Metal-on-metal implant analysis yields clues for understanding osteolysis and wear
Implant retrieval analysis can connote failure with devices, processes, and technique. Implants retrieved from hip resurfacings, however, present opportunities to develop better devices and processes for patients, according to Pat Campbell, PhD.
Dr. Campbell serves as the director of the Implant Retrieval Laboratory of the J. Vernon Luck Research Center, Orthopaedic Hospital at the University of California, Los Angeles. This multidisciplinary team has examined hundreds of implants from histologic, material, and biomechanic points of view.
Since 1991, orthopaedic surgeons and manufacturers have been sending her their retrieved implants so they can learn how to improve the device, the procedure, and the process of hip resurfacing. Her research on retrieved hip resurfacings was recognized by the Hip Society John Charnley Award in 2006.
Looking within for the answers
Dr. Campbell and her team have collected more than 600 hip resurfacing implant specimens, including 250 of the newer implant types.
Her current research is focused on metal-on-metal hip resurfacing components. In addition to the retrieved implants, she and her team analyze bone, soft tissue, and joint fluids that are submitted with the retrieved devices.
“We cut the femoral implants into sections to analyze the depth of cement penetration and its distribution, the bone-cement interface integrity, and the viability of the bone and its remodeling responses,” Dr. Campbell explained.
“We examine soft tissues for the cellular response to the metal wear products, including metal allergy or sensitivity. The joint fluids are analyzed for metal particles and ion levels,” she said.
Implant retrieval: A multifaceted analysis
A successful hip resurfacing procedure must achieve a balance between correct mechanics and tolerant biology. Based on Dr. Campbell’s clinical data, the number one cause for hip resurfacing failure is femoral neck fracture.
If fixation is not achieved or if the bone is burned in the cementing process, femoral
loosening can result, according to Dr. Campbell. Proper acetabular placement is critical to reducing wear and avoiding the release of excessive metal particles, which can result in pain, osteolysis, and the need for revision surgery.
“Even if the surgery is successful and the implant well-positioned, a revision may be required if the patient has an allergic response to the metal. This, however, seems to be a rare occurrence,” said Dr. Campbell. “Metal sensitivity has been documented and is associated with an unusual histologic appearance and a low-wear, well-functioning bearing.”
A Web site is now available to assist patients, pathologists, and surgeons who are dealing with the problem of metal sensitivities (www.metalsensitivity.com).
Infections may develop even if the surgeon does everything right. “We don’t see very many of these and the risk of infection with resurfacing does not seem to be higher than any other type of total joint,” she said.
Fractures: Primary cause of failure
Fractures of the femoral neck are the main cause of short-term failure in resurfacing procedures and occur at a median of 2 months. Implant retrieval analysis frequently indicates that these fractures occur in new bone that is still weak. Because the bone is alive, these are not avascular failures.
“The choice of the femoral head is so important,” Dr. Campbell emphasizes. “Bone quality will affect the ability of the resurfaced head to heal and withstand the altered loads that the implant imposes. In nearly all cases, the pathology shows that the fracture occurred where the bone is healing. Fig. 1 shows vigorous new bone formation, as well as the old bone and the vascular tissue.”
Femoral fractures may also result from unseated components. By measuring cut sections and cement mantle thickness, the lab found implants that were not fully seated—putting the femoral neck at risk in several ways.
“In an effort to seat the component, the surgeon may have applied extra pressure or hammer blows—leading to stress fractures. Reamed bone may have remained uncovered, increasing the stress, and the thick mantle may have reduced bone cement penetration for fixation.
“Although reports exist of stress fractures healing under metal-on-metal surface arthroplasty components, the bone that had undergone repair is not as strong. Our histologic analysis shows that fractures occurred in these specific sites,” she continued.
The Implant Retrieval Laboratory has seen a decrease in the number of femoral fractures due to unseated components as surgical techniques improve.
Dr. Campbell also reported that ischemic fractures are relatively rare but can occur at the interface of the dead and live bone under the component. This surgery-induced osteonecrosis can occur as nutrient vessels are severed, particularly if the surgeon makes an extensive dissection and removes all of the soft tissues from the femoral neck.
Fixation and femoral loosening
Most cases of femoral loosening occur an average of 4 years following surgery. Dr. Campbell indicated that the number of cases of femoral loosening has decreased with improved cement-application techniques.
Implant retrieval analysis shows two primary causes for femoral loosening—a lack of initial fixation or a loss of that fixation.
The lack of fixation is attributed to poor quality bone and the cementing technique. “Although most current resurfacing designs use cement for femoral fixation, variations in the method and the timing of cement application, as well as in the depth of the cement mantle and bone penetration, can exist. This should be a cause for concern in light of current cementing techniques where the control of the cement mantle thickness and extent of penetration may be difficult to achieve,” said Dr. Campbell.
Excessive cement usage can lead to thermal necrosis of the surrounding bone. Results from finite element analyses conducted by one of the lab team demonstrated that when deep cement penetration was combined with a relatively small, cement-filled cyst, “peak temperatures within the femoral head interfacial bone were high enough and the duration long enough to cause thermal bone damage.”
According to studies by her British and Belgian collaborators, thermal necrosis can be reduced by reducing temperatures below the level for bone necrosis through the use of extensive pulsatile lavage, suction cannula, and early reduction.
When loss of fixation was the cause of the femoral loosening, “we observed thick interfacial membranes, bone resorption, and transformation of bone into fibrous tissue in several metal-on-metal surface arthroplasty components,” she explained. Some components completely dissociated when fibrous tissue replaced bone (Fig. 2).
Malpositioning can lead to high wear, osteolysis
“Clearly, implants that function well and are well placed have low wear. The ones that are malfunctioning are often placed badly and have a higher rate of wear,” stated Dr. Campbell.
Acetabular component malpositioning, usually defined as greater than 50 degrees to 55 degrees of lateral opening, is the main cause of this problem. According to Dr. Campbell, malpositioning is a frequent and increasing reason for revision.
“Improper positioning of the socket can result in high wear, leading to excess debris, and atypically high ion counts,” she said.
High wear from edge loading can cause metallosis, synovitis, and the formation of groin masses. Some patients experience groin pain secondary to the swelling and/or impingement. More revisions for unexplained pain are found to be associated with high wear than with metal allergy.
Implant analysis reveals the following lessons for all orthopaedic surgeons:
- Fractures are related to the bone quality of the femoral neck and to surgical technique.
- “Lasting fixation is achievable with current cementing techniques if thermal necrosis is avoided,” Dr. Campbell emphasized.
- Socket malposition remains a concern, particularly as this is increasing in frequency. Malposition leads to high wear, which can cause a myriad of problems including pain and revision.
- Metal sensitivity is a rare but continuing difficult problem.
“It is difficult to diagnose—histologic analysis of periprosthetic tissues is required. Patients with high ion levels should be considered to have malfunctioning implants and may be at higher potential risk for soft-tissue complications,” said Dr. Campbell.
Dr. Campbell has the following disclosures: Orthopaedic Hospital Foundation, Wright Medical Technology, SYNOS Foundation (Zimmer), DePuy, Smith & Nephew (Australia), Stryker, Biomet and the W.G. McGowan Foundation.
Be part of the action
Orthopaedic surgeons who perform hip resurfacing procedures are invited to send their implants to the Implant Retrieval Laboratory of the J. Vernon Luck Research Center, Orthopaedic Hospital at the University of California, Los Angeles, or other labs instead of returning them to the manufacturers.
“We bring academic expertise, background, and experience to examining these implants as ‘big projects,’” said Dr. Campbell. The lab is a nonprofit 501(c)3 organization and does charge a fee to cover its expenses. Dr. Campbell can be reached at email@example.com
Read more in JAAOS
Dr. Campbell addressed the biologic reaction of newer bearing surfaces to wear at the 2007 Osteolysis and Implant Wear Symposium, sponsored by the AAOS and the National Institutes of Health. For more on this topic, refer to the special supplement to the Journal of the AAOS, June 2008, available online at www.jaaos.org
Annie Hayashi is the senior science writer for AAOS Now. She can be reached at firstname.lastname@example.org