In the Bair Hugger Litigation, plaintiffs asserted that they contracted periprosthetic joint infections (PJIs) due to the use of 3M’s Bair Hugger, a convective (forced-air) patient-warming device, during their orthopedic-implant surgeries. The MDL court excluded plaintiffs’ general causation medical experts as well as one of their engineering experts, and then granted 3M summary judgment as to all of plaintiffs’ claims.

In particular, Dr. Samet, Dr. Jarvis, and Dr. Stonnington each offered general causation opinions – that is, opinions that the Bair Hugger is capable of causing the PJIs from which plaintiffs allegedly suffered – based on both the airflow-disruption theory and the dirty-machine theory. Dr. Elghobashi created a computational-fluid-dynamics (CFD) model to support the airflow-disruption theory. The MDL court generally treated the medical experts as a collective set (their opinions were essentially the same and were founded on much of the same evidence), and it excluded their opinions as unreliable because it concluded that: (1) there was too great an analytical gap between the literature and the experts’ general causation opinions; and (2) the causal inferences made by the experts have not been generally accepted by the scientific community. The MDL court also excluded Dr. Elghobashi’s model and opinion on the basis that: (1) his conclusion about the Bair Hugger’s effects in real-world operating rooms relied on an unproven and untested premise, (2) there was too great an analytical gap between the results of his CFD and his conclusion about the Bair Hugger’s effects in real-world operating rooms, and (3) the CFD model was developed for litigation.

On appeal, the Eighth Circuit reversed:

“When a district court excludes an expert’s opinion for being fundamentally unsupported, yet another intriguing juxtaposition is evident in our case law.  On the one hand, we have recognized that we owe significant deference to the district court’s determination that expert testimony is excessively speculative, and we can reverse only if we are convinced that the District Court made a clear error of judgment on the basis of the record before it. On the other hand, we have stated numerous times that, as a general rule, the factual basis of an expert opinion goes to the credibility of the testimony, not the admissibility.  Thus, excluding an expert’s opinion for being fundamentally unsupported is an exception to the general rule that gaps in an expert witness’s knowledge go to weight, not admissibility.  Vigorous cross-examination, presentation of contrary evidence, and careful instruction on the burden of proof are the traditional and appropriate means of addressing shaky but admissible evidence.”

In this case, all three medical experts relied on a 2011 observational epidemiological study as support for their conclusion that the Bair Hugger is capable of causing PJIs. As an observational epidemiological study, McGovern 2011 explored whether forced-air warming was associated with an increased rate of PJIs by comparing a group of individuals warmed convectively to a group of individuals warmed conductively. The MDL court found that McGovern 2011 itself was sufficiently reliable to be admitted. But the MDL court faulted the experts’ reliance on it in ways that contributed to the analytical gap it found. For instance, the MDL court deemed it unreliable for the experts to draw an inference of causation from this study when the study disclaimed having proved causation. The MDL court also faulted how the experts handled the study’s limitations.

“As for the first point, we disagree that it is per se unreliable for an expert to draw an inference of causation from an epidemiological study that disclaimed proving causation. ‘Epidemiology cannot prove causation.’ Instead, epidemiology enables experts to find associations, which by themselves do not entail causation. But an observational study such as McGovern 2011 can be brought to bear on the question of causation, and can be very useful to answering that question. Ultimately causation is a judgment for epidemiologists and others interpreting the epidemiologic data. Thus, it was not necessarily unreliable for the experts to rely on McGovern 2011 to draw an inference of causation just because the study itself recognized, consistent with these principles, that the association did not establish causation. So long as an expert does the work to bridge the gap between association and causation, a study disclaiming having proven causation may nevertheless support such a conclusion.”

In addition, McGovern 2011 was not the only basis on which the experts relied in forming their opinions. In addition to the epidemiological data from McGovern 2011, the experts also relied on studies and reports ostensibly showing plausible mechanisms by which forced-air warming can cause PJIs. “Thus, the experts’ failure to handle McGovern 2011’s limitations properly is not fatal to the admissibility of their opinions…. An inference of causation based on the totality of the evidence may be reliable even if no one line of evidence supports a reliable inference of causation by itself.”

As to the engineering expert, Dr. Elghobashi, the MDL court primarily faulted him for opining that if additional real-world conditions (such as personnel movement) that have a significant impact on airflow disruption were introduced into his model, then the Bair Hugger’s effect on the dispersion of squames would be exaggerated in a real-world operating room. The Eighth Circuit affirmed that aspect of the MDL court’s exclusion. “In neither his expert report nor his published study did Dr. Elghobashi provide support for this assertion, and at the Gareis trial he explained that he knew this based on his knowledge and said to trust him about this. Nothing in either Daubert or the Federal Rules of Evidence requires a district court to admit opinion evidence that is connected to existing data only by the ipse dixit of the expert.”

However, the Court of Appeals reversed the MDL court’s categorical exclusion of Dr. Elghobashi and his model: “Dr. Elghobashi set out to determine whether forced-air warming plays a role in transporting squame particles to the surgical site;  his CFD model tested this hypothesis; and he found that forced-air warming does play a role, at least in certain operating-room conditions with limited airflow disruptions from other sources. So limited, his conclusion was tested and supported by the CFD model, and the problematic analytical gap found by the MDL court is gone.

“Granted, the MDL court also decided to exclude Dr. Elghobashi’s testimony and model because they were developed for litigation. But, with Dr. Elghobashi’s testimony properly limited so as to eliminate the other reasons for its exclusion, this factor alone does not warrant exclusion. The scientific reliability of a ‘hired gun’ expert’s testimony can be shown by proof that the research and analysis supporting the proffered conclusions have been subjected to normal scientific scrutiny through peer review and publication. That happened here – Dr. Elghobashi’s report in this case appears in a peer-reviewed journal.  In these circumstances – where a ‘hired gun’ expert’s work has been peer reviewed and published, and the developed-for-litigation concern is the only remaining reason for excluding the testimony – we conclude that lingering questions of reliability and objectivity go to weight rather than admissibility.

“In sum, we do not dispute the MDL court’s determination that there are weaknesses in the factual basis for Plaintiffs’ medical experts’ general-causation opinions. On the one hand, they have epidemiological evidence reporting an association between Bair Hugger use and PJIs, but on the other hand they failed to grapple adequately with the shortcomings of that evidence. On the one hand, they have identified two plausible mechanisms explaining this association, but on the other hand there are weaknesses in the supports for both mechanisms. This said, the question the MDL court ultimately had to answer was whether these shortcomings left too great an analytical gap between the factual bases for the experts’ opinions and the general-causation opinions themselves; or, in other words, whether the opinions were so fundamentally unsupported that they should be excluded rather than admitted and left to be impeached through cross-examination at trial (as evidently happened effectively at the Gareis trial). While giving due deference to the MDL court’s determination, we nevertheless conclude that the MDL court committed a clear error of judgment on the basis of the record before it in finding that the experts’ general-causation opinions were so fundamentally unsupported that they had to be excluded.”

With respect to the MDL Court’s finding that there was a lack of general acceptance regarding the opinions and/or conclusions expressed, the MDL court considered three data points in its general-acceptance analysis. “Notably, however,” the Court of Appeals commented, in one of those data points, (a 2018 statement), “the authors of the rationale for the bottom-line consensus recognized that ‘the literature is conflicting,’ and they called for further study to examine the issue. And Plaintiffs’ experts’ general-causation inferences are not without support in the medical community…. Even some of the other authorities 3M calls to our attention as showing a lack of general acceptance acknowledge that ‘concerns exist’ about a link between forced-air warming and surgical-site infections, and suggest modifications to forced-air-warming devices to ‘reduce the risk’ they may pose.  To exclude the experts’ opinions here because their conclusions lacked general acceptance would be to take a side on an issue that is currently the focus of extensive scientific research and debate. Accordingly, in light of our rejection of the MDL court’s analytical-gap rationale for exclusion, we conclude that the lack of general acceptance does not independently justify exclusion of Plaintiffs’ general-causation medical experts.”


In re Bair Hugger Forced Warming Prod. Liab. Lit., No.19-2899, 2021 WL 3612753 (8th Cir. Aug. 16, 2021).