Applied Radiology

Diagnosis

Meniscal ossicle

Findings

Sagittal T1-weighted and STIR magnetic resonance (MR) images and a lateral plain radiograph are provided. A well-defined lesion is present within the posterior horn of the medial meniscus, which is hyperintense relative to meniscal tissue on T1-weighted images and causes mild enlargement of the meniscus. On STIR imaging, the lesion demonstrates low signal intensity. A hypointense rim is also present on both images. The lateral plain film demonstrates a well corticated rounded ossific density in the posterior aspect of the knee joint.

Discussion

Meniscal ossicles are very rare lesions with an indeterminate etiology. Multiple theories have been proposed regarding the lesions. They may represent vestigial structures as they have been found in animals, such as cats and primates. Alternatively, some believe that they are post-traumatic lesions, either forming as a result of heterotopic ossification or avulsion. The lesions occur almost exclusively in the posterior horn of the medial meniscus. Patients tend to present with symptoms of knee pain, although in many cases the lesions are likely an incidental finding as other causes of knee pain are found upon imaging or arthroscopy. In the case above, the lesion was incidental as an alternative cause was found to explain the patient’s symptoms. In cases where no other abnormalities can be found to explain the patients’ symptoms, the ossicles may be resected.

Meniscal ossicles are usually first detected by plain films, but cannot be definitely localized to the meniscus by this modality and may represent loose bodies, a loose osteochondral lesion, or possibly chondrocalcinosis. Localization to the meniscal substance may be achieved via athrography, computed tomography (CT) arthrography, or potentially fluoroscopy in the hands of a well-trained radiologist. However, MRI is the examination of choice as it accurately localizes the ossicle to the substance of the meniscus and allows evaluation of other causes of pain. Meniscal ossicles will characteristically follow the signal intensity of normal bone marrow and demonstrate a hypointense rim. Differentiation of a meniscal ossicle from a loose body often will spare the patient unnecessary surgery as these lesions are usually treated conservatively with surgery reserved for refractory symptoms.

1. Schnarkowski P, Tirman PF, Fuchigami KD, et al. Meniscal ossicle: Radiographic and MR imaging findings. Radiology. 1995;196:47-50.
2. Lawande MA, Tavri S, Patkar DP, et al. Meniscal ossicle. Indian J Radiol Imaging. 2008;18:128-129.