Lorenza Mattei

Predicting wear in artificial joints requires integrating joint dynamics, contact mechanics and progressive surface evolution, yet these processes are often treated separately. In total hip arthroplasty (THA), finite-element approaches remain the reference standard, but they are computationally demanding and usually rely on boundary conditions from independent musculoskeletal (MSK) models, hindering consistent coupling and feedback between wear progression and movement dynamics. As a single-subject proof of concept, we present a computational framework that embeds wear estimation within forward MSK simulations through OpenSim–MATLAB integration. Contact variables are computed using an elastic-foundation formulation, and wear is updated through the Archard law, enabling cyclic prediction of contact mechanics and surface evolution within a single workflow at a practical computational cost. The framework was evaluated in one subject with right THA during five activities of daily living and numerically benchmarked against finite-element simulations. A long-term walking analysis of 4 million cycles was also performed to assess geometry updating. Across tasks, peak contact pressures remained within 7% of finite-element predictions. Linear wear depth and volumetric loss showed maximum deviations of 16% and 13%, respectively. Accounting for progressive geometry changes yielded a maximum wear depth about 31% lower than linear extrapolation. These preliminary results support the framework’s computational feasibility and numerical consistency for the tested case; nevertheless, multi-subject evaluation is required before broader predictive or clinical use.

(1) Background: Markerless motion capture may enable scalable biomechanical assessment outside laboratory environments, yet its applicability to tasks involving elevated foot contacts and rapid loading transitions remains insufficiently characterized. This feasibility study evaluated OpenCap, an open-source smartphone-based markerless system, for estimating lower-limb kinematics and ground reaction forces during stair ascent, stair descent, squat, sit-to-stand, and forward lunge. (2) Methods: One healthy adult completed three repetitions of each task while data were acquired simultaneously with three smartphones, an eight-camera Vicon system, and two force plates. OpenCap/OpenSim outputs were adapted for stair and lunge kinetics and compared with marker-based kinematics and force-plate measurements using root mean square error, Pearson correlation, and cosine similarity. (3) Results: Agreement was strongest for sagittal-plane lower-limb kinematics, particularly knee flexion-extension (RMSE: 1.4–10.5 deg; Pearson: 0.94–0.999). Larger discrepancies occurred for hip internal-external rotation, pelvis motion during sit-to-stand, and ankle flexion-extension. Resultant GRF waveforms showed high similarity to force-plate data (cosine similarity ≥ 0.988; RMSE: 0.069–0.167 BW). Stair descent produced the largest deviations and variability, particularly for single-limb forces. (4) Conclusions: In this feasibility study, smartphone-based markerless analysis agreed best with laboratory measurements for sagittal lower-limb kinematics and resultant GRF waveforms but showed larger discrepancies in multiplanar kinematics and limb-specific loading. The adapted pipeline is technically feasible for exploratory movement analysis, although larger, repeated-session studies are needed before clinical use.

(1) Background: Markerless motion capture may enable scalable biomechanical assessment outside laboratory environments, yet its applicability to tasks involving elevated foot contacts and rapid loading transitions remains insufficiently characterized. This feasibility study evaluated OpenCap, an open-source smartphone-based markerless system, for estimating lower-limb kinematics and ground reaction forces during stair ascent, stair descent, squat, sit-to-stand, and forward lunge. (2) Methods: One healthy adult completed three repetitions of each task while data were acquired simultaneously with three smartphones, an eight-camera Vicon system, and two force plates. OpenCap/OpenSim outputs were adapted for stair and lunge kinetics and compared with marker-based kinematics and force-plate measurements using root mean square error, Pearson correlation, and cosine similarity. (3) Results Agreement was strongest for sagittal-plane lower-limb kinematics, particularly knee flexion-extension (RMSE: 1.4-10.5 deg; Pearson: 0.94-0.999). Larger discrepancies occurred for hip internal-external rotation, pelvis motion during sit-to-stand, and ankle flexion-extension. Resultant GRF waveforms showed high similarity to force-plate data (cosine similarity >=0.988; RMSE: 0.069-0.167 body weight). Stair descent produced the largest deviations and variability, particularly for single-limb forces. (4) Conclusions These findings support the technical feasibility of smartphone-based markerless functional biomechanics while highlighting the need for improved multiplanar pose estimation, contact modeling, and larger validation studies.

The validation of new hip replacement designs traditionally relies on lengthy and costly In‐Vitro wear tests that replicate basic In‐Vivo conditions, as the simplified walking recommended in the ISO 14242 guidelines. These tests overlook the diverse motor tasks and lifestyle differences among patients. This study seeks to establish the foundation for In‐Silico clinical trials of total hip replacements, enabling wear simulations of patients with different lifestyles, not feasible with In‐Vitro tests. The impact of diverse kinematic and loading histories on the wear of metal‐on‐plastic hip replacements is investigated in a novel way, considering the combined effect of six daily activities (e.g., walking, fast walking, sit/stand, stairs up/down, lunging), different activity frequencies across five patient profiles (from sedentary elderly to active young), and the effect of load sequence. The results reveal that both the type and frequency of motor tasks significantly influence implant wear. The most critical tasks and at‐risk patients were stair climbing and the most active individuals, regardless of age. Load sequence also plays a key role in long‐term wear predictions. Accuracy and computational cost were balanced by simulating walking, stair climbing, and sit/stand cycles, ensuring equivalent wear to a complete motor task sequence. ISO standards conditions notably tend to underestimate volumetric wear by up to 60% compared to the simulated patient types. They also fail to predict realistic wear patterns for activities like squatting and lunging where edge contact occurs.

Pin-on-plate and pin-on-disk wear tests are typically used for assessing the wear behavior of a given material coupling and estimating its wear coefficient using the Archard wear law. This study investigates differences in the Archard law for pin-on-plate and pin-on-disk cases, particularly for flat-ended pins. Both analytical and finite element models of the two tests were developed, assuming a 21 N normal load and a 50π mm sliding distance. In pin-on-disk simulations three different distances between pin and disk axes were considered, i.e., 1.25–2.5–5 times the pin radius (5 mm). For the results, wear volumes, pressure and wear depth maps were compared. Some interesting aspects arose: (i) the rotational effect in pin-on-disk tests causes higher wear volumes (up to 13%) with respect to pin-on-plate tests: the nearer the pin to the disk axis, the higher the wear volume; (ii) a simple quadratic formula is defined to correct the wear volume estimation for pin-on-disk tests; (iii) pressure redistribution occurs with higher values closer to disk axis, opposite to the wear depth trend. Due to the high computational costs, only the running-in phase of wear tests was considered. Numerical strategies are currently under investigation to extend this study to the steady state phase.

Pin-on-plate and pin-on-disk wear tests are typically used for assessing the wear behavior of a given material coupling. Such a behavior is frequently described by a wear coefficient k, which is estimated, according to the Archard’s law, as the ratio between the measured wear volume V and the product of the applied normal force F and the sliding distance s, i.e. k=V/(F s). This study demonstrates that such a relationship is correct for pin-on-plate but not for pin-on-disk, particularly for flat-ended pins. Both analytical and finite element models of the two tests were developed, considering three different distances pin-disk axis. As results, wear volumes, pressure and wear depth maps were examined and compared for the same sliding distance. Though this study is focused only on the initial transient part of the wear tests, some interesting aspects arose: i) rotational effect in pin-on-disk tests affects k estimation, especially when the pin is near to the disk axis; ii) a simple analytical function is defined to correct the wear volume estimation for pin-on-disk tests; iii) due to the different sliding distances of contact points in pin-on-disk tests, pressure redistribution occurs with higher values on the inner side (closer to disk axis); the opposite trend is observed in wear depth maps. From a computational point of view, the pin-on-disk problem, only apparently simple, required very small time increments to correctly describe the circular trajectories of points, with high computational costs. Numerical strategies are currently under investigation to extend the study to the steady state phase of both tests.

The present study proposes the further validation of a simple mathematical procedure recently proposed by the authors to describe contact and wear evolution in line and point contacts. The procedure assumed that the maximum contact pressure could be determined using Hertz equations and a parabolic pressure profile. The contact half-width was obtained using the equilibrium equation and the Archard wear law. Several cases were selected from the literature, reporting experimental data or Finite Element simulations, and the results were compared to those obtained with the proposed approach. This paper confirms the reliability and potentialities of the proposed analytical procedure, which is capable of providing accurate solutions in case of frictional contacts and at the borders of the contact area, where the main discrepancies were found in the previous study.