Development and validation of a computed tomography-based methodology to measure carpal kinematics

Motion of the wrist bones is complicated and difficult to measure. Noninvasive measurement of carpal kinematics using medical images has become popular This technique is difficult and most investigators employ custom software. The objective of this paper is to describe a validated methodology for measuring carpal kinematics from computed tomography (CT) scans using commercial software. Four cadaveric wrists were CT imaged in neutral, full flexion, and full extension. A registration block was attached to the distal radius and used to align the data sets from each position. From the CT data, triangulated surface models of the radius, lunate, and capitate bones were generated using commercial software. The surface models from each wrist position were read into engineering design software that was used to calculate the centroid (position) and principal mass moments of inertia (orientation) of (1) the capitate and lunate relative to the fixed radius and (2) the capitate relative to the lunate. These data were used to calculate the helical axis kinematics for the motions from neutral to extension and neutral to flexion. The kinematics were plotted in three dimensions using a data visualization software package. The accuracy of the method was quantified in a separate set of experiments in which an isolated capitate bone was subjected to two different known rotation/translation motions for ten trials each. For comparison to in vivo techniques, the error in distal radius surface matching was determined using the block technique as a gold standard. The motion that the lunate and capitate underwent was half that of the overall wrist flexion-extension range of motion. Individually, the capitate relative to the lunate and the lunate relative to the radius generally flexed or extended about 30 deg, while the entire wrist (capitate relative to radius) typically flexed or extended about 60 deg. Helical axis translations were small, ranging from 0.6 mm to 1.8 mm across all motions. The accuracy of the method was found to be within 1.4 mm and 0.5 deg (95% confidence intervals). The mean error in distal radius surface matching was 2.4 mm and 1.2 deg compared to the use of a registration block. Carpal kinematics measured using the described methodology were accurate, reproducible, and similar to findings of previous investigators. The use of commercially available software should broaden the access of researchers interested in measuring carpal kinematics using medical imaging.     

A method for in-vivo kinematic analysis of the forearm

PURPOSE: To develop a method for in-vivo kinematic study of normal forearm rotation using computed tomographic (CT) images and a custom apparatus which allows for control of amount of forearm rotation. METHODS: The forearm of one asymptomatic volunteer was CT-scanned in five positions: neutral, 60 degrees pronation, maximal pronation, 60 degrees supination, and maximal supination. Surface registration of the pronated/supinated image datasets with the neutral position was performed. The resulting transformation matrices were decomposed into finite helical axis (FHA) parameters. Kinematics were expressed as motion of the radius relative to the ulna. RESULTS: The axes of the forearm passed through the volar region of the radial head at the proximal radioulnar joint (PRUJ), extending towards the dorsal region of the ulnar head at the distal radioulnar joint (DRUJ). Distinct FHAs were calculated for each forearm position analyzed relative to neutral rotation. Forearm pronation FHAs were different from forearm supination FHAs. CONCLUSIONS: Our experimental methodology is capable of describing the in-vivo kinematics of the forearm with good accuracy and reliability. Future in-vivo studies would need to be performed using a larger sample size to further validate our preliminary results. An ideal clinical application of this methodology would be in the comparative study of patients with forearm dysfunction.     

A novel methodology to reproduce previously recorded six-degree of freedom kinematics on the same diarthrodial joint

The objective of this study was to develop a novel method to more accurately reproduce previously recorded 6-DOF kinematics of the tibia with respect to the femur using robotic technology. Furthermore, the effect of performing only a single or multiple registrations and the effect of robot joint configuration were investigated. A single registration consisted of registering the tibia and femur with respect to the robot at full extension and reproducing all kinematics while multiple registrations consisted of registering the bones at each flexion angle and reproducing only the kinematics of the corresponding flexion angle. Kinematics of the knee in response to an anterior (134 N) and combined internal/external (+/-10 N m) and varus/valgus (+/-5 N m) loads were collected at 0 degrees , 15 degrees , 30 degrees , 60 degrees , and 90 degrees of flexion. A six axes, serial-articulated robotic manipulator (PUMA Model 762) was calibrated and the working volume was reduced to improve the robot's accuracy. The effect of the robot joint configuration was determined by performing single and multiple registrations for three selected configurations. For each robot joint configuration, the accuracy in position of the reproduced kinematics improved after multiple registrations (0.7+/-0.3, 1.2+/-0.5, and 0.9+/-0.2 mm, respectively) when compared to only a single registration (1.3+/-0.9, 2.0+/-1.0, and 1.5+/-0.7 mm, respectively) (p<0.05). The accuracy in position of each robot joint configuration was unique as significant differences were detected between each of the configurations. These data demonstrate that the number of registrations and the robot joint configuration both affect the accuracy of the reproduced kinematics. Therefore, when using robotic technology to reproduce previously recorded kinematics, it may be necessary to perform these analyses for each individual robotic system and for each diarthrodial joint, as different joints will require the robot to be placed in different robot joint configurations.     

Kinematic accuracy of three surface registration methods in a three-dimensional wrist bone study

The use of registration techniques to determine motion transformations noninvasively has become more widespread with the increased availability of the necessary software. In this study, three surface registration techniques were used to generate carpal bone kinematic results from a single cadaveric wrist specimen. Surface contours were extracted from specimen computed tomography volume images of the forearm, carpal, and metacarpal bones in four arbitrary positions. Kinematic results from each of three registration techniques were compared with results derived from multiple spherical markers fixed to the specimen. Kinematic accuracy was found to depend on the registration method and bone size and shape. In general, rotation errors of the capitate and scaphoid were less than 0.5 deg for all three techniques. Rotation errors for the other bones were generally less than 2 deg, although error for the trapezoid was greater than 2 deg in one technique. Translation errors of the bones were generally less than 1 mm, although errors of the trapezoid and trapezium were greater than 1 mm for two techniques. Tradeoffs existed in each registration method between image processing time and overall kinematic accuracy. Markerless bone registration (MBR) can provide accurate measurements of carpal kinematics and can be used to study the noninvasive, three-dimensional in vivo kinematics of the wrist and other skeletal joints.     

Bone alignment using the iterative closest point algorithm

Computer assisted surgical interventions and research in joint kinematics rely heavily on the accurate registration of three-dimensional bone surface models reconstructed from various imaging technologies. Anomalous results were seen in a kinematic study of carpal bones using a principal axes alignment approach for the registration. The study was repeated using an iterative closest point algorithm, which is more accurate, but also more demanding to apply. The principal axes method showed errors between 0.35 mm and 0.49 mm for the scaphoid, and between 0.40 mm and 1.22 mm for the pisiform. The iterative closest point method produced errors of less than 0.4 mm. These results show that while the principal axes method approached the accuracy of the iterative closest point algorithm in asymmetrical bones, there were more pronounced errors in bones with some symmetry. Principal axes registration for carpal bones should be avoided.     

A digital database of wrist bone anatomy and carpal kinematics

The skeletal wrist consists of eight small, intricately shaped carpal bones. The motion of these bones is complex, occurs in three dimensions, and remains incompletely defined. Our previous efforts have been focused on determining the in vivo three-dimensional (3-D) kinematics of the normal and abnormal carpus. In so doing we have developed an extensive database of carpal bone anatomy and kinematics from a large number of healthy subjects. The purpose of this paper is to describe that database and to make it available to other researchers. CT volume images of both wrists from 30 healthy volunteers (15 males and 15 females) were acquired in multiple wrist positions throughout the normal range of wrist motion. The outer cortical surfaces of the carpal bones, radius and ulna, and proximal metacarpals were segmented and the 3-D motion of each bone was calculated for each wrist position. The database was constructed to include high-resolution surface models, measures of bone volume and shape, and the 3-D kinematics of each segmented bone. The database does not include soft tissues of the wrist. While there are numerous digital anatomical databases, this one is unique in that it includes a large number of subjects and it contains in vivo kinematic data as well as the bony anatomy.     

MRI vs CT-based 2D-3D auto-registration accuracy for quantifying shoulder motion using biplane video-radiography

Biplane 2D-3D registration approaches have been used for measuring 3D, in vivo glenohumeral (GH) joint kinematics. Computed tomography (CT) has become the gold standard for reconstructing 3D bone models, as it provides high geometric accuracy and similar tissue contrast to video-radiography. Alternatively, magnetic resonance imaging (MRI) would not expose subjects to radiation and provides the ability to add cartilage and other soft tissues to the models. However, the accuracy of MRI-based 2D-3D registration for quantifying glenohumeral kinematics is unknown. We developed an automatic 2D-3D registration program that works with both CT- and MRI-based image volumes for quantifying joint motions. The purpose of this study was to use the proposed 2D-3D auto-registration algorithm to describe the humerus and scapula tracking accuracy of CT- and MRI-based registration relative to radiostereometric analysis (RSA) during dynamic biplanar video-radiography. The GH kinematic accuracy (RMS error) was 0.6–1.0 mm and 0.6–2.2° for the CT-based registration and 1.4–2.2 mm and 1.2–2.6° for MRI-based registration. Higher kinematic accuracy of CT-based registration was expected as MRI provides lower spatial resolution and bone contrast as compared to CT and suffers from spatial distortions. However, the MRI-based registration is within an acceptable accuracy for many clinical research questions.     

Registration of 6-DOFs electrogoniometry and CT medical imaging for 3D joint modeling

The paper describes a method in which two data-collecting systems, medical imaging and electrogoniometry, are combined to allow the accurate and simultaneous modeling of both the spatial kinematics and the morphological surface of a particular joint. The joint of interest (JOI) is attached to a Plexiglas jig that includes four metallic markers defining a local reference system (R(GONIO)) for the kinematics data. Volumetric data of the JOI and the R(GONIO) markers are collected from medical imaging. The spatial location and orientation of the markers in the global reference system (R(CT)) of the medical-imaging environment are obtained by applying object-recognition and classification methods on the image dataset. Segmentation and 3D isosurfacing of the JOI are performed to produce a 3D model including two anatomical objects-the proximal and distal JOI segments. After imaging, one end of a custom-made 3D electrogoniometer is attached to the distal segment of the JOI, and the other end is placed at the R(GONIO) origin; the JOI is displaced and the spatial kinematics data is recorded by the goniometer. After recording, data registration from R(GONIO) to R(CT) occurred prior to simulation. Data analysis was performed using both joint coordinate system (JCS) and instantaneous helical axis (IHA).Finally, the 3D joint model is simulated in real time using the experimental kinematics data. The system is integrated into a computer graphics interface, allowing free manipulation of the 3D scene.The overall accuracy of the method has been validated with two other kinematics data collection methods including a 3D digitizer and interpolation of the kinematics data from discrete positions obtained from medical imaging. Validation has been performed on both superior and inferior radio-ulna joints (i.e. prono-supination motion). Maximal RMS error was 1 degrees and 1.2mm on the helical axis rotation and translation, respectively. Prono-supination of the forearm showed a total rotation of 132 degrees for 0.8mm of translation. The method reproducibility using JCS parameters was in average 1 degrees (maximal deviation=2 degrees ) for rotation, and 1mm (maximal deviation=2mm) for translation. In vitro experiments have been performed on both knee joint and ankle joint. Averaged JCS parameters for the knee were 109 degrees, 17 degrees and 4 degrees for flexion, internal rotation and abduction, respectively. Averaged maximal translation values for the knee were 12, 3 and 4mm posteriorly, medially and proximally, respectively. Averaged JCS parameters for the ankle were 43 degrees, 9 degrees and 3 degrees for plantarflexion, adduction and internal rotation, respectively. Averaged maximal translation values for the ankle were 4, 2 and 1mm anteriorly, medially and proximally, respectively.     

Measuring humeral head translation using fluoroscopy: A validation study

Numerous techniques have been employed to monitor humeral head translation due to its involvement with several shoulder pathologies. However, most of the techniques were not validated. The objective of this study is to compare the accuracy of manual digitization and contour registration in measuring superior translation of the humeral head. Eight pairs of cadaver scapulae and humerii bones were harvested for this study. Each scapula and humerus was secured in a customized jig that allowed for control of humeral head translations and a vise that permitted rotations of the scapula about three axes. Fluoroscopy was used to take images of the shoulder bones. Scapular orientation was manipulated in different positions while the humerus was at 90° of humeral elevation in the scapular plane. Humeral head translation was measured using the two methods and was compared to the known translation. Additionally, accuracy of the contour registration method to measure 2-D scapular rotations was assessed. The range for the root mean square (RMS) error for manual digitization method was 0.27 mm - 0.43 mm and the contour registration method had a RMS error ranging from 0.18 mm - 0.40 mm. In addition, the RMS error for the scapular angle rotation using the contour registration method was 2.4°. Both methods showed acceptable errors. However, on average, the contour registration method showed lesser measurement error compared to the manual digitization method. In addition, the contour registration method was able to show good accuracy in measuring rotation that is useful in 2-D image analysis.     

Mandibular kinematics represented by a non-orthogonal floating axis joint coordinate system

There are many methods used to represent joint kinematics (e.g., roll, pitch, and yaw angles; instantaneous center of rotation; kinematic center; helical axis). Often in biomechanics internal landmarks are inferred from external landmarks. This study represents mandibular kinematics using a non-orthogonal floating axis joint coordinate system based on 3-D geometric models with parameters that are "clinician friendly" and mathematically rigorous. Kinematics data for two controls were acquired from passive fiducial markers attached to a custom dental clutch. The geometric models were constructed from MRI data. The superior point along the arc of the long axis of the condyle was used to define the coordinate axes. The kinematic data and geometric models were registered through fiducial markers visible during both protocols. The mean absolute maxima across the subjects for sagittal rotation, coronal rotation, axial rotation, medial-lateral translation, anterior-posterior translation, and inferior-superior translation were 34.10 degrees, 1.82 degrees, 1.14 degrees, 2.31, 21.07, and 6.95 mm, respectively. All the parameters, except for one subject's axial rotation, were reproducible across two motion recording sessions. There was a linear correlation between sagittal rotation and translation, the dominant motion plane, with approximately 1.5 degrees of rotation per millimeter of translation. The novel approach of combining the floating axis system with geometric models succinctly described mandibular kinematics with reproducible and clinician friendly parameters.     

Simulation of extension,radial and ulnar deviation of the wrist with a rigid body spring model

A novel computational model of the wrist that predicts carpal bone motion was developed in order to investigate the complex kinematics of the human wrist.This rigid body spring model (RBSM) of the wrist was built using surface models of the eight carpal bones, the bases of the five metacarpal bones, and the distal parts of the ulna and radius, all obtained from computed tomography (CT) scans of a cadaver upper limb. Elastic contact conditions between the rigid bodies modeled the influence of the cartilage layers, and ligamentous structures were constructed using nonlinear, tension-only spring elements. Motion of the wrist was simulated by applying forces to the tendons of the five main wrist muscles modeled.Three wrist motions were simulated: extension, ulnar deviation and radial deviation. The model was tested and tuned by comparing the simulated displacement and orientation of the carpal bones with previously obtained CT-scans of the same cadaver arm in deviated (45°ulnar and 15°radial), and extended (57°) wrist positions. Simulation results for the scaphoid, lunate, capitate, hamate and triquetrum are presented here and provide credible prediction of carpal bone movement. These are the first reported results of such a model. They indicate promise that this model will assist in future wrist kinematics investigations. However, further optimization and validation are required to define and guarantee the validity of results.     

A method for measurement of joint kinematics in vivo by registration of 3-D geometric models with cine phase contrast magnetic resonance imaging data

A new method is presented for measuring joint kinematics by optimally matching modeled trajectories of geometric surface models of bones with cine phase contrast (cine-PC) magnetic resonance imaging data. The incorporation of the geometric bone models (GBMs) allows computation of kinematics based on coordinate systems placed relative to full 3-D anatomy, as well as quantification of changes in articular contact locations and relative velocities during dynamic motion. These capabilities are additional to those of cine-PC based techniques that have been used previously to measure joint kinematics during activity. Cine-PC magnitude and velocity data are collected on a fixed image plane prescribed through a repetitively moved skeletal joint. The intersection of each GBM with a simulated image plane is calculated as the model moves along a computed trajectory, and cine-PC velocity data are sampled from the regions of the velocity images within the area of this intersection. From the sampled velocity data, the instantaneous linear and angular velocities of a coordinate system fixed to the GBM are estimated, and integration of the linear and angular velocities is used to predict updated trajectories. A moving validation phantom that produces motions and velocity data similar to those observed in an experiment on human knee kinematics was designed. This phantom was used to assess cine-PC rigid body tracking performance by comparing the kinematics of the phantom measured by this method to similar measurements made using a magnetic tracking system. Average differences between the two methods were measured as 2.82 mm rms for anterior/posterior tibial position, and 2.63 deg rms for axial rotation. An intertrial repeatability study of human knee kinematics using the new method produced rms differences in anterior/posterior tibial position and axial rotation of 1.44 mm and 2.35 deg. The performance of the method is concluded to be sufficient for the effective study of kinematic changes caused to knees by soft tissue injuries.     

In vivo kinematic behavior of the radio-capitate joint during wrist flexion-extension and radio-ulnar deviation.

The capitate is often considered the "keystone" of the carpus, not simply because of its central and prominent position in the wrist, but also because of its mechanical interactions with neighboring bones. The purpose of this study was to determine in vivo three-dimensional capitate kinematics. Twenty uninjured wrists were investigated using a recently developed, non-invasive markerless bone registration (MBR) technique. Surface contours of the capitate, third metacarpal and radius were extracted from computed tomography images of seven wrist positions and the three-dimensional motions of the capitate and third metacarpal were calculated with respect to the radius in wrist flexion-extension and radio-ulnar deviation. We found that in vivo capitate motion does not simply occur about a single pivot point like a universal joint, as demonstrated by non-intersecting rotation axes for different capitate motions. The distance between flexion and ulnar deviation axes was 3.9+/-2.0 mm, and the distance between extension and ulnar deviation axes was 3.9+/-1.4 mm. Furthermore, capitate axes for males tended to be located more distally than axes for females. However, we believe that this result is related to subject size and not to gender. We also found that there is minimal relative motion between the capitate and third metacarpal during these in vivo wrist motions. These findings demonstrate the complexity of capitate kinematics, as well as the different mechanisms through which wrist flexion, extension, radial deviation and ulnar deviation occur.     

Assessment of screw displacement axis accuracy and repeatability for joint kinematic description using an electromagnetic tracking device

Screw displacement axes (SDAs) have been employed to describe joint kinematics in biomechanical studies. Previous reports have investigated the accuracy of SDAs combining various motion analysis techniques and smoothing procedures. To our knowledge, no study has assessed SDA accuracy describing the relative movement between adjacent bodies with an electromagnetic tracking system. This is important, since in relative motion, neither body is fixed and consequently sensitivity to potential measurement errors from both bodies may be significant. Therefore, this study assessed the accuracy of SDAs for describing relative motion between two moving bodies. We analyzed numerical simulated data, and physical experimental data recorded using a precision jig and electromagnetic tracking device. The numerical simulations demonstrated SDA position accuracy (p=0.04) was superior for single compared to relative body motion, whereas orientation accuracy (p=0.2) was similar. Experimental data showed data-filtering (Butterworth filter) improved SDA position and orientation accuracies for rotation magnitudes smaller or equal to 5.0 degrees, with no effect at larger rotation magnitudes (p<0.05). This suggests that in absence of a filter, SDAs should only be calculated at rotations of greater than 5.0 degrees. For rotation magnitudes of 0.5 degrees (5.0 degrees ) about the SDA, SDA position and orientation error measurements determined from filtered experimental data were 3.75+/-0.30 mm (3.31+/-0.21 mm), and 1.10+/-0.04 degrees (1.04+/-0.03 degrees ), respectively. Experimental accuracy values describing the translation along and rotation about the SDA, were 0.06+/-0.00 mm and 0.09+/-0.01 degrees, respectively. These small errors establish the capability of SDAs to detect small translations, and rotations. In conclusion, application of SDAs should be a useful tool for describing relative motion in joint kinematic studies.     

Forelimb Kinematics of Rats Using XROMM,with Implications for Small Eutherians and Their Fossil Relatives

The earliest eutherian mammals were small-bodied locomotor generalists with a forelimb morphology that strongly resembles that of extant rats. Understanding the kinematics of the humerus, radius, and ulna of extant rats can inform and constrain hypotheses concerning typical posture and mobility in early eutherian forelimbs. The locomotion of Rattus norvegicus has been extensively studied, but the three-dimensional kinematics of the bones themselves remains under-explored. Here, for the first time, we use markerless XROMM (Scientific Rotoscoping) to explore the three-dimensional long bone movements in Rattus norvegicus during a normal, symmetrical gait (walking). Our data show a basic kinematic profile that agrees with previous studies on rats and other small therians: rats maintain a crouched forelimb posture throughout the step cycle, and the ulna is confined to flexion/extension in a parasagittal plane. However, our three-dimensional data illuminate long-axis rotation (LAR) movements for both the humerus and the radius for the first time. Medial LAR of the humerus throughout stance maintains an adducted elbow with a caudally-facing olecranon process, which in turn maintains a cranially-directed manus orientation (pronation). The radius also shows significant LAR correlated with manus pronation and supination. Moreover, we report that elbow flexion and manus orientation are correlated in R. norvegicus: as the elbow angle becomes more acute, manus supination increases. Our data also suggest that manus pronation and orientation in R. norvegicus rely on a divided system of labor between the ulna and radius. Given that the radius follows the flexion and extension trajectory of the ulna, it must rotate at the elbow (on the capitulum) so that during the stance phase its distal end lies medial to ulna, ensuring that the manus remains pronated while the forelimb is supporting the body. We suggest that forelimb posture and kinematics in Juramaia, Eomaia, and other basal eutherians were grossly similar to those of rats, and that humerus and radius LAR may have always played a significant role in forelimb and manus posture in small eutherian mammals.     

In vivo determination of normal and anterior cruciate ligament-deficient knee kinematics

The objective of the current study was to use fluoroscopy to accurately determine the three-dimensional (3D), in vivo, weight-bearing kinematics of 10 normal and five anterior cruciate ligament deficient (ACLD) knees. Patient-specific bone models were derived from computed tomography (CT) data. 3D computer bone models of each subject's femur, tibia, and fibula were recreated from the CT 3D bone density data. Using a model-based 3D-to-2D imaging technique registered CT images were precisely fit onto fluoroscopic images, the full six degrees of freedom motion of the bones was measured from the images. The computer-generated 3D models of each subject's femur and tibia were precisely registered to the 2D digital fluoroscopic images using an optimization algorithm that automatically adjusts the pose of the model at various flexion/extension angles. Each subject performed a weight-bearing deep knee bend while under dynamic fluoroscopic surveillance. All 10 normal knees experienced posterior femoral translation of the lateral condyle and minimal change in position of the medial condyle with progressive knee flexion. The average amount of posterior femoral translation of the lateral condyle was 21.07 mm, whereas the average medial condyle translation was 1.94 mm, in the posterior direction. In contrast, all five ACLD knees experienced considerable change in the position of the medial condyle. The average amount of posterior femoral translation of the lateral condyle was 17.00 mm, while the medial condyle translation was 4.65 mm, in the posterior direction. In addition, the helical axis of motion was determined between maximum flexion and extension. A considerable difference was found between the center of rotation locations of the normal and ACLD subjects, with ACLD subjects exhibiting substantially higher variance in kinematic patterns.     

Accuracy and precision of a method to study kinematics of the temporomandibular joint: combination of motion data and CT imaging

The purpose of the study was to test the precision and accuracy of a method used to track selected landmarks during motion of the temporomandibular joint (TMJ). A precision phantom device was constructed and relative motions between two rigid bodies on the phantom device were measured using optoelectronic (OE) and electromagnetic (EM) motion tracking devices. The motion recordings were also combined with a 3D CT image for each type of motion tracking system (EM+CT and OE+CT) to mimic methods used in previous studies. In the OE and EM data collections, specific landmarks on the rigid bodies were determined using digitization. In the EM+CT and OE+CT data sets, the landmark locations were obtained from the CT images. 3D linear distances and 3D curvilinear path distances were calculated for the points. The accuracy and precision for all 4 methods were evaluated (EM, OE, EM+CT and OE+CT). In addition, results were compared with and without the CT imaging (EM vs. EM+CT, OE vs. OE+CT). All systems overestimated the actual 3D curvilinear path lengths. All systems also underestimated the actual rotation values. The accuracy of all methods was within 0.5mm for 3D curvilinear path calculations, 0.05mm for 3D linear distance calculations and 0.2 degrees for rotation calculations. In addition, Bland-Altman plots for each configuration of the systems suggest that measurements obtained from either system are repeatable and comparable.     

μCT-generated carpal cartilage surfaces: validation of a technique

Computational models are increasingly being used for the analysis of kinematics and contact stresses in the wrist. To this point, however, the morphology of the carpal cartilage has been modeled simply, either with non-dimensional spring elements (in rigid body spring models) or via simple bone surface extrusions (e.g. for finite element models). In this work we describe an efficient method of generating high-resolution cartilage surfaces via micro-computed tomography (μCT) and registration to CT-generated bone surface models. The error associated with μCT imaging (at 10 μm) was 0.009 mm (95% confidence interval 0.007-0.012 mm ), or ~1.6% of the cartilage thickness. Registration error averaged 0.33±0.16 mm (97.5% confidence limit of ~0.55 mm in any one direction) and 2.42±1.56° (97.5% confidence limit of ~5.5° in any direction). The technique is immediately applicable to subject-specific models driven using kinematic data obtained through in vitro testing. However, the ultimate goal would be to generate a family of cartilage surfaces that could be scaled and/or morphed for application to models from live subjects and in vivo kinematic data.     

Three-dimensional computer simulation of radiostereometric analysis (RSA) in distal radius fractures

Physical phantom models have conventionally been used to determine the accuracy and precision of radiostereometric analysis (RSA) in various orthopaedic applications. Using a phantom model of a fracture of the distal radius it has previously been shown that RSA is a highly accurate and precise method for measuring both translation and rotation in three-dimensions (3-D). The main shortcoming of a physical phantom model is its inability to mimic complex 3-D motion. The goal of this study was to create a realistic computer model for preoperative planning of RSA studies and to test the accuracy of RSA in measuring complex movements in fractures of the distal radius using this new model. The 3-D computer model was created from a set of tomographic scans. The simulation of the radiographic imaging was performed using ray-tracing software (POV-Ray). RSA measurements were performed according to standard protocol. Using a two-part fracture model (AO/ASIF type A2), it was found that for simple movements in one axis, translations in the range of 25microm-2mm could be measured with an accuracy of +/-2microm. Rotations ranging from 16 degrees to 2 degrees could be measured with an accuracy of +/-0.015 degrees . Using a three-part fracture model the corresponding values of accuracy were found to be +/-4microm and +/-0.031 degrees for translation and rotation, respectively. For complex 3-D motion in a three-part fracture model (AO/ASIF type C1) the accuracy was +/-6microm for translation and +/-0.120 degrees for rotation. The use of 3-D computer modelling can provide a method for preoperative planning of RSA studies in complex fractures of the distal radius and in other clinical situations in which the RSA method is applicable.     

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.