Synovial fluid ion levels may offer increased accuracy compared to blood and serum
Matthew T. Houdek, MD, who presented data from Paper 853 at the AAOS 2018 Annual Meeting, said, “It’s not uncommon for patients with failed metal-on-metal (MoM) hip implants or who have a metal reaction (taper corrosion) to display soft-tissue damage that doesn’t correlate with what we see in their serum or blood metal ion levels. “We thought that the ion contamination may instead be a local issue,” he continued, “so we decided to look at the synovial fluid levels to see if they were significantly elevated, and if so, could we determine a cutoff point at which we might expect to see the start of soft-tissue reactions.”

Identical time points
The research team prospectively collected synovial, blood, and serum samples from 58 revision total hip arthroplasty (THA) patients—29 with an adverse reaction to metal debris (ARMD) and 29 undergoing aseptic revision without ARMD. There was no difference across cohorts in mean age, body mass index (BMI), sex distribution, serum creatinine, or time to revision.

“We drew serum and blood at the time of surgery,” said Dr. Houdek. “At the same time, we also drew synovial fluid from their hip prior to the arthrotomy, so we had the exact same time points for all three measurements.”

In the AMRD cohort, the components included MoM THA (n = 18), hip resurfacing (n = 5), dual-modular taper THA (n = 4), and MoM and dual-modular taper THA (n = 2). At the time of revision, 26 (90 percent) patients in the AMRD group had gross evidence of metallosis in soft tissues, while three (10 percent) had evidence of corrosion of the dual taper neck or MoM bearing. Prior to revision, 21 (72 percent) patients had evidence of a periarticular fluid collection based on magnetic resonance imaging (MRI). The mean cyst size was 202.9 cm3.

In the non-ARMD cohort, bearing surfaces included metal-on-polyethylene (n = 19) and ceramic-on-polyethylene (n = 10). Indications for revision included isolated acetabular loosening (n = 11), isolated femoral component loosening (n = 11), polyethylene wear (n = 5), recurrent dislocation (n = 1), and combined femoral and acetabular component loosening (n = 1). None of the patients in the non-ARMD group had a clinical diagnosis or gross evidence of taper corrosion.

A striking difference
At the time of revision, Dr. Houdek and his colleagues found that the cobalt and chromium levels were significantly elevated in patients with an ARMD compared to those in the non-ARMD group. They found that synovial fluid cobalt level was the most accurate test for detecting pseudotumor and adverse local tissue reaction (ALTR). Metal ion analysis was 84 percent sensitive and 91 percent specific for pseudotumor formation at a synovial fluid cobalt level of 36.5 ppb, and metal ion analysis was 77 percent sensitive and 88 percent specific for ALTR at a synovial fluid cobalt level of 149.9 ppb.

“It was rather striking how much of a difference there was in synovial fluid levels compared to the serum or blood levels,” Dr. Houdek noted. “The synovial fluid cobalt and chromium levels were more than 100 times higher than the serum or whole blood levels for cobalt and chromium. We suggest that the significant difference in the levels may be the difference between developing an ALTR or a pseudotumor.

“We conclude that assessing synovial fluid levels for cobalt and chromium at the time of aspiration may help identify patients who are at risk for failure or ALTR. Since a majority of these patients are getting a hip aspiration to rule out infection anyway, it could be an easy test to add on when analyzing the synovial fluid,” he continued. “Even if their serum levels may be in the normal range, this one additional test may help reduce or prevent large soft-tissue reactions that are commonly seen at the time of a revision hip.”

Dr. Houdek’s coauthors of paper Synovial Fluid Metal Ion Levels are Superior to Whole Blood and Serum Metal Ion Levels in Predicting Local Tissue Destruction in Failed Hip Arthroplasty Constructs: A Prospective Evaluation are Cody Wyles, MD; Paul Jannetto, PhD; Daniel J. Berry, MD; and David G. Lewallen, MD.

Peter Pollack is the senior staff writer for AAOS Now. He can be reached at ppollack@aaos.org.