How Often Does This Happen?

I have posted my MRI here before but long story short - ACL Avulsion Fracture, Medial Meniscus Tear, Lateral Meniscus Tear.

Particularly with regard to the Medial Meniscus the MRI report read: There is a peripheral red zone vertical tear in the posterior horn on series 7 image 9. There is an avulsion fracture from the medial femoral condyle induced by the deep medial meniscal femoral capsular ligament, with fragment measuring 9 x 5 mm. There is no medial meniscal capsular separation. There is intact cartilage in the medial compartment.

I am now a couple of days post surgical and read the post operative report. The Orthopedist said he looked at the medial meniscus but did not find a tear. It was a little over 30 days between the injury and the surgery.

How often does the MRI have a false positive with a meniscus injury?