Michael J. Dreyer


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Dreyer

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Michael J.

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0000-0002-5511-4983

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2022 - 20247
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Publications 1 - 7 of 7
  • Modelling Every Wear: In Silico, In Vitro, and In Vivo Approaches to Predict Wear of Knee Implants
    Item type: Doctoral Thesis
    Dreyer, Michael J. (2023)
    Total Knee Arthroplasty is a common surgical procedure for managing arthritis, providing pain relief and improved functionality to patients through implantation of a joint replacement. However, wear of the polyethylene (PE) inlay of knee implants poses clinical challenges, including the potential for implant failure and revision surgeries. Not only understanding, but also the ability to predict wear outcomes in knee implants is crucial for guiding implant design, regulatory approval, and clinical decision making. In vitro wear simulations are realistic, but slow and laborious, while in silico modelling is fast and accessible, but limited by model sensitivity and lack of input data. In this work, we applied these complementary approaches to "modelling every wear": We fitted statistical models to in vivo wear outcomes, performed in vitro testing, and built state-of-the-art computational wear models. Across these domains, we provide novel tools and realistic data, striving towards accurate predictions of in vivo PE wear. To provide realistic and practically usable input data for experimental testing, we generated a representative summary of the CAMS-Knee datasets, the largest collection of knee implant kinematics and tibiofemoral contact loads measured in vivo for six subjects and five activities. The loads of the created standardized subject “Stan” are similar to earlier datasets, but valuably complemented by synchronized kinematics. Compared to the ISO 14243 standard loads, Stan’s loads are up to +56% higher, while the kinematics exhibit markedly different curve shapes. Application of Stan’s kinematics and the ISO standard loads to a knee simulator wear test revealed not only visibly different wear locations on the articulating surface, but also approximately three times higher wear rates for Stan’s boundary conditions. While further testing under Stan’s conditions is necessary to substantiate this, these initial results indicate that the ISO standards may not be fully representative of in vivo loading and damage. In a different experiment, we performed an array of pin-on-disk tests to quantify the influence of contact pressure and cross-shear, i.e. multidirectional sliding, on the mechanism and volume of PE wear. Wear was found to strongly increase when going from unidirectional to multidirectional sliding and, contrary to the classical Archard law, not proportionally increase with increasing contact pressure, but increase less at higher pressures. This was due to the formation of hardened protrusions on the PE surface at higher pressures, which afford some protection of the surface. To the wear results, an empirical model of PE wear as a function of cross-shear and contact pressure was fitted to serve as input data for computational wear models. The empirical PE wear model was then used as input to a computational wear prediction algorithm based on finite-element models of the implant under physiological load and motion. This algorithm can model the change in surface geometry due to nonlinear wear and long-term plastic creep of the PE, the following change in contact mechanics, and the resulting interaction with subsequent wear damage. Verification and validation were carried out against the knee simulator test with the Stan and ISO boundary conditions. While a force-controlled ISO model was unable to mirror the bench test kinematics and thus wear rate, displacement-controlled models accurately predicted experimental wear rates for both ISO and Stan boundary conditions. This analysis confirmed that in silico wear models are very sensitive to even small errors in relative tibiofemoral motion, and that accurate reproduction of in vivo joint kinematics is crucial while some deviation of the contact loads may be permissible. If in vivo contact kinematics can be modelled, realistic computational predictions of implant wear are thus possible. To this end, we developed a novel technique to control the load and kinematic boundary conditions applied to computational models of joints. In the combined Load- and Displacement-Controlled method with Springs (LDCS), joint contact loads are applied to the model components directly. Then, motions are applied in the same directions as the loads, which would be not possible with conventional control methods. This is achieved by applying the motions through nonlinear springs, which mediate a balance between the applied loads and kinematics and also prevent propagation of any measurement noise present in the joint kinematics to the contact locations. The LDCS method reduced load and motion errors by a factor of two or more compared to conventional approaches and thus presents clear advantages for modelling tibiofemoral contact and wear. Lastly, we analysed inlays retrieved from revision surgery to quantify in vivo wear, not only providing comparative data for in silico and in vitro wear simulations, but also providing direct evidence of the effect of clinical parameters like choice of implant design and mechanical axis limb alignment on personal wear outcomes. To this end, we validated and employed a surface reconstruction approach to obtain the distribution and volume of wear on the articulating surface of the polyethylene inlays. We found that rotating-platform inlays experienced a significant 39% lower wear rates in situ than fixed-bearing implants, likely due to the rotational freedom which mediates alignment of a rotating inlay to the femoral component, thus reducing contact stresses and motion. Limb alignment, on the other hand, was only non-significantly related to overall wear rates, though there was a slight trend of varus-aligned specimens showing more wear damage. However, limb alignment did significantly alter the mediolateral distribution of wear, with varus alignment resulting in predominantly medial compartment wear. Our analysis also revealed considerable inter-patient variability, likely due to patient-specific factors like level of activity which were not included in our statistical model. In summary, this research has both added to the wear prediction toolbox by providing validated computational tools and has presented novel in vivo data to enable and validate wear simulations. With Stan, we have provided input data for experimental and computational knee implant wear simulations, where we also found in silico wear models to be sensitive to errors in joint kinematics. With the LDCS method, we provided a modelling technique to drastically reduce such errors when replicating in vivo joint kinematics in a computational model. Lastly, investigating how various parameters affect PE wear, we found contact pressure and multidirectional sliding as well as limb alignment and implant design to influence wear outcomes. Further experimental and computational studies aiming to truly predict in vivo wear should attempt to account for the observed variability between patients and the sensitivity of models to changed input conditions by including multiple patients, activities, and repetitions in simulations of implant wear driven by accurate joint kinematics.
  • A novel method to accurately recreate in vivo loads and kinematics in computational models of the knee
    Item type: Journal Article
    Dreyer, Michael J.; Kneifel, Paul; Hosseini Nasab, Seyyed Hamed; et al. (2023)
    Computer Methods in Biomechanics and Biomedical Engineering
    Despite availability of in vivo knee loads and kinematics data, conventional load- and displacement-controlled configurations still can’t accurately predict tibiofemoral loads from kinematics or vice versa. We propose a combined load- and displacement-control method for joint-level simulations of the knee to reliably reproduce in vivo contact mechanics. Prediction errors of the new approach were compared to those of conventional purely load- or displacement-controlled models using in vivo implant loads and kinematics for multiple subjects and activities (CAMS-Knee dataset). Our method reproduced both loads and kinematics more closely than conventional models and thus demonstrates clear advantages for investigating tibiofemoral contact or wear.
  • The influence of implant design and limb alignment on in vivo wear rates of fixed‐bearing and rotating‐platform knee implant retrievals
    Item type: Journal Article
    Dreyer, Michael J.; Weisse, Bernhard; Contreras Raggio, José Ignacio; et al. (2024)
    Journal of Orthopaedic Research
    Analysis of polyethylene wear in knee implants is crucial for understanding the factors leading to revision in total knee arthroplasty. Importantly, current experimental and computational methods for predicting insert wear can only be validated against true in vivo measurements from retrievals. This study quantitatively investigated in vivo polyethylene wear rates in fixed‐bearing (n=21) and rotating‐platform (n=53) implant retrievals. 3D surface geometry of the retrievals was measured using a structured light scanner. Then, a reference surface that included the deformation, but not the wear that the retrievals had experienced in vivo , was constructed using a fully automatic surface reconstruction algorithm. Finally, wear volume was calculated from the deviation between the worn and reconstructed surfaces. The measurement and analysis techniques were validated and the algorithm was found to produce errors of only 0.2% relative to the component volumes. In addition to quantifying cohort‐level wear rates, the effect of mechanical axis limb alignment on mediolateral wear distribution was examined for a subset of the retrievals (n=14+26). Our results show that fixed‐bearing implants produce significantly (p=0.04) higher topside wear rates (24.6±10.1 mm ³ /year) than rotating‐platform implants (15.3±8.0 mm ³ /year). This effect was larger than that of limb alignment, which had a smaller and non‐significant influence on overall wear rates (+4.5±11.6 mm ³ /year, p=0.43). However, increased varus alignment was associated significantly with greater medial compartment wear in both the fixed‐bearing and rotating‐platform designs (+1.7±1.3 %/° and +1.8±1.6 %/°). Our findings emphasize the importance of implant design and limb alignment on wear outcomes, providing reference data for improving implant performance and longevity.
  • Anomalous Wear Behavior of UHMWPE During Sliding Against CoCrMo Under Varying Cross-Shear and Contact Pressure
    Item type: Journal Article
    Dreyer, Michael J.; Taylor, William R.; Wasmer, Kilian; et al. (2022)
    Tribology Letters
    Wear of ultra-high-molecular weight polyethylene (UHMWPE) in joint implant applications has been shown to increase with cross-shear (CS, i.e., multidirectional sliding) but decrease with higher contact pressure (CP). Moreover, structural changes, resulting in protrusions, are known to occur to the surface of the pin following multidirectional sliding. However, these phenomena are not yet fully understood. In this study, we simultaneously varied CP and CS to derive an empirical formula for the wear factor as a function of these parameters. The wear factor increased when going from unidirectional sliding to multidirectional sliding but decreased with increasing CP, as has been previously observed. Following these tests, the protrusions on the pin surface were chemically and mechanically characterized to gain insights into both their origin and influence on wear behavior. Micro-FT-IR confirmed that the structures consist of polyethylene, rather than adsorbed, denatured proteins. It also allowed the crystallinity of both the protrusions and unaffected UHMWPE to be estimated, showing a strong positive correlation with the hardness of these different areas on the surface. Time-of-flight secondary-ion mass spectrometry was used to probe the chemistry of the surface and near-surface region and indicated the presence of contamination from the test fluid within the structure. This suggests that the protrusions are formed by the folding of UHMWPE following plastic deformation. It is also suggested that the higher hardness of the protrusions affords some protection of the surface, leading to the observed anomalous behavior, whereby wear increases with decreasing CP.
  • Knee kinematics are primarily determined by implant alignment but knee kinetics are mainly influenced by muscle coordination strategy
    Item type: Journal Article
    Febrer-Nafría, Míriam; Dreyer, Michael J.; Maas, Allan; et al. (2023)
    Journal of Biomechanics
    Implant malalignment has been reported to be a primary reason for revision total knee arthroplasty (TKA). In addition, altered muscle coordination patterns are commonly observed in TKA patients, which is thought to alter knee contact loads. A comprehensive understanding of the influence of surgical implantation and muscle recruitment strategies on joint contact mechanics is crucial to improve surgical techniques, increase implant longevity, and inform rehabilitation protocols. In this study, a detailed musculoskeletal model with a 12 degrees of freedom knee was developed to represent a TKA subject from the CAMS-Knee datasets. Using motion capture and ground reaction force data, a level walking cycle was simulated and the joint movement and loading patterns were estimated using a novel technique for concurrent optimization of muscle activations and joint kinematics. In addition, over 12′000 Monte Carlo simulations were performed to predict knee contact mechanics during walking, considering numerous combinations of implant alignment and muscle activation scenarios. Validation of our baseline simulation showed good agreement between the model kinematics and loading patterns against the in vivo data. Our analyses reveal a considerable impact of implant alignment on the joint kinematics, while variation in muscle activation strategies mainly affects knee contact loading. Moreover, our results indicate that high knee compressive forces do not necessarily originate from extreme kinematics and vice versa. This study provides an improved understanding of the complex inter-relationships between loading and movement patterns resulting from different surgical implantation and muscle coordination strategies and presents a validated framework towards population-based modelling in TKA.
  • European Society of Biomechanics S.M. Perren Award 2022: Standardized tibio-femoral implant loads and kinematics
    Item type: Journal Article
    Dreyer, Michael J.; Trepczynski, Adam; Hosseini Nasab, Seyyed Hamed; et al. (2022)
    Journal of Biomechanics
    Knowledge of both tibio-femoral kinematics and kinetics is necessary for fully understanding knee joint biomechanics, guiding implant design and testing, and driving and validating computational models. In 2017, the CAMS-Knee datasets were presented, containing synchronized in vivo implant kinematics measured using a moving fluoroscope and tibio-femoral contact loads measured using instrumented implants from six subjects. However, to date, no representative summary of kinematics and kinetics obtained from measurements at the joint level of the same cohort of subjects exists. In this study, we present the CAMS-Knee standardized subject “Stan”, whose reference data include tibio-femoral kinematics and loading scenarios from all six subjects for level and downhill walking, stair descent, squat and sit-to-stand-to-sit. Using the peak-preserving averaging method by Bergmann and co-workers, we derived scenarios for generally high (CAMS-HIGH100), peak, and extreme loading. The CAMS-HIGH100 axial forces reached peaks between 3022 and 3856 N (3.08–3.93 body weight) for the five investigated activities. Anterior-posterior forces were about a factor of ten lower. The axial moment around the tibia was highest for level walking and squatting with peaks of 9.4 Nm and 10.5 Nm acting externally. Internal tibial rotations of up to 8.4° were observed during squat and sitting, while the walking activities showed approximately half the internal rotation. The CAMS-HIGH100 loads were comparable to Bergmann and co-workers’, but have the additional benefit of synchronized kinematics. Stan’s loads are +11 to +56% higher than the ISO 14243 wear testing standard loads, while the kinematics exhibit markedly different curve shapes. Along with the original CAMS-Knee datasets, Stan’s data can be requested at cams-knee.orthoload.com.
  • Experimental and computational evaluation of knee implant wear and creep under in vivo and ISO boundary conditions
    Item type: Journal Article
    Dreyer, Michael J.; Hosseini Nasab, Seyyed Hamed; Favre, Philippe; et al. (2024)
    BioMedical Engineering OnLine
    Background Experimental knee implant wear testing according to ISO 14243 is a standard procedure, but it inherently possesses limitations for preclinical evaluations due to extended testing periods and costly infrastructure. In an effort to overcome these limitations, we hereby develop and experimentally validate a finite-element (FE)-based algorithm, including a novel cross-shear and contact pressure dependent wear and creep model, and apply it towards understanding the sensitivity of wear outcomes to the applied boundary conditions. Methods Specifically, we investigated the application of in vivo data for level walking from the publicly available “Stan” data set, which contains single representative tibiofemoral loads and kinematics derived from in vivo measurements of six subjects, and compared wear outcomes against those obtained using the ISO standard boundary conditions. To provide validation of the numerical models, this comparison was reproduced experimentally on a six-station knee wear simulator over 5 million cycles, testing the same implant Stan’s data was obtained from. Results Experimental implementation of Stan’s boundary conditions in displacement control resulted in approximately three times higher wear rates (4.4 vs. 1.6 mm3 per million cycles) and a more anterior wear pattern compared to the ISO standard in force control. While a force-controlled ISO FE model was unable to reproduce the bench test kinematics, and thus wear rate, due to a necessarily simplified representation of the simulator machine, similar but displacement-controlled FE models accurately predicted the laboratory wear tests for both ISO and Stan boundary conditions. The credibility of the in silico wear and creep model was further established per the ASME V&V-40 standard. Conclusions The FE wear model is suitable for supporting future patient-specific models and development of novel implant designs. Incorporating the Stan data set alongside ISO boundary conditions emphasized the value of using measured kinematics in displacement control for reliably replicating in vivo joint mechanics in wear simulation. Future work should focus on expanding the range of daily activities simulated and addressing model sensitivity to contact mechanics to further enhance predictive accuracy.
Publications 1 - 7 of 7