Prediction and classification when the diagnostic classes are related

We consider prediction and classification into diagnostic classes which consist of individuals who can suffer from multiple diseases. For instance, in a cardiovascular context a patient can need bypass surgery, or a valve replacement, or both. The popular multigroup logistic model is suitable for prediction into nominal classes, but does not employ the underlying structure of the classes. Hence, this model is not entirely suitable for this situation. Also, computational difficulties often occur with the multigroup logistic model when the classes are of the above nature. A modified form of the model, applicable to some economic applications, is not appropriate for most medical applications. Instead, we suggest the n-way Dale model, also called the marginal logistic model. It is shown that this model is computationally more stable, although more involved, and allows better interpretation of the parameters. To illustrate our ideas the POPS data set is taken, where the child's abilities at the age of 2 is predicted from risk factors at delivery. A simulation study is performed to indicate the gain in classification ability in comparison with the multigroup logistic model. It is also shown that in terms of the parameter estimates the Dale model is more sensitive to the choice of the sampling scheme than the multigroup logistic model. © 1997 Elsevier Science B.V.status: publishe

Calculated moment-arm and muscle-tendon lengths during gait differ substantially using MR based versus rescaled generic lower-limb musculoskeletal models

Biomechanical analysis of gait relies on the use of lower-limb musculoskeletal models. Most models are based on a generic model which takes into account the subject's skeletal dimensions by isotropic or anisotropic rescaling. Alternatively, personalized models can be built based on information from magnetic resonance (MR) images. We have studied the effect of these approaches on muscle-tendon lengths (MTLs) and moment-arm lengths (MALs) for 16 major muscles of the lower limb of a normal adult during both normal and pathologic gait. For most muscles, the MTL and MAL calculated using the rescaled generic models showed high correlation values, but large offsets when compared to values calculated using personalized models. MTL and MAL differences with the personalized model are only slightly smaller for an anisotropic than for an isotropic rescaled model. Gait kinematics influenced the observed inter-model differences and correlations due to an altered range of joint angles in both gait patterns. In conclusion, both generic rescaling methods failed to accurately estimate absolute values for MTL and MAL calculated using the personalized model. However, the magnitude of MTL and MAL changes during normal and pathologic gait corresponded between all three models for most muscles. Since rescaling depends strongly on modelling assumptions and cannot fully take into account subject-specific musculoskeletal geometry, interpretation of MTL and MAL even in normal adult subjects requires extreme caution.Scheys L., Spaepen A., Suetens P., Jonkers I., ''Calculated moment-arm and muscle-tendon lengths during gait differ substantially using MR based versus rescaled generic lower-limb musculoskeletal models'', Gait & posture, vol. 28, no. 4, pp. 640-648, 2008.status: publishe

Level of subject-specific detail in musculoskeletal models affects hip moment arm length calculation during gait in pediatric subjects with increased femoral anteversion

Biomechanical parameters of gait such as muscle's moment arm length (MAL) and muscle-tendon length are known to be sensitive to anatomical variability. Nevertheless, most studies rely on rescaled generic models (RGMo) constructed from averaged data of cadaveric measurements in a healthy adult population. As an alternative, deformable generic models (DGMo) have been proposed. These models integrate a higher level of subject-specific detail by applying characteristic deformations to the musculoskeletal geometry. In contrast, musculoskeletal models based on magnetic resonance (MR) images (MRMo) reflect the involved subject's characteristics in every level of the model. This study investigated the effect of the varying levels of subject-specific detail in these three model types on the calculated hip MAL during gait in a pediatric population of seven cerebral palsy subjects presenting aberrant femoral geometry. Our results show large percentage differences in calculated MAL between RGMo and MRMo. Furthermore, the use of DGMo did not uniformly reduce inter-model differences in calculated MAL. The magnitude of these percentage differences stresses the need to take these effects into account when selecting the level of subject-specific detail one wants to integrate in musculoskeletal. Furthermore, the variability of these differences between subjects and between muscles makes it very difficult to a priori estimate their importance for a biomechanical analysis of a certain muscle in a given subject.Scheys L., Desloovere K., Suetens P., Jonkers I., ''Level of subject-specific detail in musculoskeletal models affects hip moment arm length calculation during gait in pediatric subjects with increased femoral anteversion'', Journal of biomechanics, vol. 44, no. 7, pp. 1346-1353, April 2011.status: publishe

How Pathological Gait Kinematics, Increased Femoral Anteversion and Neck Shaft Angle Adversely Affect the Loading Conditions of the Femoral Head During Gait in Children With Cerebral Palsy

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Calculating gait kinematics using MR-based kinematic models : what’s the benefit ?

Scheys L., Desloovere K., Spaepen A., Suetens P., Jonkers I., ''Calculating gait kinematics using MR-based kinematic models : what’s the benefit ?'', Gait & posture, vol. 30, supplement 2, pp. S79, November 2009 (18th annual meeting of the European Society of Movement Analysis for Adults and Children - ESMAC 2009, September 14-19, 2009, London, United Kingdom).status: publishe

Calculating gait kinematics using MR-based kinematic models

Rescaling generic models is the most frequently applied approach in generating biomechanical models for inverse kinematics. Nevertheless it is well known that this procedure introduces errors in calculated gait kinematics due to: (1) errors associated with palpation of anatomical landmarks, (2) inaccuracies in the definition of joint coordinate systems. Based on magnetic resonance (MR) images, more accurate, subject-specific kinematic models can be built that are significantly less sensitive to both error types. We studied the difference between the two modelling techniques by quantifying differences in calculated hip and knee joint kinematics during gait. In a clinically relevant patient group of 7 pediatric cerebral palsy (CP) subjects with increased femoral anteversion, gait kinematic were calculated using (1) rescaled generic kinematic models and (2) subject-specific MR-based models. In addition, both sets of kinematics were compared to those obtained using the standard clinical data processing workflow. Inverse kinematics, calculated using rescaled generic models or the standard clinical workflow, differed largely compared to kinematics calculated using subject-specific MR-based kinematic models. The kinematic differences were most pronounced in the sagittal and transverse planes (hip and knee flexion, hip rotation). This study shows that MR-based kinematic models improve the reliability of gait kinematics, compared to generic models based on normal subjects. This is the case especially in CP subjects where bony deformations may alter the relative configuration of joint coordinate systems. Whilst high cost impedes the implementation of this modeling technique, our results demonstrate that efforts should be made to improve the level of subject-specific detail in the joint axes determination.Scheys L., Desloovere K., Spaepen A., Suetens P., Jonkers I., ''Calculating gait kinematics using MR-based kinematic models'', Gait & posture, vol. 33, no. 2, pp. 158-164, February 2011 (18th annual meeting of the European Society of Movement Analysis for Adults and Children - ESMAC 2009, September 14-19, 2009, London, United Kingdom) (ESMAC – Best Paper award 2009 – Runner UP).status: publishe

Proximal femoral deformity favors hip extension action and impairs pelvic stability during gait: a simulation study using mri-based musculoskeletal models

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Atlas-based non-rigid image registration to automatically define line-of-action muscle models : a validation study

Research has raised a growing concern about the accuracy of rescaled generic musculoskeletal models for estimating a subject's musculoskeletal geometry. Information extracted from magnetic resonance (MR) images can improve the subject-specific detail and accuracy of musculoskeletal models. Nevertheless, methods that allow efficient, automated definition of subject-specific muscular models for use in biomechanical analysis of gait have not yet been published to the best of our knowledge. We report a novel method for automated definition of subject-specific muscle paths using non-rigid image registration between an atlas image and the subject's MR images. We validated this approach quantitatively by measuring the distance between automatically and manually defined coordinates of muscle attachment sites. Data was collected for 34 muscles in each lower limb of 5 paediatric subjects diagnosed with diplegic cerebral palsy and presenting varying degrees of increased femoral anteversion. Distances showed an overall median Euclidean error of 6.1mm: 2.0mm along the medio-lateral direction, 1.8mm along the anterior-posterior direction and 3.8mm along the superior-inferior direction. A qualitative validation between automatically defined muscle points and the muscular geometry observed in the subject's medical image data corroborated the quantitative validation. This automated approach followed by visual inspection and, if needed, correction to the muscle paths, reduced the time required for defining 34 lower-limb muscle paths from around 3.5 to 1h. Furthermore, the method was also applicable to aberrant skeletal geometry. Using the proposed method, defining MR-based musculoskeletal models becomes a time efficient and more accurate alternative to rescaling generic models.Scheys L., Loeckx D., Spaepen A., Suetens P., Jonkers I., ''Atlas-based non-rigid image registration to automatically define line-of-action muscle models : a validation study'', Journal of biomechanics, vol. 42, no. 5, pp. 565-572, March 2009.status: publishe