Do SiNx coatings bear the potential to reduce the risk of micromotion in modular taper junctions?

Fretting corrosion is one contributor to the clinical failure of modular joint arthroplasty. It is initiated by micromotion in metal junctions exposed to fluids. Omitting metal-on-metal contacts could help to reduce the corrosion risk. The coating of one metal taper partner with a ceramic-based silicon nitride (SiNx) coating might provide this separation. The aim of the study was to identify whether a SiNx coating of the male taper component influences the micromotion within a taper junction. Hip prosthesis heads made of CoCr29Mo6 (Aesculap) and Ti6Al4V (Peter Brehm) were assembled (2000 N) to SiNx-coated and uncoated stem tapers made of Ti6Al4V and CoCr29Mo6 (2×2×2 combinations, each n = 4). Consecutive sinusoidal loading representing three daily activities was applied. Contactless relative motion in six degrees of freedom was measured using six eddy-current sensors. Micromotion in the junction was determined by compensating for the elastic deformation derived from additional monoblock measurements. After pull-off, the taper surfaces were microscopically inspected. Micromotion magnitude reached up to 8.4 ± 0.8 µm during loading that represented stumbling. Ti6Al4V stems showed significantly higher micromotion than those made of CoCr29Mo6, while taper coating had no influence. Statistical differences in pull-off forces were found for none of the taper junctions. Microscopy revealed CoCr29Mo6 abrasion from the head taper surface if combined with coated stem tapers. Higher micromotion of Ti6Al4V tapers was probably caused by the lower Young's modulus. Even in the contact areas, the coating was not damaged during loading. The mechanics of coated tapers was similar to uncoated prostheses. Thus, the separation of the two metal surfaces with the objective to reduce in vivo corrosion appears to be achievable if the coating is able to withstand in vivo conditions. However, the hard ceramic-based stem coating lead to undesirable debris from the CoCr29Mo6 heads during loading.

Micromotion at the head-stem taper junction of total hip prostheses is influenced by prosthesis design-, patient- and surgeon-related factors.

Taper junctions of modular hip prostheses are susceptible to fretting and crevice corrosion. Prevalence and significance increase for cobalt-chromium heads assembled on titanium-alloy stems. Retrieval and in-vitro studies have identified micromotion between the taper components to accelerate the corrosion process. The aim of this study was to identify the most critical factors contributing to increased micromotion, which is most likely influenced by design-, patient- and surgeon-related aspects. Micromotion between head and stem taper surfaces was measured for different taper surface topographies and load orientations. Consecutive visual images were recorded through windows in the head component. By image matching analysis the local micromotions at the taper junction between head and stem tapers were determined. To extend the findings to taper regions not visible through the windows, finite element models were generated. The models were further utilized to investigate the influence of head length, taper angle difference and assembly force on micromotion. Significantly higher micromotion (+20%) was found under varus loading (7.1 µm) in comparison to valgus loading (5.9 µm). Smooth and microgrooved stem tapers exhibited equal amounts of micromotion. The numerical model revealed head tilting and recurring taper contact changes in terms of cyclic engagement/disengagement during the loading sequences. Especially long heads (+240%) and low assembly forces (+53%) were found to substantially increase micromotion (from 2.7 µm to 9.3 µm and from 4.1 µm to 8.8 µm, respectively). This study accentuates the susceptibility of taper junctions to a variety of factors, which need to be appreciated in preoperative planning and surgical procedure to reduce the amount of micromotion and such minimize the risk of critical corrosion.

Adapter sleeves are essential for ceramic heads in hip revision surgery.

BACKGROUND: Removing a head during isolated acetabular revision surgery can cause damage to the stem taper surface from extraction tool contact. Implanting a ceramic head on the damaged stem taper might elevate the fracture risk, which can be mitigated with the use of titanium adapter sleeves. The aim of this study was to investigate whether the improved fracture strength of modern generation ceramic heads allows the direct implantation on damaged stem tapers without an adapter sleeve. METHODS: Finite element models of taper junctions with and without adapter sleeve were generated. Different stem taper damages were modelled to investigate the influence on the ceramic head fracture load under axial compression. FINDINGS: Heads without adapter sleeves exhibited slightly higher or equal fracture strengths compared with sleeved heads for most scenarios. However, a small metal elevation on the stem taper caused a drastic decrease of the fracture strength if no adapter sleeve was used (-96%). The sleeved head was not influenced by the metal elevation damage. INTERPRETATION: Adapter sleeves are essential to ensure patient safety and prosthesis longevity whenever implanting ceramic heads on used stem tapers.

Determination of local micromotion at the stem-neck taper junction of a bi-modular total hip prosthesis design.

High rates of clinical complications with bi-modular hip prostheses are attributed to failure of the stem-neck taper junction. Taper wear analyses have shown extensive material loss as a result of corrosion, potentially initiated by micromotion. The purpose of the study was to determine the amount of micromotion at this junction for different loading, assembly and material conditions. Micromotion between the neck adapter (CoCr29Mo6-alloy) and the stem (TiMo12Zr6Fe2-alloy; both Rejuvenate, Stryker) within the taper junction of a bi-modular hip stem were determined by image matching analysis of consecutively recorded images through windows in the stem component. A finite element model was used to determine the micromotion in the taper regions outside the windows and validated with the measured micromotion. With the model, the influence of the load amplitude, assembly force and component materials were then investigated. Determined micromotion (14-79 µm) by far exceeded critical values (5 µm) associated with the onset of fretting corrosion. Increasing assembly forces achieved a significant reduction in micromotion. The numerical model revealed insufficient assembly to cause the neck to perform rocking motions under load, repetitively changing taper contact in combination with gap opening, which facilitates fluid ingress into the junction. Changing the stem material to a stiffer Ti-alloy achieved a reduction of the micromotion of about 30%. This study emphasises the high importance of material selection, assembly force and loading on the susceptibility of bi-modular hip stems to fretting and crevice corrosion. These findings can serve to explain the increased rate of clinically reported problems with this particular prosthesis design.

Mechanical Stability of the Taper Connection of Large Metal Femoral Heads With Adapter Sleeves in Total Hip Arthroplasty Analyzed Using Explicit Finite Element Simulations.

BACKGROUND: Large diameter heads (LDHs) of metal-on-metal bearings in total hip arthroplasty provide increased range of motion and reduced dislocation rates. However, major concerns grew over high wear rates from the modular connection between femoral stem and head, especially in combination with adapter sleeves. METHODS: A computational study on the taper connection stability of LDH (50 mm) with adapter sleeves of different lengths (S, M, L, and XL) compared with a standard femoral head (32 mm) without adapter sleeves was conducted using explicit finite element analyses. Four different impact configurations were considered resulting from varied mallet mass (0.5 vs 1.0 kg) and velocity (1.0 vs 2.0 m/s). The taper stability was evaluated by determination of the pull-off forces and micromotions due to simulated joint loads during walking (2 kN and 7.9 Nm, respectively). Moreover, the deformations of the adapter sleeves and the contact area in the taper connections were evaluated. RESULTS: Although the pull-off forces of the LDH with different-sized adapter sleeves were comparable, contact area decreased and adapter sleeve deformations increased (up to 283%) with an increasing adapter sleeve length. Moreover, the micromotions of LDH with adapter sleeves were up to 7-times higher, as compared with the standard femoral head without an adapter sleeve. CONCLUSION: The present numerical study confirms that the assembly technique of LDH with adapter sleeves reveals increased micromotions compared with standard femoral head sizes. We could demonstrate that deviations of the stem trunnion geometry and improper surgical instructions led to worse mechanical stability of the taper connection.