Injury to the extensor mechanism; the quadriceps tendon, patella, and patella tendon, or a large knee joint effusion, are potential causes that prevent the perform a straight leg raise (SLR).  Clinical examination can be equivocal and the lack of patella displacement in partial thickness injuries necessitates the need for further imaging.  

A lipohemarthrosis signifies an intra-articular injury, and whilst many will be accustomed to identifying a lipohemarthrosis on a horizontal beam lateral knee radiograph, it can be overlooked on ultrasound.  

Similarly, many will be used to identifying the different bone marrow contusion patterns in knee injuries on MRI ^1, and less so on CT.  Having a high index of suspicion and a structured approach with CT interpretation can identify the injuries sustained in a recent lateral patella dislocation.