Metal-on-Metal hip implants present diagnostic challenges
MoM Hip Implants Present Diagnostic Challenges (AAOS 2014 Annual Meeting News)
Tribocorrosion occurring as articular surface loss at metal-on-metal (MoM) junctions in hip implants has emerged as one of the most important clinical problems in orthopaedic surgery. The problem has also attracted considerable media attention focused on complications occurring over time.
During the “Metal on Metal and Modular Corrosion: Clinical Impact of Tribocorrosion” symposium Wednesday, panelists surveyed the pressing issues under investigation, including diagnostic modalities and treatment options for patients who may have adverse local tissue reactions (ALTRs) to metal debris.
Retrieving valuable information
Alister Hart, FRCS, of Royal National Orthopaedic Hospital in London, first reviewed what retrieval studies, involving 5,000 components from 22 countries, reveal about implant complications. He described such forensic analysis as akin to study of the black box from an airplane after a crash, providing clues to questions such as: What is the human wear rate? What surgical, design, and patient factors influence wear rate? How does the wear occur?
Hip simulation and imaging findings can then guide the design of future implants in such parameters as material combinations, cup articular arc angle, and clearance.
For retrieval studies, Dr. Hart described the following five ascending levels of data:
- Implant only—component size mismatch
- Intraoperative details—loosening, soft-tissue findings, impingement
- Clinical and blood details—to correlate laboratory tests with other clinical parameters
- Imaging—to distinguish between surgical and design factors
- Registry studies—“the ultimate method of understanding implant performance”
Level 1 failures involving the implant only accounted for 1 percent of retrievals and showed a prominent circumferential wear scar with an oversized head, leading to a conclusion of component size mismatch and surgeon error. Blood metal ions were greater than 7 parts per billion (ppb) in all cases, and all the mismatches were missed at surgery and on plain radiograph.
At level 2, he said, soft-tissue destruction may not be seen on radiographs, and patient sensitivity to metal debris is important. A reduced head-neck ratio increases the risk of impingement, and subluxation of the femoral head raises the risk for edge loading.
Level 3 findings for patients with Birmingham Hip Resurfacing (BHR) found that half the patients had high wear and metal ion levels and half had low wear and metal ions.
At level 4, radiographs are essential, Dr. Hart said. “A surgeon would never consider a hip revision operation without a radiograph. How can proper retrieval analysis be done without a radiograph?”
Level 5 linkage to registry data is essential because it shows the failure rate of implants. “The ASR™ implant showed us that preclinical testing cannot predict the effect of all variables,” Dr. Hart said. “We all want well-performing implants. We need to know why implants fail if early data show a poor trajectory. To avoid continued use of a poor-performing implant, registry-retrieval linkage allows safe innovation.”
He concluded that retrieval analysis enables surgeons to understand the causes of implant failure. The data helps in quantifying the impact of surgical positioning, implant design, and patient factors on clinical performance, and “linkage with registry data improves postmarket surveillance and the ability to safely innovate.”
The risk variation
Moderator Young-Min Kwon, MD, PhD, of Massachusetts General Hospital, explained that evaluation of a painful MoM hip should take a systematic approach that includes the following:
- clinical history and physical examination
- testing of erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP)
- measurement of metal ion levels
- cross-sectional imaging, such as metal artifact reduction software (MARS) MRI
An algorithm for diagnosis includes identification of the implant by type and size (greater or less than 36 mm). The threshold for problematic ion levels is probably 7 ppb.
Dr. Kwon provided a three-level risk stratification profile. The low-risk patient has a low activity level, is asymptomatic mechanically and systemically, and has no change in gait, abductor weakness, or swelling. Radiographs show optimal acetabular cup orientation and no implant osteolysis or loosening; metal ion levels are low (< 3 ppb). For this patient, annual follow-up is the recommendation.
The moderate-risk patient would typically be a woman with mild local hip symptoms and no systemic symptoms. The patient has a change in gait but still no abductor weakness or swelling. The implant would be a large-diameter femoral head (³ 36 mm), a MoM total hip, a recalled implant, a hip resurfacing with risk factors (such as dysplasia), or a dual-taper modular neck device.
Infection work-up would be normal, while metal ion levels would be moderately elevated (3–10 ppb). Cross-sectional imaging would indicate presence of abnormal tissue reaction without involvement of surrounding tissue and bone and simple cystic lesions or small cystic lesions without a thickened wall.
For this patient, follow-up in 6 months is recommended, with revision surgery considered if symptoms or imaging abnormality progress, and/or metal levels rise within 6 months.
The high-risk patient would have the signs and symptoms of the moderate-risk patient, but also with high activity level, abductor weakness and swelling, metal ions greater than 10 ppb, and presence of abnormal tissue reactions with involvement of surrounding muscles or bone. Revision should be considered for this patient.
In evaluating the symptomatic patient, the physician should consider factors both intrinsic and extrinsic to the hip, Dr. Kwon said. There should be a “low threshold to perform a systematic evaluation,” and early and accurate diagnosis is important.
Craig J. Della Valle, MD, of Rush University, also addressed the diagnostic challenges in MoM hips, particularly as they relate to periprosthetic joint infections (PJIs). “The intra-operative appearance may mimic purulence, and cell count may be likewise unreliable,” he said.
He described a study to determine the utility of blood (ESR/CRP) and synovial fluid white blood cell (WBC) count to diagnose patients with a failed MoM bearing or a corrosion reaction. The study found that automated synovial WBC counts were inaccurate in 33 percent of tests run and that 41 of 47 hips initially identified as infected by these tests were actually not infected.
His conclusion was that “the diagnostic challenges of PJI are magnified for hips with MoM bearings, corrosion or full-thickness wear. The synovial fluid WBC can frequently be falsely positive—about 10 percent of the time—but is still a good test. It should only be relied upon, however, if manual count is done or if a differential can be performed.”
Dr. Della Valle said “the threshold to aspirate preoperatively should be much lower. The intra-operative appearance can be deceiving. A request for a manual count alerts the technician to examine the specimen.”
Also appearing on the panel were Robert M. Urban, MD, of Rush University; H. John Cooper, MD, of Lenox Hill Hospital; and J. Mark Wilkinson, PhD, FRCS, of the University of Sheffield.
Details of the authors’ disclosure as submitted to the Orthopaedic Disclosure Program can be found in the Final Program; the most current disclosure information may be accessed electronically atwww.aaos.org/disclosure
2014 Annual Meeting News
Tuesday through Friday, March 11 – 14, 2014.