Master's Theses

Date of Award

2011

Document Type

Thesis

Department

Biomedical Engineering

Keywords

Osteconduction, nanocoating, self-assembled-monolayer

Abstract

"Loosening of metal is the leading cause of clinical failures of orthopedic surgeries. A novel coating of self-assembled monolayer of phosphonate molecules (SAMP) that chemically enhances metallic surfaces by covalently bonding to their naturally occurring oxidized surfaces has been developed.1 The SAMP coating is chemically stable, and its in-vitro results showed that it promotes cell proliferation, osteoblast adhesion and enhances biological attachment to metal surfaces.1 The purpose of this study is to establish the efficacy of this novel chemical surface treatment in improving the fixation of metallic implants in both cortical and cancellous bone over a conventional untreated implant. We hypothesize that a SAMP monolayer coating would lead to greater in-vivo bone on-growth and fixation of SAMP-coated stainless steel (SS) implants when compared against uncoated ones. An established in-vivo model was used. The outcomes used to establish the coating’s efficacy were the histological appearance of the interface between implant and bone and the static mechanical strength of the interface. Cylindrical 316L SS implants were machined and highly polished and half of the implants were coated with the SAMP coating. A total of 44 male, skeletally mature New Zealand White rabbits, weighing between 3.5 and 5kg, were implanted bilaterally in the distal femoral intramedullary canals. Each animal received one coated implant in one leg and one uncoated implant in the other leg. Cylindrical implants were implanted in 9 rabbits per time point. Additionally at each time-point, 2 rabbits were implanted with cylindrical pins that have 6 longitudinal crevices and their cross-section were circumferentially serrated. The rabbits were randomized to one of four time points: 4, 8, 12 and 16 weeks. In the cylindrical implants, bone formation was evident in the epiphyseal cancellous bone surrounding the coated implants after 4 weeks. Histological evaluation of early time points has showed cell types associated with new bone formation, in both the cortical and cancellous 5 bone, in higher numbers in the treated implants. A thicker layer of unmineralized osteoid and higher number of osteoblasts were observed along the interfaces with the coated surfaces than the uncoated ones in the 4-week time-point. That indicated that there is greater bone formation and a stronger initial bone response to the coating. At the later time points, however, there was no difference between the amount of bone formed around the coated implants and the uncoated ones. At the 4-week time point, the coated samples of the creviced design showed significant osteoconductive potential of the treatment, which was observed histologically as recruitment and proliferation of osteoblasts into areas of initially non-bony contact are present. Bone had formed completely into the interior crevices of the coated implants. In the uncoated implants, bone was only present in the outer area surrounding the implants. Micro-Computed Topography scanning was performed on all the cancellous bone samples in order to visualize the amount of newly formed bone around the implants and calculate bone volume to total volume within the relative effective diameter. In the early time points, it was found that bone has grown in a larger volume around the coated implants. No statistical difference in bone volume was found at the later time points. Mechanical pullout testing was performed on cylindrical implants at each time-point (n=6) of the coated and uncoated implants. A mechanical pullout apparatus that ensures pullout in a uniaxial direction at under displacement control was used, and the resistive shear force at failure was measured. Mechanical testing showed no difference in the average of failure force of the coated group when compared to that of the uncoated group at each time point. There was also no statistical difference between time points among the coated samples. The SAMP coating did not increase the shear strength of the interface. In conclusion, in a rigorous, established model, we have shown that at early time points, the SAMP treatment elicits a significant bony apposition to the implant surface. However, this effect does not appear to be maintained."

 
 

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