compatible and possesses significant adhesive properties.
Recent studies also focus on surface modification of implants, i.e. emergence of spongy structure. Spongy PLA covered with collagen promotes adhesion and growth of endothelial cells, inducing angiogenesis at the implanted sites. 3D printed PLA scaffolds used with gelatinous hydrogels were shown to stimulate osteogenetic differentiation of human fat tissue-derived stem cells if cultivated. Thus, they could be used in regenerative medicine to produce bone cavity fillers and support bone regeneration [62].
Experimental studies on animals. Ingeo ™ Biopolymer 403, polylactide, a product of L-and D-type lactide polymerization (ratio from 24:1 to 32:1) was tested on rats. Screw-shaped implants were created by means of «Ultimaker3» 3D-printer (build-up fusing method, each layer 0,1-0,2 mm wide). Biopolymer-made screws were implanted into metadiaphysis and diaphysis-located defects of femoral bone [14,34]. The material was proved biocompatible, with significant osteointegrative properties; it does not provoke inflammation of adjacent soft tissues and bone marrow, as well as destructive bone changes at the implantation sites. At the study's end (after 270 days), polylactide implants retained their shape, no biomaterial got degraded, which signals a prospect of long-term use.
When an experimental study on animals involved a fracture fixation, L-lactide, D-L-lactide, polyglycolide and trimethylene carbonate plates possess the following properties: biocompatibility, absence of bone lysis under the plate and lack of negative impact on regeneration [3]. Other studies showed angiogenic and osteogenic features of PLA-based composites [24,67]. Performed experimental studies expand extant vision of PLA and PLA-composite interaction with bone tissue.
Fracture fixation with biodegrading materials was found to be as effective as fixation with traditional metals. Nevertheless, they have certain advantages: no need of removal, controlled biodegradation etc. Formerly viewed as a perfect option for the unloaded skeletal sites [40], PLA-based composites were promoted into the field of bone surgery.
Despite the fact that orthopedic surgery has been using degrading polymers for 30 years, respective spinal implants are a recent development. However, there are a limited number of studies documenting PLA or PLA-based composite use in spinal surgery. Having summarized the revier data of degradable implants in human and animal spinal surgery [59, 60], we found indications that in animals implants are effectively resorbed, spondylodesis is formed and cages degraded in the intervertebral spaces. As for humans, biodegrading cages were found to be effective, safe and spondylodesis-promoting. Cages of PLLA:PLDA with a 70:30 ratio (Hydrosorb, produced by Medtronic Sofamor Danek, Memphis, TN.) provided some proven positive results. At the first stage of clinical trials in 2002, 60 patients undergoing transforaminal lumbar interbody fusion (TLIF) with Hydrosorb vertical cylindrical biodegrading cages [30,32] did not reveal any complications after 4,7 months. Having studied those patients who received recombinant morphogenetic rhBMP2protein cages after 12-18 months, researchers found the interbody space height to remain stable, spondylodesis to occur in 87 % of patients according to X-ray and in 97 % according to CT results [28]. There were no instances of infection or cage-induced complications recorded.
Another study of TLIF with similar cages revealed spondylodesis in 96,8 % (30 out of 31 patients) after 18,4 months on average [10]. However, there was no indication of cage biodegradation.
Along with positive results of biodegradable cageinduced spondylodesis, there is a comparative study of PLLA: PLDA cages with a ratio of 70:30 and carbon fiber cages [56] which showed an increased non-union frequency (18,2%) and post-surgery cage migration (18,2%) after TLIF.
Overall, cage technology studies require prolonged observations of the respective control groups.
Our analysis of theoretical sources has thus confirmed the prospects of further studies of PLA properties and modifications in view of experimental and clinical applications. Polymer properties even if coming from the uniform substance depend on the industrial process (namely, processing temperature, production and sterilization method etc.). It provides an opportunity to develop materials and implants based on their intended properties as products made of the same input but using different technologies would have different properties, such as strength and biodegradation rates. Data comparison as well as conclusions as to the biomaterial properties should not be based solely on the experimental results of same input material if we do not know anything about processing methods and other details. An important research track is to develop and study composite biomaterials which would give Bol', sustavy, pozvonočnik, ISSN 2224-1507 (print), ISSN 2307-1133 (online) Лекції, огляди / Lectures, Reviews rise to implants with intended properties of controlled biodegradation and osteointegration. A significant progress is brought by 3D printer of implants. This is a new filed of implantalogy enabling new approach to implant creation and clinical use.

Conflicts of interests.
Authors declare the absence of any conflicts of interests that might be construed to influence the results or interpretation of their manuscript.