Influence of ingrowth regions on bone remodelling around a cementless hip resurfacing femoral implant

Hip resurfacing arthroplasty is an alternative to traditional hip replacement that can conserve proximal bone stock and has gained popularity but bone resorption may limit implant survival and remains a clinical concern. The goal of this study was to analyze bone remodelling patterns around an uncemented resurfacing implant and the influence of ingrowth regions on resorption. A computed tomography-derived finite element model of a proximal femur with a virtually implanted resurfacing component was simulated under peak walking loads. Bone ingrowth was simulated by six interface conditions: fully bonded; fully friction; bonded cap with friction stem; a small bonded region at the stem-cup intersection with the remaining surface friction; fully frictional, except for a bonded band along the distal end of the cap and superior half of the cap bonded with the rest frictional. Interface condition had a large influence on remodelling patterns. Bone resorption was minimized when no ingrowth occurred at the bone-implant interface. Bonding only the superior half of the cap increased bone resorption slightly but allowed for a large ingrowth region to improve secondary stability.     

Bone remodelling around a cementless glenoid component

Post-operative change in the mechanical loading of bone may trigger its (mechanically induced) adaptation and hamper the mechanical stability of prostheses. This is especially important in cementless components, where the final fixation is achieved by the bone itself. The aim of this study is, first, to gain insight into the bone remodelling process around a cementless glenoid component, and second, to compare the possible bone adaptation when the implant is assumed to be fully bonded (best case scenario) or completely loose (worst case scenario). 3D finite element models of a scapula with and without a cementless glenoid component were created. 3D geometry of the scapula, material properties, and several physiological loading conditions were acquired from or estimated for a specific cadaver. Update of the bone density after implantation was done according to a node-based bone remodelling scheme. Strain energy density for different loading conditions was evaluated, weighted according to their frequencies in activities of daily life and used as a mechanical stimulus for bone adaptation. The average bone density in the glenoid increased after implantation. However, local bone resorption was significant in some regions next to the bone-implant interface, regardless of the interface condition (bonded or loose). The amount of bone resorption was determined by the condition imposed to the interface, being slightly larger when the interface was loose. An ideal screw, e.g. in which material fatigue was not considered, was enough to keep the interface micromotions small and constant during the entire bone adaptation simulation.     

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.     

Bone remodelling around the tibia due to total ankle replacement: effects of implant material and implant–bone interfacial conditions

Abstract

One of the major causes of implant loosening is due to excessive bone resorption surrounding the implant due to bone remodelling. The objective of the study is to investigate the effects of implant material and implant–bone interface conditions on bone remodelling around tibia bone due to total ankle replacement. Finite element models of intact and implanted ankles were developed using CT scan data sets. Bone remodelling algorithm was used in combination with FE analysis to predict the bone density changes around the ankle joint. Dorsiflexion, neutral, and plantar flexion positions were considered, along with muscle force and ligaments. Implant–bone interfacial conditions were assumed as debonded and bonded to represent non-osseointegration and fully osseointegration at the porous coated surface of the implant. To investigate the effect of implant material, three finite element models having different material combinations of the implant were developed. For model 1, tibial and talar components were made of Co–Cr–Mo, and meniscal bearing was made of UHMWPE. For model 2, tibial and talar components were made of ceramic and meniscal bearing was made of UHMWPE. For model 3, tibial and talar components were made of ceramic and meniscal bearing was made of CFR-PEEK. Changes in implant material showed no significant changes in bone density due to bone remodelling. Therefore, ceramic appears to be a viable alternative to metal and CFR-PEEK can be used in place of UHMWPE. This study also indicates that proper bonding between implant and bone is essential for long-term survival of the prosthetic components.     

A Femur-Implant Model for the Prediction of Bone Remodeling Behavior Induced by Cementless Stem

Bone remodeling simulation is an effective tool for the prediction of long-term effect of implant on the bone tissue, as well as the selection of an appropriate implant in terms of architecture and material. In this paper, a finite element model of proximal femur was developed to simulate the structures of internal trabecular and cortical bones by incorporating quantitative bone functional adaptation theory with finite element analysis. Cementless stems made of titanium, two types of Functionally Graded Material (FGM) and flexible ‘iso-elastic’ material as comparison were implanted in the structure of proximal femur respectively to simulate the bone remodeling behaviors of host bone. The distributions of bone density, von Mises stress, and interface shear stress were obtained. All the prosthetic stems had effects on the bone remodeling behaviors of proximal femur, but the degrees of stress shielding were different. The amount of bone loss caused by titanium implant was in agreement with the clinical observation. The FGM stems caused less bone loss than that of the titanium stem, in which FGM I stem (titanium richer at the top to more HAP/Col towards the bottom) could relieve stress shielding effectively, and the interface shear stresses were more evenly distributed in the model with FGM I stem in comparison with those in the models with FGM II (titanium and bioglass) and titanium stems. The numerical simulations in the present study provided theoretical basis for FGM as an appropriate material of femoral implant from a biomechanical point of view. The next steps are to fabricate FGM stem and to conduct animal experiments to investigate the effects of FGM stem on the remodeling behaviors using animal model.     

A nonlinear finite element analysis of interface conditions in porous coated hip endoprostheses

We used a geometrically simplified finite element model to investigate load transfer between a porous coated hip endoprosthesis and a femur. Assuming both rigidly bonded and nonlinear interfaces, we analyzed fully and partially coated stems that had coatings of different elastic moduli. Our results indicate that maximum values for relative motion in the interface between bone and implant occur for implants with the same elastic modulus as compact bone. By comparison, interface motion is reduced by about half for Co-Cr-Mo alloy stems. We also showed that the elastic modulus of the porous coating had only a small influence on bone stresses.     

Osseointegration of preformed polymethylmethacrylate craniofacial prostheses coated with bone marrow-impregnated poly (DL-lactic-co-glycolic acid) foam.

Osseointegration of bone marrow-PLGA-coated, preformed polymethylmethacrylate cranioplasties offers the possibility of reducing: operative time, periimplant seroma and infection, metallic fixation, and periprosthetic resorption following surgical skull remodeling. These alloplastic materials are FDA-approved but previously have not been used together to promote cranioplasty incorporation. The objective of this study was to determine whether the use of PLGA foam coating improves host osseointegration of preformed, textured, polymethylmethacrylate prosthetic cranioplasties. A critical-sized cranial defect was created in two groups of 10 and one group of three rabbits. The defect was filled with either a textured, preformed polymethylmethacrylate disc or a textured, preformed polymethylmethacrylate disc coated with poly (DL-lactic-co-glycolic acid). Both implants were immersed in autologous bone marrow for 20 minutes before implantation. Half of each group of 10 were killed at 3 weeks, and the remainder at 6 weeks. A third group of three rabbits with excised periosteum was evaluated at 6 weeks. Histologic analysis of the discs determined relative amounts of cancellous bone formation adjacent to the prostheses. Woven trabecular bone was present at each host bone to implant perimeter interface at 3 weeks, with fine fibrous capsular formation around the implants. Thicker, lamellar trabeculae were present at 6 weeks with an increased fibrous layer surrounding both types of implants. Bone formed on the superficial and deep implant surfaces in a noncontiguous fashion. Two of five measures showed that total bone formation was significantly greater in the PLGA-coated implants. Polymethylmethacrylate discs coated with bone marrow-impregnated PLGA foam demonstrate increased bone formation at 3 and 6 weeks as compared with non-coated preformed polymethylmethacrylate discs. Only implants with preserved periosteum showed bone formation away from the host-implant interface (centrally) on the superficial surface at 6 weeks.     

To reduce the maximum stress and the stress shielding effect around a dental implant–bone interface using radial functionally graded biomaterials

In a dental implant system, the value of stress and its distribution plays a pivotal role on the strength, durability and life of the implant–bone system. A typical implant consists of a Titanium core and a thin layer of biocompatible material such as the hydroxyapatite. This coating has a wide range of clinical applications in orthopedics and dentistry due to its biocompatibility and bioactivity characteristics. Low bonding strength and sudden variation of mechanical properties between the coating and the metallic layers are the main disadvantages of such common implants. To overcome these problems, a radial distributed functionally graded biomaterial (FGBM) was proposed in this paper and the effect of material property on the stress distribution around the dental implant–bone interface was studied. A three-dimensional finite element simulation was used to illustrate how the use of radial FGBM dental implant can reduce the maximum von Mises stress and, also the stress shielding effect in both the cortical and cancellous bones. The results, of course, give anybody an idea about optimized behaviors that can be achieved using such materials. The finite element solver was validated by familiar methods and the results were compared to previous works in the literature.     

Influence of interface condition and implant design on bone remodelling and failure risk for the resurfaced femoral head

Resurfacing of the femur has experienced a revival, particularly in younger and more active patients. The implant is generally cemented onto the reamed trabecular bone and theoretical remodelling for this configuration, as well as uncemented variations, has been studied with relation to component positioning for the most common designs. The purpose of this study was to investigate the influence of different interface conditions, for alternative interior implant geometries, on bone strains in comparison to the native femur, and its consequent remodelling. A cylindrical interior geometry, two conical geometries and a spherical cortex-preserving design were compared with a standard implant (ASR, DePuy International, Ltd., UK), which has a 3° cone. Cemented as well as uncemented line to line and press-fit conditions were modelled for each geometry. A patient-specific finite element model of the proximal femur was used with simulated walking loads. Strain energy density was compared between the reference and resurfaced femur, and input into a remodelling algorithm to predict density changes post-operatively. The common cemented designs (cylindrical, slightly conical) had strain shielding in the superior femoral head (>35% reduction) as well as strain concentrations (strain>5%) in the neck regions near the implant rim. The cortex-preserving (spherical) and strongly conical designs showed less strain shielding. In contrast to the cemented implants, line to line implants showed a density decrease at the centre of the femoral head, while all press-fit versions showed a density increase (>100%) relative to the native femur, which suggests that uncemented press-fit implants could limit bone resorption.     

Influence of the stiffness of bone defect implants on the mechanical conditions at the interface--a finite element analysis with contact

The study focused on the influence of the implant material stiffness on stress distribution and micromotion at the interface of bone defect implants. We hypothesized that a low-stiffness implant with a modulus closer to that of the surrounding trabecular bone would yield a more homogeneous stress distribution and less micromotion at the interface with the bony bed. To prove this hypothesis we generated a three-dimensional, non-linear, anisotropic finite element (FE) model. The FE model corresponded to a previously developed animal model in sheep. A prismatic implant filled a standardized defect in the load-bearing area of the trabecular bone beneath the tibial plateau. The interface was described by face-to-face contact elements, which allow press fits, friction, sliding, and gapping. We assumed a physiological load condition and calculated contact pressures, shear stresses, and shear movements at the interface for two implants of different stiffness (titanium: E=110GPa; composite: E=2.2GPa). The FE model showed that the stress distribution was more homogeneous for the low-stiffness implant. The maximum pressure for the composite implant (2.1 MPa) was lower than for the titanium implant (5.6 MPa). Contrary to our hypothesis, we found more micromotion for the composite (up to 6 microm) than for the titanium implant (up to 4.5 microm). However, for both implants peak stresses and micromotion were in a range that predicts adequate conditions for the osseointegration. This was confirmed by the histological results from the animal studies.     

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.     

Implant–bone interface healing and adaptation in resurfacing hip replacement

Hip resurfacing demonstrates good survivorship as a treatment for young patients with osteoarthritis, but occasional implant loosening failures occur. On the femoral side there is radiographic evidence suggesting that the implant stem bears load, which is thought to lead to proximal stress shielding and adaptive bone remodelling. Previous attempts aimed at reproducing clinically observed bone adaptations in response to the implant have not recreated the full set of common radiographic changes, so a modified bone adaptation algorithm was developed in an attempt to replicate more closely the effects of the prosthesis on the host bone. The algorithm features combined implant–bone interface healing and continuum bone remodelling. It was observed that remodelling simulations that accounted for progressive gap filling at the implant–bone interface predicted the closest periprosthetic bone density changes to clinical X-rays and DEXA data. This model may contribute to improved understanding of clinical failure mechanisms with traditional hip resurfacing designs and enable more detailed pre-clinical analysis of new designs.     

Prediction of bone density around orthopedic implants delivering bisphosphonate

The fixation of an orthopedic implant depends strongly upon its initial stability. Peri-implant bone may resorb shortly after the surgery. This resorption is directly followed by new bone formation and implants fixation strengthening, the so-called secondary fixation. If the initial stability is not reached, the resorption continues and the implant fixation weakens, which leads to implant loosening. Studies with rats and dogs have shown that a solution to prevent peri-implant resorption is to deliver bisphosphonate from the implant surface.The aims of the study were, first, to develop a model of bone remodeling around an implant delivering bisphosphonate, second, to predict the bisphosphonate dose that would induce the maximal peri-implant bone density, and third to verify in vivo that peri-implant bone density is maximal with the calculated dose.The model consists of a bone remodeling equation and a drug diffusion equation. The change in bone density is driven by a mechanical stimulus and a drug stimulus. The drug stimulus function and the other numerical parameters were identified from experimental data. The model predicted that a dose of 0.3 μg of zoledronate on the implant would induce a maximal bone density. Implants with 0.3 μg of zoledronate were then implanted in rat femurs for 3, 6 and 9 weeks. We measured that peri-implant bone density was 4% greater with the calculated dose compared to the dose empirically described as best.The approach presented in this paper could be used in the design and analysis processes of experiments in local delivery of drug such as bisphosphonate.     

A novel non line-of-sight method for coating hydroxyapatite onto the surfaces of support materials by biomineralization

A novel method is described for the non line-of-sight coating of hydroxyapatite onto polyurethane reticulated foam and titanium discs. This utilises a biofilm of Serratia sp. NCIMB 40259 which, when challenged with a solution containing calcium chloride and phosphatase substrate, manufactures biofilm-bound material identified as hydroxyapatite by X-ray powder diffraction analysis. Non-invasive magnetic resonance imaging was used to visualize the biofilm coating throughout the foam labyrinth and to measure the thickness of the film within reticulated foam cubes in situ. The film developed within the cube matrices was similar to that measured on the surface of a glass slide. Using LaPO(4) deposition as a model system the metallised biofilm was visualised in two-dimensional slices throughout three-dimensional images acquired by magnetic resonance imaging. A similar encrustation of hydroxyapatite on the surface of biofilm grown on titanium discs was confirmed by scanning electron microscopy. Potential applications for bio-hydroxyapatite as possible bone implant precursors are discussed.     

The influence of tibial component fixation techniques on resorption of supporting bone stock after total knee replacement

Periprosthetic bone resorption after tibial prosthesis implantation remains a concern for long-term fixation performance. The fixation techniques may inherently aggravate the "stress-shielding" effect of the implant, leading to weakened bone foundation. In this study, two cemented tibial fixation cases (fully cemented and hybrid cementing with cement applied under the tibial tray leaving the stem uncemented) and three cementless cases relying on bony ingrowth (no, partial and fully ingrown) were modelled using the finite element method with a strain-adaptive remodelling theory incorporated to predict the change in the bone apparent density after prosthesis implantation. When the models were loaded with physiological knee joint loads, the predicted patterns of bone resorption correlated well with reported densitometry results. The modelling results showed that the firm anchorage fixation formed between the prosthesis and the bone for the fully cemented and fully ingrown cases greatly increased the amount of proximal bone resorption. Bone resorption in tibial fixations with a less secure anchorage (hybrid cementing, partial and no ingrowth) occurred at almost half the rate of the changes around the fixations with a firm anchorage. The results suggested that the hybrid cementing fixation or the cementless fixation with partial bony ingrowth (into the porous-coated prosthesis surface) is preferred for preserving proximal tibial bone stock, which should help to maintain post-operative fixation stability. Specifically, the hybrid cementing fixation induced the least amount of bone resorption.     

On the optimal shape of hip implants

The success of a total hip arthroplasty is strongly related to the initial stability of the femoral component and to the stress shielding effect. In fact, for cementless stems, initial stability is essential to promote bone ingrowth into the stem coating. An inefficient primary stability is also a cause of thigh pain. In addition, the bone adaptation after the surgery can lead to an excessive bone loss and, consequently, can compromise the success of the implant. These factors depend on prosthesis design, namely on material, interface conditions and shape. Although, surgeons use stems with very different geometries, new computational tools using structural optimization methods have been used to achieve a better design in order to improve initial stability and therefore, the implant durability. In this work, a multi-criteria shape optimization process is developed to study the relationship between implants performance and geometry. The multi-criteria objective function takes into account the initial stability of the femoral stem and the effect of stress shielding on bone adaptation after the surgery. Then, the optimized stems are tested using a concurrent model for bone remodeling and osseointegration to evaluate long-term performance. Additionally, the sensitivity to misalignments is analyzed, since femoral stems are often placed in varus or valgus position. Results show that the different criteria are contradictory resulting in different characteristics for the hip stem. However, the multi-criteria formulation leads to compromise solutions, with a combination of the geometric characteristics obtained for each criterion separately.     

Improving stress shielding following total hip arthroplasty by using a femoral stem made of β type Ti-33.6Nb-4Sn with a Young’s modulus gradation

Stress shielding-related bone loss occurs after total hip arthroplasty because the stiffness of metallic implants differs from that of the host femur. Although reducing stem stiffness can ameliorate the bone resorption, it increases stress at the bone–implant interface and can inhibit fixation. To overcome this complication, a novel cementless stem with a gradient in Young’s modulus was developed using Ti-33.6Nb-4Sn (TNS) alloy. Local heat treatment applied at the neck region for increasing its strength resulted in a gradual decrease in Young’s modulus from the proximal to the distal end, from 82.1 to 51.0 GPa as calculated by a heat transfer simulation. The Young’s modulus gradient did not induce the excessive interface stress which may cause the surface debonding. The main purpose of this study was to evaluate bone remodeling with the TNS stem using a strain-adaptive bone remodeling simulation based on finite element analysis. Our predictions showed that, for the TNS stem, bone reduction in the calcar region (Gruen zone 7) would be 13.6% at 2 years, 29.0% at 5 years, and 45.8% at 10 years postoperatively. At 10 years, the bone mineral density for the TNS stem would be 42.6% higher than that for the similar Ti-6Al-4V alloy stem. The stress–strength ratio would be lower for the TNS stem than for the Ti-6Al-4V stem. These results suggest that although proximal bone loss cannot be eliminated completely, the TNS stem with a Young’s modulus gradient may have bone-preserving effects and sufficient stem strength, without the excessive interface stress.     

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.     

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.     

Implant-bone interface healing and adaptation in resurfacing hip replacement

Hip resurfacing demonstrates good survivorship as a treatment for young patients with osteoarthritis, but occasional implant loosening failures occur. On the femoral side there is radiographic evidence suggesting that the implant stem bears load, which is thought to lead to proximal stress shielding and adaptive bone remodelling. Previous attempts aimed at reproducing clinically observed bone adaptations in response to the implant have not recreated the full set of common radiographic changes, so a modified bone adaptation algorithm was developed in an attempt to replicate more closely the effects of the prosthesis on the host bone. The algorithm features combined implant-bone interface healing and continuum bone remodelling. It was observed that remodelling simulations that accounted for progressive gap filling at the implant-bone interface predicted the closest periprosthetic bone density changes to clinical X-rays and DEXA data. This model may contribute to improved understanding of clinical failure mechanisms with traditional hip resurfacing designs and enable more detailed pre-clinical analysis of new designs.