to be measured on the lower polyethylene 
tibial components. This change actually 
consisted of a combination of deformation 
and abrasion, but in this paper, we shall 
refer to it as "wear". Before photogram- 
metry was used, the measurement was per- 
formed in an X-Y-Z micrometer block, using 
micrometer screws for X and Y, and a dial 
indicator with 2.5pm (.0001") graduations 
for Z. The dial indicator had a fine ball 
point, and the spring load on this indicator 
was adjusted to be as little as possible. 
  
Figure 
a knee 
l — A tibial prosthesis mounted in 
joint. 
36 
  
Figure 2 — The M.I.T. 
testing machine simu- 
lates the action of the human knee. 
When the polyethylene component was measured 
in its aluminum block, care had to be taken 
that no foreign material came between the 
block and the base supporting surface of 
the micrometer device. Point elevations 
in Z to the nearest 2.5um were extracted 
from the concave surfaces of the poly- 
ethylene component, using a 2.5mm square 
grid pattern.  Obtaining these measurements 
for just one component was a tedious oper- 
ation, and often took as long as a week. 
The grid data was then processed with the 
MIT CONTUR contour mapping program in an 
IBM 360 computer. A contour plot as shown 
in Figure 3 was then produced off-line. 
The researchers at MIT felt that photo- 
grammetry might be a better approach 
for measuring their tibial prostheses, so 
they contacted our Kelsh Instrument 
Division. We were intrigued by the problem, 
and we readily agreed to a test of our K-460 
close-range system for the wear measurement.