Finite element analysis of retroacetabular osteolytic defects following total hip replacement

Periprosthetic osteolysis in the retroacetabular region with cancellous bone loss is a recognized phenomenon in the long-term follow-up of total hip replacement. The effects on load transfer in the presence of defects are less well known. A finite element model incorporating a retroacetabular defect behind a cementless component was validated against a 4th generation sawbone pelvis. Computational predictions of surface strain and von Mises stresses were closely correlated to experimental findings. The presence of a cancellous defect increased von Mises stress in the cortical bone of the medial wall of the pelvis. At a load of 600 N this was under the predicted failure stress for cortical bone. Increases in the cup size relative to the acetabulum caused increased stress in the cortical bone of the lateral wall of the pelvis, adjacent to the acetabulum. We are confident that our modeling approach can be applied to patient specific defects to predict pelvis stress with large loads and a range of activities.     

Subject-specific finite element model of the pelvis: development, validation and sensitivity studies

A better understanding of the three-dimensional mechanics of the pelvis, at the patient-specific level, may lead to improved treatment modalities. Although finite element (FE) models of the pelvis have been developed, validation by direct comparison with subject-specific strains has not been performed, and previous models used simplifying assumptions regarding geometry and material properties. The objectives of this study were to develop and validate a realistic FE model of the pelvis using subject-specific estimates of bone geometry, location-dependent cortical thickness and trabecular bone elastic modulus, and to assess the sensitivity of FE strain predictions to assumptions regarding cortical bone thickness as well as bone and cartilage material properties. A FE model of a cadaveric pelvis was created using subject-specific computed tomography image data. Acetabular loading was applied to the same pelvis using a prosthetic femoral stem in a fashion that could be easily duplicated in the computational model. Cortical bone strains were monitored with rosette strain gauges in ten locations on the left hemipelvis. FE strain predictions were compared directly with experimental results for validation. Overall, baseline FE predictions were strongly correlated with experimental results (r2=0.824), with a best-fit line that was not statistically different than the line y=x (experimental strains = FE predicted strains). Changes to cortical bone thickness and elastic modulus had the largest effect on cortical bone strains. The FE model was less sensitive to changes in all other parameters. The methods developed and validated in this study will be useful for creating and analyzing patient-specific FE models to better understand the biomechanics of the pelvis.     

Osteochondral graft fixation using a bioresorbable bone cement

Multiple osteochondral grafts can be used to resurface large joint defects in both humans and horses. In humans, immediate postoperative weight bearing can be prevented, however in the equine, it is unavoidable. Early weight bearing can create detrimental graft micromotion. The aim of this study was to investigate the role of a bioresorbable cement in improving the initial stability of multiple osteochondral graft repairs of large subchondral cystic lesions in the horse. Configurations employed for filling a 20mm diameter cylindrical defect included: (A) twelve 4.5mm diameter grafts with cement, (B) five 6.5mm diameter grafts with cement, (C) four each of 4.5mm and 6.5mm grafts with cement and (D) cement only. Intact bone slices (E) were also tested. Push-out tests were used to quantify construct to host sidewall interface fixation. Configuration (A) proved clinically impractical (n=3). Configurations (B) (n=6), and (C) (n=4) had statistically similar interface stiffnesses and failure stresses (43+/-8 and 30+/-12 MPa and 0.96+/-0.1 and 1.2+/-0.3 Mpa, respectively) suggesting that they are equally susceptible to interface movement in the immediate postoperative period. By way of comparison, defects filled only with cement had an average stiffness of 53+/-7MPa and failure stress of 1.8+/-0.3 MPa (n=6) while the intact femoral condyle demonstrated a stiffness of 108+/-7 MPa and failure stress of 18+/-0.4 MPa (n=6). Cement augmentation improved immediate postoperative stability of multiple osteochondral graft constructs over uncemented constructs, although in all cases the observed moduli of elasticity and yield stress values were lower than those observed for cement only and intact bone test specimens. (all numbers are mean+/-SEM).     

Pelvic stresses in vitro--II. A study of the efficacy of metal-backed acetabular prostheses

Six embalmed human cadaveric hemi-pelves with their associated proximal femurs have been tested in vitro using 25 strain-gauge rosettes on each hemi-pelvis. Loads were applied up to 2.5 kN and principal stresses were computed from the principal strain data. Acetabular prostheses, cemented in place upon a cartilage-devoid but intact subchondral bone-plate, showed little change in stress-patterns when compared with the normal data, regardless of whether or not the component employed metal-backing. The use of 30 anchoring holes of 6.4 mm diameter, in the intact subchondral bone-plate, had little effect upon the stress-patterns, regardless of whether metal-backing was employed upon the prosthesis. When the subchondral bone-plate was removed, there were notable changes in the stress-pattern in the periacetabular region and on the medial wall of pelvis. The metal-backed prosthesis did not produce such notable changes as its plastic counterpart, when the subchondral bone-plate was removed. The use of a plastic prosthesis cemented in a Protrusio ring, in an acetabulum devoid of subchondral bone, produced notable changes in the stress-patterns in the whole periacetabular region and on the medial wall.     

The influence of acetabular cup material on pelvis cortex surface strains, measured using digital image correlation

Acetabular cup loosening is a late failure mode of total hip replacements, and peri-prosthetic bone deterioration may promote earlier failure. Preservation of supporting bone quality is a goal for implant design and materials selection, to avoid stress shielding and bone resorption. Advanced polymer composite materials have closer stiffness to bone than metals, ceramics or polymers, and have been hypothesised to promote less adverse bone adaptation. Computer simulations have supported this hypothesis, and the present study aimed to verify this experimentally.A composite hemi-pelvis was implanted with Cobalt Chromium (CoCr), polyethylene (UHMWPE) and MOTIS^®carbon-fibre-reinforced polyether etherketone (CFR-PEEK) acetabular cups. In each case, load was applied to the implanted pelvis and Digital Image Correlation (DIC) was used for surface strain measurement. The test was repeated for an intact hemi-pelvis. Trends in implanted vs. intact bone principal strains were inspected to assess the average principal strain magnitude change, allowing comparison of the potential bone responses to implantation with the three cups.The CFR-PEEK cup was observed to produce the closest bone strain to the intact hip in the main load path, the superior peri-acetabular cortex (+12% on average, R²=0.84), in comparison to CoCr (+40%, R²=0.91) and UHWMPE cups (?26%, R²=0.94). Clinical observations have indicated that increased periacetabular cortex loading may result in reduced polar cancellous bone loading, leading to longer term losses in periprosthetic bone mineral density. This study provides experimental evidence to verify previous computational studies, indicating that cups produced using materials with stiffness closer to cortical bone recreate physiological cortical bone strains more closely and could, therefore, potentially promote less adverse bone adaptation than stiffer press-fitted implants in current use.     

Design considerations for ceramic resurfaced femoral head: effect of interface characteristics on failure mechanisms

Ceramic hip resurfacing may offer improved wear resistance compared to metallic components. The study is aimed at investigating the effects of stiffer ceramic components on the stress/strain-related failure mechanisms in the resurfaced femur, using three-dimensional finite element models of intact and resurfaced femurs with varying stem–bone interface conditions. Tensile stresses in the cement varied between 1 and 5 MPa. Postoperatively, 20–85% strain shielding was observed inside the resurfaced head. The variability in stem–bone interface condition strongly influenced the stresses and strains generated within the resurfaced femoral head. For full stem–bone contact, high tensile (151–158 MPa) stresses were generated at the cup–stem junction, indicating risk of fracture. Moreover, there was risk of femoral neck fracture due to elevated bone strains (0.60–0.80% strain) in the proximal femoral neck region. Stresses in the ceramic component are reduced if a frictionless gap condition exists at the stem–bone interface. High stresses, coupled with increased strain shielding in the ceramic resurfaced femur, appear to be major concerns regarding its use as an alternative material.     

Finite element assessment of block-augmented total knee arthroplasty

Loosening and migration of tibial prostheses have been identified as causes of early total knee replacement (TKR) failure. The problem is made more complex when defects occur in the proximal tibia compromising fixation and alignment. Clinical studies using metal augments have shown these to be an alternative to other means of defect treatment. Finite element (FE) analysis can be used to identify regions that may be prone to loosening and migration. In the current work, 3D FE models of TKR uncontained type-2 defects treated with block augments have been constructed and analysed. It has been shown that a metal augment is the most suitable. The use of bone cement (PMMA) to fill proximal defects is not considered suitable as stresses carried by the cement block exceed those of the fatigue limit of bone cement. It has been shown that the stresses in the proximal cancellous bone of block-augmented models are significantly below levels likely to cause damage due to overloading. Furthermore, the use of stem extensions has been shown to reduce the cancellous bone stresses in the proximal region thus increasing the likelihood of bone resorption. Given this, it is recommended that stem extensions are not required unless necessary to mitigate some other problem.     

Variations in mineralization affect the stress and strain distributions in cortical and trabecular bone

The mechanical properties of bone depend largely on its degree and distribution of mineralization. The present study analyzes the effect of an inhomogeneous distribution of mineralization on the stress and strain distributions in the human mandibular condyle during static clenching. A condyle was scanned with a micro-CT scanner to create a finite element model. For every voxel the degree of mineralization (DMB) was determined from the micro-CT scan. The Young's moduli of the elements were calculated from the DMB using constant, linear, and cubic relations, respectively. Stresses, strains, and displacements in cortical and trabecular bone, as well as the condylar deformation (extension along the antero-posterion axis) and compliance were compared. Over 90% of the bone mineral was located in the cortical bone. The DMB showed large variations in both cortical bone (mean: 884, SD: 111 mg/cm(3)) and trabecular bone (mean: 738, SD: 101 mg/cm(3)). Variations of the stresses and the strains were small in cortical bone, but large in trabecular bone. In the cortical bone an inhomogeneous mineral distribution increased the stresses and the strains. In the trabecular bone, however, it decreased the stresses and increased the strains. Furthermore, the condylar compliance remained relatively constant, but the condylar deformation doubled. It was concluded that neglect of the inhomogeneity of the mineral distribution results in a large underestimation of the stresses and strains of possibly more than 50%. The stiffness of trabecular bone strongly influences the condylar deformation. Vice versa, the condylar deformation largely determines the magnitude of the strains in the trabecular bone.     

The effect of the interface on the bone stresses beneath tibial components

It was proposed that the stresses in the layer of bone immediately beneath a tibial component are an important determinant of fixation durability. Using finite element analysis, (ANSYS), the stresses were determined as a function of the amount of bone resection, the localization or completeness of implant-bone contact, and the interface material. The model was of two-dimensional sagittal slices consisting of quadrilateral elements (1 mm) with a range of seventeen material properties determined by CT scans. Typical prosthesis designs shifted the center of pressure more centrally rather than posteriorly, and thus increased anterior bone stresses. Resection up to 10 mm could actually decrease bone stresses due to an increase in bone surface area as long as complete coverage was obtained. A cement interface or direct metal on bone produced identical stresses. However a 1 mm complian: interface significantly reduced stresses in regions of high elastic modulus gradient. For rigid interfaces, the contact can be irregular, which leads to areas of over and understressing of bone. These conclusions have implications related to implant design.     

Biomechanical response of the pubic symphysis in lateral pelvic impacts: a finite element study

Automotive side impacts are a leading cause of injuries to the pubic symphysis, yet the mechanisms of those injuries have not been clearly established. Previous mechanical testing of isolated symphyses revealed increased joint laxity following drop tower lateral impacts to isolated pelvic bone structures, which suggested that the joints were damaged by excessive stresses and/or deformations during the impact tests. In the present study, a finite element (FE) model of a female pelvis including a previously validated symphysis sub-model was developed from computed tomography data. The full pelvis model was validated against measured force-time impact responses from drop tower experiments and then used to study the biomechanical response of the symphysis during the experimental impacts. The FE models predicted that the joint underwent a combination of lateral compression, posterior bending, anterior/posterior and superior/inferior shear that exceeded normal physiological levels prior to the onset of bony fractures. Large strains occurred concurrently within the pubic ligaments. Removal of the contralateral constraints to better approximate the boundary conditions of a seated motor vehicle occupant reduced cortical stresses and deformations of the pubic symphysis; however, ligament strains, compressive and shear stresses in the interpubic disc, as well as posterior bending of the joint structure remained as potential sources of joint damage during automotive side impacts.     

Stress amplifications in dental non-carious cervical lesions

This study aims to investigate the influence of the presence, shape and depth of NCCLs on the mechanical response of a maxillary second premolar subjected to functional and non-functional occlusal loadings using 3-D finite element (FE) analysis. A three-dimensional model of a maxillary second premolar and its supporting bone was constructed based on the contours of their cross-sections. From the sound model, cervical defects having either V- or U-shapes, as found clinically, were subtracted in three different depths. The models were loaded with 105 N to simulate normal chewing forces according to a functional occlusal loading (F1) vertically applied and two non-functional loadings (F2 and F3) obliquely oriented. Two alveolar bone crest heights were tested. Ansys™ FE software was used to compute stress distributions and maximum principal stress for each of the models. The presence of a lesion had no effect on the overall stress distribution of the system, but affected local stress concentrations. Non-functional loadings exhibited tensile stresses concentrating at the cervical areas and root surfaces, while the functional loading resulted in homogeneous stress distributions within the tooth. V-shaped lesions showed higher stress levels concentrated at the zenith of the lesion, whereas in U-shaped defect stresses concentrated over a wider area. As the lesions advanced in depth, the stress was amplified at their deepest part. A trend of stress amplification was observed with decreasing bone height. These results suggest a non-linear lesion progression with time, with the progression rate increasing with patient's age (deeper lesions and lower bone support).     

On the influence of mechanical conditions in osteochondral defect healing

Despite the introduction of new surgical techniques, the treatment of cartilage defects remains challenging. Delay or complete failure of cartilage healing is associated with problems in biological regeneration. The influence of mechanical conditions on this process, however, remains unevaluated. Osteochondral defects were generated on the left femoral condyle in 18 Yucatan minipigs. After 4, 6 and 12 weeks the defect filling, trabecular orientation and bone density were compared to the intact contralateral side. The mechanical straining during this period was then analyzed using an adaptive finite element technique. Histologically, the osteochondral defects showed bone resorption at the base and bone formation from the circumference. At 12 weeks, the macroscopically healed specimens showed fibrous cartilage formation, a minimally organized trabecular structure and increased trabecular volume fraction compared to the controls (p < 0.002). The amount of cancellous, cartilagineous, and fibrous tissue and the defect size as measured in histomorphometric analysis for the three time points (4, 6 and 12 weeks) was comparable in magnitude to that predicted by finite element analysis. The simulated osteochondral healing process was not fully capable of re-establishing a hyaline-like cartilage layer. The correlation between simulation and histology allows identification of mechanical factors that appear to have a larger impact on the healing of osteochondral defects than previously considered.     

Functional significance of cortical bone distribution in anthropoid mandibles: an in vitro assessment of bone strain under combined loads

Local variation in cortical bone thickness in the postcanine mandibular corpus appears to be stereotypical among anthropoids. Specifically, at sections under the molars, lingually situated cortical bone is typically thinner than that along the lateral aspect. This pattern applies despite phylogenetic, dietary, and allometric differences among the anthropoids sampled to date. Demes et al. (Food Acquisition and Processing in Primates [1984] New York: Plenum Press, p. 369-390) employed a theoretical analysis of mastication in Gorilla and Homo to argue that this pattern could be explained with reference to biomechanical stresses. Specifically, they proposed that the combined effects of torsion and direct shear on the working-side corpus create a condition in which net stresses and strains are reduced along the lingual cortical plate. Demonstration of this effect would suggest a functional linkage between localized differences in bone mass and strain gradients in the facial skeleton. We conducted an empirical evaluation of the effects of the combined loads of torsion and direct shear in vitro on a sample of formalin-fixed human mandibles. Rosette strain gages were affixed to the lateral and medial aspects of the corpus in each specimen, and surface strains were recorded separately under controlled torsional and occlusal loads, and under simultaneous application of these loads. The hypothesis that lingual strains are reduced under combined twisting and occlusal loads was generally supported; however, we observed reduction in surface strains at some sites along the lateral aspect of the corpus under these combined loads as well. These unexpected findings are attributable to unanticipated loading conditions imposed by occlusal forces, which result from sources of stress in addition to direct shear. These experiments provide provisional support for the hypothesis that superposed sources of bone strain produce large strain gradients between buccal and lingual aspects of the mandibular corpus, and that local variation in bone mass may be associated with these gradients.     

Effect of bone graft characteristics on the mechanical behavior of the lumbar spine

There is little information about the influence of bone graft size, position and elasticity on the mechanical behavior of the lumbar spine. Intersegmental motion, intradiscal pressure and stresses in the lumbar spine were calculated using a three-dimensional, nonlinear finite element model which included an internal spinal fixation device and a bone graft. Cross-sectional area, position, and elastic modulus of the graft were varied in this study. Bone grafts, especially very stiff ones, increase stresses on adjacent endplates. Though larger grafts lead to less contact pressure, it is difficult to judge the quality of different bone graft positions. In general, ventral flexion results in lower maximum contact pressure than lateral bending. There is always little intersegmental rotation in the bridged region compared with that of an intact spine.A larger graft with low stiffness should be favored from a mechanical point of view. Patients should avoid lateral bending of the upper body shortly after surgery.     

Biomechanical evaluation of porous biodegradable scaffolds for revision knee arthroplasty

Tibial bone defect is a critical problem for revision knee arthroplasty. Instead of using metallic spacer or cement, biodegradable scaffolds could be an alternative solution. A numerical model of a revision knee arthroplasty was thus developed to estimate the mechanical resistance of the scaffold in this demanding situation. The tibia, scaffold, and prosthesis were represented by simplified parameterised geometries. The maximal gait cycle force was applied asymmetrically to simulate a critical loading. Several parameters were analysed: 1) inter-individual variability, 2) cortical bone stiffness, 3) cortical bone thickness, 4) prosthesis fixation quality, and 5) scaffold thickness. The calculated scaffold strain was compared to its experimental ultimate strain. Among the tested parameters, failure was only predicted with scaffold thickness below 5 mm. This study suggests that biodegradable bone scaffolds could be used to fill bone defects in revision knee arthroplasty, but scaffold size seems to be the limiting factor.     

A computational algorithm to simulate disorganization of collagen network in injured articular cartilage

Cartilage defects are a known risk factor for osteoarthritis. Estimation of structural changes in these defects could help us to identify high risk defects and thus to identify patients that are susceptible for the onset and progression of osteoarthritis. Here, we present an algorithm combined with computational modeling to simulate the disorganization of collagen fibril network in injured cartilage. Several potential triggers for collagen disorganization were tested in the algorithm following the assumption that disorganization is dependent on the mechanical stimulus of the tissue. We found that tensile tissue stimulus alone was unable to preserve collagen architecture in intact cartilage as collagen network reoriented throughout the cartilage thickness. However, when collagen reorientation was based on both tensile tissue stimulus and tensile collagen fibril strains or stresses, the collagen network architecture was preserved in intact cartilage. Using the same approach, substantial collagen reorientation was predicted locally near the cartilage defect and particularly at the cartilage–bone interface. The developed algorithm was able to predict similar structural findings reported in the literature that are associated with experimentally observed remodeling in articular cartilage. The proposed algorithm, if further validated, could help to predict structural changes in articular cartilage following post-traumatic injury potentially advancing to impaired cartilage function.     

Multi-organ hydatidosis with extensive involvement of the hemi-pelvis and ipsilateral femur

Bone involvement of hydatid disease is uncommon but when encountered, it presents few unique pathological features. The pattern of tissue involvement is largely different from that of visceral hydatid cyst. We describe the case of a 47 year-old man from northern India, a case of systemic hydatidosis including the liver and the right lung, presenting with an abscess like lesion in the left gluteal region with pathological fractures of the left femur. Radiographs and CT-scan images showed extensive invasion of the left hemi-pelvis and left proximal femur. Debridement of the honeycombed ilium yielded hydatid fluid, numerous small cysts and necrotic material. Multiple large devitalized and sequestrated bone pieces were recovered from the bone cavity of the affected ilium. A histopathological study of the bone sequestrums revealed the unique pattern of bone invasion by the characteristic laminated multi-layered cyst walls into areas of least resistance. Bone sequestration has not often been described or demonstrated elaborately in published studies of the past. The bone defects formed after debridement of the ilium and proximal femur were filled with bone cement along with augmentation of the femur using intra-medullary nail. The surgical technique adopted in our case although was not expected to be curative owing to the multi-system disease; it did result in significant functional improvement in the patient.     

Mechanical variables affecting balloon kyphoplasty outcome--a finite element study

It is still unclear how a vertebral fracture should be stabilised and strengthened without endangering the remaining intact bone of the augmented vertebra or the adjacent vertebrae. Numerical modelling may provide insight. To date, however, few finite element (FE) spine models have been developed which are both multi-segmental and capture a more complete anatomy of the vertebrae. A 3-D, two-functional unit, CT-based, lumbar spine, FE model was developed and used to predict load transfer and likelihood of fracture following balloon kyphoplasty. The fractured anterior wall and injected cement were modelled in a two-functional spinal unit model with osteoporotic bone properties. Parameters investigated included: cement stiffness, cement volume and height restoration. Models were assessed based on stresses and a user-defined fracture-predicting field. Augmentation altered the stress distribution; shielding was dependent on positioning of the cement; and fracture algorithm found incomplete height restoration to increase the likelihood of fracture, particularly in adjacent vertebrae.     

Micromechanics modeling of Haversian cortical bone properties.

A finite-element micromechanics model for Haversian cortical bone tissue has been developed and studied. The model is an extension of two-dimensional micromechanics techniques for fiber-reinforced composite materials. Haversian systems, or secondary osteons, are considered to be the fiber component, and interstitial lamellar bone the matrix material. The cement line is included as an 'interphase' component along the fiber/matrix interface. The model assumes a regular repeatable spacing of the longitudinally aligned continuous fibers and is, therefore, restricted to approximating Haversian cortical bone in its present form. Haversian porosity is modeled explicitly by incorporating a hollow fiber to represent the Haversian canal. Solutions have been obtained by applying uniform macroscopic stresses to the boundaries of the repeating unit cell model. Macroscopic mechanical property predictions correspond reasonably well with the experimental data for cortical bone, but are necessarily dependent on the input properties for each constituent, which are not well established. The predicted variation in the elastic modulus with porosity is not as sensitive as that observed experimentally. Stresses within the constituents can also be modeled with this method and are demonstrated to deviate from the macroscopic applied stress levels.     

Comparison of the elastic and yield properties of human femoral trabecular and cortical bone tissue

The ability to determine trabecular bone tissue elastic and failure properties has biological and clinical importance. To date, trabecular tissue yield strains remain unknown due to experimental difficulties, and elastic moduli studies have reported controversial results. We hypothesized that the elastic and tensile and compressive yield properties of trabecular tissue are similar to those of cortical tissue. Effective tissue modulus and yield strains were calibrated for cadaveric human femoral neck specimens taken from 11 donors, using a combination of apparent-level mechanical testing and specimen-specific, high-resolution, nonlinear finite element modeling. The trabecular tissue properties were then compared to measured elastic modulus and tensile yield strain of human femoral diaphyseal cortical bone specimens obtained from a similar cohort of 34 donors. Cortical tissue properties were obtained by statistically eliminating the effects of vascular porosity. Results indicated that mean elastic modulus was 10% lower (p<0.05) for the trabecular tissue (18.0+/-2.8 GPa) than for the cortical tissue (19.9+/-1.8 GPa), and the 0.2% offset tensile yield strain was 15% lower for the trabecular tissue (0.62+/-0.04% vs. 0.73+/-0.05%, p<0.001). The tensile-compressive yield strength asymmetry for the trabecular tissue, 0.62 on average, was similar to values reported in the literature for cortical bone. We conclude that while the elastic modulus and yield strains for trabecular tissue are just slightly lower than those of cortical tissue, because of the cumulative effect of these differences, tissue strength is about 25% greater for cortical bone.