Missing osteoconductive effect of a resorbable PEO/PBT copolymer in human bone defects: a clinically relevant pilot study with contrary results to previous animal studies

Comparison of osteolysis around 3 different cement restrictors in total hip arthroplasty

BACKGROUND AND AIM

Several studies reported osteolysis around polyethylene glycol/polybutylene terephthalate (PEG/PBT) based femoral cement restrictors. Our goal was to evaluate and compare osteolysis around 3 different plug designs: the slow biodegradable PEG/PBT cement restrictor; the fast biodegradable gelatin cement restrictor; and the non-biodegradable polyethylene plug.

PATIENTS AND METHODS

In a retrospective multicentre cohort study chart data were extracted of patients who received a total hip arthroplasty between 2008 and 2012. A total of 961 hips were included. Cortical ratio between inner and outer cortices at the centre of the plug was measured on routine postoperative follow-up moments. Median follow up of all 3 hospitals was 3.5 years (1.4-7.3). The primary outcome was evidence of osteolysis (i.e. the difference in cortical ratio [CR]) on anteroposterior (AP) radiographs at final follow-up.

RESULTS

Progressive osteolysis was found around the PEG/PBT cement restrictor represented by a significantly increasing cortical ratio (ΔCR 0.067 (95% CI, 0.063-0.071). Distance from tip prosthesis to plug and size of the plug were found to be independent factors in predicting increased cortical ratio.

CONCLUSIONS

Our multicentre cohort shows increase of cortical ratio around the PEG/PBT cement restrictor which progresses over time. Physicians should be aware of this fact and are advised to intensify follow-up of patients who received this cement restrictor.

Biodegradable PEG-Based Amphiphilic Block Copolymers for Tissue Engineering Applications

Biodegradable tissue engineering scaffolds have great potential for delivering cells/therapeutics and supporting tissue formation. Polyesters, the most extensively investigated biodegradable synthetic polymers, are not ideally suited for diverse tissue engineering applications due to limitations associated with their hydrophobicity. This review discusses the design and applications of amphiphilic block copolymer scaffolds integrating hydrophilic poly(ethylene glycol) (PEG) blocks with hydrophobic polyesters. Specifically, we highlight how the addition of PEG results in striking changes to the physical properties (swelling, degradation, mechanical, handling) and biological performance (protein & cell adhesion) of the degradable synthetic scaffolds in vitro. We then perform a critical review of how these in vitro characteristics translate to the performance of biodegradable amphiphilic block copolymer-based scaffolds in the repair of a variety of tissues in vivo including bone, cartilage, skin, and spinal cord/nerve. We conclude the review with recommendations for future optimizations in amphiphilic block copolymer design and the need for better-controlled in vivo studies to reveal the true benefits of the amphiphilic synthetic tissue scaffolds.

Osteolysis in cemented total hip arthroplasty involving the OptiPlug cement restrictor: more than an incident?

The case report of a severe osteolytic reaction surrounding the OptiPlug cement restrictor in a 74-year-old male patient initiated a retrospective case series and closer investigation into the OptiPlug and its active compound, PolyActive. Not only did we find several cases of severe osteolysis in our own study population of 284 patients, several articles have lately described potential harmful side effects of the PolyActive material in humans. Although none of the articles have been based on large databases, we cannot guarantee the safety of this product. More research would help in our understanding of this phenomenon. Until then, we cannot recommend the use of the OptiPlug cement restrictor.

A novel biomimetic polymer scaffold design enhances bone ingrowth

There has been recent interest in treating large bone defects with polymer scaffolds because current modalities such as autographs and allographs have limitations. Additionally, polymer scaffolds are utilized in tissue engineering applications to implant and anchor tissues in place, promoting integration with surrounding native tissue. In both applications, rapid and increased bone growth is crucial to the success of the implant. Recent studies have shown that mimicking native bone tissue morphology leads to increased osteoblastic phenotype and more rapid mineralization. The purpose of this study was to compare bone ingrowth into polymer scaffolds created with a biomimetic porous architecture to those with a simple porous design. The biomimetic architecture was designed from the inverse structure of native trabecular bone and manufactured using solid free form fabrication. Histology and muCT analysis demonstrated a 500-600% increase in bone growth into and adjacent to the biomimetic scaffold at five months post-op. This is in agreement with previous studies in which biomimetic approaches accelerated bone formation. It also supports the applicability of polymer scaffolds for the treatment of large tissue defects when implanting tissue-engineering constructs. (c) 2008 Wiley Periodicals, Inc. J Biomed Mater Res, 2009.

Ectopic bone formation in cell-seeded poly(ethylene oxide)/poly(butylene terephthalate) copolymer scaffolds of varying porosity

Scaffolds from poly(ethylene oxide) and poly(butylene terephthalate), PEOT/PBT, with a PEO molecular weight of 1,000 and a PEOT content of 70 weight% (1000PEOT70PBT30) were prepared by leaching salt particles (425-500 microm). Scaffolds of 73.5, 80.6 and 85.0% porosity were treated with a CO(2) gas plasma and seeded with rat bone marrow stromal cells (BMSCs). After in vitro culture for 7 days (d) in an osteogenic medium the scaffolds were subcutaneously implanted for 4 weeks in nude mice. Poly(D, L-lactide) (PDLLA) and biphasic calcium phosphate (BCP) scaffolds were included as references. After 4 weeks (wks) all scaffolds showed ectopic formation of bone and bone marrow. For the scaffolds of different porosities, no significant differences were observed in the relative amounts of bone (7-9%) and bone marrow (6-11%) formed, even though micro computed tomography (mu-CT) data showed considerable differences in accessible pore volume and surface area. 1000PEOT70PBT30 scaffolds with a porosity of 85% could not maintain their original shape in vivo. Surprisingly, 1000PEOT70PBT30 scaffolds with a porosity of 73.5% showed cartilage formation. This cartilage formation is most likely due to poorly accessible pores in the scaffolds, as was observed in histological sections. mu-CT data showed a considerably smaller accessible pore volume (as a fraction of the total volume) than in 1000PEOT70PBT30 scaffolds of 80.6 and 85.0% porosity. BMSC seeded PDLLA (83.5% porosity) and BCP scaffolds (29% porosity) always showed considerably more bone and bone marrow formation (bone marrow formation is approximately 40%) and less fibrous tissue ingrowth than the 1000PEOT70PBT30 scaffolds. The scaffold material itself can be of great influence. In more hydrophobic and rigid scaffolds like the PDLLA or BCP scaffolds, the accessibility of the pore structure is more likely to be preserved under the prevailing physiological conditions than in the case of hydrophilic 1000PEOT70PBT30 scaffolds. Scaffolds prepared from other PEOT/PBT polymer compositions, might prove to be more suited.

Sensate scaffolds can reliably detect joint loading

Treatment of cartilage defects is essential to the prevention of osteoarthritis. Scaffold-based cartilage tissue engineering shows promise as a viable technique to treat focal defects. Added functionality can be achieved by incorporating strain gauges into scaffolds, thereby providing a real-time diagnostic measurement of joint loading. Strain-gauged scaffolds were placed into the medial femoral condyles of 14 adult canine knees and benchtop tested. Loads between 75 and 130 N were applied to the stifle joints at 30 degrees, 50 degrees, and 70 degrees of flexion. Strain-gauged scaffolds were able to reliably assess joint loading at all applied flexion angles and loads. Pressure sensitive films were used to determine joint surface pressures during loading and to assess the effect of scaffold placement on joint pressures. A comparison of peak pressures in control knees and joints with implanted scaffolds, as well as a comparison of pressures before and after scaffold placement, showed that strain-gauged scaffold implantation did not significantly alter joint pressures. Future studies could possibly use strain-gauged scaffolds to clinically establish normal joint loads and to determine loads that are damaging to both healthy and tissue-engineered cartilage. Strain-gauged scaffolds may significantly aid the development of a functional engineered cartilage tissue substitute as well as provide insight into the native environment of cartilage.

TGF-beta1-enhanced TCP-coated sensate scaffolds can detect bone bonding

Porous polybutylene terephthalate (PBT) scaffold systems were tested as orthopedic implants to determine whether these scaffolds could be used to detect strain transfer following bone growth into the scaffold. Three types of scaffold systems were tested: porous PBT scaffolds, porous PBT scaffolds with a thin beta-tricalcium phosphate coating (LC-PBT), and porous PBT scaffolds with the TCP coating vacuum packed into the scaffold pores (VI-PBT). In addition, the effect of applying TGF-beta1 to scaffolds as an enhancement was examined. The scaffolds were placed onto the femora of rats and left in vivo for 4 months. The amount of bone ingrowth and the strain transfer through various scaffolds was evaluated by using scanning electron microscopy, histology, histomorphometry, and cantilever bend testing. The VI-PBT scaffold showed the highest and most consistent degree of mechanical interaction between bone and scaffold, providing strain transfers of 68.5% (+/-20.6) and 79.2% (+/-8.7) of control scaffolds in tension and compression, respectively. The strain transfer through the VI-PBT scaffold decreased to 29.1% (+/-24.3) and 30.4% (+/-25.8) in tension and compression when used with TGF-beta1. TGF-beta1 enhancement increased the strain transfer through LC-PBT scaffolds in compression from 9.4% (+/-8.7) to 49.7% (+/-31.0). The significant changes in mechanical strain transfer through LC-PBT and VI-PBT scaffolds correlated with changes in bone ingrowth fraction, which was increased by 39.6% in LC-PBT scaffolds and was decreased 21.3% in VI-PBT scaffolds after TGF-beta1 enhancement. Overall, the results indicate that strain transfer through TCP-coated PBT scaffolds correlate with bone ingrowth after implantation, making these instrumented scaffolds useful for monitoring bone growth by monitoring strain transfer.

Biological evaluation of carbon-fibre-reinforced polybutyleneterephthalate (CFRPBT) employed in a novel acetabular cup

Polymer composites are potentially useful materials for orthopaedic applications as they can be tailored to closely match the various moduli of bone. The Cambridge epiphyseal cup has been designed using one such material, carbon-fibre-reinforced polybutyleneterephthalate (CFRPBT), so that more of the load can be transferred to bone in order to reduce bone loss around the implant. We have evaluated the suitability of CFRPBT for this application using several standard and custom biological tests, both in vitro and in vivo, and have also started the initial clinical evaluation of the prosthesis. CFRPBT showed no toxicity in bulk form and only minimal toxicity as a particulate. It was non-haemolytic, non-immunogenic and showed no genotoxicity. No adverse responses were seen after in vivo implantation, and the results obtained following 9 months of clinical evaluation have been very encouraging with a good clinical outcome and good bone bonding to the hydroxyapatite-coated CFRPBT.

Engineering principles of clinical cell-based tissue engineering

Tissue engineering is a rapidly evolving discipline that seeks to repair, replace, or regenerate specific tissues or organs by translating fundamental knowledge in physics, chemistry, and biology into practical and effective materials, devices, systems, and clinical strategies. Stem cells and progenitors that are capable of forming new tissue with one or more connective tissue phenotypes are available from many adult tissues and are defined as connective tissue progenitors. There are four major cell-based tissue-engineering strategies: (1) targeting local connective tissue progenitors where new tissue is desired, (2) transplanting autogenous connective tissue progenitors, (3) transplanting culture-expanded or modified connective tissue progenitors, and (4) transplanting fully formed tissue generated in vitro or in vivo. Stem cell function is controlled by changes in stem cell activation and self-renewal or by changes in the proliferation, migration, differentiation, or survival of the progeny of stem cell activation, the downstream progenitor cells. Three-dimensional porous scaffolds promote new tissue formation by providing a surface and void volume that promotes the attachment, migration, proliferation, and desired differentiation of connective tissue progenitors throughout the region where new tissue is needed. Critical variables in scaffold design and function include the bulk material or materials from which it is made, the three-dimensional architecture, the surface chemistry, the mechanical properties, the initial environment in the area of the scaffold, and the late scaffold environment, which is often determined by degradation characteristics. Local presentation or delivery of bioactive molecules can change the function of connective tissue progenitors (activation, proliferation, migration, differentiation, or survival) in a manner that results in new or enhanced local tissue formation. All cells require access to substrate molecules (oxygen, glucose, and amino acids). A balance between consumption and local delivery of these substrates is needed if cells are to survive. Transplanted cells are particularly vulnerable. Theoretical calculations can be used to explore the relationships among cell density, diffusion distance, and cell viability within a graft and to design improved strategies for transplantation of connective tissue progenitors. Rational strategies for tissue engineering seek to optimize new tissue formation through the logical selection of conditions that modulate the performance of connective tissue progenitors in a graft site to produce a desired tissue. This increasingly involves strategies that combine cells, matrices, inductive stimuli, and techniques that enhance the survival and performance of local or transplanted connective tissue progenitors.

Porous PEOT/PBT scaffolds for bone tissue engineering: preparation, characterization, and in vitro bone marrow cell culturing

The preparation, characterization, and in vitro bone marrow cell culturing on porous PEOT/PBT copolymer scaffolds are described. These scaffolds are meant for use in bone tissue engineering. Previous research has shown that PEOT/PBT copolymers showed in vivo degradation, calcification, and bone bonding. Despite this, several of these copolymers do not support bone marrow cell growth in vitro. Surface modification, such as gas-plasma treatment, is needed to improve the in vitro cell attachment. Porous structures were prepared using a freeze-drying and a salt-leaching technique, the latter one resulting in highly porous interconnected structures of large pore size. Gas-plasma treatment with CO(2) generated a surface throughout the entire structure that enabled bone marrow cells to attach. The amount of DNA was determined as a measure for the amount of cells present on the scaffolds. No significant effect of pore size on the amount of DNA present was seen for scaffolds with pore sizes between 250-1000 microm. Light microscopy data showed cells in the center of the scaffolds, more cells were observed in the scaffolds of 425-500 microm and 500-710 microm pore size compared to the ones with 250-425 microm and 710-1000 microm pores.

Bone growth in biomimetic apatite coated porous Polyactive 1000PEGT70PBT30 implants

We recently, developed a simple one-day one-step incubation method to obtain bone-like apatite coating on flexible and biodegradable Polyactive 1000PEGT70PBT30. The present study reports a preliminary biological evaluation on the coated polymer after implantation in rabbit femurs. The porous cylindrical implants were produced from a block fabricated by injection molding and salt leaching. This technique provided the block necessary mechanical integrity to make small cylinders (diameter 3.5 x 5 mm2) that were suitable for implantation in rabbits. The coating continuously covered the surface of the polymer, preserving the porous architecture of outer contour of the cylinders. Two defects with a diameter of 3.5 or 4 mm were drilled in the proximal and distal part of femur diaphysis. The implants were inserted as press-fit or undersized into the cortex as well as in the marrow cavity. The polymer swelled after implantation due to hydration, leading to a tight contact with the surrounding bone in both defects. The adherence of the coating on the polymer proved to be sufficient to endure a steam sterilization process as well as the 15% swelling of the polymer in vivo. The coated Polyactive 1000PEGT70PBT30 has a good osteoconductive property, as manifested by abundant bone growth into marrow cavity along the implant surface during 4-week implantation. A favorable bioactive effect of the coating with an intimate bone contact and extensive bone bonding with this polymer was qualitatively confirmed. Concerning the bone ingrowth into the porous implant in the defect of 4 mm diameter, only marginal bone formation was observed up to 8 weeks with a maximal penetration depth of about 1 mm. The pore interconnectivity is important not only for producing a coating inside the porous structure but also for bone ingrowth into this biodegradable material. This preliminary study provided promising evidence for a further study using a bigger animal model.

Design of segmented poly(ether ester) materials and structures for the tissue engineering of bone

In this study, PEOT/PBT segmented copolymers of different compositions have been evaluated as possible scaffold materials for the tissue engineering of bone. By changing the composition of PEOT/PBT copolymers, very different mechanical and swelling behaviors are observed. Tensile strengths vary from 8 to 23 MPa and elongations at break from 500 to 1300%. Water-uptake ranges from 4 up to as high as 210%. The in vitro degradation of PEOT/PBT copolymers occurs both by hydrolysis and oxidation. In both cases degradation is more rapid for copolymers with high PEO content. PEOT/PBT scaffolds with varying porosities and pore sizes have been prepared by molding and freeze-drying techniques in combination with particulate-leaching. The most hydrophilic PEOT/PBT copolymers did not sustain goat bone marrow cell adhesion and growth. However, surface modification by gas plasma treatment showed a very much improved polymer-cell interaction for all PEOT/PBT copolymer compositions. Their mechanical properties, degradability and ability to sustain bone marrow cell growth make PEOT/PBT copolymers excellent materials for bone tissue engineering.