Is asking whether a collision caused an injury really just one question?
No: asking whether a collision caused a disc herniation, for example, asks three questions at once — whether the herniation exists and what it looks like, whether loading of the reconstructed character can produce that class of injury, and whether this person’s herniation arose from this event rather than something already underway. Only the middle question is biomechanical. The first question is radiological. The third question requires both biomechanics and medicine.
Keeping those questions separate is the difference between a biomechanical opinion that survives scrutiny and one struck in part. Being struck in part is the more common outcome and the more awkward one, since a partially excluded expert still testifies.
What does a biomechanical loading analysis actually produce?
A biomechanical loading analysis produces a description of exposure: a velocity change and principal direction of force, an acceleration time history, a ground reaction force estimate, a restraint load path, an occupant excursion. Each of those elements carries a stated source — measured, calculated, simulated or assumed — and a stated uncertainty. A biomechanical loading analysis describes what happened to a body while knowing nothing of that body’s medical history.
The second half of a biomechanical opinion compares that exposure against published human tolerance data for the tissue and mechanism at issue. The comparison yields a statement about capability and likelihood in a population. A biomechanical tolerance comparison is not a diagnosis, and does not become one by naming a claimant.
What does the clinical record contribute to an injury causation opinion that engineering can’t?
The clinical record supplies what biomechanical engineering cannot generate: the identity and extent of the injury, the imaging characterization, the timeline of symptom onset and treatment, and the baseline that prior records establish. Whether a finding is acute or chronic, and what a patient reported and when, are clinical determinations.
A biomechanical engineer who starts characterizing imaging findings has left the discipline of biomechanics. So has a physician who offers a velocity change derived from vehicle crush. Both happen, and both are what opposing counsel is trained to look for.
Can a biomechanical engineer testify to general causation, specific causation, or both?
A biomechanical engineer can address general causation, which is squarely biomechanical, but specific causation needs the clinical picture too. General causation — whether loading of a given magnitude, direction and rate can produce a category of injury — rests on reconstruction, testing and tolerance literature. Specific causation, whether an individual’s injury arose from a particular exposure, requires medicine as well, so the familiar split between general and specific causation maps onto biomechanics and medicine reasonably well.
A biomechanical engineer can properly say the reconstructed loading was or was not within the range associated with a given mechanism in the published data. Saying that a particular claimant’s injury did or did not come from a particular event goes further, and into medicine.
Why do biomechanical experts end up crossing into medical conclusions?
Biomechanical experts rarely cross into medicine through arrogance; the line is usually crossed because the retaining party asks one blunt question — did the event cause the injury — and the biomechanical expert answers it as asked. The line between biomechanics and medicine is also crossed by compression: a sentence saying the loading was insufficient to cause the reported injury has quietly asserted a clinical conclusion in engineering vocabulary.
The disciplined formulation separates the engineering finding from the clinical conclusion, along these lines: the reconstructed loading was of a magnitude and direction associated with low injury probability for this mechanism; whether the documented injury is attributable to it is for the reviewing physicians, informed by that finding.
Who decides whether a pre-existing condition explains an injury — the biomechanical engineer or the physician?
Neither discipline decides it alone: a pre-existing condition is a joint problem, in which establishing what existed beforehand is a documentary and radiological exercise, not a biomechanical one, and biomechanics contributes whether the reconstructed loading would aggravate an existing condition. Degenerative change is common, often asymptomatic, and visible on imaging in people never involved in a collision. Establishing what existed before the event turns almost entirely on whether pre-event imaging exists.
Whether the reconstructed loading would aggravate an existing condition is a different mechanical question from whether the loading would initiate injury in healthy tissue. Aggravation and initiation have different loading arguments, and conflating them weakens both.
How should a biomechanical engineer and a physician work together on an injury causation opinion?
A biomechanical engineer and a physician work best as a coordinated pair of opinions rather than in series, each staying inside its competence and each explicitly relying on the other for inputs. The biomechanist states the exposure and the tolerance comparison; the physician states the injury, the timeline and the baseline; the causation conclusion cites both and identifies which element came from where.
Serial work — a report handed across for adoption — produces opinions that read as though either biomechanics or medicine could have written them.
How is the boundary between biomechanical and medical testimony challenged?
The boundary between biomechanical and medical testimony is typically challenged by motions seeking partial rather than total exclusion of the biomechanical expert: the engineer may testify to forces and general injury mechanism but not to diagnosis. Rule 702 and its state analogues put qualification, reliable methodology and fit to the facts in issue at the center. A biomechanist’s qualifications rarely include a medical license, and practice-of-medicine statutes bear on what may be said about a specific patient.
A biomechanical opinion drafted with the split between forces and diagnosis already in place is largely immune to a motion for partial exclusion; one drafted without it hands the other side its outline.
What should a retaining party do to keep biomechanical and medical opinions within their limits?
A party retaining experts on injury causation should retain both competencies, biomechanical and medical, scope each expert’s assignment in writing to the questions that discipline answers, and read the drafts for sentences that stray. The instruction is unglamorous. The stray sentences are usually short, and sit near the conclusion, where the pressure to be useful is highest.
A retaining party should also ask what each expert finding depends on: if the conclusion would change with a different seat position, the report should say so first.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.