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.     

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.     

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.     

Strain shielding in proximal tibia of stemmed knee prosthesis: experimental study

Theoretical concerns about the use of cemented or press-fit stems in revision total knee arthroplasty (TKA) include stress shielding with adverse effects on prosthesis fixation. Revision TKA components are commonly stemmed to protect the limited autogenous bone stock remaining. Revision procedures with the use of stems can place abnormal stresses through even normal bone by their constrained design, type of materials and fixation method and may contribute for bone loss. Experimental quantification of strain shielding in the proximal synthetic tibia following TKA is the main purpose of the present study. In this study, cortical bone strains were measured experimentally with tri-axial strain gauges in synthetic tibias before and after in vitro knee surgery. Three tibias were implanted with cemented and press-fit stem augments and solely with a tibial tray (short monobloc stem) of the P.F.C. Sigma Modular Knee System. The difference between principal strains of the implanted and the intact tibia was calculated for each strain gauge position. The results demonstrated a pronounced strain-shielding effect in the proximal level, close to tibial tray with the cemented stem augment. The press-fit stem presented a minor effect of strain shielding but was more extensively throughout the stem. An increase of strains closely to the distal tip of the cemented and the press-fit stem augment was observed. This suggests for a physiological condition, a potential effect of bone resorption at the proximal region for the cemented stem augment. The localized increase of strains in stems tip can be related with the clinical finding of the pain, at the end of stem after revision TKA.     

Polymer composites for use in orthopedic surgery

Ever since Movin in (1950) and McKee in (1951) introduced the use of acrylic cement for fixation of hip prosthesis components a number of investigators have proposed various hip prosthesis designs using this cement fixation concept (Neale, 1967). This study was undertaken to support the hypothesis that certain dental materials could provide a more satisfactory bone-prosthesis bond than that presently possible with acrylic bone cement.^‡ Two restorative resins were found to have superior strength and resistance to thermal degradation when compared to acrylic bone cement. Tests of acrylic cement combined with apatite fillers suggest that restorative resin-anorganic bone composites would exhibit improved strength and toxicity properties and would also promote improved bonding due to resorption of the surface anorganic bone particles with subsequent bone infiltration and anchorage. Relatively high degradation of acrylic bone cement in accelerated aging tests suggests caution in using this material for implantation.     

Bone ingrowth simulation for a concept glenoid component design

Glenoid component loosening is the major problem of total shoulder arthroplasty. It is possible that uncemented component may be able to achieve superior fixation relative to cemented component. One option for uncemented glenoid is to use porous tantalum backing. Bone ingrowth into the porous backing requires a degree of stability to be achieved directly post-operatively. This paper investigates the feasibility of bone ingrowth with respect to the influence of primary fixation, elastic properties of the backing and friction at the bone prosthesis interface. Finite element models of three glenoid components with different primary fixation configurations are created. Bone ingrowth into the porous backing is modelled based on the magnitude of the relative interface micromotions and mechanoregulation of the mesenchymal stem cells that migrated via the bonded part of the interface. Primary fixation had the most influence on bone ingrowth. The simulation showed that its major role was not to firmly interlock the prosthesis, but rather provide such a distribution of load, that would result in reduction of the peak interface micromotions. Should primary fixation be provided, friction has a secondary importance with respect to bone ingrowth while the influence of stiffness was counter intuitive: a less stiff backing material inhibits bone ingrowth by higher interface micromotions and stimulation of fibrous tissue formation within the backing.     

Pressfit of equatorially roughened cementless acetabular components--a finite element analysis]

AIM: Does the pressfit anchorage of cementless acetabular cups depend on the roughness of the pole? To answer this question the primary pressfit of two cementless acetabular cups which differ only with regard to the roughness of their poles were compared by means of finite elements analysis. MATERIALS AND METHODS: It was assumed that the material properties of bone are homogeneous, isotropic and linearly elastic. Material-specific values of cancellous bone with three different bone densities were used. Assumption of isotropy represents an approximation. RESULTS: Comparison of the two prosthesis designs revealed that both designs/shapes cause similar patterns of bone deformation and tension. CONCLUSIONS: It can therefore be concluded that with regard to pressfit anchorage the prosthesis with milled polar surface is according to FEA mechanically equivalent to the prosthesis with non-milled polar surface.     

Epiphyseal-based designs for tibial plateau components--I. Stress analysis in the frontal plane

Two-dimensional, finite element studies were conducted of the proximal tibia before and after joint arthroplasty. Equivalent-thickness models projected onto the mid-frontal plane were created for the natural, proximal tibia and for the proximal tibia with four different types of tibial plateau components. All components simulated bony ingrowth fixation, i.e. no cement layer existed between component and bone. In addition, the interface between component and bone was assumed to be intimately connected, representing complete bony ingrowth and a rigid state of fixation. Loads consisted of bi-condylar and uni-condylar forces. Results indicated that conventional plateau designs with central posts or multiple pegs led to higher stress magnitudes in the trabecular bone near the distal ends of the post/pegs and stress shielding at more proximal locations. A design without posts or pegs whose interface geometry mimics the epiphyseal plate minimizes bone stress shielding. An implant consisting of separate components covering each condyle was found effective in limiting component tilting and the consequent tensile stresses caused by non-symmetrical, uni-condylar loading.     

Primary stability of uncemented femoral resurfacing implants for varying interface parameters and material formulations during walking and stair climbing

Primary stability of uncemented resurfacing prosthesis is provided by an interference fit between the undersized implant and the reamed bone. Dependent on the magnitude of interference, the implantation process causes high shear forces and large strains which can exceed the elastic limit of cancellous bone. Plastification of the bone causes reduced stiffness and could lead to bone damage and implant loosening. The purpose in this study was to determine press-fit conditions which allow implantation without excessive plastic bone deformation and sufficient primary stability to achieve bone ingrowth. In particular, the influence of interference, bone quality and friction on the micromotion during walking and stair-climbing was investigated. Therefore elastic and plastic finite element (FE) models of the proximal femur were developed. Implantation was realized by displacing the prosthesis onto the femur while monitoring the contact pressure, plastic bone deformation as well as implantation forces. Subsequently a physiologic gait and stair-climbing cycle was simulated calculating the micromotion at the bone-implant interface. Results indicate that plastic deformation starts at an interference of 30 μm and the amount of plastified bone at the interface increases up to 90% at 150 μm interference. This effect did not reduce the contact pressure if interference was below 80 μm. The micromotion during walking was similar for the elastic and plastic FE models. A stable situation allowing bony ingrowth was achieved for both constitutive laws (elastic, plastic) for walking and stair climbing with at least 60 μm press-fit, which is feasible with clinically used implantation forces of 4 kN.     

Analysis of bone–prosthesis interface micromotion for cementless tibial prosthesis fixation and the influence of loading conditions

A lack of initial stability of the fixation is associated with aseptic loosening of the tibial components of cementless knee prostheses. With sufficient stability after surgery, minimal relative motion between the prosthesis and bone interfaces allows osseointegation to occur thereby providing a strong prosthesis-to-bone biological attachment. Finite element modelling was used to investigate the bone–prosthesis interface micromotion and the relative risk of aseptic loosening. It was anticipated that by prescribing different joint loads representing gait and other activities, and the consideration of varying tibial–femoral contact points during knee flexion, it would influence the computational prediction of the interface micromotion. In this study, three-dimensional finite element models were set up with applied loads representing walking and stair climbing, and the relative micromotions were predicted. These results were correlated to in-vitro measurements and to the results of prior retrieval studies. Two load conditions, (i) a generic vertical joint load of 3×body weight with 70%/30% M/L load share and antero-posterior/medial-lateral shear forces, acted at the centres of the medial and lateral compartments of the tibial tray, and (ii) a peak vertical joint load at 25% of the stair climbing cycle with corresponding antero-posterior shear force applied at the tibial–femoral contact points of the specific knee flexion angle, were found to generate interface micromotion responses which corresponded to in-vivo observations. The study also found that different loads altered the interface micromotion predicted, so caution is needed when comparing the fixation performance of various reported cementless tibial prosthetic designs if each design was evaluated with a different loading condition.     

新型非骨水泥柄设计特征及临床应用效果

目的:探讨一种新型的适合于儿童的非骨水泥固定型股骨柄设计特征,并通过随访获得其临床效果。方法:选取2010年9月~2013年4月在我科植入新型非骨水泥股骨柄的6名儿童患者,其中男1例,女5例;年龄8.5±3.2岁(5~11岁)。病理诊断结果骨肉瘤患者5例,恶性神经鞘瘤患者1例;右股骨下端患者5例,左股骨下端患者1例;其中一例患者术前有病理骨折。6例患者在我科行双动半膝关节置换术,其中股骨下端均采用了新型非骨水泥假体柄。采用Enneking骨肌肉肿瘤置换后下肢功能评定标准对患肢行功能评价,影像学重点评估股骨柄在髓腔放置位置是否得当、股骨柄假体有无松动、有无应力遮挡、骨溶解等现象,并测量术后患者患肢短缩畸形数据。结果:6例患者随访时间32个月(14~54个月),除1例5岁女童术前肢体条件较差在术后14个月行膝关节融合手术,其余无翻修病例,置换关节均无感染、折断等现象。MSTS评分21.33分;射线片示所有患者股骨髓腔内假体柄放置位置满意,股骨侧及胫腓骨侧假体周围未见骨溶解。结论:新型非骨水泥固定型股骨柄设计合理,早期稳定性可,后期可取得满意的生物固定效果。     

Validation of model-predicted tibial tray-synthetic bone relative motion in cementless total knee replacement during activities of daily living

As fixation of cementless total knee replacement components during the first 4–6 weeks after surgery is crucial to establish bony ingrowth into the porous surface, several studies have quantified implant-bone micromotion. Relative motion between the tray and bone can be measured in vitro, but the full micromotion contour map cannot typically be accessed experimentally. Finite element models have been employed to estimate the full micromotion map, but have not been directly validated over a range of loading conditions. The goal of this study was to develop and validate computational models for the prediction of tray-bone micromotion under simulated activities of daily living. Gait, stair descent and deep knee bend were experimentally evaluated on four samples of a cementless tibial tray implanted into proximal tibial Sawbones™ constructs. Measurements of the relative motion between the tray and the anterior cortical shell were collected with digital image correlation and used to validate a finite element model that replicated the experiment. Additionally, a probabilistic analysis was performed to account for experimental uncertainty and determine model sensitivity to alignment and frictional parameters. The finite element models were able to distinguish between activities and capture the experimental trends. Best-matching simulations from the probabilistic analysis matched measured displacement with an average root mean square (RMS) difference of 14.3 µm and Pearson-product correlation of 0.93, while the mean model presented an average RMS difference of 27.1 µm and a correlation of 0.8. Maximum deviations from average experimental measurements were 40.5 and 87.1 µm for the best-matching and average simulations, respectively. The computational pipeline developed in this study can facilitate and enhance pre-clinical assessment of novel implant components.     

Porous surface modified bioactive bone cement for enhanced bone bonding

Background

Polymethylmethacrylate bone cement cannot provide an adhesive chemical bonding to form a stable cement-bone interface. Bioactive bone cements show bone bonding ability, but their clinical application is limited because bone resorption is observed after implantation. Porous polymethylmethacrylate can be achieved with the addition of carboxymethylcellulose, alginate and gelatin microparticles to promote bone ingrowth, but the mechanical properties are too low to be used in orthopedic applications. Bone ingrowth into cement could decrease the possibility of bone resorption and promote the formation of a stable interface. However, scarce literature is reported on bioactive bone cements that allow bone ingrowth. In this paper, we reported a porous surface modified bioactive bone cement with desired mechanical properties, which could allow for bone ingrowth.

Materials and Methods

The porous surface modified bioactive bone cement was evaluated to determine its handling characteristics, mechanical properties and behavior in a simulated body fluid. The in vitro cellular responses of the samples were also investigated in terms of cell attachment, proliferation, and osteoblastic differentiation. Furthermore, bone ingrowth was examined in a rabbit femoral condyle defect model by using micro-CT imaging and histological analysis. The strength of the implant–bone interface was also investigated by push-out tests.

Results

The modified bone cement with a low content of bioactive fillers resulted in proper handling characteristics and adequate mechanical properties, but slightly affected its bioactivity. Moreover, the degree of attachment, proliferation and osteogenic differentiation of preosteoblast cells was also increased. The results of the push-out test revealed that higher interfacial bonding strength was achieved with the modified bone cement because of the formation of the apatite layer and the osseointegration after implantation in the bony defect.

Conclusions

Our findings suggested a new bioactive bone cement for prosthetic fixation in total joint replacement.     

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.     

Epiphyseal-based designs for tibial plateau components--II. Stress analysis in the sagittal plane

A two-dimensional, finite element study was undertaken to establish the stresses in the proximal tibia before and after total knee arthroplasty. Equivalent-thickness models in a sagittal plane were created for the natural, proximal tibia and for the proximal tibia with two different types of tibial plateau components. All components simulated bony ingrowth fixation, i.e. no cement layer existed between component and bone. In addition, the interface between component and bone was assumed to be intimately connected, representing complete bony ingrowth and a rigid state of fixation. Two load cases were considered: a joint reaction force acting in conjunction with a patellar ligament force, simulating the knee at 40 degrees of flexion; and a joint reaction force directed along the long axis of the tibia. For the natural tibia model, the pattern of principal stresses for loadcase 1 more closely corresponds to the epiphyseal plate geometry and trabecular morphology than do the principal stress patterns for loadcase 2. Judging from the distribution of principal stresses, loadcase 1 represents a more severe test of implant design than does loadcase 2. The model of the component with a peg predicted that the trabecular bone near the tip of the peg will experience higher than normal stresses, while the bone stresses near the posterior aspect adjacent to the metal tray will be reduced. A component without pegs that incorporates a posterior chamfer and an anterior lip lead to stress distributions closer to those existing in the natural tibia. The interface geometry for this design is based upon the contour of the epiphyseal plate.     

Loosening of cementless femoral stems: a biomechanical analysis of immediate fixation with loading vertical, femur horizontal

Large joint implants must have immediate fixation to be successful. Unfortunately, the magnitude and consistency of achieving this remains largely unknown. For cementless femoral components it is being increasingly appreciated that torsional loading as occurs during stair climbing or rising from a chair leads to loosening and thigh pain in some cases. A biomechanical test was developed to evaluate fixation in this position. Twelve pairs of human cadaveric femora were press-fit with an AML stem. Each femur was secured in a horizontal position, and the prosthetic head cyclically loaded in a vertically downward direction. The offset of the prosthetic head resulted in a combined torsional and compressive load being applied to the stem within the proximal femur. Loosening was found to consistently occur and rapidly accelerate when the head subsided more than 0.2 mm during 100 cycles. For the AML stem, loosening developed at loads from 62 to 171% of body weight and after as few as 800 cycles. This is within the physiologic range of normal daily activities as measured by others with instrumented prostheses. This poses a challenge to the ability of press-fit stems to tolerate torsional loads in vivo. Patients with a cementless prosthesis should be protected from torsional loading until porous ingrowth and/or bone remodelling have had time to occur. Testing the same stem in paired femora demonstrated no right vs left difference (p greater than 0.6).     

Pre-clinical validation of a new partially cemented femoral prosthesis by synergetic use of numerical and experimental methods

The present work reports the pre-clinical validation of an innovative partially cemented femoral prosthesis called cement-locked uncemented (CLU) prosthesis. The inventors of the device under investigation claimed that, when compared to a comparable fully cemented stem, the new stem would present various advantages. Two previous experimental studies confirmed that primary stability and stress shielding were comparable to those of cemented stems. Aim of the present study was to investigate if the remaining claims were confirmed as well. A complete finite element model of the bone-implant complex was created from CT data. The model was validated against in vitro measurements of bone surface strains as well as against primary stability measurements. The peak stresses predicted in the CLU cement mantle were not found significantly lower than those reported in other studies on fully cemented stems. However, once the cement inlet geometry is optimised and the associated stress risers are eliminated, the CLU cement mantle should be subjected to much lower stresses. The stress induced in the stems by both load cases was well below the fatigue limit of the Ti6Al4V alloy. Finite element models predicted for all load cases relative motion between cement and metal lower than 60 microm. This amplitude may be fully accommodated by elastic deformations of the cement micro-ridges. The experimental and numerical results showed the validity of the new fixation concept, although a further optimisation of the geometry of the cement pockets is needed in order to further reduce the stresses in the cement.     

Design of partial pelvic replacement with calculation of stresses on the pelvic bone using the finite element method

Although tumors of the pelvic region are relatively rare, with regard to the provision of an individual prosthesis, they make great demands both on the engineer and on the surgeon. In the case of partial pelvic replacement, the main problem is that of fixation. The direction of the preload by the screw and counternut in the anchorage should make possible the introduction of forces into the bone that closely mimic the natural pre-operative situation, in order to keep bone remodeling and stressing to a minimum. A three-dimensional finite element (FE) analysis of the stresses at work in the pelvic bone was performed using an FE model of the pelvis constructed on the basis of computed tomographic data, since with the FE method it is possible to obtain information about deformations, internal stresses and local forces acting on the bone. The results show that the main stressing of the bony pelvis occurs in the region of the acetabulum and the iliosacral joints, and that further points of loading are located in the lower region of the ischium-and this while standing on one leg, on the side of the supporting leg.     

Load-shift--numerical evaluation of a new design philosophy for uncemented hip prostheses

All hip replacement prostheses alter the load transfer from the hip joint into the femur by changing the mechanical loading of the proximal femur from an externally to an internally loaded system. This alteration of the load transfer causes stress shielding and might lead to severe bone loss, especially with uncemented prostheses. To minimize these effects, load transfer to the femur should occur as proximal as possible. In order to support sufficient primary stability, however, directly post operative (PO) distal stabilization is reasonable. Consequently, the prostheses have to alter its mechanical characteristics after implantation. This concept is referred to as load-shift concept. Primary stability during the early PO state is provided by a prosthesis shaft, which is widened at the tip by a biodegradable pin. After resorption of the pin load transfer occurs no longer distally. The objective of this study was the numerical evaluation of the load-shift concept. The analysis was performed with a finite element model. Three-dimensional non-linear dynamic gait analyses data were used to evaluate whether the load transfer during walking can be altered effectively by insertion and resorption of a distal pin. Directly PO 38% of the transverse load is transferred through the prosthesis shaft and micromotion of the proximal prostheses tip is below 55 microm. After resorption of the pin, no transverse loads are transferred through the prosthesis shaft. Therefore, the loading of the proximal bone tissue is far more pronounced than in the case of a standard prosthesis, demonstrating the feasibility of the load-shift concept. A balanced degradation of the pin simultaneously with the ingrowth of the prosthesis is expected to reduce hip replacement complications.     

Long-term study of bone remodelling after femoral stem: a comparison between dexa and finite element simulation

A hip replacement with a cemented or cementless femoral stem produces an effect on the bone called adaptive remodelling, attributable to mechanical and biological factors. The objective of all of cementless prostheses designs has been to achieve a perfect transfer of loads in order to avoid stress-shielding, which produces an osteopenia. In order to quantify this, the long term and mass-produced study with dual energy X-ray absorptiometry (DEXA) is necessary. Finite element (FE) simulation makes possible the explanation of the biomechanical changes which are produced in the femur after stem implantation. The good correlation obtained between the results of the FE simulation and the densitometric study allow, on one hand, to explain from the point of view of biomechanical performance the changes observed in bone density in the long-term, where it is clear that these are due to a different transfer of load in the implanted model compared to the healthy femur; on the other hand, it validates the simulation model, in a way that it can be used in different conditions and at different time periods, to carry out a sufficiently precise prediction of the evolution of the bone density from the biomechanical behaviour in the interaction between the prosthesis and femur.