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) [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 an automatic, cheaper, and more accurate computer method to do the same thing.

– To simulate every control condition. 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.

– To reduce noise in early development of new methods.

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 simulatingwearing of every anteversion and abductionfrom JBJS [1].

• How does the simulator work?– Line tracing method

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.

– Why not using OpenGL or DirectX?

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.

– Absorption lawThe X-ray is a kind of radiation beam with high energy. It

follows the general rule of absorption.

Beer-Lambert law:

                         -kbcPenetration = ek: molar absorbability (a)

b: path length

c: concentration (b)

It is hard for us to use it on our simulator.

– Absorption example (Fig. 2)

– Lookup table methodThe 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 metalmodel made of titanium with 1, 2, …, 5mm thickness.(b) Its X-ray image.

PE Wear Meter• What did we do before?

In the past, we measured PE wear manually. A well accepted method is Livermore’s method (Fig. 3) [2]. He finds the center of femoral head, measures the thinnest PE, and then reads the wear.

• What’s new?

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 [3][4][5] (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. 3a. Livermore’s method, find head center. Fig. 3b. Find the thinnest polyethylene.Fig. 3c. Then measure it.

• Our method– Find head-neck junction by cross-co

rrelation

Fig. 5. Make a template and mark corresponding head-neck junction and the indicated pl

ace.

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.

– Project 200 lines from indicated place to upper right side with about 135 degrees span.

– Detect edges with vector edge detector.

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.

• Our method (cont.)– Find center from the 200 detected

edges.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.

– Project 240 lines from the new center with 270 degrees span. The lower left part is excluded.

– Find head edges with the same vector edge detector.

– Find the head center

Again, the center of femoral head is found with exclusion of

extremes.

– Check centers by “Hill-Climbcing Search” [7]

– Compare the two centers, and find the displacement vector.

•.

Materials & Methods• Simulated data

– 64 simulated X-rays with 2 different anteversion angles, 2 different abduction angles, 4 different superior wears, and 4 different medial wears.

– We measure these simulated X-rays automatically by our “Auto PE Wear Meter” program.

– The results are compared.

• Real X-rays– We digitized 88 total hip arthroplast

y X-rays using Sony S70 digital camera size (2048*1680).

– The camera was set with the same distance and same zoom.

• Real X-rays (Cont.)– There is no exclusion criteria of digi

tization. – Results classification:

• Wrong detection on cross-correlation

• Wrong detection of acetabulum• Wrong detection of head• Minor error on acetabular center

estimation• Minor error on head center estim

ation• Unmeasurable X-ray by Liverm

ore’s method• Partially measurable X-ray, whic

h can be measured by other method

• Measurable X-ray by Livermore’s method

Results• Simulated data

-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).

– The error on measuring medial directional wear using our program is -0.023~+0.176mm.

– The error on measuring upper wear using our program is

-0.152~+0.315mm.

Fig. 6. Four examples of successful detection

Fig. 7. The three misdetections.

• Real X-rays• 47 excellent detections• 2 unknown errors• 3 wrong detections on cross-corr

elation (Fig. 8)• 23 wrong detections of acetabulu

m (Fig. 9)• 3 wrong detections of head (Fig.1

0)• 10 minor errors on acetabular cen

ter estimation (Fig. 11)• 0 error on head center estimation• 19 unmeasurable X-rays by Liver

more’s method (Fig. 12)• 49 partially measurable X-rays,

which can be measured with instrument (Fig. 13)

• 21 measurable X-rays by Livermore’s method (Fig. 14)

Fig. 8. Wrong detection 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 acetabulum edges.

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 byLivermore’s method.

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. 15a. Head edges areunclear to be detect manually.

Fig. 15b. Our program can detect the edges and find the best fitting circle.

Fig. 16a. Unmeasurable by Livermore’s method.

Fig. 16b. The program can measure it.

Fig. 17a. Measurable manually.

Fig. 17b. Misdetection by our program.

Fig. 18a. Measurable by Livermore’s method.

Fig. 18b Misdetection by our program.

Discussion

• In the simulated data, error in medial wear is -0.023~+0.176mm and error in upper wear is -0.152~+0.315mm.

• Compared with literature [1][6], in which the error listed was 0.1mm, our method is as good as other computer-assisted methods.

• Our program is the first fully automatic program in which no human intervention is needed for detection.

• Detection rate:– Our program: 54.5% (40/88).If we acce

pt minor error, the rate becomes 65.9% (58/88).

– Livermore’s method: 23.9% (21/88)– Others: 78.4% (69/88)

• Reasons of misdetection– Low kv

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.

– Screw noiseThe screws of acetabulum are located just above it. That is t

he place where our program detects edges for estimating acetabulum center. Sometimes, the screws cause noises and misdetections.

– Bone noiseThe edges of bone will cause white lines on X-rays. If the w

hite lines intersect the prosthesis edges, they may cause misdetection.

• Distortion– X-ray filming

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.

– DigitizingThere are several devices for digitizing X-rays. The digital c

ameras take pictures through lens. The lens will cause distortion.

• ValidationCurrently, we have not tested our program on revision hip ar

throplasty, which is to measure wears after removal of polyethylene and compare it with preoperative X-rays. This should be our future work.

Conclusions

• We designed a program which can automatically measure polyethylene wear. We have tested it on simulated X-rays and the result is encouraging. Clinical validation is needed in future work.

Authors• Liaw Chen-Kun, MD, Visiting Staff, Department of

Orthopaedic Surgery, En Chu Kong Hospital

• Wu Tai-Yin, MD, Fellow, Department of Family Medicine, Jen-Ai General Hospital

• Hou Sheng-Mou, MD, Professor, Department of Orthopaedic Surgery, National Taiwan University Hospital

• Yang Rong-Sen, MD, Professor, Department of Orthopaedic Surgery, National Taiwan University Hospital

• Fuh Chiou-Shann, PhD., Professor, Department of Computer Science and Information Engineering, National Taiwan University

• Liaw Shong-Hon, MD, Resident, Department of Emergency, Sin-Lau Christial Hospital

• Wu Chang-Chin , MD, Visiting Staff, Department of Orthopaedic Surgery, En Chu Kong Hospital

• Tai Han-Cheng , MD, Visiting Staff, Department of Orthopaedic Surgery, En Chu Kong Hospital

• Liu Dah-Hsiang , MD, Visiting Staff, Department of Orthopaedic Surgery, En Chu Kong Hospital

Institutions• En Chu Kong Hospital

• National Taiwan University Hospital

• Department of Computer Science and Information Engineering, National Taiwan University

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