It is known that 80–85 % of all osseous injuries observed in the knee after a tear of the anterior cruciate ligament (ACL) are found in the lateral compartment [
1‐
3]. The injury mechanism that leads to an ACL tear involves an anterior subluxation of the tibia in relation to the femur. This can provoke a collision between the lateral femoral condyle and the postero-lateral edge of the tibial plateau, [
4,
5] which may result in a “kissing contusion”. The location of this engagement depends on the degree of flexion of the knee while subluxating. The more forceful such an impact is, the greater the damage to the lateral femoral condyle may be [
6]. In most cases the lateral femoral notch has only radiographic significance with no need for surgical treatment. These lesions like bone bruises may only be visible with magnetic resonance imaging (MRI), while in other cases an impaction of the subchondral cortical bone may be apparent on a conventional X-ray image. These highly developed injuries have a clinical relevance due to deformation of the articular surface of the femoral condyle, as a possible precursor of osteoarthritis [
7]. Such osseous injuries with impaction fracture of the lateral femoral condyle are referred to as the lateral femoral notch sign. These have been reported to accompany ruptures of the ACL in 20–60 % [
4,
7,
12]. Apart from being an sign for the presence of a torn ACL, [
8] these impaction fractures can also be relevant due to their long-term detrimental effect to the joint [
9,
10]. As knees with abnormal notches on the femoral condyle showed lateral meniscus injuries more frequently than those without such notches [
11]. Cobby et al. reported that patients with an intact ACL showed an average depth of the sulcus of 0.5 mm while patients with a torn ACL showed an average sulcus depth of 0.9 mm. The authors concluded that a sulcus depth exceeding 1.5 mm which corresponds to a standard deviation 3 times above the norm could be considered as a reliable indicator of an ACL rupture, a cut-off, which was also chosen in this investigation to define patients for further investigation of their MRI [
8]. In most cases, this finding may not be clinically relevant in terms of the need for a surgical repair. Although very little literature exits, it seems that more extensive impactions that lead to a relevant deformation of the articular surface of the femoral condyle can be associated with an increased risk of early-onset osteoarthritis [
7]. Attempts have been made to treat extensive osteochondral impaction fractures of the lateral femoral condyle, in order to prevent long-term consequences such as osteoarthritis [
7,
12,
13]. For this reason several authors tried to characterize the localization of this radiologic sign. Kaplan et al. reported in their study, that the bone bruise would be located in the central to anterior part of the lateral femoral condyle, but they had not divided the condyle into sub-areas [
14]. Speer et al. investigated sagittal and coronal MRIs in order to define the precise localization of the impaction fracture on the lateral femoral condyle. They reported that the lesions were predominantly located in the area of the sulcus terminalis and obviously would tend to be located laterally rather than medially [
15]. Graf et al. were the first group who divided the lateral femur in defined regions for a better specification of the location [
1]. Therefore the goal of this present study was to precisely determine the size and location of impaction fractures on the lateral femoral condyle (“lateral femoral notch sign”) in patients with an ACL rupture.