Clinical History
A painful and stiff knee with locking.
Imaging Findings
The plain radiograph appeared to be within normal limits.
MR imaging demonstrated a displaced horizontal tear of the left lateral meniscus, with a truncated anterior horn (figs 1 and 2). The lateral meniscal root remained intact, but the displaced fragment had migrated through the popliteal hiatus and could be seen posterior to the popliteal tendon (fig 3).
Discussion
Background: Traumatic tears of the menisci within the knee are the most common soft tissue injury of the knee [1]. Unlike degenerative meniscal tears, where there is controversy over the need for surgical management [2], traumatic tears are still regarded as being best treated surgically. There are a variety of approaches ranging from suturing the tear itself to partial meniscectomy; however, the arthroscopic technique is not a comprehensive review of all joint compartments [3] but centres on removing any loose bodies and attempting to restore a smooth articular surface.
Imaging perspectives: As can be seen from this case, aberrant fragments do not always lie within the larger joint compartments, and the surgeons will find it particularly valuable to be directed toward unusual positions. It therefore relies upon the radiologist to perform a thorough search for the missing portion of the meniscus, and a sound knowledge of knee joint anatomy will aid us in constructing a comprehensive search pattern. The lateral meniscus is attached to the tibia by the meniscofemoral ligaments, but two fascicles of the meniscus separate to create an approximately 1cm gap through which the popliteal tendon passes [4]. This gap is the popliteal hiatus and represents a normal anatomic weakness in the posterior knee capsule. It is usually of little clinical relevance, and is not included in a normal arthroscopy [3] so can easily be overlooked.
Teaching point: it is essential that radiologists are comfortable identifying and describing detailed anatomy of the knee to aid the surgeons when treating displaced cartilage fragments.
Differential Diagnosis List
Horizontal left lateral meniscal tear, with displacement through popliteal hiatus.
None relevant
None relevant
Final Diagnosis
Horizontal left lateral meniscal tear, with displacement through popliteal hiatus.