•Metallic osteosynthetic devices have been extensively used worldwide.
•However there are inherent problems with the use of these metallic devices like stress shielding phenomenon, pain, local irritation etc.
•Retained metallic implants are always at the risk of endogenous infection.
•Release of metallic ions from these implants has been documented, though the long term effects of these are not yet known.
•Because of these reasons there is always need for a second surgery for implant removal after the bone has healed.
•The reasons mentioned above led to the evolution of biodegradable implants aiming toward true biologic solutions to reconstructive problems.
•Biodegradable implants are derived by transforming compounds that are present in nature to structural plastics.
•Organic molecules are polymerized to form strong fibers and solid compounds.
•When these polymers are implanted in patients, they degrade and are eliminated from the body in a period of time.
STRUCTURE, STRENGTH AND PROPERTIES
• Widely used biodegradable materials include polyglycolic acid (PGA), poly-L- lactic acid (PLLA), poly-DL-lactic acid (PDLLA), PGA/trimethylene carbonate compolymers (PGA/TMC), and poly-beta-
hydroxybutyric acid (PBHBA).
• Polyglycolic acid (PGA) is a hard, tough, crystalline polymer with an average molecular weight of 20,000 to 145,000 and a melting point of 224-230°C.
• Polylactic acid on the other hand is a polymer with initial molecular weights of 180,000 to 530,000 and a melting point of about 174°C .
• In orthopaedic implants poly-L-lactic acid (PLLA) has been used more extensively because it retains its initial strength longer than poly-D-lactic-acid (PDLA).
•PGA belongs to the category of fast degrading polymers, and intraosseously implanted PGA screws have been shown to completely disappear within 6 months.
• PLLA on the other hand has a very long degradation time and has been shown to persist in tissues for as long as 5 years post implantation.
• For Orthopaedic usage, the main hindrance to development of bioabsorbable implants has been the question of obtaining sufficient initial strength and retaining this strength in the bone.
• With the use of self reinforcing (SR) technique the material was sintered together at high temperature and pressure, resulting in initial strengths 5 to 10 times higher than those implants manufactured with melt moulding technique.
•Though initial strengths of SR-PLLA screws are lower than SR-PGA, strength retention in the former is longer than the latter.
•Now a days, bioabsorbable implants show no difference in the stiffness, linear load & failure mode when compared with metallic devices.
• PGA is sterilized with ethylene oxide and PLA with gamma irradiation.
ADVANTAGES
•The biggest advantage is that since these implants have the potential for being completely absorbed, the need for a second operation for removal is overcome and long-term interference with tendons, nerves and the growing skeleton is avoided.
•Additionally, the risk of implant-associated stress shielding, peri-implant osteoporosis and infections is reduced.
•An important aspect is that these implants do not interfere with clinical imaging.
• This allows the use of modalities like MRI in knee and shoulder injuries at any stage after surgical implantation.
The other advantages include biodegradability of implants placed across mobile articular surfaces, as plus acceptable biocompatibility and resorption properties that reduces concern about complications.
• Bioabsorbable implants, due to the fact that they may resorb inside tissues, offer specific
advantages in specific fracture fixations; in the foot and ankle, where removal of the hardware is often mandatory prior to mobilization, they maybe beneficial in syndesmotic disruptions and Lisfranc's dislocations.
CURRENT USES
•Biodegradable implants are available for stabilization of fractures, osteotomies, bone grafts and fusions particularly in cancellous bones, as well as for reattachment of ligaments, tendons, meniscal tears and other soft tissue structures.
•The mechanical properties of the materials permit them to be used with metaphyseal and peri-
articular fractures where the loading is relatively low.
• Therefore, they have mainly been used for treating small-bone fractures such as ankle fractures
•Another suitable anatomic area for application is the elbow joint.
•They may be used for fixing fractures of the radial head, olecranon, capitellum and distal humerus.
•Nonetheless, comminuted fractures in these locations are not good candidates.
•Other conditions for which these implants can be used are fractures of the distal radial styloid, patella, glenoid fossa and acetabulum; osteochondral fractures in the knee, tibial plateaux, phalanx, calcaneus and talus; and also hallux valgus surgery.
•Biodegradable screws or rods may also be used for treating epiphyseal fractures.
•These have been used extensively for ACL reconstruction in the form of interference screws and transfixation screws.
•Meniscal tacks and biodegradable suture anchors have opened new avenues for soft tissue reconstruction in complex knee injuries.
•Biodegradable implants provide viable options for the repair and reconstruction of many intra-articular and extra-articular abnormalities in the shoulder, including rotator cuff tears, shoulder instability, and biceps lesions that require labrum repair or biceps tendon tenodesis.
•In spine surgeries, Bioresorbable implants can be used as interbody spacers in lumbar interbody fusion
•Bioabsorbable anterior cervical plates have been used and studied as alternatives to metal plates when a graft containment device is required.
DEGRADATION
•Crystalline polymers have a regular internal structure and because of the orderly arrangement are
slow to degrade.
•Amorphous polymers have a random structure and are completely and more easily degraded
•Semi-crystalline polymers have crystalline and amorphous (random structure) regions.
•Hydrolysis begins at the amorphous area leaving the more slowly degrading crystalline debris
•Polyglycolide (PGA), is hydrophilic and degrades very quickly, losing virtually all strength within one
month and all mass within 6-12 months.
•Adverse reactions can occur if the rate of degradation exceeds the limit of tissue tolerance.
•So PGA in isolation is rarely used these days in the manufacture of bioabsorbable implants.
•Poly L Lactic Acid (PLLA), has a much slower rate of absorption.
• This is highly crystalline & has been documented to take more than five years to absorb.
• The ideal material is perhaps one that has a "medium" degradation time of around 2 years, as by that time the purpose for which the implant was put has been served.
DISADVANTAGES
•There are quite a few problems that need to be addressed with the use of these devices.
• Primarily the inadequate stiffness of the device and weakness compared to metal implant can pose implantation difficulties like screw breakage during insertion and also make early mobilization precarious.
•The other potential disadvantages are an inflammatory response described with bioabsorbable implants, rapid loss of initial implant strength and higher refracture rates.
• Problem areas of concern regarding faster resorbed implants are due to the fact that the body mechanisms are not able to clear away the products of degradation, when they are produced at a faster rate.
• This may lead to a foreign body reaction.
FUTURE
• Covalent linking of compounds such as HRP, IL-2, and BMP-2 to plates represents a novel method for delivering concentrated levels of growth factors to a specific site and potentially extending their half-life.
•BMP-2 covalently linked to resorbable plates has been used to facilitate bone healing.
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