Influence of Assembly Force and Distraction on the Femoral Head-Taper Junction

A novel method to improve femoral head and stem taper stability intraoperatively in total hip arthroplasty - a proof of concept study

BACKGROUND

Mechanically assisted crevice corrosion (MACC) has been associated with the compromised durability and fixation of modular total hip implants, adverse reaction of local tissue, and other undesirable clinical outcomes in total hip arthroplasty (THA). MACC is primarily caused by the relative motion between the femoral head and stem. To minimize the relative motion the taper connection between the two components must be strong enough. The current study addressed the following questions: (1) Does increasing the mass of the femoral stem improve the taper connection strength intraoperatively? (2) Does increasing the mass of the femoral stem reduce the risk of periprosthetic tissue damage intraoperatively?

HYPOTHESIS

Increasing the mass of the femoral stem improve the taper connection strength intraoperatively.

MATERIALS AND METHODS

During the experiment, femoral heads were impacted onto the stem tapers with and without an additional weight attached to the stem. The femoral heads were then pulled off to investigate the strength of the taper connection. The stem displacement and acceleration at impaction were also measured to evaluate the risk of periprosthetic tissue damage.

RESULTS

The results showed that the pull-off force was increased by 24% (p=0.011, n=6) when an additional weight was attached to the stem. The additional weight also reduced the maximum stem acceleration and maximum stem displacement by 37% (p<0.001, n=6) and 14% (p=0.094, n=6), respectively.

DISCUSSION

These findings suggest that the femoral head and stem taper connection strength can be significantly improved and the risk of periprosthetic tissue damage significantly reduced intraoperatively by attaching an additional weight to the stem to increase its mass.

LEVEL OF EVIDENCE

III, comparative in vitro mechanical investigation.

An Overview of the Stability and Fretting Corrosion of Microgrooved Necks in the Taper Junction of Hip Implants

Fretting corrosion at the head-neck interface of modular hip implants, scientifically termed trunnionosis/taperosis, may cause regional inflammation, metallosis, and adverse local tissue reactions. The severity of such a deleterious process depends on various design parameters. In this review, the influence of surface topography (in some cases, called microgrooves/ridges) on the overall performance of the microgrooved head-neck junctions is investigated. The methodologies together with the assumptions and simplifications, as well as the findings from both the experimental observations (retrieval and in vitro) and the numerical approaches used in previous studies, are presented and discussed. The performance of the microgrooved junctions is compared to those with a smooth surface finish in two main categories: stability and integrity; wear, corrosion, and material loss. Existing contradictions and disagreements among the reported results are reported and discussed in order to present a comprehensive picture of the microgrooved junctions. The current research needs and possible future research directions on the microgrooved junctions are also identified and presented.

Effect of femoral head material on head-to-trunnion impaction motion and taper

The purpose of this study was to evaluate the effect of femoral head material on the impaction force, relative motion, and stability of the trunnion. There were three groups with different head materials (n = 5 per group)-CoCr Group: 36 mm CoCr heads, Ceramic Group: 36 mm ceramic heads, Ceramic + Sleeve Group: 36 mm ceramic heads with a titanium sleeve-that were all impacted twice and disengaged onto titanium alloy (Ti6al4V) trunnions in in vitro conditions. A high-speed camera system was utilized to characterize relative displacement behavior of the head-trunnion junction motion. The first impact force of Ceramic + Sleeve Group (14,241    SD, 935) was significantly lower than the first impact force in Ceramic Group (14,961 N, SD = 184). Ceramic + Sleeve Group had a lower magnitude bounce-back displacement following the first impact (17.7 μm, SD = 11), p < 0.05) compared to CoCr Group (298.8 μm, SD = 84) and Group 2 (196.5 μm, SD = 31). Ceramic + Sleeve Group sat further on the trunnion (cumulative final displacement, 366.8 μm, SD = 71, p < 0.001) compared to CoCr Group (142.5 μm, SD = 41.8) and Ceramic Group (183.8 μm, SD = 30). Ceramic + Sleeve Group demonstrated two distinct disengagement patterns-(a) the sleeve disengaged from the trunnion (pull-off force 6810 N), and (b) the femoral head disengaged from the sleeve (pull-off force 18,620 N), with large fluctuations in pull-off force. The presence of a titanium sleeve with a ceramic head resulted in significant differences in impaction force on the trunnion, motion and displacement, and unique mechanisms for disengagement. Further investigation is required to determine potential clinical impact.

The mechanics of head-neck taper junctions: What do we know from finite element analysis?

Modular hip implants are widely used in hip arthroplasty because of the advantages they can offer such as flexibility in material combinations and geometrical adjustments. The mechanical environment of the modular junction in the body is quite challenging due to the complex and varying off-axial mechanical loads of physical activities applied to a tapered interface of two contacting materials (head and neck) assembled by an impact force intraoperatively. Experimental analogies to the in-vivo condition of the taper junction are complex, expensive and time-consuming to implement; hence, computational simulations have been a preferred approach taken by researchers for studying the mechanics of these modular junctions that can help us understand their failure mechanisms and improve their design and longevity after implantation. This paper provides a clearer insight into the mechanics of the head-neck taper junction through a careful review on the finite element studies of the junction and their findings. The effects of various factors on the mechanical outputs namely: stresses, micromotions, and contact situations are reviewed and discussed. Also, the simulation methodology of the studies in the literature is compared. Research opportunities for future are scrutinised through tabulating data and information that have been carefully retrieved form the reported findings.

A review on the finite element simulation of fretting wear and corrosion in the taper junction of hip replacement implants

Taperosis/trunnionosis is a scientific term for describing tribocorrosion (fretting corrosion) at the head-neck taper junction of hip implants where two contacting surfaces are undergone oscillatory micromotions while being exposed to the body fluid. Detached ions and emitted debris, as a result of taperosis, migrate to the surrounding tissues and can cause inflammation, infection, and aseptic loosening with an ultimate possibility of implant failure. Improving the tribocorrosion performance of the head-neck junction in the light of minimising the surface damage and debris requires a better understanding of taperosis. Given its complexity associated with both the mechanical and electrochemical aspects, computational methods such as the finite element method have been recently employed for analysing fretting wear and corrosion in the taper junction. To date, there have been more efforts on the fretting wear simulation when compared with corrosion. This is because of the mechanical nature of fretting wear which is probably more straightforward for modelling. However, as a recent research advancement, corrosion has been a focus to be implemented in the finite element modelling of taper junctions. This paper aims to review finite element studies related to taperosis in the head-neck junction to provide a detailed understanding of the design parameters and their role in this failure mechanism. It also reviews and discusses the methodologies developed for simulating this complex process in the taper junction along with the simplifications, assumptions and findings reported in these studies. The current needs and future research opportunities and directions in this field are then identified and presented.

Influence of Different Damage Patterns of the Stem Taper on Fixation and Fracture Strength of Ceramic Ball Heads for Total Hip Replacement

Background

Modularity finds frequent application in total hip replacement, allowing a preferable individual configuration and a simplified revision by retaining the femoral stem and replacing the prosthetic head. However, micromotions within the interface between the head and the stem taper can arise, resulting in the release of wear debris and corrosion products. The aim of our experimental study was to evaluate the influence of different taper damages on the fixation and fracture stability of ceramic femoral heads, after static and dynamic implant loading.

Methods

Ceramic ball heads (36 mm diameter) and 12/14 stem tapers made of titanium with various mild damage patterns (intact, scratched, and truncated) were tested. The heads were assembled on the taper with a quasistatic load of 2 kN and separated into a static and a dynamic group afterwards. The dynamic group (n = 18) was loaded over 1.5 million gait cycles in a hip wear simulator (ISO 14242-1). In contrast, the static group (n = 18) was not mechanically loaded after assembly. To determine the taper stability, all heads of the dynamic and static groups were either pulled off (ASTM 2009) or turned off (ISO 7206-16). A head fracture test (ISO 7206-10) was also performed. Subsequent to the fixation stability tests, the taper surface was visually evaluated in terms of any signs of wear or corrosion after the dynamic loading.

Results

In 10 of the 18 cases, discoloration of the taper was determined after the dynamic loading and subsequent cleaning, indicating the first signs of corrosion. Pull-off forces as well as turn-off moments were increased between 23% and 54% after the dynamic loading compared to the unloaded tapers. No significant influence of taper damage was determined in terms of taper fixation strength. However, the taper damage led to a decrease in fracture strength by approximately 20% (scratched) and 40% (truncated), respectively.

Conclusion

The results suggest that careful handling and accurate manufacturing of the stem taper are crucial for the ceramic head fracture strength, even though a mild damage showed no significant influence on taper stability. Moreover, our data indicate that a further seating of the prosthetic head may occur during daily activities, when the resulting hip force increases the assembly load.