Benefits of additive manufacturing and micro and nano surface texture modifications on mechanical strength and infection resistance of skin–implant interfaces in rats

Abstract

Patients find existing exo-prostheses utilizing traditional socket connections uncomfortable and irritating, as they do not provide the dexterity and proprioception which is often expected by active patients. Transcutaneous osseointegrated implants are a potential solution, but carry risk of infection at the skin–implant interface. Histological and observational studies previously demonstrated that textured implants have both improved epidermal ingrowth and decreased skin retraction. This study aimed to determine effects on mechanical integration, barrier to bacterial colonization, and infection of the skin–implant interface using additive manufacturing and post-manufactured surface modifications. In this study, titanium alloy implants were made by either computer numerical control machining (CNC) or electron beam melting (EBM). Implants in each group were left either unaltered (CNC-control and EBM-control), acid etched (CNC-micro and EBM-micro), or oxidatively treated (CNC-nano and EBM-nano) creating six distinct surface textures. This study was divided into two phases, each utilizing 10 rats. Six implants—one of each texture—randomized for position were placed in each rat. Phase 1 animals healed for three weeks and skin-implant mechanical pull-off strength was measured. Phase 2 animals were challenged by S. aureus inoculation during the three-week healing process and serial dilutions of the sonicated implants were performed to quantify bacterial colonization. The three EBM implant groups had 830% greater force at pull-off compared to the three CNC groups. Additionally, the pull-off force of EBM-micro implants was 101% and 83% stronger, respectively, than EBM-nano and EBM-control implants. There was negligible mechanical attachment of skin to any CNC implant. Bacterial colonization counts were collectively 63% (P < 0.05) lower for EBM implants relative to CNC implants. CNC-control implants exhibited 90% (P < 0.01) greater bacterial colonization than EBM-controls. No significant differences in bacterial colonization were noted between the other implant groups. Both manufacturing technique and post-manufacturing surface texture modification affected the skin-implant interface’s mechanical integration and effectiveness as a barrier to infection. EBM manufacturing produced markedly superior transcutaneous implants compared to machining.