Background: The anterior cruciate ligament (ACL) is the most commonly injured ligament of the human body. Most ACL tears occur secondary to pivot shift injuries on the planted foot. A sudden change in direction on the flexed knee with valgus stress may tear the ligament and precipitate a cascade of osseous collisions. Bone contusions on both surfaces of the knee joint, termed kissing contusions, may result. Kissing contusions are most common in the lateral compartment of the knee following an ACL tear. As energy increases, additional contusions may occur in the medial compartment. Patellotibial contusions represent a third, less common contusion pattern. This clinical vignette aims to introduce the clinical presentation and implications of patellotibial contusions.

Case Presentation: A 22-year-old male presents to the emergency department following a fall. Upon physical examination, a positive anterior drawer sign was noted. MR imaging confirmed the diagnosis of an acute ACL tear. Three kissing contusions were of note within the knee joint, consistent with patellotibial contusion.

Discussion: Patellotibial contusions are a less common ACL tear contusion pattern at MR imaging that usually results from an axial load such as a jump or fall. In addition to lateral and medial sided contusions, impaction of the inferior patella with the anteromedial tibial rim occurs. Not all patellotibial contusions result in the classic three pairs of kissing contusions. In fact, most patients present with five total contusions: kissing contusions laterally, kissing contusions medially, and an unpaired contusion of the anteromedial tibial plateau. One of the reasons proposed for lack of edema in the patella is that it is a sesamoid bone with a thick peripheral cortex, which may make the patella more resistant to bone marrow edema. Furthermore, patellotibial contusions are often associated with other knee injuries. While tears of the lateral meniscus are less common than those of the medial meniscus in the setting of most ACL tears, patellotibial contusions have an increased risk of tears of the posterior horn lateral meniscus. In addition, posterolateral corner (PLC) injuries are strongly associated with patellotibial contusions. As the anteromedial aspect of the knee narrows during patellotibial impaction, the posterolateral aspect of the knee joint must widen. If undetected on imaging, PLC injuries pose a greater risk of ACL reconstructing failure. Recognizing a patellotibial contusion on MR imaging will direct the physician to search for these associated injuries.