D. Bonneau

Volumic patient-specific reconstruction of muscular system based on a reduced dataset of medical images.

Three-dimensional mechanical modelling of muscles is essential for various biomechanical applications and clinical evaluation, but it requires a tedious manual processing of numerous images. A muscle reconstruction method is presented based on a reduced set of images to generate an approximate parametric object from basic dimensions of muscle contours. A regular volumic mesh is constructed based on this parametric object. The approximate object and the corresponding mesh are deformed to fit the exact muscles contours yielding patient-specific geometry. Evaluation was performed by comparison of geometry to that obtained by contouring all computed tomography (CT) slices, and by quantification of the mesh quality criteria. Muscle fatty infiltration was estimated using a threshold between fat and muscle. Volumic fat index (VFI) of a muscle was computed using first all the complete CT scan slices containing the muscle (VFIref) and a second time only the slices used for reconstruction (VFIrecons). Mean volume error estimation was 2.6% and hexahedron meshes fulfilled quality criteria. VFIrecons respect the individual variation of fat content.

Subject-specific musculoskeletal model of the lower limb in a lying and standing position

Accurate estimation of joint loads implies using subject-specific musculoskeletal models. Moreover, as the lines of action of the muscles are dictated by the soft tissues, which are in turn influenced by gravitational forces, we developed a method to build subject-specific models of the lower limb in a functional standing position. Bones and skin envelope were obtained in a standing position, whereas muscles and a set of bony landmarks were obtained from conventional magnetic resonance images in a lying position. These muscles were merged with the subject-specific skeletal model using a nonlinear transformation, taking into account soft tissue movements and gravitational effects. Seven asymptomatic lower limbs were modelled using this method, and results showed realistic deformations. Comparing the subject-specific skeletal model to a scaled reference model rendered differences in terms of muscle length up to 4% and in terms of moment arm for adductor muscles up to 30%. These preliminary findings enlightened the importance of subject-specific modelling in a functional position.

External and internal geometry of European adults

The primary objective of the study was to bring a deeper knowledge of the human anthropometry, investigating the external and internal body geometry of small women, mid-sized men and tall men. Sixty-four healthy European adults were recruited. External measurements were performed using classical anthropometric instruments. Internal measurements of the trunk bones were performed using a stereo-radiographic 3D reconstruction technique. Besides the original procedure presented in this paper for performing in vivo geometrical data acquisition on numerous volunteers, this study provides an extensive description of both external and internal (trunk skeleton) human body geometry for three morphotypes. Moreover, this study proposes a global external and internal geometrical description of 5th female 50th male and 95th male percentile subjects. This study resulted in a unique geometrical database enabling improvement for numerical models of the human body for crash test simulation and offering numerous possibilities in the anthropometry field.

Intervertebral disc characterization by shear wave elastography: An in vitro preliminary study

Patient-specific numerical simulation of the spine is a useful tool both in clinic and research. While geometrical personalization of the spine is no more an issue, thanks to recent technological advances, non-invasive personalization of soft tissue’s mechanical properties remains a challenge. Ultrasound elastography is a relatively recent measurement technique allowing the evaluation of soft tissue’s elastic modulus through the measurement of shear wave speed. The aim of this study was to determine the feasibility of elastographic measurements in intervertebral disc. An in vitro approach was chosen to test the hypothesis that shear wave speed can be used to evaluate intervertebral disc mechanical properties and to assess measurement repeatability. In total, 11 oxtail intervertebral discs were tested in compression to determine their stiffness and apparent elastic modulus at rest and at 400 N. Elastographic measurements were performed in these two conditions and compared to these mechanical parameters. The protocol was repeated six times to determine elastographic measurement repeatability. Average shear wave speed over all samples was 5.3 ± 1.0 m/s, with a repeatability of 7% at rest and 4.6% at 400 N; stiffness and apparent elastic modulus were 266.3 ± 70.5 N/mm and 5.4 ± 1.1 MPa at rest, respectively, while at 400 N they were 781.0 ± 153.8 N/mm and 13.2 ± 2.4 MPa, respectively. Correlations were found between elastographic measurements and intervertebral disc mechanical properties; these preliminary results are promising for further in vivo application.

Study on cervical muscle volume by means of three-dimensional reconstruction.

To quantify the cervical muscle volume variation by means of three‐dimensional reconstruction from MRI images.

A subject-specific biomechanical control model for the prediction of cervical spine muscle forces

Background: The aim of the present study is to propose a subject‐specific biomechanical control model for the estimation of active cervical spine muscle forces. Methods: The proprioception‐based regulation model developed by Pomero et al. (2004) for the lumbar spine was adapted to the cervical spine. The model assumption is that the control strategy drives muscular activation to maintain the spinal joint load below the physiological threshold, thus avoiding excessive intervertebral displacements. Model evaluation was based on the comparison with the results of two reference studies. The effect of the uncertainty on the main model input parameters on the predicted force pattern was assessed. The feasibility of building this subject‐specific model was illustrated with a case study of one subject. Findings: The model muscle force predictions, although independent from EMG recordings, were consistent with the available literature, with mean differences of 20%. Spinal loads generally remained below the physiological thresholds. Moreover, the model behavior was found robust against the uncertainty on the muscle orientation, with a maximum coefficient of variation (CV) of 10%. Interpretation: After full validation, this model should offer a relevant and efficient tool for the biomechanical and clinical study of the cervical spine, which might improve the understanding of cervical spine disorders. HighlightsA personalized proprioception‐based model estimating neck muscle forces is proposed.Consistent predictions are obtained independently from electromyogram‐recordings.The feasibility of building the subject‐specific model was assessed for one subject.Spine muscle response depends on the intervertebral joint load threshold.The model allows studying the effect of postural disorders or poor muscle quality.

Muscle parameters estimation based on biplanar radiography

The evaluation of muscle and joint forces in vivo is still a challenge. Musculo-Skeletal (musculo-skeletal) models are used to compute forces based on movement analysis. Most of them are built from a scaled-generic model based on cadaver measurements, which provides a low level of personalization, or from Magnetic Resonance Images, which provide a personalized model in lying position. This study proposed an original two steps method to access a subject-specific musculo-skeletal model in 30 min, which is based solely on biplanar X-Rays. First, the subject-specific 3D geometry of bones and skin envelopes were reconstructed from biplanar X-Rays radiography. Then, 2200 corresponding control points were identified between a reference model and the subject-specific X-Rays model. Finally, the shape of 21 lower limb muscles was estimated using a non-linear transformation between the control points in order to fit the muscle shape of the reference model to the X-Rays model. Twelfth musculo-skeletal models were reconstructed and compared to their reference. The muscle volume was not accurately estimated with a standard deviation (SD) ranging from 10 to 68%. However, this method provided an accurate estimation the muscle line of action with a SD of the length difference lower than 2% and a positioning error lower than 20 mm. The moment arm was also well estimated with SD lower than 15% for most muscle, which was significantly better than scaled-generic model for most muscle. This method open the way to a quick modeling method for gait analysis based on biplanar radiography.

Cervical muscles forces and spinal loads estimation in standing position: Asymptomatic cases

Loads acting on the spine induce muscles recruitment. Estimation of muscles activation in the cervical spine and the regulation of their corresponding intervertebral loads is important to understand neck pathologies and risk of injuries, but these parameters are not yet well known. Loads due to gravity in cervical spine can be calculated (Saintonge 2004). The aim of this study is to analyse how the muscular system of the cervical spine may conterbalance loads due to gravity, at rest and in standing position.

In vivo cervical intervertebral disc characterisation by elastography.

The invertebral disc (IVD) plays an important role in spine biomechanics. Its mechanical properties are a determinant aspect of the spine’s physiological flexibility; alterations of these properties can be the sign or the cause of a disc disease. Magnetic resonance is currently the reference technique to assess cervical IVD (Gibson et al. 1986); it allows evaluation of the disc’s morphology, water and collagen content. However, it is time consuming and difficult to include in clinical routine. Shear wave elastography is a non-invasive technique to evaluate soft tissue elasticity (Tanter et al. 2008). It measures the shear waves speed (SWS) in the tissue, which depends on its shear modulus and, under certain hypotheses, on its elastic modulus. It recently started being introduced in clinic for application to tissues such as muscles, prostate, liver and breasts. Elastography was recently applied in vitro to measure IVD in ox tails; a repeatability of 7% was determined, and a correlation was observed between SWS and disc stiffness in compression (Vergari et al. 2012; Vergari et al. 2014). In vivo feasibility in humans, however, remained to be proven. In this study, elastography was applied in vivo to measure cervical IVD and determine measurement reliability.

A framework towards personalisation and active muscle integration in a 3D finite-element neck model for orthopaedic applications.

In biomechanics, multi-segmented rigid-body and finite element (FE) models have become an essential tool to explore and quantify the dynamics of human motion through objective parameters. At the neck level, numerous segmental and parametric FE approaches have been used to quantify loads (e.g. to evaluate the design of spinal devices), but only a few studies have explored the influence of subject-specific geometric postural variations on the cervical spines behaviour (Frechede et al. 2006; Laville et al. 2009). Explorations of the influence of the muscles and the subject-specific complexity of their load sharing on the dynamic stability of the cervical spine are still restricted to rigid-body modelling approaches. At this stage, there is a need for detailed subject-specific models that would allow exploring the links between posture, dynamic stability and muscle function. This study presents the framework and current development of such a model.