The biomechanics of a validated finite element model of stress shielding in a novel hybrid total knee replacement

This study proposes a novel hybrid total knee replacement (TKR) design to improve stress transfer to bone in the distal femur and, thereby, reduce stress shielding and consequent bone loss. Three-dimensional finite element (FE) models were developed for a standard and a hybrid TKR and validated experimentally. The Duracon knee system (Stryker Canada) was the standard TKR used for the FE models and for the experimental tests. The FE hybrid device was identical to the standard TKR, except that it had an interposing layer of carbon fibre-reinforced polyamide 12 lining the back of the metallic femoral component. A series of experimental surface strain measurements were then taken to validate the FE model of the standard TKR at 3000 N of axial compression and at 0 degreeof knee flexion. Comparison of surface strain values from FE analysis with experiments demonstrated good agreement, yielding a high Pearson correlation coefficient of R(2)= 0.94. Under a 3000N axial load and knee flexion angles simulating full stance (0O degree, heel strike (200 degrees, and toe off (600 degrees during normal walking gait, the FE model showed considerable changes in maximum Von Mises stress in the region most susceptible to stress shielding (i.e. the anterior region, just behind the flange of the femoral implant). Specifically, going from a standard to a hybrid TKR caused an increase in maximum stress of 87.4 per cent (O0 degree from 0.15 to 0.28 MPa), 68.3 per cent (200 degrees from 1.02 to 1.71 MPa), and 12.6 per cent (600 degrees from 2.96 to 3.33 MPa). This can potentially decrease stress shielding and subsequent bone loss and knee implant loosening. This is the first report to propose and biomechanically to assess a novel hybrid TKR design that uses a layer of carbon fibrereinforced polyamide 12 to reduce stress shielding.     

Design forms of total knee replacement

The starting point of this article is a general design criterion applicable to all types of total knee replacement. This criterion is then expanded upon to provide more specifics of the required kinematics, and the forces which the total knee must sustain. A characteristic which differentiates total knees is the amount of constraint which is required, and whether the constraint is translational or rotational. The different forms of total knee replacement are described in terms of these constraints, starting with the least constrained unicompartments to the almost fully constrained fixed and rotating hinges. Much attention is given to the range of designs in between these two extreme types, because they constitute by far the largest in usage. This category includes condylar replacements where the cruciate ligaments are preserved or resected, posterior cruciate substituting designs and mobile bearing knees. A new term, 'guided motion knees', is applied to the growing number of designs which control the kinematics by the use of intercondylar cams or specially shaped and even additional bearing surfaces. The final section deals with the selection of an appropriate design of total knee for specific indications based on the design characteristics.     

Wear and conformity in total knee replacement

The wear on the plastic components of removed condylar replacement total knee prostheses and on total hip prostheses was studied using light microscopy, scanning electron microscopy and surface profilometry. The wear phenomena observed were smearing and stretching of the surface, cracking, “roll” formation, pitting and three-body abrasion. The type of wear apparently depended upon the stresses and the conformity between the two joint components. With high conformity there was a greater tendency for smearing and stretching, with cracking in highly stressed areas. With low conformity, e.g. with the condylar knees employing fairly flat tibial plateaux, cracking and pitting were the most common features. In medium conformity situations, roll formation and general surface disruption, with some cracking, were observed. The three-body abrasion, characterized by numerous shreds of plastic on the surface, was sometimes associated with entrapped particles of methyl methacrylate cement. Fluctuating load and motion were considered to be fundamental to the formation of the various surface phenomena. The phenomena were more or less reproduced in experiments using a special purpose test machine. The specimens were polished metal cylinders in a hemi-cylindrical plastic trough, the conformity being varied in different tests. The motion was cyclic to and fro, and the load was cyclic off and on. On the basis of these experiments and from studies of removed prostheses, conclusions about the type of wear occurring and the likelihood of severe wear were correlated with the design of the bearing (conformity) and the service conditions (stress).     

Three-dimensional dynamic model of the knee

A three-dimensional dynamic model of the knee was developed to study the interactions between the articulating surfaces of the bones and the geometrical and mechanical properties of the ligaments. The contact-surface geometry of the distal femur, proximal tibia, and patella was modeled by fitting polynomials to each of the eight articular surfaces. Twelve elastic elements were used to describe the function of the ligamentous and capsular structures of the knee. The origin and insertion sites of each model ligament were obtained from cadaveric data reported for an average-size knee. The response of the model to both anterior-posterior drawer and axial rotation suggests that the geometrical and mechanical properties of the model ligaments approximate the behavior of real ligaments in the intact knee. Comparison of the model’s response with experimental data obtained from cadaveric knee extension indicate further that the three-dimensional model reproduces the response of the real knee during movement.     

A computerized bioskills system for surgical skills training in total knee replacement

Although all agree that the results of total knee replacement (TKR) are primarily determined by surgical skill, there are few satisfactory alternatives to the 'apprenticeship' model of surgical training. A system capable of evaluating errors of instrument alignment in TKR has been developed and demonstrated. This system also makes it possible quantitatively to assess the source of errors in final component position and limb alignment. This study demonstrates the use of a computer-based system to analyse the surgical skills in TKR through detailed quantitative analysis of the technical accuracy of each step of the procedure. Twelve surgeons implanted a posterior-stabilized TKR in 12 fresh cadavers using the same set of surgical instruments. During each procedure, the position and orientation of the femur, tibia, each surgical instrument, and the trial components were measured with an infrared coordinate measurement system. Through analysis of these data, the sources and relative magnitudes of errors in position and alignment of each instrument were determined, as well as its contribution to the final limb alignment, component positioning and ligament balance. Perfect balancing of the flexion and extension gaps was uncommon (0/15). Under standardized loading, the opening of the joint laterally exceeded the opening medially by an average of approximately 4 mm in both extension (4.1 +/- 2.1 mm) and flexion (3.8 +/- 3.4 mm). In addition, the overall separation of the femur and the tibia was greater in flexion than extension by an average of 4.6 mm. The most significant errors occurred in locating the anterior/posterior position of the entry point in the distal femur (SD = 8.4 mm) and the correct rotational alignment of the tibial tray (SD = 13.2 degrees). On a case-by-case basis, the relative contributions of errors in individual instrument alignments to the final limb alignment and soft tissue balancing were identified. The results indicate that discrete steps in the surgical procedure make the largest contributions to the ultimate alignment and laxity of the prosthetic knee. Utilization of this method of analysis and feedback in orthopaedic training is expected rapidly to enhance surgical skills without the risks of patient exposure.     

Long-term implant-bone fixation of the femoral component in total knee replacement

Success of total knee replacement (TKR) depends on the prosthetic design. Aseptic loosening of the femoral component is a significant failure mode that has received little attention. Despite the clinical relevance of failures, no protocol is available to test long-term implant-bone fixation of TKR in vitro. The scope of this work was to develop and validate a protocol to assess pre-clinically the fixation of TKR femoral components. An in vitro protocol was designed to apply a simplified but relevant loading profile using a 6-degrees-of-freedom knee simulator for 1,000000 cycles. Implant-bone inducible micromotions and permanent migrations were measured at three locations throughout the test. After test completion, fatigue damage in the cement was quantified. The developed protocol was successfully applied to a commercial TKR. Additional tests were performed to exclude artefacts due to swelling or creep of the composite femur models. The components migrated distally; they tilted towards valgus in the frontal plane and in extension in the sagittal plane. The migration patterns were consistent with clinical roentgen-stereophotogrammetric recordings with TKR. Additional indicators were proposed that could quantify the tendency to loosen/stabilize. The type and amount of damage found in the cement, as well as the migration patterns, were consistent with clinical experience with the specific TKR investigated. The proposed pre-clinical test yielded repeatable results, which were consistent with the clinical literature. Therefore, its relevance and reliability was proved.     

Dynamic simulation of a displacement-controlled total knee replacement wear tester

This paper presents a dynamic finite element method (FEM) model of a commercial displacement-controlled total knee replacement (TKR) wear tester. The first goal of the study was to validate the model, which included both the wear tester and the TKR components. Convergence simulations and experimental testing were performed. These included a novel experimental determination of the coefficient of friction and an evaluation of predicted joint contact areas by comparing simulation results with experimental data collected using pressure-sensitive film. The second goal of this study was to develop a procedure for implementing force-based testing protocols on a displacement-controlled TKR wear tester. A standard force-based cyclic wear-testing protocol was simulated using the FEM model and resulting displacement waveforms were extracted. These were used as control inputs to the physical wear tester and an experimental wear test was performed. Reaction loads on the tibial components were measured and compared with the simulated results. The model was capable of accurately predicting the tibial loads throughout the test cycle, verifying the model's contact mechanics. The study demonstrated the use of computational modelling to convert a force-based testing protocol into displacement-based control parameters for use in a displacement-controlled mechanical testing system.     

A three-dimensional morphometrical study of the distal human femur.

The objective of this paper is to present a method to describe the three-dimensional variations of the geometry of the three portions forming the distal part of the human femur: the medial and lateral femoral condyles and the intercondylar fossa. The contours of equally spaced sagittal slices were digitized on the distal femur to determine its surface topography. Data collection was performed using a digitizer system which utilizes low-frequency, magnetic field technology to determine the position and orientation of a magnetic field sensor in relation to a specified reference frame. The generalized reduced gradient optimization method was used to reconstruct the profile of each slice utilizing two primitives: straight-line segments and circular arcs. The profile of each slice within the medial femoral condyle was reconstructed using two circular arcs: posterior and distal. The profile of each slice within the lateral femoral condyle was reconstructed using three circular arcs: posterior, distal and anterior. Finally, the profile of each slice within the intercondylar fossa was reconstructed using two circular arcs: proximal-posterior and anterior, and a distal-posterior straight-line segment tangent to the proximal-posterior circular arc. Combining the data describing the profiles of the different slices forming the distal femur, the posterior portions of each of the medial and lateral femoral condyles were modelled using parts of spheres having an average radius of 20 mm. The anterior portion of the lateral condyle was approximated to a right cylinder having its circular base parallel to the sagittal plane with an average radius of 26 mm. The anterior portion of the intercondylar fossa was modelled using an oblique cylinder having its circular base parallel to the sagittal plane with an average radius of 22 mm. Furthermore, it is suggested that the distal portion of the lateral femoral condyle could be modelled using parts of two oblique cones while the distal portion of the medial femoral condyle could be modelled using a part of a single oblique cone, all cones having their circular bases parallel to the sagittal plane. It is also suggested that the posterior portion of the intercondylar fossa could be modelled using two oblique cones: a proximal cone having its base parallel to the sagittal plane and a distal cone having its base parallel to the frontal plane.     

An objective tool for assessing the outcome of total knee replacement surgery

The need for an objective tool to assess the outcome of total knee replacement (TKR) surgery is widely recognized. This study investigates the potential of an objective diagnostic tool for assessing the outcome of TKR surgery based on motion analysis techniques. The diagnostic tool has two main elements: collection of data using motion analysis, and the assessment of knee function using a classifier that is based around the Dempster-Shafer theory of evidence. The tool was used to analyse the knee function of nine TKR subjects preoperatively and at three stages post-operatively. Using important measurable characteristics of the knee, the tool was able to establish the level of benefit achieved by surgery and to enable a comparison of subjects. No subject recovered normal knee function following TKR surgery. This has important implications for knee implant designs.     

Size and shape of the resection surface geometry of the osteoarthritic knee in relation to total knee replacement design

This study examines the resection surface geometry of the femur, tibia, and patella in relation to the design of total knee implants. Using a technique known as principal component analysis (PCA), the variation in the resection geometry of the knee was summarized. Of the total variation of the knee, 58 per cent was due to variation in size and 14 per cent was due to varying femoral intercondylar notch width. A PCA was performed on each bone separately and it was found that 60 per cent, 76 per cent and 71 per cent of variation was due to size for the femur, tibia, and patella respectively. Femoral and tibial size were highly correlated (r = 0.95) while patellar size had poorer correlation with both femoral and tibial size (r < 0.7). Simple linear dimensions (femoral epicondylar width or tibial mediolateral width) were reliable indicators of knee size. The effect of shape variation, which is generally not accounted for in implant design, was measured. The resected surfaces of each subject were compared with a model of the resection surfaces of the knee which varied in size but not shape. The maximum overhang and underhang of the model on the resection surfaces were measured. There was average maximum model overhang of 3.6 mm and underhang of 3.9 mm in the femur, 2.3 mm overhang and 1.9 mm underhang in the tibia, and 2.6 mm overhang and 2.5 mm underhang in the patella. The maximum coverage that an implant can be expected to provide for a population is quantified. Implant designs which include some shape as well as size variation improve on the implant fit.     

Towards a standard in assessment of bone cutting for total knee replacement

Arthroplasty outcome is influenced by the 'quality' of bone preparation for implant insertion. Surgeons face increasing choices of technique and instrumentation, yet clinical scoring methods assess the overall outcome and patient satisfaction but not the bone cuts directly. 'Quality management' of bone reshaping is needed to evaluate different bone cutting methods and computer assisted orthopaedic surgery (CAOS) systems. Analyses and experiments in this study were formulated for measurement and computation of four quantitative characteristics of bone preparation 'quality' and produced a highly condensed index for each. These represented (a) surface finish of cuts, (b) implant fit/looseness possible with the cut shape, (c) implant location/misalignment, and (d) accuracy of individual planar cuts. Assessment of synthetic bone cuts verified the robustness of the method for wide application in arthroplasty intraoperatively, in vitro and for comparing navigation systems.     

Effect of ultra-high molecular weight polyethylene thickness on contact mechanics in total knee replacement

One of the important design parameters in current knee joint replacements is the thickness of the ultra-high molecular weight polyethylene (UHMWPE) tibial insert, yet there is no clear definition of the upper limit of the 'thick' polyethylene insert. Using one design knee implant and subjecting it to the physiological loads encountered throughout the gait cycle, measurements of the lengths of contact imprints generated were compared with the corresponding theoretical predictions for different insert thicknesses under the same applied load. Multiple regression analysis was applied to test whether the dimensions of contact imprints are influenced by UHMWPE thickness. Good agreement was obtained between the theoretical predictions and the experimental measurements of the dimensions of contact imprints when the knee was at 60 degrees flexion. Therefore, it was possible to estimate the contact pressure at the articulating surface using the theoretical model. Contact imprint dimensions increased with increasing applied load. Statistical analysis of the experimental data revealed that, at 0 degree flexion, the overall imprint dimensions increased as the UHMWPE thickness increased from 8 to 20 mm. However, the increment was not significant when the thickness subinterval 10-15 mm was considered. Furthermore, at 60 degrees flexion, thickness was not a significant factor for the overall imprint dimensions. No evidence was found from the data to suggest that an increment in polyethylene thickness over 10 mm would significantly reduce the contact imprint dimensions. These findings suggest that thicker inserts can be avoided, as they require unnecessary bone resection.     

Three-dimensional contact dynamic model of the human knee joint during walking

It is well known that the geometry of the articular surface has a major role in determining the position of articular contact and the lines of action for the contact forces. The contact force calculation of the knee joint under the effect of sliding and rolling is one of the most challenging issues in this field. We present a 3-D human knee joint model including sliding and rolling motions and major ligaments to calculate the lateral and medial condyle contact forces from the recovered total internal reaction force using inverse dynamic contact modeling and the Least-Square method. As results, it is believed that the patella, muscles and tendon affect a lot for the internal reaction forces at the initial heel contact stage. With increasing flexion angles during gait, the decreasing contact area is progressively shifted to the posterior direction on the tibia plateau. In addition, the medial side contact force is larger than the lateral side contact force in the knee joint during normal human walking. The total internal forces of the knee joint are reasonabe compared to previous studies.     

A quantitative method of effective soft tissue management for varus knees in total knee replacement surgery using navigational techniques

Total knee replacement (TKR) has become the standard procedure in management of degenerative joint disease with its success depending mainly on two factors: three-dimensional alignment and soft-tissue balancing. The aim of this work was to develop and validate an algorithm to indicate appropriate medial soft tissue release during TKR for varus knees using initial kinematics quantified via navigation techniques. Kinematic data were collected intraoperatively for 46 patients with primary end-stage osteoarthritis undergoing TKR surgery using a computer-tomography-free navigation system. All patients had preoperative varus knees and medial release was made using the surgeon's experience. Based on these data an algorithm was developed. This algorithm was validated on a further set of 35 patients where it was used to define the medial release based on the kinematic data. The post-operative valgus stress angles for the two groups were compared. These results showed that the algorithm was a suitable tool to indicate the type of medial release required in varus knees based on intra-operatively measured pre-implant valgus stress and extension deficit angles. It reduced the percentage of releases made and the results were more appropriate than the decisions made by an experienced surgeon.     

A flexible multi-body dynamic model for analyzing the hysteretic characteristics and the dynamic stress of a taper leaf spring

This paper proposes a modeling technique which is able to not only reliably and easily represent the hysteretic characteristics but also analyze the dynamic stress of a taper leaf spring. The flexible multi-body dynamic model of the taper leaf spring is developed by interfacing the finite element model and computation model of the taper leaf spring. Rigid dummy parts are attached at the places where a finite element leaf model is in contact with an adjacent one in order to apply contact model. Friction is defined in the contact model to represent the hysteretic phenomenon of the taper leaf spring. The test of the taper leaf spring is conducted for the validation of the reliability of the flexible multi-body dynamic model of the taper leaf spring developed in this paper. The test is started at an unloaded state with the excitation amplitude of 1–2 mm/sec and frequency of 132 mm. First, the simulation is conducted with the same condition as the test. Then, the simulations are conducted with various amplitudes in a loaded state. The hysteretic diagram from the test is compared with the ones from the simulation for the validation of the reliability of the model. The dynamic stress analysis of the taper leaf spring is also conducted with the developed flexible multi-body dynamic model under a dynamic loading condition.     

A dynamic model of ball bearing for simulating localized defects on outer race using cubic hermite spline

In this paper a dynamic model is presented for predicting the vibration behavior of a ball bearing under the influence of localized defects on the outer race. The calculation of contact force is based on Hertzian contact deformation theory. The pulse generated by the ball striking the defect on outer race is modeled by using the blending functions of the cubic hermite spline. The effect of change in the angular position of the defect, size of the defect on outer race, multiple defects on outer race and the variation of load on the vibration amplitude is predicted by this model. A computer program in MATLAB is developed and the governing equation of motion is solved by Euler’s method. The numerical results are presented as a function of variation of the geometry of the outer race due to the impact at the defect and normal race contact w.r.t. time and the conclusion about the health of the bearing is determined by the spectral analysis. To validate the results, experimentation has also been performed.     

A sequentially-defined stiffness model of the knee

A new procedure is proposed to obtain a stiffness model of the knee, i.e. a model of the joint when static external loads are applied to the tibia and femur. A sequential approach is used to generalise a kinematic model of the passive motion of the articulation previously presented by the authors. The procedure is devised in such a way that the restraining function of the articular components which guide the passive motion of the joint is preserved after generalisation. As a result, the new stiffness model can replicate the passive motion with the same accuracy as the previous kinematic model, and it can also replicate the relative motion of the tibia and femur when the knee is loaded by static external loads. The proposed procedure is applied to a specimen and the relevant stiffness model is devised. The motion of the model under several loading conditions is then compared with the original motion of the specimen and with data obtained from the literature, in order to show the accuracy of the model and the potential of the proposed procedure.     

Effect of lubricant composition on the fatigue properties of ultra-high molecular weight polyethylene for total knee replacement

Ultrahigh molecular weight polyethylene (UHMWPE) fatigue is a critical factor affecting the longevity of total knee replacement (TKR) bearings. With the increased need for laboratory studies to mimic near in vivo conditions for accurate characterization of material performance, the present study investigated the role of hyaluronic acid (HA) in testing lubricant on the crack growth response of UHMWPE. It was hypothesized that the change in lubricant viscosity as a result of HA would affect the fatigue life of the polymer. A fracture mechanics approach as per ASTM E 647 was adopted for this study. Surface micrograph and surface chemistry analyses were employed to study the micromechanisms of fatigue failure and protein adsorption of the specimen surfaces. Rheological analysis indicated that the addition of HA to diluted bovine serum increased testing lubricant viscosity. HA concentrations of 2.22, 0.55, and 1.5 g/l closely matched the viscosity ranges reported for osteoarthritis, rheumatoid arthritic diseased joint fluid, and periprosthetic fluids respectively. Results showed that the addition of HA to standard diluted bovine serum lubricants, in concentrations similar to that of periprosthetic fluid, delayed crack initiation and crack growth during fatigue testing.     

A spatial model of the knee for the preoperative planning of knee surgery

A model on the spatial mechanical behaviour of the passive knee is presented. The femoral articular surfaces were represented by generalized, sagittally elliptical, toroidal surfaces. The medial and lateral tibial articular surfaces were represented by a dished spherical surface and the lower hemihyperbolic region of a torus respectively. Anatomical articular cartilage, knee ligaments and the posterior capsule were represented by spring-like deformable elements with non-linear load versus deflection characteristics. All the forces that act on the femur relative to the tibia were represented by three orthogonal forces and three associated moments. Spatial, articulation-dependent femorotibial kinematic constraint equations of the passive knee were formulated in an analytically explicit manner, based on the natural coordinates of the articular surfaces. The constraint equations were solved algebraically in closed form. Equations were derived that describe spatial femoro-tibial motion, ligament length, ligament strain, ligament-based elastic potential energy and the quasi-static equilibrium of the passive knee. Software was written, simulations on the motion characteristics and load versus deflection characteristics of the knee were carried out and graphical results were presented. The simulation of planar flexion/extension was almost spontaneous. The time taken to simulate spatial six-degree-of-freedom femoro-tibial motion was less than 2.5 min. The models were found to be capable of representing real-life passive knees to a high degree of satisfaction. It has been demonstrated that the models can provide knee surgeons with additional information on major aspects of the preoperative planning of knee surgery. The models can be used to enhance the preoperative planning of ligament reconstruction, articular surfaces related surgery, osteotomy and patellar tendon transfer surgery.     

Strain distribution within the human femur due to physiological and simplified loading: finite element analysis using the muscle standardized femur model

The aim of the current work was to study the effect of simplified loading on strain distribution within the intact femur using the Muscle Standardized Femur finite element model and to investigate whether the interaction between the intact human femur and the muscles which are attached to the bone surface could accurately be represented by concentrated forces, applied through the centroids of their attachment areas. An instant at 10 per cent of the gait cycle during level walking was selected as the reference physiological load case; nine load cases were analysed. Comparison of the calculated results for the physiological load case with muscle forces uniformly distributed over their attachment areas showed good agreement with in vivo measurements of strain values and femoral head displacement in humans. Simplified load cases generated unrealistic displacement results and high strain magnitudes, exceeding the physiological range. It was found that when muscles with large attachment areas are included in the model and the muscle forces are simplified, stress and strain distributions will be affected not only on the external bone surface in the vicinity of the load application node, but also on the internal surface of the cortical bone. However, applying muscle forces as concentrated loads at the centroids of the attachment areas can serve as first indicators of the physiological stress and strain levels, if results from nodes and elements in the vicinity of the load application nodes are discarded. Omitting muscle forces or fixing the femur in mid-shaft leads to large unphysiological strain values.