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--- visual3d:documentation:modeling:functional_joints:functional_joints [2026/06/09 14:52] – [Functional Joints] wikisysop
+++ visual3d:documentation:modeling:functional_joints:functional_joints [2026/09/11 18:23] (current) – Clarified that this is the Functional Joints Overview page and better organized the links to other pages. richard
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-====== Functional Joints ======
+====== Functional Joints Overview ======
-Functional joints are a method for modelling the joint between two segments according to their apparent relative movement. Functional joints are implemented in Visual3D as an alternative for defining joint landmarks and can be chosen to use in segment definitions if desired. At their core functional joints are simply landmarks. Their use is optional and can be used on a case-by-case basis, whether you can to use anatomical landmarks, regression landmarks or functional landmarks. They became popular as  a method to reduce errors from marker placement. However, their accuracy is affected by movements performed, range and speed of motion, the participant studied and the soft tissue artifact. As a result, functional joints should used carefully with their results being validated using another anatomical landmarks when possible, making these others methods often more trustworthy.
+Functional joints are a method for modelling the joint between two segments according to their apparent relative movement. Functional joints are implemented in Visual3D as an alternative for defining joint landmarks and can be chosen to use in segment definitions if desired. At their core functional joints are simply landmarks. Their use is optional and can be used on a case-by-case basis and can be used in the some model with anatomical landmarks, regression landmarks or functional landmarks. They became popular as  a method to reduce errors from marker placement. However, their accuracy is affected by movements performed, range and speed of motion, the participant studied and the soft tissue artifact. As a result, functional joints should used carefully with their results being validated using another anatomical landmarks when possible, making these others methods often more trustworthy.
 ==== Data Collection====
 Using functional joints increases data collection collections and processing time and requirements. More movement trials must be collected specifically to compute the functional joints with movements that can be challenging for certain participants, particularly patient populations. Defining functional joints requires at least 3 tracking markers are required for each segment. These tracking markers should not be near the joint to ensure that the motion measured is truly meaningful for defining the functional axis of the joint. This can mean that additional tracking markers need to be added to track segments if you're using a marker set which was not designed for defining functional joints. The movement trial used is contrived specifically for the purpose of computing the functional joint. It should not be assumed that any particular dynamic movement trial,e.g. a walking trial, is sufficient. Your “functional joint” trial should be separate from the movement data trials
 ===== The input movement =====
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 For the knee joint, we recommend projecting a lateral and medial knee marker onto the functional knee axis.
-**Warning**: The functional knee axis is not the flexion/extension axis of your knee angle. Visual3D (and most software) use right handed orthogonal coordinate systems. The z-axis (axial direction) is defined by a vector between the distal and proximal ends of the segment. The x-axis lies in the frontal plane, but is forced to be perpendicular to the z-axis, which is unlikely to be parallel to the functional knee axis (it is just in the same plane). Mathematically the functional axis computed has no direction. It could point medial or lateral to the segment, but the algorithm doesn't have enough information to know the correct direction.
 ===== Algorithms for calculating Functional Joints =====
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 >Accurate and precise knee flexion axis identification is critical for prescribing and assessing tibial and femoral derotation osteotomies, but is highly prone to marker misplacement-induced error. The purpose of this study was to develop an efficient algorithm for post-hoc correction of the knee flexion axis and test its efficacy relative to other established algorithms. Gait data were collected on twelve healthy subjects using standard marker placement as well as intentionally misplaced lateral knee markers. The efficacy of the algorithm was assessed by quantifying the reduction in knee angle errors. Crosstalk error was quantified from the coefficient of determination (r2) between knee flexion and adduction angles. Mean rotation offset error (αo) was quantified from the knee and hip rotation kinematics across the gait cycle. The principal component analysis (PCA)-based algorithm significantly reduced r2 (p<0.001) and caused αo,knee to converge toward 11.9±8.0° of external rotation, demonstrating improved certainty of the knee kinematics. The within-subject standard deviation of αo,hip between marker placements was reduced from 13.5±1.5° to 0.7±0.2° (p<0.001), demonstrating improved precision of the knee kinematics. The PCA-based algorithm performed at levels comparable to a knee abduction–adduction minimization algorithm ( Baker et al., 1999 ) and better than a null space algorithm ( Schwartz and Rozumalski, 2005 ) for this healthy subject population.
-===== Examples =====
+==== Limitations ====
-See these examples for computing and working with functional joints in Visual3D:
+Functional joint can fail with no obvious indication of failure other than unreasonable joint locations. Many laboratories compare functional joint estimates against anatomical landmarks or regression-based estimates to identify failed calculations. However, there is no defined threshold for determining when a functional joint calculations are incorrect. The results are often rejected when the values differ substantially from anatomical estimates.
+**Functional Knee Axis Warning**: The functional knee axis is not the flexion/extension axis of your knee angle. Visual3D (and most software) use right handed orthogonal coordinate systems. The z-axis (axial direction) is defined by a vector between the distal and proximal ends of the segment. The x-axis lies in the frontal plane, but is forced to be perpendicular to the z-axis, which is unlikely to be parallel to the functional knee axis (it is just in the same plane). Mathematically the functional axis computed has no direction. It could point medial or lateral to the segment, but the algorithm doesn't have enough information to know the correct direction.
+===== See Also =====
+Learn more about Visual3D's graphical interface for computing functional joints:
   * [[visual3d:documentation:modeling:functional_joints:defining_a_functional_joint|Defining a functional joint]]
   * [[visual3d:documentation:modeling:functional_joints:functional_joints_post_processing|Post-processing for functional joints]]
+Follow along with our tutorial for defining a functional joint at the right hip:
+  * [[visual3d:tutorials:modeling:functional_joints|Create a Functional Joint at the Right Hip]]
+See these examples for computing and working with functional joints in Visual3D:
   * [[visual3d:documentation:pipeline:model_commands:add_functional_joint_landmark|Add a Functional Joint landmark]]
   * [[visual3d:documentation:modeling:functional_joints:example_-_functional_hip|Compute a Functional Hip Joint]]