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Leonard D'addesi
08-21-1996, 03:12 AM
Dear Biomech-l,

I have received a few replies to my original posting and as promised I am
posting them. First, however, I wanted to restate my question because it
was not quite clear the exact question I wanted to ask. Then I will post
my original question and the replies.

New question:

My research involves locating the hip joint center under the assumption
that it acts as a ball and socket joint. However, do subluxated or
dislocated hip joints continue to act as a ball and socket joint?

Thank you once again, and I will post a summary of replies.

Leonard D'Addesi
Human Performance Lab
Thomas Jefferson University Hospital
Tel: 215-503-1646
Fax: 215-923-2475
Email: sg943dvw@dunx1.ocs.drexel.edu
__________________________________________________ _______________________

Original question:

Hello to all,

I am a masters student and my research involves locating the hip joint
center. I have a few simple questions that I am confused about:

1. I discovered in the biomech-l archives that the hip joint can be
modelled as a ball and socket joint if the centers of the femoral head
and acetabulum are the same and the contact surface is spherical. Is
this a good definition of the center of rotation?

2. Does the ball and socket model of the hip joint hold true for patients
that have neuromuscular diseases such as CP? Has anyone done a study to
show whether the model holds true?

I would greatly appreciate any references. I will post a summary of
replies.

Thank you.

__________________________________________________ ___________
Responses include:

>From Prof. Joe Mizrahi Aug 4, 96 09:05:04 am +0300

Concepts of asphericity and incongruence of the human healthy hip have been
suggested in the literature. It is believed that they help in achieving
a progressive load distribution on the contacting surfaces, and improve
lubrication within the joint. Asphericity, however renders a much less
convenient geometry to model and deals with. Nevertheless, if it can be
represented analytically this difficulty can be considerably alleviated.
Based on previously published experimental data, we have attempted to
analytically represent hip asphericity , by approximating the joint as a
tilted solid of revolution. The following two references report our
results:
- Mizrahi, J The human femoral head as a tilted solid of revolution. S.
Afr. J. Science, 73, 385-386, 1977.
- Mizrahi, J An axi-symmetrical representation of the human femoral head.
The S. Afr. Mech. Eng., 28, 206-209, 1978.

=====================================
Prof. Joseph Mizrahi , D.Sc.
Dept. of Biomedical Eng. Technion, Israel Institute of Technology
Haifa 32000, Israel
Phone: +972-4-8294128 Fax: +972-4-8234131
e-mail: jm@biomed.technion.ac.il

------------------------------------------------------------------------------------------------

>From Anne Hollister, MD Jul 31, 96 04:29:33 pm -0500

In my research on the subject, I have found only that the hip is considered
a ball and socket joint, and therefore treated as three orthogonal axes of
rotation because of its gross shape: A ball and socket. This inference was
made in the last century, and is documented in the German literature from
the early 1900's. I was unable to discover inverse kinematic studies to
validate this, but would be interested to know of any.

Anne

Anne Hollister, MD
LSUMC-S / Orthopaedic Surgery
1501 Kings Hwy.
Shreveport, LA 71130-3932
email: anne@www.ortho.lsumc.edu

------------------------------------------------------------------------------------------------

Leonard

Check the masters and doctoral theses of Slobodan Tepic at MIT (about 1980,
1982). He used ultrasound to measure cartilage geometry and thickness and
found the unloaded joint to be spherical within +/- 50 micrometers. More
recently (1993), Pat Lord developed a computer-aided surgical tool for
hip osteotomy and checked the sphericity assumption on MRI/CT data.

later

Mike Murphy