Marker Placement

Visual3D interprets all marker sets in a similar fashion. The marker configuration promoted for Visual3D consists of a combination of arrays of markers placed on a rigid surface and markers placed on anatomical landmarks. Visual3D landmarks can also be used instead of physical markers but the user should be aware that landmarks used to define one segment that are created with respect to another segment result in the two segments having less than 6 degrees of freedom each.

Care must be taken to ensure that movement artifact is not produced by the weight of the marker or the marker attachment devices moving relative to the bones (Karlsson and Tranberg, 1999). Broadly speaking, there are four categories of configurations:

• markers mounted on bone pins
• skin mounted markers on specific anatomical landmarks on the segment
• arrays of markers on a rigid surface
• combination of markers on anatomical landmarks and arrays of markers

An array of three-non-collinear markers on a pin that is inserted directly into the bone is often held as the "gold standard" against which other marker systems are held. Clearly, the direct attachment of the markers to the bone could give the most accurate measure of motion of that bone (Fuller et al., 1997; Reinschmidt et al., 1997).

The least accurate marker set would occur when the three individual, non-collinear markers are placed directly on the skin (Fuller et al., 1997; Reinschmidt et al., 1997, Karlsson and Tranberg, 1999). Because the markers would move independently of each other, a great deal of error would be introduced into subsequent calculations. Fuller et al. (1997) reported displacements of the individual markers relative to the bone of up to 20 mm while other studies have reported values up to 40 mm (Reinschmidt et al., 1997). Reinschmidt et al. (1997) reported good agreement between skin and bone markers in knee flexion/extension but a great amount of disagreement in abduction/adduction and internal/external rotation when compared to the respective ranges of motion. The average error relative to the range of motion was 21%, 63% and 70% for flexion/extension, abduction/adduction and internal/external rotation respectively. It should be noted that the motion of skin markers is also task dependent. For motions in which there is an impact transient for example, the motion of the skin-mounted markers was greater than bone markers.

In many instances, researchers have placed the non-collinear markers on a rigid structure and attached the structure to the segment (McClay and Manal, 1999; Manal et al., 2000). Because all of the markers are fixed to a rigid structure, this system has the advantage that the markers are fixed relative to each other. That is, they are not independent of each other. Angeloni et al. (1993) reported that significantly greater displacements were observed in skin mounted markers as opposed to markers mounted on rigid plates. They suggested that markers mounted on plates were preferable to skin mounted markers for both practical and accuracy reasons. Manal et al. (2000) evaluated several configurations of arrays of markers for the tibia and concluded that a marker set consisting of four markers attached to a rigid shell was optimal. It was suggested that such a shell should be placed more distally to avoid the soft tissue of the more proximal portion of the segment. However, rotational deviations of +2 degrees about the mediolateral and anteroposterior axes and +4 degrees about the longitudinal axis occurred even with the optimal marker set.

In several marker sets, the triad of markers on a segment may consist of two markers on anatomical locations and a third marker placed on a rod that projects laterally out from the segment. The marker placed on the rod is used to achieve better 3-D measurement of rotation of the segment about its longitudinal axis. Karlsson and Tranberg (1999) suggested that the inertia of markers placed on rods may dominate over skin movement when measuring faster motions.

Therefore, in deciding the marker configuration to be used it is necessary to be aware of the limitations that can be imposed on the measurement of 3-D coordinates simply as a result of the marker set. Optimally, markers should be placed on a pin inserted into the bone itself. However, this is not always feasible. Consequently, the researcher must decide which of the externally placed marker configurations will be optimal for the task under investigation. Furthermore, the accuracy of measurement from segment to segment and axis to axis is different. It is clear that relatively reliable measures can be made for all segments in terms of flexion/extension movements. However, the secondary axis motions are less reliable, particularly the internal/external rotation about the long axis of the segments.

Each segment requires at least three calibration markers (TARGETS or LANDMARKS) that are used to locate the proximal and distal ends of the segment and define the frontal plane of the LCS. The significance of this strategy lies in the fact that LANDMARKS can be created in a wide variety of ways, which allows Visual3D to mimic the models created by any marker configuration protocol whether traditional to gait analysis (e.g. Helen Hayes), or custom made in a laboratory. This has proved to be extremely useful for clinics or laboratories that are switching (or have switched) from one hardware vendor to another.

The minimum number of markers needed for 6 DOF segments.

At least 3 non colinear markers are required for tracking a segment with 6 degrees of freedom. Visual3D's hybrid model allows landmarks to be used as tracking markers.

The user should be aware that if a landmark is defined with respect to one segment coordinate system (e.g. the hip joint center landmark being defined relative to the pelvis segment coordinate system) and then used as a tracking marker for another segment (e.g. the thigh), the thigh segment is still tracked using 6 Degree of Freedom - HOWEVER, errors in tracking the pelvis contribute errors to the tracking of the thigh (so information from the pelvis is used to track the thigh).

References

Angeloni, C., Cappozzo, A., Catani, F., Leardini, A. (1993). Quantification of relative displacement of skin- and plate-mounted markers with respect to bones. Journal of Biomechanics 26:864.

Cappozzo A, Cappello A, Della Croce U, Pensalfini P (1997) Surface-Marker Cluster Design Criteria for 3-D Bone Movement Reconstruction. IEEE Transactions on Biomedical Engineering, 44 (12), p 1165-1174.

Fuller, J., Lui, L.-J., Murphy, M. C., Mann, R. W. (1997). A comparison of lower- extremity skeletal kinematics measured using skin- and pin mounted markers. Human Movement Science 16:219-242.

Karlsson, D., Tranberg, R. (1999). On skin movement artifact-resonant frequencies of skin markers attached to the leg. Human Movement Science 18:627-635.

Manal, K., McClay, I., Stanhope, S., Richards, J., Galinat, B. (2000). Comparison of surface mounted markers and attachment methods in estimating tibial rotations during walking: an in vivo study. Gait and Posture 11:38-45.

McClay, I., Manal, K. (1999). Three-dimensional kinetic analysis of running: significance of secondary planes of motion. Medicine and Science in Sports and Exercise 31:1629-1637.

Reinschmidt, C., van Den Bogert, A. J., Nigg, B. M., Lundberg, A., Murphy, N. (1997). Effect of skin movement on the analysis of skeletal knee joint motion during running. Journal of Biomechanics 30(7):729-732.