13 functional calibration (nov 2014)
TRANSCRIPT
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Functional Calibration
Richard Baker
Professor of Clinical Gait Analysis
Blog: wwRichard.net
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Model Calibration
Scaling the model to fit the person we are
measuring
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Kinematic fitting
(Inverse kinematics, Global optimisation)
Fitting a model to the marker trajectories
captured during the movement being
analysed.
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Kinematic fitting4
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Calibration
• Where are the joint
centres?
• Where are the joint
axes?
• Where are the
markers?
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Static calibration – hip
Assume
• hip joint centres can be estimated from position of pelvic markers.
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Static calibration - knee7
Assume
• knee joint centres is half-way between condyles
• line between condyles defines femur coronal plane
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Static calibration - ankle8
Assume
• anklle joint centres is half-way between malleoli
• line between malleioli defines tibia coronal plane
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Static calibration - foot9
Assume
• long axis is parallel ot the floor
• long axis is parallel to line from heel to tip of second ray
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Problems - Hip1
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Bell AL, et al. (1990). Journal of Biomechanics 23:617-621Davis R et al. (1991). Hum. Movemt Sci. 10:575-587.Leardini A et al. (1999). Journal of Biomechanics 32:99-103
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Problems - Knee1
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Functional calibration
Use data from movement trials to calibrate
the model
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Hip calibration1
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Hip calibration1
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Knee calibration1
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Knee calibration1
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Functional calibration
• Growing number of different mathematical
techniques to determine joint centres and
axes.
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Does it work?
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Errors in mm (with standard deviations)
Compared to RSA Functional Bell Davis
Anterior-posterior 3.8 (6.1) -2.5 (5.9) -11.6 (16.7)
Medio-lateral -2.1 (3.7) 5.3 (9.7) 16.5 (10.2)
Proximal-distal 2.5 (5.9) -18.7 (8.2) 8.2 (9.7)
Total 11.8 (4.1) 23.2 (10.3) 29.1 (7.6)
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Wrong regression equations?2
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3d- ultrasound2
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Sangeux, M., A. Peters, and R. Baker, Hip joint centre localization: Evaluation on normal subjects in the
context of gait analysis. Gait Posture, 2011. 34(3): p. 324-8.
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PiG HAR Geometrical Algebraic CTT SCoRE Kylie
Sphere Transformational
REGRESSION LOCAL GLOBAL
Hip validation – 19 adults
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PiG HAR Geometrical Algebraic CTT SCoRE Kylie
Sphere Transformational
REGRESSION LOCAL GLOBAL
Hip validation – 19 adults
In healthy adults functional calibration performs about as well as Harrison’s regression equations
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Hip validation – 46 Children with CP
REGRESSION
In children with CP Harrington’s regression equations outperform functional calibration
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Knee calibration
• Much bigger problem?
• Much more difficult to find gold standard
• Informal data suggests that skilled marker
placers can be reasonably accurate
(particularly using medial knee markers).
• Functional calibration is at least as
repeatable as they are.
• Should be independent of experience?
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Summary
• Functional calibration of hip does not
appear to offer much over the correct
regression equations.
• Functional calibration of knee should be
more repeatable for most marker placers.
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Practicalities
• None of the manufacturers have
implemented easy to use software for
functional calibration.
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Thanks for listening
Richard Baker
Professor of Clinical Gait Analysis
Blog: wwRichard.net
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