Products Liability Law Daily Wrap Up, EXPERT EVIDENCE—MEDICAL DEVICES—D. Mass.: Court bars expert testimony based on unreliable sample but permits other testimony, (Jun 11, 2026)

By Sara Cracau, J.D.

District court bars expert pathology from testifying about reliance on unreliable sample but allows admission of remaining testimony regarding safer alternative designs and general causation.

A district court in Massachusetts has allowed in pat and denied in part a motion to exclude the testimony of an expert witness in a multidistrict products liability action finding that the expert pathologist was qualified to opine on safer design alternatives and general causation but barred him from testifying regarding opinions based on an unreliable sample. The court found that the pathologist’s opinions concerned the safety and utility of an alternative design compared to Symbotex’s design and whether such design generally would lessen or eliminate the types of injuries at issue in the case. The court agreed with Covidien that the pathologists’ opinions based on his sample of the “Juanz sample” were unreliable because of the specter of selection bias and, therefore, barred his testimony about his examination of the Juanz sample. The court declined to deny admission of the expert’s testimony on causation finding that the testimony did not suffer from any fatal flaws related to cherry-picking (Patterson v. Covidien, Inc., No.22-cv-10153-PBS (D. Mass. June 5, 2026)).

In the first bellwether hernia mesh products case against Covidien LP and Sofradim Production SAS (collectively, Covidien) a mesh patient brought an action against two manufacturers of hernia mesh products alleging that he was implanted with a Symbotex Composite Mesh (Symbotex) during a 2017 hernia repair operation and the product adhered to his bowel ad eventually caused a bowel construction and hernia recurrence requiring a follow-up surgery in 2020 (see Surgeon’s expert opinions found largely admissible in Covidien hernia mesh dispute, May 6, 2026).The consumer brought this action against Covidien under Alabama law, including claims under the Alabama Extended Manufacturer’s Liability Doctrine (AEMLD) and for negligence, breach of warranty, and negligence and fraudulent concealment and misrepresentation. The parties filed motions to exclude the testimony of 11 expert witnesses under Federal Rule of Evidence 702. This decision addresses Covidien’s motion to exclude the testimony of a pathologist offered by the consumer.

Expert’s qualifications and opinions. The court found that the pathologist was qualified to opine on general causation and safer alternative designs. It noted that he is a board-certified pathologist and cytopathologist who is affiliated with an Oregon medical group and serves as chair of pathology at a hospital in Oregon and is clinical assistant professor at Oregon Health & Science University.

Safer alternative designs. The court found that the pathologist’s opinions concerned the safety and utility of an alternative design compared to Symbotex’s design and whether such design generally would lessen or eliminate the types of injuries at issue in the case. Covidien sough to exclude the testimony on the basis that the pathologist failed to review medical evidence about the consumer barred him from opining that an alternative design would have been reduced or eliminated the consumer’s injuries specifically. The court noted that, although this opinion may be irrelevant, unless another expert testifies that the alternative design would have lessened or eliminated the consumer’s specific injuries, the court did not address relevance in this opinion.

Juanz sample. The court agreed with Covidien that the pathologists’ opinions based on his sample of the “Juanz sample” were unreliable because of the specter of selection bias and, therefore, barred his testimony about his examination of the Juanz sample. ”Selection bias” refers to the error that can be seen in an association that results from the method used to select participants and is often caused by the existence of non-random differences between individuals who are selected as study participant and others who are not. The consumer failed to show that the pathologist may reliably draw generalizable conclusions regarding the inflammatory nature of Covidien’s polyester meshes from the Juanz sample.

General causation. The court declined to deny admission of the expert’s testimony on causation finding that the testimony did not suffer from any fatal flaws related to cherry-picking. Insofar as there may have been inconsistencies in the expert’s application of criteria in evaluating scientific literature, they were not sufficiently numerous to cause his opinions to be unreliable. Furthermore, Covidien may cross-examine him. Nor did the court find that the expert’s opinions were unreliable on the basis of his citation to animal studies. It noted that animal studies may provide “a reliable basis for reaching a scientific conclusion” whereas there could be ethical constraints on conducting comparable research in humans.

State-of-mind testimony. The court found that, although a party’s intent or state of mind is not the proper subject of expert testimony, an expert may indicate what information a defendant had within his possession. To distinguish between proper and improper testimony, it cautioned that the expert should refrain from using words such as “knew” or “aware” and instead should use language such as “possessing information” or “acknowledging information.”

The case is No. 22-cv-10153-PBS.

Judge: Saris, P.

Attorneys: Andrea R. Preston (Levin, Papantonio Rafferty) and Walter Kelley (Bernheim Kelley, LLC) for Larry Patterson and Tammy Patterson. Mary Novacheck (Nelson Mullins Riley & Scarborough LLP) for Covidien Inc., Covidien Ltd. and Covidien, LLC d/b/a Covidien, LP f/k/a Tyco Healthcare Group LP. Jessica C. Wilson (DLA Piper US LLP) for Sofradim Production SAS. No Attorney Listed for Covidien PLC, Covidien Holding Inc., Medtronic USA Inc. and Medtronic PL.

Companies: Covidien Inc.; Covidien Ltd.; Covidien, LLC d/b/a Covidien, LP f/k/a Tyco Healthcare Group LP; Sofradim Production SAS; Covidien PLC; Covidien Holding Inc.; Medtronic USA Incand Medtronic PL

Cases: CourtDecisions ExpertEvidenceNews MedicalDevicesNews DesignManufacturingNews CausationNews MassachusettsNews