In vitro assessment of strength, fatigue durability, and disassembly of Ti6Al4V and CoCrMo necks in modular total hip replacements

Abstract

Modularity in total hip replacement offers advantages with regard to biomechanical adjustments and leg lengths. Recently, modular femoral necks were introduced as an added advantage to head modularity permitting further adjustments in femoral version as well as offset and ease of revision. Currently, most necks are made of Ti6Al4V for which cases of in vivo fractures and inseparable neck-stem junctions have been reported. Therefore, we investigated CoCrMo head-Ti6Al4V stem hip replacements with necks made of CoCrMo as an alternative to Ti6Al4V. We compared the two materials with respect to (1) compressive load bearing capacity; (2) fatigue durability; and (3) component distraction. We performed in vitro fatigue-pull-off, microscopy, fatigue durability and compression investigations. The CoCrMo neck showed a load bearing capacity of 18 kN, 38% higher than 13 kN for the Ti6Al4V neck. A fatigue load of 11.2 kN for 1 million cycle failure was achieved with CoCrMo translating into nearly 1000 times longer fatigue life compared to Ti6Al4V necks. The neck-stem distraction force showed large statistical variation and was similar for both neck materials. Overall, the results suggest a superiority of CoCrMo over Ti6Al4V as neck material with regard to mechanical behavior. However, the corrosion behavior was not appropriately assessed and necessitates additional investigations.

The stability of dual-taper modular hip implants: a biomechanical analysis examining the effect of impact location on component stability

“…Introducing an angle in the neck eliminates the ability to achieve such a goal and explains the decreased stability seen in the 8° and 15° necks in this study. Failure of dual taper implants related to complications including corrosion at the NS interface, adverse local tissue reaction, and neck fracture has led to decreased utilization and recall of these implants and negated the theoretical benefits achieved with the additional modular interface [9]. Nonetheless, revision cases of failed implants will continue to be a challenge that orthopaedic surgeons face, and understanding the mechanism by which these complications and failures occur may provide us insight for future direction in designing new implants.…”

“…While several theories exist as to etiologies of wear-particle generation and subsequent corrosion at the modular interface, research suggests that the process begins with mechanical fretting and disruption of the protective oxide layer leading to the release of metal ions at the taper interface [7], [8]. Both the debris and corrosion at the modular surface can have a multitude of effects on the outcome of the prosthesis including osteolysis, adverse local tissue reactions, increased risk of neck failure or fracture, and increased distraction force requirements at revision surgery [9]. It has been postulated that the process of fretting may begin at impaction of the components at index surgery [4], thus indicating the importance of proper engagement and stability of the components to prevent micromotion and subsequent fretting corrosion.…”

Effects of hip implant modular neck material and assembly method on fatigue life and distraction force

“…Our previous studies 12,13 showed that test environments similar to body conditions (bovine serum at 37˚C body temperature) cannot replicate wear and corrosion damages observed on in vivo implant retrievals within the duration of typical in vitro tests. They would require excessive test durations (months or even years) in order to reproduce damages that occur in patients over the typical implant service life of years or decades.…”

“…2A) was applied using a MTS 858 Mini Bionix 1 II testing machine (MTS Systems Corporation; Eden Prairie, MN), with a 10 Hz cyclic loading frequency, as previously described. 12,13 About 12 h of loading and 12 h of resting cycles were used to increase the effects of wear-corrosion damage on the modular necks and to better simulate the in vivo loading/resting periods of the implant.…”

Fatigue design of a mechanically biocompatible lattice for a proof-of-concept femoral stem

“…In literature, there are several experimental and numerical studies focusing on the fatigue analysis of hip implants (Baleani et al, 1999;Hedia et al, 1996;Kayabasi and Ekici, 2007;Li et al, 2002;Nganbe et al, 2011;Ploeg et al, 2009;Raimondi and Pietrabissa, 1999;Senalp et al, 2007). For 3 example, fatigue loading conditions, ISO 7206/3, have been applied to a hip stem to predict its elastic stress via large deflection finite element analysis (Ploeg et al, 2009).…”