Susan P. James

Challenge to the concept that UHMWPE acetabular components oxidize in vivo

Severe wear of the ultra-high molecular weight polyethylene (UHMWPE) components of total joint replacements limits their long-term success. In previous studies, the infrared spectra obtained on retrieved UHMWPE components were interpreted as evidence that the UHMWPE oxidizes in vivo. In direct contrast, infrared spectroscopy of the retrieved UHMWPE acetabular components examined in this study demonstrated adsorbed esterified fatty acids, readily extractable by hexane, and no substantial evidence of in vivo oxidation. This emphasizes that special care must be taken when using infrared spectroscopy to assess retrieved components.

Assessing the dog as a model for human total hip replacement ANALYSIS OF 38 CANINE CEMENTED FEMORAL COMPONENTS RETRIEVED AT POST-MORTEM

Post-mortem retrieval of canine, cemented femoral components was analysed to assess the performance of these implants in the dog as a model for human total hip replacement (THR). Mechanical testing and radiological analysis were performed to determine the stability of the implant and the quality of the cement. Thirty-eight implants from 29 dogs were retrieved after time intervals ranging from 0.67 to 11.67 years. The incidence of aseptic loosening was 63.2%, much higher than in human patients (6% in post-mortem studies). Failure of the femoral implants began with debonding at the cement-metal interface, similar to that in implants in man. The incidence of aseptic loosening was much lower in bilateral than in unilateral implants. Significant differences were observed for three different designs of implant. While the dog remains the animal model of choice for THR, results from this study provide insight into interspecies differences in the performance of implants. For example, the performance of THR in dogs should be compared with that in young rather than in elderly human patients.

Clinical wear of 63 ultrahigh molecular weight polyethylene acetabular components: effect of starting resin and forming method.

This study compares the clinical wear rates and implant characteristics of 63 surgically retrieved acetabular components. All components were made by the same manufacturer, implanted by the same surgeon, in articulation against the same type of femoral component, and revised for the same reason; 19 were made from directly compression molded, calcium stearate free, ultrahigh molecular weight polyethylene (UHMWPE) and 44 were made from machined, ram extruded, calcium stearate containing UHMWPE. There were significant differences in wear, duration, and wear rate between the molded (type I) and machined (type II and III) components. Most importantly, the wear rates of type I (molded) components were significantly (p < 0.0001) lower than the wear rates of type II, type III, and type II and III components as a group (all machined). The machined components had wear rates 2.3 times greater than the molded components. The wear rates between the two different groups of machined components (type II and III) were not significantly different. The scanning electron microscope observations did not reveal any major differences in wear mechanisms between the three types of components, although the machined components did show more evidence of brittleness. The molded components were better consolidated (or had less fusion defects) than the ram extruded components.

Dynamic mechanical analysis and biomineralization of hyaluronan–polyethylene copolymers for potential use in osteochondral defect repair

Treatment options for damaged articular cartilage are limited due to its lack of vasculature and its unique viscoelastic properties. This study was the first to fabricate a hyaluronan (HA)-polyethylene copolymer for potential use in the replacement of articular cartilage and repair of osteochondral defects. Amphiphilic graft copolymers consisting of HA and high-density polyethylene (HA-co-HDPE) were fabricated with 10, 28 and 50 wt.% HA. Dynamic mechanical analysis was used to assess the effect of varying constituent weight ratios on the viscoelastic properties of HA-co-HDPE materials. The storage moduli of HA-co-HDPE copolymers ranged from 2.4 to 15.0 MPa at physiological loading frequencies. The viscoelastic properties of the HA-co-HDPE materials were significantly affected by varying the wt.% of HA and/or crosslinking of the HA constituent. Cytotoxicity and the ability of the materials to support mineralization were evaluated in the presence of bone marrow stromal cells. HA-co-HDPE materials were non-cytotoxic, and calcium and phosphorus were present on the surface of the HA-co-HDPE materials 2 weeks after osteogenic differentiation of the bone marrow stromal cells. This study is the first to measure the viscoelastic properties and osseocompatibility of HA-co-HDPE for potential use in orthopedic applications.

Synthesis and characterization of a hyaluronan-polyethylene copolymer for biomedical applications

Hyaluronan (HA)-based biomaterials are of interest for bone and cartilage tissue engineering because HA plays an important role in orthopedic tissue development, function, and repair. The goal of this project was to develop a biomaterial that incorporated the constituents of both a hydrogel and a hydrophobic polymer for biomedical applications. A series of amphiphilic graft copolymers consisting of HA, a glycosaminoglycan, and high-density polyethylene (HDPE), that is, HA-co-HDPE, were fabricated. The chemical characteristics, physical and viscoelastic properties, and cytocompatibility of novel HA-co-HDPE materials were characterized via Fourier Transform infrared (FTIR) spectroscopy, solid state nuclear magnetic resonance (ssNMR) spectroscopy, differential scanning calorimetry (DSC), dynamic shear testing, and an in vitro human osteoblast cell study. The esterification reaction between HA and functionalized HDPE resulted in semicrystalline, insoluble powder. The dynamic shear properties of HA-co-HDPE concentrated solutions were more like natural proteoglycans than the HA control. HA-co-HDPE was successfully compression molded into disks that swelled upon hydration. Osteoblasts were viable and expressed the osteoblast phenotype after 7 days of culture on HA-co-HDPE materials. These HA-co-HDPE materials may have several biomaterial applications in saline suspension or molded form, including orthopedic tissue repair.

Comparison of Alternate and Simultaneous Tensioning of Wires in a Single‐Ring Fixator Construct

OBJECTIVE To measure and compare the strain of wires tensioned with alternate (ALT) and simultaneous (SIM) tensioning in a single-ring fixator construct and compare the stiffness of these constructs under axial loading. STUDY DESIGN Experimental mechanical study. SAMPLE POPULATION Twenty-four, 84 mm diameter, single-ring constructs. METHODS Twenty-four, 84 mm diameter, single-ring constructs were assembled using 2 1.6 mm wires placed at a 60 degrees angle tensioned with either ALT or SIM technique to 90 kg tension. Voltage data from a strain gauge were recorded during the wire-tensioning process, cyclic axial loading, and load-to-failure testing. Wire strains were calculated for each wire and compared within constructs and between ALT and SIM groups. Construct stiffness was compared between groups. RESULTS There was no difference between the tensioning methods in final wire strains after initial tensioning for both the wire below the ring (W1; P=.698) and the wire above the ring (W2; P=.233). There was also no difference in final wire strains within each tensioning method group (ALT, P=.289; SIM, P=.583). Loss of wire strain (3.5-5%) occurred after cyclic loading for both wires in both groups. There was no difference in construct stiffness between the ALT and SIM groups (P=.126). Mode of failure was by wire breakage in all constructs and occurred most frequently in W1. CONCLUSION ALT tensioning of wires produced similar wire strains within a single-ring construct after initial tensioning to SIM tensioned wires. There was no difference in construct stiffness under axial loading between AIM and SIM tensioned constructs. CLINICAL RELEVANCE ALT tensioning of wires in a single-ring fixator construct can be used as an alternative to SIM tensioning, as similar initial wire tensions are achieved.

Biomechanical characteristics of allogeneic cortical bone pins designed for fracture fixation

The biomechanical characteristics of 1.2 mm diameter allogeneic cortical bone pins harvested from the canine tibia were evaluated and compared to 1.1 mm diameter stainless steel pins and 1.3 mm diameter polydioxanone (PDS) pins using impact testing and four-point bending. The biomechanical performance of allogeneic cortical bone pins using impact testing was uniform with no significant differences between sites, side, and gender. In four-point bending, cortical bone pins harvested from the left tibia (204.8 +/- 77.4 N/mm) were significantly stiffer than the right tibia (123.7 +/- 54.4 N/mm, P = 0.0001). The site of bone pin harvest also had a significant effect on stiffness, but this was dependent on interactions with gender and side. Site C in male dogs had the highest mean stiffness in the left tibia (224.4 +/- 40.4 N/mm), but lowest stiffness in the right tibia (84.9 +/- 24.2 N/mm). Site A in female dogs had the highest mean stiffness in the left tibia (344.9 +/- 117.4 N/mm), but lowest stiffness in the right tibia (60.8 +/- 3.7 N/mm). The raw and adjusted bending properties of 1.2 mm cortical bone pins were significantly better than 1.3 mm PDS pins, but significantly worse than 1.1 mm stainless steel pins (P < 0.0001). In conclusion, cortical bone pins may be suitable as an implant for fracture fixation based on initial biomechanical comparison to stainless steel and PDS pins used in clinical practice.

Silylation of poly-L-lysine hydrobromide to improve dissolution in apolar organic solvents

Bis(trimethylsilyl)acetamide (BSA) was used to trimethylsilylate the hydrobromide salt of the synthetic polyamide poly-L-lysine (PLL-HBr) to improve its solubility in apolar organic solvents. The resulting trimethylsilylated derivative of PLL (PLL-SiMe 3 ) was found to be soluble in methylene chloride, tetrahydrofuran, and xylenes. The PLL-SiMe 3 was eventually used in the formation of an interpenetrating polymer network (IPN) with ultrahigh molecular weight polyethylene via xylenes. Elemental analysis, FTIR, and NMR spectroscopic evidence supported highly efficient (81.7%) silylation for the BSA-treated PLL-SiMe 3 product. Spectral data also showed complete silylation at the carboxyl, N-terminal α-amine, and the e-amine groups. This chemistry provides solvent processing accessibility for PLL blends or grafting reactions not possible for the commercial hydrobromide salt of PLL.

Mechanical characteristics of cortical bone pins designed for fracture fixation.

Pins constructed from cortical bone may provide a reasonable alternative to other fracture-fixation devices by circumventing some of the complications associated with stainless steel and synthetic biodegradable implants. However, it is unknown whether cortical bone pins provide comparable strength compared to conventional pins. Using four-point bending, we compared the mechanical characteristics of 1.2-mm allogeneic cortical bone pins milled from specific regions of human tibiae and femora to commercially available 1.1-mm diameter stainless steel pins and 1.3-mm diameter polydioxanone pins. We used impact testing to identify mechanical differences in cortical bone pins between gender and harvest site. Cortical bone pins had better mechanical properties in four-point bending compared with polydioxanone pins, but not stainless steel pins. Pins milled from the right tibiae of males had the best bending characteristics. The mechanical performance of 1.2-mm cortical bone pins was comparable to those of stainless steel and polydioxanone pins regardless of site, bone, and gender. The clinical investigation of cortical bone pins as an implant for fracture fixation is warranted based on mechanical testing and comparison to commercially available polydioxanone and stainless steel pins.

Chapter 18 – UHMWPE/Hyaluronan Microcomposite Biomaterials

Publisher Summary This chapter begins with a review of other ultra-high molecular weight polyethylene (UHMWPE) orthopedic implant surface modifications, the properties of hyaluronan (HA), and the medical applications of HA. The synthesis and processing of UHMWPE/HA biomaterials is then described, followed by the chemical, physical, mechanical, and tribological characterization of the UHMWPE/HA biomaterials. The sterilization, biocompatibility, and commercialization of the UHMWPE/HA biomaterials are also covered in this chapter. UHMWPE articulating against a metallic component remains the gold standard for joint replacements. Researchers continue to develop modified and improved UHMWPE materials. The vast differences between UHMWPE total joint replacement materials and articular cartilage motivated the development of the UHMWPE/HA biomaterials. UHMWPE is a hydrophobic, “waxy” synthetic polymer, while articular cartilage is a hydrophilic material containing chondrocytes (cells) and an extracellular matrix (ECM) that is comprised of macromolecules, proteoglycans, type II collagen fibers, and water. The various bulk material and surface structural modifications that are used to enhance the wear resistance of UHMWPE do not take into consideration the surface chemistry of UHMWPE. Current total joint replacements operate in the mixed or boundary lubrication regimes, relying on the inherent low friction and wear properties of UHMWPE. UHMWPE implants do not enjoy the lower friction and wear of natural joints because the surface chemistry of UHMWPE is very different from that of articular cartilage. The extreme hydrophobicity of UHMWPE limits the ability of synovial fluid constituents to interact with and lubricate the implant surface.