A dashboard injury refers to damage of the posterior cruciate ligament (PCL) inside the knee. When a sudden impact from behind occurs, the passenger's knees are pushed forward into the dashboard. The force of the knees against the dash can cause the tibia (lower leg bone) to shift back far enough to rupture the PCL.
We've known for a long time that the main function of the PCL is to keep the tibia from sliding too far back on the femur (thighbone). But more recent studies have shown us that the PCL has some other functions as well. It also keeps the tibia from shifting side-to-side during knee motion. And it prevents too much rotation of the tibia.
Without an intact PCL, the joint shifts change the contact and pressure points. This can result in the type of early degenerative changes you are experiencing. Treatment today is with early reconstruction in the hope of preventing this from happening.
If you haven't already done so, see an orthopedic surgeon for an evaluation. Find out what are your treatment options. You are probably a good candidate for a rehab program.
The physical therapist will prescribe a program to strengthen the muscles around the knee joint. In the absence of an intact ligament, strong muscles can help support the joint during motion and possibly hold off the negative effects of a torn PCL. Other exercises can help re-tune receptors in the joint that tell you where the joint is and what motion is occurring. This can help prevent further traumatic injuries of the joint and may improve motion.