|Year : 2006 | Volume
| Issue : 4 | Page : 205-209
|Bioabsorbable implants in orthopaedics
MS Dhillon, AV Lokesh
Department of Orthopaedic Surgery, Post Graduate Institute of Medical Education and Research, Chandigarh, India
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|How to cite this article:|
Dhillon M S, Lokesh A V. Bioabsorbable implants in orthopaedics. Indian J Orthop 2006;40:205-9
| Introduction|| |
Orthopaedic surgery has evolved in the last 2 centuries from surgeons using amputation saws on mangled limbs, to modified forms of external splintages and tractions, to sophisticated internal fixation implants and devices that have allowed early mobilization and pain free locomotion. Most of the focus in modern orthopaedic implant development is on developing devices that are stronger, more acceptable to the body, cheaper and durable. In the past few decades a lot of research has been done and significant improvement has been seen in the development of bioabsorbable osteosynthetic devices. Biodegradable implants have allowed a paradigm shift away from bionic (mechanical replacement) engineering and toward true biologic solutions to reconstructive problems
With an array of bioabsorbable implants available in the market one needs to know the properties, uses and limitations of these devices. We have attempted to review the available world literature on bioabsorbable implants and present it with an orthopaedician's perspective.
Shortcomings of metallic implants
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, 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 need for a second surgery for implant removal after the bone has healed.
Low molecular weight polyglycolic acid was synthesized by Bischoff and Walden in 1893. The first synthetic absorbable suture was developed from polyglycolic acid (PGA) by American Cyanamid Co. in 1962. The 90:10 copolymer of glycolide and lactide -polygalactin 910 - has been applied as the competitive suture 'Vicryl' since 19756. Since then sutures of polyglycolide and polylactide have been used for many years and no carcinogenic, teratogenic, toxic, or allergic side effects have been observed. The only adverse reaction reported has been a mild non specific inflammation,.
Use of PGA as reinforcing pins, screws, and plates for bone surgery was first suggested by Schmitt and Polistina in 1969. Since then there has been a lot of development in manufacturing biodegradable implants with properties appropriate for osteosynthesis.
Several animal experimental studies have been done testing the use of bioabsorbable implants. Tunc et al reported successful results of experimental osteotomies of the calcaneus fixed with melt moulded poly-L-lactide screws and plates in beagles. Raiha et al reported successful fixation of trochanteric osteotomies in laboratory beagles with self reinforced polylactide screws. Bostman et al tested self reinforced PGA screws for fixation of distal osteotomies in femora of rabbits and found the fixation appropriate and well tolerated by the tissues.
Structure, strength and properties
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.
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.
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.
Knee: Arthroscopic surgery is the most recent orthopaedic discipline to embrace biodegradable implant technology. It is used extensively for ACL reconstruction in the form of interference screws and transfixation screws. The authors have personal experience of more than 4 years of biodegradable screw fixation, with no added complications visualized. Osteochondral fractures can be well fixed by using arthroscopic techniques and biodegradable pins. Meniscal tacks and biodegradable suture anchors have opened new avenues for soft tissue reconstruction in complex knee injuries; these can be used via open or arthroscopic surgical techniques.
We have found the use of bioabsorbable interference screws to be a valuable alternative to metallic implants; as MRI is the only technique which allows good visualization of the transplant and evaluation of the healing process, absence of artefacts allows use of this modality for postoperative follow-up. Lajtai et al has shown that there was minimal surgical-site edema, minimal reaction to the material, and complete replacement by new bone formation of the previous site of these bioabsorbable interference screws. At 5 years, this bioabsorbable interference screw appeared clinically safe and effective for fixation of bone blocks during ACL reconstruction and MRI showed complete absorption and replacement with new bone. Additional advantages may be in the form of decreased distal tibial and some femoral tunnel diameters at both 3- and 12-month follow-up.
Shoulder: 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 a study of arthroscopic Bankart reconstruction using either PGA polymer or PLA polymer implants the overall clinical results were comparable at two year follow up. Furthermore, the visibility of the drill holes on the 2-year radiographs was greater after using PLLA implants than after using PGACP implant.
Spine: Bioresorbable implants have significant potential for use in spine surgery. Coe and Vaccaro published the first clinical series using bioresorbable implants as interbody spacers in lumbar interbody fusion; at follow-up beyond 2 years, they were pleasantly surprised by the fact that the implant materials significantly exceeded the biologic "life expectancy" of 12-18 months. The clinical and radiographic results of their study allowed them to recommend the use of bioresorbable devices structural interbody support in the TLIF procedure.
In a study by Kandziora et al, tricortical bone grafts and the bioabsorbable polymer-calcium phosphate composite cages were implanted in sheep cervical spines; the latter showed significantly better distractive properties, a significantly higher biomechanical stiffness, and an advanced interbody fusion. However, six of eight polymer-calcium phosphate composite cages cracked. Although the fate of the foreign body reactions and the cracks is currently unclear for bioabsorbable cages, the early appearance of osteolysis associated with use of the poly(l-lactide-co-d,l-lactide) cage allows skepticism regarding the value of this bioabsorbable implant.
Bioabsorbable anterior cervical plates have been used and studied as alternatives to metal plates when a graft containment device is required-. Ames at al found that bioabsorbable plates provided better stability than resorbable mesh although the results do not necessarily indicate that a resorbable plate confers equivalent stability to a metal plate.
Paediatric Orthopaedics: Bioresorbable material use in paediatric situations was perhaps the earliest recorded use in orthopaedic literature. The applications have been widely varied, and the results very successful. Bostman et al showed that self reinforced absorbable rods were suitable for fixation of physeal fractures in children. In 1991, Hope et al had compared the self reinforced absorbable rods with metallic fixation of elbow fractures in children. Partio et al found SR-PLLA screws firm enough for fixation of subtalar extraarticular arthrodesis in children. Bioabsorbable fixation technique for pediatric olecranon fractures has been described, with the advantage of avoiding reoperation to remove hardware.
Foot and Ankle: The first series of fixation of ankle fractures with absorbable rods was reported by Rokkanen et al in 1985. Subsequently, successful results with self reinforced absorbable rods have been reported by Leixnering et alin medial malleolar fractures, Ruf et al in ankle fractures, and Kristensen et al in intra articular osteochondral fractures of talus. Outside the trauma situation, Brunetti et al used bioresorbale implants in the fixation of osteotomies for hallux valgus. Bioabsorbable implants offer specific advantages in the foot where removal of the hardware is mandatory in some fixations like syndesmotic disruptions and Lisfranc's dislocations. Partio et al reported 95% good results in 152 patients managed operatively at a mean follow up of 2 years.
Hand: The available literature at the present time is scarce about biodegradable implant usage in the hand. However mini-plating systems are available for fixation of fractures, osteotomies and arthrodesis in the wrist and hand 1 . Preliminary reports have found usage of self-reinforced polyl/dl-lactide 70/30 miniplate and 1.5-mm or 2.0-mm screws in fractures and osteotomies leading to bone union uneventful Miscellaneous: There are bioabsorbable implants now available for use in humeral condyle, distal radius and ulna, radial head and other metaphyseal areas. Bioabsorbable meshes are available for acetabular reconstructions. Bioabsorbable implants are also variously used in craniomaxillofacial surgery2 and dental surgery.
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 25 .
Some earlier biodegradable implants have had problems with degradation time and tissue reactions [Figure 1]. One commonly used material, 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 and incidence of adverse tissue reactions to implants made of PGA have been reported from 2.0 to 46.7%. 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 homopolymer of L lactide is highly crystalline due to the ordered pattern of the polymer chains and has been documented to take more than five years to absorb.
The newer generation of implants remain predominantly amorphous after manufacturing due to controlled production processes of copolymers. D Lactide when copolymerized with L Lactide increases the amorphous nature of these implants. This increases the bioabsorbability of these devices. 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.
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. Bostman et al in reported an 11% incidence of foreign body reaction to PGA screws in malleolar fractures. However the fracture fixation did not suffer in any case.
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 leads to a foreign body reaction, which however has only been recorded in the clinical situation. No experimental study has been able to document this, nor have the exact mechanisms and causes identified.
A recent animal experimental study by Bostman et al  has evaluated the kind of tissue formed at the site of the bioresorbable implants after resorption. They recorded lower levels of trabecular bone and haemopoietic elements at the site of the resorbed implants and the screw tracks, which maybe a potential area of concern. Invasion of tissue from the periphery to the center was a constatnt finding, but this was not normal tissue. Due to time limits, this study was only able to evaluate PGA screws, that degraded within the limits of the study period.
Many manufacturers are introducing coloured implants, as sometimes visualization inside the joint maybe a problem with non coloured devices. This is definitely easier to implant (personal experience), but the literature records significantly higher rates of inflammatory reactions with the use of coloured implants.
Bioabsorbable implant research is an evolving science. Resorbable plates can be covalently linked with compounds such as HRP, IL-2, and BMP-2 and represents a novel protein delivery technique. BMP-2 covalently linked to resorbable plates has been used to facilitate bone healing. Covalent linking of compounds to plates represents a novel method for delivering concentrated levels of growth factors to a specific site and potentially extending their half-life.
An area for future development would have to focus on developing implants that degrade at the "medium term". Since the screw that persists in its track for 5 years or more does not offer the advantage of bioresorbability, newer molecules may have to be studied.
In vitro studies have shown promising results of antibiotic elution from bioabsorbable microspheres and beads.
Animal in vivo tests have shown that antibiotic impregnated polymers can successfully treat induced osteomyelitis in rabbits and dogs
All in all, this is a concept that has perhaps come to stay. What the future holds in this sphere, is something we will have to wait and see.
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M S Dhillon
1090/2 Sector 39-B, Chandigarh, 160036
Source of Support: None, Conflict of Interest: None
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