Court Opinion

ID: 4766010
Source: CourtListenerOpinion
Date Created: 2021-08-16 15:01:32.125849+00
Date Added: 2024-06-11T08:09:13.630655
License: Public Domain

United States Court of Appeals
                            For the Eighth Circuit
                        ___________________________

                                No. 19-2899
                        ___________________________

  In re: Bair Hugger Forced Air Warming Devices Products Liability Litigation

                             ------------------------------

                                  George Amador

                                       Plaintiff - Appellant

                                          v.

                      3M Company; Arizant Healthcare, Inc.

                                    Defendants - Appellees
                                  ____________

                     Appeal from United States District Court
                          for the District of Minnesota
                                 ____________

                           Submitted: March 16, 2021
                              Filed: August 16, 2021
                                ____________

Before GRUENDER, KELLY, and GRASZ, Circuit Judges.
                          ____________

GRUENDER, Circuit Judge.

       In December 2015, the Judicial Panel on Multidistrict Litigation created and
centralized the In re Bair Hugger Forced Air Warming Devices Products Liability
Litigation (“MDL”) in the District of Minnesota (“MDL court”) for coordinated
pretrial proceedings. Plaintiffs 1 in the MDL have brought claims against 3M
Company and its now-defunct, wholly owned subsidiary Arizant Healthcare, Inc.
(collectively, “3M”). Plaintiffs assert that they contracted periprosthetic joint
infections (“PJIs”) due to the use of 3M’s Bair Hugger, a convective (or “forced-
air”) patient-warming device, during their orthopedic-implant surgeries. In July
2019, on 3M’s motion, the MDL court excluded Plaintiffs’ general-causation
medical experts as well as one of their engineering experts, and it then granted 3M
summary judgment as to all of Plaintiffs’ claims. Subsequently, the MDL court
entered an MDL-wide final judgment.

       Plaintiffs appeal. First, they argue that the MDL court abused its discretion
in excluding their general-causation medical experts and engineering expert.
Second, they argue that the MDL court erred in granting 3M summary judgment
whether or not those experts were properly excluded. Third, they argue that the
MDL court abused its discretion in denying Plaintiffs’ request for certain discovery.
And fourth, they argue that the MDL court abused its discretion in ordering certain
filings on its docket to remain sealed. Additionally, on appeal, Plaintiffs ask us to
unseal those parts of the appellate record that duplicate the filings whose sealing on
the MDL court’s docket they challenge.

       We reverse in full the exclusion of Plaintiffs’ general-causation medical
experts and reverse in part the exclusion of their engineering expert. We reverse the
grant of summary judgment in favor of 3M. We affirm the discovery order that
Plaintiffs challenge. We affirm the MDL court’s decision to seal the filings Plaintiffs
seek to have unsealed. And we deny Plaintiffs’ motion to unseal those same filings
on our own docket.

      1
      Although George Amador is the captioned Plaintiff-Appellant, this appeal is
brought by all Plaintiffs in the MDL to challenge several MDL-wide rulings.

                                         -2-
                                           I.

       In the mid-1980s, Dr. Scott Augustine invented the Bair Hugger, a forced-air
device used to keep patients warm during surgery so as to stave off hypothermia-
related complications that can arise during or after surgery. The device consists of
a central heating unit, a hose, and a disposable perforated blanket that is placed over
the patient. The central unit, which is often situated on or near the floor when in use,
draws in air through a filter, warms that air (usually to a temperature significantly
above the operating-room temperature), and blows it through the hose into the
perforated blanket. The air exits the blanket through the perforations and keeps the
patient warm. Typically, both the patient and the blanket are covered with surgical
draping during operations, and the blanket is placed on a part of the body away from
the surgical site, so the air does not blow directly onto the surgical site.

       Dr. Augustine marketed and sold the Bair Hugger through Augustine Medical,
Inc., the company he founded and led as CEO until 2004. Around that time, Dr.
Augustine was forced to leave Augustine Medical while under investigation for
Medicare fraud. Augustine Medical then reorganized, and the division of the
company that retained the Bair Hugger product line changed its name to Arizant
Healthcare. In 2010, 3M acquired Arizant Healthcare and the Bair Hugger product
line. Arizant Healthcare was dissolved in December 2014.

       After leaving Augustine Medical, Dr. Augustine developed the HotDog, a
patient-warming device that transfers heat conductively to the patient by direct
contact with the patient’s skin rather than by forced hot air. He then began a
campaign to discredit his old invention and promote his new one. These efforts bore
fruit. In March 2013, a plaintiff sued 3M and Arizant Healthcare in Texas state
court, claiming that he contracted a PJI due to the Bair Hugger’s use in his hip-
replacement surgery. Dr. Augustine worked with the law firm representing that
plaintiff to prepare a “litigation guide” and solicitation letter for the purpose of
fomenting more litigation against 3M. By December 2015, more than sixty
materially similar cases against 3M had been filed in or removed to federal district

                                          -3-
courts around the country. At that time, the Judicial Panel on Multidistrict Litigation
ordered these cases centralized in the District of Minnesota for consolidated pretrial
proceedings. See 28 U.S.C. § 1407(a). Nearly 6,000 lawsuits have since been filed
as part of the MDL.

       In these cases, Plaintiffs allege that they suffered PJIs from the use of the Bair
Hugger during their orthopedic-implant surgeries. PJIs are frequently caused by the
introduction of microbes into the surgical site during surgery. Bacterial
contamination is a particularly significant threat in orthopedic-implant surgeries
because a PJI can be caused by very few microbes, possibly even a single bacterium.
For this reason, it is standard for such surgeries to take place in “ultra-clean
ventilation” operating rooms, where air is blown into the operating room through
high-efficiency particulate air (“HEPA”) filtration at a uniform velocity. This
HEPA-filtered “laminar” airflow blows over the patient, reducing the likelihood that
operating-room airflow will carry ambient bacteria from nonsterile areas of the
operating room into the surgical site.

      Plaintiffs advance two theories for how the Bair Hugger caused their PJIs
during their orthopedic-implant surgeries. According to the “airflow disruption”
theory, waste heat from the Bair Hugger creates convection currents that carry
ambient bacteria from nonsterile areas of the operating room to the surgical site
despite the laminar airflow, resulting in PJIs. According to the “dirty machine”
theory, the Bair Hugger is internally contaminated with bacteria, which are blown
through the blanket into the operating room, where they become ambient and
eventually reach the surgical site, resulting in PJIs.

       In the master long-form complaint filed in the MDL, Plaintiffs asserted
fourteen state-law claims against 3M, including negligence and strict liability (for
failure to warn, defective design, and defective manufacture), among others.

     During discovery, Plaintiffs subpoenaed a third party, VitaHEAT Medical,
LLC, to produce discovery regarding its “UB3,” a conductive patient-warming

                                          -4-
device. Plaintiffs alleged that the UB3 was an alternative design to the Bair Hugger,
making this discovery ostensibly relevant to their design-defect claims. See
generally 63A Am. Jur. 2d Products Liability § 894 (May 2021 update) (“The
existence of an alternative design may be used to establish that a product was
unreasonably dangerous due to a design defect, and in some jurisdictions may be
required.”). VitaHEAT objected on relevancy grounds, arguing that the UB3 was
too different from the Bair Hugger to count as an “alternative design” for product-
liability purposes. Plaintiffs then filed what they captioned a “motion to overrule”
this relevancy objection. The MDL court denied this motion, agreeing that
conductive patient-warming devices like the UB3 are too dissimilar from the Bair
Hugger to qualify as “alternative designs,” meaning that this discovery was not
relevant. Cf. United States v. One Assortment of 93 NFA Regulated Weapons, 897
F.3d 961, 966 (8th Cir. 2018) (“The Federal Rules of Civil Procedure limit discovery
to that which ‘is relevant to any party’s claim or defense . . . .’” (quoting Fed. R. Civ.
P. 26(b)(1))).

       The parties jointly agreed to a protective order to limit the disclosure of
confidential information that might be contained in filings entered on the MDL
docket. Pursuant to this protective order, the parties submitted numerous filings
under seal over the course of the litigation. As relevant to this appeal, 3M sought to
keep seven such filings under seal over Plaintiffs’ objection, asserting that it would
suffer competitive harm if any was unsealed. The MDL court agreed and ordered
these files kept under seal.

       As the litigation progressed, 3M moved to exclude Plaintiffs’ general-
causation medical experts (Dr. Jonathan M. Samet, an epidemiologist; Dr. William
Jarvis, an infectious-disease specialist; and Dr. Michael J. Stonnington, an
orthopedic surgeon) as well as Plaintiffs’ engineering experts (including Dr. Said
Elghobashi and Michael Buck). 3M also filed a motion for summary judgment
contingent on the exclusion of Plaintiffs’ general-causation medical experts. The
MDL court denied in pertinent part the motion to exclude those experts and denied
the motion for summary judgment.

                                           -5-
       Subsequently, Gareis v. 3M Co. became the first bellwether trial in the MDL.
See generally 156 Am. Jur. Trials § 219 (May 2021 update) (explaining the
bellwether-trial process in mass-tort litigation). Ruling on pretrial motions in
Gareis, the MDL court excluded evidence of Plaintiffs’ dirty-machine theory. The
case then proceeded to trial on the airflow-disruption theory, and Plaintiffs’ experts
Dr. Jarvis, Dr. Stonnington, and Dr. Elghobashi (among others) testified. After an
approximately two-week trial, the jury returned a verdict for 3M.

       After the Gareis trial, 3M moved for reconsideration of the MDL court’s
orders refusing to exclude Plaintiffs’ general-causation medical experts as well as
Dr. Elghobashi and denying 3M summary judgment. The MDL court granted 3M’s
motion, excluding Plaintiffs’ general-causation medical experts as well as Dr.
Elghobashi and granting 3M summary judgment on all claims. The MDL court then
entered an MDL-wide final judgment. See In re Temporomandibular Joint (TMJ)
Implants Prods. Liab. Litig., 97 F.3d 1050, 1055 (8th Cir. 1996) (“A transferee court
has the authority to enter dispositive orders terminating cases consolidated under 28
U.S.C. § 1407.”).

      Plaintiffs appeal, challenging the MDL court’s exclusion of their general-
causation medical experts and Dr. Elghobashi, the MDL-wide grant of summary
judgment in favor of 3M, the discovery ruling regarding conductive patient-warming
devices, and the sealing of seven MDL docket entries. Plaintiffs also ask us on
appeal to unseal on our own docket the records that they argue should have been
unsealed on the MDL court’s docket.

                                         II.

      We begin by considering Plaintiffs’ challenge to the MDL court’s exclusion
of their general-causation medical experts (Dr. Samet, Dr. Jarvis, and Dr.
Stonnington) and engineering expert (Dr. Elghobashi). 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

                                         -6-
suffered, see Junk v. Terminix Int’l Co., 628 F.3d 439, 450 (8th Cir. 2010) 2—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 (1) it
concluded 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.” 3 The MDL
court also excluded Dr. Elghobashi’s model and opinion because (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.

      For the following reasons, we reverse in full the exclusion of the medical
experts’ opinions and reverse in part the exclusion of Dr. Elghobashi’s model and
opinion. We first recite the principles that govern our analysis. We then analyze the
reasons given by the MDL court for excluding the experts.

      2
       The opinions in question did not address specific causation—whether the
Bair Hugger “in fact caused the harm from which” any particular MDL plaintiff
suffered. See id.
      3
        The MDL court articulated a third reason; namely, that “the experts failed to
consider obvious alternative explanations.” This reason applies specifically to the
medical experts’ treatment of the epidemiological study on which they relied, see
infra Section II.B.1, that found an association between forced-air warming and PJIs,
see Federal Judicial Center, Reference Manual on Scientific Evidence at 597-600 (3d
ed. 2011) (explaining that “[c]onsideration of alternative explanations” is one of nine
factors that “guide epidemiologists in making judgments about [general] causation”
based on a study or studies that find an association). The MDL court discussed this
same issue in its analysis of the “analytical gaps” between that study and the experts’
opinions. Accordingly, we consider this point in assessing the MDL court’s
“analytical gaps” analysis.

                                         -7-
                                          A.

       As the proponent of the expert testimony in question, Plaintiffs have the
burden to prove its admissibility by a preponderance of the evidence. Lauzon v.
Senco Prods., Inc., 270 F.3d 681, 686 (8th Cir. 2001). Federal Rule of Evidence 702
governs the admissibility of expert testimony, and under this rule the district court
is “vested with a gatekeeping function, ensuring that ‘any and all scientific testimony
or evidence admitted is not only relevant, but reliable.’” Union Pac. R.R. v. Progress
Rail Servs. Corp., 778 F.3d 704, 709 (8th Cir. 2015) (quoting Daubert v. Merrell
Dow Pharms., Inc., 509 U.S. 579, 589 (1993)). In exercising this gatekeeping
function, the district court has “broad discretion,” and “on appeal we will not disturb
a decision concerning the exclusion of expert testimony absent an abuse of that
discretion.” Wagner v. Hesston Corp., 450 F.3d 756, 758 (8th Cir. 2006).

       That said, we have recognized that the “liberal thrust” of Rule 702 regarding
the admissibility of expert testimony creates “an intriguing juxtaposition with our
oft-repeated abuse-of-discretion standard of review.” Johnson v. Mead Johnson &
Co., 754 F.3d 557, 562 (8th Cir. 2014). “While we adhere to this discretionary
standard for review of the district court’s Rule 702 gatekeeping decision, cases are
legion that, correctly, under Daubert, call for the liberal admission of expert
testimony.” Id. (collecting authorities).

       Rule 702’s “screening requirement” has been “boiled down to a three-part
test.” Id. at 561. First, the testimony must be useful to the finder of fact in deciding
the ultimate issue of fact, meaning it must be relevant. See id. Second, the expert
must be qualified to assist the finder of fact. Id. Third, the testimony must be
reliable or trustworthy in an evidentiary sense. Id. At issue here is the third part of
this test—whether Plaintiffs’ experts’ proposed testimony meets Rule 702’s
reliability requirement. “The standard for judging the evidentiary reliability of
expert evidence is ‘lower than the merits standard of correctness.’” Kuhn v. Wyeth,
Inc., 686 F.3d 618, 625 (8th Cir. 2012) (quoting In re Paoli R.R. Yard PCB Litig.,
35 F.3d 717, 744 (3d Cir. 1994)).

                                          -8-
       The reliability inquiry is a “flexible” one, with “[m]any factors” bearing on it.
Daubert, 509 U.S. at 593-94. In Daubert, the Court articulated “four non-exclusive
factors” relevant to this inquiry. Johnson, 754 F.3d at 562. These factors are
(1) whether the expert’s theory or technique can be or has been tested, (2) whether
the theory or technique has been subjected to peer review or publication, (3) the
known or potential rate of error of the theory or technique, and (4) whether the
technique or theory is generally accepted. See id.; Peitzmeier v. Hennessy Indus.,
Inc., 97 F.3d 293, 297 (8th Cir. 1996). Factors recognized since Daubert include
“whether the experts are proposing to testify about matters growing naturally and
directly out of research they have conducted independent of the litigation, or whether
they have developed their opinions expressly for purposes of testifying.” Daubert
v. Merrell Dow Pharms., Inc., 43 F.3d 1311, 1317 (9th Cir. 1995).

       Additionally, while Daubert instructed that the focus of the reliability inquiry
“must be solely on principles and methodology, not on the conclusions that they
generate,” 509 U.S. at 595, the Supreme Court later clarified that “conclusions and
methodology are not entirely distinct from one another,” Gen. Elec. Co. v. Joiner,
522 U.S. 136, 146 (1997). Thus, “a district court’s focus on principles and
methodology need not completely pretermit judicial consideration of an expert’s
conclusions,” Kuhn, 686 F.3d at 625 (internal quotation marks omitted), and a
district court may exclude expert testimony if it finds “that there is simply too great
an analytical gap between the data and the opinion proffered,” Joiner, 522 U.S. at
146. Or, to put it in the language we have frequently used both before and after
Daubert and Joiner, a district court may exclude an expert’s opinion if it is “so
fundamentally unsupported” by its factual basis “that it can offer no assistance to the
jury.” E.g., Loudermill v. Dow Chem. Co., 863 F.2d 566, 570 (8th Cir. 1988); United
States v. Finch, 630 F.3d 1057, 1062 (8th Cir. 2011).

       When a district court excludes an expert’s opinion for being fundamentally
unsupported, yet another “intriguing juxtaposition” is evident in our case law. See
Johnson, 754 F.3d at 562. On the one hand, we have recognized that we owe
“significant deference” to the district court’s “determination that expert testimony is

                                          -9-
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.”
Grp. Health Plan, Inc. v. Philip Morris USA, Inc., 344 F.3d 753, 760 (8th Cir. 2003)
(internal quotation marks omitted). On the other hand, we have stated numerous
times that, “[a]s a general rule, the factual basis of an expert opinion goes to the
credibility of the testimony, not the admissibility.” E.g., United States v. Coutentos,
651 F.3d 809, 820 (8th Cir. 2011); see also Klingenberg v. Vulcan Ladder USA,
LLC, 936 F.3d 824, 829-30 (8th Cir. 2019) (distinguishing cases where we affirmed
the exclusion of experts’ opinions as too speculative because, in those cases, the
experts’ opinions were “wholly speculative,” “connected to the facts by only the
expert’s ipse dixit,” “patent speculation,” “pure conjecture,” and “vague theorizing
based upon general principles”).

      Thus, excluding an expert’s opinion for being fundamentally unsupported is
an exception to the general rule that “[g]aps in an expert witness’s . . . knowledge”
go to weight, not admissibility. See Robinson v. GEICO Gen. Ins., 447 F.3d 1096,
1100 (8th Cir. 2006); cf. Finch, 630 F.3d at 1062 (“Doubts regarding whether an
expert’s testimony will be useful should generally be resolved in favor of
admissibility.” (brackets omitted)). “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.” Daubert,
509 U.S. at 596.

       With these principles in mind, we analyze the reasons provided by the MDL
court for excluding Plaintiffs’ general-causation medical experts and Dr. Elghobashi
(whose CFD model and testimony formed part of the factual basis for the general-
causation medical experts’ opinions).

                                          B.

     We first consider the MDL court’s determination that “too great an analytical
gap” existed between “the literature” and Plaintiffs’ medical experts’ general-

                                         -10-
causation opinions. This literature falls generally into two categories: (1) an
epidemiological study reporting an association between forced-air warming and
PJIs; and (2) studies and reports ostensibly supporting both of Plaintiffs’ mechanistic
theories of causation. We consider this evidence to assess whether the MDL court
“made a clear error of judgment on the basis of the record before it” in finding the
experts’ opinions too speculative to be admitted. See Grp. Health Plan, 344 F.3d at
760 (internal quotation marks omitted).

                                          1.

       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.
See P.D. McGovern et al., Forced-Air Warming and Ultra-Clean Ventilation Do Not
Mix, 93-B J. Bone & Joint Surgery 1537 (2011) (“McGovern 2011”). 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.
See id. at 1537. See generally Reference Manual, supra, at 552, 555-56.

       Specifically, McGovern 2011 reviewed infection data from 1,437 hip- or
knee-replacement surgeries performed at a particular hospital for a 2.5-year period.
Id. at 1537, 1540. From July 2008 to March 2010, the patients were warmed with
Bair Huggers; from March 2010 to June 2010, the hospital gradually transitioned to
using conductive patient-warming devices; and from June 2010 to the end of the
study, the patients were warmed solely with conductive patient-warming devices.
Id. at 1540, 1543. The investigators found that patients warmed convectively were
nearly four times more likely to contract a PJI than patients warmed conductively.
Id. at 1541. The authors of McGovern 2011 acknowledged that the study did “not
establish a causal basis” for this association. Id. at 1543. And they acknowledged

                                         -11-
that their findings may have been “confounded” 4 by “other infection control
measures instituted by the hospital” during the study period (specifically identifying
two such potentially confounding measures) and that they were “unable to consider
all [patient-medical-history] factors” associated with PJIs, including a number of
“important predictors for deep infection,” due to limited data in the records they
reviewed. Id.

       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. “[E]pidemiology cannot prove causation.” Reference Manual,
supra, at 598. Instead, epidemiology enables experts to find associations, which by
themselves do not entail causation. See id. at 552-53, 598. But an observational
study such as McGovern 2011 “can be brought to bear” on the question of causation,
id. at 217, and “can be very useful” to answering that question, id. at 221. Ultimately
“causation is a judgment for epidemiologists and others interpreting the
epidemiologic data.” Id. at 598; see also id. at 222 (“In the end, deciding whether
associations are causal typically . . . rests on scientific judgment.”). 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

      4
        “Confounding occurs when another causal factor (the confounder) confuses
the relationship between the agent of interest and outcome of interest.” Reference
Manual, supra, at 591. For instance, if those who drink alcohol are more likely to
smoke than those who do not, then smoking may be a confounder in a study finding
an association between drinking alcohol and emphysema. See id. at 592.

                                          -12-
“to bridge the gap between association and causation,” a study disclaiming having
proven causation may nevertheless support such a conclusion. See id. at 218.

      We recognize that there is language from Joiner that, when taken out of
context, might appear to suggest otherwise. See 522 U.S. at 145 (“Given that [the
authors of the study in question] were unwilling to say that PCB exposure had caused
cancer among the workers they examined, their study did not support the experts’
conclusion that Joiner’s exposure to PCB’s caused his cancer.”). But the context
indicates that the problem with the experts’ opinions in that case was that they failed
to bridge the gap left by the study in question. See id. at 145-46 (recounting
numerous issues with the experts’ factual basis).

       As for the second issue, the MDL court rightly faulted the experts for how
they handled McGovern 2011’s limitations. “Assessing whether an association is
causal requires an understanding of the strengths and weaknesses of the study’s
design and implementation,” and “the key questions” in evaluating epidemiological
evidence “are the extent to which a study’s limitations compromise its findings and
permit inferences about causation.” Reference Manual, supra, at 553. The experts
did not adequately address McGovern 2011’s limitations. Neither Dr. Jarvis nor Dr.
Stonnington mentioned the identified potential confounders or limitations in
McGovern 2011 in their reports. Dr. Samet, on the other hand, did address in his
report the two potential confounders identified by the authors in the study itself, and
he meaningfully explained why in his view these variables did not confound the
study’s findings. But Dr. Samet did not meaningfully address the other limitations
identified by the McGovern 2011 authors except to say that confounding by other
factors “seems unlikely” for ostensibly logical reasons. 5

      5
        In the context of discussing this point, the MDL court also faulted Dr. Samet
for departing “from his own description of reliable methodology.” Specifically, it
noted that, with respect to the criterion of consistency that experts use in making
judgments about causation, see Reference Manual, supra, at 600, Dr. Samet noted
that that criterion “is generally applied as a consideration related to interpretation of
findings of multiple observational studies and hence is not applicable to the single

                                          -13-
       However, 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. See id. at 599-600, 604
(identifying the “biological plausibility” of a general-causation theory as one factor
guiding epidemiologists in “making judgments about causation” and noting that
“[w]hen biological plausibility exists, it lends credence to an inference of
causality”). Thus, the experts’ failure to handle McGovern 2011’s limitations
properly is not fatal to the admissibility of their opinions. See id. at 599-600 (listing
“factors that guide epidemiologists in making judgments about causation”; noting
that “there is no threshold number that must exist”; and including among these
factors “[b]iological plausibility” along with “[c]onsideration of alternative
explanations”). “[A]n inference of causation based on the totality of the evidence”
may be reliable even if “no one line of evidence support[s] a reliable inference of
causation” by itself. Milward v. Acuity Specialty Prods. Grp., Inc., 639 F.3d 11, 23
(1st Cir. 2011); see also United States v. W.R. Grace, 504 F.3d 745, 765 (9th Cir.
2007) (noting that whether an expert’s opinion testimony satisfies Rule 702
“requires consideration of the overall sufficiency of the underlying facts and data”).
Accordingly, we turn to the other evidence these experts considered.

study by McGovern.” Because there was only one epidemiological study here, Dr.
Samet instead “point[ed] to the consistency of the findings of studies addressing the
effect of the Bair Hugger device on particle counts at the surgical site.” In context,
we do not read this as Dr. Samet misapplying his own methodology but rather
acknowledging that the consistency factor was not relevant in its conventional sense
but nevertheless the consistency of the mechanistic studies supported an inference
of causation from McGovern 2011’s finding of an association. As he explained in
the next paragraph of his report following the language the MDL court quoted, the
mechanistic evidence is consistent with McGovern 2011’s findings. The Reference
Manual directs epidemiologists to consider whether the finding of an association is
“consistent with other relevant knowledge,” supra, at 606, and we fail to see how
mechanistic evidence would not count as “other relevant knowledge.”

                                          -14-
                                          2.

       The studies and reports ostensibly showing the “biological plausibility” of the
medical experts’ general-causation opinions broadly fall into two categories. One
set ostensibly supports Plaintiffs’ airflow-disruption theory, and the other ostensibly
supports Plaintiffs’ dirty-machine theory.

                                          a.

       All three medical experts relied on Plaintiffs’ airflow-disruption theory as a
plausible causal mechanism to support their general-causation opinions. Again,
according to this theory, waste heat generated by the Bair Hugger creates convection
currents that disrupt laminar airflow in operating rooms and transmit nonsterile air
to the surgical site, causing PJIs. The experts relied on two general categories of
evidence for this theory: (i) Dr. Elghobashi’s CFD model and (ii) published studies
examining airflow patterns in operating rooms as well as the correlation between
particles and bacteria. We consider each category in turn.

                                          i.

       To investigate whether forced-air warming “play[s] a role” in transporting
squames (skin flakes capable of carrying bacteria that are present in operating
rooms) to the surgical site, Dr. Elghobashi prepared a CFD model using large eddy
simulation (a way to model fluid turbulence) to simulate the Bair Hugger’s effect on
airflow and dispersion of squames in an ultra-clean-ventilation operating room. Dr.
Samet and Dr. Jarvis relied on Dr. Elghobashi’s model. The parties agree, and the
MDL court found, that the physics underlying Dr. Elghobashi’s model is reliable.
Dr. Elghobashi eventually published his model with several coauthors in a peer-
reviewed journal. See X. He et al., Effect of Heated-Air Blanket on the Dispersion
of Squames in an Operating Room, 34 Int’l J. Numerical Methods Biomedical
Eng’g, May 2018, at 1 (“He 2018”).

                                         -15-
        Dr. Elghobashi’s model replicated an orthopedic operating room, including
details such as laminar airflow, an operating table, surgical drapes, a patient
underneath the drapes prepared for knee surgery, four surgeons (two with hands
extended over the patient, two with hands down), two side tables, two surgical
lamps, the Bair Hugger blanket applied to the patient’s torso under the drapes, and
the Bair Hugger central unit sitting on the floor near the head of the operating table.
Dr. Elghobashi accounted for the heat generated by the Bair Hugger as well as heat
emanating from other sources, including the surgeons, patient, surgical lamps, and
even the exposed surface of the patient’s knee. He then included approximately
three million 10-µm-sized squames on the floor of the operating room near the
operating table (a person sheds on average about ten million squames a day). After
inputting a number of airflow-related details, Dr. Elghobashi simulated whether the
Bair Hugger could lift these 10-µm-sized squames—particles undisputedly large
enough to carry bacteria and thus be “dangerous”—up to four “regions of interest”
in the operating room, such as where the surgical tools are kept and the surgical site
itself.

        Dr. Elghobashi’s model showed that, with the Bair Hugger off, the laminar
airflow in the operating room was able to disperse the squames away from the
regions of interest and to airflow outlets. From this, he concluded that “without the
hot air discharged from the blower, the ventilation air circulation alone cannot
disperse the squames to the surgical site.” But with the Bair Hugger on, within less
than a minute the operating-room airflow was sufficiently disrupted by Bair-Hugger-
generated heat that convection currents generated by the Bair Hugger lifted a
statistically significant number of squames to the regions of interest.

       Dr. Elghobashi’s conclusion was that “the hot air from the blower and
resultant thermal plumes are capable of lifting [squames] and transporting them to
the side tables, above the operating table, and the surgical site.” And he added that
if other variables were introduced into the model, such as movement of medical staff,
“then the probability of dispersing the squames to the surgical site will be increased
even further.” In the published version of the study, Dr. Elghobashi noted that

                                         -16-
“several . . . complexities involving other medical equipment in an [operating room],
motion of the medical staff, opening and closing of the [operating-room] door,
among others are not accounted for,” but he asserted that “these complexities may
not impact the main conclusions of the present study.” He 2018, supra, at 18.

       The MDL court excluded Dr. Elghobashi’s opinion and his model for three
reasons. First, it found that his conclusion “relies on an unproven and untested
premise.” Second, it found that there was “too great an analytical gap between the
CFD results and Dr. Elghobashi’s conclusion that the surgical team’s movement
would only increase the Bair Hugger’s effect in the real world.” Third, Dr.
Elghobashi’s CFD model was developed for litigation, raising “concerns about its
reliability and objectivity.” We conclude that entirely excluding Dr. Elghobashi and
his model for these reasons was an abuse of discretion. See, e.g., United Fire & Cas.
Co. v. Whirlpool Corp., 704 F.3d 1338, 1341-42 (11th Cir. 2013) (per curiam)
(reversing in part the district court’s “sweeping exclusion” of an expert’s testimony
(citing, inter alia, Weisgram v. Marley Co., 169 F.3d 514, 518 (8th Cir. 1999))).

       The MDL court mainly faulted Dr. Elghobashi 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.
We affirm this 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.” “[N]othing 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.” Joiner, 522 U.S. at 146.

       But we do not affirm the MDL court’s categorical exclusion of Dr. Elghobashi
and his model. Dr. Elghobashi set out to determine whether forced-air warming
“play[s] 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

                                         -17-
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.” See generally Lauzon, 270
F.3d at 687. 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.’”
Lauzon, 270 F.3d at 693 (quoting Daubert, 43 F.3d at 1318). That happened here—
Dr. Elghobashi’s report in this case appears in a peer-reviewed journal. See He 2018,
supra. 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. See DiCarlo v.
Keller Ladders, Inc., 211 F.3d 465, 468 (8th Cir. 2000) (“An expert witness’s bias
goes to the weight, not the admissibility of the testimony, and should be brought out
on cross-examination.” (internal quotation marks omitted)).

       Accordingly, the MDL court abused its discretion insofar as it excluded all of
Dr. Elghobashi’s testimony. His testimony, properly limited as we have specified
here, is admissible. Therefore, his limited testimony and CFD model may be
considered as part of the factual basis for Plaintiffs’ medical experts’ airflow-
disruption-theory-based general-causation opinions. 6

       6
        In a footnote, the MDL court noted that if Dr. Elghobashi’s testimony were
so limited, it “would not assist the trier of fact in resolving the factual dispute” in the
cases in this MDL because every such case will require Plaintiffs to prove specific
causation. But Dr. Elghobashi’s model and limited testimony are relevant and
admissible insofar as they provide part of the factual basis for Plaintiffs’ medical
experts’ general-causation opinions. See Archer Daniels Midland Co. v. Aon Risk

                                           -18-
       As for the support that the CFD model and such limited testimony would
provide (if admissible) for the medical experts’ general-causation opinions to the
extent that they are based on the airflow-disruption theory, the MDL court noted that
there was “too great an analytical gap between the CFD results and the medical
experts’ conclusions that the Bair Hugger causes infection.” The MDL court
explained that this was because the CFD model did not account for many sources of
turbulence often present in a real-world operating room, thus leaving questions
unanswered about the real-world effects of the Bair-Hugger-created turbulence. We
agree that there are gaps between Dr. Elghobashi’s model simulating a “pure
operating room” and the opinion that the airflow-disruption theory is a plausible
mechanism for how the Bair Hugger causes PJIs in real-world operating rooms. But
Dr. Elghobashi’s CFD model is not the lone support for the airflow-disruption
theory, and whether too great an analytical gap exists here requires consideration of
the totality of the evidence on this point. See W.R. Grace, 504 F.3d at 765.

                                          ii.

       In addition to Dr. Elghobashi’s model, the medical experts relied on a number
of published studies to find the airflow-disruption theory a plausible mechanism of
how the Bair Hugger causes PJIs, thereby supporting their general-causation
opinions. The MDL court found that the studies themselves were sufficiently
reliable to be admitted. But it concluded that there remained “too great an analytical
gap between these studies and the experts’ conclusion that the Bair Hugger causes
infection” by way of this mechanism. First, the MDL court found that the
proposition that the Bair Hugger increases particle-laden airflow over the surgical
site was inadequately supported because the studies that the experts cited for this
proposition did not simulate “real world” operating-room conditions. Second, the
MDL court found that, even assuming this first proposition was correct, the
proposition that the particles in this airflow carried bacteria was inadequately

Servs., Inc. of Minn., 356 F.3d 850, 858 (8th Cir. 2004) (“An expert need not have
an opinion on an ultimate issue of fact in order for the testimony to be admissible.”).

                                         -19-
supported because Dr. Jarvis “admitted” at the Gareis trial that no study showed that
the Bair Hugger has any impact on particles that are large enough to carry bacteria
(other than “perhaps” the CFD model).

       As for whether the Bair Hugger increases particle-laden airflow over the
surgical site, the MDL court was correct that many of these studies, like Dr.
Elghobashi’s model, did not test the Bair Hugger’s effects on airflow disruption and
particle counts with all potentially relevant variables included in the analysis. See,
e.g., McGovern 2011, supra, at 1537-38 (testing the airflow-disruption hypothesis
by using a mannequin warmed underneath surgical draping with the Bair Hugger
while having a surgeon stand motionless next to the surgical site and an anesthetist
stand at the head of the operating-room table); K.B. Dasari, M. Albrecht & M.
Harper, Effect of Forced-Air Warming on the Performance of Operating Theatre
Laminar Flow Ventilation, 67 Anaesthesia 244, 245, 248 (2012) (“Dasari 2012”)
(finding that forced-air warming created significant levels of excess heat above and
around the surgical site under laminar-airflow conditions compared to conductive
warming technologies after applying forced-air warming to a mannequin underneath
surgical drapes and having two people walk around in the laminar airflow but
acknowledging that “in a working operating [room] there are more people and many
other ways by which the system might be disrupted”); A.J. Legg, T. Cannon & A.J.
Hamer, Do Forced Air Patient-Warming Devices Disrupt Unidirectional Downward
Airflow?, 94-B J. Bone & Joint Surgery 254, 255 (2012) (“Legg 2012”) (testing the
airflow-disruption theory by placing a volunteer draped for surgery with the warmer
applied under the drapes on an operating table within an enclosure meant to facilitate
laminar airflow and having a surgeon stand within the enclosure, but not including
any assistants or instrument trays in the enclosure); A.J. Legg & A.J. Hamer, Forced-
Air Patient Warming Blankets Disrupt Unidirectional Airflow, 95-B Bone & Joint
J. 407, 407 (2013) (“Legg 2013”) (using similar conditions as in Legg 2012); Kumar
G. Belani et al., Patient Warming Excess Heat: The Effects on Orthopedic Operating
Room Ventilation Performance, 117 Anesthesia & Analgesia 406, 406-07, 410
(2013) (“Belani 2013”) (testing the airflow-disruption theory by draping a
mannequin, applying warming devices underneath the drapes to the mannequin’s

                                        -20-
torso, and having an anesthetist stand motionless at the head of the mannequin, but
cautioning that their findings were “dependent on [the] exact setup” of the
experiment, which omitted “instrument trays and a working surgical team”).

        These limitations notwithstanding, a few of these studies make findings and
observations that ameliorate the problematic gap the MDL court found between the
simulated operating-room conditions in these studies and real-world operating
rooms. For instance, in McGovern 2011, the authors noted how the surgical lighting,
drapes, and personnel in their study created “fragile [airflow] conditions” that
facilitated the Bair Hugger’s ability to disrupt airflow significantly enough to
transmit air from nonsterile areas of the operating room to the surgical site.
McGovern 2011, supra, at 1542. Similarly, in Belani 2013, the authors found that
surgical lighting and drapes magnified the Bair Hugger’s effects. Belani 2013,
supra, at 410. In other words, findings in these studies provide empirical support
bridging the analytical gap from simulated operating-room conditions to real-world
operating-room conditions. This analytical gap, then, was at least partially illusory.

       As for whether particles in the increased airflow over the surgical site include
bacteria-laden particles, the MDL court made too much of Dr. Jarvis’s “admission.”
As Dr. Jarvis explained at the Gareis trial, the key study on which the medical
experts relied to correlate particles with bacteria found a statistically significant
association between the presence of bacteria and the presence of particles measuring
both 5.0-10.0 µm in diameter and ≥10.0 µm in diameter. See Gregory W. Stocks et
al., Predicting Bacterial Populations Based on Airborne Particulates: A Study
Performed in Nonlaminar Flow Operating Rooms During Joint Arthroplasty
Surgery, 38 Am. J. Infection Control 199, 199-202 (2010) (“Stocks 2010”). 7 The
Legg 2012 authors found a statistically significant increase in particles measuring
5.0 µm in size over the surgical site when forced-air warming was used. Legg 2012,
supra, at 255-56. And, as discussed above, Dr. Elghobashi’s (admissible) CFD

      7
       This study also noted that “[a]irborne bacteria-carrying particles measure 4
µm to 20 µm.” Id. at 203.

                                         -21-
model showed particles measuring 10.0 µm in size reaching “regions of interest”
(including the region simulating the surgical site) with the machine on for a short
period of time. In other words, the proposition that the increase in particles caused
by the Bair Hugger includes bacteria-laden particles finds support in the record.

       The question for the MDL court was whether there was sufficient support in
the factual basis for the experts’ opinions that the Bair Hugger is capable of causing
airflow disruption in a real-world operating room that transmits bacteria to the
surgical site. The MDL court held that there was not. But, as we have just seen,
there is significant support for the proposition that the Bair Hugger independently is
capable of disrupting airflow so as to transmit bacteria to the surgical site when other
airflow-disruptive variables are controlled for, and there also is empirical support
for the proposition that those other variables can facilitate the Bair Hugger’s airflow-
disruptive effect in a real-world operating room. Thus, notwithstanding the
significant deference owed here, we conclude that the MDL court committed a clear
error of judgment on the basis of the record before it, see Grp. Health Plan, 344 F.3d
at 760, in holding that the experts’ general-causation opinions premised on the
airflow-disruption theory were “so fundamentally unsupported” that they had to be
excluded, see Loudermill, 863 F.2d at 570. In light of the evidence the experts relied
on to find the airflow-disruption theory a plausible mechanism to explain the
association found in McGovern 2011, this was an instance in which our “general
rule” that deficiencies in an expert’s factual basis go to weight and not admissibility
should have been followed. See, e.g., Klingenberg, 936 F.3d at 830.

                                           b.

       All three medical experts also relied on the dirty-machine theory as a plausible
causation mechanism to support their general-causation opinions. Again, according
to this theory, the Bair Hugger is capable of emitting bacteria harbored within the
machine through the blanket and ultimately to the surgical site, causing PJIs. The
experts relied on published studies and reports ostensibly supporting this

                                         -22-
mechanistic theory. 8 The MDL court found “too great an analytical gap between the
experts’ conclusions” and these studies, faulting them for ignoring the limitations in
these studies, none of which examined whether contaminated air emitted from the
device “could reach the surgical site and cause infection.”

       For the dirty-machine theory to be plausible and for the experts’ opinions to
be reliably based on this theory, four premises need to be sufficiently supported in
the evidence relied on by the experts. Cf. Hirchak, 980 F.3d at 609 (noting that,
under Rule 702, “the expert opinion itself—not just one of its several premises—
must be ‘based on sufficient facts’”). First, the Bair Hugger internally must harbor
bacteria in either the central unit or the hose. Second, the Bair Hugger must be
capable of blowing that internal contamination into the blanket. Third, that internal
contamination must be capable of escaping the blanket. And fourth, that internal
contamination must be able to reach the surgical site.

       The first premise is well supported by the studies that the experts cite, many
of which found that Bair Huggers were internally contaminated with bacteria. See,
e.g., M.S. Avidan et al., Convection Warmers—Not Just Hot Air, 52 Anaesthesia

      8
        According to his deposition testimony, Dr. Jarvis also considered the
experiment done by Plaintiffs’ engineering expert Michael Buck, in which Buck
found that the Bair Hugger emitted out of the blanket 5-10 µm sized particles and
>10 µm sized particles. Seemingly because neither Dr. Jarvis nor the other two
medical experts cited Buck’s experiment in their reports, however, 3M argues that
none of these experts relied on Buck’s work and so this work cannot be considered
part of the factual basis for their opinions. See Hirchak v. W.W. Grainger, Inc., 980
F.3d 605, 609 (8th Cir. 2020) (noting that evidence an expert did not consider cannot
rescue the expert’s opinion from inadmissibility “by filling its analytical gaps”);
Turner v. Iowa Fire Equip. Co., 229 F.3d 1202, 1209 (8th Cir. 2000) (rejecting
plaintiffs’ attempt to bolster their expert’s causation opinion with materials
corroborating it because the expert “simply did not rely upon those items in
formulating his opinion”). We decline to resolve this point because we find that the
studies and reports that the experts undisputedly relied on provide enough of a
factual basis to render a clear error of judgment the MDL court’s finding of “too
great” an analytical gap here.

                                        -23-
1073, 1074-75 (1997) (“Avidan 1997”); A.T. Bernards et al., Persistent
Acinetobacter Baumannii? Look Inside Your Medical Equipment, 25 Infection
Control & Hosp. Epidemiology 1002, 1002, 1004 (2004) (“Bernards 2004”); Mark
Albrecht, Robert Gauthier & David Leaper, Forced-Air Warming: A Source of
Airborne Contamination in the Operating Room?, 1 Orthopedic Reviews 85, 85-87
(2009) (“Albrecht 2009”); Mark Albrecht et al., Forced-Air Warming Blowers: An
Evaluation of Filtration Adequacy and Airborne Contamination Emissions in the
Operating Room, 39 Am. J. Infection Control 321, 322, 324-25 (2011) (“Albrecht
2011”).

      The second premise also finds support in these studies. See, e.g., Avidan
1997, supra, at 1074 (finding that air blown out of Bair Hugger hoses contained
microbes); Albrecht 2009, supra, at 85, 87 (finding that Bair Hugger hoses were
emitting particles in the size range of airborne microbes).

       The third and fourth premises find less support but are not unsupported. On
the one hand, one of the studies expressly recognized that, for a “direct risk” from
the internal contamination to be present, the airflow from the machines would have
to reach the surgical site, and it observed that it was “presently unknown whether
this happens” because the blanket “may act as a low-efficiency microbial filter” and
“surgical drapes may act as a barrier.” See Mike Reed et al., Forced-Air Warming
Design: Evaluation of Intake Filtration, Internal Microbial Buildup, and Airborne-
Contamination Emissions, 81 Am. Ass’n Nurse Anesthetists J. 275, 279 (2013)
(“Reed 2013”). On the other hand, a subsequent report described an incident in
which a short-circuit inside a Bair Hugger generated smoke that was emitted through
the blanket and deposited as soot on the patient’s body in the pattern of the holes in
the Bair Hugger blanket, undermining the blanket-as-secondary-filter hypothesis
and supporting the third premise. See T. Moon et al., Forced Air Warming Device
Failure Resulting in Smoke and Soot on a Surgical Patient, 4 Open Access J.

                                        -24-
Surgery, May 2017, at 1 (“Moon 2017”). 9 And the fourth premise finds support in
several sources. For one, in a 2004 report on a bacterial-outbreak investigation, the
investigators reported that they traced the outbreak strain to the interior of a
ventilator and a Bair Hugger, explained that the outbreak subsided once they cleaned
the ventilator and replaced the Bair Hugger’s filter, and offered their view that “the
outbreak strain was transmitted by being carried on contaminated dust from within
the machines to the exterior during operation.” Bernards 2004, supra, at 1003.
Additionally, some of the airflow-disruption studies the experts relied on reported
that air from where the blanket exhausted waste heat reached the surgical site and
that certain draping arrangements would facilitate that. See, e.g., McGovern 2011,
supra, at 1537, 1539-40; Belani 2013, supra, at 407.

       Accordingly, we conclude that it was a clear error of judgment for the MDL
court to find that the experts’ opinions insofar as they were based on the dirty-
machine theory were so fundamentally unsupported that they should be excluded.
See Grp. Health Plan, 344 F.3d at 760; Loudermill, 863 F.2d at 570. Certainly, there
are weaknesses in the dirty-machine theory. Again, however, redress for such
weaknesses lies in cross-examination and contrary evidence rather than exclusion.
See Bonner, 259 F.3d at 929; but cf. Polski v. Quigley Corp., 538 F.3d 836, 839-41
& n.4 (8th Cir. 2008) (affirming the exclusion of an expert’s causation opinion based
on an untested mechanistic theory that the expert himself previously had effectively
stated was implausible).

                                   *      *     *

      9
        Moon 2017 does not provide unimpeachable support for the third premise, to
be sure, because it was unknown whether the particles blown out of the blanket and
onto the patient were of the size capable of carrying bacteria. We note, however,
that one of 3M’s own experts effectively conceded the validity of the third premise,
testifying when asked at deposition that “some particles” blown into the blanket will
leave it and that some of those particles “[m]ost likely” will carry bacteria.

                                        -25-
       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, see Joiner, 522
U.S. at 146; 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), see Loudermill, 863 F.2d at 570. 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. 10

     We emphasize that this conclusion is a narrow one—again, the standard for
admissibility is “lower than the merits standard of correctness,” Kuhn, 686 F.3d at

      10
         In arguing to the contrary, 3M relies heavily on our decision in Glastetter v.
Novartis Pharmaceuticals Corp., where we affirmed the exclusion of medical
experts who “lacked a proper basis” for their general-causation opinions that a
certain medication could cause intracerebral hemorrhages. 252 F.3d 986, 988-89
(8th Cir. 2001) (per curiam). We find Glastetter distinguishable, most saliently
because the experts in that case had no epidemiological evidence on which to rely to
link the medication to its purported effect, see id. at 992, unlike Plaintiffs’ general-
causation experts here, cf. id. (noting that “epidemiological studies and reports are
much desired by litigants in cases involving medical causation”); Norris v. Baxter
Healthcare Corp., 397 F.3d 878, 882 (10th Cir. 2005) (“[E]pidemiology is the best
evidence of general causation in a toxic tort case.”).

                                         -26-
625—that turns greatly on the fact that the opinions at issue here address general
causation (whether the Bair Hugger can cause a PJI) rather than specific causation
(whether the Bair Hugger did cause a particular plaintiff’s PJI). See Junk, 628 F.3d
at 450. In several places in its order excluding the medical experts, the MDL court
suggested that the weaknesses in the experts’ general-causation evidence,
particularly in the evidence regarding the mechanisms of causation, would present
significant hurdles for the specific-causation showing these Plaintiffs must make to
prevail—that is, that they would not have contracted a PJI but for use of the Bair
Hugger during their surgeries. Whether this is so is not at issue in this appeal, and
we express no view on it here. We hold only that the MDL court abused its
discretion in excluding these experts’ general-causation opinions on the basis of
excessive analytical gaps.

                                          C.

        The MDL court’s analytical-gap determination constituted the primary
justification for its decision to exclude Plaintiffs’ general-causation medical experts.
But it did briefly find as well that lack of general acceptance of the causal inferences
made by the experts also supported excluding their testimony. In Daubert, the Court
rejected “a rigid ‘general acceptance’ requirement” that could alone be dispositive,
but it did acknowledge that general acceptance (or lack thereof) “can yet have a
bearing on the inquiry.” 509 U.S. at 588, 594. However, this factor must be applied
while bearing in mind “that a rigid general acceptance requirement would be at odds
with the liberal thrust of the Federal Rules and their general approach of relaxing
traditional barriers to opinion testimony.” Lauzon, 270 F.3d at 691 (internal
quotation marks omitted).

       The MDL court considered three data points in its general-acceptance
analysis. First, it noted how, in a statement put out by the 2013 International
Consensus Meeting on Periprosthetic Joint Infection, there was a strong consensus
that, although forced-air warming devices posed a “theoretical risk,” no studies had
“shown” an increase in surgical-site infections related to the use of these devices,

                                         -27-
and although “[f]urther study” was warranted there was no need to stop using forced-
air warming devices based on the evidence at that time. Second, it considered a
letter issued by the Food and Drug Administration in 2017 reporting the agency’s
determination that it had been “unable to identify a consistently reported
association” between forced-air warming and surgical-site infection and continuing
to recommend using such devices “when clinically warranted.” Third, it noted that,
in a statement put out by the 2018 International Consensus Meeting on
Musculoskeletal Infection, there was a strong consensus that there was “no evidence
to definitively link” forced-air warming to an increased risk of PJIs.

        Notably, however, in that 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. See, e.g., A.M. Wood
et al., Infection Control Hazards Associated with the Use of Forced-Air Warming in
Operating Theatres, 88 J. Hospital Infection 132, 132 (2014) (concluding, after a
review of the literature on the issue, that forced-air warming “does contaminate
ultra-clean air ventilation,” though acknowledging that “current research” had not
shown a “definite link” between forced-air warming and surgical-site infections such
as PJIs); cf. Primiano v. Cook, 598 F.3d 558, 565 (9th Cir. 2010) (recognizing that
“medical knowledge is often uncertain” and that “[l]ack of certainty is not, for a
qualified expert, the same thing as guesswork”). 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, see Melissa D. Kellam, Loraine S. Dieckmann & Paul N. Austin, Forced-
Air Warming Devices and the Risk of Surgical Site Infections, 98 AORN J. 353, 365
(2013), and suggest modifications to forced-air-warming devices to “reduce the risk”
they may pose, see ECRI Update: You’re Getting Warm: Uncovering Forced-Air
Warming Units, ECRI Institute (May 1, 2017).

      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

                                        -28-
extensive scientific research and debate.” See Milward, 639 F.3d at 22; cf. Bonner,
259 F.3d at 929 (“[N]either Rule 702 nor Daubert requires that an expert opinion
resolve an ultimate issue of fact to a scientific absolute in order to be admissible.”).
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. Cf. Milward, 639
F.3d at 22, 26 (criticizing the district court in that case for placing “undue weight on
the lack of general acceptance” of an expert’s conclusions about causation and
ultimately reversing its exclusion of that expert).

                                          III.

       The MDL court’s grant of summary judgment to 3M was derivative of its
order excluding Plaintiffs’ general-causation medical experts and Dr. Elghobashi.
Because we reverse in relevant part the exclusion of those experts, we reverse the
grant of summary judgment. See, e.g., Kuhn, 686 F.3d at 633. 11

                                          IV.

       We next consider Plaintiffs’ challenge to the MDL court’s discovery ruling
prohibiting them on relevancy grounds from obtaining discovery concerning
conductive patient-warming devices. Plaintiffs argue that this ruling was an abuse
of discretion because the discovery is relevant to their design-defect claims. We find
no basis to reverse.

       “Appellate review of a trial court’s determination concerning discovery
matters is very narrow.” McGowan v. Gen. Dynamics Corp., 794 F.2d 361, 363 (8th
Cir. 1986). “[W]e will only reverse upon a showing of a ‘gross abuse of discretion

      11
        We thus do not reach Plaintiffs’ argument in the alternative that summary
judgment was erroneously granted even if the general-causation medical experts and
Dr. Elghobashi were properly excluded.

                                         -29-
resulting in fundamental unfairness in the trial of the case.’” Ahlberg v. Chrysler
Corp., 481 F.3d 630, 637-38 (8th Cir. 2007) (quoting Firefighters’ Inst. for Racial
Equality ex rel. Anderson v. City of St. Louis, 220 F.3d 898, 902 (8th Cir. 2000)).

       We assume without deciding that, as Plaintiffs argue, the MDL court
erroneously concluded that this discovery was irrelevant across the entire MDL
because some states would (or might allow a jury to) recognize conductive patient-
warming devices as reasonable alternative designs to convective patient-warming
devices. Even so, Plaintiffs have not even argued on appeal, let alone shown, that
the MDL court’s discovery ruling resulted in fundamental unfairness to them in
trying their cases. See Moses.com Secs., Inc. v. Comprehensive Software Sys., Inc.,
406 F.3d 1052, 1060 (8th Cir. 2005) (declining to reverse a discovery ruling where
the party seeking reversal of the discovery rulings at issue did not specify how the
rulings “resulted in fundamental unfairness” and the record did not support a finding
that the party “suffered prejudice as a result of the rulings”); Ahlberg, 481 F.3d at
634 (“[P]oints not meaningfully argued in an opening brief are waived.”).

       Even if we considered the point, we would not find fundamental unfairness
on this record. Plaintiffs apparently had other reasonable-alternative-design
evidence available to them, as is demonstrated by their recitation of studies
suggesting that filter-related modifications to the Bair Hugger would make it safer.
See, e.g., Restatement (Third) of Torts: Prod. Liab. § 2 cmt. f (Am. Law Inst. 1998)
(providing an example of a modified existing product as a reasonable alternative
design). In addition, Plaintiffs were permitted discovery regarding other convective
warming devices for reasonable-alternative-design purposes. See id. (“[O]ther
products already available on the market may serve the same or very similar function
at lower risk and at comparable cost. Such products may serve as reasonable
alternatives to the product in question.”).

                                        -30-
                                         V.

       We now turn to Plaintiffs’ challenge to the MDL court’s decision to seal
certain filings on its own docket. We review the district court’s decision to seal
records for an abuse of discretion. IDT Corp. v. eBay, 709 F.3d 1220, 1223 (8th Cir.
2013).

       Plaintiffs take issue with the sealing of seven filings: (1) MDL Docket Entry
No. 221-19 (also located at MDL Docket Entry Nos. 347, 887, 938, and 1801);
(2) MDL Docket Entry No. 221-20 (also located at MDL Docket Entry Nos. 895 and
1806); (3) MDL Docket Entry No. 340 (also located at MDL Docket Entry No. 944);
(4) MDL Docket Entry No. 377 (also located at MDL Docket Entry No. 945);
(5) MDL Docket Entry No. 901 (also located at MDL Docket Entry No. 1808);
(6) MDL Docket Entry No. 937; and (7) Docket Entry No. 1805 (which is another
version of the document at MDL Docket Entry No. 937). 12 As the MDL court found,
these documents contain sensitive business and strategic planning information. For
each, 3M asserted that it would suffer competitive harm if the document was
unsealed. The MDL court agreed and ordered these files kept under seal. Plaintiffs
argue this was an abuse of discretion. We find no abuse of discretion.

       “[T]here is ‘a common-law right of access to judicial records.’” Webster
Groves Sch. Dist. v. Pulitzer Pub. Co., 898 F.2d 1371, 1376 (8th Cir. 1990) (quoting
Nixon v. Warner Commc’ns, Inc., 435 U.S. 589, 597 (1978)). “This right of access
is not absolute,” however, “but requires a weighing of competing interests.” Id.
When this common-law right is implicated, “we give deference to the trial court
rather than taking the approach of some circuits and recognizing a ‘strong

      12
        Plaintiffs assert in passing that “[t]he MDL court erroneously sealed dozens
of court records,” but in both their opening brief and appellate motion they argue
specifically only that these seven records should be unsealed. Accordingly, we
decline to consider their challenge to the sealing of any other documents besides
these seven. See McKay v. City of St. Louis, 960 F.3d 1094, 1099 n.2 (8th Cir. 2020).

                                        -31-
presumption’ favoring access.” Id. (quoting United States v. Webbe, 791 F.2d 103,
106 (8th Cir. 1986)).

       Whether sealing is warranted, the common-law right of access
notwithstanding, turns on “the relevant facts and circumstances of the particular
case.” Warner Commc’ns, 435 U.S. at 599. The district court “must consider the
degree to which sealing a judicial record would interfere with the interests served by
the common-law right of access and balance that interference against the salutary
interests served by maintaining confidentiality of the information sought to be
sealed.” IDT Corp., 709 F.3d at 1223. Interests served by the common-law right
include bolstering public confidence in the judicial system by allowing citizens to
evaluate the reasonableness and fairness of judicial proceedings, allowing the public
to keep a watchful eye on the workings of public agencies, and providing a measure
of accountability to the public at large (which pays for the courts). Id. at 1222. But
these interests have “bowed before the power of a court to insure that its records are
not used to gratify private spite,” to “promote public scandal,” to serve “as reservoirs
of libelous statements for press consumption,” or to serve “as sources of business
information that might harm a litigant’s competitive standing.” Warner Commc’ns,
435 U.S. at 598 (internal quotation marks omitted). The “consideration of
competing values” that must be done here is “heavily reliant on the observations and
insights of the presiding judge.” Webbe, 791 F.2d at 106.

      Here, the MDL court concluded that keeping the contested records under seal
was warranted because they contained sensitive, commercially competitive material
“to which [3M’s] need to maintain confidentiality . . . outweighs the public’s right
of access.” The record shows that Dr. Augustine—who, as we noted above, created
a competitor device to the Bair Hugger and has helped foment this litigation against
3M—has attempted to exploit and misrepresent information learned in this MDL to
3M’s detriment and to his commercial benefit. In other words, the MDL court’s
sealing decision was based on “salutary interests,” see IDT Corp., 709 F.3d at 1223,
before which the common-law right of access “has bowed,” see Warner Commc’ns,
435 U.S. at 598. See also United States v. McDougal, 103 F.3d 651, 658 (8th Cir.

                                         -32-
1996) (agreeing that, “as a matter of public policy,” courts “should avoid becoming
the instrumentalities of commercial or other private pursuits”). Accordingly, we find
no abuse of discretion in this decision.

       In arguing that we should hold otherwise, Plaintiffs point out a “[m]odern”
trend that “‘the weight to be given the presumption of access’” is “‘governed by the
role of the material at issue in the exercise of Article III judicial power and resultant
value of such information to those monitoring the federal courts.’” IDT Corp., 709
F.3d at 1224 (quoting United States v. Amodeo, 71 F.3d 1044, 1049 (2d Cir. 1995)).
In these cases, “the strong weight to be accorded the public right of access” in some
instances derives from the central role the documents in question play “in
determining litigants’ substantive rights” and “from the need for public monitoring
of that conduct.” Amodeo, 71 F.3d at 1049. “Where testimony or documents play
only a negligible role in the performance of Article III duties,” however, “the weight
of the presumption is low.” Id. at 1050.

       Even assuming this is the correct framework to govern the sealing issue here,
but cf. Webster Groves Sch. Dist., 898 F.2d at 1376 (noting that we give deference
to the district court’s sealing decisions “rather than taking the approach of some
circuits and recognizing a ‘strong presumption’ favoring access”), we nonetheless
would find no abuse of discretion. The only place where Plaintiffs indicate that the
MDL court “discussed and analyzed” these documents is in a footnote in its Daubert
reconsideration order where it mentioned in passing that it was “unable to
determine” from these documents whether they undermined its general-acceptance
analysis. As noted above, the general-acceptance factor played a negligible role in
the MDL court’s Daubert decision. And these documents played a negligible role
in the MDL court’s general-acceptance analysis. Accordingly, any “presumption of
access” to these documents is “low,” and the “countervailing reason[s]” justifying
sealing here trump the right of access. See Amodeo, 71 F.3d at 1050.

                                          -33-
        Plaintiffs also ask us to join several other circuits and hold that there is a First
Amendment presumption of public access to summary-judgment materials (which
they contend the contested records are). See, e.g., Rushford v. New Yorker
Magazine, Inc., 846 F.2d 249, 253 (4th Cir. 1988). We have yet to decide “whether
there is a First Amendment right of public access to the court file in civil
proceedings.” IDT Corp., 709 F.3d at 1224 n.*. We have said, however, that for
such a right to be recognized at least two prerequisites must be satisfied: (1) there
is a historical tradition of accessibility to the records in question, and (2) there is a
significant positive role for public access in the functioning of the judicial process
in question. Id. Plaintiffs do not even mention, let alone meaningfully argue, the
first prerequisite, so we decline to consider this argument. See McKay, 960 F.3d at
1099 n.2.

                                            VI.

         Plaintiffs also have filed a motion on appeal asking us to unseal the same
documents they want us to order the MDL court to unseal. See Warner Commc’ns,
435 U.S. at 598 (“Every court has supervisory power over its own records and files
. . . .”). The rationale discussed in Section V for affirming the MDL court’s sealing
decisions regarding the seven documents at issue applies with equal force here, so
we deny Plaintiffs’ request for us to unseal these documents. 13

       13
         In their opening brief, Plaintiffs also ask us to unseal “all appellate briefs
and appendices provisionally filed under seal.” Plaintiffs filed unopposed motions
to file redacted versions of their briefs that were provisionally filed under seal and
to allow the unsealing of most of the contents in the appellate appendices that also
were provisionally filed under seal, and we already granted these motions. To the
extent they seek further unsealing, their request appears redundant of the request
they make in their appellate motion to unseal, which we deny.

                                           -34-
                                        VII.

       For the foregoing reasons, we affirm in part and reverse in part the exclusion
of Dr. Elghobashi’s CFD model and opinion, we reverse the exclusion of Plaintiffs’
general-causation medical experts’ opinions, we reverse the grant of summary
judgment to 3M, we affirm the discovery ruling challenged by Plaintiffs on appeal,
and we affirm the decision to seal those filings whose sealing Plaintiffs challenge on
appeal. Finally, we deny Plaintiffs’ motion on appeal to unseal those same filings.
                       ______________________________

                                        -35-