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