Saturday, August 7, 2010

"Pop" and an Unusual Meniscus

A common complaint that is heard in doctor's offices after a knee injury is, "I heard a pop, and then my knee swelled up". This complaint is usually related to a tear of the anterior cruciate ligament, but can sometimes also indicate a tear of the meniscus.

Occasionally, a "pop" can be felt by the examining physician as well, but this is relatively rare. Conditions that may cause painful snapping about the knee include congenital snapping knee, discoid lateral meniscus in children, a torn meniscus, intraarticular rheumatoid nodules, synovial plicae, and iliotibial band syndrome. Subluxation of tendons has been reported to cause this painful snapping syndrome about the knee, medially by the gracilis and semitendinosus tendons, and laterally by the biceps femoris and popliteus tendons (Bach and Minihane, AJSM 2001, 29:93-95).

In this case, a 32 year-old female presented to her orthopedic surgeon with knee pain. The surgeon noted that as the patient extended her knee into full extension, there was a "pop" in the posterolateral knee. She was referred for an MRI, to evaluate for internal derangement.

A sagittal proton-density weighted image reveals abnormal signal in the anterior horn of the lateral meniscus (red arrow), but is otherwise unremarkable:

The astute MRI technologist (a former naval medic) was puzzled by the relatively innocuous nature of the finding in the anterior aspect of the meniscus, particularly given the strong clinical history of a pop in the posterolateral knee. At the end of the examination, he asked the patient if she could do anything that would elicit the "pop". She obliged, and squatted downwards, and the "pop" occurred. The MRI technologist gingerly placed her back in the magnet, and repeated the sagittal:

Here, after the provocative maneuver, we see a striking peripheral tear of the posterior horn of the lateral meniscus which has been displaced anteriorly, leaving only fluid where the meniscus should be (green arrow).

An axial image better reveals the avulsed lateral meniscus (blue arrows), which has been displaced anteriorly:

Compare this to the position of the meniscus (yellow arrow) on the initial (before the provocative maneuver) axial scan:

Intermittent meniscal dislocation has been discussed in the literature (Lyle et al.Br J Radiol. 2009, 82:374-9). They described three patients with a strong history of intermittent knee locking, who had negative initial MR scans. The patients were able to reproduce locking of their knee voluntarily, as in our case. Further MR imaging of the knee in the "locked" position demonstrated meniscal dislocation in all three patients. All three were confirmed arthroscopically to have deficiency of the corresponding menisco-capsular ligaments (as was our patient).

When there is a strong clinical history of knee locking, all the structure of the knee must be carefully inspected on MRI, particularly the menisci, anterior cruciate ligament, and the hyaline cartilage. When no abnormality can be detected, it is a good idea to scrutinize the peripheral attachments of the meniscus. The meniscocapsular junction is a difficult area to analyze, with abnormalities easily missed (and overcalled as well). With higher resolution imaging now becoming increasingly common, it has become easier to detect abnormalities in this area with greater confidence.

In the exceptional case, provocative imaging can be performed, and may help demonstrate an intermittently dislocating mensicus.

Vic David MD