Injury with Low-Speed Collisions
By Jeffrey Tucker, DC, DACRB
Can pain and dysfunction develop from a low-velocity collision without attendant injury? “Low-speed” impact refers to 1-2 miles per hour and goes up to 20-25 mph. “Moderate speeds” are 25-40 mph and “high speeds” are 40 mph and over. Jackson16 and States13 estimate that 85 percent of all neck injuries seen clinically result from automobile crashes, and of those due to such collisions, 85 percent result from rear-end impacts. Morris reported that rear-end impacts of as little as five mph can give rise to significant symptoms.
Emori and Horiguchi state: “Whiplash, in some cases, persists for years but usually no obvious symptoms show up with radiological or other quantitative diagnostic techniques.”9 Our present technology does not permit precise identification of deranged soft tissues [misleading…see my comments above].
Kenna and Murtaghsay state: “It is wrong to assume that maximum neck injury occurs in a high-speed collision; it is the slow or moderate collision that causes maximum hyperextension of the cervical spine. High-speed collisions often break the back of the seat, thus minimizing the force of hyperextension.”21
A major dilemma exists for the auto manufacturer, insurance companies, and the consumer of autos. Each would like the vehicle to provide the maximum protection for the occupant with the minimum material damage to the vehicles during a collision. Stiffer cars with spring-like rear bumpers that increase the rebound have less damage costs, however the occupant experiences an increased neck snap and the potential for greater injury. When a car gets struck from the rear by another auto, the very first thing that happens is the struck car is accelerated. The occupant of the struck care experiences higher speeds as it attempts to “catch up” with the car. Navin and Romilly state: “This relative movement of the head to the shoulder during the rebound is the likely cause of neck injuries as this is the point at which dynamic loading of the neck will be maximum.”8 They conclude: “Of major concern to researchers is the lack of structural damage [to the car] present below impact speeds of 15 kmh. This indicates that the bumper system is the predominant system of energy absorption between the impact and the occupant [ie. the car takes more of the impact so the passenger’s body has less energy to contribute to whiplash]. It was also observed that deflection of the seatback tends to pitch the occupant forward, with the shoulder displacement leading the head. This relative head to shoulder motion is the likely source of whiplash injury.”
Research has shown that high impact forces are transmitted directly to the occupant in low-speed impacts and that the vehicle does not begin to crush until impact speed exceeds 15 or 20 mph.1,13 Severy1 demonstrated a 10 mph impact produced total collapse of only 2 1/2 inches in the rear structures of the impacted vehicles. Therefore, minor property damage does not necessarily equate to minor injury. The most important question is not, “What is the damage to the vehicle?” but, “What was the acceleration to the vehicle that you were in?” Injury will occur because of the acceleration differences between the different inertial parts of the occupant’s body, especially from the person’s head, versus trunk inertial acceleration differences.
Headrests are the best protection in rear-end collisions. However if the headrest is set too low, the head is able to roll over the top of the headrest, producing even more hyperextension.2
The exact position of the head at the moment of impact is important to know. If the head is turned, the injury will be greater on the side it is turned to. When head rotation is present, the pattern of tissue injury is potentially more severe.19
A surprise collision will usually cause more injury because the ligaments will be injured more than the muscles. When a person knows they are going to be struck, they will tense up the muscles and therefore injure the muscles first. MacNab states: “In impacts up to 15 mph the right front seat passenger stands in greater danger of injury than does the driver, because the driver can brace himself to some extent by holding onto the steering wheel.”14
Common predisposing factors include degenerative joint disease [including disc degeneration] and spinal stenosis. The potential for injury is increased because the neck is less resilient.
The seatback stiffness requires investigation. The harder/stiffer the seatback the less forward acceleration and therefore the less injury. The less stiffer the seatback the more forward acceleration and therefore the risk of increased injury.
Jackson states: “The belt has very little if any deterring effect on the cervical spine as the head and neck continue forward motion. Even the addition of a shoulder harness will not relieve but will only increase the forces which must be absorbed by the head and neck, although such a harness may prevent contact injuries.”12 Seat belts save lives by preventing occupants from going through the windshield, but they contribute to the neck injury.
The Office of the Chief Scientist (London), Department of Health and Social Security, had this comment regarding seat belts in 1985: “We predicted an increase in the case of two injuries: sprains of the neck and fractures of the sternum. Both were confirmed. The other apparent increase in a major injury which was not predicted was abdominal injuries of organs other than the kidney and bladder.”
Myofascial structures can be stretched; asymmetric increase in muscle tension can develop, causing altered joint movement; the facets can become affected, and posture altered.
Dunn and Blazer7 concluded: “The most injurious head deflection in an acceleration injury is hyperextension. Even though sustained in low-velocity, rear-end collisions, this acceleration injury can produce forces significant enough to produce musculoligamentous tears with resultant hemorrhage and even disk disruption and avulsion fractures of the vertebral bodies. In addition, the integrity of the apophyseal joints may be violated.” They also conclude that in head-on collisions (flexion injuries): “In low- velocity flexion accidents, because the chin strikes the chest when the full range of physiologic flexion has been reached, minimal damage occurs.”