Sunday, March 27, 2011
I tried to show the abduction angle measurement that I made myself which is identified by the yellow lines. The surgeon will do that on his own also. The target angle for all hip resurfacings is generally 40 degrees. That's the angle identifed as alpha on this image. It represents the outward tilt of the acetabular cup and is measured between a straight line across the pelvis and a straight line along the face of the acetabular cup. You can see the small indentations on the implant which designates the edge of the cup. The cup wall is about 3 mm thick and is bevelled at the face, so the "ledge" is soft and not abrupt. However, it still shows up on the image. Keep in mind that the both pieces of the implant are essentially touching and the x-ray makes it look like one piece. See the Birmingham ad in another post here to get a better understanding of how the 2 components go together.
The software that my surgeon's radiologist uses is OmniVue and the images the radiologist gave me had a free viewer version of OmniVue. It's not very robust, but it's free so I shouldn't expect much. I pulled the image up using the free viewer then drew on the lines for measuring. The software measures the angle, but it uses the edges of the image as the reference, not the line through the pelvis. The number "31.8" can be seen on the image. That's the angle provided by the software. I was not square on the x-ray table, mostly because at 2 weeks post-op I was still a little tender and getting straight on the x-ray table was not a priority. I could not export the image, so I used Faststone Capture to do a screen capture at high resolution and generated a tiff, brought it up in Paint.net and fuzzed out the personal info and personal area. BTW - The radiologist did not throw a lead codpiece on me, I suspect because of my age.
I then measured the angle between the lines, on the screen, using a protractor. The angle is about 38 degrees.
There is much talk about abduction angle of the acetabular cup in the hip resurfacing community. The reason is because the steeper angles (near 55 degrees or more) may cause increased wear on the components and metallosis (high metals ions in the fluid around the joint). There is a general belief within the community that you are more likely to have metallosis and problems with steeper angles. The well-known Belgian surgeon (De Smet) supports the argument that steeper angles cause more wear and higher metals concentrations, in his study comparing the BHR to the Conserve.
On the other hand, Pritchett does not suggest that steeper angles are directly responsible for metallosis. Pritchett suggests that there are other factors that are in play and provides details in his paper on metallosis. He also suggests that metallosis occurs in about 4% of resurfacings.
Also, there is talk that a larger component will have less wear, due to the increased surface area which spreads out the weight and thus the wear. This is why some surgeons suggest that smaller framed people are not good candidates for resurfacing. However, most of the top surgeons will do resurfacing on smaller framed people.
In summary, I'm happy with my angle and I'm not too concerned about excessive wear or metallosis.