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41MEDICAL SCIENCES
BIODEGRADABLE MATERIALS USED IN ORTHOPAEDICS
Reshma N.
Master degree 4th course student Bukovinian State Medical University Dudko O.
Ph.D., Associate Professor Traumatology and Orthopaedics Department, Bukovinian State Medical University
Abstract
As technology advances, the number of inventions and discoveries also increases in order to make life and work easier. In the field of orthopaedics, for internal fixation of bone fractures, the presence of implants is required for the time needed for fracture healing and then they should be removed. Thus to solve this problem the mechanical non-degradable implants may be replaced by biodegradable ones. Biodegradable materials are used now in the form of screws, rods, sutures, staples etc. According to the site of a fracture and patient's condition, the usage of bioabsorbable implants differs, but nowadays biodegradable screws, pins and plates are used for fixation of various fracture patterns in different parts of the body. Biodegradable implants have specific demands and limitations. They should be biocompatible, have good mechanical properties and have non toxic degradation products. Polyglycolic acid (PGA), polylactic acid (PLA) are the most researched biocompatible and biodegradable polymers. Their linear copolymer poly lactic-co-glycolic acid (PLGA) that can be prepared at different ratios between its constituent monomers lactic and glycolic acid allows to correct the physical and chemical properties of the implants. In this paper, various applications and features of different biodegradable or bioabsorbable materials used in orthopedics for fracture fixation were analysed.
Keywords: biodegradable materials, bioabsorbable materials, bone fractures, surgical treatment.
Actuality. Bioabsorbable materials have various applications for surgical procedures in different fields. Tissue engineering is one important progressively developing field which uses bioabsorbable materials as scaffolds for regenerative processes to produce the patient's own tissues (skin, muscles, bone tissues, including organs) [1]. Another common application for bio-absorbable materials is their usage as suture materials. Stents used in cardiology are made of iron based biocompatible materials or magnesium based biodegradable materials [2]. Bioabsorbable mesh is used in the repairing of ventral hernias [3]. Bioabsorbable pins and screws of special design have been used in the fixation of bone fragments, for maxillofacial surgeries and for repairing of anterior cruciate and posterior cruciate ligaments [4]. Biodegradable implants such as screws, staples, sutures, tacks, suture anchors, and devices for me-niscal repair are extensively used in sports medicine [5]. Orthopaedics and traumatology surgeons use a wide variety of implants for the fixation of bone fractures. Some of these implants have to be removed after the fixation process. However, usage of biodegradable implants and sutures provide a comfortable treatment for the patients and they exclude the need of secondary surgeries.
Aim of research: to analyse the variety of biodegradable or bioabsorbable materials used in orthopedics for fracture fixation.
Main body. Many years before, different surgeons like Sushruta of India and Galen of Greece had already used some sutures of natural origin. Later in 1868, Lister used animal tissues like sheep and goat intestine as suture material with bioabsorbable properties. In 1891 Gluck implanted ivory in places of bone gaps to improve regeneration of bone fractures [6]. Those
were biodegradable implants of natural origin, but there is no scientific data proving their safe and wide application, as well as no comparison was done with non-degradable materials. It is known that some of them have osteogenic properties, but other benefits and weak points remained unclear [7].
The first known successful research about absorb-able suture material was held in 1965 and a US patent was received [8]. In 1971, surgeons had begun to use polyglycolic acid polymer, as a new absorbable synthetic suture [9]. There are different types of bioabsorb-able materials. They can be of natural or synthetic origin. The most common natural absorbable material made of plants is alginate and those taken from animals are collagen, acellular tissue matrices including bladder mucosa and small intestinal submucosa. The most widely used in operative treatment synthetic absorbable polymers are polyglycolic acid (PGA), polylactic acid (PLA), poly lactic-co-glycolic acid (PLGA) [10].
Even though there are so many advantages of biodegradable implants, there are some limitations too. After inserting a biodegradable polymeric implant, there is no possibility of radiographic visualisation of implants because they are radiolucent. To solve this problem, it was suggested to cover bioabsorbable implants like PLGA with a layer of radiopaque barium sulphate [11].
PLGA is the most commonly used synthetic polymer. PLGA is a copolymer of lactic acid and glycolic acid. In PLGA, the most abundant monomer is lactic acid, which has slower degradation rate and drug release. Lactic acid is more hydrophobic than glycolic acid. In contrast, the 50:50 ratio of lactic acid and glycolic acid in PLGA causes a faster degradation and short half life [12].
The polymers like PLGA, collagen, chitosan, gelatin, polycaprolactone (PCL) and bioceramics are used separately or in combination for the production of scaffolds in tissue engineering [12]. Electrospinning is a process of production of fine fibers from polymeric melts.
The biodegradable materials used are poly alpha esters, polyglycolide, poly(L)lactide, poly(DL)lactide, poly(L)lactide hydroxyapatite, poly(L)lactide tri-calcium phosphate, poly(lactide-co-glycolide), polydioxa-none, poly(glycolide-co trimethylene carbonate). These materials are used as implants such as pins, screws, plates, rods, tacks, suture anchors, spine cages and scaffolds.
The biodegradable mechanisms occurring in the body are acid degradation (lactic/glycolic acid), hydrophobic implant fragmentation and particle generation. Possible adverse effects include fluid accumulation, sinus formation, swelling, osteolysis, synovitis, cyst formation [13]. Magnesium based, zinc based and iron based biodegradable metals also are available. Magnesium based biodegradable implants are most widely used clinically because magnesium alloys exhibit higher strength, ductility and formability. Elastic modulus of magnesium alloys are approximately equal to that of natural bone. Zinc has a low corrosion rate compared to magnesium. Zinc based biomaterials are used as wound closure devices, orthopedic fixation devices, cardiovascular stents and bone implants. Zinc based orthopedic implants include screws and plates [14].
Open reduction internal fracture fixation surgeries usually require a metal bone fracture fixation device. These fixators are removed by a second surgery. Their other disadvantages are discomfort for patients, rigidity of these fixators cause a shielding effect on bone tissues, post operative surgeries are destructive and they increase the expenses too [15].
According to prospective cohort study conducted for analysing the long term effects of biodegradable implants like PGA, PGA-PLA for mostly intra-periarticu-lar fracture of limbs, following results were observed.
In this study, we consider the fact that mechanical properties of polymeric material are lower than that of metal. So, biodegradable implants are not applied to load bearing joints. Initially surgeons made an incision for the realignment of bones, then biodegradable implants like screws were inserted for fixation. Later, plaster of Paris bandages were used to immobilise limbs after surgery. Follow up examinations were done with various durations. Some patients were hesitant to come for the examination after a long duration, because they didn't have any problem due these implants and can lead a normal life. The analysis result of the rest of the patients who had come for post operative examination revealed that these implants could safely maintain bone fragments in cases of avulsion fractures, joint fractures and spiral shaft fractures.
As previously reported, there were 41 patients operated with biodegradable fixation devices. Patients that were chosen for surgeries were all complicated cases. The results of biodegradable fixation devices application were well illustrated by the example of a clinical case with closed three-malleolus fracture with foot
subluxation for which open reduction and internal fixation with PGA pin was performed. As open reduction gives the possibility of better positioning of fracture fragments the X-rays showed successful results with a good position of bones. The canals into which PGA fracture fixation devices were inserted disappeared in 6-12 months. In most of the cases abundant care was given and the injured limb was immobilised with plaster bandage until the fracture healed, though there were no cases of fracture non-healing. The rehabilitation time was similar to those when metal fixation devices were used. Further X-ray examination was done periodically up to 24 years after surgery. CT scan performed in 17-24 years after surgeries revealed that the bone density was the same for both operated and opposite limbs. There were no abnormal changes and the patients were able to do all range of everyday physical and sport activities without any limitations during the whole follow-up period [15].
Biodegradable screws and pins are used for the fracture fixation of intra-articular and small bone fractures. But there are limitations in use for long bones [16]. Biodegradable implants are used for the fixation of ankle fractures too. On analysis of these patients, it was found that there was no reduction in their ability to participate in physical activities and life activity is normal [17]. These implants are also used for the fixation of pediatric cranial and facial bones using ultrasound devices [18]. Implants made of L-lactide acid and poly (L-lactide-co-DL-lactide) are used for the spinal fracture fixation. The success rate differs in the usage of different materials [19].
Bioabsorbable plates and screws are used in the linear mandibular fractures that heal well after their application [20].
Conclusion. A broad spectrum of biodegradable implants are used in the field of orthopedics. Research is going on for the discovery of ample biodegradable implants with maximum benefits. They should be biocompatible, have suitable degradation speed, non toxic degradation products and good mechanical properties for osteosynthesis of bone fractures.
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