The lateral knee x-ray shows

significant effusion in the knee joint and an indentation on the lateral femoral condyle (lateral femoral notch sign). Together, these findings should raise clinical suspicion for an ACL tear. The patient underwent an MRI scan of the knee which confirmed a tear in the ACL and was referred to specialist for further management.

Further reading – https://pubs.rsna.org/doi/abs/10.1148/radiology.219.3.r01jn12800