Programmed water-induced shape-memory of bioabsorbable poly(D,L-lactide): activation and properties in physiological temperature.

This study reports of the novel water-induced shape-memory of bioabsorbable poly(D,L-lactide). We have developed an orientation-based programming process that generates an ability for poly(D,L-lactide) to transform its shape at 37°C in an aqueous environment without external energy and to adapt to a predefined stress level by stress generation or relaxation. In this orientation-programming process, polymer material is deformed and oriented at an elevated temperature and subsequently cooled down while retaining its deformed shape, tension, and polymer chain entanglements. At body temperature and in an aqueous environment, the shape-memory is activated by the plasticizing effect of water molecules diffused into the polymer matrix causing an entropy-driven directed relaxation of oriented and preloaded polymer chains. This plasticizing effect is clearly seen as a decrease of the onset glass transition temperature by 10-13°C. We found that γ-irradiation used for sterilizing the orientation-programmed materials strongly affected the shape-recovery rate, but not the recovery ratio. Both non-γ-irradiated and γ-irradiated sample materials showed excellent shape-recovery ratios during a ten-week test period: 94 and 97%, respectively. The orientation-programmed materials generated a predefined load in a 37°C aqueous environment when their shape-recovery was restricted, but when external tension was applied to them, they adapted to the predefined level by stress relaxation. Our results show that functionality in terms of shape-memory can be generated in bioabsorbable polymers without tailoring the polymer chain structure thus shortening the time from development of technology to its utilization in medical devices.

Effects of lactide monomer on the hydrolytic degradation of poly(lactide-co-glycolide) 85L/15G.

The hydrolytic degradation of oriented poly(L-lactide-co-glycolide) 85 L/15 G (PLGA 85/15) sample materials with various amounts of lactide monomer was monitored in vitro at 37 °C. The materials were manufactured from medical grade PLGA 85/15 by a two-step melt extrusion-die drawing process. Results showed that the hydrolytic degradation rate depended highly on the lactide monomer content, which in turn influenced the retention of mechanical properties, mass loss, crystallinity, and dimensional stability. Even small quantities of lactide monomer (0.05-0.20 wt%) affected especially the retention of mechanical properties, which started to decline rapidly upon the inherent viscosity reaching 0.6-0.8 dl/g due to hydrolytic degradation. Based on our hydrolytic degradation data, we constructed a simplified mathematical model of degradation-related strength retention and recommend it as a functional quality control tool for melt-processed biodegradable medical devices manufactured from poly(L-lactide-co-glycolide) 85 L/15 G.

Drug-eluting bioabsorbable stents - an in vitro study.

The aim of this study was to investigate the drug elution properties of novel drug-eluting bioabsorbable stents in vitro with four different drugs: dexamethasone, indomethacin, simvastatin and ciprofloxacin. Braided stents of poly-lactic acid (96l/4d) fibers were coated with a solution containing the appropriate bioabsorbable polymer and drug, with acetone as the solvent. Two different drug concentrations for both non-sterile and gamma sterilized stents were used for dexamethasone and indomethacin. For ciprofloxacin and simvastatin, only one drug dose was used. The stents were placed in sodium-phosphate-buffered saline in a shaking incubator (pH 7.4, +37 degrees C) and the eluted drug was measured periodically using an ultraviolet spectrometer. The drugs were hydrophobic to different degrees, as demonstrated by their various speeds of elution. In general, the higher the drug load in the stent, the faster the drug elution and the more hydrophilic the elution profile. In the cases of dexamethasone, indomethacin and ciprofloxacin, the sterilization decreased the drug elution rate slightly and the elution started earlier. However, in the case of ciprofloxacin, the gamma sterilization increased the drug elution rate slightly. Sustained elution was achieved for all four drugs. It was also evident that both the concentration and the hydrophility of the drug had a great influence on the drug elution profile. Gamma sterilization modified the drug elution profiles of dexamethasone, indomethacin and simvastatin, but had little effect on the drug elution profile of ciprofloxacin compared to three other drugs.

Potential use of craniosynostotic osteoprogenitors and bioactive scaffolds for bone engineering.

The cranial bone has a very limited regenerative capability. Patients with craniosynostosis (the premature fusion of cranial sutures, leading to skull abnormalities) often require extensive craniofacial reconstruction and repeated surgery. The possibility of grafting autologous osteoprogenitor cells seeded on bioabsorbable matrices is of great potential for inducing regeneration of craniofacial structure and protecting the brain from external insult. To this purpose we have studied the behaviour of normal and craniosynostotic mouse osteoblast cell lines, and of human primary osteoprogenitors from craniosynostotic patients. We have monitored their ability to grow and differentiate on plastic and on a scaffold composed of bioactive glass and bioabsorbable polymer by live fluorescent labelling and expression of bone differentiation markers. Cells from syndromic patients display a behaviour very similar to that observed in the stable mouse cell line we generated by introducing the human FGFR2-C278F, a mutation found in certain craniosynostosis, into MC3T3 osteblastic cells, indicating that the mutated cell line is a valuable model for studying the cellular response of human craniosynostotic osteoblasts. Both normal and mutated calvarial osteoprogenitors can attach to the bioactive scaffold, although mutated cells display adhesion defects when cultured on plastic. Furthermore, analysis of bone differentiation markers in human osteoblasts shows that the composite mesh, unlike PLGA(80) plates, supports bone differentiation. The ability of the mesh to support homing and differentiation in both normal and mutant osteoprogenitors is important, in view of further developing autologous biohybrids to repair cranial bone deficits also in craniosynostotic patients undergoing extensive reconstructive surgery.

Innovation in multifunctional bioabsorbable osteoconductive drug-releasing hard tissue fixation devices.

We review in this paper the work performed by our group to develop multifunctional bioabsorbable ciprofloxacin releasing bone implants. Poly lactide-co-glycolide (PLGA 80/20 and polylactide (P(L/DL)LA 70/30) were used. Ciprofloxacin (CF) and bioactive glass (BaG) 13-93 were added. The mixture was then extruded and self-reinforced. CF release, mechanical strength, and the effect on S. epidermidis attachment and biofilm formation were evaluated. In rabbits, tissue reactions were assessed. Pull out strength was evaluated in cadaver bones. CF was released over 44 weeks (P(L/DL)LA) and 23-26 weeks (PLGA). Initial shear strength of the CF screws was 152 MPa (P(L/DL)LA) and 172 MPa (PLGA). Strength was retained for 12 weeks (P(L/DL)LA) and 9 weeks (PLGA). Histologically, CF releasing implants did not show much difference from control plain PLGA screws except for increased giant cells. CF miniscrews had lower pullout strength than the controls, but CF tacks had better values than controls. BaG led to a drop in pullout strength properties. Bacterial growth, attachment and biofilm formation on CF implants was significantly reduced when compared to controls. Accordingly, bioabsorbable multifunctional implants with appropriate CF release, mechanical, and biocompatibility properties are possible to develop and are considered appropriate to apply clinically.

Histomorphometric analysis of poly-L/D-lactide 96/4 sutures in the gastrocnemius tendon of rabbits.

BACKGROUND: Common Achilles tendon ruptures are not usually fixed by bioabsorbable sutures due to limitations in their strength retention properties. Modern technology has made it possible to develop bioabsorbable sutures with prolonged strength retention. AIMS: To evaluate histologically tissue reactions of poly-L/D-lactide (PLDLA) sutures implanted in Achilles tendon of rabbits. MATERIAL AND METHODS: Fifteen rabbits were evaluated at 2, 6 and 12 weeks postoperatively, with five rabbits in each follow-up group. PLDLA monofilament sutures were implanted into the medial gastrocnemius tendon. Polyglyconate monofilament sutures with similar diameter (Maxon 4-0, Cyanamid of Great Britain Ltd., Gosport, UK) were implanted in the contralateral gastrocnemius tendon. The histology was studied in hard-resin embedded samples. The thickness of the formed fibrous tissue capsule was determined histomorphometrically. RESULTS: PLDLA led to formation of significantly thinner fibrous tissue capsule than Maxon sutures of the same diameter. Median thickness (PLDLA vs. Maxon) at two weeks was 5.26 vs.13.22 microm, at six weeks 11.66 vs. 80.97 microm, and at 12 weeks 10.63 vs. 17.59 microm (p<0.01). CONCLUSIONS: During the 12 week follow-up period, PLDLA sutures implanted intratendineously formed thinner fibrous capsule than Maxon sutures of the same diameter. The suture materials were not totally absorbed by 12 weeks.

Material and knot properties of braided polyester (Ticron) and bioabsorbable poly-L/D-lactide (PLDLA) 96/4 sutures.

The purpose of the present study was to investigate in vitro the biomechanical material and knot properties and histomorphometrical knot properties of 3-0 braided polyester suture (Ticron) and bioabsorbable poly-L/D-lactide (PLDLA) 96/4 suture. In Ticron five throws are needed to form a secure knot, and the 1 = 1 = 1 = 1 = 1 and the 2 = 1 = 1 = 1 configurations are recommended. For PLDLA several granny and square knots formed a secure knot, but the 1 = 1 and 1 = 1 = 1 knots were the best. These PLDLA knots had lower yield force and strain at yield point, but higher stiffness than the recommended Ticron knots. The ultimate force values did not differ, but PLDLA knots had significantly higher strain at ultimate point. In the histomorphometrical analysis of the recommended knots, the PLDLA knots had a significantly smaller knot surface area than the Ticron knots. According to these results, PLDLA suture proved to be suitable for flexor tendon repair.

Manufacturing, mechanical characterization, and in vitro performance of bioactive glass 13-93 fibers.

Fibers were manufactured from the bioactive glass 13-93 by melt spinning. The fibers were further characterized by measuring their tensile and flexural strength, and their in vitro performance was characterized by immersing them in simulated body fluid, which analyzed changes in their mass, their flexural strength, and surface reactions. The strength of glass fibers is highly dependent on fiber diameter, test method, and possible surface flaws, for example, cracks due to abrasion. In this study, the thinnest fibers (diameter between 24 and 33 microm) possessed the highest average tensile strength of 861 MPa. The flexural strength was initially 1353.5 MPa and it remained at that level for 2 weeks. The Weibull modulus for both tensile and flexural strength values was initially about 2.1. The flexural strength started to decrease and was only approximately 20% of the initial strength after 5 weeks. During the weeks 5-40, only a slight decrease was detected. The flexural modulus decreased steadily from 68 to 40 GPa during this period. The weight of the samples initially decreased due to leaching of ions and further started to increase due to precipitation of calcium phosphate on the fiber surfaces. The mass change of the bioactive glass fibers was dependent on the surface area rather than initial weight of the sample. The compositional analysis of the fiber surface after 24 h and 5 weeks immersion did confirm the initial leaching of ions and later the precipitation of a calcium phosphate layer on the bioactive glass 13-93 fiber surface in vitro.

Characterization of human primary osteoblast response on bioactive glass (BaG 13-93)- coated poly-L,DL-lactide (SR-PLA70) surface in vitro.

Bioabsorbable polylactide-based polymers are commonly used for bone reconstruction. Although these polymers have proven successful in many applications, they do not have the capacity to induce osteoconduction. Therefore, several strategies have been developed to manufacture osteoconductive polylactide-based composites. In this study, we have investigated in vitro response of human primary osteoblasts for self-reinforced poly-L,DL-lactide 70/30 (SR-PLA70) plates coated with spheres of bioactive glass 13-93 (SR-PLA70 + BaG). Osteoblasts were cultured on SR-PLA70 and SR-PLA70 + BaG plates for 2, 7, or 14 days. By day 7, both materials induced a reduction in total cell population. However, by day 14 the proliferative response of osteoblasts on SR-PLA70 + BaG surface was such that the cell population had regained similar levels as that of day 2 controls. Alkaline phosphatase activity was higher on SR-PLA70 at day 7 but declined to control levels by day 14. There were no significant time-dependent variations in alkaline phosphatase activity on SR-PLA70 + BaG. After in vitro hydrolysis for 7 days, the elemental analysis of SR-PLA70 + BaG surface showed the presence of mineral precipitates that were confirmed as crystalline hydroxyapatite. This was accompanied by osteoblast spreading, protrusions of microvilli adhered to BaG 19-39 surface, cuboidal phenotype and cell surface associated formation of hydroxyapatite microspheres. In conclusion, the SR-PLA70 + BaG composite is capable of inducing a proliferative response of human primary osteoblasts, and appears to support the development of mature osteoblast phenotype. Therefore, the SR-PLA70 + BaG composites appear as promising osteoconductive scaffold candidates for reconstruction and regeneration of bone matrix.

A novel bioabsorbable composite membrane of Polyactive 70/30 and bioactive glass number 13--93 in repair of experimental maxillary alveolar cleft defects.

A novel bioabsorbable composite membrane of polyethylene oxide terephthalate and polybutylene terephthalate copolymer (Polyactive 70/30) combined with bioactive glass No. 13--93 was tested in the repair of experimental maxillary alveolar cleft defects. In this pilot study, the possible ability of the membrane to promote bone formation by guided tissue regeneration was investigated. Standard alveolar defects were made bilaterally in the maxilla of 12 growing rabbits and were filled with autogenous bone grafts. The test defect was covered with the composite membrane and the other defect was left uncovered to serve as a control. The follow-up time was 10 weeks. Radiological, histological, and histomorphometric evaluations were performed. Radiologically, no statistically significant differences between test and control defects at 10 weeks were found. Histologically, the membrane enhanced osteogenic activity locally at the membrane-bone interface. Swelling of the membrane was observed. Histomorphometrically, no significant promotion of bone formation by the membrane was observed. The composite membrane was found to be biocompatible and surgically easy to use, but its osteopromotive effect was limited in this experimental cleft model. Further studies are necessary to assess its suitability for reconstructive surgical applications.

In vitro properties of PLLA screws and novel bioabsorbable implant with elastic nucleus to replace intervertebral disc.

The suitability of two different implant types for the replacement of the intervertebral disc was studied in vitro. Self-reinforced poly-L-lactide (SR-PLLA) screws Ø 4.5 mm were studied 24 weeks in vitro and cylindrical implants with elastic nucleus made of poly(L/D)lactide 96/4, poly(L/DL)lactide 70/30, Bioactive Glass n:o 13-93 and Polyactive 1000PEOT70PBT30 were studied 15 weeks in vitro. The cylindrical implant mimics the size and shape of the intervertebral disc. During the in vitro, there were no changes in compression properties with either implant types. The screws had sufficient modulus for intervertebral ossification in the canine model and the cylindrical implant showed also sufficient mechanical properties. These results suggest that both implant types could be used in clinical testing.

Coralline hydroxyapatite reinforced with polylactide fibres in lumbar interbody implantation.

Porous hydroxyapatite HA blocks reinforced with poly-l/dl-lactide fibres were used to maintain the lumbar disc space and to start to create intercorporeal fusion in 23 growing pigs. In four pigs two emptied non adjacent disc spaces were left open. After 3, 6, 12 and 16 weeks the implanted disc blocks were studied radiologically, histologically, histomorphometrically, microradiographically, and with oxytetracycline fluorescence. In plain films slight to moderate ossification of the implanted disc spaces was detected at 12 and 16 weeks. Resorption of the implants was seen radiologically from 3 weeks and fragmentation from 12 weeks onwards. In microradiographs disintegration of the coralline inner structure started at 3 weeks. Histologically, connective tissue ingrowth was seen inside the porous structure from three weeks onwards. Small amounts of new bone were visible and connective tissue inside the implant increased from a mean of 65.6% at 3 weeks to a mean of 79.4% at 16 weeks histomorphometrically. The bone ingrowth varied from 0.7 to 1.7%. A loss of height in the implanted disc spaces was seen (p < 0.05, linear regression analysis). In control pigs the emptied disc spaces lost their height similarly. The implants used were not strong enough to maintain the lumbar disc height.

Efficacy of bioabsorbable antibiotic containing bone screw in the prevention of biomaterial-related infection due to Staphylococcus aureus.

Impregnation of antimicrobial agents within biodegradable orthopedic implants provides a possibility for local antimicrobial prophylaxis of biomaterial-related infections. The objective of this study was to evaluate the efficacy of a bioabsorbable ciprofloxacin containing bone screw (Ab-PLGA) in the prevention of biomaterial-related infection due to Staphylococcus aureus in a rabbit model. Animals in Group I (n=8) received a Ab-PLGA screw contaminated with S. aureus, while animals in Group II (n=8) received a stainless steel (SS) screw contaminated with S. aureus. In two negative control groups, the animals received a Ab-PLGA screw (Group III, n=4) or a SS screw (Group IV, n=4) without bacterial contamination. 18F-FDG-PET imaging, performed at 6 weeks, was applied as a novel quantitative in vivo imaging modality of implant-related infection. Infection was verified by swab cultures, direct cultures of the retrieved implant, and quantitative cultures of pulverized bone. The concentrations of ciprofloxacin in serum and local bone tissue were determined by a high performance liquid chromatographic (HPLC) method with fluorescence (FLD) detection. In the group of contaminated Ab-PLGA screws, all cultures were negative. In the group of contaminated SS screws, all cultures of retrieved implants and six cultures out of eight of pulverized bone were positive for inoculated S. aureus. In negative control groups, all cultures were negative except one contaminant (S. cohnii) found in a SS screw culture. Verified infection of contaminated SS screws was collaborated by the increased 18F-FDG-PET uptake (P=0.004 compared with the group of contaminated Ab-PLGA screws). The mean bone tissue concentration of ciprofloxacin varied from 2.54 to 0.83 microg/g bone as a function of distance from the implantation site. The serum concentration of ciprofloxacin remained undetectable and below the resolution of the analytic method (<5.0 ng/ml). This study confirmed the in vivo efficacy of bioabsorbable antibiotic containing bone screw in the prevention of biomaterial-related infection due to S. aureus.

Self-reinforced composites of bioabsorbable polymer and bioactive glass with different bioactive glass contents. Part I: Initial mechanical properties and bioactivity.

Spherical bioactive glass 13-93 particles, with a particle size distribution of 50-125 microm, were combined with bioabsorbable poly-L,DL-lactide 70/30 using twin-screw extrusion. The composite rods containing 0, 20, 30, 40 and 50 wt% of bioactive glass were further self-reinforced by drawing to a diameter of approximately 3 mm. The bioactive glass spheres were well dispersed and the open pores were formed on the composite surface during drawing. The initial mechanical properties were studied. The addition of bioactive glass reduced the bending strength, bending modulus, shear strength, compression strength and torsion strength of poly-L,DL-lactide. However, the strain at maximum bending load increased in self-reinforced composites. Initially brittle composites became ductile in self-reinforcing. The bioactivity was studied in phosphate buffered saline for up to 12 days. The formation of calcium phosphate precipitation was followed using scanning electron microscopy and energy dispersive X-ray analysis. Results showed that the bioactive glass addition affected the initial mechanical properties and bioactivity of the composites. It was concluded that the optimal bioactive glass content depends on the applications of the composites.

Intramedullary fixation of femoral cortical osteotomies with interlocked biodegradable self-reinforced poly-96L/4D-lactide (SR-PLA96) nails.

Femoral diaphyseal osteotomies of adult sheep were fixed with intramedullary (i.m.) biodegradable self-reinforced poly-96L/4D-lactide (SR-PLA96) nails (Phi 10.66-10.78 mmx195-199 mm) that were interlocked with four Phi 1.5 mm metallic Kirschner (K) wires. Hollow i.m. nails used in the pilot study resulted in implant failure and only two of these sheep were followed long term (18 months and 3 years). In the main study, solid nails were used and this resulted in consolidation in 5/6 cases and in one non-union due to failure of both proximal K wires at 3 weeks. Sheep in the main study were followed for 6 and 12 months. Femurs were studied radiographically, microradiographically, histologically and by oxytetracycline labelling. Good bone healing was achieved and histology showed only a mild tissue reaction to the implant at 6-18 months. At 3 years, the implant had almost degraded and numerous foamy macrophages were ingesting the disintegrated material. No accumulations of lymphocytes implying an immunological inflammatory reaction were seen. This investigation demonstrated that absorbable SR-PLA96 i.m. nails can be used together with interlocking metallic K wires in fixation of simple cortical osteotomies in large animals weighing up to 63 kg. The long-term biocompatibility should be studied in a larger population.

Bioreconstructive joint scaffold implant arthroplasty in metacarpophalangeal joints: short-term results of a new treatment concept in rheumatoid arthritis patients.

Swanson silicone implant is the "gold standard" of metacarpophalangeal joint reconstruction in rheumatoid arthritis (RA) patients. However, durability problems of silicone implants have led us to develop a new technique based on bioreconstructive implants. PLA96 (poly-L,D-lactide copolymer, L:D ratio of 96:4) scaffolds were engineered. Bioabsorption and substitution of porous PLA96 scaffold with living tissue eventually produce a neojoint. In the current prospective study, 23 RA patients (80 joints) were operated on, using PLA96 implants. Fifteen patients (54 joints) have been monitored for at least 1 year. Pain alleviation was well achieved. Range of motion improvement was emphasized to extension direction of functional arc. The average ulnar deviation was preoperatively 26 degrees, and at follow-up it was 6 degrees. Volar subluxation was noticeable in 56% of joints preoperatively and in 6% at 1-year follow-up. This is the first report of the formation of a living, functional joint in situ by means of a synthetic bioreconstructive joint scaffold. Results of this preliminary short-term study are comparable with previously published data on silicone arthroplasty. However, bioreconstructive prostheses can aid in preventing problems that occur with biostable prostheses. Tissue engineering has created a new era in the reconstruction of damaged joints.

Use of hydroxylapatite/ polymer-composite in facial bone augmentation. An experimental study.

On purpose to achieve better and more controlled alveolar ridge augmentation, biodegradable polyglycolide (PGA) curved implants with porous, particulate hydroxylapatite (HA) were developed to be used in an experimental augmentation in sheep mandible. Prior to surgery curved PGA implants were loaded with particulate HA with help of a rather fast degrading adhesive, polyglycolide/polylactide (PGA/PLA) copolymer. This augmentation device was inserted into the ridge using a slight modification of the tunnelling technique. Macroscopically, a dehiscence with loss of HA particles was observed in 3 of the 16 augmentations. Bone ingrowth was seen in a noticeable degree in one sheep at 24 weeks. Instead, foreign body-type cells were shown at the interface of mandibular bone and hydroxylapatite deposit, as well as abundant connective tissue reaction inside HA deposits. The results of the study give rise to concern about bony integration in the presence of biodegradable polyglycolide substances in HA augmentation. The findings call in question even their use as a carrier for bone forming agents in combination with HA.

Treatment of pectus excavatum with bioabsorbable polylactide plates: Preliminary results.

BACKGROUND/PURPOSE: Pectus excavatum usually is corrected by thoracoplasty using metal plates. Recently bioabsorbabe polylactide plates have been developed. The aim of this study was to compare outcome after use of metal and bioasorbable plates in thoracoplasty performed for correction of pectus excavatum. METHODS: Eighty-three children (<16 years old) underwent thoracoplasty (Sulamaa's technique). In 75 patients, metallic plates, and in 8 patients, self-reinforced poly-L-lactide (SR-PLLA) plates, were used. Seven patients in the SR-PLLA group and 13 patients in the metal plate group were assessed 0.5 to 13 years postoperatively. RESULTS: The mean operating time was 121 minutes in the metal plate group, and 87 minutes in the SR-PLLA plate group. In the metal plate group, complications were pain caused by instability of the metal plates (n = 17), wound infection (n = 3), recurrence of deformity (n = 3), postoperative pain (n = 3), pneumothorax (n = 1), and nonspecific postoperative fever (n = 1). Thirteen patients underwent reoperation to refix the position of the metallic plates. In the SR-PLLA group, one case of pneumothorax occurred, and plate fragment palpability caused local pain in one patient. Cosmetic results and lung function values were similar. CONCLUSION: Our preliminary results show that bioabsorbable plates are a useful option in the treatment of pectus excavatum in children.

Viscoelastic memory and self-expansion of self-reinforced bioabsorbable stents.

The possibility to decide the speed and rate of expansion of stents is of great clinical importance by reason of the varying requirements for different indications to use stents. Self-reinforced bioabsorbable stents can be made self-expanding owing to the viscoelastic memory of the material. Stents are stable at room temperature and expansion occurs at body temperature. The level at which the expansion stops depends on the material, crystallinity, initial diameter of spira and annealing temperature. The expansion rate can be estimated by logarithmic equation, if material, draw ratio and diameter of stent wire are constant. This is, however, possible only if processing parameters are constant. Based on the present results annealing temperature and expansion time were seen to be directly proportional to the expansion rate of the stent.