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Beth Israel Deaconess Medical Center and Harvard Medical School Boston, MA 02215
I commend De Smet et al. [1] on their recent article and fully agree with their conclusion that "the presence of superior popliteomeniscal fascicle abnormalities...is significantly associated with a tear of the lateral meniscus." However, one might quibble with these authors' definition of an abnormal superior fascicle as being either "disrupted or absent." The absence of this fascicle or strut on static MR images may not always be abnormal because during the passage of the popliteus tendon peripheral to the posterior lateral meniscus, portions of both the superior and inferior fascicles are noncontinuous [2].
Difficulties in assessing the relationship between the lateral meniscal fascicles and a tear of the lateral meniscal have been recognized for many years. I quote from an article I wrote about knee arthrography two decades ago [3]:
The superior and inferior meniscal fascicles, or struts, of the lateral meniscus correspond to the attachments of the medial meniscus to the femur/ joint capsule and to the tibia/coronary ligament, respectively. A tear of one of these fascicles is, therefore, equivalent to a partial peripheral separation of the superior or inferior portion of the posterior medial meniscus. A tear of both fascicles would correspond to a complete peripheral focal separation of the posterior medial meniscus. The superior fascicle is noncontinuous laterally and the inferior fascicle is noncontinuous posteriorly, allowing for the entrance and exit of the popliteal tendon....The normal variation in the continuity of these fascicles makes many arthrographers reluctant to diagnose an isolated tear in the absence of abnormalities of the body of the meniscus or popliteal bursa. Although such a diagnosis is difficult to make, particularly on the basis of a single radiograph, there is little doubt that such tears do exist. Often the diagnosis can be made only at fluoroscopy by observing the freely moving fragments of the torn fascicles or the abnormal inward motion of the lateral meniscus. The latter is invariably observed when both fascicles are torn. Isolated tears involving only these fascicles, and not the body of the meniscus, are rare, but are probably more common than their medial meniscal counterparts.... The therapeutic significance of a (isolated) fascicle tear of the lateral meniscus, like that of a partial peripheral tear of the medial meniscus, is controversial.
References
University of Wisconsin Hospital Madison, WI 53792
Suburban Radiologic Consultants Minneapolis, MN 55437
Diagnostic Radiologists Cedar Rapids, IA 52402
We appreciate the opportunity to clarify the concept of the disrupted or absent superior popliteomeniscal fascicle [1]. Our article describes the MR appearance of the normal and abnormal popliteomeniscal fascicles as previously reported for knee arthrography and described by Dr. Hall [2] and in the anatomic dissections by Jelasco [3]. As the popliteal tendon penetrates the posterior horn of the lateral meniscus, the superior fascicle is formed. Both fascicles are then seen as the popliteal tendon passes towards the superior surface of the meniscus. As Dr. Hall notes, the superior fascicle is normally absent as the tendon passes through the superior surface of the meniscus. With careful positioning and traction on the patient's knee, an arthrographer can usually display these changes in appearance of the fascicles.
As we reported in our study on the normal MR appearance of the fascicles [4], consecutive sagittal MR images show the same configuration of the fascicles as seen on arthrography. In the posteromedial aspect of the lateral meniscus, the popliteal tendon is seen on MR imaging to penetrate the capsule. Immediately lateral to this area, the popliteal tendon penetrates the meniscus, forming the superior popliteomeniscal fascicle. On MR images at the lateral edge of the knee joint, the tendon passes through the superior portion of the meniscus, and there is normal absence of the superior fascicle at this point. We consider the superior popliteomeniscal fascicle to be abnormal when the fascicle is absent or disrupted on the MR images on which the popliteal tendon penetrates the posterior margin of the posterior horn of the lateral meniscus and the fascicle should be seen. Our experience continues to show that absence or disruption of the superior popliteomeniscal fascicle on MR imaging is a useful sign suggesting the presence of a tear of the posterior horn of the lateral meniscus.
References
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