Limited hip resurfacing showing promise

Looking to the past helps map out the future.

As a renewed interest grows in metal-on-metal hip prosthesis articulations, limited femoral hip resurfacing is once again gaining attention in the orthopedic community.

In a presentation at the Orthopedics Today NY 2005 meeting in New York, Michael R. Dayton, MD, assistant professor of orthopedic surgery at the University of Colorado at Denver and Health Sciences Center, told the audience of the history, benefits and drawbacks of the hip resurfacing procedure.

Compared to traditional total hip arthroplasty, hip resurfacing replaces only the femoral head, thus preserving bone stock. Head resurfacing components are available for use in the United States. The acetabular components, however, are involved in ongoing investigational device exemption (IDE) studies.

“Two things that need to be considered with femoral resurfacing. One is the concept and the comfort level of metal-on-metal as a bearing surface,” Dayton said. “Second is the concept of femoral resurfacing in and of itself as a design feature.”

He said that metal-on-metal articulation grew out of favor in the past two decades due to design and manufacturing issues and the advent of metal-polyethylene prostheses. “However, looking in hindsight, the 20-year survival data of the metal-on-metal actually compares very favorably with the metal-on-polyethylene.”

Recent improvements to the resurfacing prostheses include greater cup-polar contact and more lubrication around the equatorial region.

The advantages of metal-on-metal articulations include a lower wear rate based on laboratory studies, claims that it is self-polishing, and the potential to use larger head sizes thus potentially lowering dislocation rates. “Plus,” Dayton added, “it eliminates concerns about volumetric polyethylene wear, and there is a potential for the particles to be less inflammatory.”

He noted that research has revealed the possibility of metal-on-metal generated particles being smaller and may incite a bit more inflammatory reaction. “So, the jury is certainly still out there,” he said. “However, long-term outcome of nearly 30 years demonstrate that there are some very favorable wear rates even in a first-generation metal-on-metal.”

The FDA has not completely approved procedure and concern about metal ion levels on a long-term basis. “Certainly it may have limited applications in terms of pathological morphology of the femur as well as bone type and consistency,” Dayton said. “It is considered a relatively technically difficult procedure because of it not being performed in any volume at all.”

Not a good MIS candidate

Although it would seem otherwise, hip resurfacing may not make a good choice for minimally invasive surgery (MIS). “It is a pretty technically difficult procedure and may not coincide with MIS at this time,” he said. “It usually involves longer operative times, potentially just as long or longer incisions and more extensive dissection, which is difficult to do in poor visualization cases.”

He noted a subset of results from a U.K. surgeon, which has shown no difference in blood loss, implant position or in rehabilitation. “So one must really question how necessary MIS would really be for this particular type of implant.”

Dayton said there have been two main modes of failure with these implants — aseptic loosening and fracture — in the investigational trials. “Dr. Mont had a 22% fracture rate in his first 50 cases due to risk factors, including notching at the femoral neck, reaming out of the axis of the femoral neck, and aggressive osteophyte removal. Dr. Amstutz has recently reported only a 1% reported failure due to fracture, which is more favorable.”

Factors for aseptic loosening included the following:

smaller-than-necessary femoral component;
advanced osteonecrosis; and
varus positioning of the femoral component.

“Five-year survival in this group in the U.K. actually was very favorable with 98% overall and 99% for aseptic revisions, which is very good and what we would expect for a viable prosthesis,” he said. “On an investigational basis, Dr. Amstutz reported a nearly 95% survival at 3.5 years.”

Indications/limitations

Indications for resurfacing are mainly degenerative disease in most study groups, he said.

“In terms of limitations there really can be little change in the femoral offset, which can be affected with the technology,” Dayton said. “Extremity length changes can be performed by up to only about 1 cm, so they are not really great procedures for people with really low offset or leg length disparities of greater than 1 cm. It is not a great consideration for people who are interested in and strongly want MIS procedures for a complex pathology or morphology.”

He also recommended avoiding the following:

significant varus placement;
use in late stage collapse or osteonecrosis, which has a poor outcome;
cases with extensive bone loss proximally in the femur; and
intraoperative femoral neck notching.

For more information:
Dayton MR. Limited hip resurfacing. Presented at Orthopedics Today NY 2005. Nov. 19-20, 2005. New York.