Additive manufacturing of a cementless hip stem demonstrator with local drug delivery functionality

Periprosthetic joint infection from implant surface colonization by pathogenic bacteria and subsequent biofilm formation remains a serious complication following total hip arthroplasty. Local delivery of antimicrobial drugs is an effective approach to prevent and treat such infection. This paper presents a prototype development process chain for a cementless hip stem from which an antimicrobial drug can be delivered directly to the implant surface. A systematic approach was followed in which the external surface of the hip stem represents an area requiring antibacterial coverage. Individual features were tested first to determine the zone of inhibition, represented by the two-dimensional area surrounding the drug release source where the pathogen cannot survive. This zone, determined for the antibiotic vancomycin against strains of the pathogen Staphylococcus aureus to be 15 mm in diameter, was simplified geometrically to circles of uniform radius which should cover the necessary implant surface. A freeform internal channel layout was then designed with source points for drug delivery at the centre of each of these circles. The final cementless stem design was manufactured with laser powder bed fusion from Ti6Al4V ELI. Internal features were free from any blockages and ∼3 mL of 15 mg/mL aqueous vancomycin could be loaded into the demonstrator stem without obstruction. Antimicrobial efficacy of the demonstrator was qualitatively evaluated in a custom experimental setup against Staphylococcus aureus Xen 31 incubated for 24 hours with a commercial hip stem as negative control. Qualitative evaluation of the inoculum revealed that surface colonization was prevented, indicating that the effective released vancomycin concentration exceeded the minimum inhibitory concentration of ∼5 µg/mL. The results contribute evidence to the applicability of laser powder bed fusion for implants with local drug delivery functionality. Further development of the concept could increase the efficacy and resource efficiency of periprosthetic joint infection prophylaxis and treatment.