Sagittal profile of the femoral condyles and its application to femorotibial contact analysis

Effects of Obesity on Medial Tibiofemoral Cartilage Mechanics in Females-An Exploration Using Musculoskeletal Simulation and Probabilistic Cartilage Failure Modelling

This study examined the effects of obesity on cartilage mechanics and longitudinal failure probability at the medial tibiofemoral compartment, using combined musculoskeletal simulation and probabilistic failure modelling approaches. The current investigation examined twenty obese females (BMI > 30.0 kg/m²) and 20 healthy weight (BMI < 25.0 kg/m²) females. Walking kinematics were obtained via an 8-camera optoelectric system, and a force plate was used to collect ground reaction forces. Musculoskeletal simulation and probabilistic failure modelling were utilized to explore medial tibiofemoral forces and cartilage probability. Comparisons between groups were undertaken using linear mixed-effects models. Net peak cartilage forces, stress and strain were significantly larger in the obese group (force = 2013.92 N, stress = 3.03 MPa & strain = 0.25), compared to health weight (force = 1493.21 N, stress 2.26 MPa & strain = 0.19). In addition, medial tibiofemoral cartilage failure probability was also significantly larger in the obese group (42.98%) compared to healthy weight (11.63%). The findings from the current investigation show that obesity has a profoundly negative influence on longitudinal medial knee cartilage health and strongly advocates for the implementation of effective weight management programs into long-term musculoskeletal management strategies.

Effects of Running in Minimal and Conventional Footwear on Medial Tibiofemoral Cartilage Failure Probability in Habitual and Non-Habitual Users

This study examined the effects of minimal and conventional running footwear on medial tibiofemoral cartilage mechanics and longitudinal failure probability. The current investigation examined twenty males who habitually ran in minimal footwear and 20 males who habitually ran in conventional footwear. Kinematic data during overground running were collected using a motion-capture system and ground reaction forces using a force plate. Medial tibiofemoral loading was examined using musculoskeletal simulation and cartilage failure probability via probabilistic modelling. In habitual minimal footwear users, peak medial tibiofemoral cartilage force, stress and strain were significantly greater in conventional (force = 7.43 BW, stress = 5.12 MPa and strain = 0.30), compared to minimal footwear (force = 7.11 BW, stress 4.65 MPa and strain = 0.28), though no significant differences in these parameters were evident in non-habitual minimal footwear users (conventional: force = 7.50 BW, stress = 5.05 MPa and strain = 0.30; minimal: force = 7.40 BW, stress = 4.77 MPa and strain = 0.29). However, in both habitual and non-habitual minimal footwear users, the probability of medial tibiofemoral cartilage failure was significantly greater in conventional (habitual = 47.19% and non-habitual = 50.00%) compared to minimal footwear (habitual = 33.18% and non-habitual = 32.81%) users. The observations from this investigation show that compared to minimal footwear, conventional footwear appears to have a negative influence on medial tibiofemoral cartilage health.

Sagittal femoral condylar shape varies along a continuum from spherical to ovoid: a systematic review and meta-analysis

INTRODUCTION

Considerable anatomic variations of sagittal femoral condylar shape have been reported, with a continuum between spherical (or single-radius) and ovoid (or multi-radius) condyles. The purpose of this systematic review and meta-analysis was to critically appraise and synthesise the available literature on the sagittal femoral profile. The hypothesis was that studies would reveal considerable variability among individuals, but also in their methodology to quantify sagittal profiles.

METHODS

This systematic review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. On 10 September 2021 two authors searched for Level I to IV studies that reported on the sagittal curvature of the medial and/or lateral femoral condyles using the MEDLINE®, EMBASE® and Cochrane Library. Results were summarised by tabulating means, standard deviations and/or ranges for the reported radii-of-curvature, or ellipsoidal semi-major and semi-minor lengths of the condyles. To quantify sagittal 'ovoidicity' and asymmetry, results were stratified according to coordinate reference frame (posterior condylar axis (PCA), clinical and surgical transepicondylar axis (cTEA and sTEA), unified sagittal plane (USP), or unclear) and summarised in forest plots as standardised mean differences (SMD).

RESULTS

Thirty-eight articles were eligible for full text extraction, quantifying sagittal radii-of-curvature by best-fit circles (BFC), ellipsoids, polynomials, spherical or cylindrical fitting. Studies with clear definition of the measurement plane revealed that both condyles were generally ovoid, with considerably greater 'ovoidicity' at the medial condyle (SMD, 4.09) versus the lateral condyle (SMD, 3.33). In addition, distal condylar radii were greater medially when measured normal to the TEA (cTEA: SMD, 0.81; sTEA: SMD, 0.79), but greater laterally when measured in a USP (SMD, - 0.83). Posterior condylar radii were greater laterally when measured in a USP (SMD, - 0.60).

CONCLUSION

Studies reported considerable variability of sagittal femoral condylar radii-of-curvature, which are not incremental, but rather a continuum that ranges from spherical to ovoid. Although this variation could be accommodated by single-, dual- and multi-radii femoral components, a surgeon typically uses only one or two TKA designs. Hence, there is a risk of mismatch between the native and prosthetic sagittal profile that could result in mid-flexion ligament imbalance unless other parameters are changed. These findings support the drive towards patient-specific implants to potentially achieve accurate sagittal bone-implant fit through implant customisation.

LEVEL OF EVIDENCE

IV.

A developed multibody knee model for unloading knee with cartilage penetration depth control

Unloader knee braces could relieve pain by decreasing the medial knee loading. Particularly for knee osteoarthritis (KOA) patients, this study investigates the relevance of the knee model after identifying the most influential parameter. Since KOA causes the cartilage in a joint to lose its elasticity and thickness, the lack of normal bone-to-bone separation can be painful. We believe that cartilage penetration depth control is an impactful strategy in this research. Moreover, the knee contact force in KOA is fewer than in healthy knees, confirming that the contact force control cannot be a straight factor. Therefore, a biomechanical human knee model is developed, and a generic procedure for dynamic analysis of contact problems in combination with the musculoskeletal model is introduced. The developed model includes the geometric expression of collision curves and an algorithm for determining collision points. This presentation addresses cartilage penetration depth and contact force calculation through nonlinear discontinuous contact law. In view of this, femur and tibia’s relative motion is analyzed through the combined collision reactions of cartilage and bone in the knee. In the simulation, maximum penetration depth in a healthy knee is reported to be 0.795 mm, while in a 75% KOA is 0.521 mm, including 0.5 mm cartilage-cartilage contact and 0.021 mm bone-bone contact. The top unloading 852 N is achieved, reducing penetration depth to 0.45 mm, avoiding bone-bone contact. This proposed procedure with low computation gives us a suitable analysis method for designing knee assistive devices.

Functional Evaluation of a Personalized Orthosis for Knee Osteoarthritis: A Motion Capture Analysis

Orthotic treatments for knee osteoarthritis (OA) typically rely on simple mechanisms such as three-point bending straps and single-pin hinges. These commonly prescribed braces cannot treat bicompartmental knee OA, do not consider the muscle weakness that typically accompanies the condition, and employ hinges that restrict the knee's natural biomechanics. Utilizing a novel, personalized joint mechanism in conjunction with magnetorheological dampers, we have developed and evaluated a brace which attempts to address these shortcomings. This process has respected three principal design goals: reducing the load experienced across the entire knee joint, generating a supportive moment to aid the thigh muscles in shock absorption, and interfering minimally with gait kinematics. Two healthy volunteers were chosen to test the system's basic functionality through gait analysis in a motion capture laboratory. Combining the collected kinematic and force-plate data with data taken from sensors onboard the brace, we integrated the brace and leg system into a single inverse dynamics analysis, from which we were able to evaluate the effect of the brace design on the subjects' knee loads and moments. Of the three design goals: a reduction in knee contact forces was demonstrated; increased shock absorption was observed, but not to statistical significance; and natural gait was largely preserved. Taken in total, the outcome of this study supports additional investigation into the system's clinical effectiveness, and suggests that further refinement of the techniques presented in this paper could open the doors to more effective OA treatment through patient specific braces.

Variation of the Three-Dimensional Femoral J-Curve in the Native Knee

The native femoral J-Curve is known to be a relevant determinant of knee biomechanics. Similarly, after total knee arthroplasty, the J-Curve of the femoral implant component is reported to have a high impact on knee kinematics. The shape of the native femoral J-Curve has previously been analyzed in 2D, however, the knee motion is not planar. In this study, we investigated the J-Curve in 3D by principal component analysis (PCA) and the resulting mean shapes and modes by geometric parameter analysis. Surface models of 90 cadaveric femora were available, 56 male, 32 female and two without respective information. After the translation to a bone-specific coordinate system, relevant contours of the femoral condyles were derived using virtual rotating cutting planes. For each derived contour, an extremum search was performed. The extremum points were used to define the 3D J-Curve of each condyle. Afterwards a PCA and a geometric parameter analysis were performed on the medial and lateral 3D J-Curves. The normalized measures of the mean shapes and the aspects of shape variation of the male and female 3D J-Curves were found to be similar. When considering both female and male J-Curves in a combined analysis, the first mode of the PCA primarily consisted of changes in size, highlighting size differences between female and male femora. Apart from changes in size, variation regarding aspect ratio, arc lengths, orientation, circularity, as well as regarding relative location of the 3D J-Curves was found. The results of this study are in agreement with those of previous 2D analyses on shape and shape variation of the femoral J-Curves. The presented 3D analysis highlights new aspects of shape variability, e.g., regarding curvature and relative location in the transversal plane. Finally, the analysis presented may support the design of (patient-specific) femoral implant components for TKA.

Anthropometry of the medial femoral condyle in the Chinese population: the morphometric analysis to design unicomparmental knee component

BACKGROUND

This study aimed to assess the radii of the distal and posterior articular surfaces of the medial femoral condyle in a Chinese population and provide detailed parameters of the knee joint for the future design of UKA components.

METHODS

This study included 500 consecutive Chinese patients who underwent knee MRI from Jan 2019 to Jan 2020. The two most appropriate circles were used to reveal the distal and posterior joint surfaces in the sagittal plane of the MRI images. The radius of the circle representing the distal articular surface in the sagittal plane was measured as R1, and the radius of the posterior articular surface was measured as R2. The distance between the centers of the two rotation circles was recorded as d. An independent t test was used to compare the differences between men and women. The Pearson correlation coefficient was calculated to analyze the correlation between R1 and R2. SPSS v19.0 software was used for statistical analysis.

RESULTS

The average values of R1, R2, R1/R2 and d were calculated. Scatter plots were constructed to show the trend of changes in the radius of the distal and posterior articular surfaces of the femoral condyle. R1, R2 and d differed significantly between men and women (p < 0.05). Correlation analysis showed that R1 was positively correlated with R2 (r = 0.61, p < 0.05).

CONCLUSIONS

The data of the radii of the distal and posterior articular surfaces of the medial femoral condyle were provided. In the UKA design, the relationships between the radii of the distal and posterior articular surfaces should be taken into account.

Flexible versus standard intramedullary rod in posterior stabilized primary total knee arthroplasty: protocol for a randomized controlled trial

Background

In total knee arthroplasty (TKA) a flexible intramedullary rod can be used to account for sagittal bowing of the distal femur. Although patients report better post-operative functional outcome when the flexible rod was used, it is unknown how the use of the flexible rod affects the placement of the femoral TKA component, and how this relates to activities of daily living. It is expected that the use of the flexible rod will result in a more flexed femoral component, a larger patellar tendon moment arm, and consequently in better functional outcome. The goal of this study is to compare the flexible rod to the standard intramedullary rod in primary TKA in terms of fit of the TKA, functional outcome, and sizing of the femoral component.

Methods

A single-blind randomized controlled trial with two groups (flexible vs standard rod), with patients blinded for group allocation, and 2 years post-operative follow-up. The fit of the TKA is quantified by two parameters: (1) the flexion angle of the TKA in the sagittal plane and (2) the sagittal profile of the distal femur compared between the pre-operative bone and the TKA. Both parameters are calculated in 3D volume images obtained using fluoroscopy. Functional outcome will be measured using (1) the timed Get-up and Go test (2), the stair climbing test (3), knee power output, and (4) patient and clinician reported outcomes. Different parameters will be measured during the TKA procedure to account for the invisibility of cartilage in the 3D volumes and to study if the amount of bone removed during the procedure is affected by group allocation.

Discussion

The sagittal fit of TKA is not a standardized outcome measure. We discuss our choice of parameters to define the sagittal fit (i.e., flexion angle and sagittal profile), our choice for the parameters we measure during the TKA procedure to account for the lack of cartilage thickness in fluoroscopy, and our choice for the parameters to study if the amount of bone removed during the procedure is affected by group allocation. Lastly, we discuss the merits of this planned trial.

Trial registration

Netherlands Trial Register, 4888, registered 30 March 2015. https://www.trialregister.nl/trial/4888

Medial knee cartilage is unlikely to withstand a lifetime of running without positive adaptation: a theoretical biomechanical model of failure phenomena

Runners on average do not have a high risk of developing knee osteoarthritis, even though running places very high loads on the knee joint. Here we used gait analysis, musculoskeletal modeling, and a discrete-element model of knee contact mechanics to estimate strains of the medial knee cartilage in walking and running in 22 young adults (age 23 ± 3 years). A phenomenological model of cartilage damage, repair, and adaptation in response to these strains then estimated the failure probability of the medial knee cartilage over an adult lifespan (age 23-83 years) for 6 km/day of walking vs. walking and running 3 km/day each. With no running, by age 55 the cumulative probability of medial knee cartilage failure averaged 36% without repair and 13% with repair, similar to reports on incidence of knee osteoarthritis in non-obese adults with no knee injuries, but the probability for running was very high without repair or adaptation (98%) and remained high after including repair (95%). Adaptation of the cartilage compressive modulus, cartilage thickness, and the tibiofemoral bone congruence in response to running (+1.15 standard deviations of their baseline values) was necessary for the failure probability of walking and running 3 km/day each to equal the failure probability of walking 6 km/day. The model results suggest two conclusions for further testing: (i) unlike previous findings on the load per unit distance, damage per unit distance on the medial knee cartilage is greater in running vs. walking, refuting the "cumulative load" hypothesis for long-term joint health; (ii) medial knee cartilage is unlikely to withstand a lifetime of mechanical loading from running without a natural adaptation process, supporting the "cartilage conditioning" hypothesis for long-term joint health.

Deciphering the "Art" in Modeling and Simulation of the Knee Joint: Overall Strategy

Recent explorations of knee biomechanics have benefited from computational modeling, specifically leveraging advancements in finite element analysis and rigid body dynamics of joint and tissue mechanics. A large number of models have emerged with different levels of fidelity in anatomical and mechanical representation. Adapted modeling and simulation processes vary widely, based on justifiable choices in relation to anticipated use of the model. However, there are situations where modelers' decisions seem to be subjective, arbitrary, and difficult to rationalize. Regardless of the basis, these decisions form the "art" of modeling, which impact the conclusions of simulation-based studies on knee function. These decisions may also hinder the reproducibility of models and simulations, impeding their broader use in areas such as clinical decision making and personalized medicine. This document summarizes an ongoing project that aims to capture the modeling and simulation workflow in its entirety-operation procedures, deviations, models, by-products of modeling, simulation results, and comparative evaluations of case studies and applications. The ultimate goal of the project is to delineate the art of a cohort of knee modeling teams through a publicly accessible, transparent approach and begin to unravel the complex array of factors that may lead to a lack of reproducibility. This manuscript outlines our approach along with progress made so far. Potential implications on reproducibility, on science, engineering, and training of modeling and simulation, on modeling standards, and on regulatory affairs are also noted.

Gender and condylar differences in distal femur morphometry clarified by automated computer analyses

We elucidated the gender and condylar effects on distal femur morphology (DFM) while evaluating a newly developed computational framework that enables fully automated analyses of DFM in an objectively defined sagittal plane. Ninety high-resolution CT-acquired distal femur models from 51 males and 39 females were analyzed. The models were accurately characterized (mean least-squares fitting residual <0.16 mm), and re-oriented to a unified sagittal plane; three morphometric measures were extracted from each model: the semi-major (a) and semi-minor (b) axis lengths of the best-fitted ellipse, and the radius (r) of the smallest flexion facet-a circle with the smallest radius best-fitted to the posterior articulating surface. Statistical analyses employing nonparametric repeated-measures ANOVA found: no significance difference between condyles or between limbs in any of the morphometric measures; significant gender effects on a, b, and r, but no gender effect on the aspect ratio (a/b). An inspection of statistical distributions of medial-lateral condyle size differences also revealed a gender difference. The findings promote a better understanding of DFM and its relation to knee mechanics and have implications on computer-aided surgery of the knee and gender-specific implant design.

Mathematical reconstruction of human femoral condyles

There is a direct correlation between ligament function and the articulating surface of the normal knee, and changes to any of these structures can affect the other. This is also true for knee replacements, where the articulating surface is greatly changed compared to the natural knee. This study investigated the morphometry of healthy knees and proposes a method to predict original normal knee profiles. A variety of mathematical techniques are compared in terms of the accuracy with which they can represent the original knee joint geometry. Additionally, a method of predicting the irregular femoral condyle geometry for an individual knee is described by making use of the mathematical techniques presented, and the accuracy of this method is also investigated. The mathematical approach using B-splines provides flexibility and can accurately describe the complex geometry of the femoral condyles in both the sagittal and transverse planes. It was further found that the condyles are highly asymmetrical; therefore simpler methods cannot portray the condyles sufficiently and are especially inaccurate in representing the lateral condyle. The study proposes a method for predicting the geometry of the femoral condyles with good accuracy. The B-spline model showed best results.

Automating analyses of the distal femur articular geometry based on three-dimensional surface data

Quantitative knowledge of the distal femur morphology is critical to understanding the relation between the anatomy and function of the knee joint. Prior knowledge was contaminated by manual procedures and subjective visual inspections in extracting geometric information from image data. This article proposes a new computational framework to enable automated analysis of the distal femur articular geometry based on 3D surface data. The framework consists of a pattern recognition algorithm for sectioning the sagittal-view condyle profiles, a least-squares algorithm for fitting and analyzing the profiles, and an optimization algorithm for establishing a unified sagittal plane. An application of the proposed framework to 12 knee surface models demonstrated that it can analyze the condyle contour profiles and extract geometric measures automatically and accurately. The proposed framework also facilitated a simulation-based analysis of the uncertainty associated with conventional manual approaches, elucidating how subjective determination of the sagittal plane and flexion facet can hinder accurate understanding of the distal femur morphology and related kinematics.

Use of anthropometric data from the medial tibial and femoral condyles to design unicondylar knee prostheses in the Chinese population

Anthropometric data on medial tibial condyles and medial femoral condyles of 172 normal knees (94 male knees, 78 female knees) were obtained using three-dimensional computer tomographic measurements. In the medial tibial condyle, we measured the anteroposterior (AP) and widest dimension (WD), and compared the measurements with the similar dimensions of five tibial unicondylar knee prostheses conventionally used in China. In the femur, we used best-fit two-circular arcs to measure the morphology of the sagittal plane of the medial femoral condyle. We found that three of the prostheses showed WD overhang for all ranges of the AP dimension, while two of them showed WD underhang. We also found a progressive decrease in the condylar aspect ratio (WD/AP%) in parallel with an increase in the AP dimension in the medial tibial condyle. However, none of the conventional tibial prosthesis showed a similar change. Furthermore, males had larger values in aspect ratio than females with the same values for AP dimension. There were definite correlations between the radius of the curvature for the posterior part (R1) and distal part (R2) in the sagittal plane of medial femoral condyle. Both of these values were smaller than in the Caucasian population. Both radiuses of curvature for the posterior and distal components showed definite correlations with the AP dimension. The results of this study may provide guidelines for designing unicondylar knee prostheses suitable for the Chinese population.

Predicting Knee Replacement Damage in a Simulator Machine Using a Computational Model With a Consistent Wear Factor

Wear of ultrahigh molecular weight polyethylene remains a primary factor limiting the longevity of total knee replacements (TKRs). However, wear testing on a simulator machine is time consuming and expensive, making it impractical for iterative design purposes. The objectives of this paper were first, to evaluate whether a computational model using a wear factor consistent with the TKR material pair can predict accurate TKR damage measured in a simulator machine, and second, to investigate how choice of surface evolution method (fixed or variable step) and material model (linear or nonlinear) affect the prediction. An iterative computational damage model was constructed for a commercial knee implant in an AMTI simulator machine. The damage model combined a dynamic contact model with a surface evolution model to predict how wear plus creep progressively alter tibial insert geometry over multiple simulations. The computational framework was validated by predicting wear in a cylinder-on-plate system for which an analytical solution was derived. The implant damage model was evaluated for 5 million cycles of simulated gait using damage measurements made on the same implant in an AMTI machine. Using a pin-on-plate wear factor for the same material pair as the implant, the model predicted tibial insert wear volume to within 2% error and damage depths and areas to within 18% and 10% error, respectively. Choice of material model had little influence, while inclusion of surface evolution affected damage depth and area but not wear volume predictions. Surface evolution method was important only during the initial cycles, where variable step was needed to capture rapid geometry changes due to the creep. Overall, our results indicate that accurate TKR damage predictions can be made with a computational model using a constant wear factor obtained from pin-on-plate tests for the same material pair, and furthermore, that surface evolution method matters only during the initial “break in” period of the simulation.

In vivo medial and lateral tibial loads during dynamic and high flexion activities

Though asymmetric loading between the medial and lateral compartments of total knee replacements may contribute to implant loosening and failure, the in vivo contact force distribution during dynamic daily activities remains unknown. This study reports in vivo medial and lateral contact forces experienced by a well-aligned knee implant for a variety of activities. In vivo implant motion and total axial load data were collected from a single knee replacement patient performing treadmill gait (hands resting on handlebars), step up/down, lunge, and kneel activities. In vivo motion was measured using video fluoroscopy, while in vivo axial loads were collected simultaneously using an instrumented tibial component. An elastic foundation contact model employing linear and nonlinear polyethylene material properties was constructed to calculate medial and lateral contact forces based on the measured kinematics, total axial loads, and centers of pressure. For all activities, the predicted medial and lateral contact forces were insensitive to the selected material model. The percentage of medial to total contact force ranged from 18 to 60 for gait, 47 to 65 for step up/down, and 55 to 60 for kneel and lunge. At maximum load during the motion cycle, medial force was 1.2 BW for gait and 2.0 BW for step up/down, while the corresponding lateral forces were 1.0 and 1.5 BW, respectively. At mean load in the final static pose, medial force was 0.2 BW for kneel and 0.9 BW for lunge, with corresponding lateral forces of 0.1 and 0.7 BW, respectively. For this patient, a constant load split of 55% medial-45% lateral during loaded activity would be a reasonable approximation for these test conditions.

Multibody dynamic simulation of knee contact mechanics

Multibody dynamic musculoskeletal models capable of predicting muscle forces and joint contact pressures simultaneously would be valuable for studying clinical issues related to knee joint degeneration and restoration. Current three-dimensional multibody knee models are either quasi-static with deformable contact or dynamic with rigid contact. This study proposes a computationally efficient methodology for combining multibody dynamic simulation methods with a deformable contact knee model. The methodology requires preparation of the articular surface geometry, development of efficient methods to calculate distances between contact surfaces, implementation of an efficient contact solver that accounts for the unique characteristics of human joints, and specification of an application programming interface for integration with any multibody dynamic simulation environment. The current implementation accommodates natural or artificial tibiofemoral joint models, small or large strain contact models, and linear or nonlinear material models. Applications are presented for static analysis (via dynamic simulation) of a natural knee model created from MRI and CT data and dynamic simulation of an artificial knee model produced from manufacturer's CAD data. Small and large strain natural knee static analyses required 1 min of CPU time and predicted similar contact conditions except for peak pressure, which was higher for the large strain model. Linear and nonlinear artificial knee dynamic simulations required 10 min of CPU time and predicted similar contact force and torque but different contact pressures, which were lower for the nonlinear model due to increased contact area. This methodology provides an important step toward the realization of dynamic musculoskeletal models that can predict in vivo knee joint motion and loading simultaneously.

Experimental evaluation of an elastic foundation model to predict contact pressures in knee replacements

Computational wear prediction is an attractive concept for evaluating new total knee replacement designs prior to physical testing and implementation. An important hurdle to such technology is the lack of in vivo contact pressure predictions. To address this issue, this study evaluates a computationally efficient simulation approach that combines the advantages of rigid and deformable body modeling. The hybrid method uses rigid body dynamics to predict body positions and orientations and elastic foundation theory to predict contact pressures between general three-dimensional surfaces. To evaluate the method, we performed static pressure experiments with a commercial knee implant in neutral alignment using flexion angles of 0, 30, 60, and 90 degrees and loads of 750, 1500, 2250, and 3000N. Using manufacturer CAD geometry for the same implant, an elastic foundation model with linear or nonlinear polyethylene material properties was implemented within a commercial multibody dynamics software program. The model's ability to predict experimental peak and average contact pressures simultaneously was evaluated by performing dynamic simulations to find the static configuration. Both the linear and nonlinear material models predicted the average contact pressure data well, while only the linear material model could simultaneously predict the trends in the peak contact pressure data. This novel modeling approach is sufficiently fast and accurate to be used in design sensitivity and optimization studies of knee implant mechanics and ultimately wear.

Effects of sterilization on the Tekscan digital pressure sensor

Investigations into the effects of sterilization on a new biomechanical pressure sensor are necessary before contemplating in vivo use. Ten, designated Experimental, "K-Scan" digital pressure sensor arrays were sterilized with ethylene oxide gas (EtO), and their ability to accurately and reproducibly measure an applied load of 2225 N (500 lb) was assessed. Simultaneously, 10 un-sterilized sensor arrays, designated Control, were assessed. Each array was loaded 10 times inside a two-dimensional curved surface, and all arrays exhibited high reproducibility (coefficients of variation<2.0%). Following sterilization, the Experimental sensors showed a 22.2% average decrease in recorded force, a statistically significant difference from the pre-sterile data (p<0.002). However, when the Experimental sensors were re-calibrated post-sterilization, they showed only a 0.1% average decrease in recorded force, not a statistically significant difference (p>0.05, beta<0.05). Following 1-week storage, trial 2 data of the Control sensors showed a less dramatic yet significant 3.4% average decrease in recorded force when compared to trial 1 data (p<0.02). Control trial 2, once re-calibrated, showed a 0.5% average decrease in recorded force, not a statistically significant difference (p>0.05, beta<0.05). Results suggest that, following EtO sterilization, accurate and reproducible pressure measurements can be obtained from K-Scan sensors when calibration is performed at time of use.

Influences of variation in force application on tibial displacement and strain in the anterior cruciate ligament during the Lachman test

OBJECTIVE

The purpose of this study was to examine the influence of Lachman test performance technique on tibial displacement and strain in the anterior cruciate ligament.

DESIGN

Model simulation of experimental Lachman test performance by trained clinicians.

BACKGROUND

Differences in clinician hand placement during Lachman test performance have been observed.

METHODS

A two-dimensional computer sagittal plane model of the knee was designed to simulate experimentally observed Lachman test performance, and determine anterior cruciate ligament strain and tibial translation that occurred during variation in clinician hand placement and force magnitude.

RESULTS

Anterior cruciate ligament strain and tibial translation were greater under conditions mimicking clinician hand placement utilizing a more proximal force application on the tibia.

CONCLUSIONS

Tibial translation and strain behavior of the anterior cruciate ligament during the Lachman test appear to be influenced by clinician hand position used in the application of force to the tibia.

Three-dimensional morphometry of the femoral condyles

OBJECTIVE

The present study describes the geometry of the three-dimensional articular surfaces of the human femoral condyles based on measurements of surface coordinates.

DESIGN

The purpose was not to obtain a complex representation of one single condyle, but to describe the femoral condyles using simple geometric parameters based on measurements using a number of specimens.

BACKGROUND

In joint modeling, a representative knee joint geometry is often desired which requires an approximation of the irregular joint geometry while taking into account interspecimen variations.

METHODS

An optical device was used to measure the condylar articular surfaces of 12 human femurs in the femorotibial contact region. The sagittal profiles were reconstructed by means of two circular arcs and the radial profiles by means of one circular arc.

RESULTS

The results provide the geometric parameters necessary for the three-dimensional reconstruction of the articular surfaces of the femoral condyles. The results indicate that the medial and lateral condyles of the distal femur are significantly asymmetric in a number of morphological features.

CONCLUSION

The primary application of the results is expected to be in the formulation of finite element models of the knee joint for static contact problems.

RELEVANCE

Numerical models of the knee joint are being widely used to study the mechanics of the joint. However, formulation of such models demands a prior knowledge of the complex three-dimensional geometry of the articular surfaces of the natural joint to establish the input parameters of the model.

Tibiofemoral Joint-Surface Motions in Weight-Bearing and Non-Weight-Bearing Movement

Context:

Analyses of the path of instant center of rotation (PICR) can be used to infer joint-surface rolling and sliding motion (arthrokinematics). Previous PICR research has not quantified arthrokinematics during weight-bearing (WB) movement conditions or studied the association of muscle activity with arthrokinematics.

Objective:

To examine tibiofemoral arthrokinematics and thigh-muscle EMG during WB and non-weight-bearing (NWB) movement.

Design:

2 x 9 repeated-measures experiment.

Setting:

Laboratory.

Participants:

11 healthy adults (mean age 24 years).

Main Outcome Measures:

Tibiofemoral percentage rolling arthrokinematics and quadriceps: hamstring EMG activity.

Results:

WB percentage rolling (76.0% ± 4.7%) exceeded that of NWB (57.5% ± 1.8%) through terminal knee extension (F8,80= 8.99,P< .001). Quadriceps:hamstring EMG ratios accounted for 45.1% and 34.7% of the variance in arthrokinematics throughout the WB and NWB movement conditions, respectively (P< .001).

Conclusions:

More joint-surface rolling occurs through terminal knee extension during WB movement and is associated with an increase in hamstring activity.

Modelling debonded stem-cement interface for hip implants: effect of residual stresses

OBJECTIVE

To assess the effect of the residual stresses due to cement curing on the load transfer of cemented hip implants.

DESIGN

The load transfer at the stem-cement interface of an idealized hip stem surrounded by cortical bone was investigated using a three-dimensional finite element analysis. A debonded stem-cement interface was considered to simulate a highly polished stem in contact with cement; Coulomb friction at the stem-cement interface was considered.

BACKGROUND

Numerical analyses on the load transfer of cemented hip implants do not include residual stresses due to cement curing at the stem-cement interface.

METHODS

The magnitude of the residual stresses was determined experimentally. In the finite element model, non-linear contact elements modelled the debonded stem-cement interface. In particular, the compressive radial residual stresses that are generated at the interface, due to the cement expansion during curing, were treated similar to a press-fit problem.

RESULTS

The cement stress distributions were affected by the magnitude of the residual stresses. Failing to include residual stresses underestimated the cement stresses at the interface, mainly affecting the radial and hoop stresses. The load was transferred from the stem to the cement more uniformly along the interface once residual stresses were included.

CONCLUSIONS

Because there is no chemical bond at the interface between the stem and cement, the interface resistance depends on friction thus radial residual compressive stresses developed by the cement curing play a direct role.

RELEVANCE

Implant loosening of cemented hip implants is one of the major causes of late failure of the arthroplasty. The load is transferred from the stem to the bone primarily across the interfaces, consequently modelling accurately the interface is essential in predicting the load transfer.