Sixth Circuit Clarifies No Shortcuts Under Daubert or the TPLA: Expertise Must Match the Theory; Malfunction Theory and Consumer-Expectation Shortcuts Fail for Complex Medical Implants

Case: Joseph Hill and Tracy Hill v. Medical Device Business Services, Inc. (MDBS), No. 24-5797

Court: United States Court of Appeals for the Sixth Circuit

Date: July 16, 2025

Panel: Judges Clay, Thapar, and Readler (Opinion by Judge Clay)

Disposition: District court affirmed; exclusion of plaintiffs’ experts and summary judgment for MDBS upheld; plaintiffs’ motion in limine denied as moot.

Note: Not recommended for publication (persuasive, not precedential, authority).

Introduction

This appeal arises from a Tennessee Products Liability Act (TPLA) manufacturing-defect suit brought by Joseph and Tracy Hill against Medical Device Business Services, Inc. after Mr. Hill’s femoral stem (part of a modular total hip replacement) fractured six years post-implantation. Plaintiffs abandoned design-defect claims and proceeded solely on a manufacturing-defect theory.

The district court excluded two of plaintiffs’ experts (a metallurgist and a biomedical engineer) under Federal Rule of Evidence 702 and Daubert, then granted summary judgment to MDBS on the TPLA claims. On appeal, the Hills challenged both the expert exclusions and summary judgment, and argued that their motion in limine (invoking Tennessee’s nonparty fault pleading requirements) should have barred MDBS’s evidence implicating the surgeons. The Sixth Circuit affirmed across the board.

The decision delivers clear guidance on three recurring issues in medical device litigation under Tennessee law:

Daubert’s “fit” and reliability demands: process-of-elimination causation opinions must stay within an expert’s actual domain, be grounded in sufficient facts and methods, and cannot be rescued by untimely or contradictory claims of reliance on another expert.
The limits of “malfunction theory” and consumer-expectations for complex medical devices: plaintiffs must trace injury to a specific construction error and supply evidence of ordinary consumer expectations; patient/surgeon-specific expectations and general device failure do not suffice.
Federal evidentiary obligations are independent of state-law pleading rules: excluding a defendant’s alternative-causation evidence via motion in limine does not lighten the plaintiff’s Rule 702 burden.

Summary of the Opinion

Expert exclusions affirmed. The court upheld exclusion of plaintiffs’ metallurgist (Dr. Greer) and biomedical engineer (Dr. Merryman). Dr. Greer, while qualified in metallurgy, lacked the surgical/biomechanical expertise necessary to eliminate surgical causes and was unfamiliar with MDBS’s manufacturing process. Her report included a key contradiction regarding reliance on the orthopedic surgeon’s opinions, which she disclaimed at deposition. Dr. Merryman mischaracterized Dr. Greer’s “critical flaw” opinion as a manufacturing-process flaw and largely parroted her conclusions without independent analysis.
Summary judgment affirmed. Plaintiffs failed to present direct or circumstantial evidence tracing the fracture to a specific manufacturing error or proving the product was unreasonably dangerous under either the consumer-expectations or prudent-manufacturer tests. Microporosity is ubiquitous in metals; plaintiffs’ own experts acknowledged idiopathic failures and could not identify a manufacturing deviation. Malfunction theory did not apply where multiple unknown causes are plausible and plaintiffs cannot show manufacturing defect is more likely than alternatives.
Motion in limine denied as moot and immaterial to Daubert burdens. Even if MDBS’s surgeon-causation evidence were excluded, plaintiffs still had to present reliable expert causation testimony; Rule 702 obligations are not contingent upon suppression of the defense’s theories.

Analysis

Precedents and Authorities Cited

The opinion is anchored in Rule 702 and Daubert’s gatekeeping framework, as recently applied in In re Onglyza (Saxagliptin) and Kombiglyze (Saxagliptin and Metformin) Products Liability Litigation, 93 F.4th 339 (6th Cir. 2024). Key points include:

Daubert and Kumho Tire. Expert testimony must be relevant and rest on reliable methods applied reliably to the facts; courts may exclude opinions supported only by the expert’s ipse dixit or featuring an “analytical gap.” See Daubert v. Merrell Dow, Kumho Tire Co. v. Carmichael, and Gen. Elec. Co. v. Joiner.
“Fit” of expertise to the question. Berry v. City of Detroit emphasizes that qualifications must match the specific question posed. The court relied on analogous medical-device cases excluding overbroad engineering testimony where the expert lacked implant biomechanics or procedure-specific knowledge (Botnick v. Zimmer; Jaske v. Zimmer).
Parroting other experts. An expert may rely on another’s work, but cannot merely regurgitate it without independent analysis (Buck v. Ford Motor Co.).
TPLA elements and tests. The court drew on Sigler v. American Honda Motor Co. for the TPLA elements; Fulton v. Pfizer for “defective condition” and the principle that injury alone is not proof of defect; Brown v. Crown Equipment for the idea that products need not be accident-proof.
Consumer-expectations and prudent-manufacturer tests. Sigler and Newell Rubbermaid v. Raymond Corp. require proof of ordinary consumer expectations; Young v. Olympus indicates the consumer-expectations test often ill fits complex medical devices absent such evidence. For the prudent-manufacturer test, expert testimony is “essential” and must address the prudence of marketing decisions and industry standards (Ray by Holman v. BIC; Irion v. Sun Lighting; Shoemake v. Omniquip).
Malfunction theory limits. Hill v. Kia Motors underscores that inferring defect from failure requires specific circumstantial evidence pointing to manufacturing defect over other causes; if a device is known to fail for unknown reasons, speculation is insufficient.
Procedural constraints on expert supplementation. Pride v. BIC and related authority confirm courts’ broad discretion to exclude untimely expert supplementation; supplementation cannot be used to cure methodological gaps post-deadline (Bowles v. City of Mansfield).
Motions in limine and federal procedure. Louzon v. Ford Motor Co. and Luce v. United States frame in limine rulings; the court also notes the inapplicability of state pleading rules (e.g., Crotty v. Flora; George v. Alexander) to relieve a federal Rule 702 burden.

Legal Reasoning

1) Excluding the metallurgist (Dr. Greer): “fit” and reliable methodology.

Dr. Greer was qualified to opine on metallurgy and the presence of microporosity, but not on surgical technique, biomechanics of hip implantation (angles, loads), the instruments used, or MDBS’s actual manufacturing steps. She used a process-of-elimination approach to attribute a “critical flaw” to manufacturing, yet admitted unfamiliarity with MDBS’s process and the surgeries.
Her report claimed reliance on the orthopedic surgeon’s opinion to rule out surgical causes, but her deposition testimony conceded she did not rely on it “in any meaningful way.” The late “supplemental” declaration reasserting reliance came months past the expert deadline and was within the court’s discretion to disregard. The internal contradiction further undermined reliability.
Because her elimination of alternative causes rested outside her domain and was not anchored by qualified reliance or process knowledge, the court found an analytical gap between the data (micropores) and the conclusion (manufacturing-process defect).

2) Excluding the biomedical engineer (Dr. Merryman): mischaracterization and parroting.

Dr. Merryman did not microscopically examine the device and expressly deferred to Dr. Greer, but miscast her “critical flaw” in the metal as a “critical flaw in the manufacturing process.” Since Dr. Greer did not analyze or criticize MDBS’s process, that mischaracterization alone undermined reliability.
His report added no independent analysis tying the fracture to a process deviation; under Buck, mere regurgitation of another expert’s unsupported view is not admissible.

3) Summary judgment: failure to prove “defect” or “unreasonably dangerous.”

No specific manufacturing error. The TPLA requires tracing injury to a specific construction error. Plaintiffs’ own experts acknowledged that microporosity exists in “every metal,” that fatigue failures can be idiopathic, and that they could not identify a step in MDBS’s process that caused the fracture. Dr. Greer even described MDBS’s process as “state-of-the-art.”
Malfunction theory rejected. With a medical device known to risk fatigue fracture and expert concessions that such failures may occur without identifiable cause, the record lacked specific circumstantial evidence making a manufacturing error more likely than other causes.
Consumer-expectations test failed. Plaintiffs offered only the patient’s and surgeon’s subjective expectations; they provided no evidence of the ordinary consumer’s expectations for a complex implant’s risk profile. That is insufficient under Tennessee law for complex devices.
Prudent-manufacturer test failed. Plaintiffs provided no expert testimony on industry standards, MDBS’s decision to market the product, or deviations from customary manufacturing/inspection/testing. Expert testimony is essential for this test; none was provided.
Temporal element. Having failed the first TPLA element, plaintiffs necessarily could not show the defect existed when the product left MDBS’s control.

4) Motion in limine: moot and immaterial to plaintiff’s Rule 702 burden.

Because summary judgment mooted trial evidentiary disputes, the motion was properly denied. More fundamentally, excluding MDBS’s alternative-causation evidence would not cure the lack of reliable plaintiff-side expert proof. Rule 702 requires plaintiffs to carry their own evidentiary burden irrespective of what the defense may present.

Impact

Although unpublished, this decision crystallizes several practical rules in the Sixth Circuit for TPLA medical device cases and Daubert practice:

Process-of-elimination opinions must be housed in the expert’s domain and anchored in facts. A metallurgist can explain how metal fails, but cannot reliably eliminate surgical or biomechanical causes absent adequate medical/biomechanical expertise or properly grounded reliance on such expertise.
Clear, timely reliance across disciplines is critical. Complex device cases often require a team: metallurgy + biomechanics + surgical practice + manufacturing-process engineering. Reliance must be genuine, documented, and timely under scheduling orders.
Malfunction theory is not a shortcut for complex products. Plaintiffs must supply specific circumstantial evidence making manufacturing defect more probable than other plausible causes, especially where “idiopathic” failures are acknowledged in the literature and by experts.
Consumer-expectations test is demanding for medical devices. Plaintiffs should expect to develop evidence about what an ordinary consumer would contemplate regarding implant risks; surgeon/patient subjectivity is not the measure. In many complex-device cases, this test may be inapposite or untenable without survey or market evidence.
Prudent-manufacturer test requires true industry evidence. Courts will look for expert testimony on industry customs, standards, and the prudence of the decision to market. Generalities, or concessions that the process is “state-of-the-art,” can be fatal.
Motions in limine cannot lighten Rule 702 burdens. Even if a defendant’s alternative-causation theory were excluded, plaintiffs must still meet Daubert with their own experts. Federal evidentiary standards govern, regardless of state procedural allocation of nonparty fault.

The opinion thus incentivizes careful expert selection and coordination, early and disciplined discovery into specific manufacturing steps, and realistic selection of TPLA theories that can be evidentially supported for complex medical devices.

Complex Concepts Simplified

Rule 702 and Daubert “gatekeeping.” Before an expert can testify, the judge must ensure the testimony is based on sufficient facts/data, uses reliable principles and methods, and applies those methods reliably to the case. The expert’s specialty must “fit” the questions they answer.
Process-of-elimination causation. An expert may sometimes infer a cause by ruling out alternatives, but only if the expert is qualified to evaluate each alternative and the reasoning is transparent and grounded in sufficient facts. Unsupported “because I say so” reasoning (ipse dixit) is inadmissible.
Manufacturing defect (TPLA). Unlike design defect, a manufacturing defect requires proof that the specific product unit departed from its intended design due to an error in construction. Evidence must trace the injury to that specific error, not to general risks.
Consumer-expectations vs. prudent-manufacturer tests.
- Consumer-expectations asks: was the product more dangerous than an ordinary consumer would expect? For complex devices, plaintiffs must show what an “ordinary consumer” would actually know/expect.
- Prudent-manufacturer asks: would a reasonable manufacturer have marketed the product if it knew of the risk? This typically requires expert testimony on industry standards and risk-utility tradeoffs.
Malfunction theory. A limited inference that a product was defective based on its failure in normal use and elimination of other causes. It requires specific circumstantial evidence pointing to a defect; mere occurrence of failure or possibility of many alternative causes will not suffice.
Microporosity and fatigue fracture. Metals naturally contain tiny voids (micropores). Under cyclic loading, cracks can initiate at stress concentrators and propagate over time (fatigue). The mere presence of micropores does not prove a manufacturing defect; the question is whether a specific process deviation made this unit unreasonably susceptible.

Conclusion

The Sixth Circuit’s decision in Hill v. MDBS sends a crisp, practitioner-focused message for complex medical device litigation under Tennessee law:

Expert “fit” and reliability are non-negotiable. A metallurgist cannot opine beyond metallurgy to exclude surgical or biomechanical causes without proper foundation; a biomedical engineer cannot simply adopt and misstate another expert’s conclusions.
TPLA plaintiffs must prove more than failure. They must identify a specific construction error and show the product was defective or unreasonably dangerous under either consumer-expectations (with evidence of ordinary expectations) or prudent-manufacturer (with industry-standard evidence).
Malfunction theory is narrow. It cannot bridge the gap where multiple unknown causes exist and plaintiffs cannot show manufacturing defect is the most probable cause.
Federal evidentiary standards govern regardless of state pleading rules. Motions in limine targeting a defendant’s alternative-causation evidence do not relieve plaintiffs of proving reliability under Rule 702.

Strategically, plaintiffs in medical implant cases should assemble a cross-disciplinary expert team early, secure detailed discovery on actual manufacturing steps, and select TPLA theories that they can substantiate with concrete, domain-matched expert analysis. Defendants, for their part, will find in this decision ample support for challenging overbroad or “parroting” expert testimony and for resisting attempts to leverage malfunction or consumer-expectation shortcuts in the context of complex medical devices.