Liying Zheng

The inaccuracy of surface-measured model-derived tibiofemoral kinematics

This study assessed the accuracy of surface-measured OpenSim-derived tibiofemoral kinematics in functional activities. Ten subjects with unilateral, isolated grade II PCL deficiency performed level running and stair ascent. A dynamic stereo radiography (DSX) system and a Vicon motion capture system simultaneously measured their knee or lower extremity movement. Surface marker motion data from the Vicon system were used to create subject-specific models in OpenSim and derive the tibiofemoral kinematics. The surface-measured model-derived tibiofemoral kinematics in all six degrees of freedom (DOFs) were then compared with those measured by the DSX as the benchmarks. The differences between surface- and DSX-measured tibiofemoral kinematics were found to be substantial: the overall mean (±SD) RMS differences during running were 9.1±3.2°, 2.0±1.2°, and 6.4±4.5° for the flexion-extension, abduction-adduction, and internal-external rotations, respectively, and 7.1±3.2 mm, 8.8±3.7 mm, and 1.9±1.2 mm for anterior-posterior, proximal-distal, and medial-lateral translations, respectively. The differences were more pronounced in relatively higher speed running than in stair ascent. It was also found that surface-based measures significantly underestimated the mean as well as inter-subject variability of the differences between PCL-injured and intact knees in abduction-adduction, internal-external rotations, and anterior-posterior translation.

Sagittal plane kinematics of the adult hyoid bone

The hyoid bone is a unique bone in the skeleton not articulated to any other bone. The hyoid muscles, which attach to the hyoid bone, may play a role in neck mechanics, but analysis of their function requires quantifying hyoid bone mechanics. The goal of this study was to obtain the detailed kinematics of the hyoid bone over a large range of flexion-extension motion using radiographs at 5 postures. The position of the hyoid bone in the sagittal plane was characterized with respect to head, jaw, and vertebral movements. Sex differences in hyoid kinematics were also investigated. We hypothesized that (1) the position of the hyoid bone in the sagittal plane is linearly correlated with motion of the head, jaw, and vertebrae, and (2) the hyoid position, size, and kinematics are sex-specific. We found that the hyoid bone X, Y, and angular position generally had strong linear correlations with the positions of the head, jaw, and the cervical vertebrae C1-C4. Hyoid X and angular position was also correlated to C5. Sex differences were found in some regressions of the hyoid bone with respect to C1-C5. The angular and linear measurements of the hyoid bone showed sex differences in absolute values, which were not evident after normalization by posture or neck size. Incorporating these results to neck models would enable accurate modeling of the hyoid muscles. This may have implications for analyzing the mechanics of the cervical spine, including loads on neck structures and implants.

Integrating dynamic stereo-radiography and surface-based motion data for subject-specific musculoskeletal dynamic modeling

This manuscript presents a new subject-specific musculoskeletal dynamic modeling approach that integrates high-accuracy dynamic stereo-radiography (DSX) joint kinematics and surface-based full-body motion data. We illustrate this approach by building a model in OpenSim for a patient who participated in a meniscus transplantation efficacy study, incorporating DSX data of the tibiofemoral joint kinematics. We compared this DSX-incorporated (DSXI) model to a default OpenSim model built using surface-measured data alone. The architectures and parameters of the two models were identical, while the differences in (time-averaged) tibiofemoral kinematics were of the order of magnitude of 10° in rotation and 10mm in translation. Model-predicted tibiofemoral compressive forces and knee muscle activations were compared against literature data acquired from instrumented total knee replacement components (Fregly et al., 2012) and the patients EMG recording. The comparison demonstrated that the incorporation of DSX data improves the veracity of musculoskeletal dynamic modeling.

Apportionment of lumbar L2–S1 rotation across individual motion segments during a dynamic lifting task

Segmental apportionment of lumbar (L2-S1) rotation is a critical input parameter for musculoskeletal models and a candidate metric for clinical assessment of spinal health, but such data are sparse. This paper aims to quantify the time-variant and load-dependent characteristics of intervertebral contributions to L2-S1 extension during a dynamic lifting task. Eleven healthy participants lifted multiple weights (4.5, 9.1, and 13.6 kg) from a trunk-flexed to an upright position while being imaged by a dynamic stereo X-ray system at 30 frames/s. Vertebral (L2-S1) motion was tracked using a previously validated volumetric model-based tracking method that employs 3D bone models reconstructed from subject-specific CT images to obtain high-accuracy (≤0.26°, 0.2 mm) 3D vertebral kinematics. Individual intervertebral motions as percentages of the total L2-S1 extension were computed at each % increment of the motion to show the segmental apportionment. Results showed L3-L4 (25.8±2.2%) and L4-L5 (31±3.1%) together contributed a larger share (∼60% combined) compared to L2-L3 (21.7±3.7%) and L5-S1 (22.6±4.7%); L4-L5 consistently provided the largest contribution of the measured segments. Relative changes over time in L3-L4 (6±12.5%) and L4-L5 (0.5±10.2%) contribution were minimal; in contrast, L2-L3 (18±20.1%) contribution increased while L5-S1 (-33±22.9%) contribution decreased in a somewhat complementary fashion as motion progressed. No significant effect of the magnitude of load lifted on individual segmental contribution patterns was detected. The current study updated the knowledge regarding apportionment of lumbar (L2-S1) motion among individual segments, serving both as input into musculoskeletal models and as potential biomechanical markers of low back disorders.

Sex-specific prediction of neck muscle volumes.

Biomechanical analyses of the head and neck system require knowledge of neck muscle forces, which are often estimated from neck muscle volumes. Here we use magnetic resonance images (MRIs) of 17 subjects (6 females, 11 males) to develop a method to predict the volumes of 16 neck muscles by first predicting the total neck muscle volume (TMV) from subject sex and anthropometry, and then predicting individual neck muscle volumes using fixed volume proportions for each neck muscle. We hypothesized that the regression equations for total muscle volume as well as individual muscle volume proportions would be sex specific. We found that females have 59% lower TMV compared to males (females: 510±43cm(3), males: 814±64cm(3); p<0.0001) and that TMV (in cm(3)) was best predicted by a regression equation that included sex (male=0, female=1) and neck circumference (NC, in cm): TMV=269+13.7NC-233Sex (adjusted R(2)=0.868; p<0.01). Individual muscle volume proportions were not sex specific for most neck muscles, although small sex differences existed for three neck muscles (obliqus capitis inferior, longus capitis, and sternocleidomastoid). When predicting individual muscle volumes in subjects not used to develop the model, coefficients of concordance ranged from 0.91 to 0.99. This method of predicting individual neck muscle volumes has the advantage of using only one sex-specific regression equation and one set of sex-specific volume proportions. These data can be used in biomechanical models to estimate muscle forces and tissue loads in the cervical spine.

The Effects of Anterior Cruciate Ligament Deficiency on the Meniscus and Articular Cartilage A Novel Dynamic In Vitro Pilot Study

Background: Anterior cruciate ligament (ACL) injury increases the risk of meniscus and articular cartilage damage, but the causes are not well understood. Previous in vitro studies were static, required extensive knee dissection, and likely altered meniscal and cartilage contact due to the insertion of pressure sensing devices. Hypothesis: ACL deficiency will lead to increased translation of the lateral meniscus and increased deformation of the medial meniscus as well as alter cartilage contact location, strain, and area. Study Design: Descriptive laboratory study. Methods: With minimally invasive techniques, six 1.0-mm tantalum beads were implanted into the medial and lateral menisci of 6 fresh-frozen cadaveric knees. Dynamic stereo x-rays (DSXs) were obtained during dynamic knee flexion (from 15° to 60°, simulating a standing squat) with a 46-kg load in intact and ACL-deficient states. Knee kinematics, meniscal movement and deformation, and cartilage contact were compared by novel imaging coregistration. Results: During dynamic knee flexion from 15° to 60°, the tibia translated 2.6 mm (P = .05) more anteriorly, with 2.3° more internal rotation (P = .04) with ACL deficiency. The medial and lateral menisci, respectively, translated posteriorly an additional 0.7 mm (P = .05) and 1.0 mm (P = .03). Medial and lateral compartment cartilage contact location moved posteriorly (2.0 mm [P = .05] and 2.0 mm [P = .04], respectively). Conclusion: The lateral meniscus showed greater translation with ACL deficiency compared with the medial meniscus, which may explain the greater incidences of acute lateral meniscus tears and chronic medial meniscus tears. Furthermore, cartilage contact location moved further posteriorly than that of the meniscus in both compartments, possibly imparting more meniscal stresses that may lead to early degeneration. This new, minimally invasive, dynamic in vitro model allows the study of meniscus function and cartilage contact and can be applied to evaluate different pathologies and surgical techniques. Clinical Relevance: This novel model illustrates that ACL injury may lead to significant meniscus and cartilage abnormalities acutely, and these parameters are dynamically measurable while maintaining native anatomy.

The Morphometry of Soft Tissue Insertions on the Tibial Plateau: Data Acquisition and Statistical Shape Analysis

This study characterized the soft tissue insertion morphometrics on the tibial plateau and their inter-relationships as well as variabilities. The outlines of the cruciate ligament and meniscal root insertions along with the medial and lateral cartilage on 20 cadaveric tibias (10 left and 10 right knees) were digitized and co-registered with corresponding CT-based 3D bone models. Generalized Procrustes Analysis was employed in conjunction with Principal Components Analysis to first create a geometric consensus based on tibial cartilage and then determine the means and variations of insertion morphometrics including shape, size, location, and inter-relationship measures. Step-wise regression analysis was conducted in search of parsimonious models relating the morphometric measures to the tibial plateau width and depth, and basic anthropometric and gender factors. The analyses resulted in statistical morphometric representations for Procrustes-superimposed cruciate ligament and meniscus insertions, and identified only a few moderate correlations (R 2: 0.37–0.49). The study provided evidence challenging the isometric scaling based on a single dimension frequently employed in related morphometric studies, and data for evaluating cruciate ligament reconstruction strategies in terms of re-creating the native anatomy and minimizing the risk of iatrogenic injury. It paved the way for future development of computer-aided personalized orthopaedic surgery applications improving the quality of care and patient safety, and biomechanical models with a better population or average representation.

Inter-individual variation in vertebral kinematics affects predictions of neck musculoskeletal models

Experimental studies have found significant variation in cervical intervertebral kinematics (IVK) among healthy subjects, but the effect of this variation on biomechanical properties, such as neck strength, has not been explored. The goal of this study was to quantify variation in model predictions of extension strength, flexion strength and gravitational demand (the ratio of gravitational load from the weight of the head to neck muscle extension strength), due to inter-subject variation in IVK. IVK were measured from sagittal radiographs of 24 subjects (14F, 10M) in five postures: maximal extension, mid-extension, neutral, mid-flexion, and maximal flexion. IVK were defined by the position (anterior-posterior and superior-inferior) of each cervical vertebra with respect to T1 and its angle with respect to horizontal, and fit with a cubic polynomial over the range of motion. The IVK of each subject were scaled and incorporated into musculoskeletal models to create models that were identical in muscle force- and moment-generating properties but had subject-specific kinematics. The effect of inter-subject variation in IVK was quantified using the coefficient of variation (COV), the ratio of the standard deviation to the mean. COV of extension strength ranged from 8% to 15% over the range of motion, but COV of flexion strength was 20-80%. Moreover, the COV of gravitational demand was 80-90%, because the gravitational demand is affected by head position as well as neck strength. These results indicate that including inter-individual variation in models is important for evaluating neck musculoskeletal biomechanical properties.

Instantaneous centers of rotation for lumbar segmental extension in vivo

The study aimed to map instantaneous centers of rotation (ICRs) of lumbar motion segments during a functional lifting task and examine differences across segments and variations caused by magnitude of weight lifted. Eleven healthy participants lifted loads of three different magnitudes (4.5, 9, and 13.5kg) from a trunk-flexed (~75°) to an upright position, while being imaged by a dynamic stereo X-ray (DSX) system. Tracked lumbar vertebral (L2-S1) motion data were processed into highly accurate 6DOF intervertebral (L2L3, L3L4, L4L5, L5S1) kinematics. ICRs were computed using the finite helical axis method. Effects of segment level and load magnitude on the anterior-posterior (AP) and superior-inferior (SI) ICR migration ranges were assessed with a mixed-effects model. Further, ICRs were averaged to a single center of rotation (COR) to assess segment-specific differences in COR AP- and SI-coordinates. The AP range was found to be significantly larger for L2L3 compared to L3L4 (p=0.02), L4L5 and L5S1 (p<0.001). Average ICR SI location was relatively higher - near the superior endplate of the inferior vertebra - for L4L5 and L5SI compared to L2L3 and L3L4 (p≤0.001) - located between the mid-transverse plane and superior endplate of the inferior vertebra - but differences were not significant amongst themselves (p>0.9). Load magnitude had a significant effect only on the SI component of ICR migration range (13.5kg>9kg and 4.5kg; p=0.049 and 0.017 respectively). The reported segment-specific ICR data exemplify improved input parameters for lumbar spine biomechanical models and design of disc replacements, and base-line references for potential diagnostic applications.

Fluoroscopic Determination of the Tibial Insertion of the Posterior Cruciate Ligament in the Sagittal Plane

Background: Currently, placement of the tibial tunnel for arthroscopic transtibial posterior cruciate ligament (PCL) reconstruction relies on a limited arthroscopic view of the native insertion or the use of intraoperative imaging. No widely accepted method exists for intraoperative determination of PCL tibial tunnel placement, and current descriptions are cumbersome. Purpose: To identify the center of the PCL’s anatomic tibial insertion site as a percentage of the PCL facet length on a lateral radiograph of the knee so that it may be reliably located in the sagittal plane during surgical reconstruction. Study Design: Descriptive laboratory study. Methods: Twenty fresh-frozen cadaveric knees were dissected and the tibial insertions of the PCL were digitized with an optical tracing system. The digitized PCL footprints were mapped onto 3-dimensional computed tomography–acquired tibial models, and their center points were determined. A K-wire was then inserted into the center of the PCL’s tibial insertion under direct visualization, a direct lateral radiograph was obtained, and the center point was measured. The center locations for both methods were defined as a percentage of PCL facet length from anterior and proximal to posterior and distal, and intraobserver and interobserver reliability was tested with 4 different observers. Results: The average location of the PCL center on the 3-dimensional bone model method was 71.7% ± 5.6% along the PCL facet from anterior/proximal to posterior/distal. In the lateral radiographic method, the center of the PCL was at an average of 69.7% ± 4.9% of the facet length. There was no significant difference between the percentage measurements of the 2 methods (P = .13). Interobserver reliability (κ = 0.57) and intraobserver reliability (κ = 0.71) were moderate to strong. Conclusion: Locating the center of the tibial PCL insertion with fluoroscopy at a point that is 70% of the PCL tibial facet length on a true lateral radiograph is a reliable method for locating the PCL tibial insertion. Clinical Relevance: The method described in this study enables clinicians to identify the tibial location of the PCL insertion, which must be accurately determined during PCL reconstruction.