Trauma Biomechanics

The human body is exposed to mechanical loads throughout its life. Aside from forces deriving from ubiquitous and penetrating fields, such as gravity, or forces due to electromagnetic fields that are non-contact in nature and as such effective over distances, there is a great variety of forces acting on the human body from contact with the surrounding environment. In addition, numerous forces are generated in the course of physiological processes inside the body, in the different organs and tissues. Throughout evolution, all forms of life, including plants, animals and humans, adapted their physiology to mechanical interactions; some of them to the extent that proper function requires the influence of forces, for example bone remodelling. Mechanical forces are known to modulate cell development even in utero.

Work in trauma biomechanics is subject to a number of limitations that are less stringent, or even totally absent, in other fields of the technical and life sciences. First, experiments involving loading situations with humans that could cause injury are excluded. Secondly, animal models are of limited use because of the difficulty of scaling trauma events reliably from animals up or down to humans. Questionable representativeness of animal models with respect to human biomechanics, in spite of some similarities, poses another problem. And finally, cost, public awareness and, above all, ethical considerations further limit how and what type of experiments can be conducted.

Traumatic brain injury (TBI) remains a significant source of mortality and morbidity throughout the world, in part because of the complex mechanisms. The complexity arises not only from the complicated intracranial biomechanics, but because that mechanical stimulus is only the start of a series of biological responses that unfold over minutes to days after the event. In this chapter, co-authors Nevin Varghese and Barclay Morrison explore the effect on cells of the primary injury that occurs during the mechanical event and the ensuing biological mechanisms of the secondary injury process.

Head injury sustained in accidents remains a leading cause of death and disability, even though considerable advancement has been made with respect to injury prevention. Successful prevention relies on a sound understanding of injury mechanisms and knowledge of the biomechanical responses, as investigated in various experimental studies. More recently, research addressing the consequences of rotational head loading has received significant attention. This holds particularly true for sports, where concussion is a major topic, but also for applications such as helmets and corresponding test standards in other fields where the rotational component is subject to scientific debate. Furthermore, the increased use of human body models (HBMs) continues to offer new insights. Accordingly, previous research has been critically questioned, new experiments have been conducted and new injury criteria have been proposed.

The potential for long-term impairment, including para- and quadriplegia, is always inherent in injuries to the spine, in particular to the spinal cord. Of all spinal segments, the cervical spine is the most frequently injured. Considering that the head and the neck form one functional entity, head loading often also implies neck loading.

Injury to the thorax commonly occurs in front and side vehicle impacts, as well as in all impact directions intermediate to these two. Impact to the thorax is frequently observed due to contact with, for example, various components of the vehicle interior, like the steering assembly, safety belt, the door or the dashboard. In the case of sports injuries, thorax impacts may be due to contact with an opponent player or to direct blows.

The abdominal cavity is a vulnerable region of the human body. In general, trauma to the abdomen is caused by blunt impact or by penetration. In automotive accidents, blunt impact is observed frequently, although the injury might not be apparent initially.

Injuries to the lower extremities play a major role in sports such as soccer or skiing. They have also emerged as the most frequent non-minor injury resulting from frontal vehicle crashes. Since injuries of the extremities are often the reason for long-term impairment, occupant safety systems dedicated to protecting the extremities (e.g. vehicle knee airbags) have been introduced to the commercial market in recent years.

Injuries to the upper extremities are common in sports and therefore receive considerable attention. Various studies have addressed the kinematics of the upper extremities in different motion patterns, such as throwing, a golf swing or a tennis stroke. Many studies can also be found on the diagnosis and treatment of upper extremity sports injuries. This is not the case with regard to injury mechanisms, however. Many questions remain unanswered and in terms of injury criteria and injury threshold levels, no conclusive literature has become available yet.

An accident is defined as a violent, unusual and possibly harmful event that occurs suddenly and unexpectedly and is mostly of a short duration. Persons involved in an accident can generally not react, or their reaction is insufficient, to prevent injury. The term “chronic”, by contrast, implies that a process extends over durations that are long in comparison with typical accident time intervals. Accordingly, the persons involved will always have physical and mental reactions to the event and these cannot be disregarded or ignored. Of primary importance, however, is the fact that under conditions of chronic mechanical overexposure and/or overuse, impairment of health and injury may result from a level of mechanical load to which the body is exposed or from functional misuse in the course of some physical activity that is well below the tolerance limit for a single, isolated exposure (such as is described in the other chapters of this book), but whose effect is aggravated and outperformed by the extended duration during which it acts. This is particularly the case in sports, in the workplace and in the household. The exposure may be interrupted often and limited to regularly or irregularly occurring time intervals, e.g. to training periods (sports) or work assignments (jackhammer), which may however extend over years.

Exposure to blast and ballistic threats occurs in both defence and civilian environments and is, unfortunately, still common today (see also Chap. 1). Ballistic injury refers to the interaction of a projectile and the human body leading to penetrating or blunt trauma, while blast injury refers to detonation of an explosive and the subsequent complex interaction of the blast, fragments and debris with the human body. There is, of course, overlap between ballistic injury and blast fragmentation. Some common areas of blast injury include the lower extremity, thorax and head, although all body regions may be exposed to blast loading. Similarly, all body regions are susceptible to ballistic injury where protection is often first focused on life-sustaining organs (heart, lungs and brain). Active areas of research include head injury in blast, vehicle and vehicle occupant blast protection, and fragmentation and ballistic protection of the head, face and thorax, including behind armour blunt trauma (BABT).

Most chapters include questions which allow the reader to test and deepen his/her understanding. Exercises start with the letter E. Here, solutions to these exercises are presented. In addition, problems (labelled with P) are also provided. Solutions to those problems are available to lecturers only and can be obtained directly from the publishers.