Defendants’ Expert Testimony Refuting Causation Need Not Be To a Degree of Reasonable Medical Probability

By editor on July 18, 2022

A California Court of Appeal, on May 26, 2022, held in a 3-0 opinion in Kline v. Zimmer, Inc. (2022) 79 Cal.App.5th 123, that to refute a plaintiff’s expert testimony on causation, a defendant’s expert causation testimony need not be provided to a degree of “reasonable medical probability.” In short, the reasonable medical probability requirement applies only to the party bearing the burden of proof (typically plaintiffs in personal injury cases).

In 2007, Gary Kline underwent total hip replacement surgery, during which he was implanted with a Durom Acetabular Component (“Durom Cup”), manufactured by Zimmer, Inc. (“Zimmer”). In 2008, Kline underwent a revision surgery in 2008 to replace the Durom Cup, but his hip pain returned in September 2010 and persisted despite eight years of treatment.

Kline sued Zimmer alleging the Durom Cup was defective and the cause of his hip pain.

In Kline’s second trial against Zimmer, his expert testified that, to a reasonable degree of medical probability, a defect in the Durom Cup caused his pain. To refute this, Zimmer introduced Dr. Sah who was prepared to testify as to alterative causes of Kline’s pain. The trial court excluded Dr. Sah’s testimony on the basis his expert opinion was not offered to a reasonable degree of medical probability. The court also excluded the testimony of Plaintiff’s treating physicians as to potential alternative causes of Kline’s pain on the same basis. Consequently, Plaintiff’s expert testimony as to causation was the only expert testimony heard by the jury. Unsurprisingly, the jury found in favor of Kline.

The Court of Appeal reversed and remanded for a new trial as to Kline’s damages caused by the Durom Cup’s design defect. The appellate court found that the trial court erred in excluding the testimony of Zimmer’s expert and Plaintiff’s treating physicians’ testimony as to possible other causes of Plaintiff’s hip pain and held that such testimony need not be offered to a degree of reasonable medical probability.

As the Court explained, once a plaintiff presents a prima facie case, she retains the burden of proving causation – the burden does not shift to the defendant. Thus, because the burden does not shift, to refute causation Zimmer was entitled to present expert testimony that Kline failed to satisfy his burden. Specifically, “[a]ll that Zimmer need to show what that Kline’s evidence was insufficient to prove Kline’s injuries were more likely than not caused by Zimmer.”  To make this showing, the Court found Zimmer “should have been permitted to do so by offering expert opinions offered to less than a reasonable medical probability that Kline’s injuries may have been attributable to other causes.”

Such defense expert testimony, “could cast doubt on the accuracy and reliability of a plaintiff’s expert.” It was for the jury to consider such evidence in deciding whether plaintiff’s expert is exaggerating their opinion. Given that complex questions of medical causation are prone to uncertainty, it is “imperative” the party not bearing the burden of proof “be allowed to suggest alternative causes, or the uncertainty of causation, to less than a reasonable medical probability.”  Otherwise, “to withhold such information from the jury is to deprive it of relevant information in assessing whether the plaintiff has met its ultimate burden of persuasion.”

A copy of the Court’s opinion can be found here:

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