Biomechanical Testimony: When Should it be Admitted?

There are lots of examples of things that are really helpful for one thing. But when they’re misused, they produce bad information and/or poor results.

Misuse of Data and the Ecological Fallacy

Take demographics for instance. Demographics is the study of statistical data relating to the population and particular groups within it.

Population-level statistics are really helpful to understand broad social trends, plan city services, public health responses, school funding, etc.

But these statistics are worthless when drawing conclusions about specific individuals in specific situations. That’s because demographic statistics describe groups, not individuals.

Statistics can’t answer the question of whether a child is good at math or bad at reading because just because other children in the same demographic group tend to score higher or lower on standardized tests.

This sounds more than a little bit like stereotyping.

Stereotyping provides the bedrock for racism. It’s also a great example of the ecological fallacy: The ecological fallacy is the mistake of inferring something about an individual based solely on data about a group to which that individual belongs.

Use and Misuse of Biomechanical Engineering

No question: Biomechanical engineering is a powerful tool where forces, materials, and human tissue responses are well-quantified, reproducible, and mechanically predictable. For example, it’s helped engineers design safer vehicles.

Biomechanics helps predict the probability of injuries that have established mechanical thresholds (like long-bone fractures). It also can be used to mitigate against them.

But the brain is not a long-bone like the femur. Its consistency is more like finger-jello.

Soft tissues like muscle, facia, etc., don’t have the same predictable tencile strength as long-bones or ligaments.

Biomechanical experts estimate force and acceleration at impact. But translating those estimates into brain (or other soft-tissue) injury predictions is intellectually dishonest on two levels.

First, there is the ecological fallacy. Population-level statistics don’t tell us whether an individual was injured. Estimates about force and acceleration don’t provide a jury any more insight into whether the plaintiff suffered a brain injury than race-based statistics from standardized testings do about whether a kid can do math problems or read a story.

Second, research has found no clear universal threshold of force for concussion. Some high-force impacts cause no concussion, while some concussions occur from low forces. Factors like impact direction, rotation of the head and individual susceptibility all influence whether an mTBI occurs.

(These factors help explain why it’s so common to see one person in a vehicle badly injured and another walk away unharmed.)

A Better Framework

To be admissible, expert testimony must be based on facts or data, be the product of reliable principles and methods and be credibly applied to the facts of the case. It also must be relevant and helpful to the jury.

Biomechanical engineering shines when:

• The forces are high.

• The structures are mechanical (bone, ligaments).

• Loading can be quantified.

• Population-wide thresholds exist.

It’s worthless (or even sinister) when:

• The injury involves soft tissues with large individual variability.

• Loading is unmeasured or estimated.

• The injury is neurological, psychological, or complexly biological.

Judges have a lot on their plates. So they tend to operate based on macros. (I’m not sure whether any judges read this newsletter. But I suspect at least one looks at it.)

If there’s a question about the admissibility of biomechanical testimony, most judges will say:

A biomechanical expert may testify about the physical forces involved in a collision and how the human body generally responds to such forces. But the expert may not offer medical causation testimony (i.e., whether the plaintiff was or was not injured). Only health care providers can testify about causation.

The premise behind allowing biomechanical testimony is that it will help the jury determine whether the plaintiff could have been injured.

So what happens when it’s already been admitted by the biomechanical expert that a low-speed collision can cause a brain or other soft-tissue injury?

That eliminates the (legitimate) need for the biomechanical engineer to testify. The jury no longer needs to answer whether the collision could have caused the injury. It’s not an issue any more.

Same thing if the defense doctors have acknowledged that the collision could have caused the injury. A fortiori if they not only acknowledged the collision could have caused injury but in fact did cause a certain injury (with the only debate being the extent or duration of the symptoms).

Based on these realities, biomechanical testimony should only be allowed when three criteria are met:

1. There is an injury involving long-bones, ligaments or other body parts with well-established mechanical failure points.

2. There is a legitimate question whether the forces of a collision could have caused a specific injury.

3. The biomechanical witness only testifies about forces involved in the specific collision and the empirical bases for his opinions that the same forces are incapable of causing the claimed injury.