The Use of Silver-Coated Orthopaedic Implants

Are All Silvers the Same?

Guy V. Morris, BSc, MBChB, FRCS (Tr & Orth); Jakub Kozdryk, MBChB, FRCS (Tr & Orth); Jonathan Gregory, FRCS BSc, MBChB, (Tr & Orth); Lee Jeys, MBChB, MSc (Orth Engin), FRCS (Tr & Orth)

Disclosures

Curr Orthop Pract. 2017;28(6):532-536. 

In This Article

Methods of Coating

Implants can be modified with silver in a number of ways. The two most common in orthopaedic practice are anodization and galvanic electroplating. Other methods are magnetron sputtering and impregnation for nonmetallic implants.[41–44]

Anodization, as used in the Agluna® (Accentus Medical, Oxfordshire, UK) coating process, involves the absorption of silver from an aqueous solution onto a titanium implant. This high voltage anodization forms an oxide layer that is integral with the metal itself. This is followed by silver ion exchange through immersion in an aqueous solution.[45] Scanning electron microscopy shows silver reservoirs in pits on an implant of 5 μm in diameter, which contain the bulk of the silver aggregate with a circumferential surface anodization (Figure 2). Because this process blends the silver as positively charged ions into the titanium itself, it is deemed a surface modification and not a coating. The surface appearance of the oxide layer is a shiny violet and appears to enhance the durability in lab tests. The anodization process results in a deposition of 6 mg of silver, which is 300 times lower than the No Observed Adverse Effect Level.[46,47] This is thought to result in delayed gradual dispersal of the silver coating into body tissues, leaving a titanium implant with all the normal material properties.

Figure 2.

Scanning electron micrographs of silver and nonsilver-coated titanium by Agluna® process. (A) No treatment. (B) Agluna® treated; circular pits of silver are seen at the surface.

Galvanic electroplating, such as the Modular Universal Tumour and Revision System (MUTARS® system, Implantcast GmBH, Buxtehude, Germany) involves the deposit of a large amount of high-percentage purity silver onto a titanium-vandium prosthesis in a 10–15 mm layer.[48] This is then coated again with an additional 0.2-mm layer of gold to ensure a sustained release of silver ions.

The drop in pH in the surrounding tissues that occurs during a bacterial infection results in increased silver ion release.[39,40] This can be observed clinically, giving the implant a tarnished or darkened surface (Figure 3), which allows surgeons to determine at revision whether an infection has been present in the past and raises the suspicion of ongoing infection. The developers of this implant noted the importance of avoiding hematoma and seroma formation because silver ions can bind to proteins and become inactivated.[49] They also stated that infections may only become clinically apparent later compared with standard implants because the silver can mask the initial process. As positively bonded silver ions dissolve into a negatively charged surrounding infected solution, the silver coating on the surface will eventually deplete. The authors concluded that silver coating only works close to the implant surface and not in the surrounding soft tissues that are not close to the implant. The rationale behind this process is that is covers the time period for acute infections that occur while the wounds are healing. This property also prevents the formation of the biofilm on the implant.

Figure 3.

An Implantcast® silver-coated distal femur. Areas of silver coating remain and localized elution of silver gives rise to areas of tarnishing, suggesting previous localized infection or contamination.

Magnetron sputtering is another technique for creating a dense silver film coating with increased adhesion. Sputtering is physical vapor deposition (PVD) coating technology that uses a strong magnetic or electrical field. During this process, particles are ejected from a solid target material by energetic bombardment of its surface layers by ions or neutral particles in a vacuum.[44] The PVD process results in good adhesiveness and wear resistance of metallic or ceramic coatings and is widely used in technical and medical applications.[50] Ewald et al.[39] achieved titanium/silver hard coatings through PVD with significant antimicrobial potency with no cytotoxic effects on osteoblasts or epithelial cells. The coatings released sufficient silver ions (0.5–2.3 ppb) for antimicrobial potency against Staphylococcus epidermis and Klebsiella pneumoniae strains.

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