Studies clearly show that wearing seat belts saves lives and prevents injuries. For the most part, they also reduce the severity of many injuries.
However, it's also true that some injuries are more likely to happen when wearing a three-point seat belt system. The three-point system includes the shoulder-lap combo.
For example, the number of cases of neck sprain is higher in seat belt wearers. Compression/burst fractures occur more often in restrained front-seat passengers. Scientists aren't sure why this happens. They suspect that the shoulder strap holds the chest back as the body is moving forward.
The restraining force acts against the middle part of the spine called the thoracic spine. The thoracic spine tends to be in a forward-curved position. Suddenly and forcibly straightening the thoracic spine increases the load down through the spine. It's this force that can result in vertebral fractures.