Strength and strength retention in vitro, of absorbable, self-reinforced polyglycolide (PGA) rods for fracture fixation

Degradation behaviour of porous poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) scaffolds in cell culture

Exploring the degradation behaviour of biomaterials in a complex in vitro physiological environment can assist in predicting their performance in vivo, yet this aspect remains largely unexplored. In this study, the in vitro degradation over 12 weeks of porous poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) bone scaffolds in human osteoblast (hOB) culture was investigated. The objective was to evaluate how the presence of cells influenced both the degradation behaviour and mechanical stability of these scaffolds. The molecular weight (Mw) of the scaffolds decreased with increasing incubation time and the Mw reduction rate (6.2 ± 0.4 kg mol-1 week-1) was similar to that observed when incubated in phosphate buffered saline (PBS) solution, implying that the scaffolds underwent hydrolytic degradation in hOB culture. The mass of the scaffolds increased by 0.8 ± 0.2 % in the first 4 weeks, attributed to cells attachment and extracellular matrix (ECM) deposition including biomineralisation. During the first 8 weeks, the nominal compressive modulus, E⁎, of the scaffolds remained constant. However, it increased significantly from Week 8 to 12, with increments of 55 % and 42 % in normal and lateral directions, respectively, attributed to the reinforcement effect of cells, ECM and minerals attached on the surface of the scaffold. This study has highlighted, that while the use of PBS in degradation studies is suitable for evaluating Mw changes it cannot predict changes in mechanical properties to PHBV scaffolds in the presence of cells and culture media. Furthermore, the PHBV scaffolds had mechanical stability in cell culture for 12 weeks validating their suitability for tissue engineering applications.

Processing and characterization of phosphate glass fiber/polylactic acid commingled yarn composites for commercial production

This study investigated the production of phosphate glass fiber/polylactic acid (PGF/PLA) commingled yarns, textiles and composites for biomedical applications. The PGF volume contents of the composites investigated were 25% and 40%. Plain weave textiles with yarn counts of 10 warp/cm and 6 weft/cm were produced using a commercial weaving machine. An orthogonal array design (OAD) was employed as a statistical method to investigate the effects of compression molding parameters (processing temperature, preheating time, compression time, and pressure) on flexural strength and porosity of PGF/PLA textile composites. Processing temperature showed the most significant effect in achieving maximum laminate flexural strength and molecular weight of PLA. Processing models were developed using regression techniques to predict the laminate flexural strength and the molecular weight of PLA. Composites with fiber contents of 25 and 40 vol% produced using optimized processing conditions identified by the processing models, provided flexural strengths of 236 MPa and 293 MPa, respectively.

Biodegradable Materials for Bone Repair and Tissue Engineering Applications

This review discusses and summarizes the recent developments and advances in the use of biodegradable materials for bone repair purposes. The choice between using degradable and non-degradable devices for orthopedic and maxillofacial applications must be carefully weighed. Traditional biodegradable devices for osteosynthesis have been successful in low or mild load bearing applications. However, continuing research and recent developments in the field of material science has resulted in development of biomaterials with improved strength and mechanical properties. For this purpose, biodegradable materials, including polymers, ceramics and magnesium alloys have attracted much attention for osteologic repair and applications. The next generation of biodegradable materials would benefit from recent knowledge gained regarding cell material interactions, with better control of interfacing between the material and the surrounding bone tissue. The next generations of biodegradable materials for bone repair and regeneration applications require better control of interfacing between the material and the surrounding bone tissue. Also, the mechanical properties and degradation/resorption profiles of these materials require further improvement to broaden their use and achieve better clinical results.

Absorbable biologically based internal fixation

Absorbable devices for use in internal fixation have advanced over the years to become reliable and cost-effective alternatives to metallic hardware. In the past, biodegradable fixation involved a laborious implantation process, and induced osteolysis and inflammatory reactions. Modern iterations exhibit increased strength, smoother resorption, and lower rates of reactivity. A newer generation manufactured from silk has emerged that may address existing limitations and provide a greater range of fixation applications.

Rapid manufacturing techniques for the tissue engineering of human heart valves

Three-dimensional (3D) printing technologies have reached a level of quality that justifies considering rapid manufacturing for medical applications. Herein, we introduce a new approach using 3D printing to simplify and improve the fabrication of human heart valve scaffolds by tissue engineering (TE). Custom-made human heart valve scaffolds are to be fabricated on a selective laser-sintering 3D printer for subsequent seeding with vascular cells from human umbilical cords. The scaffolds will be produced from resorbable polymers that must feature a number of specific properties: the structure, i.e. particle granularity and shape, and thermic properties must be feasible for the printing process. They must be suitable for the cell-seeding process and at the same time should be resorbable. They must be applicable for implementation in the human body and flexible enough to support the full functionality of the valve. The research focuses mainly on the search for a suitable scaffold material that allows the implementation of both the printing process to produce the scaffolds and the cell-seeding process, while meeting all of the above requirements. Computer tomographic data from patients were transformed into a 3D data model suitable for the 3D printer. Our current activities involve various aspects of the printing process, material research and the implementation of the cell-seeding process. Different resorbable polymeric materials have been examined and used to fabricate heart valve scaffolds by rapid manufacturing. Human vascular cells attached to the scaffold surface should migrate additionally into the inner structure of the polymeric samples. The ultimate intention of our approach is to establish a heart valve fabrication process based on 3D rapid manufacturing and TE. Based on the computer tomographic data of a patient, a custom-made scaffold for a valve will be produced on a 3D printer and populated preferably by autologous cells. The long-term goal is to support the growth of a new valve by a 3D structure resorbed by the human body in the course of the growth process. Our current activities can be characterized as basic research in which the fundamental steps of the technical process and its feasibility are investigated.

In vitro degradation, flexural, compressive and shear properties of fully bioresorbable composite rods

Several studies have investigated self-reinforced polylactic acid (SR-PLA) and polyglycolic acid (SR-PGA) rods which could be used as intramedullary (IM) fixation devices to align and stabilise bone fractures. This study investigated totally bioresorbable composite rods manufactured via compression moulding at ~100 °C using phosphate glass fibres (of composition 50P(2)O(5)-40CaO-5Na(2)O-5Fe(2)O(3) in mol%) to reinforce PLA with an approximate fibre volume fraction (v(f)) of 30%. Different fibre architectures (random and unidirectional) were investigated and pure PLA rods were used as control samples. The degradation profiles and retention of mechanical properties were investigated and PBS was selected as the degradation medium. Unidirectional (P50 UD) composite rods had 50% higher initial flexural strength as compared to PLA and 60% higher in comparison to the random mat (P50 RM) composite rods. Similar initial profiles for flexural modulus were also seen comparing the P50 UD and P50 RM rods. Higher shear strength properties were seen for P50 UD in comparison to P50 RM and PLA rods. However, shear stiffness values decreased rapidly (after a week) whereas the PLA remained approximately constant. For the compressive strength studies, P50 RM and PLA rods remained approximately constant, whilst for the P50 UD rods a significantly higher initial value was obtained, which decreased rapidly after 3 days immersion in PBS. However, the mechanical properties decreased after immersion in PBS as a result of the plasticisation effect of water within the composite and degradation of the fibres. The fibres within the random and unidirectional composite rods (P50 RM and P50 UD) degraded leaving behind microtubes as seen from the SEM micrographs (after 28 days degradation) which in turn created a porous structure within the rods. This was the main reason attributed for the increase seen in mass loss and water uptake for the composite rods (~17% and ~16%, respectively).

Biomaterials for craniofacial reconstruction

Biomaterials for reconstruction of bony defects of the skull comprise of osteosynthetic materials applied after osteotomies or traumatic fractures and materials to fill bony defects which result from malformation, trauma or tumor resections. Other applications concern functional augmentations for dental implants or aesthetic augmentations in the facial region. For ostheosynthesis, mini- and microplates made from titanium alloys provide major advantages concerning biocompatibility, stability and individual fitting to the implant bed. The necessity of removing asymptomatic plates and screws after fracture healing is still a controversial issue. Risks and costs of secondary surgery for removal face a low rate of complications (due to corrosion products) when the material remains in situ. Resorbable osteosynthesis systems have similar mechanical stability and are especially useful in the growing skull. The huge variety of biomaterials for the reconstruction of bony defects makes it difficult to decide which material is adequate for which indication and for which site. The optimal biomaterial that meets every requirement (e.g. biocompatibility, stability, intraoperative fitting, product safety, low costs etc.) does not exist. The different material types are (autogenic) bone and many alloplastics such as metals (mainly titanium), ceramics, plastics and composites. Future developments aim to improve physical and biological properties, especially regarding surface interactions. To date, tissue engineered bone is far from routine clinical application.

Degradation behaviors of electrospun resorbable polyester nanofibers

Biodegradable materials are widely used in the biomedical field because there is no postoperative surgery after implantation. Widely used synthetic biodegradable materials are polyesters, especially those used in tissue engineering. Advances in the tissue engineering field have brought much attention in terms of scaffold fabrication, such as with biodegradable polyester nanofibers. The rationale for using nanofibers for tissue engineering is that the nonwoven polymeric meshwork is a close representation of the nanoscale protein fiber meshwork in native extracellular matrix (ECM). Electrospinning technique is a promising way to fabricate controllable continuous nanofiber scaffold mimicking the ECM structure. Electrospun nanofibers provide high surface-to-volume ratio and high porosity as a promising scaffold for tissue engineering. Because the degradation behaviors of scaffolds significantly affect new tissue regeneration, the degradation of the material becomes one of the crucial factors when considering using polyester nanofibers as scaffolds in tissue engineering. In this review paper, we focus on the degradation studies of several bioresorbable polyester nanofibrous scaffolds used in tissue engineering. The degradable properties of nanofibers were compared with the corresponding degradable materials in macroscale. The factors that might affect the degradation behaviors were analyzed.

Biomechanical in vitro evaluation of the effect of cyclic loading on the postoperative fixation stability and degradation of a biodegradable ankle plate

The effect of cyclic loading on the postoperative fixation stability of a biodegradable ankle plate was tested biomechanically during 12 weeks of hydrolytic degradation. Fracture of the lateral malleolus was simulated, and the parameters of cyclic loading were chosen to represent the physiological conditions during the healing period. Additionally, the effect of cyclic loading on degradation was investigated by measuring the inherent viscosities. In Group I, the cyclic loading was conducted in four phases with gradual increases in estimated walking distance and speed during the healing period. In Group II, cyclic loading was conducted after 12 weeks. Group III was used as a control for inherent viscosity measurements. None of the specimens failed under cyclic loading. No significant differences were found between the loaded groups in any of the parameters measured. Additionally, no significant difference was found in inherent viscosities at 12 weeks. The initial fixation stability provided by the biodegradable ankle plate remains biomechanically unchanged over 12 weeks. Cyclic loading, applied either during or after 12 weeks of hydrolytic degradation, does not seem to have any clinically relevant effect on the fixation stability or the degradation properties.

Biodegradable Materials in Arthroscopy

The use of biodegradable materials as implants has revolutionized the way medicine is practiced today. This review provides a general description of salient biodegradable polymeric materials currently used in arthroscopy. These materials include polyglycolic acid, self-reinforced polyglycolic acid, poly-L-lactic acid, self-reinforced polylactic-L-acid, poly-D-L-lactic acid, copolymer of poly-D-L-lactic acid polyglycolic acid, and polyglyconate. The mechanical strength, degradation properties, and widespread use of these materials, especially in the knee and shoulder, are discussed individually. Also discussed are the relatively few complications that are related to these materials' arthroscopic use. Future directions in biodegradable materials, including smart polymers, are also considered. In the future, novel techniques to identify the ideal polymer for a particular application will need to be developed to minimize the risk for implant complications.

Finalizing the properties of porous scaffolds of aliphatic polyesters through radiation sterilization

Porous scaffolds made of various L,L-lactide (LLA), 1,5-dioxepane-2-one (DXO) and epsilon-caprolactone (CL) copolymers were sterilized by EB- and gamma-irradiation. Differences in the comonomers, composition and the microstructure of the starting materials were used to influence the degradation mechanism and susceptibility towards irradiation and by this means to achieve sterilized scaffolds with predicted end-properties. The chemical changes and the formation of low-molecular-weight products were determined by SEC, 1H nuclear magnetic resonance (NMR), 13C NMR and gas chromatography-mass spectrometry (GC-MS). The degradation mechanism changed from random chain scission to cross-linking depending on the choice of monomers, the copolymer composition and the monomer sequences. Copolymerization of LLA with small amounts of CL or DXO increased the stability compared to that of the LLA homopolymer. Changing DXO to CL in a LLA copolymer also increased the stability. The type of radiation and the microstructure of the copolymer chains determined which of the monomer sequences were more prone to degrade. The most abundant low-molecular-weight product identified after sterilization was DXO monomer. Traces of LLA and CL monomers were also identified. Modification of the copolyester microstructure changed the degradation mechanism and the susceptibility towards irradiation. This allows the use of radiation sterilization to finalize the scaffold properties.

Recent Developments in Ring Opening Polymerization of Lactones for Biomedical Applications

Aliphatic polyesters prepared by ring-opening polymerization of lactones are now used worldwide as bioresorbable devices in surgery (orthopaedic devices, sutures, stents, tissue engineering, and adhesion barriers) and in pharmacology (control drug delivery). This review presents the various methods of the synthesis of polyesters and tailoring the properties by proper control of molecular weight, composition, and architecture so as to meet the stringent requirements of devices in the medical field. The effect of structure on properties and degradation has been discussed. The applications of these polymers in the biomedical field are described in detail.

Polyhydroxyalkanonate derivatives in current clinical applications and trials

Polyhydroxyalkanonate is a typical biodegradable material, which is permitted for use in the medical and pharmaceutical fields. For its biodegradability, biocompatibility, and toxicological safety, the majority of products practically used are composed of homo-polymers of poly(lactic acid), poly(glycolic acid), and poly(epsilon-caprolactone) and their co-polymers. On the market, suture strings are still the main usage. The needs of biodegradable materials have been being gradually increased by the development of drug delivery systems, tissue engineering, and regenerative medicine. Some types of formulation, that is, mono-fibers, twisted fibers, films, fabrics, sponges, and injectable particles are developed to match each purpose. This article reviews the current clinical applications and trials of polyhydroxyalcanonate products.

Biological reactions to a poly(L-lactide)-hydroxyapatite composite: a study in canine mandible

In this study, bone response, possible use and ultimate fate of a chemically-synthesized poly(L-lactide)-hydroxyapatite (PLLA-HA) composite was experimented in canine mandible. Bilateral mandibular second premolars were extracted in four dogs. The PLLA-HA composite was placed into left surgical sites, and right extraction sites were used as controls. After three months of healing, bone specimens were harvested from each animal and processed for histological evaluation. Bone uptake of methylene diphosphonate (99mTc-MDP) was calculated as indicators of osteoblastic activity in the surgical sites. Histological evaluation and the amount of 99mTc-MDP uptake showed that all surgical sites had similar levels of cellular activity and the material was biocompatible. The experimental PLLA-HA composite studied is safe to be used as a small-defect filler in applications such as repair of alveolar defects, ridge augmentations, and sinus lift procedures.

Dynamic mechanical characterization of biodegradable composites of hydroxyapatite and polylactides

The thermo-mechanical analysis of some novel, totally bioactive, three-component composite systems intended for the repair and augmentation of bone has been performed. The composites are composed of hydroxyapatite (HA), particulate reinforced biodegradable polylactic acid (PDLLA), with a second reinforcing phase of semi-crystalline, unidirectional fibers of polylactic acid (PLLA) homopolymer. Dynamic mechanical analysis (DMA) applied small-strain cyclic deformations to simulate physiological loading. The effect of the two fillers on the glass transition temperature of the matrix (Tg-PDLLA) and on the viscoelastic parameters, storage modulus (EI), and tan delta were analyzed. Thermal characterization with differential scanning calorimetry (DSC) was compared to the DMA data. It was established that the addition of HA and/or PLLA fibers increased EI, both below and above Tg-PDLLA. The combination of both reinforcing agents was the most effective. The damping was reduced with the addition of HA alone and in combination with PLLA fibers. Tg-PDLLA was increased by the addition of HA, PLLA fibers, and the combination of the two reinforcing agents. The magnitude of the increase in Tg-PDLLA was similar for both types of reinforcement.

Pre-clinical in vivo evaluation of orthopaedic bioabsorbable devices

The presence of bioabsorbable materials in orthopaedics has grown significantly over the past two decades with applications in fracture fixation, bone replacement, cartilage repair, meniscal repair, fixation of ligaments, and drug delivery. Numerous biocompatible, biodegradable polymers are now available for both experimental and clinical use. Not surprisingly, there have been a wealth of studies investigating the biomechanical properties, biocompatibility, degradation characteristics, osteoconductivity, potential toxicity, and histologic effects of various materials. Promising results have been reported in the areas of fracture fixation, ligament repair, and drug delivery. In this article we review the pre-clinical in vivo testing of bioabsorbable devices with particular emphasis on implants used for these applications.

Bioabsorbable scaffolds for guided bone regeneration and generation

Several different bioabsorbable scaffolds designed and manufactured for guided bone regeneration and generation have been developed. In order to enhance the bioactivity and potential osteoconductivity of the scaffolds, different bioabsorbable polymers, composites of polymer and bioactive glass, and textured surface structures of the manufactured devices and composites were investigated in in vitro studies and experimental animal models. Solid, self-reinforced polyglycolide (SR-PGA) rods and self-reinforced poly L-lactide (SR-PLLA) rods were successfully used as scaffolds for bone formation in muscle by free tibial periosteal grafts in animal experiments. In an experimental maxillary cleft model, a bioabsorbable composite membrane of epsilon-caprolactone and L-lactic acid 50/50 copolymer (PCL/LLA) film and mesh and poly 96L,4D-lactide (PLA96) mesh were found to be suitable materials for guiding bone regeneration in the cleft defect area. The idea of solid layer and porous layer combined together was also transferred to stiff composite of poly 70L,30DL-lactide (PLA70) plate and PLA96 mesh which structure is introduced. The osteoconductivity of several different biodegradable composites of polymers and bioactive glass (BG) was shown by apatite formation in vitro. Three composites studied were self-reinforced composite of PLA70 and bioactive glass (SR-(PLA70 + BG)), SR-PLA70 plate coated with BG spheres, and Polyactive with BG.

Preliminary in vivo studies on the osteogenic potential of bone morphogenetic proteins delivered from an absorbable puttylike polymer matrix

This article describes preliminary in vivo studies evaluating the osteogeneic potential of bone morphogenetic proteins (BMPs) delivered from an absorbable puttylike polymer matrix. In the first study, bovine-derived bone morphogenetic proteins were incorporated in an polymer matrix consisting of 50:50 poly(DL-lactide-co-glycolide) dissolved in N-methyl-2-pyrrolidone. The matrix was implanted in an 8 mm critical-size calvarial defect created in the skull of adult Sprague-Dawley rats (n = 5 per treatment group). After 28 days, the implant sites were removed and examined for new bone formation, polymer degradation, and tissue reaction. Gamma-irradiated polymer matrices appeared to give more bone formation than nonirradiated samples (histological analysis; 2. 76 + 1.34 mm(2) of bone versus 1.30 + 0.90 mm(2) of bone, respectively and x-ray analysis; 27.2 + 15.9 mm(2) of bone versus 20. 7 + 16.7 mm(2) of bone, respectively) and less residual polymer (0.0 + 0.0 versus 0.2 + 0.4, respectively). The polymer implants with bone morphogenetic protein also gave less inflammatory response than the polymer controls (gamma irradiated polymer/BMP = 1.8 + 0.4 and nonirradiated polymer/BMP = 1.2 + 0.4 versus polymer only = 3.0 + 1. 2, respectively). However, despite trends in both the x-ray and histological data there was no statistical difference in the amount of new bone formed among the four treatment groups (P > 0.05). This was most likely due to the large variance in the data scatter and the small number of animals per group. In the second animal study, bovine-derived BMPs and the polymeric carrier were gamma irradiated separately, at doses of 1.5 or 2.5 Mrad, and their ability to form bone in a rat skull onlay model was evaluated using Sprague-Dawley rats (n = 5 per treatment group). Histomorphometry of skull caps harvested 28 days after implantation showed no significant differences as compared to non-irradiated samples, in implant area, new bone area, and percent new bone (P > 0.05). These results suggest gamma irradiation may be useful in sterilization of the bovine-derived BMPs and the polymeric carrier for potential bone repair and/or regeneration applications.

The future of biodegradable osteosyntheses

In the last 3 decades, much progress has been made in the development of biodegradable osteosyntheses. Despite this progress, these materials are still only used in small numbers, and the scope of their application has been limited. The limitations of biodegradable osteosyntheses mainly are related to problems with their mechanical properties and, in particular, biocompatibility. These problems need to be solved so that biodegradable osteosyntheses can perform up to their full potential and thus, eventually, make their general clinical application routine. This paper presents a historical perspective on the development of biodegradable osteosyntheses, discusses the successful developmental achievements and the still-existing problems, and gives a perspective on their future development.

Real-time dissolution measurement of sized and unsized calcium phosphate glass fibers

The objective of this study was to develop an efficient "real time" measurement system able to directly measure, with microgram resolution, the dissolution rate of absorbable glass fibers, and utilize the system to evaluate the effectiveness of silane-based sizing as a means to delay the fiber dissolution process. The absorbable glass fiber used was calcium phosphate (CaP), with tetramethoxysilane selected as the sizing agent. E-glass fiber was used as a relatively nondegrading control. Both the unsized-CaP and sized-CaP degraded linearly at both the 37 degrees C and 60 degrees C test temperature levels used. No significant decrease in weight-loss rate was recorded when the CaP fiber tows were pretreated, using conventional application methods, with the tetramethoxysilane sizing for either temperature condition. The unsized-CaP and sized-CaP weight loss rates were each significantly higher at 60 than at 37 degrees C (both p < 0.02), as expected from dissolution kinetics. In terms of actual weight loss rate measured using our system for phosphate glass fiber, the unsized-CaP fiber we studied dissolved at a rate of 10.90 x 10(-09) and 41.20 x 10(-09) g/min-cm(2) at 37 degrees C and 60 degrees C, respectively. Considering performance validation of the developed system, the slope of the weight loss vs. time plot for the tested E-glass fiber was not significantly different compared to a slope equal to zero for both test temperatures.

Plasma surface modification of synthetic absorbable sutures

The aim of the study was to examine the feasibility of using plasma surface modification technology to alter the hydrolytic degradation rate of commercial synthetic absorbable sutures. Size 2-0 Dexon, Vicryl, PDSII, and Maxon sutures were tested. They were treated by two different surface modification techniques: parylene deposition and plasma gases (Methane, trimethylsilane, and tetrafluoroethene). The thickness of surface treatment ranged from 200 to 1000 A. The treated sutures were subject to in vitro hydrolytic degradation in phosphate buffer of pH = 7.4 at 37 degrees C for up to 120 days. The tensile breaking strength, weight loss, surface wettability, bending stiffness, and surface morphology were evaluated. The results indicated that the concept of plasma surface treatment for altering the hydrolytic degradation of synthetic absorbable sutures was feasible, and the level of improvement depended on the type of sutures, the treatment conditions, and the duration of hydrolysis. Vicryl and PDSII sutures showed overall the best improvement in tensile strength retention among the four commercial sutures. Dexon and Maxon sutures, however, exhibited only marginal improvement. The observed improvement in tensile strength retention appeared to be related to the increasing hydrophobicity of the sutures. The surface treatments did not adversely affect the bending stiffness of the sutures and no visible surface morphological changes were observed. Refinements and optimization of the surface treatment conditions are needed for achieving the maximum advantage of the proposed concept, particularly shielding the harmful effect of uv during plasma treatment.

Biocompatibility and mechanical properties of a totally absorbable composite material for orthopaedic fixation devices

Bioabsorbable polymer/inorganic phosphate fiber composites are prone to rapid degradation due to water sensitivity of the interface between the degradable polymer and the degradable fiber. This article describes successful fabrication and laboratory evaluation of a candidate bioabsorbable composite implant material with mechanical properties similar to bone. The composite studied was poly(ortho ester) reinforced with randomly-oriented, crystalline microfibers of calcium-sodium-metaphosphate. The component materials showed no acute cytotoxicity as determined by tissue culture agar overlay. Treating the microfibers with a diamine-silane coupling agent improved mechanical properties and slowed degradation in saline, but strength still decreased 50% in 1 week. When the composite material was then coated with a layer of matrix polymer alone it retained 70% of its strength and 70% of its stiffness after 4 weeks exposure to 7.4 pH Tris-buffered saline at body temperature. The marked improvement with the coating can be attributed to the hydrophobicity of poly(ortho esters).

Mechanical properties of biodegradable polymers and composites proposed for internal fixation of bone

The mechanical properties of biodegradable polymers and composites proposed for use in internal fixation (in place of stainless steel) are crucial to the performance of devices made from them for support of healing bone. To assess the reported range of properties and degradation rates, we searched and reviewed papers and abstracts published in English from 1980 through 1988. Mechanical property data were found for poly(lactic acid), poly(glycolic acid), poly(epsilon-caprolactone), polydioxanone, poly(ortho ester), poly(ethylene oxide), and/or their copolymers. Reports of composites based on several of these materials, reinforced with nondegradable and degradable fibers, were also found. The largest group of studies involved poly(lactic acid). Mechanical test methods varied widely, and studies of the degradation of mechanical properties were performed under a variety of conditions, mostly in vitro rather than in vivo. Compared to annealed stainless steel, unreinforced biodegradable polymers were initially up to 36% as strong in tension and 54% in bending, but only about 3% as stiff in either test mode. With fiber reinforcement, reported highest initial strengths exceeded that of stainless steel. Stiffness reached 62% of stainless steel with nondegradable carbon fibers, 15% with degradable inorganic fibers, but only 5% with degradable polymeric fibers. The slowest-degrading unreinforced biodegradable polymers were poly(L-lactic acid) and poly(ortho ester). Biodegradable composites with carbon or inorganic fibers generally lost strength rapidly, with a slower loss of stiffness, suggesting the difficulty of fiber-matrix coupling in these systems. The strength of composites reinforced with (lower modulus) degradable polymeric fibers decreased more slowly. Low implant stiffness might be expected to allow too much bone motion for satisfactory healing. However, unreinforced or degradable polymeric fiber reinforced materials have been used successfully clinically. The key has been careful selection of applications, plus use of designs and fixation methods distinctly different from those appropriate for stainless steel devices.

Preliminary biocompatibility screening of several biodegradable phosphate fiber reinforced polymers

This article describes preliminary biocompatibility screening of three degradable phosphate fibers containing K +, Ca +2/Na + and Na +/Ca +2/Al +3 ions in the polymer chain, and of several different degradable polymers reinforced with these fibers. Biodegradable phosphate fibers of calcium-sodium-metaphosphate (CSM) and sodium-calcium-aluminum-polyphosphate (NCAP) were acutely nontoxic in cellular, tissue, and whole animal evaluations, as determined by standard acute toxicity tests. Histological studies of bone implants sites fabricated from composites of copolymers of poly(E-caprolactone/L-lactide) and poly(ortho ester) reinforced with either CSM or NCAP fibers showed these composite materials to be nontoxic, with no abnormal inflammatory response. However, histological evaluation of muscle implants sites revealed the appearance of necrotic foci associated with implant sites in 12 of 22 NCAP containing composite specimens (p less than 0.05). Results of this preliminary biocompatibility screening suggest CSM fibers may be useful in reinforcing degradable polymers for production of completely biodegradable composites for implant use.

Biomechanical properties of absorbable implants in finger fractures

The mechanical rigidity of five different methods of pin fixation in two proximal phalangeal fracture models was studied and absorbable implants were compared with metallic implants in a biomechanical cadaver study. Thirty phalanges were tested in apex palmar bending, compression and torsion. Results showed that rigidity of absorbable implants was comparable with metallic implants, except in torsion.

Ex vivo biomechanical comparison of pin fixation techniques for canine distal femoral physeal fractures

OBJECTIVE

To compare the biomechanical properties of five intramedullary (IM) pin fixation techniques for Salter-Harris type I fractures of the distal femur in dogs.

STUDY DESIGN

Randomized, one-way factorial design composed of five treatment groups: (1) single IM pin, (2) dynamic IM crossed pins, (3) paired convergent pins, (4) crossed pins, and (5) crossed polyglycolic acid (PGA) rods.

SAMPLE POPULATION

Forty pairs of cadaver canine femurs.

MATERIALS

One femur of each pair was manually fractured and subsequently repaired; the contralateral intact femur served as its control. Each femur was loaded in torsion until-failure occurred and load-deformation curves were generated.

RESULTS

The crossed-pin technique sustained the greatest load to failure (116.8%) followed by the paired convergent pins (104.8%), dynamic IM pins (90.6%), single IM pin (72.1%), and crossed PGA rods (71.9%). Statistically significant differences in strength at failure were detected between the crossed-pin and single IM pin and the crossed-pin and crossed PGA rod techniques. All fixation techniques underwent greater deformation (1.5 times as much) and had a lower stiffness (66% to 75%) compared with the intact controls; however, there was no significant difference between techniques. Failure in the paired convergent and crossed-pin techniques occurred by fracture of the bone; failure in the other techniques occurred by distraction at the fracture site.

CONCLUSION

The rotational stability of any of the fixation techniques appears to be primarily determined by the ability to prevent distraction and maintain interdigitation of the physis.

CLINICAL RELEVANCE

When choosing a particular fixation technique for repair of a distal femoral physeal fracture, consideration should be given to the technique's relative biomechanical merits.

Absorbable polyglycolide devices in trauma and bone surgery

Poly(glycolic acid) or polyglycolide (PGA) is a polymer of glycolic acid. Glycolic acid is produced during normal body metabolism and is known as hydroxyacetic acid. Strong implants can be manufactured from this polymer with a self-reinforcing (SR) technique and used in the treatment of fractures and osteotomies. Since 1984, SR-PGA implants have been used routinely in our hospital for internal fixation of bone fractures. These implants were studied extensively in experimental animals and proved biocompatible. In 1.7% of human cases, sinus formation may develop after the use of these implants, which does not disturb healing. Use of these absorbable implants is justified as it obviates the need for a second operation for implant removal and avoids the risks associated with biostable implants.

Postoperative irradiation treatment and bioresorbable implants in orthopaedic surgery: an experimental in vitro study

The influence of applied radiation on the degradation of a polyglycolide (PGA) test specimen was studied in vitro. There was no significant difference in the time-dependent degradation behaviour between the irradiated test specimen and controls. Mandatory irradiation of an operation site following surgery therefore does not contradict the employment of bioresorbable implants in orthopaedic surgery.

Composite biomaterials based on ceramic polymers. I. Reinforced systems based on Al2O3/PMMA/PLLA

Composite biomaterials with good mechanical response and a partially biodegradable character were prepared by the free radical polymerization of mixtures of alpha-Al2O3, low-molecular-weight but crystalline poly(L-lactic acid) (PLLA), and methyl methacrylate (MMA). Cylindrical specimens prepared with different composition were characterized by thermogravimetry, calorimetry, 1H-NMR spectroscopy, and x-ray diffraction (XRD). The in vitro biodegradative process was studied in different media, following variations of the pH, gravimetric weight loss of the specimens, and crystalline domain change by XRD after immersion in pure water and buffered solutions at pH 4.0 and pH 8.0 for 90 days. Formation of a relatively porous structure with good cohesion after the biodegradative treatment (confirmed by SEM) was observed. These systems can be considered for applications in orthopedic surgery as filling biomaterials and even as control drug-delivery systems.

Sterilization, toxicity, biocompatibility and clinical applications of polylactic acid/polyglycolic acid copolymers

This is a review of salient studies of sterilization, toxicity, biocompatibility, clinical applications and current work in the field of orthopaedics, using implants made of polylactic acid (PLA), polyglycolic acid (PGA) and their copolymers. The intrinsic nature of these biomaterials renders them suitable for applications where temporally slow releases of bioactive agents in situ may be required. They are also desirable as fixation devices of bone, because they can virtually eliminate osteopenia associated with stress shielding or additional surgery. The majority of currently available sterilization techniques are not suitable for these thermoplastic materials and it may be desirable to develop new sterilization standards, which can account for the special character of PLA-PGA materials. Biocompatibility and toxicity studies suggest that, overall, PLA-PGA biomaterials may be suitable for orthopaedic applications, although certain problems, especially pertaining to reduction in cell proliferation, have been reported. Clinical applications are also promising, albeit not without problems usually associated with transient tissue inflammation. The future of these materials appears bright, especially in soft tissues. They may be used to address the exceedingly complex problem of osteochondral repair, but also as a means to enhance fixation and repair processes in tendons and ligaments.

Effect of annealing temperature on the degradation of reinforcing fibers for absorbable implants

Calcium phosphate fibers designed for reinforcement of bioabsorbable fracture fixation devices were evaluated for their properties upon annealing. The composition of these fibers were 54% PO4, 27% Ca, 12% ZnO, 2.5% NaPO3, and 4.5% Fe2O3, and they were either not annealed, annealed at 250 degrees C, or annealed at 420 degrees C. Chemical degradation, mass loss, and morphology upon degradation were studied. Chemical degradation was performed in Tris-buffered HCl, while mass loss and morphologic studies were performed in both physiologic and nonphysiologic solutions. The results showed that degradation rates for fibers were inversely proportional to the annealing temperature. Mass loss analysis of fibers immersed in the two physiologic solutions (calf serum and simulated body fluid) revealed little change in fiber diameter up to 60 days. Morphologic examination revealed little change in fibers immersed in the two physiologic solutions until 60 days, after which thin shells were found to be peeling off the outer coating of the fiber. Samples in tris-buffered HCl revealed a dramatic difference in mode of degradation among the three fibers. Fibers not annealed and those annealed at lower temperatures underwent a delaminating type of degradation that appeared to destroy the overall integrity of the fiber, whereas fibers annealed at 420 degrees C underwent crater-like deterioration in which the overall alignment of the fiber remained intact. It is therefore concluded that annealing fibers at higher temperatures also undergo a mode of degradation that allows them to maintain their structural integrity. Although annealing fibers close to glass transition temperature may produce an initially weaker fiber, chemical and physical degradation occur much slower, making these fibers most suitable for reinforcement of biodegradable implants.

[Resorbable osteosynthesis rods. An experimental study of the biomechanics and degradation of various rods of polyglycolide and poly (-L-lactide)]

Six different polyglycolide and poly(-L-lactide) rods were tested biomechanically: native, in vitro and in vivo. They were Biofix-C and Biofix-CG rods and 4 poly(-L-lactide) rods with a molecular weight between 70,000 and 700,000. Their flexual strength and elasticity module were determined by the 3-point flexion test. A total of 70 pins were implanted in the soft tissue and intramedullary to splint tibial osteotomies of 30 rabbits. The Biofix rods' flexural strength was high. There was no difference due to coating (346 to 402 N/mm2). In vitro experiments showed a rapid loss strength by 75 and 90%, equivalent to 2 to 3 weeks' implantation. The initial strength of the lactide pins was about 60% lower than that of the polyglycolid rods. The stability of the former after implantation decreased at a lower rate, with 25% of the initial strength being retained after 14 weeks. The histological findings were commensurate with the loss of flexural strength.

Biodegradable internal fixation

The objective of this article was to assess the value of the use of biodegradable materials in internal fracture fixation according to the current literature. Research methods included a computerized Medline search and a hand check of references of identified articles. Also, a complete reference list was obtained from the manufacturer of Biofix (Bioscience Ltd., Tampere, Finland). The reviewers abstracted descriptive information about population, materials, complications, follow-up times in clinical trials and strength of fixation, complications, and population for animal experiments. Results indicated that sterile sinus formation is mostly associated with polyglycolic acid, with rates up to 25%, and to a lesser extent, polylactic acid. Volume of implanted material and vascularity of bone appeared to affect the rate of sinus formation. Absorbable fixation was equivalent to or better than steel fixation for functional outcome refracture rate, and in transepiphyseal femoral and humoral fractures. Polyglycolic acid and polylactic acid both became toxic between 10 days and 4 weeks of hydrolysis. Polyglycolic acid had the highest initial strength at 370 MPa compared with other polymers.

The effect of site of implantation and animal age on properties of polydioxanone pins

Absorbable polymeric orthopaedic pins (Orthosorb) of 2.0 mm diameter were implanted at different sites in mature (3.5 kg, > 5 months) and immature (5 weeks old) rabbits (total 36) for 2, 4, and 5 weeks. The sites of implantation were the medullary canal of the femur, muscles of the thigh and subcutaneous tissue of the dorsum. In mature rabbits, 1.3 mm diameter pins were also implanted in the medullary canal of the femur. The shear strength of the pins harvested from the rabbits, was measured at each time period using a fixture that shears the pins into three parts symmetrically about the load axis. In both mature and immature rabbits the rate of degradation in mechanical properties was higher in the medullary canal of bone than in the muscle and in the subcutaneous tissue (p < 0.05). The strength retention was lower in immature than in mature rabbits after 4 and 5 weeks. The 1.3-mm pins had higher initial strength (174.7 +/- 7 MPa), higher strength retention and slower degradation within the medullary canal of femur of mature rabbits as compared to the 2.0-mm pins (157.5 +/- 4.8). DSC and X-ray diffraction results of control and implanted pins showed higher initial crystallinity and a wider range of crystallite size in the 1.3-mm pins. After 5 weeks in vivo, the crystallinity increased indicating degradation within the amorphous phase. The smaller crystallites underwent recrystallization to form larger crystallites. The results indicate that site of implantation and age of recipient influence the degradation and associated effects on mechanical properties of absorbable implants. The size of the implant, though important in determining its properties, should be considered in association with its microstructure, which also plays an important role in determining strength and strength retention of absorbable polymeric systems.

Resorbable synthetic polymers as replacements for bone graft

The potential of resorbable synthetic polymers derived from the poly(alpha-hydroxy acids), poly(lactide) and poly(glycolide), to fulfill a role as bone graft substitutes is reviewed. The various elements of the relationship between the degradation behaviour of resorbable implants and polymer synthesis and chain structure, implant morphology, processing and dimensions have been defined. The production of resorbable polymeric implants has been extensively documented so as to provide a wide basis for selection of an appropriate manufacturing technique. The key requirement of implant dimensional stability over the early stages of bone healing is emphasised so as to provide a stable surface on which osteoblasts and/or their precursor cells may migrate and secrete bone matrix. Minimisation of the content of slow resorbing polymers such as poly(L-lactide) is recommended, consistent with retention of an adequate implant degradation characteristic. The review concludes with a summary of alternative resorbable polymers such as the polyphosphazines which are interesting candidate materials for bone repair and reconstruction.