Abstract
Recently, a special type of surface pitting found on metal implants was proposed to arise from “inflammatory cell-induced” corrosion (ICI, Figure 1) (1, 2). The actual mechanism of this was unknown, but similar features were suggested to be artefacts of electrocautery damage from revision surgery (3). Under lab conditions and without the influence of any cells, we aimed to reproduce the same surface pits and structures with electrocautery.
Methods
A polished cobalt-chromium disk (40 mm diameter, 8 mm thick) was marked into 8 sections for various testing conditions (Figure 2a). A stainless steel Bovie tip with a unipolar electrocautery machine (SYSTEM 5000, ConMed, USA) was used at typical surgical coagulation conditions: (70 volt, 120 watts, 562 KHz frequency). We mimicked three types of surgical techniques with the electrocautery: “Dotting” was repeated, on and off, direct surface contact; “Dragging” was constant, direct surface contact; “Hovering” was pausing several millimeters above the surface. We also examined the interplay of these practices on diamond-tip-induced scratches and either dry or wet (normal saline) conditions. High magnification images (Keyence VHX-2000E) were taken after the disk was cleaned with laboratory soap, light mechanical scrubbing, and formalin soak.
Results
Coagulation mode generated electrical sparks when dotting/dragging and electrical arcs when hovering. These left seared marks that persisted even after cleaning (Figure 2b). At higher magnification, the surface features were comparable in size and shape to those attributed to ICI (1, 2). Areas wet with saline (Figure 3a) showed an abundance of ringed pits with raised edges that closely resembled those observed in Figure 1. Furthermore we obtained images similar to the phenomenon of “cellular tracks” (Figure 3b) (1). Premade scratches did not influence the pit arrangement but scratches made by the Bovie tip produced the characteristic scratch-associated ICI features as observed on implant retrievals in the past (Figure 3c) (4).
Discussion
In the absence of cells, pitting equivalent to proposed ICI features was successfully replicated using an electrocautery in coagulation mode. Previously (4), we found a high incidence but small surface area of these features on the majority of retrievals, predominantly located in a focal area of the superior aspect of the femoral ball next to the junction of the stem. There were fewer on the inferior aspect which is consistent with electocautery damage when dissecting the hip capsule. The effect of this damage on retained parts is unknown, but electrocautery damage around areas of implant fractures has been reported (3).
Conclusion
The striking similarities of the recreated pit structures imaged here suggest that the noted features of “inflammatory cell induced corrosion” were artefacts of the electrocautery during revision surgery. Future implant retrieval analysis should acknowledge these structures are not related to any particular mode of failure but should check for them around implant fracture sites.
