For those of you that do not recognize the visual allusion above, it is a simulacrum of a famous bit of Americana: Lucy pulling the football away from Charlie Brown. Once I became a doctor, I began to look at this a bit differently, and began wondering what kind of injuries Charlie Brown might sustain thanks to Lucy's wicked intentions. Ah, the loss of innocence....
In this case, a 17 year old female sustained a knee hyperextension injury one day ago, and heard a "pop". The orthopedic surgeon suspected an ACL tear, and referred the patient for an MRI scan.
A sagittal proton-density image reveals that the ACL (pink arrow) and posterior cruciate ligament (PCL, green arrow) are intact, and identifies a tear of the posterior capsule (red arrow).
In a normal patient, the posterior capsule (blue arrows) is easily seen:
A sagittal T2 fatsat image of our patient depicts the tear and the associated edema:
An axial intermediate image also identifies the tear:
In a normal patient, the posterior capsule green arrows) is easily seen:
Posterior capsular tears are associated with hyperextension injuries of the knee. Hyperextension of the knee can result when direct force is applied to the anterior tibia while the foot is planted or from an indirect force, such as a forceful kicking motion (Sanders et al., Radiographics 20, S135-S151, 2000). Hyperextension injuries can result in tears of the posterior capsule, ACL, PCL, anterior bone contusions, and injuries of the posterolateral corner. There may be associated meniscal tears as well. The pattern of injury correlates with the mechanism of injury (Hayes et al., Radiographics 20, S121-S134, 2000).
MRI signs of a tear of the posterior capsule include frank disruption and high signal intensity in or adjacent to the capsule on T2-weighted MR images. In the intercondylar area, however, normal openings for the vascular structures and nerves may be present, and these should not be confused with tears. (De Maeseneer et al., AJR 182, 955-962, 2004).
Posterior capsular tears are commonly associated with PCL injuries, but can occur without significant ligament damage, as in this case.
Vic David MD