Tibial and femoral articular cartilage exhibit opposite outcomes for T1ρ and T2* relaxation times in response to acute compressive loading in healthy knees.

Abnormal loading is thought to play a key role in the disease progression of cartilage, but our understanding of how cartilage compositional measurements respond to acute compressive loading in-vivo is limited. Ten healthy subjects were scanned at two timepoints (7 ± 3 days apart) with a 3 T magnetic resonance imaging (MRI) scanner. Scanning sessions included T1ρ and T2* acquisitions of each knee in two conditions: unloaded (traditional MRI setup) and loaded in compression at 40 % bodyweight as applied by an MRI-compatible loading device. T1ρ and T2* parameters were quantified for contacting cartilage (tibial and femoral) and non-contacting cartilage (posterior femoral condyle) regions. Significant effects of load were found in contacting regions for both T1ρ and T2*. The effect of load (loaded minus unloaded) in femoral contacting regions ranged from 4.1 to 6.9 ms for T1ρ, and 3.5 to 13.7 ms for T2*, whereas tibial contacting regions ranged from -5.6 to -1.7 ms for T1ρ, and -2.1 to 0.7 ms for T2*. Notably, the responses to load in the femoral and tibial cartilage revealed opposite effects. No significant differences were found in response to load between the two visits. This is the first study that analyzed the effects of acute loading on T1ρ and T2* measurements in human femoral and tibial cartilage separately. The results suggest the effect of acute compressive loading on T1ρ and T2* was: 1) opposite in the femoral and tibial cartilage; 2) larger in contacting regions than in non-contacting regions of the femoral cartilage; and 3) not different visit-to-visit.

Athlete Muscular Phenotypes Identified and Compared with High-Dimensional Clustering of Lower Limb Muscle Volume Measurements.

INTRODUCTION: Athletes use their skeletal muscles to demonstrate performance. Muscle force generating capacity is correlated with volume, meaning that variations in sizes of different muscles may be indicative of how athletes meet different demands in their sports. Medical imaging enables in vivo quantification of muscle volumes; however, muscle volume distribution has not been compared across athletes of different sports. PURPOSE: The goal of this work was to define "muscular phenotypes" in athletes of different sports and compare these using hierarchical clustering. METHODS: Muscle volumes normalized by body mass of athletes (football, baseball, basketball, or track) were compared with control participants to quantify size differences using z -scores. z -Scores of 35 muscles described the pattern of volume deviation within each athlete's lower limb, characterizing their muscular phenotype. Data-driven high-dimensional clustering analysis was used to group athletes presenting similar phenotypes. Efficacy of clustering to identify similar phenotypes was demonstrated by grouping athletes' contralateral limbs before other athletes' limbs. RESULTS: Analyses revealed that athletes did not tend to cluster with others competing in the same sport. Basketball players with similar phenotypes grouped by clustering also demonstrated similarities in performance. Clustering also identified muscles with similar volume variation patterns across athletes, and principal component analysis revealed specific muscles that accounted for most of the variance (gluteus maximus, sartorius, semitendinosus, vastus medialis, vastus lateralis, and rectus femoris). CONCLUSIONS: Athletes exhibit heterogeneous lower limb muscle volumes that can be characterized and compared as individual muscular phenotypes. Clustering revealed that athletes with the most similar phenotypes do not always play the same sport such that patterns of muscular heterogeneity across a group of athletes reflect factors beyond their specific sports.

Does interpolation and intra-user variability affect the accuracy of arthrokinematic measurements in the knee? A dual fluoroscopic imaging and model-based tracking study.

Model-based tracking (MBT) is a time-consuming and semiautomatic approach, and thus subject to errors during the tracking process. The present study aimed primarily to quantify the effects that interpolation and intra-user variability associated with MBT have on the kinematic and arthrokinematic measurements in comparison to a gold standard radiostereometric analysis (RSA). Cadaveric knee specimens were imaged at 125 Hz while simulating standing, walking, jogging, and lunging motions. (Arthro)kinematic metrics were calculated via MBT without interpolation, MBT with two interpolation techniques when every fifth or tenth frame was analyzed, and RSA. Tracking the same activity multiple times affected (p-value, largest mean difference) the flexion-extension (FE) joint angle during walking (0.03, 0.6°), and the internal-external joint angle during jogging (0.048, -0.9°). Only during jogging for the FE joint angle was there an effect of interpolation (0.046, 0.3°). Neither tracking multiple times nor interpolation affected arthrokinematic metrics (contact path locations and excursions). The present study is the first to quantify the effects that intra-user variability and interpolation have on the (arthro)kinematic measurement accuracy using MBT. Results suggest interpolation may be used without sacrificing (arthro)kinematic outcome measurement accuracy and the errors associated with intra-user variability, while small, were larger than errors due to interpolation.

Acetabular labrum and cartilage contact mechanics during pivoting and walking tasks in individuals with cam femoroacetabular impingement syndrome.

Femoroacetabular impingement syndrome (FAIS) is a motion-related pathology of the hip characterized by pain, morphological abnormalities of the proximal femur, and an elevated risk of joint deterioration and hip osteoarthritis. Activities that require deep flexion are understood to induce impingement in cam FAIS patients, however, less demanding activities such as walking and pivoting may induce pain as well as alterations in kinematics and joint stability. Still, the paucity of quantitative descriptions of cam FAIS has hindered understanding underlying hip joint mechanics during such activities. Previous in silico studies have employed generalized model geometry or kinematics to simulate impingement between the femur and acetabulum, which may not accurately capture the interplay between morphology and motion. In this study, we utilized models with participant-specific bone and articular soft tissue anatomy and kinematics measured by dual-fluoroscopy to compare hip contact mechanics of cam FAIS patients to controls during four activities of daily living (internal/external pivoting and level/incline walking). Averaged across the gait cycle during incline walking, patients displayed increased strain in the anterior joint (labrum strain: p-value = 0.038, patients: 11.7 ± 6.7 %, controls: 5.0 ± 3.6 %; cartilage strain: p-value = 0.029, patients: 9.1 ± 3.3 %, controls: 4.2 ± 2.3). Patients also exhibited increased average anterior cartilage strains during external pivoting (p-value = 0.039; patients: 13.0 ± 9.2 %, controls: 3.9 ± 3.2 %]). No significant differences between patient and control contact area and strain were found for level walking and internal pivoting. Our study provides new insights into the biomechanics of cam FAIS, including spatiotemporal hip joint contact mechanics during activities of daily living.

Patients with cam-type femoroacetabular impingement demonstrate increased change in bone-to-bone distance during walking: A dual fluoroscopy study.

Cam-type femoroacetabular impingement (FAI) syndrome is a painful, structural hip disorder. Herein, we investigated hip joint mechanics through in vivo, dynamic measurement of the bone-to-bone distance between the femoral head and acetabulum in patients with cam FAI syndrome and morphologically screened controls. We hypothesized that individuals with cam FAI syndrome would have larger changes in bone-to-bone distance compared to the control group, which we would interpret as altered joint mechanics as signified by greater movement of the femoral head as it articulates within the acetabulum. Seven patients with cam FAI syndrome and 11 asymptomatic individuals with typical morphology underwent dual fluoroscopy imaging during level and inclined walking (upward slope). The change in bone-to-bone distance between femoral and acetabular bone surfaces was evaluated for five anatomical regions of the acetabulum at each timepoint of gait. Linear regression analysis of the bone-to-bone distance considered two within-subject factors (activity and region) and one between-subjects factor (group). Across activities, the change in minimum bone-to-bone distance was 1.38-2.54 mm for the cam FAI group and 1.16-1.84 mm for controls. In all regions except the anterior-superior region, the change in bone-to-bone distance was larger in the cam group than the control group (p ≤ 0.024). An effect of activity was detected only in the posterior-superior region where larger changes were noted during level walking than incline walking. Statement of clinical significance: Patients with cam FAI syndrome exhibit altered hip joint mechanics during the low-demand activity of walking; these alterations could affect load transmission, and contribute to pain, tissue damage, and osteoarthritis.

Articular cartilage thickness changes differ between males and females 4 years following anterior cruciate ligament reconstruction.

Anterior cruciate ligament injury and reconstruction (ACLR) affects articular cartilage thickness profiles about the tibial, femoral, and patellar surfaces; however, it's unclear whether the magnitudes of change in cartilage thickness, as well as the locations and areas over which these changes occur, differ between males and females. This is important to consider as differences exist between the sexes with regard to knee biomechanics, patellofemoral pain, and anatomic alignment, which influence risk of an index and repeated injury. Subjects underwent ACLR with a bone-patella tendon-bone autograft. At 4-year follow-up, they had asymptomatic knees; however, significant ACL injured-to-contralateral normal knee differences in articular cartilage thickness values were observed. Both thickening and thinning of cartilage occurred about the tibiofemoral and patellofemoral joints, relative to matched control subjects with normal knees. Further, the location of the areas and magnitudes of thickening and thinning were different between females and males. Thickening (swelling) of articular cartilage is an early finding associated with the onset of posttraumatic osteoarthritis (PTOA). Therefore, the increases in cartilage thickness that were observed in this cohort may represent early signs of the onset of PTOA that occur prior to the patient developing symptoms and radiographic evidence of this disease. The different locations of areas that underwent a change in cartilage thicknesses between males and females suggest that each sex responds differently to knee ligament trauma, reconstruction, rehabilitation, and return to activity, and indicates that sex-specific analysis should be utilized in studies of PTOA.

Wearables-Only Analysis of Muscle and Joint Mechanics: An EMG-Driven Approach.

Complex sensor arrays prohibit practical deployment of existing wearables-based algorithms for free-living analysis of muscle and joint mechanics. Machine learning techniques have been proposed as a potential solution, however, they are less interpretable and generalizable when compared to physics-based techniques. Herein, we propose a hybrid method utilizing inertial sensor- and electromyography (EMG)-driven simulation of muscle contraction to characterize knee joint and muscle mechanics during walking gait. Machine learning is used only to map a subset of measured muscle excitations to a full set thereby reducing the number of required sensors. We demonstrate the utility of the approach for estimating net knee flexion moment (KFM) as well as individual muscle moment and work during the stance phase of gait across nine unimpaired subjects. Across all subjects, KFM was estimated with 0.91%BW•H RMSE and strong correlations (r = 0.87) compared to ground truth inverse dynamics analysis. Estimates of individual muscle moments were strongly correlated (r = 0.81-0.99) with a reference EMG-driven technique using optical motion capture and a full set of electrodes as were estimates of muscle work (r = 0.88-0.99). Implementation of the proposed technique in the current work included instrumenting only three muscles with surface electrodes (lateral and medial gastrocnemius and vastus medialis) and both the thigh and shank segments with inertial sensors. These sensor locations permit instrumentation of a knee brace/sleeve facilitating a practically deployable mechanism for monitoring muscle and joint mechanics with performance comparable to the current state-of-the-art.

In Vivo Quantification of Hip Arthrokinematics during Dynamic Weight-bearing Activities using Dual Fluoroscopy.

Several hip pathologies have been attributed to abnormal morphology with an underlying assumption of aberrant biomechanics. However, structure-function relationships at the joint level remain challenging to quantify due to difficulties in accurately measuring dynamic joint motion. The soft tissue artifact errors inherent in optical skin marker motion capture are exacerbated by the depth of the hip joint within the body and the large mass of soft tissue surrounding the joint. Thus, the complex relationship between bone shape and hip joint kinematics is more difficult to study accurately than in other joints. Herein, a protocol incorporating computed tomography (CT) arthrography, three-dimensional (3D) reconstruction of volumetric images, dual fluoroscopy, and optical motion capture to accurately measure the dynamic motion of the hip joint is presented. The technical and clinical studies that have applied dual fluoroscopy to study form-function relationships of the hip using this protocol are summarized, and the specific steps and future considerations for data acquisition, processing, and analysis are described.

Soft tissue artifact causes underestimation of hip joint kinematics and kinetics in a rigid-body musculoskeletal model.

Rigid body musculoskeletal models have been applied to study kinematics, moments, muscle forces, and joint reaction forces in the hip. Most often, models are driven with segment motions calculated through optical tracking of markers adhered to the skin. One limitation of optical tracking is soft tissue artifact (STA), which occurs due to motion of the skin surface relative to the underlying skeleton. The purpose of this study was to quantify differences in musculoskeletal model outputs when tracking body segment positions with skin markers as compared to bony landmarks measured by direct imaging of bone motion with dual fluoroscopy (DF). Eleven asymptomatic participants with normally developed hip anatomy were imaged with DF during level treadmill walking at a self-selected speed. Hip joint kinematics and kinetics were generated using inverse kinematics, inverse dynamics, static optimization and joint reaction force analysis. The effect of STA was assessed by comparing the difference in estimates from simulations based on skin marker positions (SM) versus virtual markers on bony landmarks from DF. While patterns were similar, STA caused underestimation of kinematics, range of motion (ROM), moments, and reaction forces at the hip, including flexion-extension ROM, maximum internal rotation joint moment and peak joint reaction force magnitude. Still, kinetic differences were relatively small, and thus they may not be relevant nor clinically meaningful.

Combined Injury to the ACL and Lateral Meniscus Alters the Geometry of Articular Cartilage and Meniscus Soon After Initial Trauma.

Combined injury to the anterior cruciate ligament (ACL) and meniscus is associated with earlier onset and increased rates of post-traumatic osteoarthritis compared with isolated ACL injury. However, little is known about the initial changes in joint structure associated with these different types of trauma. We hypothesized that trauma to the ACL and lateral meniscus has an immediate effect on morphometry of the articular cartilage and meniscus about the entire tibial plateau that is more pronounced than an ACL tear without meniscus injury. Subjects underwent magnetic resonance imaging scanning soon after injury and prior to surgery. Those that suffered injury to the ACL and lateral meniscus underwent changes in the lateral compartment (increases in the posterior-inferior directed slopes of the articular cartilage surface, and the wedge angle of the posterior horn of the meniscus) and medial compartment (the cartilage-to-bone height decreased in the region located under the posterior horn of the meniscus, and the thickness of cartilage increased and decreased in the mid and posterior regions of the plateau, respectively). Subjects that suffered an isolated ACL tear did not undergo the same magnitude of change to these articular structures. A majority of the changes in morphometry occurred in the lateral compartment of the knee; however, change in the medial compartment of the knee with a normal appearing meniscus also occurred. Statement of clinical significance: Knee injuries that involve combined trauma to the ACL and meniscus directly affect both compartments of the knee, even if the meniscus and articular cartilage appears normal upon arthroscopic examination. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:759-767, 2020.

In Vivo Pelvic and Hip Joint Kinematics in Patients With Cam Femoroacetabular Impingement Syndrome: A Dual Fluoroscopy Study.

Femoroacetabular impingement syndrome (FAIS) may alter the kinematic function of the hip, resulting in pain and tissue damage. Previous motion analysis studies of FAIS have employed skin markers, which are prone to soft tissue artifact and inaccurate calculation of the hip joint center. This may explain why the evidence linking FAIS with deleterious kinematics is contradictory. The purpose of this study was to employ dual fluoroscopy (DF) to quantify in vivo kinematics of patients with cam FAIS relative to asymptomatic, morphologically normal control participants during various activities. Eleven asymptomatic, morphologically normal controls and seven patients with cam FAIS were imaged with DF during standing, level walking, incline walking, and functional range of motion activities. Model-based tracking calculated the kinematic position of the hip by registering projections of three-dimensional computed tomography models with DF images. Patients with FAIS stood with their hip extended (mean [95% confidence interval], -2.2 [-7.4, 3.1]°, flexion positive), whereas controls were flexed (5.3 [2.6, 8.0]°; p = 0.013). Male patients with cam FAIS had less peak internal rotation than the male control participants during self-selected speed level-walking (-0.2 [-6.5, 6.1]° vs. -9.8 [-12.2, -7.3]°; p = 0.007) and less anterior pelvic tilt at heel-strike of incline (5°) walking (3.4 [-1.0, -7.9]° vs. 9.8 [6.4, 13.2]°; p = 0.032). Even during submaximal range of motion activities, such as incline walking, patients may alter pelvic motion to avoid positions that approximate the cam lesion and the acetabular labrum. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:823-833, 2020.

Validation of Novel Relative Orientation and Inertial Sensor-to-Segment Alignment Algorithms for Estimating 3D Hip Joint Angles.

Wearable sensor-based algorithms for estimating joint angles have seen great improvements in recent years. While the knee joint has garnered most of the attention in this area, algorithms for estimating hip joint angles are less available. Herein, we propose and validate a novel algorithm for this purpose with innovations in sensor-to-sensor orientation and sensor-to-segment alignment. The proposed approach is robust to sensor placement and does not require specific calibration motions. The accuracy of the proposed approach is established relative to optical motion capture and compared to existing methods for estimating relative orientation, hip joint angles, and range of motion (ROM) during a task designed to exercise the full hip range of motion (ROM) and fast walking using root mean square error (RMSE) and regression analysis. The RMSE of the proposed approach was less than that for existing methods when estimating sensor orientation ( 12 . 32 ∘ and 11 . 82 ∘ vs. 24 . 61 ∘ and 23 . 76 ∘ ) and flexion/extension joint angles ( 7 . 88 ∘ and 8 . 62 ∘ vs. 14 . 14 ∘ and 15 . 64 ∘ ). Also, ROM estimation error was less than 2 . 2 ∘ during the walking trial using the proposed method. These results suggest the proposed approach presents an improvement to existing methods and provides a promising technique for remote monitoring of hip joint angles.

Hip rotation during standing and dynamic activities and the compensatory effect of femoral anteversion: An in-vivo analysis of asymptomatic young adults using three-dimensional computed tomography models and dual fluoroscopy.

BACKGROUND: Individuals are thought to compensate for femoral anteversion by altering hip rotation. However, the relationship between hip rotation in a neutral position (i.e. static rotation) and dynamic hip rotation is poorly understood, as is the relationship between anteversion and hip rotation. RESEARCH OBJECTIVE: Herein, anteversion and in-vivo hip rotation during standing, walking, and pivoting were measured in eleven asymptomatic, morphologically normal, young adults using three-dimensional computed tomography models and dual fluoroscopy. METHODS: Using correlation analyses, we: 1) determined the relationship between hip rotation in the static position to that measured during dynamic activities, and 2) evaluated the association between femoral anteversion and hip rotation during dynamic activities. Hip rotation was calculated while standing (static-rotation), throughout gait, as a mean during gait (mean gait rotation), and as a mean (mid-pivot rotation), maximum (max-rotation) and minimum (min-rotation) during pivoting. RESULTS: Static-rotation (mean ±â€¯standard deviation; 11.3°â€¯±â€¯7.3°) and mean gait rotation (7.8°â€¯±â€¯4.7°) were positively correlated (r = 0.679, p = 0.022). Likewise, static-rotation was strongly correlated with mid-pivot rotation (r = 0.837, p = 0.001), max-rotation (r = 0.754, p = 0.007), and min-rotation (r = 0.835, p = 0.001). Strong positive correlations were found between anteversion and hip internal rotation during all of the stance phase (0-60% gait) and during mid- and terminal-swing (86-100% gait) (all r > 0.607, p < 0.05). CONCLUSIONS: Our results suggest that the static position may be used cautiously to express the neutral rotational position of the femur for dynamic movements. Further, our results indicate that femoral anteversion is compensated for by altering hip rotation. As such, both anteversion and hip rotation may be important to consider when diagnosing hip pathology and planning for surgical procedures.

In Vivo Measurements of the Ischiofemoral Space in Recreationally Active Participants During Dynamic Activities: A High-Speed Dual Fluoroscopy Study.

BACKGROUND: Ischiofemoral impingement (IFI) is a dynamic process, but its diagnosis is often based on static, supine images. PURPOSE: To couple 3-dimensional (3D) computed tomography (CT) models with dual fluoroscopy (DF) images to quantify in vivo hip motion and the ischiofemoral space (IFS) in asymptomatic participants during weightbearing activities and evaluate the relationship of dynamic measurements with sex, hip kinematics, and the IFS measured from axial magnetic resonance imaging (MRI). STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: Eleven young, asymptomatic adults (5 female) were recruited. 3D reconstructions of the femur and pelvis were generated from MRI and CT. The axial and 3D IFS were measured from supine MRI. In vivo hip motion during weightbearing activities was quantified using DF. The bone-to-bone distance between the lesser trochanter and ischium was measured dynamically. The minimum and maximum IFS were determined and evaluated against hip joint angles using a linear mixed-effects model. RESULTS: The minimum IFS occurred during external rotation for 10 of 11 participants. The IFS measured from axial MRI (mean, 23.7 mm [95% CI, 19.9-27.9]) was significantly greater than the minimum IFS observed during external rotation (mean, 10.8 mm [95% CI, 8.3-13.7]; P < .001), level walking (mean, 15.5 mm [95% CI, 11.4-19.7]; P = .007), and incline walking (mean, 15.8 mm [95% CI, 11.6-20.1]; P = .004) but not for standing. The IFS was reduced with extension (ß = 0.66), adduction (ß = 0.22), and external rotation (ß = 0.21) ( P < .001 for all) during the dynamic activities observed. The IFS was smaller in female than male participants for standing (mean, 20.9 mm [95% CI, 19.3-22.3] vs 30.4 mm [95% CI, 27.2-33.8], respectively; P = .034), level walking (mean, 8.8 mm [95% CI, 7.5-9.9] vs 21.1 mm [95% CI, 18.7-23.6], respectively; P = .001), and incline walking (mean, 9.1 mm [95% CI, 7.4-10.8] vs 21.3 mm [95% CI, 18.8-24.1], respectively; P = .003). Joint angles between the sexes were not significantly different for any of the dynamic positions of interest. CONCLUSION: The minimum IFS during dynamic activities was smaller than axial MRI measurements. Compared with male participants, the IFS in female participants was reduced during standing and walking, despite a lack of kinematic differences between the sexes. The relationship between the IFS and hip joint angles suggests that the hip should be placed into greater extension, adduction, and external rotation in clinical examinations and imaging, as the IFS measured from static images, especially in a neutral orientation, may not accurately represent the minimum IFS during dynamic motion. Nevertheless, this statement must be interpreted with caution, as only asymptomatic participants were analyzed herein.

Subject-Specific Axes of Rotation Based on Talar Morphology Do Not Improve Predictions of Tibiotalar and Subtalar Joint Kinematics.

Use of subject-specific axes of rotation may improve predictions generated by kinematic models, especially for joints with complex anatomy, such as the tibiotalar and subtalar joints of the ankle. The objective of this study was twofold. First, we compared the axes of rotation between generic and subject-specific ankle models for ten control subjects. Second, we quantified the accuracy of generic and subject-specific models for predicting tibiotalar and subtalar joint motion during level walking using inverse kinematics. Here, tibiotalar and subtalar joint kinematics measured in vivo by dual-fluoroscopy served as the reference standard. The generic model was based on a cadaver study, while the subject-specific models were derived from each subject's talus reconstructed from computed tomography images. The subject-specific and generic axes of rotation were significantly different. The average angle between the modeled axes was 12.9° ± 4.3° and 24.4° ± 5.9° at the tibiotalar and subtalar joints, respectively. However, predictions from both models did not agree well with dynamic dual-fluoroscopy data, where errors ranged from 1.0° to 8.9° and 0.6° to 7.6° for the generic and subject-specific models, respectively. Our results suggest that methods that rely on talar morphology to define subject-specific axes may be inadequate for accurately predicting tibiotalar and subtalar joint kinematics.

Soft tissue artifact causes significant errors in the calculation of joint angles and range of motion at the hip.

Soft tissue movement between reflective skin markers and underlying bone induces errors in gait analysis. These errors are known as soft tissue artifact (STA). Prior studies have not examined how STA affects hip joint angles and range of motion (ROM) during dynamic activities. Herein, we: 1) measured STA of skin markers on the pelvis and thigh during walking, hip abduction and hip rotation, 2) quantified errors in tracking the thigh, pelvis and hip joint angles/ROM, and 3) determined whether model constraints on hip joint degrees of freedom mitigated errors. Eleven asymptomatic young adults were imaged simultaneously with retroreflective skin markers (SM) and dual fluoroscopy (DF), an X-ray technique with sub-millimeter and sub-degree accuracy. STA, defined as the range of SM positions in the DF-measured bone anatomical frame, varied based on marker location, activity and subject. Considering all skin markers and activities, mean STA ranged from 0.3cm to 5.4cm. STA caused the hip joint angle tracked with SM to be 1.9° more extended, 0.6° more adducted, and 5.8° more internally rotated than the hip tracked with DF. ROM was reduced for SM measurements relative to DF, with the largest difference of 21.8° about the internal-external axis during hip rotation. Constraining the model did not consistently reduce angle errors. Our results indicate STA causes substantial errors, particularly for markers tracking the femur and during hip internal-external rotation. This study establishes the need for future research to develop methods minimizing STA of markers on the thigh and pelvis.

In-vivo quantification of dynamic hip joint center errors and soft tissue artifact.

Hip joint center (HJC) measurement error can adversely affect predictions from biomechanical models. Soft tissue artifact (STA) may exacerbate HJC errors during dynamic motions. We quantified HJC error and the effect of STA in 11 young, asymptomatic adults during six activities. Subjects were imaged simultaneously with reflective skin markers (SM) and dual fluoroscopy (DF), an x-ray based technique with submillimeter accuracy that does not suffer from STA. Five HJCs were defined from locations of SM using three predictive (i.e., based on regression) and two functional methods; these calculations were repeated using the DF solutions. Hip joint center motion was analyzed during six degrees-of-freedom (default) and three degrees-of-freedom hip joint kinematics. The position of the DF-measured femoral head center (FHC), served as the reference to calculate HJC error. The effect of STA was quantified with mean absolute deviation. HJC errors were (mean±SD) 16.6±8.4mm and 11.7±11.0mm using SM and DF solutions, respectively. HJC errors from SM measurements were all significantly different from the FHC in at least one anatomical direction during multiple activities. The mean absolute deviation of SM-based HJCs was 2.8±0.7mm, which was greater than that for the FHC (0.6±0.1mm), suggesting that STA caused approximately 2.2mm of spurious HJC motion. Constraining the hip joint to three degrees-of-freedom led to approximately 3.1mm of spurious HJC motion. Our results indicate that STA-induced motion of the HJC contributes to the overall error, but inaccuracies inherent with predictive and functional methods appear to be a larger source of error.

Predicting tibiotalar and subtalar joint angles from skin-marker data with dual-fluoroscopy as a reference standard.

Evidence suggests that the tibiotalar and subtalar joints provide near six degree-of-freedom (DOF) motion. Yet, kinematic models frequently assume one DOF at each of these joints. In this study, we quantified the accuracy of kinematic models to predict joint angles at the tibiotalar and subtalar joints from skin-marker data. Models included 1 or 3 DOF at each joint. Ten asymptomatic subjects, screened for deformities, performed 1.0m/s treadmill walking and a balanced, single-leg heel-rise. Tibiotalar and subtalar joint angles calculated by inverse kinematics for the 1 and 3 DOF models were compared to those measured directly in vivo using dual-fluoroscopy. Results demonstrated that, for each activity, the average error in tibiotalar joint angles predicted by the 1 DOF model were significantly smaller than those predicted by the 3 DOF model for inversion/eversion and internal/external rotation. In contrast, neither model consistently demonstrated smaller errors when predicting subtalar joint angles. Additionally, neither model could accurately predict discrete angles for the tibiotalar and subtalar joints on a per-subject basis. Differences between model predictions and dual-fluoroscopy measurements were highly variable across subjects, with joint angle errors in at least one rotation direction surpassing 10° for 9 out of 10 subjects. Our results suggest that both the 1 and 3 DOF models can predict trends in tibiotalar joint angles on a limited basis. However, as currently implemented, neither model can predict discrete tibiotalar or subtalar joint angles for individual subjects. Inclusion of subject-specific attributes may improve the accuracy of these models.

In Vivo Kinematics of the Tibiotalar and Subtalar Joints in Asymptomatic Subjects: A High-Speed Dual Fluoroscopy Study.

Measurements of joint kinematics are essential to understand the pathomechanics of ankle disease and the effects of treatment. Traditional motion capture techniques do not provide measurements of independent tibiotalar and subtalar joint motion. In this study, high-speed dual fluoroscopy images of ten asymptomatic adults were acquired during treadmill walking at 0.5 m/s and 1.0 m/s and a single-leg, balanced heel-rise. Three-dimensional (3D) CT models of each bone and dual fluoroscopy images were used to quantify in vivo kinematics for the tibiotalar and subtalar joints. Dynamic tibiotalar and subtalar mean joint angles often exhibited opposing trends during captured stance. During both speeds of walking, the tibiotalar joint had significantly greater dorsi/plantarflexion (D/P) angular ROM than the subtalar joint while the subtalar joint demonstrated greater inversion/eversion (In/Ev) and internal/external rotation (IR/ER) than the tibiotalar joint. During balanced heel-rise, only D/P and In/Ev were significantly different between the tibiotalar and subtalar joints. Translational ROM in the anterior/posterior (AP) direction was significantly greater in the subtalar than the tibiotalar joint during walking at 0.5 m/s. Overall, our results support the long-held belief that the tibiotalar joint is primarily responsible for D/P, while the subtalar joint facilitates In/Ev and IR/ER. However, the subtalar joint provided considerable D/P rotation, and the tibiotalar joint rotated about all three axes, which, along with translational motion, suggests that each joint undergoes complex, 3D motion.

Accuracy of Functional and Predictive Methods to Calculate the Hip Joint Center in Young Non-pathologic Asymptomatic Adults with Dual Fluoroscopy as a Reference Standard.

Predictions from biomechanical models of gait may be sensitive to joint center locations. Most often, the hip joint center (HJC) is derived from locations of reflective markers adhered to the skin. Here, predictive techniques use regression equations of pelvic anatomy to estimate the HJC, whereas functional methods track motion of markers placed at the pelvis and femur during a coordinated motion. Skin motion artifact may introduce errors in the estimate of HJC for both techniques. Quantifying the accuracy of these methods is an area of open investigation. In this study, we used dual fluoroscopy (DF) (a dynamic X-ray imaging technique) and three-dimensional reconstructions from computed tomography images, to measure HJC locations in vivo. Using dual fluoroscopy as the reference standard, we then assessed the accuracy of three predictive and two functional methods. Eleven non-pathologic subjects were imaged with DF and reflective skin marker motion capture. Additionally, DF-based solutions generated virtual markers placed on bony landmarks, which were input to the predictive and functional methods to determine if estimates of the HJC improved. Using skin markers, functional methods had better mean agreement with the HJC measured by DF (11.0 ± 3.3 mm) than predictive methods (18.1 ± 9.5 mm); estimates from functional and predictive methods improved when using the DF-based solutions (1.3 ± 0.9 and 17.5 ± 8.6 mm, respectively). The Harrington method was the best predictive technique using both skin markers (13.2 ± 6.5 mm) and DF-based solutions (10.6 ± 2.5 mm). The two functional methods had similar accuracy using skin makers (11.1 ± 3.6 and 10.8 ± 3.2 mm) and DF-based solutions (1.2 ± 0.8 and 1.4 ± 1.0 mm). Overall, functional methods were superior to predictive methods for HJC estimation. However, the improvements observed when using the DF-based solutions suggest that skin motion artifact is a large source of error for the functional methods.