Design, Material and Surgical Assembly Effects on Fretting Corrosion Behavior of Modular Tapers in Orthopedic Implants

Field: Engineering (Physical Sciences)

Mechanically assisted crevice corrosion (MACC) has become a significant problem in the orthopedic device industry, particularly in modular devices with metal-on-metal (MoM) tapered interfaces. Oxide film abrasion, leading to fretting corrosion in the presence of a crevice, accelerates the corrosion process and in some instances may lead to failure of the implant. Failure is defined as the need for revision surgery. The basic processes of oxide film abrasion and repassivation, and the associated effects (chemical, transport, mechanical and electrical) of these processes have been well described and studied over time.\nDespite these advancements, there is much that is still not well understood about modular taper performance and the role of design, materials, surgical techniques and biological factors, and their effects on MACC performance. For example, the load-displacement behavior of taper junctions during seating, the effect of seating load magnitude, rate of loading and loading orientation, and the role of taper contamination on seating mechanics, pull-off mechanics and MACC behavior remain to be well understood. In addition, few studies have been carried out to improve the understanding of the relationship between MACC, local tissue reaction, taper design and material combination.\nThus, with continued concerns surrounding fretting corrosion and MACC of modular taper junctions there is a continuing need to develop appropriate in vitro tests to evaluate the roles of specific design, material and surgical techniques. These concepts are complex, interdependent and need to be clearly understood for effective design of, and surgical practice using modular taper junctions. Therefore, the goals of this dissertation are to systematically assess the effects of seating mechanics in terms of load magnitude, load rate and load orientation, contamination, taper design and material combination on the MACC behavior of head-neck taper junctions and taper locking stability (pull-off behavior) for commercially-used total hip replacements. The information provided from these studies offers a more detailed understanding of the interactions that arise between taper design and material combination based on surgical techniques and taper contamination in reference to taper fretting motions and corrosion.\nPreliminary testing consisted of a novel test protocol in which the seating (load-displacement) behavior and taper pull-off load were monitored in 12/14 Ti6Al4V/CoCrMo modular taper junctions. The seating behavior for four load magnitudes, three load orientations and five contamination groups was reported. The results showed an increase in seating load magnitude increased seating displacement, work of seating and the taper pull-off load while an increase in load orientation (to 20°) had no significant effect. Taper contamination testing presented findings which suggest the inclusion of lipids into the junction resulted in increased taper stability. The presence of contaminants increased seating displacement and work of seating compared to a control (dry) taper.\nFretting corrosion testing incorporated identical taper testing conditions (load magnitude, orientation and contamination) and underwent incremental cyclic fretting corrosion testing. The goal was to investigate the effects of taper seating conditions on fretting motions and fretting corrosion. The outcome of testing highlighted the significant effects of taper seating conditions on each sample group. In the seating load magnitude groups, a correlation between subsidence and current at the end of testing was reported. An increased seating load orientation reduced micromotion throughout testing and the average onset load but had no effect on subsidence, current at the end of testing or taper pull-off. And the introduction of lipid contaminants into the taper junction reduced fretting corrosion currents.\nLastly, the effects of taper geometry (C taper vs. V40 taper) and material combination (Ti-6Al-4V/CoCrMo vs. TMZF (Ti-Mo-Zr-Fe)/CoCrMo) as well as load rate and compliance were investigated using novel methodologies. The findings from these studies showed changes in the taper geometry affected the stiffness of the construct, fretting motions and the fretting corrosion performance while material combination had no significant effect. In terms of load rate and compliance, the study presented evidence to suggest load rate and system compliance had no significant effect on work of seating or taper locking stability.