Silver Coatings in Reconstructive Orthopaedics: Basic Science and Clinical Rationale.

Prosthetic joint infection (PJI) is one of the most devastating complications that can occur following total hip and total knee arthroplasty. Despite the remarkable advances that have been made in surgical techniques and implant technology, the incidence of PJI has remained largely unchanged over the past two decades. One approach that has been described in the literature to minimize the risk of PJI has been the use of silver-coated prostheses. Silver has been reported to have antimicrobial properties when added to a variety of orthopaedic materials including bone cement, hydroxyapatite coatings and wound dressings. Silver is also being increasingly used as a surface coating for endoprostheses used for reconstruction around the hip and the knee with the specific aim of reducing the incidence of prosthetic joint infection. Despite the increasing adoption of this technology, the use of silver coatings remains controversial. The optimal method for preparation and the thickness of the coating, as well as the mechanism(s) of action in reducing the incidence of PJI, are unclear. The issue of silver toxicity is also an important consideration. This paper provides an overview of the use of silver coatings in reconstructive orthopaedics, as well as the types available and techniques used to coat endoprostheses. We also review the basic science as well as the clinical applications of silver coatings in the prevention of PJIs.

Pre-clinical characterization of a novel flexible surface stem design for total knee replacements.

Primary stability is crucial for implant osseointegration and the long-term stability of cementless total joint replacements. Biomechanical studies have shown the potential of femoral stems for total knee replacements to reduce micromotions at the bone-implant interface. However, approaches such as focusing on the structural elasticity of the femoral stems are rarely described. Three groups with different femoral stem designs were investigated: group 1: flexible surface stem, group 2: flexible surface stem with open-porous structured lamellas, and group 3: solid stem (reference). The stems were implanted into bone substitute material and dynamically loaded for 1000 cycles. Relative movement and subsidence were measured optically, and axial pull-out forces were determined after dynamic testing. Relative movements increased to 0.10 mm (groups 1 and 2) compared to 0.03 mm (group 3). Subsidence increased to 0.08 mm (group 1) and 0.11 mm (group 2) compared to 0.06 mm (group 3). For each group, subsidence mainly occurred during the first 500 cycles. A similar convergence was observed in the further course. Pull-out forces increased to 1815.0 N (group 1) and 1347.1 N (group 2) compared to 1306.4 N (group 3). The flexible surface stem design resulted in higher relative movements and subsidence, but also exhibited increased pull-out forces. The relative movements were below the critical limit of 0.15 mm and represent a superposition of the elastic deformations of the interacting implant components as well as the micromotion at the bone-implant interface. Therefore, the novel flexible surface stem design appears to offer promising primary implant fixation.

3D-printed porous Ti6Al4V alloys with silver coating combine osteocompatibility and antimicrobial properties.

Additive manufacturing allows for the production of porous metallic implants for use in orthopaedics, providing excellent mechanical stability and osseointegration. However, the increased surface area of such porous implants also renders them susceptible to bacterial colonization. In this work, two trabecular porous Ti6Al4V alloys produced by electron beam melting were investigated for their osteocompatibility and antimicrobial effects, comparing samples with a silver-coated surface to uncoated samples. Dense grit-blasted Ti samples were used for comparison. The porous samples had pore sizes of 500-600 µm and 5 to 10 µm surface roughness, the silver-coated samples contained 7 at.% Ag, resulting in a cumulative Ag release of 3.5 ppm up to 28 days. Silver reduced the adhesion of Staphylococcus aureus to porous samples and inhibited 72 h biofilm formation by Staphylococcus epidermidis but not that of S. aureus. Primary human osteoblast adhesion, proliferation and differentiation were not impaired in the presence of silver, and expression of osteogenic genes as well as production of mineralized matrix were similar on silver-coated and uncoated samples. Our findings indicate that silver coating of porous titanium implants can achieve antimicrobial effects without compromising osteocompatibility, but higher silver contents may be needed to yield a sustained protection against fast-growing bacteria.

Evaluation of topographical and chemical modified TiAl6V4 implant surfaces in a weight-bearing intramedullary femur model in rabbit.

For cementless total joint replacement implants, the biological response to physicochemical surface characteristics is crucial for their success that depends on fixation by newly formed bone. In this study, the surface of TiAl6V4 (Tilastan®) implants was modified by (a) corundum blasting, (b) corundum blasting followed by electrochemical calcium phosphate (CaP) deposition, and (c) titanium plasma spraying followed by electrochemical CaP deposition. All modifications resulted in a surface roughness suitable to enhance primary implant stabilization and to favor osteoblast adhesion and function; the thin, biomimetic CaP coating is characterized by fast resorbability and served as chemical cue to stimulate osteogenesis. After implantation in a full weight-bearing rabbit intramedullary distal femur model, osseointegration was investigated after 3, 6, and 12 weeks. For all modifications, new bone formation, occurring from the endosteum of the femoral cortical bone, was observed in direct contact to the implant surface after 3 weeks. At the later time points, maturation of the woven bone into lamellar bone with clearly defined osteons was visible; the remodeling process was accelerated by the CaP coating. The ingrowth of newly formed bone into the pores of the titanium plasma sprayed surfaces indicates a strong interlock and finally implant fixation. Our findings indicate a positive impact of the tested surface modifications on osseointegration.

New antibacterial agents derived from the DNA gyrase inhibitor cyclothialidine.

Cyclothialidine (1, Ro 09-1437) is a potent DNA gyrase inhibitor that was isolated from Streptomyces filipinensis NR0484 and is a member of a new family of natural products. It acts by competitively inhibiting the ATPase activity exerted by the B subunit of DNA gyrase but barely exhibits any growth inhibitory activity against intact bacterial cells, presumably due to insufficient permeation of the cytoplasmic membrane. To explore the antibacterial potential of 1, we developed a flexible synthetic route allowing for the systematic modification of its structure. From a first set of analogues, structure-activity relationships (SAR) were established for different substitution patterns, and the 14-hydroxylated, bicyclic core (X) of 1 seemed to be the structural prerequisite for DNA gyrase inhibitory activity. The variation of the lactone ring size, however, revealed that activity can be found among 11- to 16-membered lactones, and even seco-analogues were shown to maintain some enzyme inhibitory properties, thereby reducing the minimal structural requirements to a rather simple, hydroxylated benzyl sulfide (XI). On the basis of these "minimal structures" a modification program afforded a number of inhibitors that showed in vitro activity against Gram-positive bacteria. The best activities were displayed by 14-membered lactones, and representatives of this subclass exhibit excellent and broad in vitro antibacterial activity against Gram-positive pathogens, including Staphylococcus aureus, Streptococcus pyogenes, and Enterococcus faecalis, and overcome resistance against clinically used drugs. By improving the pharmacokinetic properties of the most active compounds (94, 97), in particular by lowering their lipophilic properties, we were able to identify congeners of cyclothialidine (1) that showed efficacy in vivo.

Choosing a cervical disc replacement.

BACKGROUND CONTENT: Three important basic scientific studies are presented that measured the volumetric density of longitudinal bony columns within the cervical vertebra. The most solid bone is lateral, adjacent to the uncovertebral joints in a radial pattern. PURPOSE: To characterize the best footprint, profile and biomaterials to construct a cervical disc replacement. STUDY DESIGN: A compilation of biomechanical and anatomical basic scientific studies. METHODS: Microcomputed tomographic imaging, trabecular density and mineral distribution were quantitated from human cervical vertebra. RESULTS: The lateral portions of the cervical vertebra are subjected to higher bending loads than the lumbar vertebral bodies. Therefore, the optimal prosthesis needs to be anchored in the lateral uncovertebral bone. To reduce the incidence of cervical subsidence, the prosthesis needs to be more rectangular than round to take advantage of the radially oriented lateral trabeculae. TiCaP (titanium/calcium phosphate) (Cervitech, Inc., Rockaway, NJ) bony ingrowth coating leads to 10% to 15% greater bony integration than plasma-sprayed titanium. TiCaP causes a supersaturated solution of CaP at the metal-bone interface, which enables reprecipitation of hydroxyapatite and superior bony integration. The optimum pore size of the ingrowth coating of the lumbar spine is 75 to 300 microns, whereas in the cervical spine the optimal ingrowth coating is 20 to 30 microns. This is an order of magnitude lower in pore size to match the smaller cervical trabecular architecture. CONCLUSIONS: Kinematic considerations for the cervical spine show the load is 1/9th the load carried by the lumbar spine or 50 N per segment. Knowing the sliding distance and wear characteristics of conventional biomaterials (ultrahigh molecular weight polyethylene and cobalt chrome) demonstrates that the generation of particulate debris should be a very minor consideration with cervical arthroplasty.

Clinical experience with the new artificial cervical PCM (Cervitech) disc.

The results of a pilot study performed between December 2002 and October 2003 in which 82 cervical disc arthroplasties were implanted in 53 patients are reviewed in detail. Visual Analog Scale (VAS) pain scale, Neck Disability Index (NDI), and Treatment Intensity Gradient Test (TIGT) scales were evaluated as were static and dynamic radiographs. Significant improvents in all scales were seen postoperatively. One device migration of 4 millimeters was seen at 3 months and observed. Eighty percent of patients had a good or excellent result at one week, improving to ninety percent of patients being judged to have a good or excellent result by one month (Odom's criteria), which then remained at ninety percent at 3 months.

History, design and biomechanics of the LINK SB Charité artificial disc.

The SB Charité I artificial disc was developed in 1982 by Schellnack and Büttner-Janz and modified as the Mark II version in 1984. Both types were manufactured in the former German Democratic Republic (GDR). Today's design, the SB Charité III, was first produced by LINK in 1987. Five sizes of the artificial disc in various angulations are available today, with a double coating of titanium/calciumphosphate. Designed with a three-component set-up, the SB Charité mimics the physiological segmental motion. The possibility of translation in the SB Charité provides proper biomechanical function and protects the zygapophysial joints. Results of biomechanical testing showed a sufficient cold-flow resistance of the UHMWPE (Ultra High Molecular Weight Polyethylene) sliding core and confirmed the negligible abrasion rate. The LINK SB Charité disc is a safe and effective operative treatment for discogenic low back pain. Long-term results (10 years and more) have been published.

Biomaterial optimization in total disc arthroplasty.

STUDY: Knowledge gained through the clinical history of total joint replacement materials combined with the current promise of new biomaterials provides improved guidelines for biomaterial selection in total disc arthroplasty. OBJECTIVES: The following will detail: 1) current biomaterials technology; 2) how current designs of total disc arthroplasty seek to optimize implant performance through judicious biomaterial selection; and 3) what technical obstacles and clinical concerns remain. METHODS: Metals and polymers remain the central material components of state-of-the-art total joint arthroplasties. Polymers provide low friction surfaces for articulating bearings and some degree of shock absorption. Metals provide appropriate material properties such as high strength, ductility, fracture toughness, hardness, corrosion resistance, formability, and biocompatibility necessary for use in load-bearing roles required total disc replacement. There are three principal metal alloys used in orthopaedics and particularly in total joint replacement: 1) titanium based alloys; 2) cobalt based alloys; and 3) stainless steel alloys. Alloy specific differences in strength, ductility, and hardness generally determine which of these three alloys is used for a particular application or implant component. RESULTS: Current designs. Two examples of current lumbar (Charitè and Prodisc) and cervical (Bryan and Prestige) disc replacements are compared. The similarities and differences in the biomaterials used for each demonstrate prevailing consensus and some idea of how to best optimize implant performance through biomaterial selection. CONCLUSION: The primary factors governing total disc arthroplasty biomaterials are similar to those of all total joint arthroplasties: generation of wear debris is the primary source of implant degradation, and the subsequent tissue reaction to such debris is the primary factor limiting the longevity of joint replacement prostheses. Particulate debris generated by wear, fretting, or fragmentation induces the formation of an inflammatory reaction, which at a certain point promotes a foreign-body granulation tissue response that has the ability to invade the bone-implant interface. This commonly results in progressive, local bone loss that threatens the fixation of both cemented and cementless devices alike. All metal alloy implants corrode in vivo. When severe, the degradative process may reduce structural integrity of the implant, and the release of corrosion products is potentially toxic to the host. The corrosion resistance of implant alloys is primarily due to the formation of passive oxide films to prevent significant electrochemical dissolution from taking place. The result of this knowledge is a consensus of opinion as to which materials are best suited for use in current total disc arthroplasty designs, where most total disc replacement designs incorporate cobalt-chromium-molybdenum alloy endplates articulating internally on a relatively soft polymeric core and externally coated with titanium or titanium alloy for enhanced bone fixation.

Cómo seleccionar basado en conceptos biomecánicos un disco artificial cervical / Selecting based on biomechanical concepts cervical artificial disc

La artroplastía cervical con reemplazo total de disco por una prótesis artificial dinámica es en la actualidad una importante opción de tratamiento de la enfermedad degenerativa de la columna cervical; permite conservar el movimiento, generando estabilidad y evitando degeneración del segmento adyacente. Son múltiples las opciones de discos artificiales disponibles actualmente en el mercado. Aplicamos los principales conceptos biomecánicos y estudio de materiales para analizar cinco de los más utilizados discos cervicales artificiales ( PCM®, Bryan Disc®, Cervidisc®, Prodisc C® y Prestige I ®); damos una puntuación y unas conclusiones para que el cirujano de columna tenga herramientas a la hora de escoger un disco artificial.