Pelvis

 

Visual3D Pelvis

 

The Visual3D pelvis refers to a pelvis segment defined as a default Visual3D segments. Markers on the Iliac Crest specify the proximal end of the segment. Markers on the Greater Trochanter specify the distal end of the segment. The length of the Pelvis segment is defined as the distance from the proximal end to the distal end.

One of the advantages of this definition of the pelvis segment is repeatability. The greater trochanter landmarks can be identified reliably on most subjects (including patients). The Iliac Crest marker is placed pragmatically (i.e. such that it is placed to have anatomical meaning). For example, the Iliac Crest marker is placed to ensure that the angle of the pelvis relative to the laboratory computed in Visual3D is the same as the clinical measure of pelvis tilt. This can be accomplished by placing the Iliac Crest marker along the Iliac Crest such that the position of the marker relative to the Greater Trochanter marker yields pelvis tilt. In the figure to the left the Iliac Crest marker is placed slightly anterior to the greater trochanter marker to yield a slightly forward leaning pelvis tilt.

Note that the Visual3D pelvis does not generate landmarks at the Hip Joint Center automatically.

 

In the Visual 3D pelvis what part of the iliac crest is used as the landmark (i.e. do you just pick the point on the iliac crest that is at the midline -so inline with the greater trochanter, or is there something more particular that is palpated)? I am a little concerned about repeatability of marker placement on the crest since it is a larger bony protuberance and several spots could be selected?

The greater trochanter can be located quite reliably. As you point out, however, there isn't an identifiable landmark on the iliac crest. The Iliac Crest landmark should be placed consistently with a clinical measure of pelvic tilt. For example, if the pelvic tilt during the physical examination is considered to be zero degrees. The Iliac Crest marker is placed directly superior to the greater trochanter marker. This is a bit of a reliance on common sense, but it is always important that the results of your motion analysis are consistent with your clinical measures.

 

CODA Pelvis

The CODA pelvis refers to a pelvis segment model used by Charnwood Dynamics that they requested that we implement. The pelvis segment is defined using the anatomical locations of the ASIS (Anterior Superior Iliac Spine) and the PSIS (Posterior Superior Iliac Spine). These landmarks are bony protuberances on the Pelvis bones that can be palpated on most (but not all) subjects. There are particularly useful if the subject is very thin, a cadaver, or a skeleton, and historically have been the reference points for defining the segment coordinate system for many experimental studies focused on determining regression equations for locating the hip joint centers. Unfortunately, these landmarks are not easily palpated, or more importantly, cannot have motion capture markers placed accurately on them, for overweight subjects. This challenge notwithstanding, the CODA pelvis and HELEN HAYES pelvis segments are still popular in many laboratories.

In the Visual3D implementation of this pelvis segment, the (x-y) plane of the segment coordinate system is defined as the plane passing through the right and left ASIS markers, and the mid-point of the right and left PSIS markers. Note that the midpoint of the PSIS markers is the same point as the SACRUM marker used for the Helen Hayes pelvis. Therefore, if a Helen Hayes markers set was used for the motion capture, a CODA pelvis can be created using the SACRUM marker twice (once for the left PSIS and once for the right PSIS).

 

The origin of the pelvis segment coordinate system is defined as the mid-point between the ASIS markers. The user should consider creating ASIS landmarks, offset by the radius of the motion capture markers, in the lateral direction of a plane comprised of the ASIS markers and SACRUM. This will create the ORIGIN at the midpoint of the ASIS anatomically rather than the mid-point of the motion capture markers. The x-axis is defined from the ORIGIN towards the Right ASIS. The z-axis is perpendicular to the (x-y) plane. The y-axis is then the crossproduct of the x-axis and z-axis. If the markers have been placed accurately, the pelvis segment coordinate system will typically be tilted forward about 20 degrees, which is unfortunate for the calculation of the hip angle, but can be resolved using virtual segments.

Estimates for the Right and Left Hip Joint Center are represented as Landmarks that are created automatically when the CODA pelvis segment is created. The location of the landmark is defined as:
RHJC=(0.36*ASIS_Distance,-0.19*ASIS_Distance,-0.3*ASIS_Distance)
LHJC=(-0.36*ASIS_Distance,-0.19*ASIS_Distance,-0.3*ASIS_Distance)

These estimates are adapted from the article:
Bell AL, Pedersen DR, Brand RA
(1990) A Comparison of the Accuracy of Several hip Center Location Prediction Methods. J Biomech. 23, 617-621.

Note that if the ASIS markers have been placed medial or lateral to the palpated landmark because the subject is fat or because the markers can not be placed at these locations, it is important to measure the inter-ASIS distance and enter this value into the Subject Properties TAB.

 

Helen Hayes (Davis) Pelvis

 

The Helen Hayes pelvis refers to a pelvis segment model defined by Davis et. at. (1991) and used by many manufacturers, including the VCM model by Vicon. The pelvis segment is defined using the anatomical locations of the ASIS (Anterior Superior Iliac Spine) and the midpoint between the PSIS (Posterior Superior Iliac Spine) commonly referred to as the Sacrum marker. These landmarks are bony protuberances on the Pelvis bones that can be palpated on most (but not all) subjects. There are particularly useful if the subject is very thin, a cadaver, or a skeleton, and historically have been the reference points for defining the segment coordinate system for many experimental studies focused on determining regression equations for locating the hip joint centers. Unfortunately, these landmarks are not easily palpated, or more importantly, cannot have motion capture markers placed accurately on them, for overweight subjects. This challenge notwithstanding, the CODA pelvis and HELEN HAYES pelvis segments are still popular in many laboratories.

In the Visual3D implementation of this pelvis segment, the (x-y) plane of the segment coordinate system is defined as the plane passing through the right and left ASIS markers, and the mid-point of the right and left PSIS markers.

 

 

The origin of the pelvis segment coordinate system is defined as the mid-point between the ASIS markers. The marker radius value is incorporated into the computation of the origin so that the ORIGIN is at the midpoint of the ASIS anatomically rather than the mid-point of the motion capture markers. NOTE that the marker radius is used differently than with the CODA pelvis, which requires the use of a landmark to create the offset.

The x-axis is defined from the ORIGIN towards the Right ASIS. The z-axis is perpendicular to the (x-y) plane. The y-axis is then the crossproduct of the x-axis and z-axis. If the markers have been placed accurately, the pelvis segment coordinate system will typically be tilted forward about 20 degrees, which is unfortunate for the calculation of the hip angle, but can be resolved using virtual segments.

If no clinical measurements are taken, the following values are calculated:

Estimates for the Right and Left Hip Joint Center are represented as Landmarks that are created automatically when the Helen Hayes pelvis segment is created. The location of the landmarks are defined in the Pelvis Segment Coordinate System as:

Hip X=

-S (C sin(theta)-0.5*distASIS)

 

Hip Y=

(-Xdis-Rmarker) cos(beta)+ C cos(theta) sin(beta)

 

Hip Z=

(-Xdis-Rmarker) sin(beta)+ C cos(theta) cos(beta)

 

Where:

 

 

C=

0.115*LegLength - 0.0153 (in meters)

 

theta=

28.4 +- 6.6

 

beta=

18 +- 4

 

distASIS=

ASIS to ASIS distance, measured during clinical exam

 

Xdis=

anterior/posterior componend of the ASIS/hip center distance in the sagittal plane of the pelivs and measured during the clinical exam

 

If not measured:

 

 

Xdis=

0.1288*LegLength - 0.04856

 

Rmarker=

marker Radius (in meters)

 

S=

+1 for the right side and -1 for the left side

 

 

These equations are adapted from the article:
Davis RB, Ounpuu S, Tyburski D, Gage JR.
(1991) "A Gait Analysis Data Collection and Reduction Technique." Human Movement Science, 10: 575-587.

 

Choosing between Helen Hayes and CODA Pelvis

I usually recommend choosing the CODA Pelvis within Visual3D mostly because it does not require an estimate of the Leg Length, which can be a bit of a problem for some patients. The CODA pelvis also does not require the measurement of the ASIS-greater trochanter distance, which I find that very few laboratories ever make.

 

For the CODA/DAVIS pelvis the segment coordinate system typically will be tilted forward about 20 degrees forward. It is possible, however, to create a Visual3D pelvis from this same data set such that the pelvic tilt matches the clinical measure of pelvic tilt.

The Visual3D pelvis is created typically using Iliac Crest markers and Greater Trochanter markers. The Conventional Gait marker set, however, does not include these markers. One of the consequences of creating a CODA/DAVIS pelvis, however, is that Hip Joint Centers are created automatically based on regression equations.

The following process can be used to create a Visual3D pelvis segment coordinate system using the Conventional Gait marker set.

First, create a CODA pelvis and note the creation of Hip Joint Landmarks.

<---->
Remove the animation model associated with the CODA pelvis. Switch to the Segment Properties tab and remove the assignment to the obj and col files.

Create landmarks to represent the location of the Iliac Crest markers. In this example, I elected to have a segment coordinate system aligned with the laboratory, resulting in a pelvic tilt of zero degrees. Modifying the location of the landmarks will allow the creation of any orientation.

<---->

Create the Visual3D Pelvis

Note the difference in orientation of the two pelvis segment coordinate systems.

 

The previous process created a pelvis segment coordinate system that was aligned vertically by creating landmarks located vertically above the hip joint centers. Any pelvic tile, however, can be created by a modest change to the landmark definitions.

Note: Visual3D allows the use of mathematical expressions in most of the edit boxes in model builder model. In the previous section the following landmarks were created.

Landmark Name=Right Iliac Crest
Starting Point=
RIGHT_HIP
Offset Using the Following ML/AP/AXIAL Offsets=
Checked
X=
0.0
Y=
0.0
Z=
0.14

Landmark Name=Left Iliac Crest
Starting Point=
LEFT_HIP
Offset Using the Following ML/AP/AXIAL Offsets=
Checked
X=
0.0
Y=
0.0
Z=
0.14

Assuming the y-direction is Anterior. We can create a forward tilt of the pelvis of 0.3 radians by modifying the landmarks as follows:

Landmark Name=Right Iliac Crest
Starting Point=
RIGHT_HIP
Offset Using the Following ML/AP/AXIAL Offsets=
Checked
X=
0.0
Y=
0.14*sin(0.3)
Z=
0.14*cos(0.3)

Landmark Name=Left Iliac Crest
Starting Point=
LEFT_HIP
Offset Using the Following ML/AP/AXIAL Offsets=
Checked
X=
0.0
Y=
0.14*sin(0.3)
Z=
0.14*cos(0.3)

Pelvis Animation Models (obj, v3g, wrl)

There are two common obj files used to animate the bones of the pelvis.

Pelvis.obj is typically used for the conventional Visual3D pelvis, which is defined using the Iliac Crest markers to define the proximal end, and the greater trochanter markers to define the distal end.

PelvisWrtASIS.obj is typically used for either the CODA or Helen Hayes pelvis segments. The Helen Hayes pelvis is defined with respect to the left and right ASIS landmarks and a Sacrum marker (mid point between the PSIS landmarks). The CODA pelvis is defined with respect to the left and right ASIS landmarks and the left and right PSIS landmarks. The CODA pelvis can also be built using the SACRUM marker twice (once for each PSIS landmark).

In all 3 default pelvis segments, any number of tracking markers fixed to the pelvis can be used to determine the segment pose.