Head & Cup Simulator. Why do we use simulator? To verify the accuracy of new methods.
When we invent a new measuring method, it is important to verify its accuracy and reliability. Many methods have been published, including using a special machine (see left picture, Fig. 1) , in which anteversion, inclination, and three directions’ wearing can be adjusted manually. Another method is to fabricate a cup inside plastic material with fixed orientation and wearing parameters. All these published methods are good but expensive. We propose anautomatic, cheaper, and more accurate computer method to do the same thing.
For mechanical devices, to accurately adjust the orientation and wearing parameters is difficult. It is also hard to prove it. With computer, every condition is easy to simulate.
In reality, noise is inevitable, but in simulated situation, noise is removable. In the early stage of developing a method, noise will always bother us. With the use of simulator, noise is removed, and we can focus more on the real measurement, thus facilitating the development.
Fig. 1. A mechanical device for simulating
wearing of every anteversion and abduction
from JBJS .
We simulate every X-ray beam traveling from source to the film. The simulated prosthesis projection will be calculated. The total thickness of metal is estimated and then we translate to gray level of the simulated X-ray.
DirectX is the 3D tool of Windows. The OpenGL is the open source version of 3D tool. They are useful and effective in virtual reality. They are good in drawing non-transparent object. In our case, the X-ray beam is a highly penetrating radiation. We are drawing a semi-transparent object. It is hard to use OpenGL or DirectX, although they are quite good in performance.
The X-ray is a kind of radiation beam with high energy. It follows the general rule of absorption.
-kbcPenetration = ek: molar absorbability (a)
b: path length
c: concentration (b)
It is hard for us to use it on our simulator.
The simulated X-ray was analyzed. We build a table with known thickness, in which we can deduce its photo-density on X-ray. We then look up the table, interpolate the data and then get the result of photo-density.
Fig. 2. (a)A metal
model made of titanium with 1, 2,
…, 5mm thickness.
(b) Its X-ray image.
In the past, we measured PE wear manually. A well accepted method is Livermore’s method (Fig. 3) . He finds the center of femoral head, measures the thinnest PE, and then reads the wear.
Fig. 4a. Hardinge’s method Fig. 4b. Shaver’s method (need to point out p1, p2, p3)
Fig. 4c. John’s method (need to point out ischial tuberoses).
As the computer technology progresses, some digitalized methods  (Fig. 4) have been proposed. All of them need human hand to point out several important landmarks. With the new invention on computer vision, we propose a new method to make the computer find the prosthesis by itself and then compute. No manual intervention is needed.
Fig. 3c. Then measure it.
Fig. 3b. Find the thinnest polyethylene.
Fig. 3a. Livermore’s method, find head center.
Fig. 5. Make a template and mark corresponding head-neck junction and the indicated place.
At first, we make an ordinary X-ray with template. Then we mark the important landmark and pointout the indication from it. The program will use cross-correlation method, find the corresponding landmark on the other X-rays, and then find the indicated place. This procedure is the most time-consuming procedure and costs about 60 seconds (Pentium 500MHz computer). This is also our original contribution to this method.
Using vector edge detector subroutine, we detect the shell edges. These edges are rechecked again and again in order to remove noise such as screw or others. We consider currently published edge detectors unsuitable, so we invent a new one. It detects vectory edges, so we called it vector edge detector.
The center of the 40 edges are found by calculus. Then the program will exclude the extremes and find center again and again, until there are no extremes.
Again, the center of femoral head is found with exclusion of extremes.
-Our program can detect 61 of 64 simulated X-rays without misdetection (Fig. 6).
-The three mis-detected X-rays images were due to misdetection of acetabulum (Fig. 7).
Fig. 6. Four examples of successful
Fig. 7. The three misdetections.
due to cross correlation.
Fig. 9. Wrong detection
on finding acetabulum.
Fig. 10. Wrong detection
on finding head.
Fig. 11. Minor error on
fitting the circle with
Fig. 12. Head-acetabulum
edges are too unclear to
use Livermore’s method.
Fig. 13. With instrument,
the thinnest edges can be
detected by other edges.
Fig. 14. Measurable by
unclear to be detect
Fig. 15b. Our program
can detect the edges
and find the best fitting
Fig. 16a. Unmeasurable
by Livermore’s method.
Fig. 16b. The program
can measure it.
2 X-rays measurable by program but unmeasurable manually
27 X-rays measurable by program but unmeasurable by Livermore’s method
24 X-rays measurable manually but unmeasurable by program
8 X-rays measurable by Livermore’s method but unmeasurable by program
Fig. 17a. Measurable
Fig. 17b. Misdetection
by our program.
Fig. 18b Misdetection
by our program.
Fig. 18a. Measurable
by Livermore’s method.
As we know, the kv of X-rays will change the penetration power. When the kv is low, the penetration power is low. On the other hand, when the kv is high, the penetration power is high. If the kv is too low, the bone and prosthesis will be saturated as white. If the kv is adjusted high, the prosthesis will be whiter. That makes detection easy.
The screws of acetabulum are located just above it. That is the place where our program detects edges for estimating acetabulum center. Sometimes, the screws cause noises and misdetections.
The edges of bone will cause white lines on X-rays. If the white lines intersect the prosthesis edges, they may cause misdetection.
When the X-rays pass through body to film, there are grids between body and film to filtrate scattering. The grids may cause torsions and make circles became ellipses.
There are several devices for digitizing X-rays. The digital cameras take pictures through lens. The lens will cause distortion.
Currently, we have not tested our program on revision hip arthroplasty, which is to measure wears after removal of polyethylene and compare it with preoperative X-rays. This should be our future work.