Vital Protocol for Reliability and Accuracy of PolyWareTM Measurements

Background: PolyWareTM software (PW) has been exclusively used in most of the polyethylene wear studies of total hip arthroplasty (THA). But, we found that PolyWareTM (PW) measurements can be signicantly inaccurate and unrepeatable depending on imaging conditions or subjective manipulation choices. In this regard, this study reveals the required conditions to achieve the best accuracy and reliability of the PW measurements. Methods: The experiment examined the dependency of PW on several measurement conditions. The Xray images of in-vitro THA prostheses were acquired under a clinical X-ray scanning condition. A liner wear of 6.67 mm, an acetabular lateral inclination of 36.7° and an anteversion of 9.0° were simulated. Results: Among all the imported X-ray images, those with a resolution of 1076×1076 exhibited the best standard deviation in wear measurements as small as 0.01 mm and the least occurrences of blurriness. The edge detection area specied as non-squared and off the femoral head center exhibited the most occurrence of blurriness. At the X-ray scanning moment, an eccentric placement of the femoral head center by 15 cm superior to the X-ray beam center led to an acetabular anteversion error up to 5.3°. Conclusion: The results request researchers to observe following conditions; 1) the original Xray image be 1076×1076 squared X-ray images, 2) the edge detection area be specied as a square with edge lengths of 5 times the diameter of the femoral head centered at the femoral head center, 3) the femoral head center or acetabular center be placed as close to the center line of the X-ray beam as possible, at the X-ray scanning moment.


Abstract
Background: PolyWareTM software (PW) has been exclusively used in most of the polyethylene wear studies of total hip arthroplasty (THA). But, we found that PolyWareTM (PW) measurements can be signi cantly inaccurate and unrepeatable depending on imaging conditions or subjective manipulation choices. In this regard, this study reveals the required conditions to achieve the best accuracy and reliability of the PW measurements.
Methods: The experiment examined the dependency of PW on several measurement conditions. The Xray images of in-vitro THA prostheses were acquired under a clinical X-ray scanning condition. A liner wear of 6.67 mm, an acetabular lateral inclination of 36.7° and an anteversion of 9.0° were simulated.
Results: Among all the imported X-ray images, those with a resolution of 1076×1076 exhibited the best standard deviation in wear measurements as small as 0.01 mm and the least occurrences of blurriness. The edge detection area speci ed as non-squared and off the femoral head center exhibited the most occurrence of blurriness. At the X-ray scanning moment, an eccentric placement of the femoral head center by 15 cm superior to the X-ray beam center led to an acetabular anteversion error up to 5.3°.
Conclusion: The results request researchers to observe following conditions; 1) the original Xray image be 1076×1076 squared X-ray images, 2) the edge detection area be speci ed as a square with edge lengths of 5 times the diameter of the femoral head centered at the femoral head center, 3) the femoral head center or acetabular center be placed as close to the center line of the X-ray beam as possible, at the X-ray scanning moment.

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However, the manuscript can be downloaded and accessed as a PDF. Figure 1 Error (in absolute value) in the wear for spatial eccentricity modes of the femoral head in the original Xray images Figure 2 Errors (in absolute value) in the acetabular anteversion for spatial eccentricity modes of the femoral head in the original X-ray images Measurement of acetabular anteversion using a CAD to investigate the effect eccentricity of the prosthesis from the center of X-ray beam on the acetabular anteversion. The same X-ray images used for Polyethylene measurements were also used for the measurement using a CAD software, i.e. Rapidform 2006®(INUSTechnology, Seoul, Korea). The superior and inferior placements of the prosthesis bring about errors in acetabular anteversion, by the nature of perspective X-ray imaging.

Figure 4
Overall process scheme of the current study X-ray images of the initial (the left) and nal (the right) positions, simulating the wear of the cup by a translation of the femoral stem of 6.67 mmnormal to the equator plane of AC.

Figure 7
Measured values for Ployware evaluation. (a) Wear X-ray images of the initial (the left) and nal (the right) positions, simulating the wear of the cup by a translation of the femoral stem of 6.67 mmnormal to the equator plane of AC.

Figure 8
Eccentricity comparison test setup, i.e. nine spatial eccentricity modes. With respect to the center of the Xray detector, nine spatial eccentricity locations of the THA prostheses were set up to gure out how the eccentricity of the component location affected PolyWare measurement results.

Figure 9
Blur of the edge detection area. For the same X-ray image, different speci cations of rectangular edge detection areas result in different image sharpness. The left is shown normal, but the right is shown blurred. In the normal case, the rectangular edge detection area is speci ed such that its center positions at the very center of the femoral head. In the blurred case, in contrast, the rectangular edge detection area is speci ed such that its center positions considerably off the center of the femoral head, and the edge detection area becomes blurred.

Figure 10
Three ways of speci cation of the edge detection area. Edge detection area assigned as a rectangle whose edge lengths were 5 times (5Dh) square, 7 times (7Dh) square of the diameter of the femoral head