Many types of incidents, including low-speed vehicle impacts, elevator stops, and trips or slips and falls, result in injury claims that may or may not be attributable to the incident. Biomechanical engineers can answer questions of injury causation by comparing the mechanism of the claimed injuries with the mechanical forces present in the incident. These questions can range from determining whether an incident caused a specific claimed injury to reconstructing an incident from the injuries to determine if it occurred as claimed. Answering these questions requires evaluation of the forces applied to the human body, understanding of the body’s motions in response to forces, and knowledge of body tissue tolerance to these applied motions and forces. Many injury claims involve pathologies that can either occur acutely in a traumatic event or develop over time as a result of wear and tear and/or degenerative processes. Therefore, understanding how mechanisms and injury patterns differ between traumatic injury and degeneration is also important when analyzing injury causation.
To assess the loads acting on the body, one generally starts by analyzing the subject incident. For incidents involving motor vehicles, accident reconstruction is performed to determine the forces acting on the vehicle and the consequent vehicle kinematics. These, in turn, are used to assess the motions and forces experienced by the occupants. For many low-speed vehicle interactions, the biomechanical engineer can conservatively estimate the motions and forces of the vehicle based on repair records, photographs, and/or witness testimony, along with vehicle specific data and a fundamental understanding of vehicle structure mechanics. For some other types of incidents, direct measurements of forces or accelerations can be made during a recreation of the event. For example, for a claim of injury during an emergency elevator stop, the emergency stop can be recreated and the motions of the elevator directly measured. For more sophisticated reconstruction, biomechanical engineers can draw upon expertise and resources throughout Exponent’s practices to analyze even the most complex accidents.
Once the motions and forces associated with an incident are known, their effects on the human body can be evaluated. For low-speed vehicle interactions, loads applied to the body and the body’s motion in response to those loads can be evaluated by comparison with appropriate low-speed crash tests of similar severity. Exponent has performed numerous tests with instrumented anthropomorphic test devices (ATDs), also known as crash test dummies, which allow for the direct measurement of the motions and loads at specific body parts. Research studies, including several performed by Exponent, with human volunteers wearing instrumentation has also allowed measurement of motions and loads at specific body parts. For other types of incidents, such as a trip or slip and fall, or impact with an object, appropriate testing, either performed by Exponent or available in the literature, can be referenced to determine the loads acting on either the body as a whole or on a specific body part. Exponent’s biomechanical consultants can also employ computer modeling, when appropriate, to asses a body’s motions and response to applied forces.
In order to determine whether a claimed injury occurred acutely during a specific incident, the biomechanical engineer analyzes the mechanisms required to cause the injury traumatically and investigates whether those mechanisms were present in the subject incident. This includes an examination of the direction, magnitude and point of application of forces, and an examination of the presence or absence of related injures. Is the direction of impact or the relative motion of components within a joint consistent with the injury? Is the force sufficient, relative to established injury tolerances to cause the injury? Are there associated injuries that correspond to the same traumatic loading mechanism, or associated pathologies consistent with degeneration?
Spinal disc injuries are a common claim arising from low energy incidents. A disc, located between two bony vertebral bodies, is comprised of a gelatinous center, the nucleus pulposus, which is encapsulated by a fibrous structure, the annulus fibrosis. A disc herniation is a protrusion of the nucleus polpusus through the annulus fibrosis, which may impinge upon the spinal cord or nerve roots and lead to pain. Disc herniations typically result from a degenerative process and are often caused by the loads repeatedly applied to the spine over a long period of time during everyday activities. However, acute loading of the spinal column can also cause traumatic herniation. To determine whether a particular incident caused a traumatic herniation, the biomechanical engineer first assesses whether the incident resulted in loading of the vertebral disc. If a potential mechanism for disc loading exists, the magnitude of the load can be compared not only with established tolerance values for the general population, but also with the individual’s tolerance as demonstrated by his or her daily activities. Additionally, the presence or absence of adjacent vertebral injuries can assist in the determination of whether a disc herniation was traumatic.
Shoulder pathology is another example of a common injury claim. Often the specific anatomy of a shoulder lesion, such as a labral tear, indicates the specific manner in which the shoulder was loaded. The shoulder is a ball and socket joint, in which the “ball” of the humeral head articulates with the shallow “socket” of the glenoid. The socket is deepened by a rim of cartilage called the glenoid labrum. Labral tears can occur traumatically when the humeral head compresses against and slides relative to the glenoid, shearing a portion of the labrum. A biomechanical engineer can analyze the anatomy of the tear, determine the direction of force application required to cause a labral tear in a particular location, and evaluate whether a specific incident provided the requisite loading mechanism either directly to the shoulder or indirectly through the arm. Often there are a variety of potential mechanisms, traumatic and degenerative, which need to be evaluated in the context of the incident and in the existence and absence of associated trauma or pathology. Similar differential analyses can be performed for other shoulder pathologies, such as rotator cuff tears and impingement syndrome.
Claims of knee injury are another common subject of biomechanical injury causation analysis. The knee joint is comprised of the distal femur, which articulates with the tibial plateau and the patella. The joint is supported by various soft tissues, including the cartilaginous menisci and the collateral and cruciate ligaments. As in other joints of the body, the bones and the soft tissues of the knee can fail acutely in trauma or can deteriorate over time with mechanical wear and tear and/or through degenerative pathologies. Traumatic injuries of the knee require force applications to load the tissue and generally reveal a pattern that indicates the combination and direction of the applied forces. Other knee pathologies, such as ligamentous tears, are associated with a variety of possible loading combinations, including knee shear, torsion, hyperextension or varus or valgus bending. In evaluating whether knee injury was caused by a particular event, the incident is first evaluated to determine whether direct knee contact or indirect loading of the joint occurred. If loading was possible, the biomechanical engineer then evaluates whether the direction and magnitude of the forces in the event are consistent with the anatomy of the specific lesion (e.g., is the direction of a contact force consistent with the location and direction of a meniscal tear?). Also, as in spine and shoulder pathologies, the presence or absence of associated knee injuries or degenerative pathologies can inform whether a claimed injury occurred as a result of traumatic loading or as a part of a degenerative process.
The above are just a few examples of questions biomechanical engineers can answer with regard to injury causation. Using our extensive resources and expertise, Exponent can reconstruct and analyze the prevalent forces in a wide array of incidents, and then compare the magnitudes, directions and combinations of those forces with motions of the human body and mechanisms necessary to cause many types of claimed injury.