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Osteonecrosis Most Common Cause of Hip Resurfacing Arthroplasty Periprosthetic Fractures

Posted on: 02/18/2010
Hip replacements, or arthroplasties, are becoming increasingly common in developed nations. Some replacements are partial and others are total, depending on the extent of the damage and the surgeon's choice. However, in younger people, the less that has to be replaced, the better because the younger the people are at the time of the replacement, the higher the risk of them having to have revision surgery later on - perhaps more than one. Every time revision surgery is done, bone is removed, so if too much is removed early, then revision surgeries may not be possible.

One way around having to replace the whole hip is by performing a hip resurfacing arthroplasty, which involves replacing on the damaged part of the hip, in essence, resurfacing it. Resurfacing allows the surgeon to attach the replacement in the joint without having to go into the leg bone, the femur, which may add to its popularity when it is feasible to do it. According to statistics, five years after resurfacing, 96 percent of patients have functioning prosthesis (implant) and after seven years, 95 percent do. Although this surgery is often very successful, resurfacing surgery does have complications. In one to three percent of cases, the femoral neck, the top of the femur, fractures. The prosthesis loosens in one to two percent of cases.

The authors of this study wanted to see if they could identify the different types of fractures, using prosthesis that had been removed from the patients, in order to understand why they broke in the first place.

To perform the study, researchers obtained 152 femoral head parts (remnants) and the femoral part of the hip replacement that failed. Of the 152 hips, 107 had to be revised because the prosthetic had broken. The remaining hips were replaced because a component failed, the prosthesis loosened, or the patients had groin pain.

The researchers did not know what the original diagnosis was for all the patients, but 62 patients had been diagnosed with advanced osteoarthritis. A few others had been injured, had rheumatoid arthritis, or had osteonecrosis of the femoral head, a condition where the bone cells die off for an unknown reason.

After analyzing the parts they had and the information that was available (sex, age, body mass index, the type of device used and its design, and how many surgeries had been done for each patient before this revision), the researchers found that the average time that the prosthesis broke was about five months after surgery. The researchers broke down the fractures into different categories:

- Type A: acute biomechanical fracture
- Type B: acute postnecrotic fracture
- Type C: chronic biomechanical fracture

Type A fractures did not have any evidence of osteonecrosis or other biological issue that could have caused the break. This break was found in nine of the hips. Type B, on the other hand, had a fracture line related to the osteonecrosis of the bone and this occurred in 55 of the hips. Type C fractures occurred in 43 of the hips. While there weren't as many type A fractures, those that did occur happened much earlier than the other two, around 41 days instead of 149 to 179 days. The researchers also found that the type A fractures almost always occurred in the femoral neck.

The authors concluded that osteonecrosis was the most common cause of fracture-related failures of the prosthesis. As for the biomechanical failures, they suggest that this could be due to incidents that may occur during surgery, such as notching the femoral head or not positioning it properly.

Jozef Zustin, MD, et al. Morphologic Analysis of Periprosthetic Fractures After Hip Resurfacing Arthroplasty. In The Journal of Bone & Joint Surgery. February 2010.  Vol. 92-A. No. 2. Pp. 404-410.

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