Porous Coatings in Retrieved Acetabular Components

BACKGROUND

We evaluated bone volume fraction in retrieved acetabular shells with 2 types of porous coatings: (1) titanium fiber mesh (HG) components and (2) tantalum metal coating (TM) components.

METHODS

Eight HG shells were matched to 8 TM shells for patient age, body mass index, and gender. The mean age at index surgery was 69 (51-82) years, mean body mass index was 28 (21-40), and patients were evenly divided between male and female (4:4). The length of implantation was 40 (16-96) months for the TM group and 156 (108-216) months for the HG group. Shells were embedded and two 5-mm thick cross-sections were cut through the apex of each component for backscatter scanning electron microscopy assessment. Backscatter scanning electron microscopy images were segmented to threshold for metal, bone, and available space for ingrowth. Slices were assessed regionally for ingrowth at the rim, equator, and pole of the acetabular shell. Differences were assessed using general estimating equations, and P values were adjusted for multiple comparisons using the Holm-Bonferroni step-down procedure.

RESULTS

The mean bone volume fraction was 21 ± 17% for the HG shell and 7 ± 4% for the TM shell (P < .0001). The rim and pole regions both had less bone ingrowth than the equator. No association was found between bone ingrowth and length of implantation for either design.

CONCLUSION

Adequate bone ingrowth is a requirement for successful biological fixation, but the amount of ingrowth may not be a driving factor. Both implants studied had successful outcomes and long-term fixation despite the observation of low amounts of ingrowth.

Patient and surgical variability in the primary stability of cementless acetabular cups: A finite element study

Aseptic loosening is the most common indication for revision of cementless acetabular cups and often depends on the primary stability achieved following surgery. Cup designs must be capable of achieving primary stability for a wide variety of individuals and surgical conditions to be successful. Typically, preclinical finite element (FE) testing of cups involves assessing the performance in a single patient and under a limited set of idealized conditions. The aim of this study was to assess the effect of patient and surgical parameters on the primary stability of an acetabular cup design in a set of subject-specific FE models. Interference fit was varied in a representative set of 12 patient-specific models of the implanted hemipelvis. Linear mixed models showed a significant association with micromotion for interference fit (P < .0001), acetabular bone elastic modulus (P < .001), native acetabular diameter (P  = .03), and the interference fit-elastic modulus interaction (P = .01). There were no significant associations between the polar gap and any of the parameters considered. The significant interference fit-elastic modulus interaction suggests that increasing the interference fit in patients with low bone quality leads to a greater reduction in micromotion than in patients with higher bone quality. However, the significant association between percentage bone yielding and interference fit (P < .0001) suggests a higher periacetabular fracture risk at higher interference fits. This work supports the development of preclinical testing of cup designs for the broad range patients and surgical conditions a cup may face following surgery.

Primary stability of a cementless acetabular cup in a cohort of patient-specific finite element models

The primary stability achieved during total hip arthroplasty determines the long-term success of cementless acetabular cups. Pre-clinical finite element testing of cups typically use a model of a single patient and assume the results can be extrapolated to the general population. This study explored the variability in predicted primary stability of a Pinnacle^® cementless acetabular cup in 103 patient-specific finite element models of the hemipelvis and examined the association between patient-related factors and the observed variability. Cups were inserted by displacement-control into the FE models and then a loading configuration simulating a complete level gait cycle was applied. The cohort showed a range of polar gap of 284-1112 μm and 95th percentile composite peak micromotion (CPM) of 18-624 μm. Regression analysis was not conclusive on the relationship between patient-related factors and primary stability. No relationship was found between polar gap and micromotion. However, when the patient-related factors were categorised into quartile groups, trends suggested higher polar gaps occurred in subjects with small and shallow acetabular geometries and cup motion during gait was affected most by low elastic modulus and high bodyweight. The variation in primary stability in the cohort for an acetabular cup with a proven clinical track record may provide benchmark data when evaluating new cup designs. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1012-1023, 2018.

The effects of musculoskeletal loading regimes on numerical evaluations of acetabular component

The importance of clinical studies notwithstanding, the failure assessment of implant–bone structure has alternatively been carried out using finite element analysis. However, the accuracy of the finite element predicted results is dependent on the applied loading and boundary conditions. Nevertheless, most finite element–based evaluations on acetabular component used a few selective load cases instead of the eight load cases representing the entire gait cycle. These in silico evaluations often suffer from limitations regarding the use of simplified musculoskeletal loading regimes. This study attempts to analyse the influence of three different loading regimes representing a gait cycle, on numerical evaluations of acetabular component. Patient-specific computer tomography scan-based models of intact and resurfaced pelvises were used. One such loading regime consisted of the second load case that corresponded to peak hip joint reaction force. Whereas the other loading regime consisted of the second and fifth load cases, which corresponded to peak hip joint reaction force and peak muscle forces, respectively. The third loading regime included all the eight load cases. Considerable deviations in peri-acetabular strains, standard error ranging between 115 and 400 µε, were observed for different loading regimes. The predicted bone strains were lower when selective loading regimes were used. Despite minor quantitative variations in bone density changes (less than 0.15 g cm−3), the final bone density pattern after bone remodelling was found to be similar for all the loading regimes. Underestimations in implant–bone micromotions (40–50 µm) were observed for selective loading regimes after bone remodelling. However, at immediate post-operative condition, such underestimations were found to be less (less than 5 µm). The predicted results highlight the importance of inclusion of eight load cases representing the gait cycle for in silico evaluations of resurfaced pelvis.

Four decades of finite element analysis of orthopaedic devices: where are we now and what are the opportunities?

Finite element has been used for more than four decades to study and evaluate the mechanical behaviour total joint replacements. In Huiskes seminal paper "Failed innovation in total hip replacement: diagnosis and proposals for a cure", finite element modelling was one of the potential cures to avoid poorly performing designs reaching the market place. The size and sophistication of models has increased significantly since that paper and a range of techniques are available from predicting the initial mechanical environment through to advanced adaptive simulations including bone adaptation, tissue differentiation, damage accumulation and wear. However, are we any closer to FE becoming an effective screening tool for new devices? This review contains a critical analysis of currently available finite element modelling techniques including (i) development of the basic model, the application of appropriate material properties, loading and boundary conditions, (ii) describing the initial mechanical environment of the bone-implant system, (iii) capturing the time dependent behaviour in adaptive simulations, (iv) the design and implementation of computer based experiments and (v) determining suitable performance metrics. The development of the underlying tools and techniques appears to have plateaued and further advances appear to be limited either by a lack of data to populate the models or the need to better understand the fundamentals of the mechanical and biological processes. There has been progress in the design of computer based experiments. Historically, FE has been used in a similar way to in vitro tests, by running only a limited set of analyses, typically of a single bone segment or joint under idealised conditions. The power of finite element is the ability to run multiple simulations and explore the performance of a device under a variety of conditions. There has been increasing usage of design of experiments, probabilistic techniques and more recently population based modelling to account for patient and surgical variability. In order to have effective screening methods, we need to continue to develop these approaches to examine the behaviour and performance of total joint replacements and benchmark them for devices with known clinical performance. Finite element will increasingly be used in the design, development and pre-clinical testing of total joint replacements. However, simulations must include holistic, closely corroborated, multi-domain analyses which account for real world variability.

Primary stability of two uncementedacetabular components of different geometry: hemispherical or peripherallyenhanced?

Objective

This study compared the primary stability of two commercially available acetabular components from the same manufacturer, which differ only in geometry; a hemispherical and a peripherally enhanced design (peripheral self-locking (PSL)). The objective was to determine whether altered geometry resulted in better primary stability.

Methods

Acetabular components were seated with 0.8 mm to 2 mm interference fits in reamed polyethylene bone substrate of two different densities (0.22 g/cm³ and 0.45 g/cm³). The primary stability of each component design was investigated by measuring the peak failure load during uniaxial pull-out and tangential lever-out tests.

Results

There was no statistically significant difference in seating force (p = 0.104) or primary stability (pull-out p = 0.171, lever-out p = 0.087) of the two components in the low-density substrate. Similarly, in the high-density substrate, there was no statistically significant difference in the peak pull-out force (p = 0.154) or lever-out moment (p = 0.574) between the designs. However, the PSL component required a significantly higher seating force than the hemispherical cup in the high-density bone analogue (p = 0.006).

Conclusions

Higher seating forces associated with the PSL design may result in inadequate seating and increased risk of component malpositioning or acetabular fracture in the intra-operative setting in high-density bone stock. Our results, if translated clinically, suggest that a purely hemispherical geometry may have an advantage over a peripherally enhanced geometry in high density bone stock.

Cite this article: Bone Joint Res 2013;2:264–9.