What is Kinematics?
Kinematics comes from the Greek word “kuma” or “kumat”, which means movement. It is the study of movement without regard to the forces or masses involved. Trauma is all about movement as all trauma, whether it be blunt or penetration trauma, involves the phenomenon of movement.
All injury relates to movement and its relationship to the body. The source of the movement may be as commonplace as a motor vehicle or a penetrating foreign body. It can also be as esoteric as the moving particles found in heat injuries, blast injuries, or ionizing radiation. Kinematics is all about velocity and acceleration. When a car accelerates and strikes a pedestrian, the trauma received is all about kinematics.
Take this scenario:
You and your partner are EMTs dispatched to a collision involving two vehicles. The first automobile is in a ditch at the right hand side of the road and has hit a tree on the passenger side of the door. There are two bullet holes. In the left driver’s side door. There are two people inside the car. The second car is an old one without airbags. It, too, has veered off of the road and has hit a utility pole on the left side of the road. The utility pole struck the car in the front, between the two headlights. There are two people inside that car as well. The steering wheel is bent and there is a fractured windshield on the driver’s side. There is also an indentation on the lower part of the passenger-side dashboard. No one in either car was wearing a seat belt. All people in the car are injured and none have been ejected. As the head EMT, it is your job to decide the priority of treating the patients. You take each one based on the kinematics of each person’s injuries.
In the scenario, the driver of the car with the side impact has two left-sided bullet wounds, one of which is below the rib cage and one is the area of the rib cage. The blood pressure is low so you suspect a pneumothorax. There could also be a hemothorax (blood in the lung area), penetration of the heart and possibly a puncture of one of the major blood vessels. The lower bullet wound could have struck any area of the abdomen and there could be bleeding.
In patient number 2, the passenger of the first vehicle, there is energy exchanged between the door and the individual. There could be injury to areas such as the clavicle, the chest wall, the lungs, the abdomen, and the pelvis. There could be fractured clavicles, fractured ribs, lung bruising, sheering of the aorta, pneumothorax (air in the lung space), fractured liver, and fractured spleen. There could also be a deceleration injury to the kidney, a fractured pelvis, and an injury caused by rotation of the cervical spine.
In patient number 3, the driver of the second vehicle, you suspect the ca went up and over the side of the road before it hit the utility pole. There could be blunt chest injury or head injury. There could also be a bruising of the heart, pneumothorax, flail chest, bruising of the lungs and a pressure injury to the abdomen. The liver and spleen could be fractured and there could be cervical spine fracture or brain injury.
In patient number 4, you suspect the patient went down and under the dashboard causing ankle, femur, or hip injuries. There can also be facial injuries, and cervical spine injuries. You wonder where the bullets come from and decide to search each victim for weapons.
Unexpected traumatic injuries cause greater than 169,000 deaths in the US per year. Motor vehicle crashes account for more than 37,000 deaths and more than four million injuries per year. Injuries from firearms are also high with 31,000 deaths due to firearms. Thirteen thousand of these injuries were fatal and 78,000 were not fatal. In some parts of the world, blast injuries are more common than in the US.
In traumatic injuries, it is the history of the impact and the energy exchange that results from the impact that determines what injuries are present. Understanding the kinematics or the mechanisms of injuries will lead to the correct suspicion of 95 percent of potential injuries. Injuries that are not obvious but are still dangerous can be fatal if they are not managed at the scene or on the way to the hospital.
Much information can be gathered even before seeing the patient. In this section, we look at the kinematics of trauma, including blunt and penetrating trauma. The laws of physics involved are those that are all about energy exchange and the general effects of energy change. Mechanical principles play into the interaction between the human body and things like cars, three-wheeled vehicles, two-wheeled vehicles and falls, as well as penetrating trauma and blasts.
A crash is nothing more than the exchange of energy between two objects when something solid like a human body hits a motor vehicle or pavement.
There are three phases:
• Precrash—conditions leading up to the impact, including the person’s medical condition and state of mind.
• Crash—from the time of impact from one moving object to another.
• Postcrash—when information is gathered about the crash and precrash phases so a patient can be managed.
Energy involves the initial events that occurred at the time of the crash; it is important to understand the amount of energy that was exchanged with the human body. According to the First Law of Motion, a body will stay at rest and a body will remain in motion unless acted on by an outside force.
There are three phases: 1) the car hitting an object; 2) the possible patient inside the vehicle crashing into the vehicle; and 2: the internal organs being torn from their supporting structures.
According to the Second Law of Motion, energy can’t be created or destroyed, just changed in form. Energy goes from the vehicle to the patient in a crash and through to the organs in the body. The change in energy takes the form of vehicular damage and injury to the patient unless a seatbelt or other restrained can make the dissipation of energy less harmful. There is a greater energy exchange when the victim is bigger and when the velocity of the crash is greater.
The shorter the stopping distance and the faster the vehicle stops increases the injury to the patient. When brakes are applied, the force is dissipated differently as when the vehicle crashes into a brick wall without the breaks. A related thing applies when a person falls on a firm surface or onto powdered snow. Concrete provides more injury than a compressible material, such as snow. When a vehicle strikes a pedestrian, it is slowed a little bit, while the pedestrian is thrown a lot because they don’t weigh much. The amount of energy exchanged will depend on the type of organ that is injured. Lung tissue injures differently than, say, liver tissue.
In a trauma, two cavities are created: 1) a temporary one caused by stretching of the tissues and 2) a permanent cavity caused by tissue destruction. The difference between the two is the elasticity of the tissues. More elastic tissue has more temporary cavitation when compared to less elastic tissue. The energy exchange is the same between blunt or penetrating trauma.
The important things to think about are the direction of the impact, the external damage to the vehicle and the intrusion of the energy into the patient’s space.
There are many types of blunt trauma, but motor vehicle crashes are the most common. About 86 percent of fatalities were to occupants of vehicles. The rest were pedestrians or cyclists. There are front impact collisions, rear impact, lateral impact, and rotational impact (rollover injuries). In a front impact injury, some of the energy is absorbed by the bending of the vehicle and more is absorbed by the impact of the steering wheel hitting the chest. This energy goes back through to the back of the individual.
The use of the seatbelt and the deployment of an air bag will absorb much of the energy so that the patient’s injuries are less. In an up-and-over injury, the person goes up and over the steering wheel, striking usually the windshield. The force is absorbed along the spine. In a down-and-under path, the person moves forward and downward and out of the seat, into the dash. Lower extremity injuries can be of the foot, the knee, the hip, the pelvis, or of major vessels that are torn during impact. Some of these injuries can be subtle.
In rear-impact collisions, a slower vehicle is overtaken by a “bullet vehicle” that is travelling faster. The greater the difference in speed of the two vehicles, the greater is the force of the impact and the energy transferred. Some energy is stored by the springs in the seats and the headrest. The neck is hyperextended and can be injured.
In a T-bone injury, there is lateral impact. The vehicle struck absorbs some energy by being moved out of the way of the striking vehicle and stops when the energy is absorbed or when it hits another object. There can be injuries because of side trauma. Injuries are worse when the victim is on the near side of the accident as opposed to being from the side of the impact.
In a rollover, the car is impacted on all sides and energy can come from any direction. There can be fractured organs and ribs. More serious injuries come from being unrestrained. In one study, a total of 77 percent of ejected individuals were killed.
In lateral impacts, there is more injury to the lateral injuries because there is less protection from the car than from a frontal impact. SUVs and pickups have less injury because they are struck lower in relationship to the patient.
Seatbelts save lives by transferring energy to the firmer pelvis and across the trunk, when they are worn correctly. Airbags offer even more protection. Side airbags are effective in lateral collisions.
Pedestrian injuries have three phases: 1) the initial impact to the legs; 2) the torso rolls onto the hood of the car; and 3) the victim falls on the ground, usually head first. Kids, being curious, are likely to be struck in the front, while adults are struck in the back attempting to escape. The pedestrian can be struck by another vehicle in addition to the initial vehicle, increasing injury.
In a fall, the victim who falls from greater heights will have a higher incidence of injury than when falling from a shorter height. Where the victim lands (hard or compressible) makes a difference. Energy forces travel from the feet to the knees to the pelvis, the hips and finally to the spine. Any part can be injured. If the fall is to the outstretched hand, arm and hand injuries can occur. The forces are the same if a person is ejected from an ATV, bicycle, skateboard, or motorcycle. Athletes are less injured than non-athletes because of protective equipment that absorbs impact.
The aorta is a source of injury because it can be torn from its foundation. About 80 percent of the time, the patients die at the scene, while 20 percent die within three days.
When the abdomen is injured, the diaphragm is the weakest part and can be torn and ruptured as the force within the abdomen increases. Organs can shear, such as the kidneys, small and large intestine and the spleen. The liver can be lacerated. The pelvis is a ring so it is generally broken in two places.
There are also bullet-type injuries and explosive injuries that fall under the realm of kinematics but these go beyond the scope of this section.
Kinematics help EMTs and emergency doctors discover whether or not the injury is life threatening. About 95 percent of injuries can be anticipating by understanding the kinematics of a collision. If injuries are left untreated, they can contribute to injury and death secondary to trauma. Damage to the body tissue is a function of both the kinetic energy applied and the tissue’s ability to tolerate the forces applied to it. Ejection causes a reduction in the protection at the time of impact. Energy-absorbing devices can save lives. Pedestrian injuries are different from person to person, depending on the height of the victim and where the impact hits the victim. The target at the end of the fall may absorb some of the impact, such as snow and bushes.
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