Rigid-body modeling of knee cartilage and meniscus using experimental pressure-strain curves.

Rigid-body knee models have gained popularity thanks to computational speed and ease of setup compared to finite element models-showing exciting potential for clinical patient-specific models in the future. However, Rigid-body studies in general have encountered difficulty in modeling cartilage and especially meniscus material properties, often relying on computationally costly optimization techniques. This paper presents two new methods to alleviate the difficulty-one to define model contact pressure and one to define meniscus internal forces-and is the first to our knowledge to use experimental pressure-strain curves from the literature to simulate cartilage and meniscus behavior in a rigid body model. This paper describes the methodology to derive the proof of concept model and preliminary results from a gait simulation based on ISO 14243-1.

Diagnosis of bone marrow metastases in children with solid tumors and lymphomas. Aspiration, or unilateral or bilateral biopsy?

BACKGROUND: Malignancies are among the most common causes of death in children. The present study was undertaken to evaluate and compare bone marrow aspiration and unilateral biopsy to detect bone marrow metastases in pediatric patients, using bilateral biopsy as the gold standard. METHODS: During a 6-month period, 63 consecutive newly diagnosed children with confirmed malignant diseases other than leukemia were evaluated for bone marrow metastases or infiltration. Biopsies were obtained from both right and left posterior iliac crests whereas aspiration was performed only at the right crest. Interpretation to the right-side biopsy was considered as the unilateral biopsy result, whereas the bilateral biopsy result was as follows: positively was accepted if one or both of the two-side samples were qualified as positive, while a negative result was considered only if both sides were negative. The bilateral biopsy was considered the gold standard, and sensitivity, specificity, positive and negative predictive value, and false positive and negative rates were computed for the unilateral biopsy and aspiration procedure. RESULTS: We identified bone marrow metastases in 11 (17.5%) patients. The sensitivity was the only significant difference (p <0.05) observed between unilateral biopsy and aspiration. Finally, of the 63 patients, unilateral biopsy was reported as inadequate in one patient (1.6%), while aspiration was inadequate in two (3.2%). CONCLUSION: Unilateral biopsy was better than bone marrow aspiration. However, because bilateral biopsy is the gold standard, we recommend using this and bone marrow aspiration simultaneously to evaluate a pediatric patient with any malignancy potentially infiltrating bone marrow.

Muscle activity in rapid multi-degree-of-freedom elbow movements: solutions from a musculoskeletal model.

The activity of certain muscles that cross the elbow joint complex (EJC) are affected by forearm position and forearm movement during elbow flexion/extension. To investigate whether these changes are based on the musculoskeletal geometry of the joint, a three-dimensional musculotendinoskeletal computer model of the EJC was used to estimate individual muscle activity in multi-degree-of-freedom (df) rapid (ballistic) elbow movements. It is hypothesized that this model could reproduce the major features of elbow muscle activity during multi-df elbow movements using dynamic optimal control theory, given a minimum-time performance criterion. Results from the model are presented and verified with experimental kinematic and electromyographic data from movements that involved both one-df elbow flexion/extension and two-df flexion/extension with forearm pronation/supination. The model demonstrated how the activity of particular muscles is affected by both forearm position and movement, as measured in these experiments and as previously reported by others. These changes were most evident in the flexor muscles and least evident in the extensor muscles. The model also indicated that, for specific one- and two-df movements, activating a muscle that is antagonistic or noncontributory to the movement could reduce the movement time. The major features of muscle activity in multi-df elbow movements appear to be highly dependent on the joint's musculoskeletal geometry and are not strictly based on neural influences or neuroanatomical substrates.

How muscle architecture and moment arms affect wrist flexion-extension moments.

The purpose of this investigation was to determine how the moment arms and architecture of the wrist muscles influence their isometric moment-generating characteristics. A three-dimensional computer graphic model was developed that estimates the moment arms, maximum isometric forces, and maximum isometric flexion-extension moments generated by 15 muscles about the wrist over a range of wrist flexion angles. In combination with measurements of muscle strength, we used this model to answer three questions: (1) why is peak wrist flexion moment greater than peak extension moment, (2) why does flexion moment vary more with wrist flexion angle than does extension moment, and (3) why does flexion moment peak with the wrist in a flexed position? Analysis of the model revealed that the peak flexion moment is greater than the peak extension moment primarily because of the larger (110%) summed physiologic cross-sectional area of the flexors. The larger variation of flexion moment with flexion angle is caused mainly by greater variation of the moment arms of the major wrist flexors with flexion angle. The location of the peak flexion moment is determined by the wrist flexion moment arms (which tend to increase with wrist flexion) in combination with the force-length characteristics of these muscles.

Development and evaluation of a musculoskeletal model of the elbow joint complex.

This paper describes the development and evaluation of a musculoskeletal model that represents human elbow flexion-extension and forearm pronation-supination. The length, velocity, and moment arm for each of the eight musculotendon actuators were based on skeletal anatomy and joint position. Musculotendon parameters were determined for each actuator and verified by comparing analytical moment-angle curves with experimental joint torque data. The parameters and skeletal geometry were also utilized in the musculoskeletal model for the analysis of ballistic (rapid-directed) elbow joint complex movements. The key objective was to develop a computational model, guided by parameterized optimal control, to investigate the relationship among patterns of muscle excitation, individual muscle forces, and to determine the effects of forearm and elbow position on the recruitment of individual muscles during a variety of ballistic movements. The model was partially verified using experimental kinematic, torque, and electromyographic data from volunteer subjects performing both isometric and ballistic elbow joint complex movements. This verification lends credibility to the time-varying muscle force predictions and the recruitment of muscles that contribute to both elbow flexion-extension and forearm pronation-supination.

Comparison of experimental and analytical torque-angle relationships of the human elbow joint complex.

The human elbow joint complex (EJC) is an intricate joint that is currently being modeled with eight musculotendon actuators for the analysis of flexion-extension and pronation-supination movements. The musculotendon length (LMT), musculotendon velocity (VMT), and muscular moment arm (MA) for each actuator have been investigated based on joint anatomy and joint angle position. Musculotendon parameters necessary for the muscle model have been collected from various sources. These parameters, along with the musculoskeletal geometry, are used in a static muscle model to compute the force and joint torque generated by each muscle. The results are plotted as torque-angle curves and compared with experimental joint torque data.

Comparison of experimental and predicted muscle activation patterns in ballistic elbow joint movements.

The human elbow joint complex (EJC) is an intricate joint which produces combinations of movements that are unique within the human body and that are involved in performing many important tasks. This paper discusses an on-going study to predict muscle force patterns in elbow flexion-extension. Four musculotendon actuators are included in this preliminary musculoskeletal model for the analysis of ballistic elbow flexion-extension movements. The key objective is to develop a computational model, guided by optimal control, to investigate the relationship among patterns of muscle excitation, individual muscle forces, and movement kinematics. Model verification is attempted using experimental data from volunteer subjects performing a ballistic elbow flexion movement.