Analysis of a femoral hip prosthesis designed to reduce stress shielding

The natural stress distribution in the femur is significantly altered after total hip arthroplasty (THA). When an implant is introduced, it will carry a portion of the load, causing a reduction of stress in some regions of the remaining bone. This phenomenon is commonly known as stress shielding. In response to the changed mechanical environment the shielded bone will remodel according to Wolff's law, resulting in a loss of bone mass through the biological process called resorption. Resorption can, in turn, cause or contribute to loosening of the prosthesis. The problem is particularly common among younger THA recipients. This study explores the hypothesis that through redesign, a total hip prosthesis can be developed to substantially reduce stress shielding. First, we describe the development of a new femoral hip prosthesis designed to alleviate this problem through a new geometry and system of proximal fixation. A numerical comparison with a conventional intramedullary prosthesis as well as another proximally fixed prosthesis, recently developed by Munting and Verhelpen (1995. Journal of Biomechanics 28(8), 949–961) is presented. The results show that the new design produces a more physiological stress state in the proximal femur.     

Discriminating the loosening behaviour of cemented hip prostheses using measurements of migration and inducible displacement.

In vitro pre-clinical tests of hip prostheses have not yet been developed to the extent that inferior prostheses can be 'screened-out' prior to animal or clinical trials. This paper reports the experimental part of a project to develop a pre-clinical testing platform for cemented femoral hip implants. It is based on the clinical observation (K?rrholm et al. JBJS, 76B (1994) 912-916) that higher subsidence (distal migration) correlates with early revision of hip prostheses. A protocol to measure the relative movement between implant and bone was designed to test whether or not such a measurement, if made in a laboratory, could discriminate between hip prostheses. The protocol was applied to the Lubinus SPII prosthesis (W. Link, Germany) and a Müller Curved Stem (JRI Ltd., UK)-these prostheses were chosen because they are known to have different loosening rates in vivo. Five prostheses of each design were tested. The migration, the rate-of-migration, and the inducible displacement of each prosthesis was recorded over two million cycles of loading.For each implant, rapid initial migration was found, followed by a period of steady-state migration. In the majority of cases, the prostheses migrated medially, distally and posteriorly. On average, the Lubinus migrated less than the Müller in all directions. The average Lubinus migration was less than half that of the Müller, and this difference was significant at a level of p=0.05. Inducible displacement was greater for the Müller compared to the Lubinus. Furthermore, the inducible displacement decreased over time for the majority of Lubinus prostheses whereas it increased over time for the majority of the Müller prostheses leading to the conclusion that a rapid pre-clinical test based on measurement of inducible displacement may be possible.     

Shape Optimization of a Cementless Hip Stem for a Minimum of Interface Stress and Displacement

The primary stem stability is an essential factor for success of cementless hip stems. A correct choice of the stem geometry can improve the stem stability and, consequently, increase the life time of a hip implant. In this work, it is proposed a computational model for shape optimization of cementless hip stems. The optimization problem is formulated by the minimization of relative displacement and stress on bone/stem interface using a multi-criteria objective function. Also multiple loads are considered to incorporate several daily life activities. Design variables are parameters that characterize the geometry of selected cross sections, which are subject to geometric constraints to ensure a clinically admissible shape. The stem/bone set is considered a structure in equilibrium with contact conditions on interface. The contact formulation allows us to analyze different lengths of porous coating. The optimization problem is solved numerically by a steepest descent method. The interface stress and relative displacement are obtained solving the contact problem by the finite element method. Numerical examples are presented for a two-dimensional model of a hip stem, however, the formulation is general and can be applied to the three-dimensional case. The model gives indications about the relation between shape, porous coating and prosthesis stability.     

Experimental method for the in vitro testing of the initial stability of cementless hip prostheses

Micromotions at the interface between bone and prosthesis are believed to induce bone resorption and ultimately lead to loosening of the implant. Thus the initial stability achieved by a hip prosthesis is an important factor for the long-term function of the implant. Knowing the biological consequences of the mechanical conditions, it appears to be mandatory to measure the extent of these three-dimensional movements. An in vitro dynamic method for measurement of the micromotion of the femoral component of hip prostheses has been developed. Tests in cemented prostheses have confirmed that the use of cement reduces sinkage and rotation manyfold and have yielded reference values for stability. Comparison with two types of cementless prostheses has shown that certain cementless implants may achieve stability comparable to cemented ones in some load directions.     

A model of otolith stimulation

A new model of otolithic stimulation by linear acceleration is presented and compared to previous models, based upon anatomical evidence and on the ability of normal subjects to sense the direction of a linear acceleration vector acting in the coronal plane (roll-tilt perception). There are two basic methods of generating roll-tilt stimuli: 1) tilt-chairs either inside or outside a centrifuge and 2) fixed-chair centrifuges. The present model is based on consideration of the probable otoconial displacement produced by these two different methods of stimulation and the model incorporates a major role for the elastic restoring force of the otolith membrane. When this force is taken into account, and most previous models have ignored it, the model predicts that different patterns of otoconial displacement will be produced in tilt-chair and in fixed-chair centrifuge experiments. The different roll-tilt perception produced by these two methods may be caused by the different otoconial displacement patterns. It is suggested that the elastic restoring force of the otoconial membrane may contribute to space motion sickness.     

Topological optimization in hip prosthesis design

With particular interest on total hip arthroplasty (THA), optimization of orthopedic prostheses is employed in this work to minimize the probability of implant failure or maximize prosthesis reliability. This goal is often identified with the reduction of stress concentrations at the interface between bone and these devices. However, aseptic loosening of the implant is mainly influenced by bone resorption phenomena revealed in some regions of the femur when a prosthesis is introduced. As a consequence, bone resorption appears due to stress shielding, that is to say the decrease of the stress level in the implanted femur caused by the significant load carrying of the prosthesis due to its higher stiffness. A maximum stiffness topological optimization-based (TO) strategy is utilized for non-linear static finite element (FE) analyses of the femur–implant assembly, with the goal of reducing stress shielding in the femur and to furnish guidelines for re-designing hip prostheses. This is accomplished by employing an extreme accuracy for both the three-dimensional reconstruction of the femur geometry and the material properties maps assigned as explicit functions of the local densities.     

Use of an absorbable membrane to position biologically inductive materials in the periprosthetic space of cemented joints

A device is presented that positions ultrahigh molecular weight polyethylene (UHMWPE) debris against periprosthetic bone surfaces. This can facilitate the study of aseptic loosening associated with cemented joint prostheses by speeding the appearance of this debris within the periprosthetic space. The device, composed of a 100 microm thick bioabsorbable membrane impregnated with 1.4 x 10(9) sub-micron particles of UHMWPE debris, is positioned on the endosteum of the bone prior to the insertion of the cemented orthopedic implant. An in vitro pullout study and an in vivo canine pilot study were performed to investigate its potential to accelerate "time to aseptic loosening" of cemented prosthetic joints. Pullout studies characterized the influence of the membrane on initial implant fixation. The tensile stresses (mean+/-std.dev.) required to withdraw a prosthesis cemented into canine femurs with and without the membrane were 1.15+/-0.3 and 1.54+/-0.01 MPa, respectively; these findings were not significantly different (p > 0.4). The in vivo pilot study, involving five dogs, was performed to evaluate the efficacy of the debris to accelerate loosening in a canine cemented hip arthroplasty. Aseptic loosening and lameness occurred within 12 months, quicker than the 30 months reported in a retrospective clinical review of canine hip arthroplasty.     

Bone remodelling of the scapula after a total shoulder arthroplasty

According to Wolff’s law, the changes in stress after a prosthesis implantation may modify the shape and internal structure of bone, thus compromising the long-term prosthesis fixation and, consequently, be a significant factor for glenoid loosening. The aim of the present study is to evaluate the changes in the bone adaptation process of the scapula after an anatomical and reverse total shoulder arthroplasty. Five finite element models of the implanted scapula are developed considering the implantation of three anatomical, cemented, all-polyethylene components; an anatomical, cementless, metal-backed component; and a reverse, all-metal component. The methodology followed to simulate the bone adaptation of the scapula was previously validated for the intact model, prior to the prosthesis implantation. Additionally, the influence of the bone quality on the adaptation process is also investigated by considering an osteoporotic condition. The results show that the stress shielding phenomenon is more concerning in cementless, metal-based components than in cemented, all-polyethylene components, regardless of the bone quality. Consequently, as far as the bone adaptation process of the bone is concerned, cemented, all-polyethylene components are better suited for the treatment of the shoulder joint.     

Shape optimal design of the stem of a cemented hip prosthesis to minimize stress concentration in the cement layer

An optimal shape of the metal stem of a cemented total hip prosthesis minimizing stress concentration in the cement layer was searched for. A gradient projection method of numerical optimization and a finite element method of stress analysis were employed. A two-dimensional model of the femoral part of a total hip prosthesis was derived equivalent to a simplified three-dimensional axisymmetric model. The result of the stress analysis of the two-dimensional model compared favorably with that of the three-dimensional axisymmetric model. Using this two-dimensional model, an optimal shape of the stem, minimizing stress concentration in the cement layer, was obtained by a gradient projection method and the shape was checked again by the three-dimensional finite element analysis. The resulting optimal shape of the stem profile was in good agreement with conventional ones, except in the proximal region where a significant amount of stress reduction in the cement layer was achieved by tapering the stem to the limit that the stem still could withstand the increased stem stress.     

Residual stresses at the stem-cement interface of an idealized cemented hip stem

During the operation of total hip arthroplasty, when the cement polymerizes between the stem implant and the bone, residual stresses are generated in the cement. The purpose of this study was to determine whether including residual stresses at the stem-cement interface of cemented hip implants affected the cement stress distributions due to externally applied loads. An idealized cemented hip implant subjected to bending was numerically investigated for an early post-operative situation. The finite element analysis was three-dimensional and used non-linear contact elements to represent the debonded stem-cement interface. The results showed that the inclusion of the residual stresses at the interface had up to a 4-fold increase in the von Mises cement stresses compared to the case without residual stresses.     

Evaluation of cement stresses in finite element analyses of cemented orthopaedic implants.

Stress analysis of the cement fixation of orthopaedic implants to bone is frequently carried out using finite element analysis. However the stress distribution in the cement layer is usually intricate, and it is difficult to report it in a way that facilitates comparison of implants for pre-clinical testing. To study this problem, and make recommendations for stress reporting, a finite element analysis of a hip prosthesis implanted into a synthetic composite femur is developed. Three cases are analyzed: a fully bonded implant, a debonded implant, and a debonded implant where the cement is removed distal to the stem tip. In addition to peak stresses, and contour and vector plots, a stressed volume and probability-of-failure analysis is reported. It is predicted that the peak stress is highest for the debonded stem, and that removal of the distal cement more than halves this peak stress. This would suggest that omission of the distal cement is good for polished prostheses (as practiced for the Exeter design). However, if the percentage of cement stressed above a certain threshold (say 3 MPa) is considered, then the removal of distal cement is shown to be disadvantageous because a higher volume of cement is stressed to above the threshold. Vector plots clearly demonstrate the different load transfer for bonded and debonded prostheses: A bonded stem generates maximum tensile stresses in the longitudinal direction, whereas a debonded stem generates most tensile stresses in the hoop direction, except near the tip where tensile longitudinal stresses occur due to subsidence of the stem. Removal of the cement distal to the tip allows greater subsidence but alleviates these large stresses at the tip, albeit at the expense of increased hoop stresses throughout the mantle. It is concluded that a thorough analysis of cemented implants should not report peak stress, which can be misleading, but rather stressed volume, and that vector plots should be reported if a precise analysis of the load transfer mechanism is required.     

Shape optimization of a cementless hip stem for a minimum of interface stress and displacement

The primary stem stability is an essential factor for success of cementless hip stems. A correct choice of the stem geometry can improve the stem stability and, consequently, increase the life time of a hip implant. In this work, it is proposed a computational model for shape optimization of cementless hip stems. The optimization problem is formulated by the minimization of relative displacement and stress on bone/stem interface using a multi-criteria objective function. Also multiple loads are considered to incorporate several daily life activities. Design variables are parameters that characterize the geometry of selected cross sections, which are subject to geometric constraints to ensure a clinically admissible shape. The stem/bone set is considered a structure in equilibrium with contact conditions on interface. The contact formulation allows us to analyze different lengths of porous coating. The optimization problem is solved numerically by a steepest descent method. The interface stress and relative displacement are obtained solving the contact problem by the finite element method. Numerical examples are presented for a two-dimensional model of a hip stem, however, the formulation is general and can be applied to the three-dimensional case. The model gives indications about the relation between shape, porous coating and prosthesis stability.     

Mechanical Response of Bone under Short-term Loading of a Dental Implant with an Internal Layer Simulating the Nonlinear Behaviour of the Periodontal Ligament

We consider a non-standard design for a fixed dental implant, incorporating a soft layer which simulates the presence of the periodontal ligament (PDL). Instead of being aimed at causing an a priori defined stress/strain field within the surrounding bone, upon loading, such a design simply tries to better reproduce the natural tooth–PDL configuration. To do this, the mechanical properties of the internal layer match those of the PDL, determined experimentally to be strongly nonlinear. Three-dimensional finite element analyses show that the presence of such a layer produces (i) a prosthesis mobility very similar to that of a healthy tooth, for several loading conditions, and (ii) a stress/strain distribution substantially different from that arising, upon loading, around a conventional implant. The lack of knowledge of the real mechanical fields existing, under loading, in the bone around a healthy tooth makes it very difficult to state that the stress distribution produced by the modified implant is “better” than that produced by the standard one. Nevertheless, the comparison of the results obtained here, with those of previous refined analyses of the tooth–PDL–bone system, indicates that the modified implant tends to produce a stress distribution in the bone, upon loading, closer to “natural” than that given by the standard one, within the limits imposed by the presence of threads coupling the implant with the bone.     

Flexible device for vertebral body replacement

A novel vertebral prosthesis is presented. The prosthesis was developed for surgical procedures requiring the resection of a complete vertebral body and the adjacent intervertebral discs, the design objective being to develop a flexible implant that would be robust enought to withstand the in vivo stress environment of the human spine. In theory, a flexible implant should preserve a more normal range of motion and apply less stress to surrounding tissue than a rigid implant. A prototype implant was constructed so as to combine a rigid stainless steel structure with flexible silicon rubber elements in order to form an implant with static and dynamic mechanical characteristics similar to those of the anterior spinal column. Implant flexibility characteristics were determined from ex vivo stress-strain behaviour during bending and compressive creep testing. Results from the bending tests indicated good agreement for the lateral and sagittal bending characteristics in comparison with in vitro bending tests of human lumbar motion segments. Comparison of the implant compressive creep responsé with similar in vitro tests on human lumbar intervertebral discs also demonstrated similarities in the time-dependent mechanical parameters.     

Application of an anisotropic bone-remodelling model based on a damage-repair theory to the analysis of the proximal femur before and after total hip replacement

In this work, a new model for internal anisotropic bone remodelling is applied to the study of the remodelling behaviour of the proximal femur before and after total hip replacement (THR). This model considers bone remodelling under the scope of a general damage-repair theory following the principles of continuum damage mechanics. A "damage-repair" tensor is defined in terms of the apparent density and Cowin's "fabric tensor", respectively, associated with porosity and directionality of the trabeculae. The different elements of a thermodynamically consistent damage theory are established, including resorption and apposition criteria, evolution law and rate of remodelling. All of these elements were introduced and discussed in detail in a previous paper (García, J. M., Martinez, M. A., Doblaré, M., 2001. An anisotrophic internal-external bone adaptation model based on a combination of CAO and continuum damage mechanics technologies. Computer Methods in Biomechanics and Biomedical Engineering 4(4), 355-378.), including the definition of the proposed mechanical stimulus and the qualitative properties of the model. In this paper, the fundamentals of the proposed model are briefly reviewed and the computational aspects of its implementation are discussed. This model is then applied to the analysis of the remodelling behaviour of the intact femur obtaining densities and mass principal values and directions very close to the experimental data. The second application involved the proximal femoral extremity after THR and the inclusion of an Exeter prosthesis. As a result of the simulation process, some well-known features previously detected in medical clinics were recovered, such as the stress yielding effect in the proximal part of the implant or the enlargement of the cortical layer at the distal part of the implant. With respect to the anisotropic properties, bone microstructure and local stiffness are known to tend to align with the stress principal directions. This experimental fact is mathematically proved in the framework of this remodelling model and clearly shown in the results corresponding to the intact femur. After THR the degree of anisotropy decreases tending, specifically in the proximal femur, to a more isotropic behaviour.     

The transport of wear particles in the prosthetic hip joint: a computational fluid dynamics investigation

The joint fluid mechanics and transport of wear particles in the prosthetic hip joint were analyzed for subluxation and flexion motion using computational fluid dynamics (CFD). The entire joint space including a moving capsule boundary was considered. It was found that particles suspended in the joint space are drawn into the joint gap between prosthesis cup and head during subluxation, which was also documented by Lundberg et al. (2007; Journal of Biomechanics 40, 1676-1685), however, wear particles remain in the joint gap. Wear particles leave the joint gap during flexion and can finally migrate to the proximal boundaries including the acetabular bone, where the particle deposition can cause osteolysis according to the established literature. Thus, the present study supports the theory of polyethylene wear particle induced osteolysis of the acetabular bone as a major factor in the loosening of hip prosthesis cups.     

The canal fill ratio as a factor influencing the aseptic loosening of the Müller-type cemented stem--preliminary report

The Müller type cemented hip prosthesis, also called self-locking prosthesis, is one of the most extensively copied and utilized hip prostheses. Considering the implantation of the cemented stem, the author originally suggested achieving a press fit between the stem and endostal cortex by implanting a prosthesis of the largest possible size. Analyzing our group of patients who undergo aseptic stem revision, we try to quantify the term "press fit" by measuring the filling of the femoral canal by the metal stem, the so-called canal-fill ratio. Our preliminary results suggest that press-fit is achieved when the stem occupies more than 90% of the canal diameter in the anterior-posterior projection.     

Implant Optimisation for Primary Hip Replacement in Patients over 60 Years with Osteoarthritis: A Cohort Study of Clinical Outcomes and Implant Costs Using Data from England and Wales

Background

Hip replacement is one of the most commonly performed surgical procedures worldwide; hundreds of implant configurations provide options for femoral head size, joint surface material and fixation method with dramatically varying costs. Robust comparative evidence to inform the choice of implant is needed. This retrospective cohort study uses linked national databases from England and Wales to determine the optimal type of replacement for patients over 60 years undergoing hip replacement for osteoarthritis.

Methods and Findings

Implants included were the commonest brand from each of the four types of replacement (cemented, cementless, hybrid and resurfacing); the reference prosthesis was the cemented hip procedure. Patient reported outcome scores (PROMs), costs and risk of repeat (revision) surgery were examined. Multivariable analyses included analysis of covariance to assess improvement in PROMs (Oxford hip score, OHS, and EQ5D index) (9159 linked episodes) and competing risks modelling of implant survival (79,775 procedures). Cost of implants and ancillary equipment were obtained from National Health Service procurement data.

Results

EQ5D score improvements (at 6 months) were similar for all hip replacement types. In females, revision risk was significantly higher in cementless hip prostheses (hazard ratio, HR = 2.22, p<0.001), when compared to the reference hip. Although improvement in OHS was statistically higher (22.1 versus 20.5, p<0.001) for cementless implants, this small difference is unlikely to be clinically important. In males, revision risk was significantly higher in cementless (HR = 1.95, p = 0.003) and resurfacing implants, HR = 3.46, p<0.001), with no differences in OHS. Material costs were lowest with the reference implant (cemented, range £1103 to £1524) and highest with cementless implants (£1928 to £4285).Limitations include the design of the study, which is intrinsically vulnerable to omitted variables, a paucity of long-term implant survival data (reflecting the duration of data collection), the possibility of revision under-reporting, response bias within PROMs data, and issues associated with current outcome scoring systems, which may not accurately reflect level of improvement in some patients.

Conclusions

Cement fixation, using a polyethylene cup and a standard sized head offers good outcomes, with the lowest risks and at the lowest costs. The most commonly used cementless and resurfacing implants were associated with higher risk of revision and were more costly, while perceptions of improved function and longevity were unsupported.     

A Eulerian method to analyze wall shear stress fixed points and manifolds in cardiovascular flows

Biomechanics and Modeling in Mechanobiology - Based upon dynamical systems theory, a fixed point of a vector field such as the wall shear stress (WSS) at the luminal surface of a vessel is a point...     

Vector calibration in Australian desert ants,Melophorus bagoti: Effects of a delay after the acquisition of vector information

Desert ants navigate by using two chief strategies: path integration, keeping track of the straight‐line distance and direction to the starting point as they travel, and landmark guidance, orientation based on the visual panorama. Both Cataglyphis ants in North Africa and Melophorus bagoti in Central Australia are known to adjust their vectors derived from path integration to compensate for mismatches between their outbound direction of travel and (the reverse of) the inbound direction of travel that takes them home, a process known as vector calibration. We created mismatches of 90° between the outbound vector and the homebound direction by displacing ants from a feeder before their homebound run. We examined temporal factors in vector calibration by varying the delay (0, 1 or 3 hr) between the outbound run to the feeder and the homebound run from the displacement site. According to the temporal weighting rule, such a delay should decrease the weight given to the vector information obtained from the outbound run. This in turn should favour reliance on the visual panorama and thus speed up calibration. Results did not support this prediction. At the displacement site, a delay had little effect on the extent of calibration or the speed of calibration (the number of trials to reach maximum calibration). Just before being displaced, ants were also tested in a test ring surrounded by high walls that obliterated the visual scenery. In the test ring, a delay made the ants less likely to rely on their vector: ants were often not oriented as a group. Otherwise, the ants in the test ring also did not calibrate any more or any faster.