Periprosthetic modelling of femoral component fit using computed tomography data for total hip arthroplasty: a feasibility study

The aim of the work was to create a new three-dimensional periprosthetic multi-criteria optimisation technique to identify the best six degrees of freedom transform to position a porous-coated anatomic cementless femoral component for three factors, including: first, maximisation of the degree of contact achieved between designated bone ingrowth surfaces and the periprosthetic bone; secondly, minimisation of the bone mass to be removed to accommodate the component and thirdly, the extreme constraint of the component to be positioned so that it does not project beyond the periosteum. Discrete integrals were computed over regions of interest derived from the polyhedral component mesh in transaxial CT scan planes, using a polygon scan-conversion algorithm. A new biomedical imaging volume rendering technique utilising dynamic virtual textures was developed to visualise the design trade-offs. Pareto-optima were identified for four femora that matched an average-sized component. The non-linear, multi-modal fit metric was quadratic near minima, with a narrow trough of equivalent fit values within 3mm of translation and 3 degrees of rotation with respect to the canal axis, and possessed a dependence most pronounced for distal-directed insertion against varus/valgus rotation. The study gives previously unavailable data on the three-dimensional femoral component fit and is the first report that demonstrates that fitting the implant using several design criteria in a multi-criteria optimisation scheme is feasible.

Design and analysis of robust total joint replacements: finite element model experiments with environmental variables

Computer simulation of orthopaedic devices can be prohibitively time consuming, particularly when assessing multiple design and environmental factors. Chang et al. (1999) address these computational challenges using an efficient statistical predictor to optimize a flexible hip implant, defined by a midstem reduction, subjected to multiple environmental conditions. Here, we extend this methodology by: (1) explicitly considering constraint equations in the optimization formulation, (2) showing that the optimal design for one environmental distribution is robust to alternate distributions, and (3) illustrating a sensitivity analysis technique to determine influential design and environmental factors. A thin midstem diameter with a short stabilizing distal tip minimized the bone remodeling signal while maintaining satisfactory stability. Hip joint force orientation was more influential than the effect of the controllable design variables on bone remodeling and the cancellous bone elastic modulus had the most influence on relative motion, both results indicating the importance of including uncontrollable environmental factors. The optimal search indicated that only 16 to 22 computer simulations were necessary to predict the optimal design, a significant savings over traditional search techniques.

Predictive model for tensile true stress-strain behavior of chemically and mechanically degraded ultrahigh molecular weight polyethylene

The gamma radiation sterilization of ultrahigh molecular weight polyethylene (UHMWPE) components in air generates long-lived free radicals that oxidize slowly over time during shelf storage and after implantation. To investigate the combined effects of chemical and mechanical degradation on the mechanical behavior of UHMWPE, sterilized tensile specimens were immersed in 0.5% hydrogen peroxide solution at 37 degrees C for up to 9 months and concurrently subjected to cyclic stress levels of 0 (control), 0 to 5, and 0 to 10 MPa. After chemical and mechanical preconditioning, specimen density was measured using the density gradient column technique. The true stress-strain behavior was measured up to 0.12 true strain and characterized using a multilinear material model, the parameters of which were found to vary linearly with density and cyclic stress history. The mechanical behavior of as-irradiated and degraded UHMWPE was accurately predicted by an analytical composite beam model of the tensile specimens. The results of this study support the hypothesis that chemical and mechanical degradation affect the true stress-strain behavior of UHMWPE. In the future, the material model data presented in this study will enable more accurate prediction of the stresses and strains in UHMWPE components following gamma sterilization in air and subsequent in vivo degradation.

A reconciliation of local and global models for bone remodeling through optimization theory

Remodeling rules with either a global or a local mathematical form have been proposed for load-bearing bones in the literature. In the local models, the bone architecture (shape, density) is related to the strains/energies sensed at any point in the bone, while in the global models, a criterion believed to be applicable to the whole bone is used. In the present paper, a local remodeling rule with a strain "error" form is derived as the necessary condition for the optimum of a global remodeling criterion, suggesting that many of the local error-driven remodeling rules may have corresponding global optimization-based criteria. The global criterion proposed in the present study is a trade-off between the cost of metabolic growth and use, mathematically represented by the mass, and the cost of failure, mathematically represented by the total strain energy. The proposed global criterion is shown to be related to the optimality criteria methods of structural optimization by the equivalence of the model solution and the fully stressed solution for statically determinate structures. In related work, the global criterion is applied to simulate the strength recovery in bones with screw holes left behind after removal of fracture fixation plates. The results predicted by the model are shown to be in good agreement with experimental results, leading to the conclusion that load-bearing bones are structures with optimal shape and property for their function.

Preclinical cost analysis of orthopaedic implants: a custom versus standard cementless femoral component for revision total hip arthroplasty

A preclinical cost analysis method was introduced to assess the cost effectiveness of using a custom implant instead of standard "off-the-shelf" implants for revision total hip arthroplasty. Finite element models of proximal femur-implant systems were constructed and an array of environmental factors, including loads and bone properties, was incorporated into a computer experiment to evaluate relative motion between implant and bone. Implant performance related cost was then determined from relative motion measures using a quality loss function. Unit manufacturing cost was added to implant performance cost to determine the cost difference between the two implants. The reduction in relative motion achieved by the custom implant with respect to an equivalent-lengthed standard implant justified its additional unit manufacturing costs. In response to these results and suggestions by surgeons, we increased the length of the standard implant by 50 mm and performed an identical series of analyses. We found that increasing the stem length to 120 mm substantially decreased the relative motion of the standard implant to values less than for the custom implant. This case study provides preliminary evidence that a surgical inventory consisting of longer-stemmed standard implants or modular distal stems is more cost effective than designing custom devices on a case-by-case basis. Additional design studies are warranted before generalizing such a claim.

Robust optimization of total joint replacements incorporating environmental variables

Direct search techniques for the optimal design of biomechanical devices are computationally intensive requiring many iterations before converging to a global solution. This, along with the incorporation of environmental variables such as multiple loading conditions and bone properties, makes direct search techniques infeasible. In this study, we introduced new methods that are based on the statistical design and analysis of computer experiments to account efficiently for environmental variables. Using data collected at a relatively small set of training sites, the method employs a computationally inexpensive predictor of the structural response that is statistically motivated. By using this predictor in place of the simulator (e.g., finite element model), a sufficient number of iterations can be performed to facilitate the optimization of the complex system. The applicability of these methods was demonstrated through the design of a femoral component for total hip arthroplasty incorporating variations in joint force orientation and cancellous bone properties. Beams on elastic foundation (BOEF) finite element models were developed to simulate the structural response. These simple models were chosen for their short computation time. This allowed us to represent the actual structural response surface by an exhaustive enumeration of the design and environmental variable space, and provided a means by which to validate the statistical predictor. We were able to predict the structural response and the optimal design accurately using only 16 runs of the computer code. The general trends predicted by the BOEF models were in agreement with previous three-dimensional finite element computer simulations, and experimental and clinical results, which demonstrated that the important features of intramedullary fixation systems were captured. These results indicate that the statistically based optimization methods are appropriate for optimization studies using computationally demanding models.

Cemented femoral stem performance. Effects of proximal bonding, geometry, and neck length

The effects of proximal bonding, distal stem geometry, and femoral neck length on cement and interface stresses were determined to understand better their role in clinical performance. The effects of stem design were compared with the effects of environmental variables, patient weight, and patient activity. Finite element models were used to determine peak cement and interface stresses, and an experimental layout was used to separate design and environmental effects. Bonding reduced cement mantle stresses by 35% to 60%, to levels below the cement fatigue strength. A flat sided implant provided more torsional resistance, reducing shear stresses at the proximal cement-prosthesis interface by 22% to 73% with respect to a distal round implant. Neck length had minimal effects on stresses compared with bonding or implant geometry. Cement-bone interface stresses were more sensitive to patient activity than to the design variables. Therefore, claims that a strong cement and prosthesis bond may be harmful to the bone-cement interface are unjustified based on these results. The best combination of design variables was a proximally bonded, flat sided implant with neck length left to the surgeon's discretion. This combination was most effective at protecting the cement mantle and prosthesis interface and perhaps the cement-bone interface by minimizing stresses associated with cement debris generation.

Postirradiation aging affects stress and strain in polyethylene components

Ultrahigh molecular weight polyethylene components oxidatively degrade because of gamma radiation sterilization and subsequent shelf aging in air. The effects of shelf aging on the stresses and strains associated with surface damage in tibial and acetabular components were examined. A material model was developed to predict the stress and strain relationship of oxidatively degraded polyethylene as a function of density using samples of polyethylene that were gamma radiation sterilized and evaluated immediately after irradiation and after 42 months of shelf aging. The finite element method was used to determine the stresses and strains before and after shelf aging for two tibial components with different conformities between the articulating surfaces and for an acetabular component. The stresses increased by 10% to 14% in the conforming tibial model after 42 months of aging, whereas the stresses in the nonconforming tibial model and in the acetabular model increased by only 4% to 8%. Aging decreased the principal strains by 5% to 10% in both tibial models and by 15% to 17% in the acetabular model. Postirradiation aging during shelf storage of polyethylene joint components is likely to worsen long term wear, based on the increased stresses and decreased strains predicted to occur as a result of aging.

Residual stresses in ultra-high molecular weight polyethylene loaded cyclically by a rigid moving indenter in nonconforming geometries

The characterization of stress and deformation fields that incorporate moving cyclic loads and nonlinear material response in ultra-high molecular weight polyethylene components for total knee replacements is required to quantify mechanisms of surface damage. A simulation of stresses in polyethylene components for total knee replacement subjected to cyclic moving loads was performed with use of nonlinear finite element analysis. Convergence to a steady-state cycle of stress and deformation was observed within five cycles of loading. Differential plastic deformation under the surface of the polyethylene led to horizontal residual stresses that were tensile at the surface and compressive in the subsurface. The magnitudes of the residual stresses indicate their importance in surface failure mechanisms. Horizontal residual tensile stresses at the surface are consistent with the initiation and propagation of surface cracks that could cause pitting in polyethylene. Horizontal residual compressive stresses under the surface could cause such cracks to arrest or turn and thus limit damage to a region just beneath the surface. The results emphasize the importance of incorporating nonlinear effects to simulate long-term stress fields associated with surface damage in polyethylene.

Influence of stem geometry on mechanics of cemented femoral hip components with a proximal bond

Nonlinear, three-dimensional, finite element models of cemented femoral hip components with a proximal stem-cement bond were developed with use of a Charnley stem geometry and a modified Charnley stem geometry that had a cylindrical cross section over the distal two-thirds of the stem (Distal-Round). Peak tensile stresses in the proximal cement mantle increased 63 and 74% for the Charnley and Distal-Round stems, respectively, when the proximal stem-cement interface was debonded. The shear stresses over the stem-cement interface with a proximal bond were 29% larger for the Distal-Round stem than for the Charnley stem. After the proximal stem-cement interface was debonded, the peak tensile stresses in the cement mantle were 15% larger for the Distal-Round stem than for the Charnley stem. The results illustrate that stresses within the proximal cement mantle could be substantially reduced for both Charnley and Distal-Round stems through use of a proximal stem-cement bond. However, the risk of debonding may be higher for the Distal-Round stem because of increased shear stresses, and once debonded the risk of further loosening due to failure of the cement mantle would also be higher for the Distal-Round stem.

The role of backside polishing, cup angle, and polyethylene thickness on the contact stresses in metal-backed acetabular components

Mechanical interactions between the polyethylene liner and the metal-backing play an important role in the load transfer and debris-generation mechanisms of an acetabular component. Insert thickness, cup orientation, and insert-shell interface conditions affect the resulting contact stresses at the articulating and backside surfaces of the polyethylene component. The objective of this study was to determine the variation in contact stresses on a hemispherical acetabular component as a function of the friction coefficient of the line-shell interface, the thickness of the insert, and the load application angle. Three-dimensional finite element models of a metal-backed acetabular component with liner thicknesses of 3-12 mm were developed. The insert-shell interface was modeled as either matte or highly polished, and the load angle of the joint reaction force was changed from 36 to 63 degrees with respect to the dome. We found that the contact stresses at the articulating and backside surfaces of the insert were relatively insensitive to changes in the coefficient of friction at the insert-shell interface (resulting in approximately 1-10% variation in contact stress), when compared to the effect of changing the insert's thickness (approximately 80% variation in contact stress) or changing the direction of the joint reaction force (approximately 20% variation in contact stress). The results of this study suggest that polishing the metal at the insert-shell interface does not substantially change the contact stresses at either surface of the component. Of the design variables available for selective modification by either the surgeon or the engineer, insert thickness and shell orientation play a greater role in determining the magnitude of the resulting contact stresses.

Degradation rate of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene components for total joint replacement chemically degrade before and after implantation, and the degradation is associated with an increase in density. The goal of this study was to determine the average rate of density change in these components following sterilization by gamma radiation in air as a function of shelf age and implantation time. Using the density gradient column method, density profiles were obtained through the thickness from loaded and unloaded regions of 10 retrieved Insall-Burstein/Posterior-Stabilized II tibial components and one operating-room inventory component for which the initial density profile and patient history (if applicable) were known. The average density of the components increased at a constant rate of 0.000186 g/cc/month during the first 50 months after sterilization (r2 = 0.54) but was not significantly affected by loading (p > 0.05). The quantitative degradation rates may be useful to help verify kinetic models to predict bulk degradative changes on the basis of micro-structural and chemical processes. This research also suggests the hypothesis that degradation of ultra-high molecular weight polyethylene can be modeled in terms of changes in bulk or average properties.

Exponential model for the tensile true stress-strain behavior of as-irradiated and oxidatively degraded ultra high molecular weight polyethylene

Following sterilization by gamma radiation, ultra high molecular weight polyethylene components for total joint replacement undergo oxidative degradation upon exposure to air and the in vivo environment. Oxidative degradation is accompanied by an increase in density. The primary objective of this study was to develop a mathematical model to predict the monotonic tensile mechanical behavior of these sterilized components as a function of changes in density arising from oxidative degradation. Tensile specimens of ultra high molecular weight polyethylene were sterilized with gamma radiation and then oxidatively degraded in an air furnace. The average density of each specimen was measured using a density gradient column. Differential scanning calorimetry and Fourier transform infrared spectroscopy were conducted on selected specimens to characterize the physical and chemical changes due to accelerated aging as opposed to ambient shelf aging. Mechanical testing was conducted in monotonic uniaxial tension. An exponential model was fitted to the true stress-strain data (up to a true strain of 0.12). The observed fitted stress had a correlation coefficient of 0.996. The model permits a quantitative prediction of the association between the true stress-strain curve and density for the ultra high molecular weight polyethylene components. The proposed exponential model effectively describes changes in the large-strain monotonic tensile behavior of as-irradiated and oxidatively degraded ultra high molecular weight polyethylene components.

Surface geometry of retrieved McKee-Farrar total hip replacements

A coordinate measurement machine was used to determine the surface geometry of 22 retrieved McKee-Farrar total hip replacements. The radial clearance of each acetabular cup-femoral head pair was computed to see whether there was potential for hydrodynamic lubrication. The surface geometry was used to estimate the volumetric wear for the acetabular cup and the femoral head, and to assess whether those prostheses that had the potential for fluid film lubrication also displayed lower wear. The volumetric wear rate for each prosthesis was estimated by dividing the volumetric wear by the corresponding service life. Two distinct wear patterns were seen on the femoral head. Prostheses displaying polar wear on the femoral head showed significantly greater service lives than those displaying non-polar wear. Although several prostheses had clearances that provided for the possibility of hydrodynamic lubrication, no correlation could be found between clearance and the measured wear.

Coulomb frictional interfaces in modeling cemented total hip replacements: a more realistic model

Loosening of cemented femoral hip stems could be initiated by failure of the cement mantle due to high cement stresses. The goals of this study were to determine if realistic stem-cement interface characteristics could result in high cement stresses when compared to a bonded stem-cement interface and to determine if stem design parameters could be chosen to reduce peak cement stresses. Three-dimensional finite-element models of cemented femoral hip components were studied with bonded or realistic Coulomb friction stem-cement interfaces. The results showed that the use of a non-bonded, non-linear Coulomb friction interface resulted in substantially different stress fields in the cement when compared to a bonded stem-cement interface. Tensile stresses in the proximal cement mantel for the Coulomb friction interface case (10.8 MPa) were greater than the fatigue strength of the cement. In contrast, the tensile stresses in the cement mantle were not greater than the fatigue strength for the bonded case (7.5 MPa). Failure of the cement mantle in the proximal femur could therefore be initiated by a lack of a bond at the stem-cement interface. The effect of different cross-sectional stem geometries (medial radii of 3.0, 4.9 and 5.5 mm and antero-posterior widths of 9.8 and 13.7 mm) and different elastic moduli (cobalt chromium alloy and titanium alloy) for the stem material were also evaluated for models with a Coulomb friction interface. Changes in the stem cross-section and elastic modulus had only limited effects on the stress distributions in the cement. Of the parameters evaluated in this study, the characteristics of the stem-cement interface had the largest effect on cement mantle stresses.

Stresses in polyethylene components of contemporary total knee replacements

Contemporary knee designs differ considerably in the conformity that exists between the articulating surfaces of the femoral and tibial components. The thickness of the polyethylene components also varies from design to design. Conformity and thickness affect the stresses associated with surface damage and the subsequent generation of harmful polyethylene debris. In this study, the stresses and strains caused by contact were calculated for 8 contemporary knee prostheses. Finite element analysis using large-strain theory was used to determine the stresses and strains for the minimum available polyethylene thickness and for the knee in flexion. The greatest differences among designs was for the von Mises strain, which reached its maximum beneath the surface. The differences in stresses were less notable because of the nonlinear material behavior of the polyethylene. This study also confirmed the advantages of designs that have more conforming articulating surfaces and thicker polyethylene components.

Mechanical consequences of bone ingrowth in a hip prosthesis inserted without cement

Long-term biomechanical problems associated with the use of sintered porous coating on prosthetic femoral stems inserted without cement include proximal loss of bone and a risk of fatigue fracture of the prosthesis. We sought to identify groups of patients in whom these problems are accentuated and in whom the use of porous coating may thus jeopardize the success of the arthroplasty. We attempted to develop clinical guidelines for the use of sintered porous coating by investigating the long-term biomechanical effects of bone growth into partially (two-thirds) porous-coated anatomic medullary locking hip prostheses that fit well. More specifically, we used a detailed finite element analysis and a composite beam theory to determine the dependence of proximal loading of the bone and maximum stresses on the stem on the development of clinically observed patterns of bone ingrowth and the dependence of the risk of fatigue fracture of the stem on the diameter of the stem, the diameter of the periosteal bone, and the material from which the prosthesis was made. We found that bone ingrowth per se substantially reduced proximal loading of the bone. With typical bone ingrowth, axial and torsional loads acting on the proximal end of the bone were reduced aa much as twofold compared with when there was no ingrowth; bending loads on the proximal end of the bone were also reduced. The risk of fatigue fracture of the stem was insensitive to the development of bone ingrowth. However, the risk of fatigue fracture of the stem increased with decreased diameters of the stem and the periosteal bone and with increased modulus of the stem. The maximum risk of fracture was found in active patients in whom a cobalt-chromium-alloy stem with a small diameter had been implanted in a bone with a small diameter.

Fundamental load transfer patterns for press-fit, surface-treated intramedullary fixation stems

To extend our understanding of the concentric cylinder idealization of a cementless hip stem, we used finite element analysis to study the effects of various types and amounts of surface treatments on the mechanical environment of the bone-stem interface for different load cases in the early post-operative situation. All analyses used no-tension interface conditions with various values of the frictional coefficient. We found that the shear stresses along the medial bone-stem interface were most sensitive to the type and amount of surface treatment, while the contact regions were relatively insensitive to the surface treatment. Consequently, the surface treatment had a negligible effect on the transfer of bending loads. By contrast, the axial load (when combined with a bending load) was transferred by shear stresses at the lateral stem tip for fully coated stems, but by shear stresses at the medial coating junction for partially coated stems. These findings therefore indicate that transfer of bending loads from the stem to the diaphysis cannot be controlled in the early post-operative situation by surface treatments, while transfer of axial loads can be controlled with the appropriate choice and distribution of a coating. In addition, the correspondence between the predicted shear stress distributions at the porous coating junction of partially coated devices and observed bone hypertrophy patterns that are apparently biased towards the medial aspect of the junction suggest that bone remodeling at the interface around porous coated hip implants may be initially stimulated by the development of small shear stresses.

Exposure and postoperative stability of three medial surgical approaches to the canine elbow

Articular cartilage exposure and immediate postoperative stability provided by three medial surgical approaches in canine cadaver elbows were compared. The approaches evaluated were a desmotomy of the medial collateral ligament (DMCL) that included a tenotomy of the pronator teres muscle, a longitudinal myotomy of the flexor carpi radialis (MFCR), and an osteotomy of the medial epicondyle (OME). Nondestructive biomechanical testing was performed before the surgical approach and repeated after surgery. The stiffness at 13 degrees valgus deviation of the elbow and energy absorption up to 13 degrees valgus deviation of the elbow were determined from the preoperative and postoperative torque-rotation curves. The perimeters of the ulnar and humeral articular cartilage that were visualized through the approach were scored with a dental pick. Latex casts were made of articular surfaces of the elbow. The humeral and ulnar articular exposures were determined by computerized planimetric analysis of latex cast photocopies. The humeral cartilage exposure of the OME approach was significantly greater than either the MFCR or DMCL approaches. The DMCL approach provided a significantly greater humeral cartilage exposure than the MFCR approach. All three approaches provided statistically similar percentages of ulnar cartilage exposure. The stiffness and energy absorption of the OME and MFCR approaches were similar and significantly greater than the DMCL approach. The OME approach provided the best combination of exposure and immediate postoperative stability.

Fatigue crack propagation behavior of ultra high molecular weight polyethylene under mixed mode conditions

Analytical studies of the stresses on and within ultra high molecular weight polyethylene joint components suggest that damage modes associated with polyethylene fatigue failure are caused by a combination of surface and subsurface crack propagation. Fatigue crack propagation tests under mixed mode loading conditions were conducted on center-cracked tension specimens machined from extruded blocks of sterilized polyethylene in an attempt to determine how fatigue cracks change direction in this material. Cyclic testing was performed using a sinusoidal wave form at a frequency of 5 Hz and an R-ratio (minimum load/maximum load) of 0.15. Specimens had the notch oriented perpendicular to the direction of applied load and at angles of 60 degrees and 45 degrees to the loading direction. Numerical analyses were used to interpret the experimental test and to predict the fatigue behavior of polyethylene under mixed mode conditions. It was found that all cracks eventually propagated horizontally, regardless of the initial angle of inclination of the notch to the direction of applied cyclic load. In fact, the extent of the curvilinear crack growth was quite limited. An effective range of cyclic stress intensity factor was calculated for correlation with the rate of crack growth. The results followed a Paris relation, with crack growth rate linearly related to a power of the range of stress intensity, for all three crack orientations. The numerical analyses adequately modeled the experimental fatigue crack growth results.

Effects of porous coating, with and without collar support, on early relative motion for a cementless hip prosthesis

In theory, porous or rough coatings could be used to reduce early post-operative relative motion about cementless hip prostheses. To investigate this theory, we used detailed, non-linear finite element analysis to compare early relative motion about a well-fit Anatomical Medullary Locking (AML) prosthesis for different amounts of porous coating (full, proximal 2/3, and no coating), both with and without collar support. Details of the model included quantitative computed tomography-derived (QCT-derived) geometric and material properties for the bone, and a no-tension interface condition at all bone-prosthesis interfaces, with Coulomb friction (mu = 1.73) over coated surfaces and zero friction elsewhere. Predicted values of relative motion for this well-fit device were in the range of approximately 1-550 microns. The distribution of relative motion was relatively insensitive to the amount of porous coating but was sensitive to collar support, while the magnitude of relative motion was sensitive to the porous coating and collar support. In addition, a reduction in the porous coating caused larger increases in relative motion when there was no collar support, indicating an interaction between the effects of porous coating and collar support. For example, distal twist increased (full vs 2/3 partial coating) by 38% with collar support and by 58% without collar support. These data suggest that porous coating, or other surface treatments which result in a high coefficient of friction at the bone-prosthesis interface, may well be used to control the magnitude of early relative motion, particularly when there is no collar support.

Effects of porous coating and collar support on early load transfer for a cementless hip prosthesis

We used a new postprocessing method with the results from a three-dimensional finite element analysis to describe the general load transfer patterns for a cementless hip arthroplasty in the early postoperative situation, and to determine the effects of porous coating [full, partial (2/3), and none] and calcar-collar support (ideal initial contact with separation allowed upon loading, no collar) on this early load transfer. No-tension interfaces were modeled over the entire bone-prosthesis interface, with an upper bound on the Coulomb-friction over coated surfaces, and zero friction over smooth surfaces to accentuate the frictional effects of the coating. The results indicate that the anteroposterior, mediolateral, and axial forces acting on each cross section of the bone were substantially different from the corresponding homeostatic (no prosthesis) forces for the fully coated device with collar support. The frontal bending moments acting on the bone were substantially less than the homeostatic values all along the prosthesis, while the sagittal bending and torsional loads were relatively similar to the homeostatic values. By far, the largest change in these loading patterns occurred with the loss of collar support, where axial loads acting on the bone were so low that over half the bone was in net tension because appreciable transfer of the compressive head force did not occur until well below the lesser trochanter. Both axial and torsional loads were transferred more distally for devices with more coating, and torsional loading of the bone was also sensitive to the degree of collar support. The frontal bending moments acting over most of the bone were insensitive to the coating or collar support. The strain energy density in the endosteal bone was most sensitive to these design variables in the proximal region, and the largest values occurred without a collar and without coating. These findings indicate that all load components acting on the proximal bone in the early postoperative situation (no bone-ingrowth or fibrous tissue at the interface) are altered by the frictional coefficient of the bone-prosthesis interface (i.e. the presence of porous coating or some other surface treatment) and the degree of collar support, while only the axial and torsional loads are altered in the distal bone. From a prosthesis design perspective this implies that surface treatments and collar support can be used to control the axial forces and the torsional moments acting all along the bone. By contrast, the distal frontal bending moment, which dominates stresses in the diaphysis, cannot be altered by these design variables.

Load transfer with the Austin Moore cementless hip prosthesis

More than 1,300 Austin Moore hemiarthroplasties have been reviewed in the literature, with no reports of fracture of the stem. Many patients with these hip implants had good function. The lack of stem fractures in patients with good functions has not been explained and contrasts with stem fractures that have occurred in patients with cemented prostheses of other designs during the same time. We used three-dimensional finite-element analysis and free-body diagrams to explain the lack of fractures for this device by a description of the probable load-transfer mechanisms between the prosthesis and the bone. Results from our finite-element analysis indicate that, with good calcar-collar support, the stresses in the stem are small because the stem portion of the prosthesis and the bone are uncoupled and, consequently, do not share the resultant bending moment of the head and abductor forces. If the stem is coupled to the bone so that the resultant bending moment is shared, high stresses in the stem are predicted; such stresses are inconsistent with the complete absence of fractures of these prostheses. The results of the finite-element analysis further showed that loss of calcar-collar support with proximal fixation through the fenestrations resulted in high stresses in the stem and stress shielding of the proximal medial cortex. The uncoupled prosthesis also may be modeled with a free-body diagram as a three-force member loaded at the head, stem tip, and in the proximal region. With this model, it can be shown that the reaction force of the stem tip, and thus the peak bending stress in the stem, increases as calcar-collar support is decreased. If there is no calcar-collar support, proximal support must be provided by some combination of integration of bone in the fenestrations and wedging due to the lateral-medial taper of the device. Stresses in the stem are largest when there is no wedging, but high stresses develop in the cancellous bone in the fenestrations. When there is wedging, stresses in the stem can be low, but stresses in the supporting cancellous bone can be high; additional proximal support through the fenestrations substantially reduces these bone stresses. If reduced stresses in the cancellous bone are indicative of a stable device, these mechanisms indicate that fractures of the Austin Moore prosthesis have not occurred in normally loaded hips because load was transferred primarily either through the collar or by wedging, with additional support at the fenestrations.

Consequences of an interference fit on the fixation of porous-coated tibial components in total knee replacement

A linearly elastic, axisymmetrical finite-element model was developed in an attempt to explain observed long-term patterns of growth of bone into tibial components. This model, which represents a portion of the tibial tray, one peg, and the surrounding cancellous bone, was used to examine two conditions of fixation in the immediate postoperative period. The first was characterized by the use of an interference fit for initial fixation of the component and the second, by the use of an interference fit with the hole in the bone deeper than the length of the peg. Two conditions of long-term fixation were also examined. In one, the bone was assumed to have grown into all of the porous coating. In the other, the bone was assumed to have grown only into the peg, and a layer of fibrous tissue was assumed to have developed between the tray and the bone. An interference fit between the peg and the cancellous bone produced considerable residual radial stresses in the bone. These stresses provide conditions that are favorable for ingrowth of bone into the pegs because the bone at the interface is stressed, and these stresses inhibit relative motion at the bone-peg interface. However, the interference fit of the peg relieved the stresses in the cancellous bone under the tray of the implant. Lack of stress at this interface is consistent with relative motion and subsequent formation of a layer of fibrous tissue. Deepening of the hole for the peg in the cancellous bone did not diminish the effects of the interference fit. Stresses in the bone under the metal tray were relieved when a layer of fibrous tissue under the tibial tray was represented in the model.

Mechanical characteristics of the stem-cement interface

The mechanical characteristics of the interface between a metallic stem and the surrounding poly(methyl methacrylate) bone cement were determined from experimental tests and finite element analyses. Push-through-stem tests of straight and tapered titanium alloy stems, surrounded by cement columns, were performed and the resulting load-displacement behavior and strain distribution on the surface of the cement column were measured for loading, unloading, and reloading. Test geometries were modelled using nonlinear, axisymmetric, finite element analyses, which incorporated Coulomb friction elements at the titanium alloy-cement interface. Initial residual stresses, due to curing of the cement column, were modeled by thermal contraction of the cement. Good agreement was obtained between load-displacement curves and surface strains predicted from the nonlinear analysis and those obtained from experiments, when a coefficient of friction of 0.3 was assumed for the stem-cement interface. These results show that, in the absence of chemical adhesion, the load-displacement behavior of a stem-cement composite can be described completely in terms of the friction at the interface and the residual stresses normal to the interface.

Effects of sodium hyaluronate on tendon healing and adhesion formation in horses

Sodium hyaluronate reduces adhesions after tendon repair in rodents and dogs, and has been used in limited clinical trials in people. To evaluate its effect on tendon healing and adhesion formation in horses and to compare these effects with those of a compound of similar visco-elastic properties, a study was performed in horses, using a model of collagenase injection in the flexor tendons within the digital sheath. Eight clinically normal horses were randomly allotted to 2 groups. Adhesion formation between the deep digital flexor tendon and the tendon sheath at the pastern region was induced in the forelimbs of all horses. Using tenoscopic control, a 20-gauge needle was inserted into the deep digital flexor tendon of horses under general anesthesia and 0.2 ml of collagenase (2.5 mg/ml) was injected. The procedure was repeated proximally at 2 other sites, spaced 1.5 cm apart. A biopsy forceps was introduced, and a 5-mm tendon defect was created at each injection site. Group-A horses had 120 mg of sodium hyaluronate (NaHA) gel injected into the tendon sheath of one limb. Group-B horses had methylcellulose gel injected at the same sites. The contralateral limbs of horses in both groups served as surgical, but noninjected, controls. Horses were euthanatized after 8 weeks of stall rest. Ultrasonographic evaluation revealed improved tendon healing after NaHa injection, but no difference in peritendinous adhesion formation. Tendon sheath fluid volume and hyaluronic acid (HA) content were greater in NaHA-treated limbs. Gross pathologic examination revealed considerably fewer and smaller adhesions when limbs were treated with NaHA. However, significant difference in pull-out strengths was not evident between NaHA-treated and control limbs. Histologically, the deep digital flexor tendon from the NaHA-treated limbs had reduced inflammatory cell infiltration, improved tendon structure, and less intratendinous hemorrhage. Treatment with methylcullulose had no significant effect on tendon healing, adhesion size, quantity, or strength or on the volume and composition of the tendon sheath fluid. Sodium hyaluronate, administered intrathecally, appears to have a pharmaceutically beneficial action in this collagenase-induced tendinitis and adhesion model in horses.

Development of a model to demonstrate photosensitizer-mediated viral inactivation in blood

A model has been developed to demonstrate the use of photodynamic treatment (PDT) to eradicate viral contaminants from donated blood and blood products. Whole blood, spiked with vesicular stomatitis virus (VSV), was treated with the photosensitizer benzoporphyrin derivative-monoacid ring A (BPD-MA). After light activation of BPD-MA, a neutral red dye uptake assay was carried out to determine virus inactivation. Various drug incubation times and light intensities were tested as well as red cell lysis and distribution of VSV in blood. At BPD-MA concentrations between 2 and 4 micrograms per mL in whole blood, up to 10(7) VSV were inactivated. Several photosensitizers were also tested with this model to determine their relative efficacy in viral inactivation.

The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement

Debris resulting from damage to the surface of polyethylene components of total joint replacements has previously been shown to contribute to long-term problems such as loosening and infection. Surface damage has been associated with fatigue processes due to stresses arising from contact between the metal and polyethylene components in these prostheses. In the present study, we used elasticity and finite-element solutions to determine these stresses for total hip replacements with head diameters of twenty-two and twenty-eight millimeters and for a condylar total knee replacement. We also examined the effect on these stresses of using carbon-fiber-reinforced polyethylene instead of plain polyethylene. Stresses associated with surface damage in the tibial component of the total knee replacement were much larger than those in the hip replacements. The analysis of contact stress as a function of thickness of the polyethylene insert for tibial components showed that a thickness of more than eight to ten millimeters should be maintained when possible. The contact stress in the tibial components was reduced most when the articulating surfaces were more conforming in the medial-lateral direction. Contact stresses were much less sensitive to changes in geometry in the anterior-posterior direction. For the hip components, the stresses were lower in the acetabular component of the twenty-eight-millimeter hip replacement than in the twenty-two-millimeter replacement. The use of carbon-fiber-reinforced polyethylene resulted in stresses that were higher by as much as 40 per cent. Because the contact area between articulating surfaces moves during flexion, portions of the surface will be subjected to cyclic stresses. The contact area for the knee replacements in flexion was smaller than for the hip replacements, and the range of the maximum principal stress was larger. Consequently, the combination of the higher stress and the moving contact area is more likely to cause surface damage due to fatigue in tibial components than in acetabular components, which is consistent with clinical observations.

The problem of surface damage in polyethylene total knee components

Observations of surface damage on retrieved total knee polyethylene components have been combined with experimental and analytical studies of the contact problem to identify important clinical and design factors. The amount and severity of damage occurring on the articulating surfaces of knee components increases significantly with patient weight and with the length of time the component is implanted. The design variables that affect the amount of damage are component thickness, the conformity of the articulating surfaces, and the type of polyethylene material used. Surface damage will be more severe in thin (less than 4-6 mm) components and in components with relatively flat tibial articulating surfaces. Surface damage is also expected to be more extreme for carbon-reinforced polyethylene components than for components made from plain polyethylene.

Elastic constitutive laws for cow teat tissue undergoing finite deformations

Five constitutive laws are investigated to model the effect of machine milking. A nonlinear least squares procedure is employed to estimate material constants from in vivo teat inflation data. An exponential form is found to be statistically adequate as a constitutive law, and is used to determine the mechanical stresses in teat tissue during finite deformations.

The effect of conformity and plastic thickness on contact stresses in metal-backed plastic implants

Surface damage in polyethylene components for total joint replacement is associated with large contact stresses. An elasticity solution and finite element analyses were used to determine the influence of design parameters on the stresses due to contact in metal-backed components. For nearly conforming contact surfaces, it was found that the stresses in the plastic are very sensitive to clearance, that minimum plastic thickness of 4-6 mm should be maintained for metal-backed components, and that bonding the plastic to the metal backing reduces tensile stresses in the plastic at the edge of the contact zone.

Performance of the tibial component in total knee replacement

In patients with deficient bone in the proximal end of the tibia, the mechanical support of a conventional total knee replacement may be inadequate. We have developed a custom design for use in situations in which there is extensive deficiency of tibial cancellous bone. To do this, we examined conventional and custom prosthetic tibial components using finite-element analysis. Several loading configurations were tested, and the worst loading conditions were found to be those in which eccentric loads were placed on the margin of the tibial component. The results showed that the stresses on the cancellous bone beneath a conventional-design prosthesis may be lowered if a metal tray and metal peg are employed. A salvage-design concept for revision in the presence of deficient cancellous bone was tested analytically and used successfully. This concept requires that some portion of the applied load be transferred directly to the tibial cortical shell. Stresses in the remaining cancellous bone were lowered by the combination of a thickened metal tray and a metal support buttress.

CLINICAL RELEVANCE

This study demonstrates the advantage of metal trays for the tibial plateau and suggests that they should be used whenever the supporting tibial bone is insufficient. In knees in which there are large defects in the bone, direct transfer of the load to the cortical shell through the prosthesis, made possible by a custom design, appears to be necessary.

Surgical repositioning of the medial collateral ligament. An anatomical and mechanical analysis

Analytical techniques using multiple-exposure roentgenograms were employed to investigate surgical repositioning of either the femoral or the tibial attachment of the medial collateral ligament. The motion of the femoral attachment of the ligament with respect to the tibial attachment was used to compute the changes in length of the borders of the ligament for normal knees and for knees with repositioned attachments. The results support the conclusion that when advancement of the medial collateral ligament is utilized in the treatment of medial instability, optimization is accomplished by distal and anterior advancement with the knee in 30 degrees of flexion. Femoral displacement (proximal realignment) or tibial displacement at knee-flexion angles greater than 45 degrees is not recommended.

An Automated Method for Dynamic Analysis of Spatial Linkages for Biomechanical Applications

A computer-aided analysis procedure is developed to obtain biomechanical information for unassisted and assisted upper extremity systems. An automated assembly procedure is derived which determines the equations of motion for a linkage consisting of N links connected by ball and socket joints and/or pin joints. Springs may be attached between the links or between the links and ground. The major advantage of this procedure is that the lengthy and time-consuming derivations required to obtain the equations of motions using other methods are eliminated. The procedure automatically writes the equations of motion based on the input data alone. Changes in the linkage being analyzed require changes in input data; the basic program does not have to be modified. In particular the basic program does not depend upon the nature of the links or the types of joints.

The Optimum Design of Mechanical Systems With Competing Design Objectives

A general method is developed for analyzing optimum design problems with multiple, competing objective junctions. Methods are presented for generating trade-off curves for problems with competing objectives. The usefulness of these methods is demonstrated by applying them to the optimum design of hydrodynamic journal bearings.