Chitosan-coated stainless steel screws for fixation in contaminated fractures

Abstract

Stainless steel screws and other internal fixation devices are used routinely to stabilize bacteria-contaminated bone fractures from multiple injury mechanisms. In this preliminary study, we hypothesize that a chitosan coating either unloaded or loaded with an antibiotic, gentamicin, could lessen or prevent these devices from becoming an initial nidus for infection. The questions investigated for this hypothesis were: (1) how much of the sterilized coating remains on the screw with simulated functional use; (2) is the unloaded or loaded chitosan coating bacteriostatic and biocompatible; and (3) what amount and rate does an antibiotic elute from the coating? In this study, the gentamicin eluted from the coating at a detectable level during 72 to 96 hours. The coating was retained at the 90% level in simulated bone screw fixation and the unloaded and loaded chitosan coatings had encouraging in vitro biocompatibility with fibroblasts and stem cells and were bacteriostatic against at least one strain of Staphylococcus aureus. The use of an antibiotic-loaded chitosan coating on stainless steel bone screws and internal fixation devices in contaminated bone fracture fixation may be considered after optimization of antibiotic loading and elution and more expanded in vitro and in vivo investigations with other organisms and antibiotics.

Fig. 1

Testing with human dermal fibroblasts revealed similar growth rates between the chitosan-coated stainless steel and the uncoated stainless steel at all times.

Fig. 2

Cytotoxicity testing with an osteoblastic precursor cell line revealed similar growth rates between chitosan-coated stainless steel coupons and uncoated stainless steel coupons at all times.

Fig. 3A–C

Compared with the (A) stainless steel control, zone of inhibition testing confirmed the inhibition of bacteria in the presence of (B) chitosan and (C) gentamicin-loaded chitosan.

Fig. 4A–B

(A) Light microscopy confirmed the establishment of a loaded chitosan coating on the surface of a stainless steel screw. (B) Scanning electron microscopy also confirmed a loaded chitosan coating covering the surface of the bone screw.

Fig. 5A–B

(A) After functional bone simulation testing, scanning electron microscopy revealed some of the coating was lost during implantation. (B) Higher magnification scanning electron microcopy shows where the coating became separated.

Fig. 6

No statistical differences were seen among unloaded chitosan coatings, dry loaded chitosan coatings, and wet loaded chitosan coatings, and all coatings were retained well above the desired 80% retention rate.

Fig. 7

Elution testing confirmed a measurable release of gentamicin up to 96 hours characterized by a burst release of antibiotic within the first 4 hours.