Biomedical subjects

R Huiskes

Publications and source records attributed to R Huiskes.

At least 73 records · Page 4Linked to original sources

PubMed1998

Effects of prosthesis surface roughness on the failure process of cemented hip implants after stem-cement debonding.

Retrieval studies suggest that the loosening process of the cemented femoral components of total hip arthroplasties is initiated by failure of the bond between the prosthesis and the cement mantle. Finite element (FE) analyses have demonstrated that stem-cement debonding has stress-producing effects in the cement mantle. High interface friction, which corresponds to a degree of surface roughness, reduces these stresses. In experiments, however, debonded rough stems produced more cement damage than polished ones; in the Swedish Hip Register polished stems were clinically superior with respect to stems with a mat surface finish. The purpose of the present study was to investigate this contradiction. For this purpose, global and local FE models with debonded stem-cement interfaces were used to study the effects of prosthesis surface roughness on the cement stresses on a global scale and microscale, respectively. Similar to earlier numerical studies, the global FE model predicted that an increased surface roughness of the stem reduced the stresses in the cement mantle. The local model provided insight in the load-transfer mechanism on a microscale and could explain the experimental and clinical findings. The local cement peak stresses around the asperities of the surface roughness profile increased with increasing surface roughness and decreased again beyond a particular roughness value. Cement abrasion is caused by localized stresses in combination with micromotion. From this study it can be concluded that to minimize cement abrasion, debonded stems should either have a polished microstructure to minimize the local cement stresses or have a profiled macrostructure to minimize micromotions at the stem-cement interface.

Arthroplasty, Replacement, Hip↗

PubMed1998

[The causes of failure for hip and knee arthroplasties].

Most joint replacement prostheses in the hip or knee survive for longer than 10 years. Failure of a prosthesis may be due to infection or a fracture of bone or prosthesis, but much more often it has no clear cause, after a process of aseptic loosening. Mechanisms of aseptic loosening are: bone resorption as a reaction of bone to migrated foreign body particles, material fatigue due to repeated mechanical stress on the prosthesis and its connections with the bone, failed ingrowth due to a poor fit of the (uncemented) prosthesis in the bone bed, resorption of bone mass due to the prosthesis taking over part of the mechanical bone stresses, and wear of the material. The risk of early failure depends on the patient's bone quality and life expectation, on the surgical technique, the weight bearing on the prosthesis and the fixation, and the shape and materials of the prostheses. Selection of durable prosthetic types and techniques is being done more and more by means of stepwise introduction, (pre)clinical tests and post-marketing surveillance.

Aged↗

PubMed1998

Relationships between bone morphology and bone elastic properties can be accurately quantified using high-resolution computer reconstructions.

It would be advantageous if the mechanical properties of trabecular bone could be directly inferred from stereomorphometric parameters. For that purpose, apparent density and mean intercept length, as measures of bone mass and directionality (fabric), are commonly correlated with the elastic characteristics of bone samples, as determined in compression tests. However, complete and accurate relationships have not yet been established in this way. This may be due not only to the occurrence of artifacts in both the stereomorphometric and the mechanical assessments but also to an inherent inadequacy of mean intercept length in characterizing the full mechanical significance of bone architecture or nonhomogeneities in trabecular tissue properties not accounted for in stereomorphometry. In this study, we introduce a computer modeling approach allowing these biases to be eliminated. With use of high-resolution three-dimensional computer reconstructions of trabecular bone specimens for stereomorphometry and for microstructural finite element models to simulate mechanical tests, unbiased comparisons become feasible. The purpose was to investigate if accurate and complete relationships can be established in this way. Four different fabric measures were considered: mean intercept length and three volume-based ones. Compliance matrices were calculated from fabric tensors, with use of the mathematical relationship proposed by Cowin for 29 vertebral whale-bone specimens. These were correlated with the compliance constants determined directly from the microstructural finite element model simulation. The nine orthotropic elastic constants of all 29 specimens were well predicted from their stereomorphometric fabric and volume fraction values, with correlation coefficients ranging from R2adj = 0.9934 to 0.9963. When individual compliance components were considered (1/Ei, 1/Gij, or -v[ij]/Ei), correlation coefficients ranged from R2adj = 0.924 to 0.982. All four fabric measures performed equally well. It is concluded that volume fraction and fabric measures correlate highly with the apparent elastic properties of bone samples, provided that anisotropy and nonhomogeneity in the elastic properties of the trabecular tissue itself have negligible effects on the apparent properties. Whether this is true for bone in general remains to be seen, as only a subset was analyzed here. These methods, however, can be valuable in similar assessments of other subsets.

Animals↗

PubMed1998

Surface roughness of debonded straight-tapered stems in cemented THA reduces subsidence but not cement damage.

Although stress analyses have shown that the mechanical endurance of cemented femoral THA reconstructions is served by stems that firmly bond to their cement mantles, retrieval studies suggest that this may be difficult to achieve. Clinical studies with roentgen stereophotogrammetric analyses have shown that stems may gradually debond from their cement mantle. Accepting the fact that stem debonding is unavoidable, stem subsidence and cement stresses can be reduced by increasing stem-cement friction, as indicated by finite element stress analyses. Hence, it can be hypothesized that debonded stems with high surface roughness values would damage the cement mantle to a lesser extent as compared to polished ones. To confirm this hypothesis, tapered stems with polished and rough surface finishes were implanted in cement mantles and cyclically loaded for 1.7 million times. It was investigated how surface roughness affected the damage in the cement mantle, and how it was related to prosthetic subsidence. The polished taper subsided considerably more than the rough one (630 vs. 270 microm at the end of the experiments). In addition, it was found that the polished taper displayed step-wise subsidence, which is probably due to the interaction of stick-slip processes at the interface, associated with creep of the acrylic cement. The rough taper subsided monotonously. Scanning Electron Microscopic (SEM) analysis of the taper-cement structures showed that the rough taper was completely debonded from the cement mantle, creating a gap at the interface, and that many large cement cracks and particles were created. Around the polished taper, only a few cracks were found and the taper-cement interface seemed undamaged. It was concluded that an increased surface roughness does not necessarily lead to a reduction in cement damage. On the contrary, compared to polished ones, debonded rough stems may produce more cement cracks and acrylic cement debris, and provide routes to transport these wear products. Hence, after failure of the stem-cement interface, straight-tapered stems with an increased surface roughness accelerate the failure process due to inferior fail-safe features. Consequently, in vivo subsidence patterns at the stem-cement interface should be considered in combination with the surface finish of the implant. An amount of post-operative subsidence of a rough stem may be much more damaging for the reconstruction than the same amount for a polished stem.

Bone Cements↗

PubMed1998

Acetabular reconstruction with impacted morselized cancellous allografts in cemented hip arthroplasty: a histological and biomechanical study on the goat.

Bone defects in total hip arthroplasty revision surgery can be restored with different types of bone graft. The use of impacted morselized allograft chips in combination with cement is the treatment of our choice. To establish the incorporation capacity of the grafts and mechanical stability of the implant, an animal model in the goat was developed. An acetabular defect was created and restored with morselized grafts and a cemented cup. Postoperative performance of the reconstruction was followed both histologically and biomechanically. Histology showed that consolidation of the graft with the host bone bed had occurred within 3 weeks. In the following period a front of vascular sprouts infiltrated the graft. Graft resorption, woven bone deposition, and subsequent remodeling resulted in a new trabecular structure. This structure contained only scarce remnants of the original dead graft material. At the graft-cement interface, graft resorption and new bone formation had resulted in areas of direct vital bone-cement contact. Locally, a soft tissue interface was present. After longer follow-up periods, progressive interface formation and loosening of the cups were found in most animals. Mechanical testing showed that the stability of the reconstruction increased during the first 12 postoperative weeks. Thereafter, the stability decreased, probably by soft-tissue interface formation at the graft cement interface. We conclude that cemented morselized allografts have a high capacity to incorporate. Initial cup stability is adequate to provoke graft incorporation with decreasing stability after the incorporation process has been completed.

Acetabulum↗

PubMed1998

Effect of mechanical set point of bone cells on mechanical control of trabecular bone architecture.

The architecture of trabecular bone is thought to be controlled by mechanosensitive bone cells, where hormones provide a background for their responses to mechanical signals. It has been suggested that, in osteoporosis, this response is hampered by changed hormonal levels, thereby increasing the mechanical set point of the cells, which would lead to bone loss. We have investigated if a temporary increase of the mechanical set point causes deterioration of trabecular bone architecture, such as seen in osteoporosis. Furthermore, the effects of a changed loading pattern were investigated for the same reason. For this purpose, we used a computer simulation model, which was based on the regulation of bone architecture by mechanosensitive osteocytes. It was found that a temporary shift of the mechanical set point causes no lasting changes in architecture. Although an increase of the mechanical set point induces bone loss, the mechanism of bone loss (trabecular thinning) differs from what is observed in osteoporosis (loss of whole trabeculae). Hence, a change of the mechanical set point alone cannot explain bone loss as seen in osteoporosis. On the other hand, the removal of load components in a particular direction resulted in irreversible loss of whole trabeculae. These results indicate that such temporary changes in loading patterns could be important risk factors for osteoporosis.

Adult↗

PubMed1998

Migration, stem shape, and surface finish in cemented total hip arthroplasty.

In many recent publications it was suggested that the amount of early subsidence of a femoral stem in total hip arthroplasty is indicative for later revision. In this article it is argued that stems can be designed according to alternative objectives, resulting in different shapes and surface roughness, each producing its own characteristic postoperative subsidence pattern. It was investigated whether these inherent subsidence patterns can be estimated in preclinical testing. For that purpose two stems, both without a collar, relying on cement fixation only, were compared regarding their stress transfer, migration, and induced micromotion behavior. Finite element analysis, cyclic bench testing of substitute bone reconstructions, and clinical radiostereophotogrammetric analysis were applied. The stems investigated were the Exeter, which is assumed to be a force closed fixation design, relying on subsidence under load as a method of maintaining stability, and the SHP, as a shape closed fixation design, meant to be contained by the cement mantle. Both designs were true to their design concepts in the analyses, in the sense that migrations and micromotions of the Exeter stems far exceeded those of the SHP stems. It was found that preclinical studies such as finite element analysis or bench tests give reasonable indications of in vivo postoperative behavior. It is concluded that early clinical migration values should be considered relative to stem shape and surface finish, when prediction of later revision probability is the issue.

Aged↗

PubMed1998

Acetabular reconstruction with impacted morsellised cancellous bone graft and cement. A 10- to 15-year follow-up of 60 revision arthroplasties.

We report a long-term review of 60 acetabular components revised using impacted, morsellised bone allografts and a cemented polyethylene cup. The acetabular defects were cavitary (37) or combined (23). Follow-up was for a mean 11.8 years (10 to 15). Further revision was needed in five hips, two for septic and three for aseptic loosening. The overall survival rate at 11.8 years was 90%; excluding the septic cases it was 94%. Acetabular reconstruction with impacted morsellised cancellous grafts and cement gives satisfactory long-term results.

Acetabulum↗

PubMed1998

Three-dimensional displacement of the hip joint after triple pelvic osteotomy. A postmortem radiostereometric study.

Triple pelvic osteotomy reorients the acetabulum relative to the pelvis in order to improve acetabular coverage of the femoral head in cases of acetabular dysplasia. We undertook a radiostereometric analysis (RSA) on 6 osteotomized cadaver hips to determine the actual three-dimensional reorientation obtained. The centers of the femoral head were all translated posteriorly between 11 and 41 mm, and distally up to 13 mm. 4 were lateralized up to 8 mm, and 2 were medialized up to 5 mm. All acetabuli rotated anteriorly about the lateral to medial axis (X-axis), and 4 rotated outwards around the distal to proximal axis (Y-axis). The correlations between measurements performed on conventional anteroposterior radiographs and the RSA measurements were poor: variations in the lateral-medial direction ranged from -16 to +6 mm, and in the distal-proximal direction between -10 and +12 mm. The changes in orientations measured will significantly affect the load across the hip joint, since the dimensions of the pelvis change and the moment arms of the muscles, their lengths and lines of action are changed as well. We conclude that, with the procedures presently performed, the loads across the hip joint are bound to change, and that the reorientation can hardly be checked with conventional radiographs.

Acetabulum↗

PubMed1997

An inverse dynamics modeling approach to determine the restraining function of human knee ligament bundles.

During knee motion, the fiber bundles of ligaments are nonuniformly loaded in a recruitment pattern which is different for successive knee-joint positions. As a result, the restraining functions of these ligaments are variable. To analyze the relative restraint contributions of the fiber bundles in different knee-joint positions, a new method was developed. Its application was illustrated for the cruciate ligaments of one knee-joint specimen. The methods developed to estimate bundle forces comprise five steps. First, the three-dimensional motions of a knee specimen are measured for anterior-posterior forces, using Röntgen Stereophotogrammetric Analysis. Second, bone-ligament-bone tensile tests are performed to evaluate the mechanical properties of these structures in several relative orientations of the bones. Third, multiple fiber bundles are identified in each ligament, based on the main fiber orientations. Fourth, the nonlinear force-length relationship of each functional bundle, as defined by a stiffness and a recruitment parameter, is determined by combining the multidirectional tensile tests with a multiline-element ligament model. Finally, the information obtained is combined in a whole-joint computer model of the knee, to determine the internal forces in the initial kinematic experiment, using an inverse dynamics approach. The technique appeared to be extremely time consuming and technologically involved. However, it was demonstrated to be useful and effective. The preliminary results reveal that the fiber bundle restraints are extremely sensitive to the knee flexion angle and the restraining forces are highly variable within the ligaments. For both cruciate ligaments, a gradual transition was demonstrated in load transfer from the posterior bundles to the more anteriorly positioned ones during knee flexion. Furthermore, it appeared that relatively high forces were carried by only a few fiber bundles at each flexion angle. Based on these preliminary results, it is concluded that the determination of forces in multiple ligament bundles is important for the understanding of failure mechanisms of ligaments. In particular, alternate loading of different fiber bundles suggests that successful operative reconstruction of the cruciate ligaments may not be achieved simply by a one-bundle preparation.

Algorithms↗

PubMed1997

ESB Research Award 1996. Biophysical stimuli on cells during tissue differentiation at implant interfaces.

If musculoskeletal tissues are indeed efficient for their mechanical function, it is most reasonable to assume that this is achieved because the mechanical environment in the tissue influences cell differentiation and expression. Although mechanical stimuli can influence the transport of bioactive factors, cell deformation and cytoskeletal strain, the question of whether or not they have the potential to regulate tissue differentiation sequences (for example, during fracture healing or embryogenesis) has not been answered. To assess the feasibility of biophysical stimuli as mediators of tissue differentiation, we analysed interfacial tissue formation adjacent to a micromotion device implanted into the condyles of dogs. A biphasic finite element model was used and the mechanical environment in the tissue was characterised in terms of (i) forces opposing implant motion, (ii) relative velocity between constituents, (iii) fluid pressure, (iv) deformation of the tissue and (v) strain in the tissue. It was predicted that, as tissue differentiation progressed, subtle but systematic mechanical changes occur on cells in the interfacial tissue. Specifically, as the forces opposing motion increase, the implant changes from being controlled by the maximum-allowable displacement (motion-control) to being controlled by the maximum-available load (force-control). This causes a decrease in the velocity of the fluid phase relative to the solid phase and a drop in interstitial fluid pressure accompanied by a reduction in peri-prosthetic tissue strains. The variation of biophysical stimuli within the tissue can be plotted as 'mechano-regulatory pathway', which identifies the transition from motion-control to force-control as a branching event in the tissue differentiation sequence.

Animals↗

PubMed1997

Fabric and elastic principal directions of cancellous bone are closely related.

Cancellous bone architecture and mechanics are intimately related. The trabecular architecture of cancellous bone is considered determined by its mechanical environment (Wolff's law), and the mechanical properties of cancellous bone are inversely determined by the trabecular architecture and material properties. Much effort has been spent in expressing these relations, but the techniques and variables necessary for this have not been fully identified. It is obvious, however, that some measure of architectural anisotropy (fabric) is needed. Within the last few years, volume-based measures of fabric have been introduced as alternatives to the mean intercept length method, which has some theoretical problems. This paper seeks to answer which of four different fabric measures best predicts finite element calculated mechanical anisotropy directions. Twenty-nine cancellous bone specimens were three-dimensionally reconstructed using the automated serial sectioning technique. A series of large-scale finite-element analyses were performed on each of the three-dimensional reconstructions to calculate the compliance matrix for each specimen, from which the mechanical principal directions were derived. The architectural anisotropy was determined in three-dimensional space for each specimen using mean intercept length (MIL), volume orientation (VO), star volume distribution (SVD) and star length distribution (SLD). Each of the architectural anisotropy results were expressed by a fabric tensor. Architectural main directions were determined from the fabric tensors and compared with the FE-calculated mechanical anisotropy directions. All architectural measures predicted the mechanical main directions rather well, which supports the assumption that mechanical anisotropy directions are aligned with fabric directions. MIL showed a significant, though very small (1.4 degrees), deviation from the primary mechanical direction. VO had difficulty in determining secondary and tertiary mechanical directions; its mean deviation was 8.9 degrees. SVD and SLD provided marginally better predictors of mechanical anisotropy directions than MIL and VO.

Animals↗

PubMed1997

The effects of cement-stem debonding in THA on the long-term failure probability of cement.

The damage accumulation failure scenario is one of the most prominent ones of cemented THA reconstruction, and involves the accumulation of mechanical damage in materials and interfaces due to repetitive dynamic loading eventually resulting in gross loosening. This study addresses this scenario by combining finite element techniques with the theory of continuum damage mechanics, to analyze the damage accumulation process in the cement mantle. It was investigated how damage accumulation was affected by stem-cement debonding, and what the effects of a layer with poor bone quality around the cement mantle were. For the unbonded stem, it was determined if clinical migration rates can be explained by failure of the cement mantle, and whether cement failure promotes the formation of a pathway for debris at the stem-cement interface. It was found that stem-cement debonding not only elevated the initial stress levels with a factor of about two to three as demonstrated in earlier studies, but remained to have an impact on the failure process of the cement mantle. Stem-cement debonding accelerated the failure process by a factor of four, and promoted the formation of a pathway for debris at the stem-cement interface, particularly when the bone support to the cement mantle was reduced. The amount of subsidence was only substantial when the damaged cement mantle was surrounded by a layer of bone with reduced stiffness. This study supports the hypothesis that the survival of cemented THA is enhanced by a firm and lasting bond between the stem and the cement mantle, although this may be difficult to realize clinically.

Bone Cements↗

PubMed1997

Histological and biomechanical analysis of bone and interface reactions around hydroxyapatite-coated intramedullary implants of different stiffness: a pilot study on the goat.

We hypothesized that reduced stem stiffness of orthopaedic implants contributes to a high risk of loosening, since interface stresses and relative motions may exceed a tolerable range. To study this hypothesis, three types of load-bearing implant with different stiffnesses were inserted into the tibia of the goat. Histological analysis was performed of bone repair after insertion of the implant, bone ingrowth, interface disruption and loosening. A finite element model of the configuration provided the quantitative range of interface stresses and relative motions for the present experiment. The implants were made out of stainless steel, hollow titanium and a thin titanium core covered with a polyacetal coating. The stiffness ratios of these implants were approximately 10:4:1, respectively. All implants were coated with a layer of hydroxyapatite (HA) in order to minimize the possible biological effects of the different implant materials. Irrespective of the type of implant, there was a repair phase that lasted 6-12 weeks. The stiff implants functioned well. Large areas of bone bonding to the HA layer were found after the repair phase at 12 weeks postoperatively. After 24 weeks, some signs of loosening were observed. More loosening occurred with the hollow titanium and polyacetal implants, mainly during the repair phase. Three hollow titanium and three polyacetal coated implants survived this period, and were killed after 24 weeks. The integrity of the HA layer at the bone-implant interface of the titanium implants was good. In the polyacetal implants, the repair reaction of the cortical bone was incomplete. Bone ingrowth into HA was largely lacking. In conclusion, we found significant differences in the repair and interface reactions around implants of different stiffness. Stiff implants showed favourable initial interface conditions for bone ingrowth. Intermediate and flexible implants provoked unfavourable interface conditions for initial bone ingrowth. The finite element study showed that the flexible stems produce larger micromotions and higher interface stresses at the bone-prosthesis interface than the stiff stems, indicating an explanation for the histological findings.

Acetals↗

PubMed1997

Osteocytes and bone lining cells: which are the best candidates for mechano-sensors in cancellous bone?

Previously, we have investigated the possible role of osteocytes as mechano-sensors, and mediators of bone turnover. It was found that the proposed regulatory mechanism produced morphologies of trabecular bone, under particular loading conditions, which were consistent with morphogenesis and adaptation as seen in reality. The main objective of this study was to discern whether lining cells or osteoblasts could possibly play a similar role as effectively with regard to their capacity for self-optimization of the trabecular architecture, in terms of a low apparent mass to stiffness ratio. For that purpose the earlier analyses with osteocytes as mechano-sensors, distributed throughout the bone, were repeated for mechano-sensors located at bone surfaces only. Compared to the osteocyte model, the surface cell remodeling algorithm was reluctant to change its architecture, which implies that it is less sensitive to changes in the loading pattern. This resulted in less efficient bone adaptation, which was reflected by a considerably higher relative mass for a similar apparent stiffness in the loading direction. In other words, more mass is needed to obtain an equally stiff structure, at the apparent level, with respect to the externally applied loads. Furthermore, stresses and strains at the tissue level vary across a much wider range, relative to the osteocyte model, where the higher incidence of elevated strains indicates an increased failure risk. Therefore, we conclude that mechanical information at the bone surface may not be sufficient to adequately regulate functional bone adaptation.

Animals↗

PubMed1997

A biomechanical regulatory model for periprosthetic fibrous-tissue differentiation.

Loosening of implants in bone is commonly associated with a development of fibrous interface tissues, due to interface gaps and a lack of mechanical stability. It has been postulated that the differentiation of these tissues to fibrocartilage or bone is governed by mechanical stimuli. The objective of our research is to unravel these relationships to the extent that the question whether an implant will loosen can be answered from initial conditions determined by implant and interface morphology, and functional loads. In this project we studied the hypothesis that distortional strain and interstitial fluid flow are the mechanical stimuli governing tissue differentiation. For that purpose, a biomechanical regulatory model was developed and used in association with a finite element code to simulate animal experiments with implants moving in bone. The similarities between the implant incorporation process found in the experiment and its simulation with the computer model demonstrate that our hypothesis is viable from a regulatory point of view.

Journal Article↗

PubMed1997

Cement debonding process of total hip arthroplasty stems.

Retrieval studies have indicated that debonding of the stem cement interface in total hip arthroplasty precedes clinical failure of femoral components. This study addressed the mechanisms that play a role in the debonding process by analyzing how debonding is likely to proceed in the course of time. It was investigated whether debonding is an immediate process or if it is likely to develop slowly with time, which interface stress components contribute particularly to its progression, and whether the mechanical integrity of the cement mantle is likely to be compromised by the debonding process. To answer these questions, a 3-dimensional finite element model of a femoral total hip arthroplasty reconstruction was developed and used to simulate the debonding process. The results showed that debonding was governed by the shear stress component at the interface. Debonding started in the tip region and the proximal, medial anterior region. These debonded regions expanded until the whole interface was de bonded. Cement stresses slowly increased at the end of the debonding process to a level twice as high as the initial one. The probability of debonding, as measured by an interface failure index, remained constant as debonding progressed. This indicates that, for this particular design, much less surface area is required for load transfer than is provided by the stem, and the debonding process does not necessarily accelerate quickly once debonding is initiated.

Biomechanical Phenomena↗

PubMed1997

Mathematical optimization of elastic properties: application to cementless hip stem design.

The designer of a cementless hip stem in total hip replacement must find a balance between two conflicting demands. On the one hand, a stiff stem shields the surrounding bone from mechanical loading (stress shielding), which may lead to bone loss, particularly around the proximal part of the stem. Reducing the stem stiffness decreases the amount of stress shielding and hence the amount of bone loss. However, this measure inevitably promotes higher proximal interface stresses and thereby increases the risk of proximal interface failure. The designer's task therefore is to optimize the stem stiffness in order to find the best compromise in the conflict. Yet, a better compromise might be found when the stem material was nonhomogeneous, in other words when an arbitrary distribution of the elastic properties inside the stem was allowed. The number of conceivable designs would increase enormously, making the designer's task almost impossible. In the present paper, we develop a numerical design optimization method to determine the optimal stiffness characteristics for a hip stem. A finite element program is coupled with a numerical optimization method, thus producing a design optimization scheme. The scheme minimizes the probability for interface failure while limiting the amount of bone loss, by adapting the parameters describing the nonhomogeneous elastic modulus distribution. As an example, a simplified model of a hip stem is made, whose modulus distribution is optimized. Assuming equal long-term bone loss, the maximum interface stress can be reduced by over 50 percent when compared to a homogeneous flexible stem, thus demonstrating the value of the new approach.

Elasticity↗