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SCALING OF A SHOULDER MUSCULOSKELETAL MODEL TO INDIVIDUAL SUBJECT DATA
Presentation SB01, Upper Extremity 1 – Models. 8:30, Room 201DEF
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SCALING OF A SHOULDER MUSCULOSKELETAL MODEL TO INDIVIDUAL SUBJECT DATA S. Martelli1, H.E.J. Veeger2,3, and F.C.T. Van der Helm2 1 2 3
Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Bologna, Italy
Department of Mechanical Engineering, Delft University of Technology, Delft, The Netherlands
Dep. of Human Movement Sciences, Inst. for Fundamental and Clinical Movement Sciences, Amsterdam, The Netherlands
INTRODUCTION To optimize the use of musculoskeletal models for clinical practice, individualized models are a prerequisite. This would allow for a more exact estimation of muscle contributions to a specific motion task and would enable researchers and clinicians to distinguish between general and subject-specific effects of anthropometry on the performance in a particular task. An individualized model will help to quantify muscle actions in-vivo. To date, only limited data sets are available for the construction of anatomically accurate models. These data sets are either based on cadaver data, or the quantification using imaging techniques (MRI). Both methods have their pro’s and cons, but the general conclusion should be that neither methods can produce ‘complete’ sets of individualized data and that optimization of models using scaling principles is still the most feasible procedure. It is, however, unknown what scaling procedures will lead to valid results. The current study compares different scaling techniques for fitting an extensive musculoskeletal shoulder model onto the anatomy of 30 volunteers METHODS A detailed musculoskeletal shoulder model [1] created for the inverse dynamic simulation of the human shoulder district was scaled onto the anatomy of 30 volunteers. Any bone segment is defined by a local coordinate system calculated from the palpated bony landmark [2] plus a rigidly connected node cloud that include muscle and ligament attachment sites, joint center locations and palpated landmarks. For each volunteer the full set of bony landmarks according ISB standards was recorded plus a set of 40 thorax locations along the ribs, the sternum and the spine. Scaling was based upon the geometrical relationship between bony landmarks of both the model and individual. Three different methods were implemented to register the musculoskeletal model onto individual anatomy: uniform scaling method, intrasegmental uniform scaling and intrasegmental non-uniform scaling method. Uniform scaling: The whole model was scaled by a single scaling factor based on a length ratio of the arm which is easy to measure in clinics. The arm length was defined as the sum distance angulus acromialis (AA)-lateral epicondyle (EL) plus lateral epicondyle-radial styloid. Inertial parameters and mass parameters were scaled by the second power and the third power scaling factor respectively. Intrasegmental uniform scaling: Any segment node cloud was scaled using a single scaling factor based on a representative segment dimension. The shape of the thorax was approximated by an ellipsoid thus the thorax scaling factor was defined as the ratio of the Journal of Biomechanics 40(S2)
vertical axes of the thorax surface between the subject and the model. The distance between the recorded landmark and the surface was up to 2cm. The clavicle, the humerus and the forearm scaling factors were based upon the segment length defined as the joint to joint distance. Lastly, the scapula scaling factor was calculated based upon the distance between the angulus acromialis (AA) and the trigonum scapulae (TS). Intrasegmental non uniform scaling: The thorax scaling matrix was defined as the ratio between the thorax ellipsoid axes for each direction. Clavicle, humerus and forearm scaling matrix were defined using a longitudinal and a transversal scaling factor. The scapula scaling matrix was based on the edge length of the bounding box parallel to the local coordinate system. RESULTS AND DISCUSSION
Figure 1: Individual data (labeled landmarks and thorax ellipsoid) overlapping the original cadaver model (left) and the scaled model (right) by the third method. The work shows that it is possible to scale a complete musculoskeletal model to a subject anatomy described by a set of bony locations easily measurable in clinics. The results for the effect of scaling on the difference between model- and individual kinematics will be presented, as well as the effect of scaling on the estimation of individual muscle forces. However, whether this procedure will lead to more accurate results will require a validation study in which both kinematics and muscle activity are recorded. CONCLUSIONS The presented methods allow the simple generation of subject specific musculoskeletal models for clinical purposes. Nevertheless, before to apply it to clinical practice the validity of model predictions has to be determined. REFERENCES 1. van der Helm, F.C., J Biomech, 1994. 27(5): p. 551-69.. 2. Wu, G., et al., J Biomech, 2005. 38(5): p. 981-992 XXI ISB Congress, Podium Sessions, Monday 2 July 2007
S68
SCALING OF A SHOULDER MUSCULOSKELETAL MODEL TO INDIVIDUAL SUBJECT DATA S. Martelli1, H.E.J. Veeger2,3, and F.C.T. Van der Helm2 1 2 3
Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Bologna, Italy
Department of Mechanical Engineering, Delft University of Technology, Delft, The Netherlands
Dep. of Human Movement Sciences, Inst. for Fundamental and Clinical Movement Sciences, Amsterdam, The Netherlands
INTRODUCTION To optimize the use of musculoskeletal models for clinical practice, individualized models are a prerequisite. This would allow for a more exact estimation of muscle contributions to a specific motion task and would enable researchers and clinicians to distinguish between general and subject-specific effects of anthropometry on the performance in a particular task. An individualized model will help to quantify muscle actions in-vivo. To date, only limited data sets are available for the construction of anatomically accurate models. These data sets are either based on cadaver data, or the quantification using imaging techniques (MRI). Both methods have their pro’s and cons, but the general conclusion should be that neither methods can produce ‘complete’ sets of individualized data and that optimization of models using scaling principles is still the most feasible procedure. It is, however, unknown what scaling procedures will lead to valid results. The current study compares different scaling techniques for fitting an extensive musculoskeletal shoulder model onto the anatomy of 30 volunteers METHODS A detailed musculoskeletal shoulder model [1] created for the inverse dynamic simulation of the human shoulder district was scaled onto the anatomy of 30 volunteers. Any bone segment is defined by a local coordinate system calculated from the palpated bony landmark [2] plus a rigidly connected node cloud that include muscle and ligament attachment sites, joint center locations and palpated landmarks. For each volunteer the full set of bony landmarks according ISB standards was recorded plus a set of 40 thorax locations along the ribs, the sternum and the spine. Scaling was based upon the geometrical relationship between bony landmarks of both the model and individual. Three different methods were implemented to register the musculoskeletal model onto individual anatomy: uniform scaling method, intrasegmental uniform scaling and intrasegmental non-uniform scaling method. Uniform scaling: The whole model was scaled by a single scaling factor based on a length ratio of the arm which is easy to measure in clinics. The arm length was defined as the sum distance angulus acromialis (AA)-lateral epicondyle (EL) plus lateral epicondyle-radial styloid. Inertial parameters and mass parameters were scaled by the second power and the third power scaling factor respectively. Intrasegmental uniform scaling: Any segment node cloud was scaled using a single scaling factor based on a representative segment dimension. The shape of the thorax was approximated by an ellipsoid thus the thorax scaling factor was defined as the ratio of the Journal of Biomechanics 40(S2)
vertical axes of the thorax surface between the subject and the model. The distance between the recorded landmark and the surface was up to 2cm. The clavicle, the humerus and the forearm scaling factors were based upon the segment length defined as the joint to joint distance. Lastly, the scapula scaling factor was calculated based upon the distance between the angulus acromialis (AA) and the trigonum scapulae (TS). Intrasegmental non uniform scaling: The thorax scaling matrix was defined as the ratio between the thorax ellipsoid axes for each direction. Clavicle, humerus and forearm scaling matrix were defined using a longitudinal and a transversal scaling factor. The scapula scaling matrix was based on the edge length of the bounding box parallel to the local coordinate system. RESULTS AND DISCUSSION
Figure 1: Individual data (labeled landmarks and thorax ellipsoid) overlapping the original cadaver model (left) and the scaled model (right) by the third method. The work shows that it is possible to scale a complete musculoskeletal model to a subject anatomy described by a set of bony locations easily measurable in clinics. The results for the effect of scaling on the difference between model- and individual kinematics will be presented, as well as the effect of scaling on the estimation of individual muscle forces. However, whether this procedure will lead to more accurate results will require a validation study in which both kinematics and muscle activity are recorded. CONCLUSIONS The presented methods allow the simple generation of subject specific musculoskeletal models for clinical purposes. Nevertheless, before to apply it to clinical practice the validity of model predictions has to be determined. REFERENCES 1. van der Helm, F.C., J Biomech, 1994. 27(5): p. 551-69.. 2. Wu, G., et al., J Biomech, 2005. 38(5): p. 981-992 XXI ISB Congress, Podium Sessions, Monday 2 July 2007