C. Mauli Agrawal

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.

Biodegradable polymeric scaffolds for musculoskeletal tissue engineering

Biodegradable scaffolds have played an important role in a number of tissue engineering attempts over the past decade. The goal of this review article is to provide a brief overview of some of the important issues related to scaffolds fabricated from synthetic biodegradable polymers. Various types of such materials are available; some are commercialized and others are still in the laboratories. The properties of the most common of these polymers are discussed here. A variety of fabrication techniques were developed to fashion polymeric materials into porous scaffolds, and a selection of these is presented. The very important issue of scaffold architecture, including the topic of porosity and permeability, is discussed. Other areas such as cell growth on scaffolds, surface modification, scaffold mechanics, and the release of growths factors are also reviewed. A summary outlining the common themes in scaffold-related science that are found in the literature is presented.

Age-related changes in the collagen network and toughness of bone

The hypothesis of this study is that the mechanical integrity of the collagen network in bone deteriorates with age, and such adverse changes correlate with the decreased toughness of aged bone. To test the hypothesis, 30 human cadaveric femurs from donors ranging from 19 to 89 years of age were tested to determine the age-related changes in the mechanical properties of demineralized bone and fresh bone samples. Along with bone porosity, bone density, and weight fractions of the mineral and organic phases, collagen denaturation and concentrations of collagen cross-links (HP, hydroxylysylpyridinoline; LP, lysylpyridinoline; PE, pentosidine) were determined for these bone specimens as a function age. Analysis of variance (ANOVA) showed that age-dependent changes were reflected in the decreased strength, work to fracture, and fracture toughness of bone; in the decreased strength, elastic modulus, and work to fracture of the collagen network; as well as in the increased concentration of pentosidine (a marker of nonenzymatic glycation) and increased bone porosity. Regression analyses of the measured parameters showed that the age-related decrease in work to fracture of bone (especially its postyield portion) correlated significantly with deterioration in the mechanical integrity of the collagen network. The results of this study indicate that the adverse changes in the collagen network occur as people age and such changes may lead to the decreased toughness of bone. Also, the results suggest that nonenzymatic glycation may be an important contributing factor causing changes in collagen and, consequently, leading to the age-related deterioration of bone quality.

Technique to control pH in vicinity of biodegrading PLA‐PGA implants

This in vitro study was performed to examine if the pH decrease in the vicinity of degrading polylactic acid (PLA) and polyglycolic acid (PGA) polymers can be offset by incorporation of basic salts within PLA-PGA implants. It has been suggested that such pH lowering results in adverse effects, which may be responsible for biocompatibility concerns raised recently about PLA and PGA polymers. The results indicated that all three salts investigated in this study were successful in controlling the decrease in pH due to the acidic degradation products of the copolymer. The pH of the test media for the control group fell to a value of 3.0 at 9 weeks. Implants containing calcium carbonate maintained the pH value between 7.4 and 6.3 throughout the degradation process. Implants with calcium hydroxyapatite and sodium bicarbonate controlled the pH values between 6.9 and 4.3 and 8.2 and 4.5, respectively. At 3 weeks, marked swelling of implants containing calcium carbonate or sodium bicarbonate was observed relative to the control implants. The molecular weight and mass changes in the implants did not show any significant differences at 9 weeks. Thus, results from this in vitro model show that a significant decrease in pH in the vicinity of PLA-PGA implants can be avoided by incorporating basic salts.

Diffusion in musculoskeletal tissue engineering scaffolds: Design issues related to porosity, permeability, architecture, and nutrient mixing

The field of tissue engineering continues to advance with the discovery of new biomaterials, growth factors and scaffold fabrication techniques. However, for the ultimate success of a tissue engineered construct the issue of nutrient transport to the scaffold interior needs to be addressed. Often, the requirements for adequate nutrient supply are at odds with other scaffold design parameters such as mechanical properties as well as scaffold fabrication techniques, leading to incongruities in finding optimal solutions. The goal of this review article is to provide an overview of the various engineering design factors that promote movement of nutrients, waste and other biomolecules in scaffolds for musculoskeletal tissue engineering applications. The importance of diffusion in scaffolds and how it is influenced by porosity, permeability, architecture, and nutrient mixing has been emphasized. Methods for measuring porosity and permeability have also been outlined. The different types of biomaterials used, scaffold fabrication techniques implemented and the pore sizes/porosities obtained over the past 5 years have also been addressed.

The role of collagen in determining bone mechanical properties

The hypothesis of this study was that collagen denaturation would lead to a significant decrease in the toughness of bone, but has little effect on the stiffness of bone. Using a heating model, effects of collagen denaturation on the biomechanical properties of human cadaveric bone were examined. Prior to testing, bone specimens were heat treated at varied temperatures (37–200°C) to induce different degrees of collagen denaturation. Collagen denaturation and mechanical properties of bone were determined using a selective digestion technique and three‐point bending tests, respectively. The densities and weight fractions of the mineral and organic phases in bone also were determined. A repeated measures analysis of variance showed that heating had a significant effect on the biomechanical integrity of bone, corresponding to the degree of collagen denaturation. The results of this study indicate that the toughness and strength of bone decreases significantly with increasing collagen denaturation, whereas the elastic modulus of bone is almost constant irrespective of collagen denaturation. These results suggest that the collagen network plays an important role in the toughness of bone, but has little effect on the stiffness of bone, thereby supporting the hypothesis of this study.

Preventing diabetic foot ulcer recurrence in high-risk patients : Use of temperature monitoring as a self-assessment tool

OBJECTIVE—The purpose of this study was to evaluate the effectiveness of a temperature monitoring instrument to reduce the incidence of foot ulcers in individuals with diabetes who have a high risk for lower extremity complications. RESEARCH DESIGN AND METHODS—In this physician-blinded, randomized, 15-month, multicenter trial, 173 subjects with a previous history of diabetic foot ulceration were assigned to standard therapy, structured foot examination, or enhanced therapy groups. Each group received therapeutic footwear, diabetic foot education, and regular foot care. Subjects in the structured foot examination group performed a structured foot inspection daily and recorded their findings in a logbook. If standard therapy or structured foot examinations identified any foot abnormalities, subjects were instructed to contact the study nurse immediately. Subjects in the enhanced therapy group used an infrared skin thermometer to measure temperatures on six foot sites each day. Temperature differences >4°F (>2.2°C) between left and right corresponding sites triggered patients to contact the study nurse and reduce activity until temperatures normalized. RESULTS—The enhanced therapy group had fewer foot ulcers than the standard therapy and structured foot examination groups (enhanced therapy 8.5 vs. standard therapy 29.3%, P = 0.0046 and enhanced therapy vs. structured foot examination 30.4%, P = 0.0029). Patients in the standard therapy and structured foot examination groups were 4.37 and 4.71 times more likely to develop ulcers than patients in the enhanced therapy group. CONCLUSIONS—Infrared temperature home monitoring, in serving as an “early warning sign,” appears to be a simple and useful adjunct in the prevention of diabetic foot ulcerations.

Stability of self-assembled monolayers on titanium and gold.

Methyl- and hydroxyl-terminated phosphonic acid self-assembled monolayers (SAMs) were coated on Ti from aqueous solution. Dodecyl phosphate and dodecyltrichlorosilane SAMs were also coated on Ti using solution-phase deposition. The stability of SAMs on Ti was investigated in Tris-buffered saline (TBS) at 37 degrees C using X-ray photoelectron spectroscopy, contact angle goniometry, and atomic force microscopy. For comparison purposes, a hydroxyl-terminated thiol SAM was coated on Au, and its stability was also investigated under similar conditions. In TBS, a significant proportion of phosphonic acid or phosphate molecules were desorbed from the Ti surface within 1 day, while the trichlorosilane SAM on Ti or thiol SAM on Au was stable for up to 7 days under similar conditions. The stability of hydroxyl-terminated phosphonic acid SAM coated Ti and thiol SAM coated Au was investigated in ambient air and ultraviolet (UV) light. In ambient air, the phosphonic acid SAM on Ti was stable for up to 14 days, while the thiol SAM on Au was not stable for 1 day. Under UV-radiation exposure, the alkyl chains of the phosphonic acid SAM were decomposed, leaving only the phosphonate groups on the Ti surface after 12 h. Under similar conditions, decomposition of alkyl chains of the thiol SAM was observed on the Au surface accompanied by oxidation of thiolates.

Isolation and Characterization of Polyethylene Wear Debris Associated with Osteolysis Following Total Shoulder Arthroplasty

We evaluated the interface membranes surrounding three total shoulder prostheses that had been removed because of progressive aseptic loosening associated with osteolysis. The mean time between the uncomplicated initial arthroplasty and the revision procedure was twelve years (10.5, 10.5, and 16.0 years). Membranes from around both the humeral and the glenoid component were obtained from all three shoulders and were studied histologically to determine the biological response involved in the development of aseptic loosening. For the purpose of comparison, periprosthetic tissue was also obtained from the sites of four failed total hip prostheses that were associated with osteolysis. Polyethylene particles were retrieved with an enzymatic digestion technique that involved the use of papain. Raman vibrational spectroscopy verified that the particles were ultra-high molecular weight polyethylene. The particles were isolated from the tissue, and a computerized image-analysis system characterized 582 of them in terms of size and morphology. Each particle was defined with the use of six shape descriptors: equivalent circle diameter, roundness, form factor, aspect ratio, elongation, and outline fractal dimension. The particles from the hips had a mean equivalent circle diameter (and standard error of the mean) of 0.62 +/- 0.03 micrometer, were predominantly globular in shape, and had low mean values for aspect ratio (1.46 +/- 0.02) and elongation (1.85 +/- 0.03) and relatively high values for roundness (0.74 +/- 0.01) and form factor (0.87 +/- 0.01). In contrast, the particles from the shoulders had a mean equivalent circle diameter of 1.04 +/- 0.03 micrometers. In addition, they had relatively high values for aspect ratio (2.36 +/- 0.07) and elongation (4.96 +/- 0.23) and correspondingly low values for roundness (0.54 +/- 0.01) and form factor (0.67 +/- 0.01), indicating that they were more fibrillar in shape. The particles from the shoulders and those from the hips were significantly different (p < 0.0001) with respect to all of the descriptors except outline fractal dimension. The particles from the shoulders, in general, were larger and more fibrillar than the particles from the hips.

Drug delivery from gold and titanium surfaces using self-assembled monolayers.

Currently available drug-eluting stents (DES) use polymers for coating and releasing drugs. Increasing evidence suggests that inflammatory and hypersensitive reactions are caused by such polymer coatings. This study focused on developing new techniques for delivering drugs directly from metal implant surfaces. Hydroxyl-terminated self-assembled monolayers (SAMs) were coated on Au and Ti surfaces. Therapeutic self-assembled monolayers (TSAMs) were prepared by chemically attaching the model drug, flufenamic acid, to SAM coated metal surfaces. Three different methods of esterification (acid chloride esterification, dry heat esterification, and direct esterification) were explored to attach flufenamic acid to SAMs. TSAMs were characterized using X-ray photoelectron spectroscopy, fluorescence microscopy, atomic force microscopy, and contact angle goniometry. These techniques collectively confirmed the attachment of drug onto SAM coated metal surfaces. In vitro drug release was investigated by immersing TSAM coated metal specimens in tris-buffered saline (TBS) at 37 degrees C for 28 days. TBS was analyzed at 1, 3, 7, 14, 21, and 28 days for the amount of drug eluted using high performance liquid chromatography. Large data scatter was observed for the release profiles of TSAMs prepared by acid chloride esterification. TSAMs prepared by dry heat and direct esterification methods showed an initial burst release of the drug followed by a sustained slow release for up to 2 weeks. Thus, this study suggests the potential for using self-assembled monolayers as an alternate system for delivering drugs from coronary stents and other metal implants.