Knee pain after an accident can take many forms.  There are two types: 

1.  Pain Due to Direct Trauma (Knee hitting the dash)

2.  Referred or overload pain from the low back

The most common injury is the knee cap hitting the dash.  This can cause a gouging of the cartilage on the undersurface of the knee cap.  This problem usually escapes detection on routine MRI scanning.  A surgical scope of the knee can identify and fix this problem.  There are also assorted ligament tears to the major ligaments of the knee including:

1.  PCL-Posterior Cruciate Ligament (lower part of knee hits the dash)

2.  ACL-Anterior Cruciate Ligament (upper part of knee hits dash or knee takes a side impact)

3.  MCL/LCL-Medial Collateral Ligament or Lateral Collateral Ligament (knee takes a side impact)

4.  Meniscus Tear-torn cartilage (knee takes a side impact or is jammed suddenly)

Referred pain from the low back is the most common cause of knee pain after an accident. 

Usually, problems within the SI Joint or with weakness in the multifidus muscles in the lumbosacral spine causes the hamstrings muscle to get inhibited (loose the ability to contract fully) and this leads to the knee cap rubbing against it's groove in the femur (patello-femoral syndrome).  Treatment usually involves:

1.  Fix the problem in the SI joint or low back (this problem will not resolve unless this problem is mitagated).

2.  Tape the knee cap so that it's pain-free with movement.

3.  Strengthen the VMO muscle only with standing exercises (no sitting strengthening with the foot free).

4.  In more recalcitrant cases, a new brace called Protronics can be helpful in retraining the hamstrings.

5.  Stay away from lateral release surgery (99% of these cases can be treated without undergoing knee surgery).