## Hip Joint Center

Hello everyone,

I have a question for the gait people out there. Is there a good way
to locate the center of the hip joint without measuring
BOTH ASIS points and the spine. Due to camera
limitations I can only get one ASIS. I have been collecting data on
normals and patients and am now in the process of predicting ankle,
knee and hip loads during various tasks. The markers that I have been
collecting (which are relative to the hip joint) are the knee, greater
trochanter, a point on the front of the thigh-midway between the knee
and hip, the ASIS, the PSIS.

I have looked in the archives and found references to work by Bell et
al. (J. Biomech 23(6), 1990 and Hum. Mvmt Sci (8), 1989) and Seidel et
al. (J. Biomech 28(8), 1995). Although these papers are good, they
all depend on knowing the pelvic width. I have seen reference to work
by Cappozzo (Hum. Mvmt, Sci 3, 1984) which is based on the assumption
of the thigh being rigid and rotating about the hip joint. This
method may prove useful but it's not ideal because we are studying
walking and stair climbing, and not simply moving the joint through a
simple arc.

In trying to calculate the HJC from the data that I have, I have
formulated a system of equations. However, I have a made a couple of
assumptions in order to complete them. Thus I appeal to you in the
community for any input. I will describe the equations.

1. Since I do know the location of the ASIS on the side that I'm
interested in... and can measure pelvic width with a tape measure, one
of the equations is a sphere centered around the ASIS with a radius of
38*(Pelvic Width). This is based on the Bell paper stating the
location of the HJC in relation to the ASIS. Using Pythagoras, I came
up with a radius of .38* Pelvic Width.

2. The markers placed laterally on the knee and greater trochanter
define the long axis of the thigh. I have found reference to the
angle between the long axis and the femoral head to be 115 to 125
degrees. Incorporating that information, I can say the dot product
between the shaft and the line joining the greater trochanter to the
HJC is sin of 120 degrees. There will be some error associated with
this, which will depend on the length of the femoral neck.

3. In the third equation, I want to relate the distance from the
greater trochanter to the HJC, to the distance from the knee to the
greater trochanter. This is the information that I have been looking
for, unsuccessfully.

If this data is available somewhere (femoral neck length as a
proportion of shaft length), can you please point me in the
right direction. This nonlinear system of equations can solved
iteratively for the three dimensional coordinates of the HJC. I would
appreciate any input into alternative methods, errors that I will
encounter, or insights into the limitations of what I am attempting.
Thank you for reading through this post and your thoughts on the
matter. I will summarise replies and redistribute shortly.

Sincerely,

David