Strain Shielding Research Articles

Highly stretchable and mechanically robust copolymer-based strain-engineered substrate for wearable electronics

The challenge of integrating electronic components into soft substrates to consolidate diverse device functionalities into a single wearable unit impedes the advancement and application of emerging soft and stretchable electronics. This is primarily due to the tendency of rigid components to be detached from deformable substrates under mechanical stress such as repetitive stretching, due to significant differences in the elastic modulus of dissimilar materials. Therefore, it is essential to employ strategies that prevent electrical and mechanical failures when incorporating rigid components into stretchable systems. To address this issue, stiff islands of a copolymer were incorporated into a soft copolymer substrate, ensuring stable interfaces between rigid and stretchable regions. In this work, polyimide-siloxane copolymers with adjustable compositions of hard and soft blocks with mechanical modulus tuneability were employed to fabricate a strain-engineered substrate (SES). The resulting SES demonstrated exceptional strain shielding properties, preventing electronic components from being detached under strain levels of up to 266%. This substrate, engineered to endure strain, exhibited a resistance variation of less than 4% for the gold electrode on it when stretched cyclically up to 5000 cycles at 15% strain. A stretchable supercapacitor and triboelectric sensor with high stability could be fabricated on the SES. The SES enabled a robust interface capable of mitigating local strain on rigid components while preserving the stretchability of the soft substrates, which helps prevent potential damage to the rigid components. By alleviating the issues of unstable interfaces and mechanical and electrical failure, our approach of the SES has great potential in wearable electronics.

Load transfer in bone after partial, multi-compartmental, and total knee arthroplasty.

Introduction: Arthroplasty-associated bone loss remains a clinical problem: stiff metallic implants disrupt load transfer to bone and, hence, its remodeling stimulus. The aim of this research was to analyze how load transfer to bone is affected by different forms of knee arthroplasty: isolated partial knee arthroplasty (PKA), compartmental arthroplasty [combined partial knee arthroplasty (CPKA), two or more PKAs in the same knee], and total knee arthroplasty (TKA). Methods: An experimentally validated subject-specific finite element model was analyzed native and with medial unicondylar, lateral unicondylar, patellofemoral, bi-unicondylar, medial bicompartmental, lateral bicompartmental, tricompartmental, and total knee arthroplasty. Three load cases were simulated for each: gait, stair ascent, and sit-to-stand. Strain shielding and overstraining were calculated from the differences between the native and implanted states. Results: For gait, the TKA femoral component led to mean strain shielding (30%) more than three times higher than that of PKA (4%-7%) and CPKA (5%-8%). Overstraining was predicted in the proximal tibia (TKA 21%; PKA/CPKA 0%-6%). The variance in the distribution for TKA was an order of magnitude greater than for PKA/CPKA, indicating less physiological load transfer. Only the TKA-implanted femur was sensitive to the load case: for stair ascent and gait, almost the entire distal femur was strain-shielded, whereas during sit-to-stand, the posterior femoral condyles were overstrained. Discussion: TKA requires more bone resection than PKA and CPKA. These finite element analyses suggest that a longer-term benefit for bone is probable as partial and multi-compartmental knee procedures lead to more natural load transfer compared to TKA. High-flexion activity following TKA may be protective of posterior condyle bone resorption, which may help explain why bone loss affects some patients more than others. The male and female bone models used for this research are provided open access to facilitate future research elsewhere.

Investigation on the mechanical genes of “rigidity-flexibility balance” composite materials similar to feather shaft by SR-CT in-situ experiments

Feather shaft is a fiber reinforced lightweight bio-composite materials with good strength and toughness. It provides possibility for the design of carbon fiber composite that combine rigidity and flexibility, and the key to achieve this is to study the corresponding mechanical gene. Here, inspired by the ring array fiber in the feather shaft, a carbon fiber composite was designed. Through experimental investigation, the “ring array” induced shear strain shielding, progressive damage caused by shear strain reversal and “rigidity-flexibility balance” caused by progressive damage was revealed, which will help establish an approach from structure to mechanical properties through strain and damage.

Numerical Evaluations of an Uncemented Acetabular Component in Total Hip Arthroplasty: Effects of Loading and Interface Conditions.

Using finite element (FE) models of intact and implanted hemipelvises, the study aimed to investigate the influences of musculoskeletal loading and implant-bone interface conditions on preclinical analysis of an uncemented acetabular component after total hip arthroplasty (THA). A new musculoskeletal loading dataset, corresponding to daily activities of sitting up-down, stairs up-down and normal walking, for a pelvic bone was generated based on previously validated Gait2392 model. Three implant-bone interface conditions, fully bonded and debonded having two rim press-fits (1 mm and 2 mm), were analyzed. High tensile (2000-2415 μϵ) and compressive strains (900-1035 μϵ) were predicted for 2 mm press-fit, which might evoke microdamage in pelvic cortex. Strain shielding in periprosthetic cancellous bone was higher for bonded condition during sitting up activity, compared to other combinations of interface and loading conditions. Only the nodes around acetabular rim (less than 6%) were susceptible to interfacial debonding. Although maximum micromotion increased with increase in press-fit, postoperatively for all load cases, these were within a favorable range (52-143 μm) for bone ingrowth. Micromotions reduced (39-105 μm) with bone remodeling, indicating lesser chances of implant migration. Bone apposition was predominant around acetabular rim, compared to dome, for all interface conditions. Periprosthetic bone resorption of 10-20% and bone apposition of 10-15% were predicted for bonded condition. Whereas for press-fit (1 mm and 2 mm), predominant bone apposition of 200-300% was observed. This study highlights the importance of variations in loading and interface conditions on in silico evaluations of an uncemented acetabular component.

Innovative Design Methodology for Patient-Specific Short Femoral Stems.

The biomechanical performance of hip prostheses is often suboptimal, which leads to problems such as strain shielding, bone resorption and implant loosening, affecting the long-term viability of these implants for articular repair. Different studies have highlighted the interest of short stems for preserving bone stock and minimizing shielding, hence providing an alternative to conventional hip prostheses with long stems. Such short stems are especially valuable for younger patients, as they may require additional surgical interventions and replacements in the future, for which the preservation of bone stock is fundamental. Arguably, enhanced results may be achieved by combining the benefits of short stems with the possibilities of personalization, which are now empowered by a wise combination of medical images, computer-aided design and engineering resources and automated manufacturing tools. In this study, an innovative design methodology for custom-made short femoral stems is presented. The design process is enhanced through a novel app employing elliptical adjustment for the quasi-automated CAD modeling of personalized short femoral stems. The proposed methodology is validated by completely developing two personalized short femoral stems, which are evaluated by combining in silico studies (finite element method (FEM) simulations), for quantifying their biomechanical performance, and rapid prototyping, for evaluating implantability.

Topology Optimisation for Compliant Hip Implant Design and Reduced Strain Shielding.

Stiff total hip arthroplasty implants can lead to strain shielding, bone loss and complex revision surgery. The aim of this study was to develop topology optimisation techniques for more compliant hip implant design. The Solid Isotropic Material with Penalisation (SIMP) method was adapted, and two hip stems were designed and additive manufactured: (1) a stem based on a stochastic porous structure, and (2) a selectively hollowed approach. Finite element analyses and experimental measurements were conducted to measure stem stiffness and predict the reduction in stress shielding. The selectively hollowed implant increased peri-implanted femur surface strains by up to 25 percentage points compared to a solid implant without compromising predicted strength. Despite the stark differences in design, the experimentally measured stiffness results were near identical for the two optimised stems, with 39% and 40% reductions in the equivalent stiffness for the porous and selectively hollowed implants, respectively, compared to the solid implant. The selectively hollowed implant’s internal structure had a striking resemblance to the trabecular bone structures found in the femur, hinting at intrinsic congruency between nature’s design process and topology optimisation. The developed topology optimisation process enables compliant hip implant design for more natural load transfer, reduced strain shielding and improved implant survivorship.

Computational analysis of the effects of interprosthetic distance on normal and reduced cortical thickness femur models.

Distal femoral fractures associated with the femoral stem in a well-fixed hip arthroplasty pose a risk of an interprosthetic fracture, the treatment of which is known as difficult. To effectively prevent and treat IP fractures, biomechanical effects must be demonstrated. We defined eight variations of the interprosthetic distance ranging from 48 mm overlap to 128 mm gap. Femoral geometries with normal and reduced cortical thickness were modeled to evaluate the effects of cortical thickness. In addition to the intact model, a total of 16 finite element models were analyzed under physiological boundary conditions. Maximum and minimum principal strains on the lateral and medial cortex surfaces were always found to be greater in models with reduced cortical thickness than in normal femurs. The model with 48 mm overlapping interprosthetic distance produced the least peak strain and the model with 16 mm interprosthetic gap produced the greatest strain with both normal and reduced cortical thickness. The screw holes produced local strain concentrations and increased the peak strains on the cortex surfaces, especially close to the stem tip. Statistically, a significant correlation (R2 = 0.9483) was found between strain shielding and interprosthetic distance. Axial stiffness, interfragmentary shear motion, and maximum von-Mises stress on the distal plate showed a high correlation with the interprosthetic distance. It was concluded that the overlapping structures are superior to other fixations we analyzed in that they offer better mechanical stability and eliminates the local strain concentrations.

A novel intramedullary nail to control interfragmentary motion in diaphyseal tibial fractures.

Numerous animal and human studies have demonstrated the benefit of controlled interfragmentary motion on fracture healing. In this study, we quantified interfragmentary motion and load transfer in tibial fractures fixed using a novel intramedullary nail (IMN) that allows controlled axial motion. Fifty composite tibias with various fracture patterns were utilized. For all test conditions, two interlocking screws were used to fix the nail in the proximal metaphysis, and two interlocking screws through the distal metaphysis. The nail allowed either no motion (static mode) or 1 mm (dynamic mode) of cyclic axial motion between the two fracture fragments for every fracture pattern tested. As expected, strain shielding was more prominent under static nail conditions. In contrast, specimens tested under dynamic nail conditions transferred axial load between the fracture fragments such that strains near the fracture site were generally similar to those measured on an intact tibia. Maximum shear strains proximal to the fracture were significantly lower in specimens with oblique or butterfly fracture patterns (p < 0.01) compared to intact specimens. This decrease in shear strain indicates that strain shielding effects were likely present due to the implant. However, strain shielding appeared to be reduced in tensile and compressive principal strains. In summary, the novel IMN allowed controlled axial motion between the fragments in a variety of common diaphyseal tibial fracture patterns.Clinical Significance: The present in vitro biomechanical study investigated a novel intramedullary nail capable of controlled axial interfragmentary motion which may potentially enhance fracture healing.

Decreased stress shielding with a PEEK femoral total knee prosthesis measured in validated computational models

Due to their high stiffness, metal femoral implants in total knee arthroplasty may cause stress shielding of the peri-prosthetic bone, which can lead to loss of bone stock. Using a polymer (PEEK) femoral implant reduces the stiffness mismatch between implant and bone, and therefore has the potential to decrease strain shielding. The goal of the current study was to evaluate this potential benefit of PEEK femoral components in cadaveric experiments. Cadaveric femurs were loaded in a materials testing device, while a 3-D digital image correlation set-up captured strains on the surface of the intact femurs and femurs implanted with PEEK and CoCr components. These experimental results were used to validate specimen-specific finite element models, which subsequently were used to assess the effect of metal and PEEK femoral components on the bone strain energy density. The finite element models showed strain maps that were highly comparable to the experimental measurements. The PEEK implant increased strain energy density, relative to the preoperative bone and compared to CoCr. This was most pronounced in the regions directly under the implant and near load contact sites. These data confirm the hypothesis that a PEEK femoral implant can reduce peri-prosthetic stress shielding.

Stress and strain distribution in femoral heads for hip resurfacing arthroplasty with different materials: A finite element analysis

Femoral bone loss due to stress and strain shielding is a common problem in hip resurfacing arthroplasty (HRA), which arises from the different stiffness of implant materials and the adjacent bone. Usually, the implants used in HRA are made of cobalt-chromium alloy (CoCr). As a novel concept, implants may also be made of ceramics, whose stiffness exceeds that of the adjacent bone by a multiple. Therefore, this computational study aimed to evaluate whether poly (ether-ether-ketone) (PEEK) or a hybrid material with a PEEK body and ceramic surface made of alumina toughened zirconia (ATZ) might be more suitable implant alternatives for HRA, as they can avoid stress and strain shielding. A reconstructed model of a human femur with an HRA implant was simulated, whereby the material of the HRA was varied between CoCr, ATZ, zirconia toughened alumina (ZTA), PEEK, and a hybrid PEEK-ATZ material. The implant fixation method also varied (cemented or cementless). The simulated models were compared with an intact model to analyze stress and strain distribution in the femoral head and neck. The strain distribution was evaluated at a total of 30,344 (cemented HRA) and 63,531 (uncemented HRA) nodes in the femoral head and neck region and divided into different strain regions (<400 µm/m: atrophy; 400–3000 μm/m: bone preserving and building; 3000–20,000 μm/m: yielding and >20,000 μm/m fracture). In addition, the mechanical stability of the implants was evaluated.When the material of the HRA implant was simulated as metal or ceramic while evaluating the strains, it was seen that around 22–26% of the analyzed nodes in the femoral head and neck were in an atrophic region, 47–51% were in a preserving or building region, and 27–28% were in a yielding region. In the case of PEEK implant, less than 0.5% of the analyzed nodes were in an atrophic region, 66–69% in a preserving or building region, and 31–34% in a yielding region. The fixation technique also had a small influence. When a hybrid HRA was simulated, the strains at the analyzed nodes depended on the thickness of the ceramic material.In conclusion, the material of the HRA implant was crucial in terms of stress and strain distribution in the adjacent bone. HRA made of PEEK or a hybrid material leads to decisively reduced stress and strain alteration compared to stiffer materials such as CoCr, ATZ, and ZTA. This confirms the potential for reduction in stress and strain shielding in the femoral head with the use of a hybrid material with a PEEK body for HRA.

Strain shielding for cemented hip implants

BackgroundLong-term survival of hip implants is of increasing relevance due to the rising life expectancy. The biomechanical effect of strain shielding as a result of implant insertion may lead to bone resorption, thus increasing risk for implant loosening and periprosthetic fractures. Patient-specific quantification of strain shielding could assist orthopedic surgeons in choosing the biomechanically most appropriate prosthesis. MethodsValidated quantitative CT-based finite element models of five femurs in intact and implanted states were considered to propose a systematic algorithm for strain shielding quantification. Three different strain measures were investigated and the most appropriate measure for strain shielding quantification is recommended. It is used to demonstrate a practical femur-specific implant selection among three common designs. FindingsStrain shielding measures demonstrated similar trends in all Gruen zones except zone 1, where the volumetric strain measure differed from von-Mises and maximum principal strains. The volumetric strain measure is in better agreement with clinical bone resorption records. It is also consistent with the biological mechanism of bone remodeling so it is recommended for strain shielding quantification. Applying the strain shielding algorithm on three different implants for a specific femur suggests that the collared design is preferable. Such quantitative biomechanical input is valuable for practical patient specific implant selection. InterpretationVolumetric strain should be considered for strain shielding examination. The presented methodology may potentially enable patient-specific pre-operative strain shielding evaluation so to minimize strain shielding. It should be further used in a longitudinal study so to correlate between strain shielding predictions and clinical bone resorption.

Femoral revision knee Arthroplasty with Metaphyseal sleeves: the use of a stem is not mandatory of a structural point of view

PurposeAlthough metaphyseal sleeves are usually used with stems, little is known about the exact contribution/need of the stem for the initial sleeve-bone interface stability, particularly in the femur, if the intramedullary canal is deformed or bowed. The aim of the present study is (1) to determine the contribution of the diaphyseal-stem on sleeve-femur interface stability and (2) to determine experimentally the strain shielding effect on the metaphyseal femur with and without diaphyseal-stem. It is hypothesised that diaphyseal-stem addition increases the sleeve-femur interface stability and the strain-shielding effect on the metaphyseal femur relatively to the stemless condition.Material and methodsThe study was developed through a combined experimental and finite-element analysis approach. Five synthetic femurs were used to measure cortex strain (triaxial-rosette-gages) behaviour and implant cortex micromotions (Digital Image Correlation) for three techniques: only femoral-component, stemless-sleeve and stemmed-sleeve. Paired t-tests were performed to evaluate the statistical significance of the difference of cortex strains and micromotions. Finite-element models were developed to assess the cancellous bone strain behaviour and sleeve-bone interface micromotions; these models were validated against the measurements.ResultsCortex strains are significantly reduced (p < 0.05) on the stemmed-sleeve with a 150 μstrain mean reduction at the medial and lateral distal sides which compares with a 60 μstrain mean reduction (p > 0.05) on the stemless condition. Both techniques presented a mean cancellous bone strain reduction of 700 μstrain (50%) at the distal region and a mean increase of 2500 μstrain (4x) at the sleeve proximal region relative to the model only with the femoral component. Both techniques presented sleeve-bone micromotions amplitude below 50-150 μm, suitable for bone ingrowth.ConclusionsThe use of a supplemental diaphyseal-stem potentiates the risk of cortex bone resorption as compared to the stemless-sleeve condition; however, the stem is not essential for the enhancement of the initial sleeve-bone stability and has minor effect on the cancellous bone strain behaviour. Based on a purely structural point view, it appears that the use of a diaphyseal-femoral-stem with the metaphyseal sleeve is not mandatory in the revision TKA, which is particularly relevant in cases where the use of stems is impracticable.

Anatomic grooved stem mitigates strain shielding compared to established total hip arthroplasty stem designs in finite-element models

Aseptic loosening remains a major problem for uncemented femoral components in primary total hip arthroplasty (THA). Ideally, bone adaptation after THA manifests minimally and local bone density reduction is widely avoided. Different design features may help to approximate initial, post-THA bone strain to levels pre-THA. Strain-shielding effects of different SP-CL stem design features are systematically analyzed and compared to CLS Spotorno and CORAIL using finite element models and physiological musculoskeletal loading conditions. All designs show substantial proximal strain-shielding: 50% reduced medial surface strain, 40–50% reduction at lateral surface, >120 µm/m root mean square error (RMSE) compared to intact bone in Gruen zone 1 and >60 µm/m RMSE in Gruen zones 2, 6, and 7. Geometrical changes (ribs, grooves, cross sections, stem length, anatomic curvature) have a considerable effect on strain-shielding; up to 20%. Combinations of reduced stem stiffness with larger proximal contact area (anatomically curved, grooves) lead to less strain-shielding compared to clinically established implant designs. We found that only the combination of a structurally flexible stem with anatomical curvature and grooves improves strain-shielding compared to other designs. The clinical implications in vivo of this initial strain-shielding difference are currently under evaluation in an ongoing clinical analysis.

Effects of implant orientation and implant material on tibia bone strain, implant-bone micromotion, contact pressure, and wear depth due to total ankle replacement.

The aim of this study is to investigate the effects of implant orientation and implant material on tibia bone strain, implant-bone micromotion, maximum contact pressure, and wear depth at the articulating surface due to total ankle replacement. Three-dimensional finite element models of intact and implanted ankle were developed from computed tomography scan data. Four implanted models were developed having varus and valgus orientations of 5° and 10°, respectively. In order to determine the effect of implant material combination on tibia bone strain, micromotion, contact pressure, and wear depth, three other finite element models were developed having a different material combination of the implant. Dorsiflexion, neutral, and plantarflexion positions were considered as applied loading condition, along with muscle force and ligaments. Implant orientation alters the strain distribution in tibia bone. Strain shielding was found to be less in the case of the optimally positioned implant. Apart from the strain, implant orientation also affects implant-bone micromotion, contact pressure, and wear depth. Implant materials have less influence on tibia bone strain and micromotion. However, wear depth was reduced when ceramic and carbon fibre-reinforced polyetheretherketone material combination was used. Proper orientation of the implant is important to reduce the strain shielding. The present result suggested that ceramic can be used as an alternative to metal and carbon fibre-reinforced polyetheretherketone as an alternative to ultra-high molecular weight polyethylene to reduce wear, which would be beneficial for long-term success and fixation of the implant.

Ex-vivo and in vitro validation of an innovative mandibular condyle implant concept

PurposeThe purpose of this study is to pre-validate an inovative implant concept, and to compare the behavior of the mandibular condyle against a commercial Biomet implant in an ex vivo model and present results of the first cadaveric studies. Materials and methodsThree experimental cadaveric condyles were tested under three conditions: one intact, another with the Biomet model, and one with the innovative concept. The condyle was tested with a reaction of 300 N in all situations and the principal strains were measured. Before the geometry of the cadaveric condyle was reconstructed from a microCT scan, and a finite element model was created. Finally, a procedure was carried out with the new implant by two expert surgeons on a two cadaveric head model. ResultsIn vitro the mandible condyle presents a linear behavior until maximum load. The strain measured with Biomet implant indicates a strain shielding effect in the proximal region, inducing bone loss in the long term. The lingual side of the Biomet implanted condyle presents an increase of +44% in strain. ConclusionThe new concept was evaluated and showed a similar behavior to the intact model, and better behavior than the Biomet. The innovative concept proves that it is possible to avoid screws for a TMJ fixation and improve the TMJ alloplastic behavior.

Numerical comparison of two different tibial nails: expert tibial nail and innovative nail

The extended usage of unreamed tibial nailing resulted in reports of an increased rate of complications, especially for the distal portion of the tibia. Unreamed nailing favors biology at the expense of the achievable mechanical stability, it is therefore of interest to define the limits of the clinical indications for this method. Extra-articular fractures of the distal tibial metaphysis, meta-diaphyseal junction, and adjacent diaphysis are distinct in their management from impaction derived “pilon” type fractures and mid-diaphyseal fractures. With this purpose a complete model of the human tibia was realized, simulating a mid-diaphyseal fracture, classified as A2 type 1, according to the AO classification. Two different FE models of nail were developed in order to carry on a mechanical comparison between an innovative design of nail for fractures occurring on long bones and a traditional intra-medullary nail expert tibial nail (DePuy Synthes $$^{\textregistered }$$ ). The innovative nail’s functioning is based essentially on sliding of conical surfaces, located in a spindle and in holding pins. Spindle and holding pins are connected together by a sleeve. The sliding transforms the rotational and translational motion of the spindle to a radial expansion of the holding pins, protruding inside the intramedullary canal. Results obtained by a non-linear finite element analysis of the models were performed with Abaqus version 5.4 (Hibbitt, Karlsson and Sorensen, Inc., Pawtucket, RI, USA) using the geometric non-linearity and automatic time stepping options. In particular, analyses confirmed a satisfactory behavior of the nail in terms of stress and strain shielding, comparable to the other traditional system of prosthesis. In conclusion, this kind of nail appears to offer a good solution for elderly patients, which could not endure complications due to a complex surgery, as distal or medial screws are not necessary.

Characterization of an innovative intramedullary nail for diaphyseal fractures of long bones

In this paper, an innovative design of nail for fractures occurring on long bones has been investigated. Its functioning is based essentially on sliding of conical surfaces, located in a spindle and in holding pins. Spindle and holding pins are connected together by a sleeve. The sliding transforms the rotational and translational motion of the spindle to a radial expansion of the holding pins, protruding inside the intramedullary canal. In order to evaluate mechanical behavior of the prosthesis different benchmarks and tests were numerically performed by an FE code. Results confirm good performances in terms of strength, under compression, bending and torque loading. Moreover, a complete model of the nail implanted on a tibia, has been developed and tested evaluating two loading configurations. Results confirmed a satisfactory behavior of the nail in terms of stress and strain shielding, comparable to the others traditional systems of prosthesis. In conclusion, this kind of nail appears to offer a good solution for elderly patients, which could not endure complications due to a complex surgery, as distal or medial screws are not necessary.

On stress/strain shielding and the material stiffness paradigm for dental implants.

Stress shielding considerations suggest that the dental implant material's compliance should be matched to that of the host bone. However, this belief has not been confirmed from a general perspective, either clinically or numerically. To characterize the influence of the implant stiffness on its functionality using the failure envelope concept that examines all possible combinations of mechanical load and application angle for selected stress, strain and displacement-based bone failure criteria. Those criteria represent bone yielding, remodeling, and implant primary stability, respectively MATERIALS AND METHODS: We performed numerical simulations to generate failure envelopes for all possible loading configurations of dental implants, with stiffness ranging from very low (polymer) to extremely high, through that of bone, titanium, and ceramics. Irrespective of the failure criterion, stiffer implants allow for improved implant functionality. The latter reduces with increasing compliance, while the trabecular bone experiences higher strains, albeit of an overall small level. Micromotions remain quite small irrespective of the implant's stiffness. The current paradigm favoring reduced implant material's stiffness out of concern for stress or strain shielding, or even excessive micromotions, is not supported by the present calculations, that point exactly to the opposite.

Strain shielding inspired re‐design of proximal femoral stems for total hip arthroplasty

A large number of hip prosthesis with different designs have been developed. However, the influence of hip implant design changes on the strains induced in the bone remains unclear. The purpose of this study is to better understand the mechanics of short stem total hip arthroplasty. Specifically, it investigates whether strain shielding can be avoided by changing implant shape and/or material properties. It is hypothesized that the re-design of existing implant designs can result in further reduction of strain shielding and thus keep bone loss minimal following total hip replacement. Finite element methods were used to compare healthy and implanted models. The local mechanics strains/stresses in the intact and implanted femurs were determined under patient-specific muscle and joint contact forces. Results suggest that small changes in implant geometry and material properties have no major effect on strain shielding. Furthermore, it was found that improvement depends on a dramatic re-design of the original implant design. Whereas the benefit of this strategy of modification of the original geometry of a given short-stemmed hip consists in reduced bone remodeling, care should be taken with regard to long-term bone anchorage and implant fatigue strength. It is also shown that geometrical and material changes have a limited potential in avoiding strain shielding even in short-stemmed implants. Finally, it is suggested that an understanding of the influence of these changes on the strain distribution within the bone can guide in the process of optimizing the current stem designs toward minimal strain shielding effects. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2534-2544, 2017.

Strain shielding in distal radius after wrist arthroplasty with a current generation implant: An in vitro analysis

A systematic review of peer reviewed articles has shown that the main cause for wrist arthroplasty revision is carpal and radial prosthetic loosening and instability. To improve arthroplasty outcomes, successive generations of implants have been developed over time. The problem with the current generation of implants is the lack of long-term outcomes data. The aim of the present work was to test the hypothesis that the current generation Maestro WRS implant has a stress, strain and stability behaviour which may be associated with a reduced risk of long-term radial component loosening. This study was performed using synthetic radii to experimentally predict the cortex strain behaviour and implant stability considering different load conditions for both intact and implanted conditions. Finite element models were developed to assess the structural behaviour of cancellous-bone and bone-cement, these models were validated against experimentally measured cortex strains. Measured cortex strains showed a significant reduction between intact and implanted states. Cancellous bone adjacent to the radial body component suffers a two to threefold strain reduction, comparing with the intact condition, while along the radial stem, in the axial direction, a strain increase was observed. It is concluded that the use of contemporary Maestro WRS implant changes the biomechanical behaviour of the radius and is associated with a potential risk of bone resorption by stress-shielding in the distal radius region for wrist loads in the range of daily activities.