This month’s Garagram examines the question-could an injury have been prevented by seatbelt use?
The injury accident occurred when a small car was struck at the left front by a van. The driver was listed on the police report as unbelted.
The driver suffered a number of injuries from the impact, most of which were minor in nature. The single significant injury to the driver was a split fracture of the lateral aspect of the right tibial plateau (a broken leg).
The question was, “Would the injury have had a significantly lower probability of occurrence if the seat belt had been used?” To answer the question, our biomechanical expert examined three areas:
1. The driver’s whole body motion due to the impact (kinematics).
2. The loading mechanism that produces the specific injury suffered (lateral fracture of the tibial plateau).
3. The experimental and epidemiological evidence related to restraint used and knee injuries.
1. Kinematics: At impact the driver would move toward the point of impact. In this case, that was forward and to the left. This motion would result in left side head contact with the B-pillar or left front window, left lateral knee/thigh contact with the left front door panel, and right knee impact with the steering column.
The described kinematic motion would be modified to some extent by reflexive bracing. Because the sequence of events leading to impact happened in front of the driver, the driver would have reflexively braced. Bracing the right foot on the brake (or against the floor pan) would force the pelvis rearward stabilizing and fixing the right lower extremity between the floor pan and the seat back.
2. The loading mechanism (lateral fracture of the tibial plateau): Fractures of the tibial plateau occur as a result of strong varus (inward) or valgus (outward) forces combined with axial loading (such as pressing hard on the brake pedal). When varus or valgus forces are combined with axial loading, the respective condyle (the knuckle of the joint, the round projection or rounded articular area) exerts both a shearing and compressive force on the underlying condyle. The applied forces produce either a failure fracture or ligament injury, but rarely both. It is generally believed that intact collateral ligaments are the determining factors with respect to whether fracture or ligament damage occurs (in other words, an already loose joint will tear the ligaments, a strong/tight joint will transfer the impact forces to the bone). Accordingly, intact collateral ligaments on one side of the knee are necessary to create a fracture of the contralateral plateau. The medial collateral ligament acts to limit medial opening of the joint and increases the valgus force as it drives the lateral femoral condyle into the lateral aspect of the tibial plateau, causing a fracture. The result can be a split fracture, a depressed fracture, or both. The magnitude of the forces applied determines both the degree of fracture comminution (the process of grinding or crushing a solid into fine particles) and the degree of fracture depression.
The frontal collision with pre-impact bracing of the right lower extremity, right knee fixed by the steering column, and leftward motion of the torso, describes exactly this well established mechanism for producing lateral split fracture of the right tibial plateau.
3. The experimental and epidemiological evidence: Studies by Pattimore et al. (1991), Thomas et al. (1995), Kuppa and Fessahaie (2003), Morgan et al. (1991), McGovern et al. (2000), and Rastogi et al. (1986) were examined in detail and found to be applicable to this particular accident. The conclusion drawn from the review of the scientific literature was that the incidence of femoral fracture was not significantly different for restrained or unrestrained occupants.
Based on his review of the evidence and applicable literature, as well as his education, training, and experience, our expert noted that the driver’s kinematics created the precise mix of loading conditions recognized as producing lateral tibial plateau fracture. Therefore, he concluded that if the driver had been wearing the seat belt the probability of a broken leg from this accident would have been the same.