The Mid Back
The mid back or thoracic spine is most commonly injured as a result of the asymmetry of most seat belt designs. As the torso moves forward the seat belt locks and the torso is forced to stop at the end of the shoulder strap. Because the shoulder strap crosses only one shoulder (usually the left), the side without a strap (usually right) is able to travel forward further which results in a forward flexing and twisting injury to the mid back.
The Lower Back
The lumbar spine is also frequently injured, but the mechanism differs from the above. As the torso and pelvis move forward the pelvis is stopped by the lap belt. However, the torso continues to move forward which producing a shearing force in the lumbar spine. These injuries often result in lumbar disc injuries.
Spinal injuries can damage a number of pain sensitive structures located in the spinal region. Some of the more common structures that are damaged and that have innervation which allows them to transmit pain include:
- anterior longitudinal ligament (runs down the front of the vertebral bodies-prevents excessive extension)
- outer layers of intervertebral disc (located between adjacent vertebrae)
- posterior longitudinal ligament (runs down the back of the vertebral bodies-prevents excessive flexion)
- nerve root dura
- capsule of the facet joints
- intervertebral ligaments
- extensor musculature
- flexor musculature: the colli and scalenes
When these tissues are injured, pain is experienced. Damage to these structures also results in tissue inflammation, edema, microscopic hemorrhage, and the release of noxious chemicals such as histamine, prostaglandins, substance P, and kinins which further hypersensitize already painful and injured tissues. In addition, when spinal injury occurs spinal muscles often reflexively spasm to protect the area from further injury – whether or not the muscles are injured. This spasming can lead to muscle ischemia (lack of oxygen) due to increased energy demands.