Evaluation of an electromagnetic position tracking device for measuring in vivo, dynamic joint kinematics

An electromagnetic position tracking device was evaluated to determine its static and dynamic accuracy and reliability for applications related to measuring in vivo joint kinematics. The device detected the position and orientation of small coiled sensors, maintained in an electromagnetic field. System output was measured against known translations or rotations throughout the measurement volume. Average translational errors during static testing were 0.1 +/- 0.04, 0.2 +/- 0.17, and 0.8 +/- 0.81 mm (mean+/-SD) for sensors 50, 300, and 550 mm away from the field generator, respectively. Average rotational errors were 0.4 +/- 0.31 degrees, 0.4 +/- 0.21 degrees, and 0.9 +/- 0.85 degrees (mean +/- SD) for sensors located at the same distances. Since we intended to use this system in an animal walking on a treadmill, we incrementally moved the sensors under various treadmill conditions. The effects of treadmill operation on translational accuracy were found to be negligible. The effects of dynamic motions on sensor-to-sensor distance were also assessed for future data collection in the animal. Sensor-to-sensor distance showed standard deviations of 2.6 mm and a range of 13 mm for the highest frequency tested (0.23 Hz). We conclude that this system is useful for static or slow dynamic motions, but is of limited use for obtaining gait kinematics at higher speeds.     

Improvements in measuring shoulder joint kinematics

For many clinical applications it is necessary to non-invasively determine shoulder motion during dynamic movements, and in such cases skin markers are favoured. However, as skin markers may not accurately track the underlying bone motion the methods currently used must be refined. Furthermore, to determine the motion of the shoulder a model is required to relate the obtained marker trajectories to the shoulder kinematics. In Wu et al. (2005) the International Society of Biomechanics (ISB) proposed a shoulder model based on the position of bony landmarks. A limitation of the ISB recommendations is that the reference positions of the shoulder joints are not standardized. The aims of this research project were to develop a method to accurately determine shoulder kinematics using skin markers, and to investigate the effect of introduction of a standardized reference configuration. Fifteen subjects, free from shoulder pathology, performed arm elevations while skin marker trajectories were tracked. Shoulder kinematics were reconstructed using a chain model and extended Kalman filter. The results revealed significant differences between the kinematics obtained with and without introduction of the reference configuration. The curves of joint angle tended towards 0° for 0° of humerus elevation when the reference configuration was introduced. In conclusion, the shoulder kinematics obtained with introduction of the reference configuration were found to be easier to interpret than those obtained without introduction of the reference configuration.     

Feasibility of using a magnetic tracking device for measuring carpal kinematics

While several different methods have been used to measure carpal kinematics, biplanar radiography is generally considered to be the most accurate and popular one. However, biplanar radiography is tedious and so only pseudo-dynamic kinematics can be measured. Recently, magnetic tracking system has been developed for the measurement of joint kinematics which is versatile and easy to use and so the possibility of measuring motions dynamically. In this study, the capability of a magnetic tracking device to accurately measure carpal kinematics was investigated by comparing it with biplanar radiography. The kinematics of the third metacarpal, scaphoid, and lunate in five fresh cadaveric specimens were measured using both methods as the wrists were placed in eight positions. The finite screw rotation of each bone with respect to the distal radius during selecting the seven wrist motions was calculated for both measuring techniques and compared. In general, the kinematics for all three bones measured by using either magnetic tracking device or biplanar radiography was identical and showed no statistical difference. The averaged differences ranged from 0.0 to 2.0°. These differences were due to the potential effect of the weight of the sensors and the interference of the attaching rod to the surrounding tissue. It is concluded that the application of the magnetic tracking device to carpal kinematics is warranted, if proper technical procedures as suggested are followed.     

The glenohumeral joint rotation centre in vivo

Within the framework of the current call for standardization in upper extremity research, three methods to determine the glenohumeral joint rotation centre in vivo were tested. Therefore, subjects performed humeral movements, while a 3D electromagnetic tracking device registered the motion of the humerus with respect to the scapula. For the first method to estimate the glenohumeral joint rotation centre five scapular bony landmarks served as input to regression equations. The second method fitted a sphere through the humeral position data and the third method calculated the rotation centre determining an optimal helical axis. The experiment consisted of two parts, at first one subject was measured 10 times, subsequently one observer measured 10 subjects twice and another observer measured these subjects once. The first part of the experiment demonstrated that all methods are capable to reproduce the rotation centre within 4 mm, but the location of the centre differed significantly between methods (p<0.001). The second part, showed that inter- and intra-observer reliability was sufficiently for the sphere-fitting method and for the helical-axes method. The two observations of one observer differed significantly (p<0.008) using the regression method. The authors prefer the helical-axes method, it is a reliable and valid method which can be applied in movement registration of healthy subjects and patients with a shoulder endoprosthesis, it can be applied in hinge joints to determine a rotation axis instead of a rotation centre which is desirable in standardized upper extremity research, and calculation time is short.     

A new in vivo technique for determination of 3D kinematics and contact areas of the patello-femoral and tibio-femoral joint

Patello-femoral disorders are often caused by changes of patello-femoral and/or tibio-femoral kinematics. However, until now there has been no quantitative in vivo technique, that is able to obtain 3D kinematics and contact areas of all knee compartments simultaneously on a non-invasive basis. The aim of this study was therefore to develop and apply a technique which allows for determination of 3D kinematics and contact areas of the patello-femoral and tibio-femoral joint during different knee flexion angles and under neuromuscular activation patterns. One knee of each of the 10 healthy volunteers was examined in an open MR system under flexing isometric muscle activity at 30 degrees and 90 degrees. Three-dimensional kinematics and contact areas of the patello-femoral and tibio-femoral joints were analyzed by 3D image postprocessing. The reproducibility of the imaging technique yielded a coefficient of variation of 4.6% for patello-femoral, 4.7% for femoro-tibial displacement and 8.6% for contact areas. During knee flexion (30-90 degrees ), patella tilt (opened to medial) decreased (8.8+/-3.4 degrees vs. 4.6+/-3.1 degrees, p<0.05), while lateral patellar shift increased significantly (1.6+/-2.3mm vs. 3.4+/-3.0mm, p<0.05). Furthermore, a significant posterior translation and external rotation of the femur relative to the tibia was observed. Patello-femoral contact areas increased significantly in size (134+/-60mm(2) vs. 205+/-96 mm(2)) during knee flexion. This technique shows a high reproducibility and provides physiologic in vivo data of 3D kinematics and contact areas of the patello-femoral and the tibio-femoral joint during knee flexion. This allows for advanced in vivo diagnostics, and may help to improve therapy of patello-femoral disorders in the future.     

Validation of a single camera three-dimensional motion tracking system

The ability to analyze human movement is an essential tool of biomechanical analysis for both sport and clinical applications. Traditional 3D motion capture technology limits the feasibility of large scale data collections and therefore the ability to address clinical questions. Ideally, the measurement system/protocol should be non-invasive, mobile, generate nearly instantaneous feedback to the clinician and athlete, and be relatively inexpensive. The retro-grate reflector (RGR) is a new technology that allows for three-dimensional motion capture using a single camera. Previous studies have shown that orientation and position information recorded by the RGR system has high measurement precision and is strongly correlated with a traditional multi-camera system across a series of static poses. The technology has since been refined to record moving pose information from multiple RGR targets at sampling rates adequate for assessment of athletic movements. The purpose of this study was to compare motion data for a standard athletic movement recorded simultaneously with the RGR and multi-camera (Motion Analysis Eagle) systems. Nine subjects performed three single-leg land-and-cut maneuvers. Thigh and shank three-dimensional kinematics were collected with the RGR and Eagle camera systems simultaneously at 100 Hz. Results showed a strong agreement between the two systems in all three planes, which demonstrates the ability of the RGR system to record moving pose information from multiple RGR targets at a sampling rate adequate for assessment of human movement and supports the ability to use the RGR technology as a valid 3D motion capture system.     

Theoretical accuracy of model-based shape matching for measuring natural knee kinematics with single-plane fluoroscopy

Quantification of knee motion under dynamic, in vivo loaded conditions is necessary to understand how knee kinematics influence joint injury, disease, and rehabilitation. Though recent studies have measured three-dimensional knee kinematics by matching geometric bone models to single-plane fluoroscopic images, factors limiting the accuracy of this approach have not been thoroughly investigated. This study used a three-step computational approach to evaluate theoretical accuracy limitations due to the shape matching process alone. First, cortical bone models of the femur tibia/fibula, and patella were created from CT data. Next, synthetic (i.e., computer generated) fluoroscopic images were created by ray tracing the bone models in known poses. Finally, an automated matching algorithm utilizing edge detection methods was developed to align flat-shaded bone models to the synthetic images. Accuracy of the recovered pose parameters was assessed in terms of measurement bias and precision. Under these ideal conditions where other sources of error were eliminated, tibiofemoral poses were within 2 mm for sagittal plane translations and 1.5 deg for all rotations while patellofemoral poses were within 2 mm and 3 deg. However, statistically significant bias was found in most relative pose parameters. Bias disappeared and precision improved by a factor of two when the synthetic images were regenerated using flat shading (i.e., sharp bone edges) instead of ray tracing (i.e., attenuated bone edges). Analysis of absolute pose parameter errors revealed that the automated matching algorithm systematically pushed the flat-shaded bone models too far into the image plane to match the attenuated edges of the synthetic ray-traced images. These results suggest that biased edge detection is the primary factor limiting the theoretical accuracy of this single-plane shape matching procedure.     

Magnetic resonance imaging for in vivo assessment of three-dimensional patellar tracking

We have developed a non-invasive measurement technique which can ultimately be used to quantify three-dimensional patellar kinematics of human subjects for a range of static positions of loaded flexion and assessed its accuracy. Knee models obtained by segmenting and reconstructing one high-resolution scan of the knee were registered to bone outlines obtained by segmenting fast, low-resolution scans of the knee in static loaded flexion. We compared patellar tracking measurements made using the new method to measurements made using Roentgen stereophotogrammetric analysis in three cadaver knee specimens loaded through a range of flexion in a test rig. The error in patellar spin and tilt measurements was less than 1.02 degrees and the error in lateral patellar shift was 0.88 mm. Sagittal plane scans provided more accurate final measurements of patellar spin and tilt, whereas axial plane scans provided more accurate measurements of lateral translation and patellar flexion. Halving the number of slices did not increase measurement error significantly, which suggests that scan times can be reduced without reducing accuracy significantly. The method is particularly useful for multiple measurements on the same subject because the high-resolution bone-models need only be created once; thus, the potential variability in coordinate axes assignment and model segmentation during subsequent measurements is removed.     

Evaluation of a new immunoturbidimetry technique for measuring microalbuminuria

Subclinical elevation of urinary albumin excretion is a good predictor of later clinical proteinuria. A simple, sensitive and rapid immunoturbidimetric method was developed to quantify urinary albumin excretion (URIN-PAK ImmunoMICRO LAB, Miles Italia Spa). In the presence of polyethylene glycol 6000, immunocomplex between human albumin and its specific antibody are rapidly formed (5-50 min, at room temperature). Absorbance reading are mode U 340 nm (Automatic Analyzer RA 1000, Technicon). The test is specific for albumin failing to cross react with other plasma proteins present in urine, as well as with glibenclamide, chlorpropamide, phenformin, hemoglobin, glucose, urea and thymol. The present method correlates with SCLAVO H-ALBUMIN RIA Kit (r = 0.9917). The test is suitable for clinical use.     

The effect of load magnitude on three-dimensional patellar kinematics in vivo

Studies of three-dimensional patellar kinematics done with little or no applied load may not accurately reflect kinematics at physiological load levels, and may provide different results to those acquired with greater applied loads or in physiologic weightbearing. We report the effect of load magnitude on three-dimensional patellar kinematics (flexion, spin and tilt; proximal, lateral and anterior translation) using a validated, sequential static, MRI-based method. Ten healthy subjects loaded their study knee to 0% (no load), 15% and 30% bodyweight (BW) using a custom designed loading rig. Differences between loading levels were determined as a function of knee flexion for each kinematic parameter using linear hierarchical random-effects models. Quadratic and random slope terms were included in the models when significant. We found that the patellae flexed less with knee flexion at 30% BW load compared to 0% BW load (p<0.001) and 15% BW (p=0.004) load. The patellae showed a slight medial tilt with knee flexion at 30% BW load which was significantly less than the medial tilt seen at 0% BW load (p=0.017) and 15% BW load (p=0.043) with knee flexion. Small but statistically significant differences were also observed for proximal and anterior translation; the patellae were in a more proximal and posterior position at 30% BW load than at 0% BW load (p=0.010 and p=0.005, respectively) and 15% BW load (p<0.001 and p=0.029, respectively). Since differences in three-dimensional patellar kinematics were observed between loading levels, magnitudes of prescribed loads must be considered when designing studies and comparing results between studies.     

A new system for three-dimensional tracking of motile microorganisms

A new three-dimensional (3D)-tracking system with optimized dark-field illumination is presented. It allows simultaneous 3D tracking of several free-swimming microorganisms with diameters of >10 microm. Resolution limits and illumination efficiencies for different size classes of microorganisms are treated analytically. First applications for 3D tracking of protists are demonstrated.     

A new in vivo technique for determination of femoro-tibial and femoro-patellar 3D kinematics in total knee arthroplasty

Aim was to develop an in vivo technique which allows determination of femoro-tibial and of femoro-patellar 3D-kinematics in TKA simultaneously. The knees of 20 healthy volunteers and of eight patients with TKA (PCR, rotating platform) were investigated. Kinematics analysis was performed in an open MR-system at different flexion angles with external loads being applied. The TKA components were identified using a 3D-fitting technique, which allows an automated 3D-3D-registration of the TKA. Femoro-patellar and femoro-tibial 3D-kinematics were analyzed by image postprocessing. The validity of the postprocessing technique demonstrated a coefficient of determination of 0.98 for translation and of 0.97 for rotation. The reproducibility yielded a coefficient of variation (CV%) for patella kinematics between 0.17% (patello-femoral angle) and 6.8% (patella tilt). The femoro-tibial displacement also showed a high reproducibility with CV% of 4.0% for translation and of 7.1% for rotation. While in the healthy knees the typical screw-home mechanism was observed, a paradoxical anterior translation of the femur relative to the tibia combined with an external rotation occurred after TKA. Fifty percent of the TKA's experienced a condylar lift-off of >1mm predominately on the medial side. Regarding patellar kinematics significant changes were found in both planes in TKA with an increased patella height in the sagittal plane and patella tilt and shift in the transversal plane. The results demonstrate that the presented 3D MR-open based method is highly reproducible and valid for image acquisition and postprocessing and provides--for the first time--in vivo data of 3D-kinematics of the tibio-femoral and simultaneously of the patello-femoral joint during knee flexion.     

Comparison of the SCoRE and HA methods for locating in vivo the glenohumeral joint centre

A recent paper has described a new functional method, the symmetrical centre of rotation (SCoRE), for locating joint centre position [Ehrig, R.M., Taylor, W.R., Duda, G.N., Heller, M.O., 2006. A survey of formal methods for determining the centre of rotation of ball joints. Journal of Biomechanics 39 (15), 2798-2809]. For in vitro analyses, the SCoRE method showed better precision than helical axis (HA) or sphere fitting methods. Despites HA determination is very sensitive to small angular velocity, the International Society of Biomechanics has recommended to use HA for locating the glenohumeral joint centre. This paper aims at comparing the SCoRE method with the HA method for locating in vivo the glenohumeral joint centre according to the movement characteristics. Nine subjects performed 10 cycles of three different movements at two different velocities. For each test (combination of movements) the location of the centre of rotation was estimated with both methods (SCoRE and HA). Analyses focused on the 3D location of the glenohumeral joint centre and on the repeatability of location (standard deviation). This study showed that SCoRE and HA methods yielded the same GH location. Nevertheless, with SCoRE method, the location of the glenohumeral joint centre was different according to the test. This study evidenced that the SCoRE method was more precise than HA method (error of 3 mm versus 4.6 mm) and that the GH location with the SCoRE method was not affected by movements with slow velocities.     

Effect of abducting and adducting muscle activity on glenohumeral translation, scapular kinematics and subacromial space width in vivo

It is currently unknown in which ways activity of the ab- and adductor shoulder muscles affects shoulder biomechanics (scapular kinematics and glenohumeral translation), and whether these changes are relevant for alterations of the subacromial space width. The objective of this experimental in vivo study was thus to test the hypotheses that potential changes of the subacromial space width (during antagonistic muscle activity) are caused by alterations of scapular kinematics and/or glenohumeral translation. The shoulders of 12 healthy subjects were investigated with an open MRI-system at 30 degrees, 60 degrees, 90 degrees, 120 degrees and 150 degrees of arm elevation. A force of 15N was applied to the distal humerus, once causing isometric contraction of the abductors and once contraction of the adductors. The scapulo-humeral rhythm, scapular tilting and glenohumeral translation were calculated from the MR image data for both abducting and adducting muscle activity. Adducting muscle activity led to significant increase of the subacromial space width in all arm positions. The scapulo-humeral rhythm (2.2-2.5) and scapular tilting (2-4 degrees) remained relatively constant during elevation, no significant difference was found between abducting and adducting muscle activity. The position of the humerus relative to the glenoid was, however, significantly (p < 0.05) different (inferior and anterior) for adducting versus abducting muscle activity in midrange elevation (60-120 degrees). These data show that the subacromial space can be effectively widened by adducting muscle activity, by affecting the position of the humerus relative to the glenoid. This effect may be employed for conservative treatment of the impingement syndrome.     

Validation of ethylene oxide-sterilization using a new measuring system.

Up to now the parametric release of medical devices sterilized with EO was not possible due the lack of an online method of measurement for EO. With the developed mobile system it is possible to measure all physical parameters of the sterilization process online in the chamber. The accuracy of the EO concentration and humidity measurement is < +/- 1%, of the temperature < +/- 0.05 degree C and of pressure < 0.5% of the measured value.     

Comparison of different calculations of three-dimensional joint kinematics from video-based system data

When skin-fixed marker trajectories are used to calculate 3D joint kinematics, the measurement errors (i.e. the difference between the trajectories of the external markers and those of the skeleton) influence to some extent the accuracy of the results, depending both on the calculation method and on the axes about which the rotations are expressed. The purpose of this paper is to compare several expressions of joint angular variations. Two kinematic concepts are used to calculate the changes in the orientation of the distal segment versus the proximal one: the first method consists of computing the components of the spatial attitude vector, the second one deals with the determination of elementary rotations about successive axes. For each of these methods, two sets of three axes are tested to express the results: the axes forming the reference frame affixed to the body segment adjacent to the joint (named fixed axes), and a set consisting of a first axis belonging to the proximal segment, a third axis belonging to the distal segment and a second (floating) axis defined as the cross-product between the two other ones (named mobile axes). To compare these four distinct expressions on the knee joint, numerical simulations of perturbed skin marker trajectories are performed, based on experimental data recorded by a Motion Analysis system during a normal gait cycle. A significant difference is pointed out only for the internal–external rotation angle, for which the best expression — from the viewpoint of sensitivity to experimental errors — is obtained using the components of the attitude vector in a segment-embedded reference frame.     

Validation of the Delft Shoulder and Elbow Model using in-vivo glenohumeral joint contact forces

The Delft Shoulder and Elbow Model (DSEM), a large-scale musculoskeletal model, is used for the estimation of muscle and joint reaction forces in the shoulder and elbow complex. Although the model has been qualitatively verified using EMG-signals, quantitative validation has until recently not been feasible. The development of an instrumented shoulder endoprosthesis has now made this possible. To this end, motion data, EMG-signals, external forces, and in-vivo glenohumeral joint reaction forces (GH-JRF) were recorded for two patients with an instrumented shoulder hemi-arthroplasty, during dynamic tasks (including abduction and anteflexion) and force tasks with the arm held in a static position. Motions and external forces served as the model inputs to estimate the GH-JRF. In the modeling process, the effect of two different (stress and energy) optimization cost functions and uniform size and mass scaling were evaluated. The model-estimated GH-JRF followed the in-vivo measured force for dynamic tasks up to about 90° arm elevations, but generally underestimates the peak forces up to 31%; whereas a different behavior (ascending measured but descending estimated force) was found for angles above 90°. For the force tasks the model generally overestimated the peak GH-JRF for most directions (on average up to 34%). Applying the energy cost function improved model predictions for the dynamic anteflexion task (up to 9%) and for the force task (on average up to 23%). Scaling also led to improvement of the model predictions during the dynamic tasks (up to 26%), but had a negligible effect (<2%) on the force task results. Although results indicated a reasonable compatibility between model and measured data, adjustments will be necessary to individualize the generic model with the patient-specific characteristics.     

Simultaneous measurement of three-dimensional joint kinematics and ligament strains with optical methods

The objective of this study was to assess the precision and accuracy of a nonproprietary, optical three-dimensional (3D) motion analysis system for the simultaneous measurement of soft tissue strains and joint kinematics. The system consisted of two high-resolution digital cameras and software for calculating the 3D coordinates of contrast markers. System precision was assessed by examining the variation in the coordinates of static markers over time. Three-dimensional strain measurement accuracy was assessed by moving contrast markers fixed distances in the field of view and calculating the error in predicted strain. Three-dimensional accuracy for kinematic measurements was assessed by simulating the measurements that are required for recording knee kinematics. The field of view (190 mm) was chosen to allow simultaneous recording of markers for soft tissue strain measurement and knee joint kinematics. Average system precision was between +/-0.004 mm and +/-0.035 mm, depending on marker size and camera angle. Absolute error in strain measurement varied from a minimum of +/-0.025% to a maximum of +/-0.142%, depending on the angle between cameras and the direction of strain with respect to the camera axes. Kinematic accuracy for translations was between +/-0.008 mm and +/-0.034 mm, while rotational accuracy was +/-0.082 deg to +/-0.160 deg. These results demonstrate that simultaneous optical measurement of 3D soft tissue strain and 3D joint kinematics can be performed while achieving excellent accuracy for both sets of measurements.