Purpose: This study presents cases of fracture-dislocation of the thoracic spine in extension during severe rear impacts. The mechanism of injury was investigated.Methods: Four crashes were investigated where a lap-shoulder-belted, front-seat occupant experienced fracture-dislocation of the thoracic spine and paraplegia in a severe rear impact. Police, investigator and medical records were reviewed, the vehicle was inspected and the seat detrimmed. Vehicle dynamics, occupant kinematics and injury mechanisms were determined in this case study.Results: Each case involved a lap-shoulder-belted occupant in a high retention seat with ≻1,700 Nm moment or ≻5.5 kN strength for rearward loading. The crashes were offset rear impacts with 40-56 km/h delta V involving under-ride or override by the impacting vehicle and yaw of the struck vehicle. In each case, the occupant's pelvis was restrained on the seat by the open perimeter frame of the seatback and lap belt. The rear loading caused the head, neck and upper body to displace off the side or top of the seatback. The seatback frame acted like a fulcrum as the unsupported head and upper body was accelerated forward causing extension of the spine around the seatback frame as the head and shoulders moved rearward of the frame. In each case, there was fracture-dislocation of the thoracic spine in extension with spinal cord injury resulting in paraplegia. Two occupants were overweight and two were obese, which increased inertial loads on the spine.Conclusions: High retention seats have improved safety of occupants in rear crashes, but there are situations where the upper body becomes unsupported in a severe rear crash. This can lead to extension loads on the spine causing fracture-dislocation, spinal cord injury and paraplegia. Injury is a result of the strong seat frame remaining upright, the lap belt holding the pelvis on the seat and the upper body moving off the seatback concentrating load on the thoracic spine. The seatback frame acts like a fulcrum resulting in fracture-dislocation of the spine in extension.