“Sir, are you a medical doctor?”

How to prevent the defense biomechanics expert from rendering medical opinions at trial

George Ellard
2022 December

It is impermissible for a defense biomechanical engineer to testify as to medical opinions. If these opinions are not challenged by the plaintiff’s attorney, it will perpetuate the continued use of this impermissible expert opinion testimony by defense counsel. All too often, the plaintiff’s attorney will not attack such impermissible testimony because they do not understand the issue. The trial court must always be briefed as to limitations of a biomechanist’s allowable scope of testimony and its limitations to non-medical opinions. Medical opinions are solely the purview of medical doctors. If these opinions are not challenged by the plaintiff’s attorney, it will perpetuate an abuse of allowable expert opinion.

Biomechanics is essentially the application of engineering principles to the human body or to biology. Defense biomechanical experts will invariably attempt to testify that plaintiff’s injuries were not caused by a collision’s forces, even though they have no formal medical training. They should be precluded from giving such opinions by a motion in limine because opinions on medical causation must be offered by properly licensed medical doctors. Only if the biomechanical expert has proper medical professional qualifications should they be allowed to testify regarding medical causation.

Blackletter law dictates that experts must have “special knowledge, skill, experience, training, or education” to qualify as an expert. (Evid. Code, § 720, subd. (a).) Excludable expert opinion testimony is “(1) based on matter of a type on which an expert may not reasonably rely, (2) based on reasons unsupported by the material on which the expert relies, or (3) speculative.” (Sargon Enterprises, Inc. v. University of Southern Cal. (2012) 55 Cal.4th 747, 771-72.)

All too commonly, defense biomechanical experts opine in deposition that the forces involved in a collision or other traumatic event are insufficient to cause the claimed injury. The biomechanical expert will typically rely on publications and mechanical engineering principles to support their causation opinions. However, while a biomechanist may testify as to forces involved, the ultimate opinion of cause and extent of injury is actually a medical opinion and only medical professionals should be allowed to render them. (Salasquevara v. Wyeth Laboratories, Inc. (1990) 222 Cal.App.3d. 379.)

The defense biomechanist will attempt to dance around the acceptable limits of their opinions in various ways. One important practice tip for plaintiffs’ attorneys is to attack absence of injury opinions. An example is defense expert testimony that for the forces plaintiff’s expert claims were involved, a specific body part was not “injured.” Such opinion supposedly supports the lack of sufficient injury-causing forces. The reality is that absence of injury is also a medical professional opinion. Those opinions are the purview of only medical doctors.

The proper use of biomechanical opinion with medical doctor opinion should limit the role of engineers to engineering principles and permit only medical doctors to opine on medical causation of particular injuries. The appropriate limitation of biomechanical testimony was discussed in People v. Roehler (1985) 167 Cal.App.3d 353, a murder case. There, defendant claimed the victim’s head injury and drowning were accidental. The prosecution’s engineers recreated the victim’s body movements to measure forces applied to the victim’s head. The trial court then allowed only medical doctors to testify regarding the injuries to the head. The Court of Appeal approved the trial court’s limitation of the biomechanical engineering testimony:

In addition, the trial court judge made an important ruling with respect to the experimental evidence testimony that should be noted: he ruled that the engineering experts could testify concerning engineering principles and that only appropriately qualified medical experts could testify concerning the injuries to human heads involved.

(People v. Roehler, supra, 167 Cal.App.3d 353 at 388-389)

It is uncommon for biomechanical engineers to have the requisite medical training and experience to testify as to medical opinions. Therefore, the biomechanist must be thoroughly questioned at deposition regarding their qualifications to expose their inability to render competent medical opinions.

Plaintiff’s motion in limine should also pose a choice to defendant. If defendant contends its biomechanical expert is qualified to testify as to causation and/or extent of injury, is defendant willing to withdraw its medical doctor opinions to preclude duplicative and time-consuming opinions?

George Ellard George Ellard

Senior Associate Attorney George W. Ellard brings over 25 years of experience to the Rains Lucia Stern St. Phalle & Silver Injury Resource Group. George represents plaintiffs in catastrophic personal injury cases arising from motorcycle, automobile and trucking collisions, maritime activities, dangerous conditions of governmental and private properties, injuries occurring at work, defective products, elder abuse, medical and legal malpractice, and aviation accidents.


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