Aspiration Cell Counts May Rule Out Aseptic Loosening

Terry Stanton

A study to determine the value of synovial cell count in the diagnosis of aseptic loosening in patients undergoing revision total hip arthroplasty (THA) found that cell counts might be useful in ruling out aseptic loosening, with a sensitivity and negative predictive value (NPV) exceeding that of radiographs and paralleling that of bone scintigraphy.

Scientific Presentation 277, “Correlation of aspiration results with aseptic loosening in total hip arthroplasty” was presented yesterday by Peter N. Chalmers, MD, of Rush University Medical Center in Chicago. Dr. Chalmers and colleagues report that in patients with painful THA, aspiration data (white blood cell [WBC] and lymphocyte counts) can help rule out the diagnosis of aseptic loosening.

Aseptic loosening has been cited as the reason for 20 percent to 70 percent of revision THA procedures, but current methods to evaluate for aseptic loosening are insensitive and nonspecific, the authors say. Plain radiographs have an 82 percent sensitivity and an 81 percent specificity, although in some circumstances these may be as low as 76 percent and 73 percent, respectively. Bone scintigraphy, which is often used if radiographs are unrevealing, does not perform much better, with an 85 percent sensitivity and a 72 percent specificity, dipping to 79 percent and 64 percent in some circumstances.

According to Dr. Chalmers, a physiologic basis exists to suggest that cell counts may correlate with aseptic loosening, because this process involves the activation of inflammatory cascades and cytokines, leading to infiltration of both lymphocytes and monocytes. The bone-implant interfaces at which these reactions occur have been hypothesized to be in continuity with the joint space itself. However, whether synovial analysis might differentiate aseptic loosening from other causes of hip pain after THA has not been verified.

Dr. Chalmers and colleagues hypothesized that in patients undergoing revision THA, high synovial lymphocytic and monocytic differential cell counts would correlate with the diagnosis of aseptic loosening.

Retrospective review
Researchers conducted a retrospective chart review to identify revision THA patients who had preoperative or intraoperative synovial aspiration for cell count, cell differential, and culture. Patients with a synovial WBC count of less than 100 were excluded.

These and other data were used to develop a single cause of failure from among the following:

Aseptic loosening (intraoperative gross implant movement with no infection)
Periprosthetic sepsis
Reimplantation (placement of an antibiotic-implanted spacer with aspiration performed at the time of spacer removal and reimplantation)
Component wear (significant wear in the absence of aseptic loosening or infection)
Periprosthetic fracture (fracture without aseptic loosening or infection)
Instability (history of dislocation without aseptic loosening or infection)
Other

Of 253 patients who met the inclusion criteria, 76 (30 percent) had aseptic loosening; periprosthetic sepsis was the next most common cause of failure (52 patients; 20.6 percent).

In examining the data, the researchers found that patients with aseptic loosening had significantly lower WBC counts (P < 0.001), significantly lower percentages of segmented cell counts (P = 0.001), significantly higher percentages of lymphocyte cell counts (P < 0.001), and significantly higher percentages of monocyte cell counts (P = 0.039).

Negative predictive value
“Our study demonstrated that synovial aspiration characteristics do provide information helpful in the diagnosis of aseptic loosening,” reported Dr. Chalmers. “Our hypothesis that lymphocyte count might be used to determine the presence or absence of aseptic loosening was validated: A lymphocyte count of greater than 10 has an 86 percent sensitivity and a negative predictive value of 87 percent for aseptic loosening. Combining a WBC count of less than 2,500 with a lymphocyte count of greater than 10 further improved sensitivity to 96 percent and negative predictive value to 95 percent. This sensitivity well exceeds that of either radiographs or bone scintigraphy, the current gold standards in the diagnosis of aseptic loosening.”

As a result, when aspirates have neither a WBC less than 2,500 nor a lymphocyte count greater than 10, aseptic loosening can be effectively ruled out, say the researchers. Fewer than 5 percent of patients with these levels will have aseptic loosening.

The researchers noted that the specificity (33 percent) and positive predictive value (38 percent) “are [low] for this combination, which could complicate the interpretation of positive results” and that monocyte counts were less predictive than lymphocyte counts and thus less useful diagnostically.

Among the study limitations acknowledged by the authors were the use of strict criteria to subcategorize patients into modes of failure, which may have underestimated the complexity of presentations; for example, component wear might lead to aseptic loosening, which contributes to component malpositioning and instability. Also, they noted, further studies are needed to determine whether WBC and lymphocyte counts in hips with aseptic loosening differ from those in normal hips with pain due to an extra-articular etiology, such as spinal stenosis.

Coauthors with Dr. Chalmers are Kevin Hudak, MD; Scott M. Sporer, MD, MS; and Brett R. Levine, MD, MS.

Disclosure information: Dr. Sporer—Smith & Nephew, Zimmer, Central DuPage Hospital, SLACK Inc.; Dr. Levine—DePuy, Zimmer, Biomet, Human Kinetics, Orthopedics, AAOS. Dr. Chalmers and Dr. Hudak reported no conflicts.

2013 Annual Meeting News
Tuesday through Friday, February 19 – 23, 2013.
http://www.aaos.org/news/acadnews/2013/AAOS15_3_22.asp