Metallosis and Pseudotumor of the Hip after Resurfacing Prosthesis

Case and Explanation courtesy of Dr. William Denison

"Pseudotumor," is a problem that occurs with metal on metal resurfacing and any other metal on metal prosthetics. Apparently, it is some sort or nonneoplastic but aggressive immune type process that eats through bone, nerves, etc, and manifests as an enlarging cystic mass, per Joe McCarthy. If diagnosed, it needs to be excised and the joint revised.  Joe seemed to think that we will be seeing these, that in Britain they are expecting to see them in 10-15% of cases, or so he says. They occur with resurfacing procedures, more frequently when the acetabular cup is greater that 45 degrees angulated, which is less desirable; the cup should be less than 45 degrees off axial plane. Bottom line: If you see a weird erosive cystic mass around a replaced joint, think about this diagnosis, and call the orthopedist.
Here is a radiograph of a hip resurfacing case where the acetabular cup component is angulated > 45 degrees off of the axial plane.  This pt had pseudotumor (see below MRI image).  >45 degrees apparently increases the risk of metallosis and pseudotumor.  Further below are some internet search results of some articles. 

http://radiology.case reports.net/index.php/rcr/article/viewArticle/115/316  (good imaging example)

 

http://www.ejbjs.org/cgi/reprint/70/8/1238  (old article but good on the pathologic explanation)

 

http://www.ejbjs.org/cgi/content/abstract/88/6/1183  (abstract only regarding this disorder)

 

http://www.hipresurfacingsite.com/Doctors-Articles/Doctors-Articles/Mr.-Derek-McMinn-on-Pseudotumors/menu-id-108.html   (patient oriented article, does not appear to be peer critiqued, but offers some info)

 

I had a question regarding the radiographic measurement I sent. To clarify the angle: One makes a line through the "tear drop" appearance seen at the inferior medial aspect of both hips, and then another line through the plane of the acetabular cup. In the example I gave that measurement is 135 degrees due to the pacs angle software function. Dr McCarthy is actually referring to the more acute angle at the intersection of those two lines; ie, on the other side. So, you subtract that number (135) from 180, and get 45 degrees, which is borderline too steep. In this case the pt had metallosis, and it was felt that the acetabular cup angle was too steep by the orthopedists.