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.

["Water Hammer effect": a rare mechanism of hydrocephalus]. / Un mécanisme rare d'hydrocéphalie : l'effet coup de marteau.

We are reporting a case of functional hydrocephalus in a 66-year-old male patient presenting for gait disturbance. The etiology of the disease is a cerebrospinal fluid flow disturbance due to an ectatic basilar artery at the level of Monro foramen. Different pathophysiological mechanisms are discussed below.

Estimating foot inertial parameters: a new regression approach.

BACKGROUND: Estimating the inertial parameters for the foot (mass, center of mass position and inertia tensor) is important for applications involving the ankle joint such as inverse dynamics or stiffness measurement techniques (e.g. Quick-release). Scaling equations relying on foot length and body mass are widely used. However, because of the complex foot geometry, such equations may represent an oversimplified solution. Our aim was to evaluate these approaches and propose a new method. METHODS: Thirty-four right feet (17 Males, mean age and weight 30 years, 75 kg; 17 Females, 32 years, 61.5 kg) were reconstructed using a 3D surface scanner and used as geometrical references. Associated inertial parameters were calculated directly on each reference assuming a uniform density distribution and were compared to corresponding scaling and multiple regression estimates. Finally, an alternative method, based on multiple non-linear regressions, was proposed considering both foot length (L) and ankle width (W). FINDINGS: Comparisons showed that reference mass and moments of inertia were greater than scaling predictions with mean difference up to 33 and 16% for mass and moments of inertia respectively. The maximum standard errors of estimate for scaled moments of inertia reached 26%. The alternative solution involving ankle width in the equations lowered the gap with reference data (8.7% max standard errors of estimate) for both genders. INTERPRETATION: This strategy, requiring two simple and accessible measurements, may offer a better practicality/relevance compromise for clinical routine use, in regards to existing scaling and regression equations.