Answer to Radiologic Case Study
Medial Meniscal Ossicle
A 33-year-old woman presented with persistent right knee pain that had lasted for 2 years. The patient reported that the pain had started after she fell while ice skating. Radiographs of the right knee (Figure 1) showed a well-corticated small ossified density in the knee joint posteromedially. On magnetic resonance imaging, a triangular ossification that conformed to the shape of the medial meniscus about the root and posterior horn (Figure 2) showed fatty marrow signal. No meniscal tear was detected on magnetic resonance imaging, although this evaluation was limited by the presence of an ossicle. Because of the patient’s symptoms, a decision was made to proceed with surgical intervention.
Anteroposterior (A) and lateral (B) radiographs of the right knee showing a well-corticated ossicle in the knee joint posteromedially (arrows). Note the triangle shape on the lateral radiograph, similar to the meniscus.
T1-weighted (A) and T2-weighted (B) fat-suppressed magnetic resonance images of the right knee showing a focal ossification in the posterior horn of the medial meniscus near the meniscal root (arrows), consistent with a meniscal ossicle.
During the arthroscopic surgery, a meniscal ossicle was found at the root of the posterior horn of the medial meniscus. In addition, there was a radial tear adjacent to the meniscal root. The torn medial meniscal root was repaired arthroscopically with debridement of the meniscal ossicle. The patient’s symptoms improved after surgery.
Initial reports of meniscal ossicles date back to the early 19th century. Burrows1 described the first case: a 25-year-old man who was found to have an ossicle in the medial meniscus.2,3 The estimated prevalence of meniscal ossicles in the general population has been reported to be 0.15%.4 Meniscal ossicles are typically found in the posterior horn, at or near the meniscal root; it is extremely unusual to see ossicles in other locations.5,6
The etiology of meniscal ossicles has not been clearly defined. One theory suggests that meniscal ossicles are vestigial structures. This theory is supported by the presence of meniscal ossicles in animals such as rodents.7–9 Other theories for the etiology of meniscal ossicles include mucoid degeneration secondary to ossification9 and trauma resulting in heterotopic ossification.10 The current authors’ patient had a history of knee trauma and was found to have a meniscal tear during arthroscopic surgery.
Mohankumar et al11 recently suggested that the most likely etiology of meniscal ossicles is the occurrence of remote single or multiple injuries resulting in metaplasia of fibrocartilage and secondary heterotopic ossification. Similar to the findings in the current case, Mohankumar et al11 also observed an association between meniscal ossicles and meniscal tears, particularly posterior root tears.
No specific symptoms are associated with meniscal ossicles.12 Most patients with meniscal ossicles present with pain. Mechanical symptoms, such as locking of the knee, are rarely seen.13 Apart from knee pain, there was no other prominent symptom in the current case.
The differential diagnosis for meniscal ossicles includes meniscal calcification, osteochondritis dissecans, chondrocalcinosis, loose body, and avulsion of the semimembranosus or popliteus tendons.2,5 On histological examination, meniscal ossicles are seen as cancellous bone structures with fatty bone marrow surrounded by a cortex covered with hyaline cartilage.14 On radiographs, meniscal ossicles appear as triangular or rectangular calcified lesions.5 On magnetic resonance imaging, demonstration of ossification within the meniscal substance confirms the diagnosis of a meniscal ossicle. A fatty bone marrow signal is typically seen in meniscal ossicles on T1-weighted images,13 and homogeneous suppression of signal on fat-suppressed images is also expected.
Characteristic imaging findings and typical location of meniscal ossicles permit a straightforward diagnosis of this condition on imaging. Further interventions such as arthroscopy should be guided by clinical symptoms and may reveal additional findings, such as the meniscal tear seen in this case.
- Burrows HJ. Two cases of ossification in the internal semilunar cartilage. Br J Surg. 1934; 21(83):404–410. doi:10.1002/bjs.1800218306 [CrossRef]
- Yu JS, Resnick D. Meniscal ossicle: MR imaging appearance in three patients. Skeletal Radiol. 1994; 23(8):637–639. doi:10.1007/BF02580385 [CrossRef]
- Conforty B, Lotem M. Ossicles in human menisci: report of two cases. Clin Orthop Relat Res. 1979; (144):272–275.
- Schnarkowski P, Tirman PF, Fuchigami KD, Crues JV, Butler MG, Genant HK. Meniscal ossicle: radiographic and MR imaging findings. Radiology. 1995; 196(1):47–50. doi:10.1148/radiology.196.1.7784586 [CrossRef]
- Adams JE, Lopez-Ben R, Meyer RD, Siegal GP. A 32-year-old man with knee pain. Clin Orthop Relat Res. 2004; (427):272–277. doi:10.1097/01.blo.0000128673.93147.73 [CrossRef]
- Yen P, Hunter J, Escobedo E. Meniscal ossicle in the lateral meniscus: an unusual variant of a rare entity. Radiol Case Rep. 2009; 4(2):289. doi:10.2484/rcr.v4i2.289 [CrossRef]
- Pedersen HE. The ossicles of the semilunar cartilage of rodents. Anat Rec. 1949; 105(1):1–9. doi:10.1002/ar.1091050102 [CrossRef]
- Walker M, Phalan D, Jensen J, et al. Meniscal ossicles in large non-domestic cats. Vet Radiol Ultrasound. 2002; 43(3):249–254. doi:10.1111/j.1740-8261.2002.tb00998.x [CrossRef]
- Weber NA. Apparent primary ossification of the menisci in a dog. J Am Vet Med Assoc. 1998; 212(12):1892–1894.
- Harris HA. Calcification and ossification in the semilunar cartilages. Lancet. 1934; 223(5778):1114–1116. doi:10.1016/S0140-6736(00)94122-9 [CrossRef]
- Mohankumar R, Palisch A, Khan W, White LM, Morrison WB. Meniscal ossicle: post-traumatic origin and association with posterior meniscal root tears. AJR Am J Roentgenol. 2014; 203(5):1040–1046. doi:10.2214/AJR.13.11821 [CrossRef]
- Glass RS, Barnes WM, Kells DU, Thomas S, Campbell C. Ossicles of knee menisci: report of seven cases. Clin Orthop Relat Res. 1975; (111):163–171. doi:10.1097/00003086-197509000-00023 [CrossRef]
- Tuite MJ, De Smet AA, Swan JS, Keene JS. MR imaging of a meniscal ossicle. Skeletal Radiol. 1995; 24(7):543–545. doi:10.1007/BF00202157 [CrossRef]
- Yoo JH, Yang BK, Son BK. Meniscal ossicle: a case report. Knee. 2007; 14(6):493–496. doi:10.1016/j.knee.2007.07.008 [CrossRef]