In the

        United States Court of Appeals
                      For the Seventh Circuit
                          ____________________
No. 17-2068
CHARLOTTE ROBINSON and
BOBBY DON BOWERSOCK as co-personal
representatives of the Estate of
Georgia J. Bowersock, deceased, and
MARK BOWERSOCK, individually,
                                                     Plaintiffs-Appellants,

                                       v.

DAVOL INC. and C.R. BARD, INC.,
                                                    Defendants-Appellees.
                          ____________________

            Appeal from the United States District Court for the
            Southern District of Indiana, Indianapolis Division.
          No. 1:08-cv-01313-LJM-TAB — Larry J. McKinney, Judge.
                          ____________________

     ARGUED FEBRUARY 13, 2018 — DECIDED JANUARY 22, 2019
                   ____________________

   Before SYKES and BARRETT, Circuit Judges, and GRIESBACH,
Chief District Judge. ∗



∗   Of the Eastern District of Wisconsin, sitting by designation.
2                                                  No. 17-2068

    SYKES, Circuit Judge. C.R. Bard, Inc., manufactures a sur-
gical mesh patch used to repair hernias by implantation. The
patch consists of two pieces of mesh that surround a flexible
plastic ring. During a hernia repair, the patch is folded to fit
through a small incision, then the plastic ring springs back
into its original shape and flattens the mesh against the
abdominal wall.
   Bard recalled several versions of the patch in late 2005
and early 2006 following reports that the plastic ring was
defective. Sometimes the ring broke, exposing a sharp edge
that could perforate the patient’s intestines. Other times the
ring caused the patch to bend and warp, exposing the patch’s
adhesive to a patient’s viscera.
   Prior to the recall, Georgia Bowersock underwent sur-
gery to repair a hernia, and her surgeon implanted a Bard
patch. Roughly one year later, on October 31, 2006, she died
of complications arising from an abdominal-wall abscess.
Her estate and family members sued Bard and Davol Inc.,
the patent holder for the patch, alleging that a defect in the
patch caused her death. To establish medical causation, the
plaintiffs retained three experts to opine on the defect and
the likely cause of Mrs. Bowersock’s death.
    But the experts had trouble establishing causation. Un-
like defective patches in other injured patients,
Mrs. Bowersock’s patch did not adhere to her bowel or
perforate her organs with a broken, sharp edge. One expert
tried to present a new theory of causation: the patch had
“buckled,” forming a stiff edge that rubbed against and
imperceptibly perforated her internal organs.
No. 17-2068                                                  3

   The defendants moved to exclude the expert testimony.
The judge granted the motion, finding that the “buckling”
theory was not sufficiently reliable. Lacking expert testimo-
ny to establish causation, the plaintiffs could not prove their
case, and the judge entered summary judgment for Bard and
Davol.
   We affirm. The novel theory of causation was not peer
reviewed, professionally presented, consistent with
Mrs. Bowersock’s medical records or autopsy, or substantiat-
ed by other cases. The judge therefore did not abuse his
discretion in excluding the expert testimony. Summary
judgment for the defendants necessarily followed.
                        I. Background
    The Composix® Kugel Patch is a prescription medical
device designed to repair hernias. Bard manufactures the
patch and Davol owns the patent. (We refer to them collec-
tively as “Bard.”) The patch consists of two layers of mesh
that surround one or two flexible plastic rings called
memory rings. The top layer is made of polypropylene; it
adheres to the abdominal wall under the hernia and facili-
tates healing. The bottom layer is made of smooth expanded
polytetrafluorethylene; it faces the bowel to prevent the
patch from attaching to the viscera. To implant the patch, a
physician folds the device and then inserts it into the patient
via a small incision. After insertion the memory ring springs
back and flattens the patch against the abdominal wall. The
patch remains in the body after the hernia heals.
    The patch hit the market in 2001. Users soon began re-
porting problems with the plastic ring. Sometimes it would
altogether fail. Other times the ring would experience “buck-
4                                                 No. 17-2068

ling”—that is, the mesh components of the patch would
contract, causing the ring to resist and bend, kink, break, or
buckle. Although the patch came with instructions for use
that contained user warnings, none of the warnings men-
tioned any of these problems with the plastic ring. On
December 22, 2005, Bard recalled all extra-large models.
Several months later Bard expanded the recall to include
other models.
    On May 25, 2005, Mrs. Bowersock sought medical treat-
ment for an abdominal-wall hernia. On July 22 Dr. Mark O.
Lynch performed surgery and implanted a Bard patch, using
a model that was included in the second recall. Dr. Lynch
testified that he would not have implanted the patch if he
had known about the defective memory rings.
    On October 4, 2006, Mrs. Bowersock went to the emer-
gency room with an abdominal-wall abscess. The hospital
cultured the abscess, and the lab results returned positive for
staphylococcus aureus. Doctors administered antibiotics,
drained the abscess, and released her from the hospital. She
returned several days later with a large wound infection.
While hospitalized she suffered a cardiac arrest. She was
resuscitated and placed on a ventilator. The hospital took a
second culture that indicated the presence of staphylococcus
epidermidis and enterococcus faecalis, or fecal bacteria. A
third culture returned positive for pseudomonas aeruginosa
and yeast. Her condition deteriorated until her death on
October 31, 2006.
   Dr. Roland Kohr, the county coroner, performed an au-
topsy that same day and determined that pneumonia and
complications of that disease ultimately caused
Mrs. Bowersock’s death. In his report Dr. Kohr noted
No. 17-2068                                                     5

“abdominal adhesions” and an “abdominal wall fistula.” He
also noted that the “small bowel and colon [were] intact
without perforation, diverticula or palpable tumors.”
Dr. Kohr later exhumed Mrs. Bowersock’s body and re-
trieved the implanted patch for further study.
    Bobby and Mark Bowersock (Mrs. Bowersock’s sons) and
Charlotte Robinson (her sister) sued Bard in federal court
raising claims of negligence, failure to warn, breach of
implied warranty, fraud, and intentional infliction of emo-
tional distress. They also asserted a statutory claim for
violation of the Indiana Deceptive Consumer Sales Act, IND.
CODE § 24-5-5. Bobby and Charlotte are co-representatives of
Mrs. Bowersock’s estate; Mark also asserted an individual
claim under the Indiana Wrongful Death Act, 

All of the claims rested on the same essential allegations: the
patch implanted in Mrs. Bowersock was defective and
ultimately caused her death. The district court consolidated
the claims under the Indiana Products Liability Act, 

et seq., which “govern[s] all product liability
actions, whether the theory of liability is negligence or strict
liability in tort,” Dague v. Piper Aircraft Corp., 

,
212 (Ind. 1981).
    The plaintiffs retained Dr. Stephen Ferzoco to opine on
the cause of death. Dr. Ferzoco has experience treating
patients who had problems with the patches. He also has
testified in cases where the memory ring broke or the poly-
propylene side of the patch adhered to the intestines. After
examining the patch that was retrieved from
Mrs. Bowersock, however, Dr. Ferzoco conceded that neither
of those problems had occurred here. He instead developed
a new theory to account for her injury: the ring had buckled
6                                                 No. 17-2068

but stayed intact, and the raised portion of the mesh
“rubb[ed] up against the bowel causing a fistula or break
and then seal[ed] up prior to explantation or discovery of the
mesh in the bowel.” The parties and the district judge re-
ferred to this as the “nidus” theory (meaning the location or
focus of an infection), so we do the same; here it describes
the location where Dr. Ferzoco theorized that the buckled
ring rubbed against the bowel. Dr. Ferzoco also testified that
he could rule out several other possible causes of death,
including cross-contamination of fecal matter, fecal matter
entering through the skin, obesity, diabetes, and chronic
obstructive pulmonary disease.
   Dr. Ferzoco’s theory was novel: he had never before pre-
sented it in a formal or professional setting and could not
identify published medical literature discussing it. Though
he claimed to have seen this particular malfunction occur in
other patients, he declined to identify the patients or pro-
duce their medical records. Crucially, he admitted that there
was no evidence in the medical records or autopsy report of
bowel erosion or perforation.
   The plaintiffs also retained Dr. William Hyman, a profes-
sor of biomedical engineering. He opined that the memory
ring’s design was inherently dangerous, that Bard failed to
adequately test the patch, and that feasible alternative
designs were available. He also speculated that based on the
defective design and Dr. Ferzoco’s medical testimony, the
ring buckling likely caused the bowel injury. He identified
two important limitations in his testimony, however. First,
he admitted that he never examined or viewed images of
Mrs. Bowersock’s patch. Second, he is “not a microbiologist
No. 17-2068                                                   7

and [was] not offering an independent opinion on the mi-
crobiology of her infection.”
    Finally, the plaintiffs retained Dr. Kohr, the coroner. He
reiterated in his deposition that the autopsy did not reveal
any visible breaches of the small bowel or colon. He clari-
fied, however, that “there could have been superficial
breaches scarred over with additional inflammation” and
that “extensive adhesions in the suprapubic area [and] lower
abdomen” suggested the “possibility” of a breach. He also
testified that at the time of the autopsy, he wasn’t aware of
the problems with the patch or Bard’s recalls. Dr. Kohr
concluded that there was a “reasonable medical probability”
that the patch caused Mrs. Bowersock’s death.
    Bard moved to exclude the causation opinions offered by
each of these experts, arguing that (1) Dr. Ferzoco’s nidus
theory was not reliable; (2) Dr. Hyman’s opinion was un-
supported by the medical records; and (3) the plaintiffs
failed to timely disclose Dr. Kohr as an expert under Rule 26
of the Federal Rules of Civil Procedure. Bard also sought
summary judgment, arguing the plaintiffs could not prove
that the patch or its warnings were defective or caused
Mrs. Bowersock’s death.
     The judge granted the motion to exclude the experts. He
ruled that Dr. Ferzoco’s nidus theory failed to meet the
reliability threshold under Rule 702 of the Federal Rules of
Evidence. He also held that Dr. Hyman was not qualified to
offer an opinion about medical causation and that the plain-
tiffs’ failure to disclose Dr. Kohr as an expert precluded
them from calling him to testify in that capacity. That left the
plaintiffs without a causation expert—a requirement to
8                                                 No. 17-2068

prove the element of medical causation under Indiana law—
so the judge entered summary judgment for Bard.
                        II. Discussion
    We normally review a summary judgment de novo, but
our review is “slightly more nuanced” when summary
judgment follows from a decision to exclude expert testimo-
ny. Higgins v. Koch Dev. Corp., 

, 701 (7th Cir.
2015). Our first question is whether the judge properly
applied the Daubert framework for evaluating the admissibil-
ity of expert testimony. 

v. Merrell Dow
Pharm., Inc., 

(1993)). We then review for abuse of
discretion the judge’s decision to exclude the expert witness.

must establish causation to prove a viola-
tion of the Indiana Products Liability Act. IND. CODE § 34-20-
1-1. Under Indiana law “questions of medical causation of a
particular injury are questions of science necessarily de-
pendent on the testimony of physicians and surgeons
learned in such matters.” 

(quoting
Armstrong v. Cerestar USA, Inc., 

, 366 (Ind. Ct.
App. 2002)). “[W]hen there is no obvious origin to an injury
and it has multiple potential etiologies, expert testimony is
necessary to establish causation.” 

omitted).
    The key expert testimony is that of Dr. Ferzoco; without
it the plaintiffs cannot establish medical causation. They
concede as much. They do not challenge the exclusion of
Dr. Kohr as an expert based on their procedural violation,
and they acknowledge that Dr. Hyman’s opinion “does not,
in and of itself, establish medical causation.” We therefore
No. 17-2068                                                    9

focus our attention on the exclusion of Dr. Ferzoco’s testi-
mony under Rule 702.
   An expert’s opinion is permitted if:
       (a) the expert’s scientific, technical, or other
       specialized knowledge will help the trier of
       fact to understand the evidence or to determine
       a fact in issue;
       (b) the testimony is based on sufficient facts or
       data;
       (c) the testimony is the product of reliable
       principles and methods; and
       (d) the expert has reliably applied the princi-
       ples and methods to the facts of the case.
FED. R. EVID. 702.
    The familiar Daubert two-step framework applies to de-
termine whether the requirements of Rule 702 have been

–94. The proponent of the expert
testimony must first establish that “the proposed witness
would testify to valid scientific, technical, or other special-
ized knowledge.” Ammons v. Aramark Unif. Servs., Inc.,

, 816 (7th Cir. 2004) (internal quotation marks
omitted). The proponent must then show that the expert
testimony will assist the trier of fact. 

one the
judge evaluates whether the expert’s theory has been
“(1) tested, (2) subjected to peer review and publication,
(3) analyzed for known or potential error rate, and/or is
(4) generally accepted within the specific scientific field.”
Lapsley v. Xtek, Inc., 

, 810 (7th Cir. 2012). At step
two the judge evaluates “whether the proposed scientific
10                                                  No. 17-2068

testimony fits the issue to which the expert is testifying.”
United States v. Hall, 

, 1102 (7th Cir. 1999).
    The judge properly applied the Rule 702 and Daubert
standards in addressing Bard’s motion. He summarized
Dr. Ferzoco’s theory that the patch buckled and rubbed
against Mrs. Bowersock’s colon, causing fecal matter to
escape through an opening that either closed prior to dis-
covery or was not visible to the naked eye. He then ex-
plained why this novel theory of causation wasn’t reliable.
To begin, the theory wasn’t tested, subjected to peer review,
or described in medical literature. See 

. Moreover, the phenomena that Dr. Ferzoco described
were not found in Mrs. Bowersock’s medical records or
autopsy report. Last, the judge discounted Dr. Ferzoco’s
contention that he had previously treated patients injured in
this manner, explaining that the claim was not substantiated
with identified patients or records. See Olinger v. U.S. Golf
Ass’n, 

, 950 (N.D. Ind. 1999) (“The court
cannot evaluate the reliability of the undisclosed methodol-
ogy or of the principles that support the methodology.”).
    On appeal the plaintiffs contend that Dr. Ferzoco’s meth-
od was the equivalent of a differential diagnosis, which is an
“accepted and valid methodology.” Myers v. Ill. Cent. R.R.
Co., 

, 644 (7th Cir. 2010). Put in simple terms, a
differential diagnosis “provides a framework in which all
reasonable hypotheses are ‘ruled in’ as possible causes of a
medical problem and some of these possible causes are then
‘ruled out’ to the extent scientific evidence makes it appro-
priate to do so.” Ervin v. Johnson & Johnson, Inc., 

,
903 (7th Cir. 2007). The plaintiffs didn’t raise this argument
at summary judgment, however. They first used the term
No. 17-2068                                                    11

“differential diagnosis” in their motion to alter or amend the
judgment. That’s too late to preserve an argument for ap-
peal. Cf. Green v. Whiteco Indus., Inc., 

, 201 n.4 (7th
Cir. 1994) (“[R]aising [an] argument for the first time in the
motion for reconsideration is not adequate to preserve the
issue for appeal and definitively waives it.”).
    The plaintiffs insist that they presented the argument be-
low, just without using the term “differential diagnosis.”
They point to their argument at summary judgment that
Dr. Ferzoco’s opinions were “founded on reliable methods,
experience[,] and data.” That’s far too general a statement to
situate their expert’s opinion in the specific domain of
differential-diagnosis methodology. See Fednav Int’l Ltd. v.
Cont’l Ins. Co., 

, 841 (7th Cir. 2010) (explaining
that the failure to present a specific argument below results
in waiver, even if the argument “may have been before the
district court in more general terms”).
    Even if preserved, the argument fails on the merits.
Though differential diagnosis is widely accepted as a general
matter, an expert’s decision to “rule in” or “rule out” poten-
tial causes must itself be “scientifically valid.” 

. In other words, Dr. Ferzoco needed to establish the
reliability of his nidus theory in order to “rule in” the buck-
ling as a potential cause of Mrs. Bowersock’s death. As
we’ve noted, the judge identified several reasons why
Dr. Ferzoco’s nidus theory is not sufficiently reliable.
    The plaintiffs also argue that a scientific theory should
not be rejected solely because it lacks peer review. See Smith
v. Ford Motor Co., 

, 720 (7th Cir. 2000). But the
judge gave multiple reasons for his decision, including the
lack of corroborating evidence in Mrs. Bowersock’s medical
12                                                No. 17-2068

records and autopsy report. Along the same lines, the plain-
tiffs repeatedly assert that the lack of scientific literature
supporting the expert’s theory goes to the weight, not the
admissibility, of his testimony. That’s not the correct stand-
ard. Rule 702 and Daubert require the judge to act as a vigor-
ous gatekeeper to ensure the reliability of expert testimony.
See Kumho Tire Co. v. Carmichael, 

, 149 (1999).
   In sum, the plaintiffs cannot prove medical causation
without Dr. Ferzoco’s testimony. The record reflects that the
judge properly applied the Daubert framework and soundly
exercised his discretion to exclude it. It follows that Bard
was entitled to summary judgment.
                                                   AFFIRMED.