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Hydrolytic degradation behavior of biodegradable polyetheresteramide-based polyurethane copolymers
Liu, C., Y. Gu, et al. (2005), J Biomed Mater Res A 75(2): 465-71.
Abstract: In this article, a new kind of biodegradable polyetheresteramide-based polyurethane (PEEA-U) copolymers were prepared by the melt polycondensation method from epsilon-caprolactone, 6-aminocaproic acid, poly(ethylene glycol), and toluene diisocyanate. The water absorption of PEEA-U was affected strongly by the reaction time and the content of chain extender; and the hydrolytic degradation behavior of the copolymers was mainly determined by the reaction time, chain extender content, and the pH value of the degradation medium. Also, DSC, (1)H-NMR, and inherent viscosity were used to characterize the degradation behavior of the copolymers.

Hydrolytic degradation of tyrosine-derived polycarbonates, a class of new biomaterials. Part I: study of model compounds
Tangpasuthadol, V., S. M. Pendharkar, et al. (2000), Biomaterials 21(23): 2371-8.
Abstract: Tyrosine-derived polycarbonates have been identified as promising, degradable polymers for use in orthopedic applications. These polymers are non-toxic, biocompatible, and exhibit good bone apposition when in contact with hard tissue. Tyrosine-derived polycarbonates were designed to incorporate two hydrolytically labile bonds in each repeat unit, a carbonate bond that connects the monomer units and an ester bond connecting a pendent chain. The relative hydrolysis rate of the two bonds will determine the type of degradation products and the degradation pathway of the polymers. In order to study the degradation mechanism of these polycarbonates in more detail, a series of small model compounds were designed that mimic the repeat unit of the polymer. Results obtained from the use of these model compounds suggested that the backbone carbonate bond is hydrolyzed at a faster rate than the pendent chain ester bond. Increasing the length of the alkyl pendent chain lowered the hydrolysis rates of both hydrolyzable linkages, possibly by hindering the access of water molecules to those sites. The hydrolysis rates of both linkages were pH dependent with the lowest rate at pH about 5. The results from this study can be used to explain the degradation behavior of the corresponding polycarbonates as well as their degradation mechanisms. This information is essential when evaluating the utility of tyrosine-derived polycarbonates as degradable medical implant materials.

Hydrolytic degradation of tyrosine-derived polycarbonates, a class of new biomaterials. Part II: 3-yr study of polymeric devices
Tangpasuthadol, V., S. M. Pendharkar, et al. (2000), Biomaterials 21(23): 2379-87.
Abstract: The kinetics and mechanisms of in vitro degradation of tyrosine-derived polycarbonates, a new class of polymeric biomaterials, were studied extensively at 37 degrees C. These polymers carry an alkyl ester pendent chain that allows the fine-tuning of the polymer's material properties, its biological interactions with cells and tissue, and its degradation behavior. The polymer carrying an ethyl ester pendent chain, poly(DTE carbonate), has been established as a promising orthopedic implant material, exhibiting bone apposition when in contact with hard tissue. Tyrosine-derived polycarbonates are relatively stable and degrade only very slowly in vitro. Therefore, accelerated studies were conducted at 50 and 65 degrees C to observe the behavior of polymers during the later stages of degradation. Varying the pendent chain length affected the rate of water uptake, initial degradation rate, and physical stability of the polymeric devices. During the 3-yr study, the polymer degraded by random chain cleavage of the carbonate bonds, accompanied by a relatively small amount of pendent chain de-esterification. No mass loss was observed during this period at 37 degrees C, but mass loss was readily evident during the accelerated studies at 50 and 65 degrees C. Thus, it is reasonable to assume that mass loss will occur also at 37 degrees C, albeit only after extensive backbone carbonate cleavage and pendent chain ester hydrolysis. The dimension and surface area of the devices influenced the initial degradation rate, but did not significantly affect the overall rate of degradation. No evidence of "acid dumping" or the release of acidic residues found during the degradation of poly(D,L-lactic acid) were observed for this family of tyrosine-derived polycarbonates.

Hydrophilic acrylic biomaterials derived from vitamin E with antioxidant properties
Ortiz, C., B. Vazquez, et al. (1999), J Biomed Mater Res 45(3): 184-91.
Abstract: Hydrogels based on polymeric derivatives of vitamin E for biomedical purposes have been prepared by copolymerization reaction of the alpha-tocopheryl methacrylate (V) with 2-hydroxyethyl methacrylate (H) in a range of composition between 5-20 wt % of V. The swelling behavior of the hydrogels in water, alkaline, and acidic media showed a slight decrease in the equilibrium water content with the content of V in the copolymer although in all cases it was superior to an EWC > 20%. The diffusion mechanism followed a Fickian behavior in all media. The values of the diffusion coefficients were in the range 2.5-1.6 10(-7) cm2/s. The states of water in the hydrogels were determined by DSC. A decrease in the content of freezing water was obtained with the V content for all media, and for all compositions lower values of freezing water were obtained in acidic or basic pHs than in distilled water. The copolymeric xerogels, analyzed by contact angle measurements, deviated from those expected taking into consideration those of the homopolymers and the average fraction of the monomers in the copolymer. The polar contribution dropped with the introduction of a small content (4 wt %) of the vitamin E-containing monomer, and it reached a value similar to that of poly-V for a composition of 49 wt % of V in the copolymer. This behavior is accounted for by the segregation of the macromolecular chains of both kinds of monomers, due mainly to differences in their polarity, molecular weights, and the reactivity of both monomers. Finally, thermogravimetric analysis showed a higher thermal (antioxidant) stability of the poly-V with respect to poly-H, giving rise to a residue of 18 wt %. The V-containing copolymers also showed an improved stability (antioxidant behavior), indicating the possibility of the V unit's interfering with the oxidative process, based on free-radical species, and, therefore, with the aging process at the cellular level.

Hydrophilicity of 3-D biomaterials: the Washburn equation
Jackson, P. V., J. A. Hunt, et al. (2004), J Mater Sci Mater Med 15(4): 507-11.
Abstract: Characterisation and quantification of the surface energy of biomaterials used as tissue engineering scaffolds is important, but many of the techniques available to examine these properties are only applicable to smooth flat samples, not porous materials. This paper describes the application of the Washburn equation to measure the surface energy of a range of porous polyether polyurethane scaffolds with three test liquids; n-Hexane was used to measure a material constant, whilst ethanol and xylene were used to measure contact angles. The results show that the Washburn equation is not applicable in its current form, reasons for this could be that the voids in the materials are too wide for effective capillarity; absorption of the solvents into the polymer matrix may further complicate the measured imbibition profile. Another possible reason is the differences between the sizes of the interconnecting pores in scaffolds with varying void sizes; this could affect the capillary effect of the test liquids through the material. The repeatability of the results and the similar patterns observed with the different liquids suggest that if these issues could be quantified and incorporated into the Washburn equation, it may be possible to generate useful results for similar materials.

Hydrophilized poly(lactide-co-glycolide) nanospheres with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer
Kim, B. K., D. Kim, et al. (2004), J Microencapsul 21(7): 697-707.
Abstract: A novel method for preparing the PLGA nanospheres with hydrophilic surface has been designed and characterized. Because of good solubility of tetraglycol in water, PLGA (poly(lactide-co-glycolide)) nanospheres were formed by spraying the PLGA/tetraglycol solution into water. The size of PLGA nanospheres was manipulated by changing the concentration of PLGA/tetraglycol solution. Based on the hydrophobic interaction between PLGA and poly(propylene oxide) domain of F-127 (one of Pluronics, poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) triblock copolymer, F-127-coated PLGA nanospheres was prepared to enhance the stability of PLGA nanospheres in the aqueous media. For the application as a drug delivery vehicle, it was characterized by measuring the loading amount, the encapsulation efficiency and the release pattern of drug. Paclitaxel used as a potent anti-cancer drug was selected as a model drug.

Hydrophobic Bronsted acid-base ionic liquids based on PAMAM dendrimers with high proton conductivity and blue photoluminescence
Huang, J. F., H. Luo, et al. (2005), J Am Chem Soc 127(37): 12784-5.

Hydrophobic drug delivery by self-assembling triblock copolymer-derived nanospheres
Sheihet, L., R. A. Dubin, et al. (2005), Biomacromolecules 6(5): 2726-31.
Abstract: We describe the synthesis and characterization of a family of biocompatible ABA-triblock copolymers that comprised of hydrophilic A-blocks of poly(ethylene glycol) and hydrophobic B-blocks of oligomers of suberic acid and desaminotyrosyl-tyrosine esters. The triblock copolymers spontaneously self-assemble in aqueous solution into nanospheres, with hydrodynamic diameters between 40 and 70 nm, that do not dissociate under chromatographic and ultracentrifugation conditions. These nanospheres form strong complexes with hydrophobic molecules, including the fluorescent dye 5-dodecanoylaminofluorescein (DAF) and the antitumor drug, paclitaxel, but not with hydrophilic molecules such as fluorescein and Oregon Green. The nanosphere-paclitaxel complexes retain in vitro the high antiproliferative activity of paclitaxel, demonstrating that these nanospheres may be useful for delivery of the hydrophobic drugs.

Hydrophobic interaction and hydrogen bonding cooperatively confer a vancomycin hydrogel: a potential candidate for biomaterials
Xing, B., C. W. Yu, et al. (2002), J Am Chem Soc 124(50): 14846-7.

Hydroxy apatite microspheres enhance gap junctional intercellular communication of human osteoblasts composed of connexin 43 and 45
Nakaoka, R., S. Ahmed, et al. (2005), J Biomed Mater Res A 74(2): 181-6.
Abstract: The aseptic loosening of artificial joints with associated periprosthetic bone resorption may be partly due to the suppression of osteoblast function to form new bone by wear debris from the joint. To assess the effect of wear debris on osteoblasts, effects of model wear debris on gap junctional intercellular communication (GJIC) of normal human osteoblasts were estimated. The GJIC activity of the osteoblasts after a 1-day incubation with the microspheres was similar to that of normal osteoblasts. However, hydroxy apatite particles, which have been reported to enhance the differentiation of osteoblasts in contact with them, enhanced the GJIC function of the osteoblasts. From RT-PCR studies, not only connexin 43 but also connexin 45 is suggested to play a role in the GJIC of the osteoblasts in an early stage of coculture with the microspheres, although it is still unclear how these connexins work and are regulated in the GJIC and differentiation. However, this study suggests that there is a relationship between the early levels of GJIC and the differentiation of the cells. Therefore, estimating the effect of biomaterials, even in the microsphere form, on the GJIC of model cells, with which the biomaterials may be in contact in vivo, can provide important information about their biocompatibility.

Hydroxyapatite as a biomaterial for alveolar ridge augmentation
Frame, J. W. (1987), Int J Oral Maxillofac Surg 16(6): 642-55.
Abstract: Hydroxyapatite is a useful biomaterial because of its excellent biocompatibility. It is extremely well tolerated by the hard and soft tissues of the mouth and jaws, and offers great potential for the future. This paper reviews the various physical forms of the material, both solid and porous, its biological behaviour in different implant sites, and the surgical techniques for its implantation. Some of the controversies and doubts about the material are discussed in relation to its use in alveolar ridge augmentation.

Hydroxyapatite biomaterial implanted in human periodontal defects: an histological and ultrastructural study
Orly, I., B. Kerebel, et al. (1989), Bull Group Int Rech Sci Stomatol Odontol 32(2): 79-86.
Abstract: The purpose of the present work was to study the response of human periodontium to hydroxyapatite biomaterial particles (180-200 microns). The biomaterial was implanted in two infra-osseous periodontal defects (two patients) after clearing of the granulation tissue. At two months post-surgery, biopsies were studied using light and electron microscopy. No sign of inflammation was observed, the biomaterial aggregates were surrounded either by typical fibroblasts or larger phagocytotic cells with phagocytosis vesicles containing biomaterial crystals. These intracellular crystals were noticeably smaller than the non-phagocytized ones. Some of the phagocytized crystals showed morphological signs of intracellular dissolution. The spaces between the crystals constitutive of the aggregates were filled with organic substance containing collagen fibers.

Hydroxyapatite Cement, a Smart Biomaterial for Craniofacial Skeletal Tissue Engineering
Friedman, C. D. and P. D. Costantino (1998), Surg Technol Int VII: 421-423.

Hydroxyapatite crystals and rotator cuff disorders: comment on the article by Gomoll et al
O'Shea, F. D. and G. M. McCarthy (2005), Arthritis Rheum 52(11): 3681; author reply 3681-3682.

Hydroxyapatite promotes superior keratocyte adhesion and proliferation in comparison with current keratoprosthesis skirt materials
Mehta, J. S., C. E. Futter, et al. (2005), Br J Ophthalmol 89(10): 1356-62.
Abstract: AIM: Published clinical series suggest the osteoodontokeratoprosthesis (OOKP) may have a lower extrusion rate than current synthetic keratoprostheses. The OOKP is anchored in the eye wall by autologous tooth. The authors' aim was to compare adhesion, proliferation, and morphology for telomerase transformed keratocytes seeded on calcium hydroxyapatite (the principal mineral constituent of tooth) and materials used in the anchoring elements of commercially available synthetic keratoprostheses. METHODS: Test materials were hydroxyapatite, polytetrafluoroethylene (PTFE), polyhydroxyethyl methacrylate (HEMA), and glass (control). Cell adhesion and viability were quantified at 4 hours, 24 hours, and 1 week using a calcein-AM/EthD-1 viability/cytotoxicity assay. Focal contact expression and cytoskeletal organisation were studied at 24 hours by confocal microscopy with immunoflourescent labelling. Further studies of cell morphology were performed using light and scanning electron microscopy. RESULTS: Live cell counts were significantly greater on hydroxyapatite surfaces at each time point (p<0.04). Dead cell counts were significantly higher for PTFE at 7 days (p<0.002). ss(1) integrin expression was highest on hydroxyapatite. Adhesion structures were well expressed in flat, spread out keratocytes on both HA and glass. Keratocytes tended to be thinner and spindle shaped on PTFE. The relatively few keratocytes visible on HEMA test surfaces were rounded and poorly adherent. CONCLUSIONS: Keratocyte adhesion, spreading, and viability on hydroxyapatite test surfaces is superior to that seen on PTFE and HEMA. Improving the initial cell adhesion environment in the skirt element of keratoprostheses may enhance tissue integration and reduce device failure rates.

Hydroxyapatite-ceramic-coated femoral stems in revision hip surgery
Trikha, S. P., S. Singh, et al. (2005), J Bone Joint Surg Br 87(8): 1055-60.
Abstract: We describe the clinical and radiological results of 120 consecutive revision hip replacements in 107 patients, using the JRI Furlong hydroxyapatite-ceramic-coated femoral component. The mean age of the patients at operation was 71 years (36 to 92) and the mean length of follow-up 8.0 years (5.0 to 12.4). We included patients on whom previous revision hip surgery had taken place. The patients were independently reviewed and scored using the Harris hip score, the Western Ontario and McMaster Universities osteoarthritis index (WOMAC) and the Charnley modification of the Merle d'Aubigne and Postel score. Radiographs were assessed by three reviewers for the formation of new bone, osteolysis, osseointegration and radiolucent lines in each Gruen zone.The mean Harris hip score was 85.8 (42 to 100) at the latest post-operative review. The mean WOMAC and Merle d'Aubigne and Postel scores were 34.5 and 14.8, respectively. The mean visual analogue score for pain (possible range 0 to 10) was 1.2 overall, but 0.5 specifically for mid-thigh pain. There were no revisions of the femoral component for aseptic loosening. There were four re-revisions, three for infection and one for recurrent dislocation. Radiological review of all the femoral components, including the four re-revisions showed stable bony ingrowth and no new radiolucent lines in any zone. Using revision or impending revision for aseptic loosening as an end-point, the cumulative survival of the femoral component at ten years was 100% (95% confidence interval 94 to 100). We present excellent medium- to long-term clinical, radiological and survivorship results with the fully hydroxyapatite-ceramic-coated femoral component in revision hip surgery.

Hydroxyapatite-coated external fixation pins
Moroni, A., F. Pegreffi, et al. (2005), Expert Rev Med Devices 2(4): 465-71.
Abstract: The aim of this review is to report on studies of hydroxyapatite-coated external fixation pins as a solution to enhance pin fixation. In a highly loaded animal study, three tapered pin types were compared: Type A uncoated, Type B coated with hydroxyapatite and Type C coated with titanium. There was a 13-fold increase in the extraction torque of Type B pins compared with Type A, and a twofold increase compared with Type C pins. Extraction torque was significantly lower compared with the corresponding insertion torque in both Types A (p < 0.001) and C (p = 0.003). Conversely, with the hydroxyapatite-coated pins there was no difference between extraction and insertion torque. In a clinical study of 76 external fixation pins in 19 patients treated with hemicallotasis for osteoarthritis on the medial side of the knee, pin insertion and extraction torque forces were measured. The patients were randomized to be treated with either standard tapered pins or tapered pins coated with hydroxyapatite. Extraction torque of the hydroxyapatite-coated pins was higher than the extraction torque of the standard pins in both cancellous and cortical bone (p < 0.005). In a prospective, randomized clinical study of osteoporotic wrist fractures, extraction torque of the coated pins was higher than with standard pins (p < 0.0001). These studies demonstrate that with the use of hydroxyapatite-coated pins, no deterioration of pin fixation occurs, and that there is no significant pin loosening and infection, regardless of bone type and loading conditions.

Hydroxyapatite-coated femoral implant in metal-on-metal resurfacing hip arthroplasty: minimum of two years follow-up
Lilikakis, A. K., S. L. Vowler, et al. (2005), Orthop Clin North Am 36(2): 215-22, ix.
Abstract: The authors report preliminary results of an uncemented, hydroxyapatite-coated femoral implant for metal-on-metal hip resurfacing. The survival rate of 70 implants after at least 2 years follow-up was 98.6%, with an excellent clinical outcome. There have been no femoral fractures, aseptic loosening, or radiolucencies around the stem. Thinning of the femoral neck at the inferomedial cup-neck rim has been a frequent radiologic finding but with no clinical implication so far. Longer follow-up is needed to confirm the results.

Hydroxyapatite-coated total knee arthroplasty in rheumatoid arthritis patients: a short-term follow-up study
Siddiqi, N., H. Sugiyama, et al. (2005), J Coll Physicians Surg Pak 15(6): 333-7.
Abstract: OBJECTIVE: To determine the outcome of cementless hydroxyapatite-coated total knee prosthesis in rheumatoid patients. DESIGN: Analytical. PLACE AND DURATION OF STUDY: University of Yamanashi Hospital, Japan, from October 1999 to September 2002. PATIENTS AND METHODS: Twenty total knee arthroplasties using HA-coated Scorpio Superflex CR and Delta 7000 series were performed in 17 rheumatoid patients. Pre and postoperative results were compared using JOA scores. RESULTS: Male: female ratio was 16:1, average age was 67 years and mean duration of follow-up was two years. JOA scores improved significantly (p<0.05) postoperatively from an average of 37 to 80. Varus and valgus angular deformities improved significantly and 90% of the knees had alignment between zero and 5 degrees of valgus angulation with an average tibiofemoral angle (FTA) of 172. The radiographic findings revealed no radiolucent lines at two years follow-up. Distal femur osteolysis was not observed. CONCLUSION: The clinical results were excellent at 2-year follow-up, which are in agreement with other reports using hydroxyapatite-coated knee prosthesis.

Hydroxyapatite-coating of pedicle screws improves resistance against pull-out force in the osteoporotic canine lumbar spine model: a pilot study
Hasegawa, T., A. Inufusa, et al. (2005), Spine J 5(3): 239-43.
Abstract: BACKGROUND CONTEXT: In patients with spinal osteoporosis, the early achievement and maintenance of a biological bond between the pedicle screw and bone is important to avoid screw loosening complications. There are few reports of in vivo investigations involving biomechanical and histological evaluations in the osteoporotic spine. PURPOSE: To evaluate the effect of hydroxyapatite (HA)-coating on the pedicle screw in the osteoporotic lumbar spine and to investigate the relationship between resistance against the screw pull-out force and bone mineral density (BMD) of the vertebral body. STUDY DESIGN/SETTING: Mechanical and pathological investigations in the lumbar spine. METHODS: Two 24-month-old female beagle dogs were fed a calcium-free dog chow for 6 months after ovariectomy (OVX). BMD (in g/cm2) was measured by dual energy X-ray absorptiometry at pre-OVX and 6 months after OVX. Pedicle screws were placed from L1 to L6 at 6 months after OVX. Twenty-four pure titanium cortical screws (Synthes, #401-114) were used as pedicle screws (Ti-PS). Of these, 12 screws had HA-coating (HA-PS). The HA-PS screws were inserted into the right pedicles and the Ti-PS were inserted into the left pedicles. Ten days after this procedure, the lumbar spines were removed en bloc for screw pull-out testing and histological evaluation. RESULTS: The mean BMD value of the lumbar vertebrae 6 months after the OVX was 0.549+/-0.087 g/cm2, which was significantly less than the pre-OVX mean BMD of 0.603+/-0.092 g/cm2 (p < 0.001). The mean resistance against the pull-out force for the HA-PS was significantly greater at 165.6+/-26.5N than in the Ti-PS (103.1+/-30.2N, p <.001). The histological sections in the HA-PS clearly revealed new bone bonding with the apatite coating but only fibrous tissue bonding in the Ti-PS. CONCLUSIONS: The results of this study showed that the resistance to the pull-out force of HA-PS is 1.6 times that of Ti-PS. Furthermore, HA-PS has superior biological bonding to the surrounding bone, as early as 10 days after surgery in this osteoporotic spine model. Thus, in patients with osteoporosis, coating of the pedicle screw with HA may provide better stability and bonding between the pedicle screw and bone in the early postoperative period.

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Last Modified: 8 February 2006