Device simulates all three motions of walking and provides realistic variable loading during each step. Simulator will enable laboratory evaluation of all known types of total hip prostheses

Johnson, B. L.

Swikert, M. A.

English

NASA Technical Reports Server

October 1975	 B75-10148 
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Hip-Joint Simulator Accurately Duplicates 
Human Walking Pattern
Friction force 
transducer 
Wear proximity	 Fluid 
probe	 supply 
Adduction - 
abduction 
Alternate specimen
configuration
0 
Acetabular cup 
•
1 Load 
Femoral ball
	
	
I	 It 
External-internal 
rotation	 ii> 
Flexion 
Load
Extension 
FULL PROSTHESIS HIP SIMULATOR 
The Problem: 
Laboratory methods are needed to establish reliable 
minimum wear lives and performance for orthopedic total 
hip prostheses. 
There have been many simulators built for bioengi-
neering laboratories of prosthesis manufacturers, univer-
sities, clinics and research institutes. There is no standard 
device, and virtually all of the present devices are not in 
operation because of difficulties in obtaining significant 
data with reasonable effort or because they are deficient 
in that the application is not adequately simulated. Most 
devices only provide for one type of motion and uniform
loading; they are difficult to use because precise alignment 
of the two prosthesis parts is essential to obtain useful 
data. 
The Solution: 
A hip joint simulator that can be used to evaluate the 
relative friction, wear and life of orthopedic internal 
prostheses for human hip joints, and which can be 
modified for use with knee and possibly other joints. The 
complex motions and dynamic load patterns of human 
walking are accurately duplicated and can be varied while 
(continued overleaf) 
This document-was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States	 will be free from privately owned rights.
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providing unique self-alignment features that are critical 
to significant simulation and convenient operation. 
In fatigue wear, the magnitude of maximum stresses 
and the direction and frequency of stress cycles determine 
wear •rates. Thus, for fatigue wear, stem fracture, and 
other life limiting phenomena, an effective simulator 
evaluation must duplicate real load patterns as well as the 
motion rates, magnitude, and frequencies of real walking 
by humans. All of these are accomplished by this 
newly-developed simulator. 
How It's Done: 
This device simulates all three motions of walking and 
provides realistic variable loading during each step. The 
diagrammatic sketch of the apparatus in the figure shows 
the femoral ball component mounted in a fixture ex-
tending from the end of an oscillating shaft. The shaft 
oscillates ±200 simulating the extension and flexion in the 
sagittal plane of the primary walking motion. An arm 
extending radially from the fixture restrains the mounting 
assembly and, in turn, causes the ball to oscillate ±70 in a 
supporting bearing simulating the rotational mode in the 
transverse plane of the second walking motion as indi-
cated by turning the feet out and in. The acetabular cup 
of the prosthesis is mounted in a fixture that is supported 
and stabilized with self-aligning tension flexures. The 
flexures carry the loading applied through the cup. The 
third motion is provided through the cup with ±60 
abduction and deduction simulating the sideways exten-
sion of the leg during walking. That motion is transmitted 
by a push rod follower from an axial cam attached to the 
main drive shaft. The drive shaft crank arm is driven by a 
fractional horsepower electric motor through a gear box 
assembly at up to 60 cycles per minutee. 
The flexure suspension and a force transducer provides 
continuous measurement of friction force in the direction 
of primary motion. Wear displacement is continuously 
measured from the center of the acetabular cup at null 
position. 
Load is applied hydraulically to the acetabular cup 
through two cables and the main flexures. The hydraulic 
load cylinder is actuated by a servo system controlled 
from a commercial electronic programmer. A load pro-
gram to simulate the variable loads during each step of 
walking (e.g., heel impact may cause loads that approach 
five times body weight) can be diagramed on an alumi-
nized mylar chart attached to a rotating drum that is 
synchronized with the apparatus drive shaft. A capaci-
tance probe in the instrument follows the load diagram of 
the chart and actuates the servo system. 
An elastomeric bellows can enclose the ball and socket 
simulating the capsule of the joint. Fluid to simulate the 
synovial fluid may be pumped into the ball and socket 
enclosure and be retained within the bellows or circulated 
through an external flow system. A tubing pump is 
utilized for the simple supply system. The primary test 
components are in a chamber so the environment can be 
controlled.
Notes: 
1. The availability of this simulator will enable more 
significant laboratory evaluation of all known types of 
total hip prostheses. 
2. The wear life of present hip prostheses has been 
estimated at from 10 to 20 years and surgeons will not 
deliberately entertain the prospect for re-operation to 
install new appliances. Patients become sensitized to 
the bone cement and the surgical procedures are severe. 
Total hip replacement surgery is most often considered 
only for older persons. Extended wear life, which can 
be established with the simulator, may make the 
operation available to many younger patients now 
suffering pain and lack of mobility. 
3. A primary advantage of this simulator is ease of set up 
for test. The apparatus can be modified to alter the 
operational parameters described. The self-aligning 
feature is especially important. Also, the device has 
adapters to fit the variety of important prosthesis types 
now used and they can be mounted in the adapters 
without the use of jigs or fixtures and bone cement 
mountings. A special adapter is also available so the 
femoral ball component can be installed without the 
stem, thus a reduced cost specimen can be used when 
test objectives permit. 
4. No additional documentation is available. Specific 
technical questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B75-10148 
Patent Status: 
NASA has decided not to apply for a patent. 
Source: R.L. Johnson and M.A. Swikert

Lewis Research Center

(LEW-1 2515)
. 
B75-10148	 Category 05


Hip-joint simulator accurately duplicates human walking pattern

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