By Loren K. Peck[1]
Accidents happen. When they do, injuries may become symptomatic immediately or manifest months later. Insurance coverage often depends on whether injuries can be attributed to a particular accident. Litigators attempting to prove causation in the courtroom have traditionally relied on medical practitioners to testify as expert witnesses.
Some scientists now claim, however, that engineers trained in biomechanics are better equipped than doctors to determine injury causation. Biomechanical engineers study the way mechanical forces affect the human body. While mechanical engineers measure the failure strength of buildings, biomechanical engineers attempt to measure the tolerance of structures inside the human body.
Biomechanics came into the spotlight around the 1970s, when the National Highway Safety Bureau began funding biomechanical research to improve the crashworthiness of vehicles. In the late 1970s, litigators began hiring biomechanical experts to testify about design defects in products liability cases. Over time, biomechanical engineers’ testimony strayed into the realm of injury causation. This prompted litigators to challenge biomechanical engineers’ opinions under Federal Rule of Evidence 702 and Daubert v. Merrell Dow Pharmaceuticals, Inc.[2]
Biomechanical Methods
Biomechanical engineers who attempt to determine injury causation must uncover two variables: (1) the amount of trauma the biological structure in question could have endured without injury, and (2) the amount of force applied to that structure in the accident. Engineers have sought answers to the first variable through limited testing on volunteers, cadavers, and animals. Based on these studies, researchers have proposed risk curves for some structures.[3]
The research underlying risk curves is imperfect. Research on living human subjects is critical to the accuracy of these curves; yet, exposing humans to non-trivial trauma is unethical. Gathering data from real-world accidents has limited value because critical variables are uncontrolled, undiscoverable, or depend on the accuracy of subjects’ memory.
Engineers extrapolate from risk curves to opine whether an accident involved enough force to cause injury to a specific region. This is the most controversial part of biomechanical experts’ methods. Differences between individuals including age, weight, pre-existing injuries, athleticism, and genetic predisposition to injury, among other things can affect the validity of such extrapolations. For example, a college football player suffered a concussion after a hit that generated forces well below those predicted to cause such an injury. In fact, the force was near the 1% mark on the risk curve.[4] Other players in the study sustained impacts far above the predicted threshold with no injury. Id. The researchers had no explanation except that “[t]he variation in injury tolerance between individuals explains why high severity impacts cause [mild traumatic brain injury] in some players but not others.” Id. at 358.
In another example, biomechanical research predicts that a force resulting in a Head Injury Criteria (HIC) of 1000 will cause serious but non-fatal head injuries. Experience has shown, however, that “it is possible for a person to walk away uninjured from an accident with an HIC greater than 1000 while a person can die from a head injury in a crash where the HIC was significantly less than 1000.”[5]
In an attempt to find answers to the second variable, biomechanical experts employ two methodologies in combination: accident reconstruction and occupant simulation. Accident reconstruction has become typical in personal injury cases. Occupant simulation is less common and requires complex computer programs that apply laws of physics in virtual environments. The “people” in these simulations are exist as ellipsoids connected by joints that are based on crash test dummies. The same limitations of real-world dummy crash tests appear in these simulations, with the additional problem that thousands of parameters are required to return realistic results. Many parameters are difficult or impossible to obtain following real-world accidents.[6]
Courts’ Response
Courts that have considered the admissibility of biomechanical experts’ injury causation opinions generally follow one of two approaches. A growing majority of courts around the country have adopted the Sixth Circuit’s approach in Smelser v. Norfolk Southern Railway.[7] A second approach was developed by the Supreme Court of Delaware in Eskin v. Carden,[8] but this approach has not found much acceptance outside Delaware.
The Smelser Approach
The Sixth Circuit in Smelser made a distinction between general and specific causation, a separation uncommon in personal injury cases but typical in toxic tort litigation. Courts following Smelser find biomechanical engineers qualified only to testify regarding general aspects of injury causation. This is the same limitation imposed on toxicologists and epidemiologists. The reason for this limitation is that biomechanical risk curves were intended to predict the risk of injury in a population. As the Sixth Circuit in Smelser explained, biomechanical engineers are unqualified to consider “the different tolerance levels . . . of individuals” or the “pre-existing medical conditions of individuals.”[9] Medical doctors, on the other hand, are trained to diagnose injuries in unique individuals.
There is a fine line between general and specific causation opinions. For example, a court found the following opinion “general”: The force “was sufficient to have caused [plaintiff’s] mild traumatic brain injury.”[10] In contrast, another court labeled this opinion “specific”: The force “did not result in [plaintiff’s] concussion injury.”[11] The difference between the two may seem trivial, but the general-specific distinction has significant legal consequences. For example, evidence of general causation alone is usually not enough to make out a prima facie case or to succeed on summary judgment.
The Eskin Approach
Courts following Eskin do not recognize the general-specific distinction, and typically find biomechanical engineers qualified to give injury causation opinions. However, Delaware courts scrutinize biomechanical expert methodologies under the reliability and application prongs of Rule 702. Most notably, when a plaintiff is unlike subjects in the biomechanical research, Delaware courts have excluded biomechanical expert opinions for lack of fit.[12]
Conclusion
In conclusion, judges are struggling to find the proper place for biomechanical engineers in the courtroom. A majority of courts have allowed biomechanical engineers to explain general aspects of injury causation. However, a growing number of courts are finding biomechanical engineers unqualified to opine whether a specific injury is attributable to an accident. In most jurisdictions except Delaware, medical doctors remain the only experts qualified to testify to both general and specific injury causation.
Loren Peck participated in the drafting of Daubert motions regarding biomechanical experts while employed at Peck Hadfield Baxter & Moore in Logan, Utah. Loren recently graduated from Washington & Lee University School of Law and is currently employed as a judicial clerk in the federal judiciary.
[1] This article is adapted from a Note for the Washington and Lee Law Review (73 Wash & Lee L. Rev. 1063).
[2] 509 U.S. 579 (1993)
[3] See, e.g., Liying Zhang, King H. Yang & Albert I. King, A Proposed Injury Threshold for Traumatic Brain Injury, 126 J. Biomechanical Engineering 226, 226 (2004) (explaining the development of the Head Injury Criterion (HIC)).
[4] J.R. Funk et al., Biomechanical Risk Estimates for Mild Traumatic Brain Injury, 51 Ann. Proc. Ass’n Advancement Automotive Med. 343, 357—58 (2007).
[5] Steven C. Batterman & Scott D. Batterman, Forensic Engineering and Science, in Forensic Science and Law: Investigative Applications in Criminal, Civil, and Family Justice 566 (Cyril H. Wecht & John T. Rago eds., 2006).
[6] See Michael B. James et al., Limitations of ATB/CVS as an Accident Reconstruction Tool, SAE TECH. PAPER 971045, Feb. 24, 1997, at 1.
[7] 105 F.3d 299 (6th Cir. 1997).
[8] 842 A.2d 1222, 1229 (Del. 2004).
[9] Smelser, 105 F.3d at 305.
[10] Berner v. Carnival Corp., 632 F. Supp. 2d 1208, 1213 (S.D. Fla. 2009) (internal quotation marks omitted).
[11] Roach v. Hughes, No. 4:13-CV-00136-JHM, 2015 WL 3970739, at *12 (W.D. Ky. June 30, 2015).
[12] See Eskin, 842 A.2d at 1229 (excluding biomechanical expert testimony that failed to establish a link between generic injury thresholds and a unique individual).
